robot hand redesign/menu/...robot hand redesign - fingertip force sensing implementation and...

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.

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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

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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.

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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].

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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.

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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

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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.

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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.

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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.

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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.

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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


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.

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Figure 18. Proposed and built distal phalanx prototype 2.1


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

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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

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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.

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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

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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.

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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

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• 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

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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.

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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.

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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.


Servo cutout – to cut the space for servo


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:



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:



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


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:



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:



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


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.

robot hand redesign/menu/...robot hand redesign - fingertip force sensing implementation and...

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