=Paper=
{{Paper
|id=Vol-1484/paper20
|storemode=property
|title=Lessons from the Design and Testing of a Novel Spring Powered Passive Robot Joint
|pdfUrl=https://ceur-ws.org/Vol-1484/paper20.pdf
|volume=Vol-1484
|dblpUrl=https://dblp.org/rec/conf/iros/ShortPLTA15
}}
==Lessons from the Design and Testing of a Novel Spring Powered Passive Robot Joint==
https://ceur-ws.org/Vol-1484/paper20.pdf
Lessons from the Design and Testing of a
Novel Spring Powered Passive Robot Joint
Joel Stephen Short1 , Aun Neow Poo2 , Chow Yin Lai3 , and Pey Yuen Tao3 , Marcelo H Ang Jr2
Abstract— The design, assembly, and testing of a new tor- by [1] and expanded by [2], there arose a need for an
sional spring joint for use in underactuated robots is presented. underactuated robot, for testing of the method proposed
The joint can use an array of spring sizes and is able to adjust in [3]. The motivation behind designing and building an
the spring offset and preload independently. This work outlines
the design process with details on the troubles faced and lessons underactuated robot was twofold, first it would provide a
learned from multiple redesigns. experimental platform to test the theoretical system inversion
method mentioned above, and second, it would give insight
I. INTRODUCTION
into the general capabilities and usefulness of such a robot.
The design of new mechanical parts and assemblies is an The robot is required to perform cyclic(repeating) tasks
integral part of robotics research. Even when an engineer’s and is made up of two linkages in a planar arrangement.
research is mainly theoretical, it is typically expected that the There is an actuator at the first joint and the torsional spring
theory will be tested in an experimental setup, often requiring mechanism at the second joint, see Figure 1 for a simplified
the design of specialized pieces and devices, either for model of the robot. The actuator and passive joint placement
testing by themselves or inclusion in a larger robotic setup. ensures that the robot is underactuated but not completely
While there exist many design and testing methodologies uncontrollable, as backed up by the general serial-link robot
for mechanical and mechatronic parts and assemblies, when analysis done in [4]. There are many reliable sources to use
seeking to create a one-off prototype there is normally not
enough time for these long processes. The engineer must try
to quickly design, build and test an assembly, being efficient y
and using only as much time as is necessary to ensure the
design criteria is achieved. And this all must be done without τ1
running into dead ends or overly difficult problems during
L1
any stage of the build-up.
This work presents the design and build process of a m1
torsional spring joint with a special emphasis on the problems
encountered and the lessons learned. A short background sets
the stage for a discussion of the design goals and the resulting θ1
τs
initial design. Then the assembly and testing are discussed L2
with a presentation of the problems, attempted solutions and
final torsion joint layout. Lastly a discussion presents the key
lessons learned from this experimental work and how they
x m2
can contribute to prototype design in the future.
A. Background
θ2
While working on a stable system inversion method for the
control of underactuated robots, a technique first investigated Fig. 1. 2DOF planar robot with torsional spring joint
This work is supported by A*STAR, the Agency for Science Technology
and Research, under the Ministry of Trade and Industry of Singapore
1 Joel Stephen Short studies at the National University of Singa- when working with springs and mechanical design, though
pore and also a student member of the SIMTech-NUS Joint Lab [5] was consulted most often for this project.
(Industrial Robotics), c/o Department of Mechanical Engineering, Na- The use of torsion springs to provide a passive torque, that
tional University of Singapore, 9 Engineering Dr. 1, Singapore 117576
joel.stephen.short@u.nus.edu
depends on the position and arrangement of the spring, is a
2 Aun Neow Poo and Marcelo H Ang Jr are with the Department of very old idea and most easily seen in the common clothespin,
Mechanical Engineering, National University of Singapore, 9 Engineer- yet its use in robotics has been limited. An early study of
ing Dr. 1, Singapore 117576 and also staff members of the SIMTech- torsional springs within the dynamics of the a generalized
NUS Joint Lab (Industrial Robotics) mpepooan@nus.edu.sg;
mpeangh@nus.edu.sg; robot framework can be seen in [6]. Other closely related
3 Lai Chow Yin and Tao Pey Yuen are with the Singapore Institute of work focuses on using springs in conjunction with actuators,
Manufacturing Technology, Agency for Science, Technology and Research, normally classified as passive-compliant or variable stiffness
Singapore 638075 and also a staff member of the SIMTech-NUS Joint
Lab (Industrial Robotics), cylai@SIMTech.a-star.edu.sg; actuators. A useful survey of various passive-compliant ac-
pytao@SIMTech.a-start.edu.sg tuators, where the springs basic properties are used without
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The path to success: Failures in Real Robots October 2, 2015
adjustment, is seen in [7]. Some variable stiffness actuators The flanges can be rotated independently, along the cham-
use actively adjusted springs, as seen in [8], showing an fered slots, when the screws are loosened as seen in Figure
additional connection to biomechanical design. 2. The flange slots allow both the preload and offset of the
The design presented here is unique in two ways; first spring to be adjusted within a limited range of positions. The
it is very versatile, capable of using many different size range of motion and possible adjustment is outlined in Table
springs, second, it is highly adjustable, allowing the offset I while the setting ranges of the offset and preload are shown
and preload to be set independently. Though experimental, in more detail in Figure 3. The graph helps show that as the
this joint allows for greater investigation into the capabilities offset is adjusted the available offset range also changes, this
and usefulness of torsional springs within the serial-link is due to the limits of the mechanical setup.
robot framework. The spring sits around the joint axle with the second link
mounted to the axle using a mini-bush clamp, this allows the
II. DESIGN linkage to be placed higher or lower on the axle depending
The design of the torsional joint was performed using the on the size of spring used. The axle is secured to the first
traditional tools and methods of the mechanical engineer. joint with a single ball bearing.
After developing a few possible ideas that led to sketches and
drawings, the most promising one was built up in a computer
aided drafting (CAD) program (Autodesk Inventor) with the
creation and virtual assembly of the parts. The completed
initial design of the prototype led to the manufacturing and
assembling of the parts. The build, test, and redesign cycle
was run through twice with the final prototype showing
reliable performance in all important areas of the design.
A. Design Goals
Adapting the basic spring principles and capabilities for
use in a torsional spring driven joint started with a review
of what was needed from the joint. The design goals were
created by reviewing the needs of the overall robot as well as
the materials and space available. The goals are built around
keeping the design simple and are listed below:
Fig. 2. Linkage 2 flange attachments
1) Use a single torsion spring
2) Offset and preload angles must be adjustable The last design goal, allowing an encoder to read the
3) Spring body width must be adjustable angular position, was fulfilled by creating a joint axle with a
4) Only use the spring in compression small protruding extension at the bottom. An encoder could
5) Allow an optical encoder to read the angular position then be mounted on the underside joint, specifically on the
The experimental nature of the joint drove the first and end cap spacer, with the optical wheel mounted at the end
second goals, to allow for adjustment of the spring position of the axle.
and initial torque. The use of different springs prompted
the third goal. The fourth design goal was created after TABLE I
investigating the proper use of torsional springs, they are T ORSIONAL SPRING JOINT PROPERTIES
not made to be used repetitively in both tension and com- Parameter Stiffness Offset Preload Link 2 Motion
pression. Most manufactures recommend only using them in Variable k θf θp θ2
Range (0.02, .01) ±50 (−20, +80) (−175, +270)
compression. The last goal is due to the experimental nature Units Nm/rad degree degree degree
of the mechanism and enables the angular position feedback
from the joint to be recorded, allowing further study and
evaluation of the robots motion in post processing. The overall design can be seen in Figure 4 with its related
parts list in Figure 5. All of the design goals were achieved
B. Implementation
in the general design layout, though only by building the
The simplest and most direct design uses only one spring torsional joint and testing it could the mechanism be deemed
and two pairs of hook and flange subassemblies. Each hook successful.
plate is attached to a flange that is stacked with another
flange with both secured to the robot linkage. There are two III. ASSEMBLY AND TESTING
flanges per link, one set has long flange arms and the other The prototype went through a cycle of assembly, testing,
short flange arms. This hook hand-off design, with the two and redesign, twice before the arriving at the final setup.
different flange arm lengths, ensures that the torsion spring Therefor are three designs, denoted alpha(original), beta, and
is only used in compression, no matter if the linkage moves final. The difficulties encountered at each stage are discussed
in the positive or negative radial direction. leading to the proposed solutions and redesign.
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1
2 D
3
18
7 4
5
6
9
10
14
15
7 11
8
16
12 17
13
SECTION D-D D
Fig. 4. Overall design of the torsional joint, optical encoder to be mounted at the joint underside, to linkage 1
PARTS LIST
2 D ITEM A. AlphaPART
QTY NUMBER
results
120
1 1 Joint 2 Axle
100 2 1 TheMini-Bush
parts Inner
for
18 the torsional spring joint were sent out
3 for
1 manufacture
Mini-Bush Outerat a local machine shop while the mini-
804
5 4 bush,
1 springs,
Linkage 2 and hardware(bolts) were procured from local
Preload(deg)
60 5 1 Upper Flange2
suppliers. Upon receiving the parts and assembling the joint a
6 6 1 Upper Flange1
40 major problem was observed; the bore in linkage 1, to house
9 7 4 M3x10 CSK
20 10 8
the
1
ball bearing was cut 1mm to short, causing the bearing to
Torsion Spring
14 protrude from the housing. This was discouraging but before
0 15 9 1 Lower Flange1
10 sending
1 theFlange2
Lower part back to be finished properly, the rest of the
7 -20 11 11 assembly
1 Linkagewas1 constructed to check for other problems.
-40 12 1 Additional
Ball Bearinginvestigation proceeded despite the improper
16
13 1
-20 -10 0
13
10 20 fit of End
theCap Spacer
17bearing and another major problem was found.
14 2 Long Hook
Offset(deg) The ball bearing tolerances were far too loose and allowed
D 15 2 Short Hook
16 the
2 axleWasher toPlate
wobble from side to side. This caused the
Fig. 3. Range of Torsion joint settings hook hand-off to sometimes miss and more importantly the
17 3 M2x6
18 8
opticalM3x6
encoder could not function reliably under such wide
tolerances. After considering this major problem of axle
PARTS LIST wobble, it was thought that by adding a roller bearing to the
ITEM QTY PART NUMBER axle the problem could be fixed with the addition of only
1 1 Joint 2 Axle
one new machined part, an extension spacer. All the original
2 1 Mini-Bush Inner
3 1 Mini-Bush Outer parts could still be used. This new design compensated for
4 1 Linkage 2 the previous machining error, a drawing of the new bearing
5 1 Upper Flange2 package can be seen in Figure 6. The tolerance limit of the
6 1 Upper Flange1 encoder was closely consulted but due to the lack of precise
7 4 M3x10 CSK bearing tolerances from the manufacturer the redesign had
8 1 Torsion Spring
to rely on the best estimates of the engineer.
9 1 Lower Flange1
10 1 Lower Flange2
Lastly, as part of the hook hand-off difficulties, the short
11 1 Linkage 1 hook trough (where the spring sits on the hook) was found
12 1 Ball Bearing to be too close to the flange, making it difficult for the
13 1 End Cap Spacer long hook plate to grab the spring leg at the hand-off. The
14 2 Long Hook new hooks would be needed to allow for easy transition, a
15 2 Short Hook comparison of the old and new hooks is seen in Figure 7.
16 2 Washer Plate
The hook plates lacked specific angular markings, so
17 3 M2x6
18 8 M3x6
preload and offset angles had to be estimated. In order to
change out the torsion spring or adjust the preload or offset
Fig. 5. Torsion joint parts list the second linkage had to be removed from its axle. This was
not difficult due to the locking mini-bushings used, though
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The path to success: Failures in Real Robots October 2, 2015
1
3
8
4
8 2
6
B 2 6
5 C
5
4
7
1 6
3 SECTION C-C
PARTS LIST
7 ITEM QTY PART NAME
1 1 Linkage 1 V2
2 1 Joint 2 Axle V2
SECTION B-B 3 1 Extension Spacer
4 1 End Cap Spacer
PARTS LIST
5 1 Bearing Spacer
B ITEM QTY PART NUMBER
6 2 Ball Bearing
1 1 Linkage 1 V2 7 3 M2x15
2 1 Joint 2 Axle V1
C
8 3 M2x6
3 1 End Cap Spacer
4 1 Extension Spacer
Fig. 8. Final bearing package
5 1 Ball Bearing
6 1 Roller Bearing
7 3 M2x15
8 3 M2x7 it made adjustments a tedious affair.
B. Beta results
Fig. 6. Second torsion joint design (Beta)
With an additional roller bearing, a new short hook, and
the extension spacer the second assembly proved to still
contain difficulties. The new short hook allowed the hook
hand-off to proceed smoothly despite the fact that the joint
axle wobble was still too great. The roller bearing did reduce
the axial play (in terms of the wobbling) but not enough
to allow for reliable readings from the optical encoder
mounted on the bottom. It was at this point decided that an
adjustable bearing package would be the best solution, then
the tolerances of the ball bearings would not be an issue.
The final bearing setup is seen in Figure 8.
The final design required a new joint axle that was slightly
longer as well as a bearing spacer for the axle. An additional
ball bearing was also needed. The measurements sent to
the machine shop, regarding the bearing package, were kept
rough such that upon assembly the engineer could adjust the
fit of the bearing package to allow an appropriate amount of
play. If the bearings package is too tight and the axle won’t
turn, the bearing spacer can be ground down, while if the
package is too loose, the machined surface of the extension
spacer (which sits against linkage 1) can be ground down.
This is a common method for tuning the bearing clearances
of large gearboxes.
C. Final results
The final build-up of the torsional joint can be seen in
Figure 9. The tuning of the bearing package was done by
hand; by using a hand file and a lathe the extension spacer
was ground down progressively, bringing the outer races
of the bearings closer together until the axle wobble was
Fig. 7. Old hook (left) with new hook (right)
eliminated, but it could still freely turn.
The final design was completely successful in achieving all
of the design goals. The optical encoder returned a reliable
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The path to success: Failures in Real Robots October 2, 2015
signal while the adjustability of the joint allowed for the use complex. The first redesign of the bearing setup only required
of different springs and numerous different offset and preload one more machined part and one additional bearing, plus it
setups. allowed the imperfection of the bearing bore in linkage 1
to be left alone. It was thought to be the most economical,
IV. DISCUSSION
yet there was little to no guarantee that it would solve the
The design, assembly and testing of the torsional joint was axle wobble problem. The adjustable bearing package was
completed as a prototype, for use in testing a theoretical slightly more complex but should have been used in the first
control methodology and some important lessons can be redesign.
learned from the process. The failures in design and the Lesson 3: Familiarity with standard engineering solutions
route taken in redesigns reveals some beneficial as well as that are related to the current design is highly beneficial.
detrimental decisions. These will each be discussed as they The design of a prototype lends itself to quick thinking
relate to either the design of the mechanism or the testing of and the use of engineering solutions that “may” work or
the assembled parts. “should” work. Though time is often of the essence and there
A. Design Lessons is not time for an in depth analysis of the parts to ascertain
if the part or assembly will meet the design goals exactly it
Lesson 1: Bring all the design constraints together, ex- is critical that the engineer have a general understanding of
plicitly listing how they need to be achieved. standard industry and engineering practices. Spending time
The design goals of a mechatronic system typically involve to become familiar with the traditional solutions, relating to
requirements from the mechanical side, such as bearings, the particular parts or assemblies under design, can save time
fits and hardware, as well as from electrical parts, such and energy later in the process. This can be readily seen in
as encoders, motors and other interface pieces. If details the example when considering the bearing setup for the joint
are left out, they will often show up as trouble during axle. The first design turned out to be inadequate and only
testing. The design goals in the example were clear when a half measure. Instead, the industry standard for bearing
the mechanism was first drawn up, the needs of the torsional packages which need tight tolerances should have been used
spring adjustments were straightforward to implement, but right away.
the optical encoder requirements were not explicitly checked
in the initial design. This lack of detail in the design goal B. Testing Lessions
contributed to the problems found in the first design, leading Lesson 4: Test and investigate all aspects of a mechanisms
the the first redesign. design, as able, before disassembly and redesign.
Lesson 2: The simplest solution is not always the best, When working with the design, assembly and testing of
choose the redesign solution that solves the problem most prototype, it is important not to get caught up with a single
completely. problem such that it distracts from overall testing. This is
When redesigning a part or assembly, the simplest and seen with regards to the machining mistake on the first
most minimal design is often the most attractive but when linkage, where the bearing bore was to short. Instead of
considering the complexity of the possible solutions, go with immediately sending the part back for correction and having
the one most likely to solve the problem, even if it is more to wait before testing the overall mechanism, the engineer
assembled the rest of the parts to examine the part interfaces,
the hook hand-off. This additional testing revealed problems
that were much more critical than the bore mistake. By
testing and examining the assembly as much as possible
before trying to fix the small mistake, time was saved and
the redesign could include the altered dimensions.
Lesson 5: Implement low risk redesigns early.
Lastly, when working with and testing an assembly of
parts that requires a redesign, take time to step back and
examine the assembly as a whole, looking for small problems
that can be improved with a low risk of affecting the overall
working of the mechanism. Including these improvements in
a first redesign can save time in later testing. An example
of this is seen in the short hook redesign. Though the
wobble of the joint axle, when supported by one ball bearing,
contributed to an unreliable hook hand-off the engineer was
able to identify a second problem area around the short hook.
The hook trough was too close to the flange, in order for a
successful spring leg hand-off the long hook was required
to pass extremely close to the short hook flange. The hook
Fig. 9. Final torsional joint setup (without encoder) was redesigned to allow for more space between the moving
FinE-R 2015 Page 40 IROS 2015, Hamburg - Germany
The path to success: Failures in Real Robots October 2, 2015
parts. This contributed to a smoother working hook hand-off
of the spring, outside of the troubles with the bearings.
V. CONCLUSIONS
When designing, building and testing a prototype mecha-
nism for robotics research there are often difficulties. The
short time schedule forces an engineer to make certain
assumptions and estimations, which can lead to trouble in
the assembly and testing phase. This paper presented the
experience of one researcher in designing, building and
testing a torsional spring joint prototype. The process faced
a few problems but through two redesigns the failures were
solved, producing a successful mechanism that met all the
required design goals. The lessons learned from this process
were discussed in detail and connected to specific examples
in the design and testing of the torsional spring joint.
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