Arm Development
Review of Existing Technologies
F. Jensen and S. Pellegrino
CUED/D-STRUCT/TR 198
NSE/GNSR/5031
Contact reference No. 400 43169
Issue No. 01
Release date: 25 June, 2001.
Confidentiality and Intellectual
Property Rights Statement
The information contained in this report has been produced BEG(UK) Ltd on
behalf of the Industry Management Committee (IMC) thus is the joint property
of British Energy Generation Ltd, British Energy Generation (UK) Ltd, British
Nuclear Fuels Limited and British Nuclear Fuels Magnox Generation Ltd, and
their successor companies. This information is to be held strictly in confidence
by the originators and recipients, unless the contract specifies otherwise. No
disclosure is to be made to a third party without written agreement from the
nominated officer from Mr R. Shead, BNFL Magnox.
Further any Intellectual Property Rights arising from or contained in the report
are the joint property of British Energy Generation Ltd, British Energy Generation (UK) Ltd, British Nuclear Fuels Limited and British Nuclear Fuels Magnox
Generation Ltd, and their successor companies, as such are subject to the same
obligation of confidence on the originator and recipients as set out above and may
only be used in accordance with the contract.
i
Disclaimer
The views expressed in this report are those of the author[s] and not necessarily
represent the views of the IMC or the Health and Safety Commission/Executive.
ii
Executive Summary
In the nuclear industry there is a requirement for remotely operated inspection
devices with a horizontal reach of 5 m that fit inside access tubes with diameter
of 0.2 m, that are able to support a tip mass of 2 kg plus a distributed mass of
0.5 kg/m.
This report presents a review of deployable boom technologies which aims to
identify technologies that —if suitably developed— would have the potential to
meet the above requirements. Fifteen different technologies are reviewed, of which
six are found to be potentially of interest. After a further down selection, it is
concluded that two technologies —push chains and thin-walled booms— are the
most promising.
It is unclear if a single system can meet all of the requirements, but the selected
technologies are seen to be complementary; the former being better suited to
short-reach systems with heavier payloads and the latter to long-reach systems
with lighter payloads. It is recommended that both technologies be further investigated.
iii
Contents
Confidentiality and Intellectual Property Rights Statement
i
Disclaimer
ii
Executive Summary
iii
List of Figures
vi
1 Introduction
1
2 Review
2.1 Telescopic booms . . . . . . . . . . . . . . . . . . .
2.1.1 Telescopic triangular or square truss . . . .
2.1.2 Telescopic cylinders . . . . . . . . . . . . . .
2.2 Folding beam . . . . . . . . . . . . . . . . . . . . .
2.3 Active/passive cable systems . . . . . . . . . . . . .
2.4 Box bellows . . . . . . . . . . . . . . . . . . . . . .
2.5 Folding articulated trusses . . . . . . . . . . . . . .
2.6 Coilable trusses . . . . . . . . . . . . . . . . . . . .
2.7 Triangular wire boom . . . . . . . . . . . . . . . . .
2.8 Tri-beam . . . . . . . . . . . . . . . . . . . . . . . .
2.9 Instarect . . . . . . . . . . . . . . . . . . . . . . . .
2.10 Flexible tether . . . . . . . . . . . . . . . . . . . . .
2.11 Thin-Walled Tubular Boom . . . . . . . . . . . . .
2.11.1 Overlapping STEM or single STEM . . . . .
2.11.2 Bi-STEM . . . . . . . . . . . . . . . . . . .
2.11.3 Interlocking Bi-STEM & Tablock Bi-STEM
2.11.4 Collapsible Tube Mast (CTM) . . . . . . . .
2.11.5 Bi-stable Tubes . . . . . . . . . . . . . . . .
2.12 STACER . . . . . . . . . . . . . . . . . . . . . . . .
2.13 Inflatables . . . . . . . . . . . . . . . . . . . . . . .
2.14 Flexible actuator . . . . . . . . . . . . . . . . . . .
2.15 Chain systems . . . . . . . . . . . . . . . . . . . . .
2.15.1 Cable carriers . . . . . . . . . . . . . . . . .
2.15.2 Push chain . . . . . . . . . . . . . . . . . . .
iv
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3 Preliminary selection
31
4 Recommendations
4.1 Rejected systems . . . . . . . . . . .
4.1.1 Hingeless mast . . . . . . . .
4.1.2 Tri-beam . . . . . . . . . . . .
4.2 Recommended systems . . . . . . . .
4.2.1 Push chain . . . . . . . . . . .
4.2.2 Thin-Walled Booms . . . . . .
4.2.3 Modified design requirements
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Bibliography
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39
v
List of Figures
1.1
Schematic diagram of two access schemes. . . . . . . . . . . . . .
2
2.1
2.2
2.3
2.4
2.5
2.6
a) Sequential and b) synchronous deployment of telescopic mast [37].
Telescopic triangular truss [39]. . . . . . . . . . . . . . . . . . . .
Telescopic cylinder [39]. . . . . . . . . . . . . . . . . . . . . . . .
Folding beam [39]. . . . . . . . . . . . . . . . . . . . . . . . . . .
View of inspection arm within a nuclear steam generator [43]. . .
Simple two-dimensional deployable structure showing the use of
active and passive cable elements [33]. . . . . . . . . . . . . . . .
Diagram of mast consisting of pantograph with triangular crosssection, see (c), deployed by smaller pantograph [45]. . . . . . . .
Box bellows [39]. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Folding principles for folding articulated trusses [38]. . . . . . . .
’HIMAT’ retractable mast for space applications [31]. . . . . . . .
Folding Articulated Square Mast (FAST) [38]. . . . . . . . . . . .
Deployment methods considered for the HIMAT [31]. . . . . . . .
X-beam concept [1]. . . . . . . . . . . . . . . . . . . . . . . . . . .
Deployment of large truss structure [42]. . . . . . . . . . . . . . .
Coilable masts deployed in (a) local coil mode and (b) motor driven
deployment-nut [38]. . . . . . . . . . . . . . . . . . . . . . . . . .
Deployment of Astromast using a lanyard [6]. . . . . . . . . . . .
Weights and dimensions of lanyard deployer for Astromast [6]. . .
Deployment of Astromast using a deployment nut [6]. . . . . . . .
Weights and dimensions of canister for nut-deployed Astromast [6].
Design chart from Astro Research Corporation [6]. . . . . . . . . .
Design chart from Astro Research Corporation [6]. . . . . . . . . .
Design chart from Japan Aircraft Manufacturing Co. [28]. . . . . .
Design chart from Japan Aircraft Manufacturing Co. [28]. . . . . .
Triangular wire boom [39]. . . . . . . . . . . . . . . . . . . . . . .
Tri-beam [39]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Result of bending test on Tri-Beam [16]. . . . . . . . . . . . . . .
Instarect [39]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Flexible tether [39]. . . . . . . . . . . . . . . . . . . . . . . . . . .
(a) Tubular boom, (b-d)different cross sections, and (e) deployment cassette for bi-STEM [37]. . . . . . . . . . . . . . . . . . . .
Overview of bending stiffness for various flexible shell systems [41].
3
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5
5
2.7
2.8
2.9
2.10
2.11
2.12
2.13
2.14
2.15
2.16
2.17
2.18
2.19
2.20
2.21
2.22
2.23
2.24
2.25
2.26
2.27
2.28
2.29
2.30
vi
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2.31 Deployment of single STEM (a) drum, (b) tip drum, and (c) Jackin-a-box [40]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.32 Illustration of the Bi-STEM [36]. . . . . . . . . . . . . . . . . . .
2.33 Packaging dimensions for STEM and Bi-STEM [7]. . . . . . . . .
2.34 Critical bending moment of STEM and Bi-STEM [7]. . . . . . . .
2.35 Bending stiffness of STEM and BI-STEM [7]. . . . . . . . . . . .
2.36 Steerable Bi-STEM unit [36]. . . . . . . . . . . . . . . . . . . . .
2.37 Steerable Bi-STEM in a waste tank [36]. . . . . . . . . . . . . . .
2.38 Fully deployed cross sections of different STEM types [5]. . . . . .
2.39 Cross-section of collapsible tube [4]. . . . . . . . . . . . . . . . . .
2.40 Collapsible tube [34]. . . . . . . . . . . . . . . . . . . . . . . . . .
2.41 Data for CuBe CTMs [4]. . . . . . . . . . . . . . . . . . . . . . .
2.42 Rolatube used for camera inspection [15]. . . . . . . . . . . . . . .
2.43 Back-wound spring helix [24]. . . . . . . . . . . . . . . . . . . . .
2.44 Critical bending moment of STACER [24]. . . . . . . . . . . . . .
2.45 Inflatable tube rolled on reel [39]. . . . . . . . . . . . . . . . . . .
2.46 Actuation principle of ”elephant trunk” [20]. . . . . . . . . . . . .
2.47 Working prototype of ”elephant trunk” [20]. . . . . . . . . . . . .
2.48 Movement of cable carrier with intermediate support [29]. . . . . .
2.49 Mechanism for links with finger [21]. . . . . . . . . . . . . . . . .
vii
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30
Chapter 1
Introduction
The nuclear industry has a requirement for a deployable “arm” to be used —
for example— as a remotely operated manipulator in an inspection system for
nuclear facilities. Key characteristics of the required device are as follows:
• Maximum length Lmax = 5 m;
• Attached at the root to a remotely operated tilting mechanism;
• Can support itself, plus a tip mass of 2 kg and 0.5 kg per metre of cabling,
when partially or fully extended, and in all configurations described below;
• Fits inside a circular access tube with a diameter of 0.2 m, in its retracted
configuration;
• Can be fully recovered in case of failure.
Figure 1.1 shows two typical configurations in which the arm is to be used. Figure 1.1(a) shows a vertical access tube, up to 15 m long and with a diameter of
0.2 m; the tilting device attached to the root of the arm can tilt up to 20◦ beyond
the horizontal, i.e. the off-axis rotation angle θ has a maximum value of 110◦ .
Figure 1.1(b) shows the second configuration, with a horizontal access tube, up
to about 5 m long, and θ —again, measured from the axis of the arm— anywhere
between 0◦ and 45◦ .
Several delivery devices, AB in the figure, already exist to position the arm, BC,
and the tilting mechanism, B, at the end of the access tube. Hence, the main
purpose of this study is to investigate possible design solutions for the arm BC.
This report provides an overview of existing technology for deployable booms
that are thought to be relevant to the present application. It is based on a review
of reference material. Fifteen methods of deployment are identified and briefly
described and documented. Robotic arms with mechanically controlled joints
were excluded from the present study.
1
A
Access tube
φmin = 0.2 m
Lmax = 15 m
Access tube
φmin = 0.2 m
Lmax = 5 m
θmax=45o
C
θ
A
B
θmax=110o
C
θ
B
(b)
(a)
Figure 1.1: Schematic diagram of two access schemes.
2
Chapter 2
Review
2.1
Telescopic booms
Telescopic booms, Figures 2.1 - 2.3, can be divided into two categories: trusses
and cylinders. In both cases they are limited by the thickness of the telescopic
elements, which must be nested into each other in the stowed configuration.
To secure sufficient stiffness in the deployed boom, sufficient overlap between
consecutive elements is required, hence it is not possible to achieve the maximum
theoretical length of the boom. The stiffness usually decreases with deployment.
The tip of the boom does not rotate during deployment.
Telescopic booms can be deployed either synchronously or sequentially, and the
deployment/retraction can be controlled by various mechanisms. Typical mechanisms are spindle-and-nut arrangements [37], cable-and-pulley arrangements [37],
chain drives [23], pneumatic systems [39], or use of a secondary deployable/retractable
system for deployment/retraction of the main structure [19].
Figure 2.1: a) Sequential and b) synchronous deployment of telescopic mast [37].
3
2.2. FOLDING BEAM
2.1.1
Telescopic triangular or square truss
Telescopic trusses, see Figure 2.2, are constructed using concentric truss sections
supported on rollers to allow them to move relative to one another. Steel truss
structures are often used for larger applications such as masts or antennas on
earth. The size and stiffness of these structures depends on the application [39].
Figure 2.2: Telescopic triangular truss [39].
2.1.2
Telescopic cylinders
These are often used for various antenna sizes and small deployable booms on
satellites. They are constructed using concentric tubular sections. Materials
used for space applications are typically aluminium alloys and CFRP. For earth
applications steel is typically used. The smallest diameter is typically about 10
mm. Typical wall thickness of approximately 0.5 mm for space applications [37].
Various systems have been constructed using telescopic cylinders. From one of
these it has been concluded that for aluminium tubes it would be possible to
construct these with lengths from 5 to 25 meters, for space applications [9, 19].
Figure 2.3: Telescopic cylinder [39].
2.2
Folding beam
An otherwise rigid beam can be made foldable by inserting one or more transverse
hinges. Deployment and retraction can be controlled by turning these hinges, either by means of a cable system or by controlling the hinges with motors and
gears. During deployment, the tip of the beam does not rotate around the longitudinal axis. The structural integrity of the system depends on the stiffness and
strength of the hinges [11, 39].
One space application of this type of beam involved the design of a 6 meter
boom with 3 hinges. It was constructed using GFRP members and Titanium
and Aluminium hinges. The boom was deployed using a tension cable [11].
4
2.3. ACTIVE/PASSIVE CABLE SYSTEMS
Figure 2.4: Folding beam [39].
A patented robotic arm for inspection of nuclear steam generators also utilizes
this type of stowage. The mechanism primarily consists of two beam elements,
but the entire system is highly complex [43].
Figure 2.5: View of inspection arm within a nuclear steam generator [43].
2.3
Active/passive cable systems
This type of structure is based on a series of rods forming a pantograph, plus a
number of ”passive” cables connected to the joints of the pantograph. One or
more active cables, following specially chosen routes along the pantograph, and
running over small pulleys, deploy and retract the structure. The passive cables
become prestressed in the fully deployed state, thus increasing the stiffness of the
structure. During deployment the only load bearing structure is the pantograph,
whose stiffness is considerably lower. So far three models have been developed
using this system. Two are based on three dimensional pantographs and the last
on foldable octahedra, neither of them show any rotation of the tip during deployment. The performance of these structures can be predicted using computational
5
2.4. BOX BELLOWS
methods [33].
Figure 2.6: Simple two-dimensional deployable structure showing the use of active and
passive cable elements [33].
Another type of pantograph structure utilizes the different deployment rates of
pantographs of different lengths [45].
Figure 2.7: Diagram of mast consisting of pantograph with triangular cross-section, see
(c), deployed by smaller pantograph [45].
2.4
Box bellows
Constructed from flat rectangular panels joined longitudinally by hinges, the box
bellows is deployed by torsion springs. The hinges open inward and outward on
alternate panels to allow for maximum storage lengthwise, it has been recently
6
2.5. FOLDING ARTICULATED TRUSSES
used to support the solar cells in a nanosatellite . Looking in plan, this folding
method increases the surface area of the system. The thin plates provide good
torsional stiffness but cause problems with buckling for long slender structures.
This type of structure is usually not retractable and has only been tested using
small scale models. One of these had a built-in rotation of 90◦ during deployment [39].
Figure 2.8: Box bellows [39].
2.5
Folding articulated trusses
This type of structure is used widely for space applications and there are many
different configurations. Several companies offer fully developed and scalable
systems. Available sizes vary from a diameter of 250 mm upwards [2, 14, 31].
Figure 2.9: Folding principles for folding articulated trusses [38].
These systems, see Figure 2.10 and 2.11, contain hinges at the joints between
the longitudinal elements, or longerons, and the transverse elements, or battens.
Bracing cables, some of which are slack during deployment are attached to the
longerons. These structures can be deployed in a number of ways, including screw
jacks, belt drives, lanyard cables and others, see Figure 2.12. The elastic energy
stored in the battens during folding can be used to provide deployment force, see
Figure 2.11 [2, 31, 44].
7
2.5. FOLDING ARTICULATED TRUSSES
Figure 2.10: ’HIMAT’ retractable mast for space applications [31].
Figure 2.11: Folding Articulated Square Mast (FAST) [38].
8
2.5. FOLDING ARTICULATED TRUSSES
Figure 2.12: Deployment methods considered for the HIMAT [31].
These structures are deployed sequentially, and hence they can be constructed
such that they maintain full stiffness during deployment. Rotation of the tip
depends on the configuration used, as illustrated in Figure 2.9. The plan area
and volume of the stowed systems vary with the system of folding, see Figures 2.13
and 2.14. Some systems have only a limited change in plan area. Systems with
both three and four longerons are available. Materials used include Aluminium
alloy, GFRP and stainless steel for the cables [1, 2, 31, 42].
Figure 2.13: X-beam concept [1].
9
2.6. COILABLE TRUSSES
Figure 2.14: Deployment of large truss structure [42].
2.6
Coilable trusses
These structures are lattice trusses with triangular cross section and continuous
longerons, in the corners, braced by cables and/or battens. They are stowed by
coiling the longerons, starting at one end. The deployment methods are either
self-extension, also called local coil mode, or motor driven extension using a
deployment nut, as shown in Figure 2.15 [22, 37].
(b)
(a)
Figure 2.15: Coilable masts deployed in (a) local coil mode and (b) motor driven
deployment-nut [38].
The first method relies on the energy stored in the elastically folded longerons
and is controlled by a lanyard, see Figure 2.16. With this deployment method
during deployment the tip of the mast rotates during deployment and the stiffness
of the structure is much lower than in the fully deployed configuration, due to the
pressure of the unsupported, partially folded transition zone. This method, however, produces the most compact stowed configuration. Weight and dimensions
of a lanyard deployer are shown in Figure 2.17.
In the second method, deployment is controlled by a deployment nut and a canister. The canister controls and contains the transition zone and thus the deployed
part of the mast has full stiffness but rotates with the deployment nut relative
10
2.6. COILABLE TRUSSES
Figure 2.16: Deployment of Astromast using a lanyard [6].
Figure 2.17: Weights and dimensions of lanyard deployer for Astromast [6].
11
2.6. COILABLE TRUSSES
to the storage drum. This system is illustrated in Figure 2.18. Dimensions and
weight of the canister are shown in Figure 2.19.
Figure 2.18: Deployment of Astromast using a deployment nut [6].
Figure 2.19: Weights and dimensions of canister for nut-deployed Astromast [6].
Deployment can also be conducted by uncoiling the entire mast in one go. This
is called the helix mode. This method is, however, not widely used [30].
Several configurations are available commercially, with dimensions from 100 mm
to much larger [3, 28]. Design charts are provided in Figures 2.20 to 2.23. A
hingeless configuration has been developed for increased reliability through fewer
mechanical joints [30,32]. For increased stiffness additional bracing cables can be
incorporated in the design; these cables span two bays instead of the normal one
bay [28]. Materials used are GFRP, Polymide and Titanium or Stainless Steel
cables [3, 28, 30].
12
2.6. COILABLE TRUSSES
Figure 2.20: Design chart from Astro Research Corporation [6].
Figure 2.21: Design chart from Astro Research Corporation [6].
13
2.6. COILABLE TRUSSES
Figure 2.22: Design chart from Japan Aircraft Manufacturing Co. [28].
Figure 2.23: Design chart from Japan Aircraft Manufacturing Co. [28].
14
2.7. TRIANGULAR WIRE BOOM
2.7
Triangular wire boom
This system utilizes triangular sections welded to three longerons. One side of
the triangular sections is made to flex or is hinged, so that the other two sides
can be brought together. The collapsed mast is rolled up on a drum. When
rolled out by a mechanical drive deployment depends on the energy stored in the
flexed or hinged members. The longerons must be flexible enough to allow them
to fold elastically around the reel. Diagonal members brace the mast. The tip
of the mast does not rotate during deployment. This system has yet to be fully
developed [39].
Figure 2.24: Triangular wire boom [39].
2.8
Tri-beam
The Tri-beam system is constructed from three identical strips of material interlocked at their common edges. The interlocking can be achieved using permanent
tabs and sockets along the edges of the strips [16]. Alternatively, it can be done
using velcro tape or snap fasteners [39].
Figure 2.25: Tri-beam [39].
Because the individual strips are very flexible before being locked together, there
is a large degree of freedom when placing them on separate storage drums. This
allows configurations varying from flat and rectangular to long and circular. As
the beam has already achieved its full stiffness when it exits the deployment
mechanism, it maintains its stiffness throughout deployment. There is no rotation
15
2.9. INSTARECT
of the tip during deployment. Materials are Beryllium Copper and Stainless Steel.
Two systems, with diameter of 75 mm and 150 mm have been commercially
developed and tested. See Figure 2.26 for results of bending tests conducted on
one such system [16, 39].
Figure 2.26: Result of bending test on Tri-Beam [16].
2.9
Instarect
This is a beam composed of 3 strips. The two outer strips are preformed tape
springs which are flattened and then rolled up; the central strip is flat. The two
outer strips have side tabs that interlock into holes along the edges of the center
strip, Figure 2.27. Two models have been made using Stainless Steel; they have
a cross-section of approx. 75 by 100 mm.
When stowed, the three strips are stored on reels. The deployment mechanism
reels out the strips and interlocks them [39].
Figure 2.27: Instarect [39].
2.10
Flexible tether
This system uses cylindrical sections with spherical seats on either end, alternating with balls. All members have a central hole running longitudinally through
16
2.11. THIN-WALLED TUBULAR BOOM
them for accommodating a tensioning cable, which is fixed at one end and, when
tensioned, forces all the sections to become aligned and form a straight boom.
This system does not have much torsional stiffness unless modified, and there is
therefore no control over the rotation of the tip during deployment. The required
shape of the sections can be achieved by machining metals such as Aluminium
alloy.
This type of deployable boom has low bending stiffness and strength and also
poor storage characteristics as the stowed and deployed volume are the same.
Some engineering models of this concept have been demonstrated [39].
Figure 2.28: Flexible tether [39].
2.11
Thin-Walled Tubular Boom
Thin-walled tubular booms are commonly used for deployable/retractable structures. These structures utilize the ability of thin walled shells to deform elastically. There are a large number of configurations for these flexible shells, each
with certain specific properties, which are highlighted below. Materials commonly
used for the shells are Beryllium Copper, Stainless Steel and CFRP [4, 37, 38, 42].
A comparison of the stiffness characteristics for various flexible shell systems is
shown in Figure 2.30. STEM systems are already being used for inspection applications in North America [36]. Note that the tubular STEM systems discussed
in 2.11.1 and 2.11.2 can be combined with an outer layer of telescopic tubes to
increase strength and stiffness. This type of hybrid configuration, however, is
only applicable for shorter systems [7].
2.11.1
Overlapping STEM or single STEM
This configuration is based on the same principle as the tape measure, but the
cross-section forms a complete circle with some overlap. This provides a stiff
tubular section but there are inherent problems due to the lack of torsional stiffness of the open section tubes. Torsion can be carried only if there is sufficient
friction in the overlap region.
The strip is coiled on a drum for storage, at the base, and can be deployed and
retracted by rotation of the drum in a compact mechanism. Strain energy is
17
2.11. THIN-WALLED TUBULAR BOOM
Figure 2.29: (a) Tubular boom, (b-d)different cross sections, and (e) deployment cassette for bi-STEM [37].
Figure 2.30: Overview of bending stiffness for various flexible shell systems [41].
18
2.11. THIN-WALLED TUBULAR BOOM
stored within the strip, this energy must be contained by the stowage cassette or
some other structure [35, 37, 40].
An example of the size of the deployment and containment mechanism is 212×212×220
mm for a tubular single STEM i.e. consisting of a single strip, with a diameter of
24 mm [17]. Overlapping STEMs have standard commercial sizes with diameters
varying between 10 mm and 130 mm [7, 40]. Design methods and calculations
are well proven and are available [5, 7, 40].
Two alternative methods of deployment, one where the STEM is deployed from
a drum at the tip of the STEM and one using the jack-in-a-box deployment,
are illustrated in Figure 2.31. But these methods are not useful if controlled
deployment is required [40].
Figure 2.31: Deployment of single STEM (a) drum, (b) tip drum, and (c) Jack-in-abox [40].
2.11.2
Bi-STEM
This configuration of the STEM is a development from the single STEM. The
Bi-STEM employs two diametrically opposed strips. One strip is placed inside
the other with an overlap, so the concave sides of each element face one another.
This configuration forms a tubular structure, see Figure 2.32.
The Bi-STEM can be stored on a single or multiple drums in the same fashion
as single STEMs. The main advantage over a single STEM of the same length
and stiffness is that the two strips in the Bi-STEM are narrower than that making up the single STEM. Thus the drums can be shorter and the design more
compact. As for the single STEM there is no rotation of the tip during deployment/retraction [7, 35, 38].
19
2.11. THIN-WALLED TUBULAR BOOM
The Bi-STEM has better mechanical properties than the single STEM. Both the
bending and torsional stiffnesses are slightly higher than for that of similar sized
single STEMs but at a small increase in weight. The properties of Bi-STEMs can
be calculated on the basis of published material; design charts are provided in
Figures 2.33 to 2.35. Note that Figure 2.33 includes typical packaging data. The
maximum diameter of a Bi-STEM is approximately 130 mm [5, 7, 35].
Systems using Bi-STEMs have been developed for inspection of nuclear facilities
and waste tanks in North America. In a particular application 3D mapping
instruments have been deployed up to 30 feet horizontally by using a steerable
version of the Bi-STEM. The steering mechanism is contained in a joint which
allows up to 120◦ about a single axis [36]. The system is illustrated in Figures
2.36 and 2.37.
Figure 2.32: Illustration of the Bi-STEM [36].
2.11.3
Interlocking Bi-STEM & Tablock Bi-STEM
The Bi-STEM configuration can be further improved by interlocking the two
deployed strips while they deploy. This is done by tabbing the inner strip along
the edges and guiding these tabs into corresponding holes in the outer strip. This
locks the two strips together and improves the torsional stiffness of the STEM
though there still is some free play within the holes.
Free play can be completely eliminated by using the tablock system, specially
designed for this purpose. Mechanical properties for both systems can be easily
calculated if it is arranged that the interconnection takes place in the deployment
mechanism [5].
These structures have been used on spacecraft in the past but, due to their
complexity and potential reliability problems, are no longer being used nowadays.
20
2.11. THIN-WALLED TUBULAR BOOM
Figure 2.33: Packaging dimensions for STEM and Bi-STEM [7].
Figure 2.34: Critical bending moment of STEM and Bi-STEM [7].
21
2.11. THIN-WALLED TUBULAR BOOM
Figure 2.35: Bending stiffness of STEM and BI-STEM [7].
Figure 2.36: Steerable Bi-STEM unit [36].
22
2.11. THIN-WALLED TUBULAR BOOM
Figure 2.37: Steerable Bi-STEM in a waste tank [36].
Figure 2.38: Fully deployed cross sections of different STEM types [5].
23
2.11. THIN-WALLED TUBULAR BOOM
2.11.4
Collapsible Tube Mast (CTM)
A collapsible tube is made by bonding two thin, transversely curved strips along
the edges. This creates a tubular boom with higher torsional stiffness than
the STEM. The tube can be collapsed and reeled on a storage drum much
like a STEM. Like the STEM, there is no rotation of the tip during deployment/retraction.
The tube is formed into a biconvex shape and is unstressed in the deployed
position. The two strips are either welded or glued together, see Figure 2.39
[4, 12, 37].
Figure 2.39: Cross-section of collapsible tube [4].
The cross-sectional dimensions of CTM’s manufactured so far go from 22 by 33
mm to 133 by 194 mm, though only larger sizes have been made in both CFRP
and CuBe. CFRP is troublesome in smaller sizes [42]. Some data for specific
CTMs are provided in Figure 2.41.
Figure 2.40: Collapsible tube [34].
24
2.12. STACER
Figure 2.41: Data for CuBe CTMs [4].
2.11.5
Bi-stable Tubes
A recent development in tubular booms is bi-stable reeled composites (BRCs).
These materials are stable in two shapes; as a straight tube and as a compact
coil. Booms produced using these composites have the advantage of more compact
stowage, as the stowed configuration is stable and thus the cassette containing the
stored structure can be made smaller and lighter. Also the deployment/retraction
mechanism can be simplified.
Most types of tubular booms could take advantage of the characteristics of bistable composites, illustrated in Figure 2.42. Open section booms using bi-stable
materials have been made with diameters of 15 mm and upwards. Booms with
a diameter of 150 mm and 200 mm diameter have been produced and used for
camera inspection of nuclear reactors [15].
2.12
STACER
The Spiral Tube & Actuator for Controlled Extension/Retraction (STACER),
Figure 2.43, is constructed of a single spiral-wound, overlapping, prestressed metal
strip, which forms a rigid tube when extended. Two methods for deployment are
used, only one of which provides a retraction capability. The repeatable roll-out
STACER is back-wound onto a drum and rolled out during deployment. The
drum can be motorized or driven manually. During deployment/retraction the
tip of the helix rotates to allow the strip to roll around the tube. [24, 39]
The spring helix is produced commercially for a large variety of uses, including
antennas with a length of more than 12 meters and minimum diameter of 10 mm.
Typical materials are Stainless Steel and Beryllium Copper. A bending strength
design chart for the STACER is provided in Figure 2.44.
25
2.12. STACER
Figure 2.42: Rolatube used for camera inspection [15].
Figure 2.43: Back-wound spring helix [24].
Figure 2.44: Critical bending moment of STACER [24].
26
2.13. INFLATABLES
2.13
Inflatables
Several types of inflatable booms have been proposed. For applications where
retraction is not a requirement these structures are usually rigidized, e.g. by
curing a thermosetting epoxy for increased stiffness after deployment. Retractable
structures rely on the stiffness of the thin skin to provide sufficient stability.
Various types of retractable configurations have been proposed and they include
rolled up and flattened tubes, jack-in-a-box and folded tubes. Skin materials
include plastic foils, such as Mylar and Kapton with or without stiffening fibres,
and low-yield stress Aluminium coated with a plastic film. The deployment is
usually controlled by the gas used for inflation [8, 10, 39].
Figure 2.45: Inflatable tube rolled on reel [39].
2.14
Flexible actuator
A FLexible Appendage for Positioning and Stabilization (FLAPS) has been proposed. It is being developed for maritime inspection, repair and maintenance.
This appendage uses a flexible actuator, called the elephant’s trunk actuator.
This works by element deflection rather than by links and joints, which improves
reliability and decreases the complexity of the system.
The operation of the trunk itself relies on the elastic deformation of cylindrical
metal bellows. These are of such a configuration that longitudinal extension is
introduced when an internal pressure is applied. By combining three such bellows,
rotation about all three axes can be achieved. At the moment only prototypes
have been constructed and these are of limited size, approx. 1 m or smaller [20].
2.15
Chain systems
Various systems utilizing the principle of the bicycle chain are available commercially. In a normal bicycle chain, the links pivot around cylindrical-joints and
are allowed to rotate freely until they come into contact with neighboring links.
As this does not provide a stiff system, various other configurations have been
developed for other applications.
27
2.15. CHAIN SYSTEMS
Figure 2.46: Actuation principle of ”elephant trunk” [20].
Figure 2.47: Working prototype of ”elephant trunk” [20].
28
2.15. CHAIN SYSTEMS
2.15.1
Cable carriers
This type of chain has been developed to allow machinery freedom to move linearly along a prescribed path while allowing electrical and other cabling to follow
along. Electrical cables can thus be permanently fixed to both the moving machinery and a fixed support.
For this type of application the behavior of the chain is governed by the supports
at either end, which restricts the rotation of the links within a certain range.
Figure 2.48 shows that beyond a certain length the upper part of the chain needs
to be supported to prevent excessive deflection; alternatively the deflection can be
reduced by pretensioning the chain. Note that the stiffness of this chain requires
proper support conditions, which prevents cable carriers from being used as a
boom.
Figure 2.48: Movement of cable carrier with intermediate support [29].
Cable carriers can be made from plastics or Steel and the unsupported length of
the upper part of the chain can reach 6 meters if a sufficiently large deflection is
allowed [27, 29].
2.15.2
Push chain
Push-pull chains are also used for linear motion but, unlike cable carriers, they
have been developed to provide an axial force to an object. To do this, the push
chain is made such that the links are only allowed to rotate one way from the
initial straight configuration, thus the push chain becomes rigid and will not fold
when subjected to an axial compression force.
A widely used mechanism for providing the rotational restraint is to provide a
finger on the links, as can be seen in Figure 2.49. This creates a locking chain link,
whose rigidity is dependent on an off-center compressive force, or other bending
moment putting the fingers into compression. This configuration could be used
for a deployable boom, as gravity loading would provide the bending moments
that rigidize the chain.
The tip deflection of a boom based on a push chain is controlled by the cumulative
play in the pivot pins and the guide rail in the drive housing. Thus, flexing will
29
2.15. CHAIN SYSTEMS
Figure 2.49: Mechanism for links with finger [21].
occur in both horizontal and vertical planes, but no data is currently available on
the magnitude of these effects. Materials are either Steel or Stainless Steel [21].
30
Chapter 3
Preliminary selection
No.
2.1.1
2.1.2
Solution type
Telescopic triangular or square
truss
Telescopic cylinders
Selected
No
Reasons
- The cylindrical shape provides much better
performance for the present scope.
No
- Performance depended on initial length,
which is probably too long.
- Well know technology and mechanics.
- Low compaction.
- Direction of tip changes during deployment.
- Performance depended on initial length.
- Reliability of large number of pulleys and
joints.
- Unsuitable for small dimension applications.
- This type of boom have been insufficiently
developed at present.
- Problems with making boom retractable.
- The large number of joints prevents it from
being scaled down.
- Reliability of the many joints is a problem.
- The hingeless configuration of coilable
masts eliminates the many joints which
might cause problems.
- The stowed configuration is parallel to the
axis of deployment, which allows larger diameter of boom unlike eg. STEM systems.
- This type of boom is not sufficiently developed.
2.2
Folding beams
No
2.3
Active/passive
cable systems
No
2.4
Box bellows
No
2.5
Folding articulated trusses
No
2.6
Coilable trusses
Yes
2.7
Triangular wire
boom
No
31
No.
2.8
Solution type
Tri-beam
2.9
Instarect
No
2.10
Flexible tether
No
Overlapping
Yes
2.11.1
Selected
Yes
STEM or single
STEM
2.11.2
Bi-STEM
Yes
2.11.3
Interlocking BiSTEM &
Tablock
BiSTEM
No
Reasons
- Closed section provides torsional stiffness.
- Various storage configurations possible as
the flat strips are very flexible and thus allows remote stowage.
- Interlocking mechanism deemed too complicated.
- Could be constructed using bi-stable materials.
- The closed section provides torsional stiffness.
- Various storage configurations possible as
the flat strips are very flexible and thus allows remote stowage.
- Interlocking mechanism has to be developed.
- Could be constructed using bi-stable materials.
- Few advantages that could not be provided
by a simpler CTM or bi-STEM.
- Very low bending stiffness.
- Very low compaction as stored volume is
the same as when deployed.
- Good stiffness though poor torsional stiffness.
- Simple mechanical principles.
- Containment canister and deployment
mechanism may be too large for present application.
- Provide good stiffness and simple mechanics.
- Smaller canister than for single STEM but
still large.
- As for other STEMs large containment canisters required at present
- Complex connection mechanism for interlocking.
32
No.
2.11.4
Solution type
Collapsible Tube
Mast (CTM)
Selected
Yes
2.11.5
Bi-stable Tubes
Yes
2.12
2.13
STACER
Inflatables
No
No
2.14
Flexible
actuator
No
Cable carriers
Push chain
No
Yes
2.15.1
2.15.2
Reasons
- Closed section provides torsional stiffness.
- Higher bending stiffness about one axis can
be utilized in the present scope.
- Presently the containment canister is relatively large compared with the deployed
boom.
- More compact stowage.
- Simpler deployment/retraction mechanism.
- Tip rotation during deployment.
- This type of boom have been insufficiently
developed at present.
- Low bending stiffness
- No compaction
- Large radius of bending
- Not appropriate for present application.
- Relatively cheap solution and known technology.
- Problems with flexing.
- Fixed angle changes about all axis in drive
housing.
33
Chapter 4
Recommendations
The review of existing technologies, in Chapter 2, has produced a large number
of solutions but only a few of them are suited for the present application. The
preliminary selection in Chapter 3 has identified seven systems of interest, but two
will be excluded in the next section. This leaves two different types of solutions,
push-chains and Bi-STEM/CTM/Bi-stable tubes, for further consideration. It is
currently unclear if these solutions can lead to working systems that meet in full
the requirements described in Chapter 1; further study of these solutions will be
required.
The following calculations are based on an applied tip load P = 20 [N], a distributed load p = 5 [N/m], and a maximum deployed length L = 5 [m]. Self
weight of the structure itself is neglected.
4.1
4.1.1
Rejected systems
Hingeless mast
The performance of the hingeless mast produced by Japan Aircraft Manufacturing
Co. was evaluated for a structure with a diameter of 200 mm, using Figure 2.22.
Calculating the tip deflection δ for a bending stiffness EI = 4.109 [Nmm2 ], from
the figure:
δ=
pL4
20 × 50003
5 × 10−3 × 50004
L
P L3
+
=
+
= 306 [mm] ≈
9
9
3EI
8EI
3 × 4 × 10
8 × 4 × 10
16
(4.1)
From Equation 4.1 it can seen that the stiffness of the structure is too low for an
application requiring precision. Thus, this system is deemed unsuitable for the
present application.
34
4.2. RECOMMENDED SYSTEMS
4.1.2
Tri-beam
Reevaluating the design of this structure it was found that designing a reliable
joining mechanism would be very difficult. It is thought that other systems will
be easier to implement.
4.2
4.2.1
Recommended systems
Push chain
The push chain systems are commercially available and based on well known
technology. They provide a simple system with a high reliability, but there are a
number of potential disadvantages that will need to be investigated.
As they are constructed from a large number of small links they will be flexible
due to free play in the joints and, because the available systems are made from
Steel, they have a high self weight. These limitations make the system potentially
less attractive for long distance horizontal deployment, but it may well prove ideal
for shorter booms.
This can be verified by a simple calculation where the constant bending moment
from the off-axis axial compression is compared with the moment caused by
loading, excluding self weight. The smallest push chain from Drives International
Ltd has a maximum push force of 5,000 N and an eccentricity of 9.5 mm.
Maximum bending moment in horizontally deployed arm:
M = P L + 21 pL2 = 20 × 5 +
1
2
× 5 × 52 = 163 [N m]
(4.2)
Bending moment due to max. axial load in push-chain:
M = N e = 5, 000 × 9.5 × 10−3 = 47.5 [N m]
(4.3)
Equations 4.2 and 4.3 show that the proposed application will cause higher bending moment than the chain is designed for, which may cause excessive flexing and
maybe even failure. The self weight of the system could be reduced by producing
the chain from a material lighter than Steel, by using a tapered design, etc.
As the available systems have a fixed angle in the drive housing it would be
necessary to develop a housing that allows some control of the pointing of the
arm.
35
4.2. RECOMMENDED SYSTEMS
4.2.2
Thin-Walled Booms
Thin-walled boom technologies are already being used for applications similar to
the present and there is quite a variety to choose from. These include single- and
bi-STEM, CTM and bi-stable tubes. All of these create an approximate circular
cross-section when deployed and in the following calculations will be treated as
such. The calculations are used to verify the feasibility of the various types of
thin-walled booms.
In the following calculations the diameter of the deployed cross-section of the
boom, the material thickness, and the length and diameter of the storage drum
are calculated for a bi-STEM/CTM made from BeCu and Steel. Finally, it is
shown that their design is driven by a deflection limit, not by stress and thus
these structures could be made from bi-stable materials, which may have a lower
stress capacity due to the existence of residual stresses. Self weight has not been
included in the calculations.
d = Tube diameter, mm
D = Drum diameter, mm
E = Modulus of Elasticity, N/mm2
l = Length of drum
σyp = Yield stress, N/mm2
t = Tube wall thickness, mm
ν = Poisson’s ratio
From [40]:
E D/d + ν
d
=
t
σyp (1 − ν 2 )D/d
(4.4)
Tip deflection of boom:
P L3
pL4
+
(4.5)
3EI
8EI
The width of the flattened Bi-STEM/CTM is approximately equal to the length
of the drum, l:
δ=
l≈
π×d
2
(4.6)
Preliminary sizes were determined for two different stiffness requirements, L/500
and L/100. For D/d = 1, BeCu and Steel are evaluated using Equations 4.4
36
4.2. RECOMMENDED SYSTEMS
Table 4.1: Results for Bi-STEM/CTM configurations
δ
d
t
D
l
BeCu
L/500 L/100
182
122
1.24 0.831
182
122
286
192
Steel
L/500 L/100
153
102
0.879 0.588
153
102
240
160
to 4.6.
As can be seen in Table 4.1, the stricter stiffness requirement (δ = L/500) cannot
be met in a system that fits inside an access tube with diameter of 200 mm.
The only feasible system is a bi-STEM/CTM made from Steel. To decrease the
deflections it would be necessary to decrease either the loading or the deployment
length.
For a tubular section of Steel with d = 102 mm and t = 0.588 mm, the maximum
bending stress due to the moment in Equation 4.2 is:
σb =
M
=
W
163 × 103
π 1024 −(102−0.588)4
32
102
£
¤
= 58 N/mm2
(4.7)
Equation 4.7 shows that the maximum stress due to bending is much smaller than
the permissible level of stress in bi-stable materials, which are several hundred
N/mm2 .
This leaves four approaches to thin-walled booms: (i) the commercial system from
SPAR Environmental Systems, Ontario, Canada, which might not satisfy the
tight dimensional requirements, (ii) developing a bi-STEM or (iii) a CTM system
using either conventional materials or bi-stable materials, the latter giving the
possibility of a more compact design of the deployment mechanism; (iv) reevaluate
the initial design requirements.
4.2.3
Modified design requirements
Considering the systems identified so far, one could imagine partitioning the initial design requirements into two parts and develop two different systems. One
would be a light deployable system which would be able to deliver a light tipmounted package with light weight cabling up to a distance of 5 meters horizontally, and one for a heavier system which would carry a heavier load but at a
shorter distance.
37
4.2. RECOMMENDED SYSTEMS
By doing this, both the push chain and STEM systems become better suited for
one of the two design envelopes.
38
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