A multifunctional pneumatic artificial muscle. Proof
of concept.
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1
2
1
Taimoor Hassan , Matteo Cianchetti , Barbara Mazzolai , Cecilia Laschi and Paolo Dario
1
The BioRobotics Institute, Scuola Superiore Sant’Anna, Italy
2
Center for Micro-Biorobotics, Instituto Italiano di Tecnologia, Italy
1
Abstract:
This paper presents a novel multifunctional pneumatic artificial muscle (PAM) with bi-directional force and
motion capabilities. PAM's are generally preferred due to their high power to weight ratio, light weight, ease of
installation, hazard free use and inherent compliance To demonstrate the concept, a prototype of the bidirectional artificial muscle has been designed and developed. The prototype was characterized using a universal
testing machine with an actuation pressure of 1 bar. Isometric and Isobaric experiments conducted on the
prototype show good force capacity and the unique bi-directional capability.
Keywords: Double acting pneumatic muscles, Mc-Kibben actuators, Soft actuators.
Introduction
Pneumatic artificial muscles (PAM), more
commonly known as fluidic or Mc-Kibben muscles
have been around for quite some time now. They
are generally preferred thanks to their high power to
weight ratio, light weight, ease of installation,
hazard free use and inherent compliance. These
properties make them an ideal choice for industrial/
robotic
applications
where
human-machine
interaction is involved. Recent and comprehensive
surveys on their applications can be found in [1] [2].
PAM usually consist of a hollow cylindrical
elastomeric chamber covered by an outer braided
sleeve, consisting of fibers made of un-stretchable
material and arranged in an anti-symmetric helical
configuration. The hollow internal chamber and the
braided sleeve are tightly sealed and attached to
rigid end fittings, usually a passage is provided
through one of these end fittings for pressurizing the
elastic chamber with air or another fluid. The
mechanical work done is transferred to an external
system through these end fittings. When the inner
elastic tube is pressurized the muscle either expands
or contracts or stiffens depending on the initial angle
of the braid fibers with the longitudinal axis of the
muscle. The existing/traditional PAMs has a fixed
initial braid fiber angle and are usually designed to
produce a contractile force upon actuation, hence
they are able to produce uni-directional force and
motion [3]. This single acting nature of traditional
PAMs poses a drawback when utilized for robotic
applications, i.e. for bi-directional actuation of a
robotic joint; two actuators are required in
antagonistic configuration. This increases the overall
size and complexity of the actuation mechanism [4].
In order to overcome this challenge previous
attempts have been made to develop a bi-directional
PAM. In [5] a spring over muscle (SOM) actuator is
reported.
Fig. 1: Actuator design schematics. Physical
prototype and sectioned CAD rendering.
The SOM utilizes a passive compression spring in
parallel with a PAM for producing bi-directional
forces. Zheng & Shen developed a double acting
sleeve muscle actuator [6], which is able to produce
bi-directional force and motion. The muscle
incorporates a unique insert at the center, inside the
hollow elastic chamber. This design results in a
significant decrease in the air consumption during
operation and an increase of force capacity over the
entire range of contraction. Furthermore the insert
consists of an additional chamber, which can
produce extension, when pressurized. Although the
fluidic muscle is able to produce bi-directional force
and motion but it has some disadvantages. First the
insert increases the weight of the actuator, secondly
the extension force and motion is comparatively low
as compared to contraction force and motion of the
fluidic muscle. The patent [7] discloses a pneumatic
actuator, which is able to achieve 2 actuation states,
that is able to contract and expand upon actuation
but it achieves this by circulating the working fluid
between two expandable fluid-containing cells.
In this paper we present the working concept and
experimental characterization of a novel pneumatic
artificial muscle which is able to contract, expand
and stiffen. The paper is organized as follows: in the
next sections the concept and design of the actuator
are explained followed by the experimental
characterization and discussion of results. A brief
section explains the future work and its application
in robotics. In the end the conclusion is presented.
Concept
In PAM's, when the inner elastic chamber is
pressurized, it tends to expand, also expanding the
braided sleeve with it. It is known from literature
that when the braided sleeve expands, its individual
fibers reorient themselves in such a manner that it
tends to attain an angle of around 54o, with the
longitudinal axis. At this particular angle the muscle
tends to stiffen only and there is no more lateral
deformation. If the braid fiber angle θ is less than
54o, upon pressurization there is an increase in the
radial direction and the muscle contracts. If the braid
fiber angle θ is greater than 54o, upon actuation there
is reduction in radial direction and the muscle tends
to expand [8].
The working concept of the multi functional muscle
is based on the traditional Mc-Kibben muscles but
some fundamental changes to the design, enables the
user to change the initial braid fiber angle θ
independently, hence achieving bi-directional
motion and forces upon actuation as shown in Fig. 2.
The ability to change the initial braid fiber angle
enables the actuator to achieve variable stiffness at
each point along the total stroke of the actuator,
whereas the traditional PAMs lack this ability.
Design
Fig. 1 shows the physical prototype and its sectioned
CAD rendering. The different parts of the muscle
were manufactured in house by utilizing rapid
prototyping technology. Commercially available
PET thread sleeving was used for the braid structure.
It consists of 72 fibers arranged in an antisymmetrical helical configuration. The inner elastic
chamber is made of Latex with an approximate
thickness of 0.2mm. The diameter of each end fitting
is 22mm and the active length of the actuator ranges
from 52mm to 120mm, depending on the position of
the movable end fitting. The movable end fitting can
slide over the stem, hence enabling the user to
change the braid fiber angle θ. For the expansion
state the fiber angle is equal to 71o while for the
contraction state it is equal to 33o. O-rings are used
to provide a sealing medium between the matting
surfaces of the movable end fitting and the stem.
Experimental characterization
The experiments were conducted to characterize the
bi-directional force output capacity, displacement
and the axial stiffness of the actuator, under a range
of internal pressure. The experimental setup
consisted of an Instron machine (4464), air
compressor and a pressure valve. An arduino® uno
board is used to control the proportional valve, while
a PC is used to collect data from the Instron machine.
Three different kinds of experiments were
performed.
Fig. 3: Schematic diagram of the experimental setup.
Fig. 2: Actuator working concept. A single braid
fiber is shown instead of the whole outer braided
sleeve for clarity. State A (extension state) , State C
(contraction state)
ACTUATOR 2016, MESSE BREMEN
First the maximum force output was measured by
keeping both ends of the actuator fixed and
pressurizing it in 0-1bar range, in increments of
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0.1bar at each stage (Isometric testing). Secondly
isobaric testing was used to measure the force output
from the actuator on elongation/contraction at a
range of pressures that were held constant for each
experiment. The pressure range was 0-1 bar with 0.1
bar increments. In the last set of experiments the
axial stiffness of the actuator was measured. The
actuator was pressurized and allowed to expand or
contract freely. It was then fixed in the Instron and
the free end was displaced 5mm, either compressing
or extending the actuator depending on the state it is
in. The required amount of force is recorded from
the instron and the axial stiffness is measured.
The schematics of the experimental setup is shown
in Fig. 3. Each experiment is repeated three times in
order to guarantee the stability and repeatability of
the measure. The maximum actuation pressure is
kept at 1 bar due to thin nature of the latex internal
chamber. Higher pressures were tested and the
membrane usually failed at a pressure of 1.5-2 bar.
during the loading and unloading cycle. This is a
common feature of PAM's and is mainly due to the
elastic nature of the inner chamber, friction between
the inner chamber and the outer braid, as well as
friction between the individual fibers of the braid
during the inflation and deflation of the actuator.
Fig. 5 shows the relation between the applied
pressure and measured axial stiffness of the actuator.
It is evident that the axial stiffness increases with
increase in pressure for both the contraction and
extension states. Almost similar values of stiffness
are measured for both modes of operation with a
maximum value of around 1.95 N/mm.
Results
The results of the experiments are shown in Fig. 4, 5
and 6. The sign convention used is as follows. The
contraction force and displacement is denoted by
positive values while the extension force and
displacement is denoted by negative values.
Fig. 5: Relation between applied pressure and axial
stiffness of the actuator.
Fig. 4: Relation between applied pressure and force
output of the bi-directional actuator.
The bi-directional force capability of the actuator
can be seen in Fig. 4. It is able to produce good force
output, both in contraction and expansion modes,
with maximum forces of around 60N (contraction)
and 49N (expansion). It is also noted that a
minimum (threshold) pressure of around 0.1bar is
required to actuate the muscle. This can be due to
the fact that the inner elastic chamber and the outer
braid are not attached. This threshold pressure is
required so that the inner chamber expands and
comes in contact with the outer braid in order to
produce a force output. Some hysteresis is also noted
ACTUATOR 2016, MESSE BREMEN
The results of the isobaric experiments are shown in
Fig. 6. It shows the force output of the actuator as a
function of displacement at different constant
pressure values, for both the contraction and
extension states. The horizontal axis in Fig. 6 is a
non-dimensional measure of displacement. It is the
ratio between the change in length upon actuation
divided by the maximum active length (120mm). It
is evident that the actuator is able to achieve bidirectional motion with a maximum displacement of
around 20% for the contraction state and 26% for
expansion state. It also shows that the force output
of the actuator decreases with increasing the
displacement at a given pressure. It can be seen that
the actuator produces zero force when fully
contracted or extended.
Applications in robotics
The multi-functional pneumatic muscle with its
novel bi-directional force and motion capabilities
can notably simplify the design of PAM actuated
robotic mechanism/joints. A single muscle can be
used to replace the antagonistic configuration of
traditional muscles, in order to operate a robotic
joint. Moreover, by combining two or more
actuators together in parallel, enhanced features can
be achieved as compared to their traditional
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Fig. 6: Force-displacement characteristics of the actuator at different pressure values.
Table 01: Summary of the motions produced by a mechanism consisting of a pair of traditional fluidic muscles.
Table 02: Summary of the motions produced by a mechanism consisting of a pair of the bi-directional fluidic
counterparts. This comparison is summarized in
Table 01 and Table 02.
Conclusion
In this study, the working concept and a physical
prototype of the multifunctional pneumatic artificial
muscle were presented as an improvement over the
traditional pneumatic muscle actuator. Experimental
characterization of the actuator shows good force
output and the unique bi-directional motion
capabilities. It is believed that the bi-directional
actuation capabilities can notably simplify the
design of PAM actuated robotic mechanism/ joints.
For the last part, some applications of the actuator
were discussed.
Acknowledgments
The authors would like to acknowledge the support by the People
Programme (Marie Curie Actions) of the European Unions
Seventh Framework Programme FP7/2007-2013/ under
REA grant agreement number #608022.
References
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