Practical Pin Tooling
MASSACHUSETTS4N TIUTE
OF TECHNOLOLGV
by
JUL 14 2014
Benjamin J. Peters
LIBRARIES
S.B. Mechanical Engineering
Massachusetts Institute of Technology, 2011
Submitted to the Program in Media Arts and Sciences
School of Architecture and Planning
in partial fulfillment of the requirements for the degree of
Master of Science in Media Arts and Sciences
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
September 2013
@
Massachusetts Institute of Technology 2013. All rights reserved.
Signature redacted
Author ...........
Program of Media
and Sciences
August 09, 2013
//
Signature redactedCertified by ...............................
......
............
Neri Oxman
Assistant Professor in Media Arts and Sciences
esis Supervisor
Accepted by................
Signature redacted
Prof. Patricia Maes
Associate Academic Head
Program in Media Arts and Sciences
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2
Practical Pin Tooling
by
Benjamin J. Peters
Submitted to the Program in Media Arts and Sciences,
School of Architecture and Planning
On August 9, 2013, in partial fulfillment of the
requirements for the degree of
Master of Science in Media Arts and Sciences
Abstract
A high resolution reconfigurable mold has been sought after for over 150 years. An ideal
reconfigurable mold would be capable of producing detailed surfaces for use in molding
and be capable of fast surface reconfiguration. Such a device would combine the fast
speed and repeatability of formative processes, like injection molding, with the flexibility
of additive processes, like 3D printing. An affordable, high resolution, reconfigurable
mold could be a practical tool for a lean, short-run shop or factory, serving as a resin
mold, fixturing surface or concrete formwork; to name only a few possible applications.
Resembling the common pin art toy, a discrete element pin bed has often been
proposed as a design for a reconfigurable mold. Conventional actuation strategies are
often cumbersome and expensive, due to the quadratic increase in number of pin
elements with increasing surface resolution. Very few designs exist that are capable of
producing a reconfigurable pin array at sufficiently high resolution to compete with the
cost of making a fixed geometry mold. A practical pin tool, an inexpensive but high
resolution molding tool, could be an asset to shops and small business everywhere, but
nothing like this currently exists.
This thesis presents a concise design strategy, backed up by analytical arguments
and empirical evidence which can used to construct an affordable, high resolution
reconfigurable pin tool. We address the challenge of quadratic scaling by edgeaddressing simple, nonlinear mechanisms to actuate each pin. Edge addressing, along
rows and columns of pins, reduces the number of control inputs to a small, linearly
increasing value. Nonlinear mechanisms at each discrete pin site allow for a predictable,
localized expression of the edge addressed inputs. Two embodiments of this design
strategy are proposed and tested. First, a diode/resistive heater array allows for
electrical edge addressing and control of a fusible alloy brake to grip or release targeted,
sliding pins. Second, an array of screws is edge addressed by mechanical oscillations to
generate nonlinear tangential forces in targeted screws, resulting in controllable screw
translations. We conclude by summarizing our results and discuss directions for future
work.
Thesis Supervisor: Neri Oxman
Title: Assistant Professor in Media Arts and Sciences
3
Practical Pin Tooling
by
Benjamin J. Peters
The following people served as readers for this thesis:
Signature redacted
.......
Thesis Reader .............
.......................
Neil Gershenfeld
Director, Center for Bits and Atoms
Program in Media Arts and Sciences
Signature redacted
Thesis Reader .....................
Joseph Jacobson
Associate Professor
Program in Media Arts and Sciences
'A
4
Acknowledgements
First, I would like sincerely thank my advisor, Neri, for her incredible,
unwavering support during the years of this work. I'm eternally grateful to be
given the opportunity to push my boundaries as a designer and engineer and
pursue my passions.
I'd also like to thank my readers, Joe and Neil, for lending
an ear and providing sage advice and inspiration. Thanks also go to Prof. David
Hardt, a pioneer in reconfigurable pin tooling, who graciously served as my thesis
advisor during my undergraduate research into reconfigurable molds and helped
to fuel my interest in this topic.
I am especially thankful for the many discussions with my talented colleagues
and friends in the MIT Media Lab, you guys bring color and energy to a
transparent and cold building. The Mediated Matter group: Steve, Liz, Yoav,
Michal, Zjenja, Markus, Jared, Jorge, Laia and Carlos, thank you for your
patience and for sharing your time and space with me. I'd also like to thank my
comrades in the Mechanical Engineering department for providing inspiration
and a helping hand when it was needed.
I'd be remiss if I didn't also specifically thank Eric, my original partner-in-crime
on the reconfigurable mold project, for his great attitude, tireless dedication and
immense brain.
Finally, I must thank my family and my girlfriend, Caitlin, for
their continued interest and support in my research projects, half-baked ideas
and endless puns.
Everyone mentioned above has made a formative and lasting impression on me
and helped to mold my experiences and discretely reconfigure my life.
5
Contents
Acknowledgem ents ........................................................................................................................ 5
Chapter 1 ...................................................................................................................................... 11
Introduction .................................................................................................................................. 11
M otivation .................................................................................................................................... 11
Background ............................................................................................................................... 12
Prior A rt ................................................................................................................................... 18
Classification of Technologies ............................................................................................ 18
Historical developm ent ....................................................................................................... 19
Conventional Actuation ...................................................................................................... 21
Serial Pin A ddressing .......................................................................................................... 23
Braking Actuation ............................................................................................................... 25
Hybrid approaches ............................................................................................................... 27
Surface interpolation ........................................................................................................... 27
Deficiencies ............................................................................................................................... 28
Requirem ents ............................................................................................................................ 28
Scope ......................................................................................................................................... 28
Chapter 2 ...................................................................................................................................... 29
Design Philosophy ....................................................................................................................... 29
Introduction .............................................................................................................................. 29
Serial Actuation in Tools ....................................................................................................... 30
Parallel Actuation in Tools .................................................................................................... 30
Distributed A ctuation ............................................................................................................. 31
Distributed Actuation in Pin Arrays ................................................................................ 32
Chapter 3 ...................................................................................................................................... 34
Electronic Brake Pin Setting ..................................................................................................... 34
Introduction .............................................................................................................................. 34
Braking M echanism s ............................................................................................................... 38
M echanical ............................................................................................................................ 38
M agnetic ............................................................................................................................... 41
Electrical ............................................................................................................................... 41
6
9
Shape M em ory Actuators ................................................................................................... 41
Piezoelectric .......................................................................................................................... 41
Active Fluids ........................................................................................................................ 41
Therm ally Rheological Fluid Braking .................................................................................. 42
Proper array cooling ............................................................................................................ 49
Chapter 4 ...................................................................................................................................... 51
Vibration Induced Pin Setting ................................................................................................... 51
Introduction .............................................................................................................................. 51
Screw bundles ........................................................................................................................... 52
Screw packing ........................................................................................................................... 53
Selective screw rotation .......................................................................................................... 55
Force Analysis .......................................................................................................................... 61
Chapter 5 ...................................................................................................................................... 67
Conclusion .................................................................................................................................... 67
7
List of Figures
Figure 1: Metal and plastic pin screen novelty toy [7] [81.................................................14
Figure 2: CGI pin based dynamic mold, Mission: Impossible - Ghost Protocol, 2011 [10]
15
.......................................................................................................................................................
Figure 3: Large reconfiguring topographical map, X-Men, 2000 [11].............................16
Figure 4: Pin based topographical map with holographic overlay, After Earth, 2013 [12]
16
.......................................................................................................................................................
Figure 5: A literal "bed of pins" medical support, The Wolverine, 2013 [13]...............16
Figure 6: Flow chart of the classification and nomenclature for the tools of interest to
18
th is research .................................................................................................................................
Figure 7: A manually adjustable spring forming device, Williams et al, 1923 [19]..........19
Figure 8: A device for taking impressions of feet and forming foot supports [21]......20
Figure 9: A high resolution mold, manually configured, with pins 0.03" in thickness [22]
21
.......................................................................................................................................................
22
Figure 10: Lead screw driven pin actuation.......................................................................
Figure 11: Vibrating stylus reconfigures positions of smooth pins [28]..........................24
24
Figure 12: Interlocking threaded pins [31]..........................................................................
Figure 13: Interlocking pin array actuation; accomplished by a three axis robot able to
25
turn screw s to desired heights [31].......................................................................................
[33].....................................26
et
al.
by
Cook,
surface
molding
resolution
Figure 14: High
Figure 15: Actuation arrangement in the device proposed by Cook, et al. [33] ........... 26
Figure 16: Wang's reconfigurable mold, before and after milling [34] ...........................
27
Figure 17: Zagar Inc. 142 spindle head [35] .......................................................................
31
Figure 18: Comparison of various actuation techniques used in a drilling operation......33
35
Figure 19: Pin braking array schematic, side cross section ............................................
35
Figure 20: Pin braking detail, side cross section ..............................................................
36
Figure 21: Pin braking actuation example .........................................................................
Figure 22: Left: an array of thermally buckling flexures, right: FEA analysis of a single
beam heated to 150C [25]...........................................................................................................39
Figure 23: Schematic of a chevron-type thermal actuator [36]; below equation was used
to approximate the deflection needed; design was further refined in FEA. [25]............39
Figure 24: Left: CAD design of flexure array, right: micro-machined array.................40
Figure 25: Single pin, fusible alloy prototype [25]............................................................
Figure 26: Cross section of thermal braking arrangement. ...............................................
Figure 27: Pins can stay fixed, move up or move down simultaneously in this
43
44
configuration. An 'X' over a joint assumes that it is clutched........................................44
Figure 28: Surface pressure testing apparatus. Pins were soldered into a PCB with low
temperature alloy, supported by acrylic and subjected to test molding pressures........45
Figure 29: A prototype of the use of shift registers to control a transistor and diode (our
nonlinear elements) row column addressing technique in heater array fabrication...........45
8
Figure 30: Top view of high resolution heater board. Shift registers controlled
transistors on rows and columns of the array, addressing current inputs to and outputs
from the grid of resistors. A diode was added in series with each resistive heater to
channel the rows and columns and prevent current from just passing through the
shortest path in the array. Variable heating and discrete element control was
accomplished via pulse width modulation of transistor inputs to the array. ................. 46
Figure 31: Bottom view of high resolution heater board. Without exceeding the
specifications for our components too much, the array was built at the maximum density
we could manage with inexpensive discrete components. Resolution was 0.13" pin to pin
46
spacing, hexagonal packing ...................................................................................................
Figure 32: Integrated into a working vacuum former, a fusible alloy micro-brake array.
47
.......................................................................................................................................................
Figure 33; Rubber pin-tips were used instead of a continuous interpolator to investigate
47
th eir feasib ility .............................................................................................................................
Figure 34: Parts were vacuum formed and the pin array held its shape......................48
Figure 35; The rubber tip, discrete interpolation seemed to work well at this resolution.
48
.......................................................................................................................................................
Figure 36: Integrated electronics underneath the rubber pin heads...............................49
Figure 37: Large copper tube branches off into many orthogonal smaller tubes, soldered
50
to th e P C B . ..................................................................................................................................
was
soldered
pipes,
Figure 38: Thin copper tubing, running from the large side manifold
directly to specially designed pads on the bottom surface of the PCB to allow for
controllable heat exchange .....................................................................................................
50
52
Figure 39: A bundle of threaded rods ................................................................................
Figure 40: Enlarged view of the helical thread engagement between adjacent screws .... 52
Figure 41: Basic types of dense pin packing (top view)...................................................53
Figure 42: Single red arrow represents a force input and direction, small green arrows
represent resulting force propagation from the input vector. ...............................................
54
55
Figure 43: A ctuator assem bly...............................................................................................
Figure 44: Nonlinear properties of forward displacement and return stroke. F1<<F2.56
Figure 45: Patterned screw actuation, dislocations are exaggerated. Left image shows
direction of initial displacement and right image shows return stroke as well as the
56
direction of rotation of targeted screw. ...............................................................................
Figure 46: Modified Piezoelectric Vibrating Feeder..........................................................58
58
Figure 47: Threaded coupling from vibrating mechanism to edge screw. ............
Figure 48: Upward screw translation test. Images show the displacement generated by
the vibrating inputs at five second intervals. Screws are 3/8-16 and one inch long........59
Figure 49: Downward screw translation test. Images show the displacement generated
by the vibrating inputs (reversed from test in figure 48) at five second intervals. ........... 60
Figure 50: Free body diagram of dislocation stroke (out) and resetting stroke (in)........61
Figure 51: Graph of dislocation (outward) stroke scaling...............................................63
Figure 52: Graph of return (inward) stroke scaling..........................................................65
9
Abbreviations
Reconfigurable Pin Tooling (RPT)
Variable Geometry Molds (VGM)
Digitally Reconfigurable Surface (DRS)
Computer Aided Design (CAD)
Numerical Control (NC)
Computer Numerical Control (CNC)
Shape Memory Actuators (SMA)
Micro Electromechanical Systems (MEMS)
Nomenclature
3D printer: An umbrella term encompassing many types of additive fabrication
machines.
Voxels:
Addressed positions in three dimensions, so-called volumetric pixels.
Serial Actuation: An actuation strategy in which a single operation is performed,
followed by another, by the same mechanism, until the process is complete.
Parallel Actuation: An actuation strategy in which many parallel mechanisms can
perform many operations at the same time.
Distributed Actuation: A second order actuation strategy for a machine tool that
includes simple actuators distributed over a parallel array, with the intention of the
array to be used as a serial device in a larger system.
"stacking" of serially operated parallel actuators.
10
A distributed actuator is the
Chapter 1
Introduction
Motivation
The driving force behind this research comes from the dream of an affordable, high
resolution reconfigurable discrete element mold, a versatile tool for directly converting
electronic designs into physical shapes.
The author and many like-minded colleagues
predict that the existence of this tool would be groundbreaking for molding and rapid
prototyping-both in the shop and at home. This work first introduces the inspiration
behind and history of reconfigurable molds. We then offer a design strategy that can be
used to construct an affordable, high resolution reconfigurable mold. The challenge of
quadratic scaling is addressed by edge addressing simple, nonlinear mechanisms at each
discrete pin. Electronically addressed braking elements and screws rotated by vibration
are proposed as likely candidates for embodiments of the proposed edge-addressed
actuation architecture.
The initiation of this research project was motivated by personal experiences in
rapid prototyping and product design. Specifically, the primary inspiration for this
research occurred during the course of a project that involved a particularly high degree
of tedium; hand carving thirty different molds to form plastic parts for a Halloween
costume-a Star Wars "Stormtrooper" armor set. Still in my early days as a student at
MIT, I didn't have the access to or knowledge of CNC tooling that would have made the
detailed carving much easier, so I had little choice but to proceed manually. It seems
11
that a great way to inspire invention is by impatiently struggling with a task that is so
tedious that one's mind can't help but imagine contraptions to make the task job easier
or faster. It wasn't long after this manual mold making that I became interested in
rapid prototyping tools, inspired in part by popular, at-home 3D printers. Hands-on
experiences in fabrication have led to a personal interest in not just making products,
but with making tools. What intrigues me is the process of improving and passing along
tools from generation to generation, providing usefulness that is compounded with each
generation.
Modern tools in this way are nothing less than functional embodiments of
the combined knowledge of countless generations of engineers and builders-we are truly
indebted to these tool-makers throughout history.
The topic of this thesis, design of a practical, high resolution reconfigurable
surface, has become somewhat of an obsession for me. I would like very much to use this
tool for vacuum forming, resin casting and composite layups; and, seeing as it does not
yet exist, I have been trying to build one. The scope of this project has been a unique
challenge and has driven a large part of my development as an engineer.
During the two
years of this Master's degree, I have been blessed with the freedom and encouragement
to pursue this self-motivated research vector (among many others). I am sincerely
grateful for the opportunity to write this document and am enthusiastic to share my
passion for fabrication tools.
Background
This section introduces the concept of a reconfigurable material, both in fact and fiction,
moving on to describe various real world applications of a simplified reconfigurable
material, the discretely reconfigurable pin array.
Along with the teleporter, artificial gravity and faster than light travel, a selfactuating, reconfigurable material is often described in science fiction. Noteworthy
examples of this shape-shifting trope are the T-1000 liquid metal terminator seen in
James Cameron's Terminator 2, or the "holodecks" of Gene Roddenberry's Star Trek
film series.
Purposefully mediating matter from one form into another describes the ultimate
goal of the material engineering disciplines-manufacturing, chemistry, biology, etc. A
perfectly reconfigurable piece of matter would be the ultimate Swiss-army knife,
containing limitless possible tools and forms inside a tangible, physical volume.
12
Speculative works of fiction often describe such a perfectly reconfigurable object
as composed of micro-scale robots, largely inspired by Drexler's Engines of Creation [1],
working in concert to produce an aggregate behavior as an intelligent and sometimes
malicious swarm [2]. Encouraging academic work into developing self-reconfigurable
objects points to intelligent blocks [3], choreographed folding chains
14] and re-engineered
biological mechanisms [5]. Surprisingly, detailed and diverse reconfigurable materials are
actually quite common and can be found hiding in plain sight. These materials are,
naturally, biological systems. Unfortunately, it seems that the more advanced a system,
the more difficult it is to control or manipulate.
Biological systems are immensely
complex and many processes that make up these systems are still not completely
understood. If a real, practical attempt is to be undertaken into making a reconfigurable
material that can be applied to processes today, it would need to involve a certain
amount of simplification.
In pursuit of this practicalreconfigurable material, a simplified mechanism has
been proposed: a bundle of rods or pins-a so-called "bed of nails"-to generate a three
dimensional surface from the topography defined by tops or bottoms of the rods in the
bundle. This arrangement constrains the three dimensional addressable positions to one
degree of freedom, set by the axial position of each rod with respect to a fixed global
reference. The resolution of this discretized surface is defined by the diameter and
packing density of these rods. A familiar example of this mechanism is the novelty Pin
Art or Pinscreen toy, patented by Ward Fleming in 1985 [6], and illustrated in figure 1.
This product is made from a dense array of metal or plastic pins held loosely in a plastic
frame.
The frame and the pins are shaped such that each pin can easily translate along
its axis, but the pins can't fall completely out of the assembly. This arrangement allows
for artistic impressions to be made of common objects (often body parts).
13
Figure 1: Metal and plastic pin screen novelty toy [7] [8]
By itself, a pin screen has limited utility. Impressions of an object are better and
more often made with a cast rubber mold, with clay or even a 3D scanner.
It has also been proposed that this pin array device would be able to
automatically configure and reconfigure its pin elements into many different surfaces
derived from an electronic input-not simply from an object's impression. Now this is
where it gets interesting. For the sake of context, it is illustrative to compare this
physical pin display to the graphical pixel displays on computers, televisions and other
media devices. Compare, for example, the camera, which can store visual impressions, to
the pin screen toy in figure 1, which stores physical impressions. As large of a
technological leap as it was from a pinhole camera to television screen, a pin screen toy
compares similarly to an electronically controlled reconfigurable pin surface. This is to
say that the construction of this "simplified" version of an ideally reconfigurable material
is still a significant challenge but could yield benefits comparable to the utility of a
television screen over a photograph.
Many applications exist for a reconfigurable surface, despite its simplified and
discrete nature.
Surfaces are what designers are most often concerned about, as this is
where a user will interact with a product. A reconfigurable surface would be an efficient
tool for simulating cases and shells for products. As for other possible applications, it has
been proposed that a reconfigurable pin surface could be used indirectly as a tool for
generating molding surfaces and forming dies, or directly as a physical display or tactile
media interface.
Complicated mechanical fixturing, such as robotic end effectors for
delicate part handling could benefit from an electronically controlled, high resolution pin
14
surface.
Precise surface deformations, such as those required in applying adaptive optics
principles to large telescope lenses, are also within the domain of a reconfigurable pin
surface [9]. Pushing the limits of imagination: reconfigurable surfaces on performance
vehicles , including aircraft , for adaptable and dynamically reconfigurable aerodynamic
properties; reconfigurable sitting or sleeping surfaces, or specially designed patient
handling beds; hyper-redundant locomotive structures , like the legs of a millipede or cilia
on a cell; adaptive acoustical structures to deaden or amplify different frequencies of
sound ; dynamic molding, where pins apply stress to a molded material to generate
internal pressures to harden or soften areas of the molded material; in-mold component
assembly where multiple parts are made in one casting and the mold reconfigures its
geometry such that the parts are cleaned and/ or assembled within the mold before being
ejected; or even discretely reconfigurable ski resorts , where the mountain would
dynamically reconfigure large columns of land, generating an infinite and endlessly
varying slope.
Looking into popular culture and the media with this specific idea of a
reconfiguring pin surface in mind , we can find many examples and various niche
applications for reconfigurable pin surfaces. Unfortunately, these examples exist almost
exclusively in science fiction. In the images below, pin surfaces are being used for
topographical maps, dynamically reconfiguring molds, and even for high tech medical
furniture.
Figure 2: CGI pin based dynamic mold, Mission : Impossible - Ghost Protocol, 2011 [10]
15
Figure 3: Large reconfiguring topographical map, X-Men, 2000 [11]
Figure 4: Pin based topographical map with holographic overlay, After Earth, 2013 [12]
Figure 5: A literal "bed of pins" medical support, The Wolverine, 2013 [13]
16
It should be noted that the movies and movie trailers that these images were
captured from are all either newly released this year, 2013, or are recent films.
Reconfigurable surfaces are featured as objects of wonder in present day science fiction
because they are still just that, fiction. Reconfigurable pin surfaces just don't physically
exist in this advanced form.
After discussing potential applications and seeing exciting images of how pin
surfaces could be useful, it may be unclear why devices like these are so incredibly rare.
The tantalizing thing about pin surfaces is that they also seem functionally simple. It's
just a bunch of rods with a bunch of motors, right? There doesn't seem to be any
intractable mechanical parts, any magical, levitating parts or use of exotic materials. So,
why can't one purchase an electronic reconfigurable pin surface? Haven't engineers tried
to make these before? The answer is yes, attempts with varying degrees of success have
been made for at least 150 years now. The next section reviews the various prior art of
reconfigurable pin surfaces and tools, cataloging various strategies and mechanisms in
attempt to develop some intuition as to which characteristics are necessary for a highperforming functional device.
17
Prior Art
Classification of Technologies
Reconfigurable Pin Tools (RPT), are surfaces made up entirely of individually
controllable discrete pins [14]. Among the various embodiments of this technology, this
research is specifically targeted at the development of electronically addressed and
automatically reconfigurable pin arrays. This excludes any pin matrix examples of
reconfigurable fixturing devices or discrete element surface gauges as are occasionally
used in sculpting or woodworking.
Also excluded are devices or mechanisms that are
manually configured, pin by pin or set in shape by an impression into an existing
"master" surface.
Figure 6 illustrates the classification of Digitally Reconfigurable
Surface (DRS), a proposed term for an automatically reconfiguring, digitally controlled,
pin matrix surface. Within the broad label of Variable Geometry Molds (VGM), the
stated tools of interest have highest potential to take full advantage of a Computer
Aided Design (CAD) input and achieve high resolutions [14] [15].
Shape changing
Variable Geometry Molds
discret ization
--..
Reco nfig-ura b Ie Pir Swapable.
Mold Inserts
Pin Tools
Pin setting
tech nique
Manual Pin
Adjustment
Pattern
refe rencing
"Master" Shape
Impression
I
Flexible MoldsI
Automatic Pin
Adjustment
Electronically
j FAddressed
Digitally Reconfigurable
Surfacej
Figure 6: Flow chart of the classification and nomenclature for the tools of interest to
this research
18
Historical development
The first records of plans for a discrete, reconfigurable molding tool emerged in the mid
1800's. As suggested by Munro and Walczyk, a reliable method of tracking the
development of these unique molding tools is by a detailed patent search [15]. The very
first appearance of a discrete pin tool in literature is the 1863 patent' by Cochrane in
which he describes a manually adjustable tool for forming thick steel sheets for ship
construction [16].
In 1892 and in the following years, several versions and improvements
on a manually adjustable spring-forming press were patented, in response to the demand
for vehicular suspension springs [17] [18] [19] [20].
Mon
o
ON--I
-.
~q1
--
*CST
_
g~o
A
Figure 7: A manually adjustable spring forming device, Williams et al, 1923 [19]
In 1931, Fritz Hess patented a novel concept for taking the natural impression of
a customer's foot in a spring-loaded pin array and then locking that shape in place with
a side clamp and using this clamped mold to form metal sheets for use in supportive
footwear.
In figure 8, the device is seen to also allow for a complimentary shape of the
formed foot-mold to be impressed onto a second mold by the first. The use of side
clamping and mold shape copying along with the unique application makes this patent a
remarkable advancement in pin tooling [21].
!
1 As the first patent issued on this technology was issued on September 15, 1863, this makes the
publication of this thesis in September of 2013 a celebration of the sesquicentennial, 1 50 th
anniversary of pin tooling. Happy Pinniversary
19
5
'F1
0
P
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Figure 8: A device for taking impressions of feet and forming foot supports [21]
In 1961, a patent was issued for the first example of a high resolution pin mold,
with pins on the order of 0.03" in diameter [22]. Although the pin setting method is still
manual, the resolution, seen in figure 9, of this tool is much higher than any previous
patent, showing the pin tools can do more than forming large curvature shapes at low
resolution.
20
FIG.2.
Figure 9: A high resolution mold, manually configured, with pins 0.03" in thickness [22]
Finally, in 1971, a device was patented that utilized Numerical Control (NC) in
automatically cutting a series of rods to specified lengths for manual arrangement into a
framework to form a hole-piercing die [23]. While the assembly of this mold is still a
manual process, the patent marks chronological transition point into the development of
devices that integrate electronic control and are self-reconfigurable.
Other examples of
digitally reconfigurable surfaces are classified by type and documented further in the
following sections.
Conventional Actuation
For the application of low detail, soft curvature formed parts; pins in a reconfigurable
pin tool can often be in excess of one inch in diameter and still afford sufficient
resolution for properly formed parts [24]. With pins as thick as many off-the-shelf
motors, the engineer's natural inclination is to adapt a system that has already proven
reliable for linear actuation and place one such system underneath each pin. This oneactuator-per-pin strategy is the most common of the prior art surveyed and also the
most common strategy employed by the few real, working devices [15] [25].
Conventional actuation works temptingly well with single pin mockups and small
arrays, but as the pin density increases and the pin diameter decreases, the method
becomes challenging to implement in a low cost device. The challenge arises in part
from requirement of an actuator or mechanical drive to be equal in diameter to the pins
of the device (actuator should ideally fit beneath the pin without overlap into adjacent
pins) and from the quadratically increasing number of actuators required in a high
21
resolution array. Assemblies of thousands or hundreds of thousands of tiny motors or
mechanical clutches, even if they are simple, result in a sharp increase in device
complexity and cost [26].
Individual Lead Screws
A common embodiment of an electric linear actuator is a helically threaded screw,
rotated by a fixed, stationary electric motor and threaded into a non-rotating, hollow
pin. Pins are packed together and intentionally shaped so that they cannot easily rotate,
allowing the rotation of the various screws to apply a vertical translation force [15].
The
most common embodiments of this actuation scheme are categorized below in Figure 10.
6::7-
I
I
a
Figure 10: Lead screw driven pin actuation.
Left: View and cross sectional view of motor, screw and translating pin and top view of
pin geometry, shaped as to discourage rotation.
Left Center: One motor per rigid screw arrangement.
Right Center: One motor is attached to a mechanical transmission that has the
ability to switch rotating outputs to various attached screws [24].
Right: One motor per long, flexible screw transmission. This arrangement allows for
the use of motors that are slightly larger in diameter than the diameter of the pins
themselves. An impressive variation of this flexible transmission was recently built by
the Tangible Media Group, in the MIT Media Lab [25].
22
Hydraulic/Pneumatic Pistons
Pin reconfiguration can also be driven by the conventional pressurization of pistons,
typically one piston per discrete pin [27]. The complexity and cost of pistons is on par
or greater than equivalent electrical transmissions; as such, few reconfigurable arrays use
pneumatics or hydraulic actuation for actuation of individual pins [26].
Serial Pin Addressing
The first published academic research in the reconfigurable pin tooling field was
conducted by the Nakajima group at the University of Tokyo [28].
Nakajima's mold
involved a matrix of tightly packed, smooth pins that were set to their intended height
by an ultrasonically vibrating stylus, sweeping across the grid's surface.
This method
has great potential for a high resolution surface, as only a single pin setting actuator is
needed and very small pins can be used.
This approach actually resembles the direct
NC machining of a mold with a cutting tool, with the benefit of cost savings on material
and tooling.
Potential problems with this method include the mechanical complexities of
a multi axis positioner, path planning and long reconfiguration time. Also, in this
arrangement, static indeterminacies in the pins may occur, resulting from small
variations in the pin diameters, causing uneven array clamping force and loose pins-this
problem was later solved by Hardt's group in the late 90's by aligning the grid of pins in
straight rows and placing a metal sheet in between columns to evenly spread out the
clamping force [29]. In Nakajima's 1969 article he mentions that in his invention, as in
all pin tooling machines, there is a practical limitation to the size of the pins used in any
array, as when the diameter of the pin decreases, so does the rigidity of the pins and
subsequently the stiffness of the array. This is an important limitation to understand as
reconfigurable pin tooling devices achieve greater resolution and incorporate pins with
higher length/diameter aspect ratios. Array stiffness may be augmented by backfilling
the spaces between pins with a fusible material as is suggested in many publications, like
Wakefield's 1943 patent [30].
23
Push-metaL
Wi~tta posit ie.rn ofwie
CveI4ttrat
11
OL3
L.CUS
push-me . er
Wires
Push-"tat
0
Of SWeep
45
Fig. 0 Ultrasonic vibrator
Figure 11: Vibrating stylus reconfigures positions of smooth pins [281
Another noteworthy example of a serially addressed device is described in a
patent by Jacques Berteau in 1994. This tool is made up of close packed screw bundles,
configured into position by an automatic NC screwdriver.
This novel arrangement of
screws allows for a bundle of similarly threaded rods be used as a self-supporting grid, a
clever arrangement that circumvents the need for a clamping apparatus to keep the pins
in position-at least for low to moderate molding pressures. [26]
1
IN
I
C
4
2
T f-K
PT
0-,,
r,
1
22
2-
T
40 4
2
0
OT
"
0, 0
6
0',-
IT o'-
f
0,-
6
Zr'2
Figure 12: Interlocking threaded pins [31]
24
I
6
x
0
Figure 13: Interlocking pin array actuation; accomplished by a three axis robot able to
turn screws to desired heights [311.
Braking Actuation
Another unique pin setting strategy is the use of braking elements, connected to a fixed
platform, to grip pins in place when loosely held pins are cycled up and down by a
motorized reference plate.
The advantage of this technique is the reduced complexity
and cost of an array of simple braking elements when compared to a similar array of
electric motors and mechanical transmissions.
Additionally, this braking technique has a
stable, solid-state nature that makes it more robust than serially addressed pin setting
robots, which require path planning and feedback control.
Proposed braking elements include electrical solenoids [32] and shape memory
actuators (SMA)
[331. A device capable of an impressive resolution using Nitinol SMA
braking elements was proposed by Cook, et al and is illustrated in the figures 14 and 15.
25
Figure 14: High resolution molding surface by Cook, et al. [33]
Pin 'header'
I
Lock 1
--
Static
datum
plate
j
Setting-platen (motor
driven through
drive-train)
Lock 2
Pin 'tail'
Figure 15: Actuation arrangement in the device proposed by Cook, et al. [33]
26
Hybrid approaches
Multi axis pin setting mechanisms (described in the previous "serial addressing" section)
have nearly the same level of complexity as conventional high speed machining of molds
and dies, with the benefit of saving much wasted stock material generated in making
single purpose molds or dies. A unique method of subtractive pin tooling developed by
Zhijian Wang incorporates both the positioning of large interlocking plastic screws and
the subsequent conventional machining of those screws to quickly create molds for
vacuum forming [34]. This method is unique because, instead of trying to directly create
a mold from a grid of pins, the intent of the pin array is to conserve mold stock material
and total machining time. If small changes are made to the design, the pins can be
moved upwards slightly and the mold re-machined.
This flexible rapid prototyping
technique saves both time and material cost, relying on complex integrated machinery
for both pin setting and machining to achieve excellent surface resolutions. [26]
Figure 16: Wang's reconfigurable mold, before and after milling [34]
Surface interpolation
Due to the discrete nature of all reconfigurable pin devices, an inter-pin interpolation
method is commonly used, usually sacrificing some resolution to provide a smooth, nondimpling forming surface. Interpolation is typically accomplished by placing a flexible
rubber layer on the tops of pins and either drawing the rubber down by vacuum or
relying on the force of molding to conform it to the tops of the pins. Other methods of
surface smoothing involve the use of pivoting pin tips, which rotate to the correct
tangential position on the solid material being formed. [26]
27
Deficiencies
The most obvious deficiency in reconfigurable pin tooling is limited resolution. If we
compare any known reconfigurable surface to a mold made using subtractive machining
methods, the reconfigurable pin tool will look grainy, dimpled or pixelated. Even with
the benefits of a reusable molding stock, a reconfigurable tool still falls short of being a
practical shop tool, as evidenced by their extreme scarcity. Without a practical method
of achieving much better resolutions, discrete pin tooling just is not a useful system for
anything but highly specialized applications.
Requirements
Poor resolution is the primary deficiency of discrete tooling. Upon review of prior art
and with knowledge of the history of fabrication tools and recent manufacturing trends,
it is logical that developing a strategy to reconfigure pins at a high resolution should be
the focus of this research if a practical device is eventually to be produced for low cost.
The question is now, "how high is high resolution?" If we take the illuminated
display industry as a representative example of what resolutions are acceptable, a lower
limit is characterized by 120 pixels per inch by the Google Android developer
community. This would be a very high physical display resolution, surpassing the
smallest reconfigurable pin tools ever built by orders of magnitude.
Somewhat ambitiously, we choose to define the target resolution of this research
to be 16 pins per linear inch (roughly 1.58 pins/mm) as this is this would yield a surface
similar to the resolution one might see on an object that was roughly cut by a small,
1/16" diameter milling bit (before any surface interpolation is applied). This equates to
about 2,300 pins needed to make a small 3" by 3" molding surface and over one billion
pins to make a 4' by 8' surface. With these sobering numbers in mind, the game is
afoot.
Scope
The purpose of this research to develop a strategy, backed up by analytical arguments
and empirical evidence, that could be used to design and build a high resolution
reconfigurable pin tool on the order of 1/16" pin diameter and pitch. This research is
also committed to developing a tool that is electronically addressable and
reconfigurable-not manually set. When such a strategy is found, a practical resolution
upper limit should be characterized, to provide a baseline for further inquiry.
28
Chapter 2
Design Philosophy
Introduction
This section categorizes different ways of designing and controlling mechanical systems
and concludes on a design strategy to be used to design digitally reconfigurable pin
surfaces.
At what point does a group of trees become a forest? When do a cluster of
bristles become a brush? It is common for a group of nearby, similar objects to be
referred to as a single, larger entity. This occurrence may reflect a simple matter of
speaking efficiency; rather than saying, "Meet me by the tree and the other tree and the
other tree and the other..." it is often more concise to say, "Meet me in the forest." In
some cases, however, a group of objects is viewed as something greater and altogether
different than the sum of its parts. As a bundle of protein strands forms a muscle, when
many small objects or mechanisms with weak properties or forces become patterned and
structured together, powerful "bulk" properties often appear. These favorable properties
are varied; some common examples are smaller size and reduced cost-demonstrated
famously by the integrated circuit. Indeed, as far as technology goes, the electronics
industry is the best example of the benefits derived from tight system integrationsystematically clustering useful elements to make new discrete assemblies with amazing
The fields of data processing, sensing and
capabilities when added into larger systems.
29
display technology have grown in recent years by taking advantage of hierarchical
systems integration, but what of mechanical systems?
Serial Actuation in Tools
Milling machines, CRT monitors and a cheese knife are all examples of serial devices. A
single action is performed, followed by another, and then another until the process is
complete. It can be difficult to distinguish a process as serial or parallel, but in general,
a serial device performs many, time-separated operations. A saw blade or a milling bit
may have many cutting teeth in parallel, but will only perform one cutting operation at
once. In the domain of machine tools, operations may be performed at very high speeds
in quick succession, as is true of high speed machining or laser engraving, but these
operations are still serial in nature. The advantage of serial tools is that, since the
operations performed are discrete and spaced out in time, serial devices are highly
reconfigurable and have the ability to behave differently every operation cycle.
Rapid
prototyping is often done on serial machines, where a single or few parts are desired and
cycle time isn't a priority.
Parallel Actuation in Tools
Lithographic deposition machines, LCD screens and cheese graters are parallel devices.
Again, serial and parallel devices can be challenging to clearly categorize-a cheese
grater has many cutters and cuts in parallel, but performs single operations on the
cheese in serial. In general, parallel devices perform many operations simultaneously.
As far as machine tools that operate in parallel, we can point to a molding tool, a
stamping tool or the especially poignant example of a aggregate tool head in Figure 17,
machining 142 specific holes at once.
30
Figure 17: Zagar Inc. 142 spindle head [35]
In parallel tools, reconfigurability is sacrificed in preference of shorter cycle times and
improved repeatability between cycles. A precision machined mold is an excellent tool
for producing copies of its complimentary shape, but a mold is generally designed for a
single purpose. The set-up cost and time to make a mold discourages fabrication of
single parts or short runs, especially if other, more flexible processes are available.
Distributed Actuation
Inkjet technology is a unique exception to the previously discussed reconfigurability-orspeed priority that characterizes serial and parallel machining processes. Inkjets have
serially operating, parallel arrays of simple actuators that yield high resolution,
reconfigurable operations at fast cycle times.
Having both reconfigurability and speed, in-home printers have become a shining
example of the incredible utility of giving the end user the ability to design and
manufacture their own products. The feedback loop between writer, editor, type-setter
and printer is bypassed and the task of printing a page of information can collapse on a
single, empowered individual.
The technology that makes inkjet printing work so well is characterized by very
small, simple but sophisticated actuators operating in parallel over a small area-the ink
head-and then in serial over a larger area-the page. The physical mechanisms that
have worked well at this scale are thermal and piezoelectric ink jets and laser-generated
electrostatic charges on toner transfer drums. These mechanisms and the hybrid
31
parallel-serial control give a hint into what may work for a digitally reconfigurable
surface that is both high resolution and fast actuating. A new classification is proposed
for devices that are both parallel and serial in nature and combine their strengths-speed
and reconfigurability. We propose to call this actuation strategy Distributed Actuation
after the distribution of operations over elements in a parallel array and with the
intention of using that array as a serial device in a larger system. A distributed actuator
benefits from the "stacking" of serially operated parallel devices, much as a biological
system performs serial operations with a parallelly structured tissue or organ. This
distributed actuation design strategy is the foundation upon which the following
reconfigurable mold designs are based.
Distributed Actuation in Pin Arrays
In order to apply distributed actuation principles in the control scheme of a
reconfigurable pin array, we must first determine the largest controllable parallel arrays
in the system so they can be grouped and treated as single components of a larger
system. The parallel array of the whole set of pins is too large-this is essentially the
entire device. A common way to address pixels in display technology is by row/column
addressing, so that, given a specific signal pattern coming from the rows and a specific
pattern coming from the columns, an image is fully defined. Inspired by this strategy,
we propose to define the rows and columns of a pin array as two, intertwined serial
entities of parallel elements. By controlling signals down rows and columns, instead of
individual pins, the number of control signals needed is reduced to a number that scales
linearly, versus quadratically-a favorable ratio at high resolutions.
If the entire set of
rows or set of columns proves too large an entity, the elements can be divided into
smaller entities as needed-for example, every other row is one group-and the signal
generators would translate between these segments to address the entire array. Now, in
order to make this control strategy work, we still need a mechanism to tie the rows and
columns together at each pin site. The nature of this mechanism is largely dependent on
the type of signal input, whether it is optical, electrical, mechanical or otherwise. A
helpful tip in designing this "coupling" mechanism is to take advantage of some kind of
nonlinear mechanism as this can be used to better isolate specific entities than if the
system behaved isotropically.
In nature, many organisms exploit nonlinearities in their
environment to accomplish feats such as walking on water (a water strider takes
advantage of surface tension) or walking right up walls (a gecko can attach or detach to
32
a surface by minutely changing the contact angle of the micro-bristle setae on the pads
of its feet). In the following chapters, we propose a method for electrical row-column
addressing using diodes and transistors as nonlinear elements to channel current down
targeted paths and another method using patterned mechanical vibrations to induce
rotations in bundled screw elements by taking advantage of geometric nonlinearities.
Serial Actuation
Parallel Actuation
Distributed Actuation
Figure 18: Comparison of various actuation techniques used in a drilling operation.
33
Chapter 3
Electronic Brake Pin Setting
Introduction
In the first of two strategies explored, an array of braking elements is proposed, one
brake for every pin being controlled.
In this design, the reconfiguring pins are
constrained to only move axially (pins may rotate along their axis as well, but pins are
considered radially symmetric, so this is of little consequence). This loose pin
arrangement is similar to the form of the common pin art toy. The pin setting method
is illustrated in the following figures. Figure 19 shows an embodiment of this braking
concept with labeled components. The brake array is fixed, while the push plate can
translate up and down via a linear motor (not shown).
34
I
I
I
I
I
I
I
I
I I
Pin Shaft
Brake Array
Push Plate
Figure 19: Pin braking array schematic, side cross section
Loose
U
U
U
U
U
U
U
U
U
U
U
U
U
U
Braked
U
U
U
U
U
U
U
Figure 20: Pin braking detail, side cross section
35
'
Pin Head
: M 0M a
0
0M
N
M
a M a0 0 M0
0
1b
M
M
M
0
00
0a
M0
MM
M
N
0
0
a
B
I I
I
I
El
I
yI
I I
I I
II
I.
MM00MM0Ma00MMaaaMM
U
U'
.1
*
II
I|
II
II
-
II
4,
YzYLYLY'
ElFigure 21: Pin braking actuation example
Figure 21 progresses through an actuation cycle step by step, progressing from A
to D. First, in A, the bottom plate moves upwards and pushes all the pins, held loosely
by the upper stationary plate. In the next step, the push plate reaches its highest point
and reverses direction. Also in B, the two inner brakes are activated, gripping the pins.
In the next step, C, the push plate moves further down and the inner pins are shown to
be held in place by their braking elements. Two more brakes are activated in C,
establishing the position of the next two pins from the reference of the moving plate.
Finally, in D, the array has been fully actuated and all pins are held in place by their
36
U
brakes.
Note that this diagram is for illustrative purposes only and many such braking
arrangements are possible. The key features are the array of braking elements and the
pin setting plate that provided a position reference for the pins.
The tricky piece of this braking actuation arrangement is the brake array itself.
The other components, the pins and the moving plate can be more easily designed and
constructed, but the high density brake array is an unavailable, unique component.
As
well as being row-column addressable, the braking elements need to fit in the area
between the pin shafts (the pin heads can be larger than the shafts, as illustrated,
allowing more room for the brake). Ideally, this design will be scalable to very small pin
sizes and be able to withstand moderate forming pressures if the surface were to be used
as a mold. Below, the functional requirements for the brake component are discussed,
followed by a discussion of actuator choice for benchmarking an example of the braking
system.
The functional requirements of this brake array are as follows:
Limited space:
Brakes can only take up an area equivalent to the area between the outer
diameter of the pin head and outside the diameter of the shaft (limited in x and y).
However, there is little limitation to length of the brake along the throw of the pin
(unlimited in z direction). The resolution of the array should be capable of exceeding 16
brakes per linear inch.
Electronic control:
Due to the large number of braking elements, robust electronic control is needed
to ensure accurate and repeatable braking over many thousands of cycles.
Clutching force:
Each brake should have the holding force necessary to allow the array to resist
forming pressures of 15 psi or greater (baseline pressure for vacuum forming).
Actuation speed:
Brakes must actuate fast enough to allow the array to reconfigure in a reasonable
amount of time. This varies with the device's cost, resolution and intended use, but we
will define it as approximately 10-20 minutes.
37
[26]
Braking Mechanisms
This section discusses various forces to use and methods of building electronically
controlled brakes.
Regrettably, many techniques were only considered briefly and then
shelved. Initial brainstorming generated so many possible brake designs; we decided to
choose the brake that was most readily built with available in-house tools. Our
investigation quickly converged on a thermal heating system, chosen for its relative ease
of fabrication on a conventional printed circuit board with off the shelf components.
The
availability of an in-house pick and place machine allowed a high resolution (over 1500
pin brakes) test prototype to be made at low cost. The dismissed braking techniques
may be more easily produced with a different approach or another set of available
fabrication tools, so we expect to revisit this discussion for future designs.
Mechanical
As a part of the author's undergraduate thesis written on this subject [26], research was
conducted into electrically controlled mechanical braking elements and promising
mechanisms were found described in the field of Micro Electromechanical Systems
(MEMS).
A thermally actuated chevron beam actuator was bench tested as representative of a
MEMS type miniature mechanical actuator. Many MEMS type actuation concepts
could not be easily bench tested in the macro-scale, such as electrostatic comb drives,
but the chevron beam actuator was possible to quickly test at the millimeter scale with
limited resources. The concept was that the crescent-moon shaped part of the beam,
pictured in figure 22, would be have threads cut into the curved surface and upon
heating the chevron beam, the flexure would thermally expand and buckle just enough
to engage the threads of a nearby, concentric threaded screw and lock that screw in
vertical position.
Finite element analysis was done on the beam to optimize its length
and width, keeping in mind the limitations of the fabrication process (in this case, micromilling). Flexure arrays were made from 6061 aluminum stock. [261
38
Figure 22: Left: an array of thermally buckling flexures, right: FEA analysis of a single
beam heated to 150C [26]
1Td I
L
11
I
-I
I
Un-heated actuator; critical dimensions
... Threaded rod, engaged and
I locked in place by actuator
Heated actuator; upward buckling resulting
from outward thermal expansion of beam
Figure 23: Schematic of a chevron-type thermal actuator [36]; below equation was used
to approximate the deflection needed; design was further refined in FEA. [26]
1
d =
L =
L'
a
d
[L2 + (2LL') - L cos(a) 2]f - Lsin(a)
Unheated single beam length,
Heated single beam length
Unheated bend angle
Displacement
39
This design worked well in simulations, but proved challenging to manufacture, even
as a simple bench test. The final array design, pictured in figure 24, was initially rough
cut with an abrasive water-jet, and then the threads in the center of each flexure were
tapped.
The device was then re-fixtured to a milling machine and the thin flexures were
milled out. Placing electrical traces and resistive heaters on the flexures was another
challenge. First, the aluminum flexure was anodized, to electrically insulate the surface.
Next, conductive paint was sprayed on over a laser-cut stencil to make a base for further
electroplating of copper. Copper was then electroplated on the conductive paint and
resistive carbon paint was applied to the flexure to allow for direct heating of the
flexures by resistive heating. Partial success was seen from this method, but due the
complexity of creating only a few braking elements, the concept was discarded as
impractical to make with conventional machinery, in the time constraints of
benchmarking and testing. Furthermore, a successful implementation of a MEMS-type
actuator as a braking element would prove complex as these types of actuators generally
require much more area than would be available under the heads of the pins. To combat
the requirements of a space hogging brake, the array in figure 24 was designed to be
vertically stacked with several other staggered brake arrays and the chevron elements
were spaced such that they wouldn't interfere with adjacent pins that were controlled by
other arrays. [26]
Figure 24: Left: CAD design of flexure array, right: micro-machined array
40
Magnetic
Magnetic brakes are common in single actuator configurations in conventional systems,
but early tests showed that in high resolution arrays, due to the difficulty in shielding
magnetic fields, sufficient braking strength is difficult to achieve without substantial
leaking of magnetic fields to adjacent brakes. This concept, if explored further, would
require the design of magnetic shielding elements to reduce interference and eddy
currents in tightly packed arrays of braking elements.
Electrical
Electroadhesive brakes show promise as a feasible braking element, but high voltageusually in excess of 1 kV-electronics are necessary to generate small amounts of
gripping force. Discrete isolation of electrical fields between brakes is substantially easier
to achieve than isolation of magnetic or thermal fields as common materials can have
very high dielectric properties, much more so than available to limit the propagation of
thermal or magnetic fields.
The high voltages and special electronic requirements of
such a system prevented initial bench testing.
Shape Memory Actuators
Shape Memory Actuators (SMA) have been successfully implemented as a clutching
system in the prior art studied [331. Prior experiences using nickel titanium SMA wire
as actuators were met with mixed success as the wire is sensitive to overheating and
overstressing and if used improperly the wire can begin to lose its shape memory,
rendering it unusable.
Electrically connecting to SMA wire is also difficult, requiring a
crimp connector or a very special solder and as such, electronic integration into a device
with thousands of such brakes is non-trivial.
Piezoelectric
Piezoelectric brakes were investigated, but within the area available to the braking
element under the heads of a high resolution pin array, only very low displacements are
possible, usually on the order of several microns. Although technically possible, high
tolerances would need to be placed on the manufacture of the pins and the brake array
so that the small displacement of the piezoelectric brake could grip a pin reliably.
Manufacturing expense prohibited bench testing of this technique.
Active Fluids
The use of an active fluid, such as a rheological fluid, was explored and it was found that
such fluids hold much promise to amplify and focus a thermal, electrical or magnetic
41
field. Experimenting with such fluids led to a promising concept using solder alloy, a
thermally rheological fluid. This further testing is described in the next section.
Thermally Rheological Fluid Braking
While working on the thermally actuating flexure concept described above, one of the
test pins became embedded in a once melted, now solidified, puddle of solder from some
nearby electronics work. The idea occurred then that one could use a solder alloy to
bond a pin to another appropriate surface. A quick bench test, illustrated in Figure 25,
confirmed this idea. After experimenting with a few different heating elements, it was
determined that a common electronic resistor should be used as a heating element. A
resistor, especially a surface mount resistor, is inexpensive to obtain in large quantities,
would have very predictable properties and be very convenient to integrate into
controlling electronic components. [26]
This solder alloy braking technique at the prototype's length scale has a roughly
calculated maximum holding force of around 70 pounds per pin. This value was
calculated from the solder's alloy's approximate shear modulus of 20 MPa and the area
that this force is applied, the surface of the 1/16" diameter pin over the thickness of the
1/8" circuit board. This equates to nearly 18,000 psi of surface force in an array of 1/16"
square packed pin spacing. In reality, the strength of the array is a composite function
based on the strength the solder used, the temperature of the solder and the strength of
the pin-brake supporting structure, but this initial estimate of the magnitude of braking
force encouraged us that this "fusible alloy brake" could hold a pin in place reliably in a
dense pin array.
42
Figure 25: Single pin, fusible alloy prototype [26]
As mentioned, a favorable property of this solder-braking technique is the ease of
attachment to an electronic control board. An entire array of brakes can be cheaply
made on a conventional printed circuit board, with no special components needed or post
processes applied. Due to the small size of available electronic components, it is also
scalable to a moderate resolution. Resolution is limited, as we soon discovered, based on
the size of cheaply available components able to handle the power requirements of
melting the solder alloy. As for molding pressure, it was stated above that the solder
alloy braking technique certainly could have sufficient holding strength for most desktop
molding applications.
Problems found with this technique include reconfiguration speed and power
consumption.
Due to the density of the array, thermal isolation between heating
elements is a significant challenge, especially when using a conventional printed circuit
board as platform for fabrication of the array.
Several devices using this fusible alloy brake were fabricated as illustrated in the
following figures. First, two schematic examples of the thermo-rheological braking
actuation are shown, and then several examples of test heater arrays and finally a
finished device with integrated fluidic cooling. The specially designed cooling system is
described in the final part of this section.
In Figure 26, the friction plate has holes lined with compliant rubber to allow
friction force to be applied to rods when the plate is moved, but not so much that the
electronic brake can't overcome the rubber-applied friction when the pins are clamped in
43
Unlike a solid pin setting plate that serves as a position reference, this
proper position.
arrangement allows for the fixed pins to slide past the moving plate, allowing more
flexibility during pin setting. Figure 27 is another arrangement that uses two electronic
brake arrays to allow pin movement both up and down simultaneously.
Rubber interpolator
I LI I I IIIIII~
.4
4
Fixed electronic
clutch array
.4
Moving
friction plate
HUUUUH
Figure 26: Cross section of thermal braking arrangement.
fixed
fixed
fixe
V
Fixed electronic
clutch array
Moving electronic
clutch array
Moving
friction plate
Figure 27: Pins can stay fixed, move up or move down simultaneously in this
configuration. An 'X' over a joint assumes that it is clutched.
44
Figure 28: Surface pressure testing apparatus. Pins were soldered into a PCB with low
temperature alloy, supported by acrylic and subjected to test molding pressures.
Figure 29: A prototype using shift registers to control a transistor and diode (our
nonlinear elements) row column addressing technique.
45
Figure 30: Top view of high resolution heater board. Shift registers controlled
transistors on rows and columns of the array, addressing current inputs to and outputs
from the grid of resistors. A diode was added in series with each resistive heater to
channel the rows and columns and prevent current from always simply passing through
the shortest path in the array. Variable heating and discrete element control was
accomplished via pulse width modulation of transistor inputs to the array.
Figure 31: Bottom view of high resolution heater board. Without exceeding the
specifications for our components too much, the array was built at the maximum density
we could manage with inexpensive discrete components. Resolution was 0.13" pin to pin
spacing, hexagonal packing.
46
Figure 32: Integrated into a working vacuum former, a fusible alloy micro-brake array.
Figure 33; Rubber pin-tips were used instead of a continuous interpolator to investigate
their feasibility.
47
Figure 34: Parts were vacuum formed and the pin array held its shape.
Figure 35; The rubber tip, discrete interpolation seemed to work well at this resolution.
48
Figure 36: Integrated electronics underneath the rubber pin heads.
Proper array cooling
Foremost in the testing of the fusible alloy braking array, issues arose from thermal
leaking from one heater to adjacent heaters.
In a high resolution array, thermal isolation
of resistors and heat sinking of excess heat is necessary to prevent heat buildup from
affecting adjacent pins. A unique copper manifold liquid cooling system was built to
allow for cooling to be applied in various amounts to characterize how much the power
requirements of the array were changed by varying the rate of cooling.
After some
tweaking, thermal isolation the array was achieved with resistor power consumption at
around 8 watts per pin with a low flow rate of room temperature water flowing through
the manifold. This high power requirement could be reduced by taking the time to
properly design and optimize the complex thermal circuit made by the heating elements,
the thermally coupled pins, the circuit board, thermal insulating elements and a proper
waste heat cooling system. The system we built was designed with the cheapest
materials and components we could find, to simply demonstrate a working, but
unfortunately inefficient, system as quickly as possible.
49
Figure 37: Large copper tube branches off into many orthogonal smaller tubes, soldered
to the PCB.
Figure 38: Thin copper tubing, running from the large side manifold pipes, was soldered
directly to specially designed pads on the bottom surface of the PCB to allow for
controllable heat exchange.
50
Chapter 4
Vibration Induced Pin Setting
Introduction
After having constructed a functional reconfigurable mold using thermal braking, we
began investigating a purely mechanical means of addressing pins, in attempt to further
reduce system complexity. Inspired by the vigorous and directional motion resulting
from oscillations of vibrating parts feeders, a new design strategy was developed around
a matrix of closely packed screws and edge addressed patterned vibrations. These
vibrations take advantage of local nonlinearities in the screw matrix to induce controlled
screw rotations, and resultant axial screw translations.
A mechanically addressed screw
actuation strategy is an exciting improvement over the braking control scheme because
this new concept doesn't require an additional element installed at the site of each
actuated pin to generate a nonlinear response.
Instead, nonlinear mechanics were found
to naturally exist in a square packed bundle of screws. A built-in nonlinearity serves to
dramatically reduce the part count and cost of such a system.
In order to visualize the mechanical interaction between elements of a close
packed screw array, a series of illustrations are presented: first, an introduction to the
concept of a screw bundle or screw matrix; next, an explanation of force propagations in
densely packed screw arrangements and finally a representative example of how a driving
torque can be applied to a selected screw within a large matrix.
51
Screw bundles
This reconfigurable pin array actuation technique is based around the unique
properties of a closely packed matrix or "bundle" of threaded rods or screws. When
threaded rods of the same thread pitch (linear axial spacing between thread teeth) are
held together in close parallel arrangement, the thread teeth engage with each other and
screws can be made to translate with respect to each other by adjacent screws acting as
composite "nut" to a rotating screw.
Figure 39: A bundle of threaded rods
Figure 40: Enlarged view of the helical thread engagement between adjacent screws
The geometry of this arrangement yields unique mechanical properties.
First of
all, if a screw is rotating, the friction it applies to the immediately adjacent screws
results in a rotation moment being applied to those adjacent screws. In essence, if you
52
turn one screw, the screws around it may start to turn as well. Preloading of the bundle
by pressing the screws together from the outer edges is a convenient way to increase the
friction between the screw elements and reduce undesirable interaction between screws.
Another unique property of a screw bundle is that the interlocking threads of two
adjacent screws will strongly resist rotating in the same direction because of the physical
interference of the threads. This "thread-locking" effect may be utilized as nonlinear
property to isolate induced moments to the array and for preventing undesired rotations.
The choice of different screw threads can also be used to change the system
characteristics. A doubly or triply threaded rod would increase the linear distance a
screw will travel when it undergoes a rotation.
Different thread/linear distance ratios
could also be used to change the drive efficiency and transmission ratio. The shape of
the thread is also an important variable. For example, a ball screw with spherical edges
could be used to reduce the friction between touching screws from an area contact (as
with standard threads, see figure 40) to a point contact. The use of ball bearings
between adjacent ball screws is an interesting, but yet unexplored possibility to further
reduce the sliding contact friction between screws to a rolling contact friction.
Screw packing
The packing arrangement of the screw elements into the matrix is of special
importance to this actuation strategy as this packing geometry will dramatically affect
how forces and displacements will be transmitted throughout the matrix. The two basic
types of screw packing are illustrated in figure 41 below.
Square
Triangular/Hexagonal
Figure 41: Basic types of dense pin packing (top view)
53
It can be noted from figure 41 that the interstitial space between screws in the
square packing is larger than the area between screws in the hexagonal packing. This is
to say that hexagonal packing is known to be denser than square packing and hexagonal
packing is higher resolution of the two. However, the actuation method proposed
involves the patterned, subtle shifting of screws within a matrix; consequently, the
propagation of force and ease of displacement of screw elements is of critical importance.
As illustrated in figure 42, a device with square packed pins has clean, linear force
propagation and would be well suited for targeting individual screw sites from the edges
of the array.
Similar targeting may be possible with a hexagonal array, but is clearly
more energy intensive and the benefits of slightly increased resolution would need to
outweigh the labyrinthian complexity and inefficiency of this tightly packed
arrangement.
The resultant friction and rotational torque generated by the
displacements of the screw elements is modeled in the next section.
Figure 42: Single red arrow represents a force input and direction, small green arrows
represent resulting force propagation from the input vector.
Left: Hexagonal packing results in an exponentially decreasing force propagation as the
screws overlap to their maximum degree.
Right: Square packing force propagation is linear along rows and columns.
54
Selective screw rotation
A selected screw or potentially several compatible screws can be rotated in a
matrix by a pattern of linear displacements from edge positions on the rows and columns
of the matrix.
Figure 43 illustrates a sample square packed array (viewed from above) with
actuators placed at every row and column edge site. The actuators have three different
positions: extended, neutral or retracted.
This could be replaced by a single direction
actuator with a spring return. This model assumes that the actuator also provides a
constant preload force in all the extended, neutral and retracted positions. The preload
is important as it defines the force of friction that the screws apply on one another and
keeps them in contact.
z
z
z
z
z
R
X
N
AkE
N
N
N
N
N
N
N
N
N
N
Actuators with three positions:
N = Neutral
E = Extended
R = Retracted
z
It is assumed that actuators also provide a constant
preload force to the screws in all positions.
z
z
z
z
Figure 43: Actuator assembly
In this square packed arrangement, a linear dislocation of a row or column causes
those pins to move slightly inwards, being pushed by the edge preload force and sinking
into a lower energy state. The inverse displacement, to return those screws to their
original position, requires more force than the initial dislocation.
This is our
advantageous nonlinearity: we see more generated tangential friction from a returningstroke than from a forward stroke.
Illustrated in a simplified model in figure 44, the
55
initial force F1 pushes the circles down into a lower energy state. As long as there is a
preload force on these elements, the magnitude of F1 is always going to be lower than
the magnitude of the return stroke, F2. While F1 lowers the normal force applied the
row, F2 serves to add to that normal force, thereby increasing the friction applied to
adjacent screws during F2.
Another nonlinearity that can be taken advantage of is the slip-stick
The slip-stick phenomenon occurs because the value of the coefficient of
phenomenon.
static friction is greater here than the coefficient of kinetic friction. In this way, a fast
dislocation of a row, followed by a slow return stroke to the initial position could also
generate unequal tangential forces on adjacent screws.
F2
F1
Figure 44: Nonlinear properties of forward displacement and return stroke. F1<<F2
N
N
N
N
N
N
E
R
N
N
N
N
R
E
N
N
N
N
N
z
z
z
z
z
z
X
z
L
N
Figure 45: Patterned screw actuation, dislocations are exaggerated. Left image shows
direction of initial displacement and right image shows return stroke as well as the
direction of rotation of targeted screw.
56
Figure 45 shows an example of possible movements of rows and columns to select
a specific screw for rotation. In order to rotate a screw counter-clockwise, a clockwise
collective translation needs to be applied to the screws surrounding the selected screw
and the rotation will occur on the stroke returning the matrix back to its initial
configuration.
Simply having uneven opposite tangential forces on all sides of the targeted screw
does not guarantee the screw's rotation. Consider this: A car's tire can rotate exactly
the same amount against a road for different weights of cargo, forward or backward.
only when the tire begins to skid or slip, do we see a different total rotation.
It's
If the four
screws applying forces to the targeted screw are always rolling against the targeted
screw, the uneven forward/return forces don't make a difference to the targeted screw's
overall rotation. A rotation can be generated when these four screws slip against the
targeted screw.
Slipping can be encouraged by reducing friction, preload forces or
making the forward, dislocating stroke as fast as possible, utilizing the slip-stick effect.
It should be noted that tangential forces are being applied to all the adjacent
screws to a dislocating row, not to the targeted screw only. Fortunately, we benefit now
from the discussed "thread locking" effect that discourages these adjacent screws from
rotating and causing an undesired rotation. Only the center, targeted screw has the
same direction of tangential friction applied from all four adjacent screws, giving it the
best chance of rotating incrementally. If undesired rotations still do occur, the edge
preload can be used as a filter and increased up to the point where only the targeted
screw rotates, as the targeted screw should feel the strongest forces.
Since there are so many things happening at once and many forces to consider,
we needed empirically verify that this theory would actually work. Four piezoelectric
linear vibrating parts feeders, Model PEF-L125A, were modified to mate with the edge
screws of a spring preloaded square lattice of 3/8-16 alloy steel machine screws. The
unloaded linear vibrators displace a maximum of approximately 0.5mm when tuned to
their resonant frequency.
The successful test is documented in figures 46, 47, 48 and 49
below.
Other screw actuation arrangements may be possible, for example, to actuate
multiple screws simultaneously; but the above description is the only example so far that
has been backed up by successful testing on a working device.
57
Figure 46: Modified Piezoelectric Vibrating Feeder
Figure 47: Threaded coupling from vibrating mechanism to edge screw.
58
Figure 48: Upward screw translation test. Images show the displacement generated by
the vibrating inputs at five second intervals. Screws are 3/8-16 and one inch long.
59
Figure 49: Downward screw translation test. Images show the displacement generated
by the vibrating inputs (reversed from test in figure 48) at five second intervals.
60
Force Analysis
To better understand how this screw bundle actuation system scales as the
number of screws increases and the diameter of each screw decreases, the following
analysis is presented.
To begin, we can analyze the free body diagram of a single screw element in the
case of a row/column dislocation, driven by Fout and the case of the return stroke,
driven by Fin.
Fou
F
F
u
F]
U
F
4.
Fln
Fut
FN
FN
FN
g
FN
FP
P
Figure 50: Free body diagram of dislocation stroke (out) and resetting stroke (in).
The edge applied preload is felt as a constant Fp, the displacement of the screw
is u, the diameter is D, the normal force between the screws is FN and the contact
friction is Ff. The x direction horizontal and positive pointing to the right of the page
and the y direction is vertical and positive pointing to the top of the page. First we will
solve for Fout.
61
First we sum the y-axis components of the upper body.
1
Fout = Ff - FN
y
Fout =
2
2
FJ(D -u )
D
F
p.IFN
FNU
2
(D2 -
2
D
FNU
)
D
3
D
Now we find FN in terms of Fp.
4
F = FNx + Ff,
F =
F
FNI(D
2
2
D
- FN
FN
+
)
5
FfU
D
2
2
(D _U ) +IFNU
D
D
6
7
Fp
2
2
+
(D -u )
D
D
Plug equation 7 into equation 3.
(2 2 -u 22))
(D
FP
p
Fout =
D
Fp
2
u
2
-u
D
D
)
(D2_.2 )(D
D
D
D
8
And after reducing, we get equation 9, the force relationship between the dislocated
screw, the preload, the coefficient of friction, the diameter of the screw and the
displacement magnitude.
-
Fut
-
Fp(I(D2_U2)_U)
(D 2 -u 2 )+[iu
9
As this is only half the force of one element, we add a factor of two. The r'esulting
equation, 10, is the relationship for a single screw element. In order to estimate the total
force on the row, the equation should be multiplied by the number of elements in that
row.
n*2Fp(pt
ut =
(D2_U2)-U)
2
(D -u
62
2
)+Ru
10
We now plot Fout/Fp against u/D to obtain a rough dimensionless comparison of how
the forces scale with displacement. The graph assumes a friction coefficient of 0.5;
increasing the coefficient of friction will translate the graph upwards, lowering the zero
point plotted below. It is important to note that this graph doesn't include the factor of
n for the number of elements being considered in a row or column. The graph, Figure
51, considers the forces on a single element only.
Forward Stroke
I
0.8
0-6
0-4
0.
LL.
0
X: 0.447
0-2
Y: 0.000
------------------------------------
0 ----------------------------------------------0.2
-0-4
III-
-
-0.6
-0-8
0
0-1
0.2
0.3
0.4
0.5
0-6
0-7
0.8
Displacement / Diameter
Figure 51: Graph of dislocation (outward) stroke scaling
Figure 51 reveals several insights. First of all, the dislocation force of a single screw is at
maximum equal to the preload force. Next, when the dislocation of the screw element is
about 44.7% of the screw's diameter, the force of the preload reverses direction and
actually helps to push the screw further along the direction of displacement. This may
not be desirable, depending on how much dislocation is allowed in the system, because
reaching this point might cause a row to fully collapse into hexagonal packing.
63
Next, we will analyze the return stroke force, Fin.
First we sum the y-axis components of the upper body.
F =Ff +F
11
2
Fin = F 1 (DD2
-U )
Fin
2
(D -u
~ *FN
12
FNU
D
2
D
)
13
+ FNU
D
Now we find FN in terms of Fp.
F, =
FNx -
F =
FN (D -u )
D
Ffu
D
15
F =
FN (D2_u2)
D
[FNU
D
16
2
FN
2
14
Fx
2
17
Fp
(D -U
2
) -9U
D
D
Plug equation 17 into equation 13.
(D 2 -u 2)
Fp
2
=(D2_g 2-2)D
Fout
D
+
D
D
-U
(D2_t
2
)
Fp
D
D
D
18
And after reducing, we get equation 9, the force relationship between the dislocated
screw, the preload, the coefficient of friction, the diameter of the screw and the
displacement magnitude.
- Fp(g, (D2_U2)+u)
F (D 2 -u 2 )-pu
19
As this is only half the force of one element, we add a factor of two. Again, this
equation, 20, is the relationship for a single screw element. In order to estimate the total
force on the row, the equation should be multiplied by the number of elements in that
row.
n*2Fy(pf(D2_U2)+U)
out -
(D 2 -u
64
2
)-
2
Now, we again plot Fout/Fp against u/D. The graph also assumes a friction coefficient
of 0.5; but increasing the coefficient of friction shows a steeper slope for high
displacements, increasing the return force and how quickly it increases with
displacement. As with the first graph, it is important to note that this graph doesn't
include the factor of n for the number of elements being considered in a row or column.
The graph, Figure 52, compares the forces on a single element only.
Return Stroke
5-5-
54.54LL
3.5U.
-3-
2.52-
1
0.8
'
1.50.7
0.6
0.5
0.3
0.4
Displacement
/
0.2
0.1
0
Diameter
Figure 52: Graph of return (inward) stroke scaling
Figure 52 shows that the returning force to reset a single screw element increases sharply
starting at displacements around 50% of the screw's diameter.
This makes sense from
what we saw in figure 51, that the preload ratio passes an origin around this same point
and becomes negative, acting here in figure 51 against the return stroke.
Assuming that a sufficiently strong actuator is always available, the maximum
possible resolution of the system is affected primarily by the material stiffness and
manufacturing tolerance of the screws. Low stiffness or loose tolerance screws can
introduce backlash into an actuated row or column and result in poor force propagation
for rows with many elements. A small amount of backlash, when summed over many,
65
many screw elements, could cause a dislocation to "fizzle out" and be prevented from
propagating along an entire row or column. These kinds of backlash errors can be
reduced by increasing the material stiffness of the screws, holding a tighter tolerance or
by increasing the magnitude of the dislocation applied. If reasonably hard materials are
used to make the screws and basic ANSI tolerances are met, we expect to see this kind of
backlash only in arrays with rows or columns on the order of 100 or more entities. A
potential strategy to reduce backlash is to coat the screws with a thin layer of compliant
material (like rubber) to help elastically average undesired backlash-this preloads every
screw site, not just the perimeter of the array.
From the above analysis, we aren't able to put a hard number on the absolute
limits to resolution, but we have discovered some helpful insights into the relationships
between the major parameters in the system-edge preload force, force of dislocation,
screw diameter and row displacement magnitude.
The force scaling graphs give an idea
where displacements might cause instability in the square packing and cause an
undesired collapse into hexagonal packing.
The above equations, 10 and 20, are useful
tools for choosing actuators based on a desired displacement, screws based on diameter
and friction properties and preload strength based on actuator strength.
66
Chapter 5
Conclusion
The design of a high resolution reconfigurable pin tool is a unique engineering challenge.
The quadratic scaling associated with this type of device results in a surprisingly large
number of pins at resolutions adequate for most molding applications and form
generation.
Conventional linear actuation strategies for translating a single pin are not
physically or financially scalable to high resolution arrays. This thesis shows the
necessity of developing a hierarchal design strategy to simplify the large number of
separately controllable pin elements into two addressable groups-rows and columns.
Breaking up the system in this way, a reconfigurable pin tool can be designed top-down,
and scaling challenges can be better anticipated.
Using this design methodology, this
thesis presents a practical strategy based on row-column addressing that uses simple,
arrayed nonlinear mechanisms to locally combine separate row-column inputs and
reconfigure individual pins. Two embodiments are proposed: the first is a thermally
actuated pin braking array and the second uses patterned mechanical vibrations to
directly translate pins.
After reviewing prior art and testing a variety of promising strategies for high
resolution pin actuation, the most promising was the vibration induced screw actuation.
This concept is favored over the fusible alloy braking mechanism because no additional
components are required in-between the pins themselves as the screws themselves act as
the nonlinear motion transmission mechanism.
67
The final behavior of a reconfiguring pin
array is always mechanical motion. It makes sense then that the vibration screw
actuation works well by using mechanical site addressing and mechanical force inputsthis keeps the driving force for the array in the same domain as long as possible, from
the vibration inputs to the linear motion of the screws.
Again, because controlled
actuation is applied only at the perimeter of the array, the design affords high resolution
scalability. For example, in an n x n device where n=100, a single pair of row-column
vibration inputs is able to individually control the motion of 100 different screws. As the
number of pin elements increase quadratically, the number of edge elements required to
actuate the array increases only linearly, one of the key insights of this research.
It may be surprising, but after completing this research, one of the most
important lessons I've learned, is that, despite our best efforts, there is no "silver bullet"
machine or process that will fulfill every manufacturing challenge. This reality is at
direct odds with the age-old desire to build a complex all-in-one machine that can
produce any conceivable object-a reliable and hands-free matter-printer, a Star Trek
replicator. Surprisingly, biology does not share this quest for the perfect printing
machine.
In biological systems, the most robust and prolific organisms are not the most
complex organisms, but rather, the most diverse [37].
When designing a machine, flexibility and robustness is achieved by the use of
simple, interchangeable parts and modular systems. Fine control is achieved by the
fragmentation of a machine system into individually adjustable, controllable entities; the
smaller the discrete element, the more capable the system. Looking back to biology, the
most powerful manufacturing system, atomic scale resolution is used in the coding of
DNA molecules.
If we ever want to make machines that come close to the capability of
biological systems, a similar emphasis on resolution should be taken, dividing a system
into as many hierarchical building blocks as possible. This emphasis on resolution is a
large part of what has driven this research into high resolution reconfigurable pin
tooling.
Pushing this screw actuation concept to the absolute resolution limit, it might be
possible to pattern a catalyst in a sufficiently dense 2D array and grow forests of
nanotubes from that pattern, generating the bundled screw elements (using the helical
arrangement of carbon atoms as a thread) in the physical vibration strategy proposed.
In an engineer's quest to freely reconfigure physical matter-a practical
reconfigurable surface is a great start.
68
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