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ROBOT HAND REDESIGN -
Fingertip force sensing implementation and assembling
simplification
Student research project
of
Siim Viilup
25.01.2010 – 28.05.2010
Supervisor: PhD. Johan Tegin, KTH Machine Design, Mechatronics Lab.
II
Abstract
The following report is written regarding a project work in KTH Machine Design department. The
general goal of the project was to develop a cost-effective robotic three finger hand. The main goal
of current project was to implement fingertip sensing for precision grasps, simplify assembling and
make the functioning more reliable.
Present report proposes different solutions for the fingertip shape and configuration, where the best
solution was chosen to be implemented in the hand. In order to simplify and improve the reliability
of the connection from the tactile sensor to the palm, a new solution was proposed. Additionally,
several other solutions were proposed and implemented. To simplify assembly, a new thumb
assembling method and a thumb turning servo nest was introduced. To improve reliability, the cables
in the palm were separated from the moving parts, which they could interfere with. Further, the
hand was made a little lighter, thinner and at the same time stronger by introducing honey comb
structure
It is important to notice that the current report is closely interrelated with [2] and before reading this
report the reader should be acquainted with it. The reason is that the general mechanical platform
was developed among this thesis work.
III
Acknowledgement
By virtue of financial support from ‘Doctoral Studies and Internationalization Programme DoRa’ it
was possible for the author to visit and stay in KTH, Stockholm and to participate in the current
project.
For the moral support, the author thanks Johan Tegin. His advice, mentoring and helping in brain
storming generated much better project result.
Additional gratitude goes to Kimblad Technology AB for manufacturing the springs and to Accurate
Nordic for supplying the Nicomatic crimp contact samples.
IV
Notation
The terms regarding the finger are taken directly from the previous report of project [2].
Distal direction The direction towards the finger tip
AWG American wire gauge
Base phalanx The one included into the hand
DC Direct Current
Distal phalanx The most distal one
DOA The term degree of actuation will be called in short.
DOF The term degree of freedom will be called in short.
FFC Flexible flat cable
Fingertip The tip of the distal phalanx
FSR Force sensitive resistors
Inner lid The cover lid inside of the hand
Middle phalanx The one between the distal an the proximal phalanx
Outer lid The cover lid on the top of the hand
Palm The housing for actuators and encoders.
PCB Printed circuit board
proximal direction The direction towards the palm
Proximal phalanx The one next to the base phalanx in distal direction
Screw Plates The plates which are screwed on the top of the links to fix the spring
are called
SLS Selective laser sintering. A rapid prototyping process
STL File format to stereolithography. Used for transferring the models to
fast prototyping.
STL Japan Supplier of the DC motors
Thumb base The part of the thumb which is fixed in the hand backside will be
called.
V
Contents
1. Previous work.................................................................................................................................. 1
1.1. The Finger................................................................................................................................ 1
1.2. The Palm.................................................................................................................................. 2
1.3. Actuators and Sensors............................................................................................................. 3
2. Specifications................................................................................................................................... 5
2.1. Task Specifications................................................................................................................... 5
2.2. Design Specifications............................................................................................................... 6
3. Design Changes ............................................................................................................................... 7
3.1. Making Changes in the Model................................................................................................. 7
3.2. Fingertip Redesign................................................................................................................... 8
First Prototype............................................................................................................................... 10
Second Prototype.......................................................................................................................... 11
3.3. Force Sensitive Resistor Sensor Connecting Improvement................................................... 13
Choosing the solution.................................................................................................................... 14
Connecting of the FSR Sensor ....................................................................................................... 15
The Printed Circuit Boards............................................................................................................. 17
FSR Circuit Diagram ....................................................................................................................... 14
3.4. Other Changes....................................................................................................................... 20
Cabling ........................................................................................................................................... 20
Thumb Assemble........................................................................................................................... 21
All the Simplifications Made.......................................................................................................... 22
All the Main Changes Made .......................................................................................................... 23
4. Manufacture and Assembly .......................................................................................................... 25
4.1. Preparation of STL Files ......................................................................................................... 25
VI
4.2. Manufacture of springs ......................................................................................................... 25
4.3. Finger Assembly Instructions ................................................................................................ 26
4.4. Evaluation.............................................................................................................................. 28
5. Conclusions.................................................................................................................................... 30
6. Works Cited ................................................................................................................................... 31
APPENDIX 1. Model descriptions .......................................................................................................... 33
APPENDIX 2.Fingertip sensor solution with separate fingertip............................................................. 39
APPENDIX 3. Test Protocol for Fingertip Prototype 1 Force Sensing Capabilities ................................ 40
APPENDIX 4. Fingertip Prototype 2 Force Sensing Capabilities ............................................................ 43
APPENDIX 5. Fingertip Prototype 2.1 Force Sensing Capabilities ......................................................... 47
1
1. Previous work
The full report of the preceding finger design can be found in [1] and for palm design in [2]. In
the current report very brief overview of previous design has been presented. See figure 1. The
general hand behavior was very good.
Figure 1. The preceding design [2].
1.1. The Finger
The finger used in the palm was designed from lightweight material and contained three parts,
the Proximal Phalanx, the Middle Phalanx and the Distal Phalanx.. To tie the phalanxes and to
unbend the finger, a steel spring was used. In addition, three screw plates and screws were used
to fasten the spring and keep the finger together. All the parts are also presented on Figure 2. To
be able to sense the forces FSR sensors were used, which were mounted on the pads and
covered with rubber foam to spread the forces evenly to the sensor area. [2]
2
Figure 2. The parts of the finger in preceding design [2].
Connections from the palm to the phalanxes were realized with AWG 26 cables, so that every
sensor used two cables. Hence, six cables lay in the phalanx ducts. To connect the FSRs to cables
epoxi glue was used.
1.2. The Palm
The palm comprised three motors for bending the fingers, optical encoders and magnetic
encoders, a servo for turning the thumb and cables.
The palm consisted basically of six different parts: the fingers, the back of the hand, thumb
joint, mounting adapter, lid for covering the hand and cable glands. Presentation of hand is
shown on Figure 3.
3
Figure 3. Preceding design in three views [2].
All the positions for the thumb, fingers and motors were very well considered.
The palm was manufactured the same way as fingers i.e. fast prototyping and the material used
was DURAFORM© PA plastic.
1.3. Actuators and Sensors
Although the hand has 10 DOF it has only 4 DOA. In order to bend the fingers three regular
micro DC motors and to turn the thumb one servo motor are used. The DC motors used are
produced by STL Japan (Figure 4). The servo used was Bluebird Servo (Figure 5).
4
Figure 4. The motor from STL Japan for bending fingers [2].
Figure 5. The Bluebird Servo for rotating the
thumb [2].
Besides the tactile sensors, additional sensors are used to sense the position of the motor. The
position and speed sensors comprise of magnetic encoder (Figure 6) and optical encoders
(Figure 7). It is important to notice that magnetic sensor is used to measure pulley position and
optical sensor to measure motor shaft position i.e. unreduced rotation. However the optic
sensors could not be used for two motors, because the cables inside the palm tend to touch the
motor encoder disks and therefore stopping the motors [2].
Figure 6. AS5050 magnetic sensor to sense the absolute position of finger bending motors. Measures pulley
position [2].
Figure 7. EE-SX1108 Photomicrosensor to sense position change of finger bending motor. Measures motor shaft
position [3].
5
2. Specifications
Whereas the intension of the project was to improve the design of the hand, only task
specification and design specification has been presented. The performance specification
and control specification can be found in [1].
In the preceding reports the weight of the hand has been considered as performance
specification. Whereas the author of this paper is in the opinion that weight is a design
specification, it is included in the design specification list.
2.1. Task Specifications
The task specifications have been the same for a long time and there were no necessity to
change them. The specification has been taken from preceding project [2].
• The hand shall be able to grasp and to lift a filled bottle with the total mass of 2
kg,
• Regardless of the height the bottle is grasped, it should be possible to rotate the
bottle at 180˚ C. That means the hand has to perform grasp 1 in the Cutkosky
grasping hierarchy [6].
• The same should be possible with a card box with a total weight of 1 kg (also
grasp 1).
• The hand should be able to grasp and to pick up a chocolate bar with one and two
forefingers and the thumb (grasp 8 and 9 in the grasping hierarchy).
• The hand should also be able to hold a credit card between one forefinger and the
thumb (grasp 9 in the grasping hierarchy).
• The hand should be able to press a button.
6
2.2. Design Specifications
The design specifications are mainly taken from [2], but some are rephrased:
• The finger has to be as light as possible to be able to disregard the inertia in the
control models;
• The dimensions and the shape of the palm (hand) shall be close to a human hand;
• The mass of the hand shall be less than 300 g;
• The palm has to house all actuators, tendons and cables;
• The whole palm should be easy to assemble and consist of as few components as
possible;
• The palm has to have modular interface to the ABB robot;
• The hand should cost less than 200 € to manufacture.
Due to the problems which were mentioned in the preceding report and other problems
which troubled the supervisor some specifications are updated or added.
• The hand has to be able to perform precision grasp i.e. finger tip has to sense
forces;
• The cabling in the finger should not hinder the movement of the finger;
• The cables in the hand should be routed neatly and so that they do not interfere with
any moving parts.
• The thumb servo should have more flexibility i.e. it should be possible to change
the servo with other similar models.
7
3. Design Changes
In this chapter are described the changes made in design. The main principle was to make and
keep things as simple as possible mainly regarding assembling. In special interest was
redesigning the fingertip to make possible force controlled precision grasp and improving the
connection of FSR sensors and their cabling to palm. Illustratively can see differences between
the changes on figure 8.
Figure 8. Comparison between old and new design
3.1. Making Changes in the Model
The model is built to be assembly driven, which means that the main features of parts are defined in the sketches in the assembly. It is mostly enough to modify sketches to change the characteristics. However the changes may result model inconsistencies. Therefore it is
8
important after the changes to look the part which the changes applied to. The failed features have red exclamation mark or gray arrow. NB! If the lines in the sketch are deleted then it also breaks the between the dependent features. Notice that it does not apply to the relationship handles. If it is necessary to delete a line, make sure to update the links in the features.
The model description is given in Appendix 1. More in detail is given the main assembly hand.asm and its subassembly fingerforallembl.asm, which is the finger.
3.2. Fingertip Redesign
The hand with the previous design was not able to sense the forces on the fingertip. Therefore it
was also impossible to perform force controlled precision grasp. In order to be able to perform it
a solution for the fingertip sensing had to be established.
While designing the fingertip the main principle was to keep it as simple as possible. While
starting the design very different solutions were proposed. One of them is shown in Figure 9.
Figure 9. Proposed distal phalanx with round fingertip.
The solution would have given smaller fingertip, which can be used for very precision grasp.
However, it would have caused at same time a dead zone between the pads. The dead zone in
arises because we cannot place the pad sensor where the finger starts narrowing. It is preferred
that there are no dead zones on the pad.
The fingertip whether had to have the same characteristics for calculating the force as the pad
or have separate sensor for fingertip which can have its own characteristics for calculating the
force. To test the characteristics, contact points were defined where to measure the sensor
readings. The first contact was perpendicular with phalanx pad, the second and fourth precision
grasp and the third parallel to spring. For graphical presentation see Figure 10.
In the preceding design the height of the distal phalanx, especially the fingertip, was very small.
Therefore it would have been very difficult to implement any kind of sensing solution there.
9
Additionally, the PCB-s, which is described more in depth in cabling improvement chapter, could
not been possible to be fitted. Therefore the fingertip had to be anyway made higher.
Figure 10 Directions of contacts for testing capability
Different ideas for fingertip sensing
The very first proposed solution consisted from two parts – the distal phalanx and separately
attached fingertip see Figure 11. For more information see Appendix 2.
Figure 11.Proposed distal phalanx idea 1.
Firstly, the proposed solution does not correspond to the philosophy as simple as possible i.e.
the fingertip consist of two parts. It could have been possible to design the distal phalanx as a
whole. Secondly, the solution works best if the force vector is parallel with the proximal
direction. However, when performing the precision grasp the contact comes in an angle. See
Figure 10 direction 4.
Taking into account the shortages mentioned in the last paragraph a new solution had to be
proposed. The second solution firstly simplifies the design by eliminating additional part.
Secondly, the normal vector of the sensor plate plane is parallel to direction 4 (see Figure 10) i.e.
the forces coming from direction 4 can be detected better. The prototype can be seen in Figure
18.
10
Figure 12. Proposed distal phalanx idea 2.
Firstly, when the force is applied to the fingertip it imposes opposite force, because the fingertip
is as spring. If we wanted to make the opposite force as small as possible then also it would have
been more vulnerable to breaking when the force came from other directions. The worst would
have been if the force came from the top of the finger direction. To solve the problem it would
have needed complicated solution.
Secondly, the slot needed to fit besides FSR also the rubber which distributes the force. It means
it would have taken a lot of room in the fingertip, which would have made it very weak.
Therefore, again another solution had to be proposed. The next proposed solutions were also
tested with prototypes and therefore described in following chapters.
First Prototype
Prototype was built as simple as possible. The FSR suppliers have said that sensor works best on
planar surfaces, but we wanted to test how the sensor works on curved surfaces the prototype
was built. The fingertip shape is given in Figure 13.
Figure 13. Proposed distal phalanx prototype 1
The full testing protocol can be seen in the appendix 3. However, the relation between force and
reading is also presented in Figure 14.
11
Figure 14. Relation between force and resistance for Prototype 1. Where 1, 2 and 3 are the contact points (See
Figure 10)
Whereas the finger tip comprises only one sensor it is impossible to tell from the reading
whether the force is coming from contact 1, 2 or 3. Therefore deciding the force upon the
reading regardless of the contact point is very inaccurate i.e. the characteristics of the contact
points are different, especially at lower forces. Although the reading is better than nothing,
which was the situation at the moment, another solution had to be found.
Second Prototype
In the second prototype the round surface has been replaced with planar surface. The angle of
the surface is chosen so that with the precision grasp the contact comes from direction 4 (Figure
16). To get the normal finger shape a rubber is to be used. See Figure 15.
Figure 15. Covering of the fingertip with additional rubber foam. The red is FSR and black is the rubber foam.
The following sections describe the solutions proposed. The solution with two sensors on distal
phalanx was chosen in the final design.
12
Prototype 2.0 - One sensor on distal phalanx
In that solution only one FSR is used. See figure 16.
Figure 16 Proposed distal phalanx prototype 2.0
The full testing protocol can be seen in the appendix 3. However, the relation between force and
reading is also presented in Figure 17. Where 1, 2 and 3 are the contact points (See Figure 10).
Figure 17. Relation between force and resistance for Prototype 2.0. Where 1, 2 and 3 are the contact points (See
Figure 10)
The results were better than for prototype 1. Because we are still using only on FSR it is still not
possible to determine accurate value of force the same time for direction 1 and 3 (See Figure
10). To be able to use different characteristics for calculating the force from the reading,
separate FSR for fingertip and pad had to be implemented.
Prototype 2.1 - Two sensors on distal phalanx
Besides that an additional FSR sensor has added, the fingertip surface has been made larger. The
reason is to make the active area of the sensor larger. See figure 18.
13
Figure 18. Proposed and built distal phalanx prototype 2.1
The full testing protocol can be seen in the appendix 3. However, the relation between force and
reading is also presented in Figure 179. Where 1, 2 and 3 are the contact points (See Figure 10).
Figure 19. Relation between force and resistance for Prototype 2.1. Where 1, 2 and 3 are the contact points (See
Figure 10)
Whereas now there were used two separate sensors it was possible to use different
characteristic formulas to get the force value from the reading. Readings were especially good
for the contact point 2 (and 4) which was the most important.
As it were mentioned the solution had good characteristics and were chosen to be implemented
on final design.
3.3. Force Sensitive Resistor Sensor Connecting Improvement
In the preceding design silver epoxi glue was used to connect FSR to the cables. The most
important was to get rid of the need for using epoxy, which is poisonous and very inconvenient
14
to use. Additionally the cables in the finger hindered a little the movement of the finger and
therefore also another solution for cabling had to be proposed.
FSR Circuit Diagram
In the preceding design two cables for every sensor was used. To save the number of cables
(channels) running in the finger a common cable has been taken into use. On the Figure 20 and 21
can be seen the notation of cables. The COM is common cable, 1 is proximal phalanx, 2 is middle
phalanx, 3 is distal phalanx and 4 is fingertip sensor connection.
Figure 20. Notation of connections on connector
Figure 21. Notation of connections on phalanxes
The circuit diagram is given on Figure 34.
Figure 22. Notation of connections on connector
It is important to notice that common is approximately 0,5V.
Choice of the solution
To reduce the effect of the cabling to finger movement a flexible flat cable (FFC) was chosen.
That kind of cables are used in a lot of miniature electronics applications e.g. notebooks, mobile
phones, cameras etc, but also in robotics applications. The cable is very flexible and is also more
durable than regular cable in moving. Additional advantage is its small dimension, which makes
the design more compact. An example of FFC is presented on Figure 23.
COM 1 2 3 4 2 1
3
4
15
Figure 23. Flat flexible cable, used to connect tactile sensors between phalanxes
The task was to get the connection from every FSR to different channel of the cable. Whereas
the pitch is very small it is very difficult to get the connection otherwise than using FFC
connector. Additionally, we also need to connect the FSR. Therefore a solution of connecting
them with PCB was proposed. In preceding design two cables for every FSR was used i.e. 6
cables were used. Because there is no need for so many cables and also additional sensor was
added for fingertip, a common channel was implemented. Hence, with four sensors five-channel
cable is to be used. The principle is given in Figure 24.
Figure 24. Principle of connecting FSR (tactile sensors).
Connection of the FSR Sensor
In the finger force sensitive resistor are used. The supplier for them in our case is Interlink
Electronics. Interlink offers different kind of standard FSR sensors (Figure 25), from where the
strip sensors are (Figure 26) best suitable by the shape for the pads.
PCB
Connector
Connector
16
Figure 25. The selection of force sensitive resistor sensors from Interlink Electronics [3].
Figure 26. Force sensitive resistor sensor [2].
Interlink use contacts with pitch 2.54 mm for their sensors. It was not possible to find any self-
fixing 2.54 mm pitched connector, which would fit in the phalanx. Therefore another kind of
fixing the FSR connector had to be found. The proposed principle is given on Figure 24. As it can
be seen on the figure, the cable is fixed with pressing the cable between PCB and phalanx
surface. The PCB is pressed towards the cable by fixing the finger spring.
Because the sensor also has to pass the tendon to get into the phalanx and we would like to
change the sensors without removing the tendon, the sensor have to have a gap between the
ways. It was also used in the preceding design. See Figure 27.
Figure 27. The gap between the ways of sensor to
bypass tendons [2].
Figure 28. Nicomatic Crimpflex© contact, used to
connect FSR
17
As it was mentioned, in the preceding design epoxy glue was used. To connect the cut sensor a
contacts had to be attached to the ways. The technology, which can be used for it, is called
crimping. The suitable solution for our need is offered by Nicomatic. Because they are supplying
only in grand amounts, a patch of samples were ordered from their distributor Accurate Nordic
AB. See the contact type on Figure 28.
The first solution proposed to use regular socket headers to connect the FSR to PCB (Figure 29).
Figure 29. Tactile sensor connecting PCB bottom layer with connectors for FSR-s.
However, ordered the first prototype, it became obvious that it was quite difficult to insert the
contacts inside the socket while assembling. Whereas, it was possible to implement more
convenient idea (Figure 30) by just turning the PCB 180° was there no need to order new PCB-s.
In the eagle files correct directions have marked. So in the built prototype the arrows point
toward palm. However, if new PCB-s are ordered the arrows will point toward fingertip as
designed.
Figure 30. Final solution of connecting FSR to PCB.
The Printed Circuit Boards
In order to fit the PCB, the phalanxes had to be redesigned. The development of PCB and
phalanx redesign was simultaneous.
18
We wanted to have the same type of PCB in all phalanxes i.e. order only the same PCB-s.
Therefore there are pads for every channel and FSR connector on the PCB-s. To select the
correct channel in the PCB, correct pads have to be soldered together. The dimensions chosen
for the PCB was 11 mm * 17 mm. The first version which is very straight-forward and simple is
given in Figure 31.
Figure 31. First version of the PCB in the phalanxes. Left- top layer. Right – bottom layer.
However, the supplier for the PCB-s, Olimex, did not find it possible to produce it. Although the
routes width and gaps satisfied the limitations (min 8 mil), the necessary distance 40 mil from
outer border was not satisfied i.e. approx 1mm. Whereas it was not possible to make the PCB
any bigger, more compact circuit design had to be found. The final version is given in Figure 32.
Figure 32. Final version of the PCB in the phalanxes. Left- top layer. Right – bottom layer.
19
As it can be seen on the circuit layout on Figure 32 there are two pads on bottom layer in top of
the picture. The two pads are used to connect the FSR. One of the pads is directly connected to
common channel, marked as GND. The second pad is drawn in the middle of the other channel
pads. On the top layer are also two pads. One of them is again connected directly to common
channel and the other directly to the channel 5. The top layer pads are meant for fingertip
sensor. Whereby, no FFC1 connector is to be soldered on the distal phalanx PCB.
Because it is not possible to take the FFC out of the hand, it was important to convert the FFC to
regular cables in the hand. Therefore another PCB had to be designed, which will be fitted in the
palm. The circuit layout is presented in Figure 33.
Figure 33. The PCB used in the palm to convert flat cables to regular wires.
It is possible to see the PCB in the phalanxes on the Figure 34.
Figure 34. PCB-s in the phalanxes
20
3.4. Other Changes
Cabling
As mentioned before, the cables in the palm got in contact with the encoder disk of motors and
hence stopping them. To solve the problem the cables are taken in to the other side of the hand.
The principle can be seen on Figure 35.
Figure 35. The principle of cabling
In the final implementation some equipment are located on the top of the palm i.e. magnetic
encoder PCB and optical encoders. The reason is their depending position from motors.
Additionally, it is also better to assemble the sensors from the top of the hand. The cables
running in the channels can be seen on Figure 36.
Figure 36. Cable channels on the top of the hand.
21
In addition to the cabling in the palm, also output of the cables has been redesigned. When in
the preceding design cable glands, which are robust and large, are used then in the new design
much more compact solution was implemented. The reasons are that the specification does not
see millions of cycles of work and also serial producing of hand. In contrary, the cable glands are
firstly meant to preserve the cable for very long time and secondly their ordering and
installation is very simple and fast. The last solution is very good for industrial uses. The new
implemented solution is shown on Figure 37.
Figure 37. Cables output
Thumb Assembly
In order to simplify the assembly of the hand the method how the thumb was connected to
palm was changed. When before the bearings had to be inserted in rectangular prisms, attached
to the thumb and fixed with screws to the palm then now the bearings, which are on thumb, fit
into nests in the palm and then fixed with the lid. The principle can be seen on Figure 38 and the
actual solution on Figure 39.
22
Figure 38. Fixing the thumb.
Figure 39. Implementation of thumb bearing fixing
All the Simplifications Made
The simplifications are as follows:
• Connecting of FSRs
o Old design- Had to be glued with epoxy.
o New design- the FSR have to crimped, put into the phalanx and cover with the PCB
• Insertion of thumb to palm
o Old design – Added two additional parts and 4 screws.
o New design – There are nests on both, palm and the lid. The palm have to be fitted
into the nest and lid closed.
Inner lid
Palm
Thumb bearing
23
• Cabling
o Old design – Cables were relatively loose in the palm and could interfere with moving
parts, especially finger motor axis. Additionally cable glands, which were very robust
and large, were used to take cable out of the palm,
o New design – Cable are routed neatly and separated from the moving parts in the
hand. The cable glands have been removed, so the design is more compact. The
cables are fixed with cable ties. Moreover, the cover lid and most of the equipment
can be removed from the hand without cutting cables/ disconnecting connectors.
• Servo
o Old design - The servo were fitted into exact nest. Whereas the thumb is fixed via the
bearing the servo nest causes over constraint, which produces tensions in the
system.
o New design – The servo is fixed with the surfaces of palm and cover lid.
• Mounting adapter
o Old design – used four screws to fix the mounting plate to the palm
o New design – The palm can be slide in the palm and fixed with one screw. It uses
dovetail joint connection.
• The number of screws
o Old design – The number of screws in palm 15
o New design - The number of screws in palm was reduced to 10, thereby improving
the fixing of the motors.
All the Main Changes Made
1. Finger
a. All phalanxes
i. Fitted PCB for connecting FSRs. Hence the change in fixing of spring.
ii. The radius of flanges between phalanxes increased
b. Distal Phalanx –
i. New tendon connecting method.
ii. Fingertip redesign
c. Cover plates
i. Hence of the PCB connection the change in the fixing of cover plates had
to be done.
2. Thumb
24
a. Thumb base made narrower and shape change a little. The position and thumb
kinematics is the same.
b. Fitted a PCB in the thumb to connect FFC to cables. Hence the change in fixing of
spring.
c. Fixing of tendon bearing simplified.
d. Collars added for the thumb base to avoid that the outer rim of bearing (thumb to
palm) collides with the thumb base.
3. Palm
a. The whole space made in solid with honeycomb structure
b. Two covers for the palm to get to both sides of the palm
c. Cables put on the top side of the palm
d. Changed the motor positions and made the palm narrower
e. Simplified the assembly of the thumb
f. Changed the servo fixing.
g. Implemented two encoders for one motor.
4. Outer lid
a. Taking the cable out of hand.
25
4. Manufacture and Assembly
4.1. Preparation of STL Files
Because preparing STL files have not been described in the reports before, it is done briefly here.
Firstly, the models are manufactured from layer to layer z-axis direction. Because the laser
accuracy is better on x-y plane than the layer thicknesses on z-axis, it is important to place the
model so that the important curves lay on x-y plane. It is possible to turn the model by placing
the part to assembly and save it as STL file. In the hand design only phalanxes positions were
changed from the default coordinates. They were revolved 90 degrees around y-axis.
Secondly, while saving the file to STL format, it is important to use smaller conversion tolerance
than the default value. In the manufactured prototypes value 0,001 mm was used.
If there is need to see the converted STL files, their accuracy and position, a freeware program
MiniMagics can be used.
4.2. Manufacture of springs
The spring has more complicated design than the preceding spring. This accrues from the design
changes induced from implementing PCB-s to phalanxes.
Although, it is possible to manufacture the spring manually i.e. using snips, it is preferred to use
industrial manufacturing method. Because the thickness of the sheet and corners in cutouts are
small, it would be best to use laser cutting. However, Kimblad Technology AB was able to help us
with progressive cutting method called water jet cutting, so we used it over laser technology.
Even though the main advantage of the technology over the other is the possibility to cut much
thicker materials, it is also possible to cut thinner materials. To get the springs, six layers of the
spring steel were cut at once.
26
Figure 40. The changes made in spring fixing due to technology. On the left original shape, on the right modified
shape.
Because the diameter of the water jet is 1,1 mm a little changes in the cut out had to be made.
The changes can be seen on Figure 40.
4.3. Finger Assembly Instructions
In this chapter the key points in assembling the finger is described to simplify the process when
assembling a new hand.
The finger design has changed quite a lot and also it comprises some more parts than before. All
the parts are shown on the Figure 41.
27
Figure 41. All the parts of the fingers
The lengths of the FFC cables from the base to fingertip are 20, 17 and 15 mm. The FSR sensor
dimensions can be found in the Table 1.
Table 1. The lengths of the FSR sensors
The sensor Overall length (mm) Pad length (mm)
Proximal 36 16
Middle 31 16
Distal 31 19
Fingertip 21 7
As mentioned in Chapter 3.3 Force Sensitive Resistor Sensor Connecting Improvement,
Connection of the FSR Sensor there is a need to put rubber foam under the PCB. How it should
be done can be seen on Figure 42.
28
Figure 42. The rubber foam in the phalanxes.
4.4. Evaluation
The manufactured prototype had only minor faults and even those could be fixed with
modifying the ordered prototype. All the faults have been fixed in models.
The first and prime deficiency was that one cable channel was to narrow and had to be made
wider. At the moment when no optical encoders are installed, there is no problem.
Secondly, the thumb and third finger bending motors were placed 1,5 mm wrongly toward top
of the hand. This caused the tendon to come from thumb and third finger guides to motor
pulleys not totally collinearly.
Thirdly, a lip was added to thumb cable channel to hold the moving cables better in the channel
Fourthly, third finger bearing base was reinforced a little to withstand the forces while bending.
Fifthly, guides for the screws were added to simplify fixing the inner lid and outer lid.
Sixthly, the third finger sensors cables channels was modified to reduce the tension in cables.
29
The others were tolerance faults, like motor nest, wire lid, bearing base and motor fixing prism.
The only test carried out for the sensors was to precision grasp a small cylindrical object (pencil)
and see the reading of the fingertip sensors. The precision grasp and readings from fingertip
sensor can be seen on figure 43 and figure 44.
Figure 43. The precision grasp of pencil.
Figure 44. The reading of sensors in precision grasp.
30
5. Conclusions
1. Fingertip has been redesigned and implemented on the final design. The hand is able to
perform force controlled precision grasps. However, the thorough testing of the sensor
capabilities still has to be conducted.
2. The assembling has been simplified. It comprises finger, thumb and mounting adapter
assembling simplification. Additionally, the number of used screws has been reduced from
28 to 20.
3. The principle of cabling inside the hand and palm has been changed. The phalanxes are
connected with flexible flat cables, which have a lot less impact in hindering the movement
of the finger. Additionally, the cables in the palm have been separated from the components
by keeping them in channels on top of the palm. Hence, the cables do not interfere with
moving parts anymore.
4. It is possible to use different servos with the new design. The main limitation of choosing the
motor is the thickness of it. However, the mentioned dimension is standard for miniature
servo motors.
5. In the future the possibility of making the hand even lighter should be considered. The hand
can be made up to 40 g lighter by making the honey comb structure sparser.
6. Completely new CAD model was developed, because the preceding model was beyond
repair.
31
6. Works Cited
1. Reiche, Jakob. Mechanical Development of a Robot Hand. 2007.
2. Schmidt, Klaus. PALM IMPROVEMENT AND A ROBOTIC HAND-ARM INTERFACE. 2008.
3. Interlink Electronics. [Online] [Cited: 05 20, 2010.] http://www.interlinkelectronics.com/.
32
APPENDICES
33
APPENDIX 1. Model descriptions
Describtion of model New palm.asm
34
Most of the sketches are interlinked. So changing one sketch may result in changes in other sketches also.
Hand on top – the outer boundaries of the palm looked on the top
Used in: hand.par, inner lid.par, outer lid.par outer lid v2.wire
Hand on side - the outer boundaries of the palm looked on the side. Also the different cutting depths of the other sketches are defined there.
Used in: hand.par
Thumb cutout – the boundary of the thumb cutout from the palm
Used in: hand.par
Finger location – the locations of the fingers. Needed for the other sketches.
Used in: Only in other sketches.
35
Tendon channels – the tendon channels from the fingers to the motors
Used in: hand.par
Thumb axis top – the location of thumb turning axis viewed from top
Used in hand.par, inner lid.par
Thumb axis side - the location of thumb turning axis viewed from right
Used in: hand.par, inner lid.par
Motors – the locations of motors and the boundaries of cutout from palm
Used in: hand.par, inner lid.par (the bearing cutout)
Thumb bearing – the location of bearing and the support of the bearing.
Used in: hand.par, inner lid.par
Servo cutout – to cut the space for servo
Used in: hand.par, inner lid.par
Inner lid side – the inner lid from side
Used in: inner lid.par
Outer lid side – the outer lid from side.
Used in: outer lid.par
Motor PCB – the cutout for the magnet encoders
Used in: hand.par
Third finger tendon guide- the guides for the tendon near to the bearing
Used in: hand.par
Cable channels – the cable channels and the cable vias
36
Used in: hand.par
Fixing – the locations of mounting bosses to fix outer lid, hand and inner lid.
Used in: outer lid.par, hand.par. inner lid.par
Palm sensor – the location of the FSR sensor in the palm
Used in: hand.par
Cable lips – the location of the lips to hold the cables in the channels
Used in: hand.par
Encoder adapters – motors optical encoders size and location
Used in: hand.par, encoder adapter.par
Outer lid right 1, Outer lid right 2 and Outer lid right 3 – the profiles of outer lid viewed from right. Consist of 3 layers.
Used in: outer lid.par, outer lid wire v2.par
Mounting slots – the dovetail profile
Used in: hand.par, mounting adapter.par
37
Model description of Fingerforallembl.asm
Phalanxes – the main sketch for the phalanxes. The main shape of the finger is defined in the sketch
Used in: other sketches, new_spring.asm
Prox, Mid and Dist – more detailed sketch of the phalanxes
Used in: respectively new_prox.par, new_mid.par and new_dist.par
Base – the sketch for the base where proximal is fitted
Used in: new_palm.asm/thumb.asm/thumbbase.par, new_palm.asm/hand.par
Tendon – the sketch for tendon channels in the phalanxes
38
Used in: new_prox.par, new_mid.par and new_dist.par, but also new_palm.asm/thumb.asm/thumbbase.par, new_palm.asm/hand.par
PCB position – the position of PCB in the phalanxes. The line is for the lower plane of PCB.
Used in: other sketches
Phalanxes_top – the walls and cutouts view from the top.
Used in: other sketches
Prox_top, mid_top, dist_top,– The walls and cutout view from the top. The lines are included from the phalanxes_ top. The sketches also comprise the triangle, which are used to fix the spring. They are located in separate sketch to give the flexibility if phalanx walls and cutouts need to be adjusted independently.
Used in: respectively new_prox.par, new_mid.par and new_dist.par. Also used in new_spring.asm
SensorCable_top – the cutout for the cable, which connects the PCB-s.
Used in: new_prox.par, new_mid.par and new_dist.par
Base triangle – the triangle which are used to fix the spring
Used in: new_palm.asm/thumb.asm/thumbbase.par, new_palm.asm/hand.par
ScrewPlates – the sketches for the plates which fix the spring and limits the finger movement.
Used in: new_ScrewPlateProx.asm, new_ScrewPlateMid.asm, new_ScrewPlateDist.asm
39
APPENDIX 2.Fingertip sensor solution with separate fingertip
40
APPENDIX 3. Test Protocol for Fingertip Prototype 1 Force Sensing Capabilities
Test protocol
Date and time: 2010-04-19 18:00
Contuctor: Siim Viilup
Measuring Equipment: Multimeter Velleman DVM890, Dynamometer Salter Super Samson
The prototype:
Test setup:
The contact points:
The contact: shape plane and width 5 mm.
Measurement range: 2 ... 15 N
41
Test data:
Contact 1 [kOhm] Contact 2 [kOhm] Contact 3 [kOhm]
Forc
e
[N]
1 2 3 1 2 3 1 2 3
2 4,1 3,87 3,17 87 60,7 70 18 10,5 13,2
5 1,83 1,72 1,8 8,71 8,65 9,2 6,85 6,03 4,9
10 1,17 1,12 1,13 3,56 3,29 3,3 3,5 3,36 3,2
15 0,94 0,95 0,94 2,25 2,17 2,1 2,3 2,84 2,46
The estimates of the resistances:
Forc
e
[N]
Contact 1
[kOhm]
Contact 2
[kOhm]
Contact 3
[kOhm]
2 3,71 72,56 13,90
5 1,78 8,85 5,92
10 1,14 3,38 3,35
15 0,94 2,17 2,53
The Graphs:
42
Conclusion:
Whereas the finger tip comprises only one sensor it is impossible to tell from the reading
whether the force is coming from contact 1, 2 or 3. Therefore deciding the force upon the
reading regardless of the contact point is very inaccurate i.e. the characteristics of the contact
points are different, especially at lower forces. Although the reading is better than nothing,
which is the situation at the moment, another solution should be found.
43
APPENDIX 4. Fingertip Prototype 2 Force Sensing Capabilities
Test protocol
Date and time: 2010-04-22 16:00
Contuctor: Siim Viilup
Measuring Equipment: Multimeter Velleman DVM890, Dynamometer Salter Super Samson
The prototype:
Test setup:
The contact points:
The contact: shape plane and width 5 mm.
Measurement range: 2 ... 15 N
44
Test data:
Contact 2 [kOhm] Contact 3 [kOhm]
Forc
e
[N]
1 2 3 1 2 3
2 7,5
7,0
1
7,7
4
12,
83
9,2
1
8,6
7
5 4,1 3,9
4,0
2
6,2
5
5,4
4 4,6
10 2,9
2,9
4
2,9
6 3,9
4,0
4 4,5
15 2,0
4
2,3
5
2,3
4 3,3
2,8
8 2,9
The estimates of the resistances:
Forc
e
[N]
Contact 2
[kOhm]
Contact 3
[kOhm]
2 7,42 10,24
5 4,01 5,43
10 2,93 4,15
15 2,24 3,03
45
The Graphs:
Protype 2 compared to prototype 1. The contact 1 is measured in the last experiment and given in prototype 2 graph for illustrative proposes.
46
Conclusion:
The same as for the prototype 1:
Whereas the finger tip comprises only one sensor it is impossible to tell from the reading
whether the force is coming from contact 1, 2 or 3. Therefore deciding the force upon the
reading regardless of the contact point is very inaccurate i.e the characteristics of the contact
points are different, especially at lower forces.
However the protype 2 is better then protype 1.
47
APPENDIX 5. Fingertip Prototype 2.1 Force Sensing Capabilities
Test protocol
Changes from last prototype 2: The fingertip and pad sensors are separate. Fingertip sensor is
12,7 mm round FSR which is cut into half. Pad sensor is the same strip FSR.
Date and time: 2010-05-05 12:00
Conductor: Siim Viilup
Measuring Equipment: Multimeter Velleman DVM890, Multimeter Velleman DVM890,
AMPROBE 30XR-A, Dynamometer Salter Super Samson
The prototype:
Test setup:
The contact points:
The contact: shape plane and width 5 mm.
Measurement range: 2 ... 15 N
48
Test data:
Contact 2 [kOhm] Contact 3 [kOhm] Contact 4 [kOhm]
Force
[N] 1 2 3 1 2 3
1 2 3
2 4,
7 6,0 5,5
12,
7
10,
2
11,
5 5,7 5,9 6,6
5 2,
4 2,7 2,6 6,8 6,5 7,3 3,3 3,4 3,9
10 1,
6 1,7 1,6 3,5 3,0 3,8 2,0 2,3 2,4
15 1,
2 1,3 1,2 2,0 2,1 2,1 1,5 1,8 1,9
The estimates of the resistances:
Forc
e
[N]
Contact 2
[kOhm]
Contact 3
[kOhm]
Contact 4
[kOhm]
2 5,40 11,47 6,07
5 2,57 6,87 3,53
10 1,63 3,43 2,23
15 1,23 2,07 1,73
49
The Graphs:
Protype 2.1 compared to prototype 1. The contact 1 is measured in the Fingertip Prototype 1
experiment and given in prototype 2 and 2.1 graph for illustrative proposes.
Conclusion:
Firstly, whereas know there is used two separate sensors it is possible to use different
characteristic formulas to get the force value from the reading. Secondly, in the prototype 2.1
we are using 12,7 mm round sensor, which has quite large sensing area (compared to strip FSR),
and therefore we get better readings than in the prototype 2. Especially the contact point 2 (and
4).
The solution is good enough and should be implemented.
Recommended