INVESTIGATION AND MODIFICATION

OF A COMMERCIALLY

AVAILABLE TACTILE SENSOR

SUITED FOR ROBOTIC APPLICATIONS

by

Carolina Creus

Rensselaer Polytechnic Institute

Electrical, Computer, and Systems Engineering Department

Troy, New York 12180-3590

August 1991

CIRSSE REPORT #100

CONTENTS

Page

LIST OF TABLES ......................................................................................................... v

LIST OF FIGURES ........................................................................................................ vii

ACKNOWLEDGEMENT ............................................................................................ x

ABSTRACT ................................................................................................................... xi

1. INTRODUCTION AND HISTORICAL REVIEW ..................................... 1

2. REVERSE ENGINEERING ............................................................................ 5

2.1 Electrical Description of the Tactile Array Pad .............................. 5

2.2 Hardware Description ......................................................................... 8

2.2.1 Description of the Analog Board Components .............. 11

2.2.2 Selecting a Site in the Tactile Array Pad .......................... 11

2.2.3 Voltage Map Circuit ............................................................. 12

2.2.4 Digital Gain for the Signal's Amplification .................... 14

3. CHARACTERISTICS OF THE TACTILE SENSOR ARRAY ................... 16

3.1 Experimental Setup ............................................................................. 17

3.2 Protecting the Tactile Pad ................................................................... 17

3.3 Force Around the Tactile Pad Borders ............................................ 18

3.4 Force Measurement Interpolation ................................................... 22

3.5 Hysteresis Analysis .............................................................................. 32

3.6 Sensitivity Analysis ............................................................................. 48

3.7 Effects of an Applied Force on its Neighbors ................................. 53

4. SENSOR INTERFACE WITH THE 68HCll EVALUATION

BOARD ............................................................................................................... 54

ii
5,

.

7.

Page

4.1 Hardware Configuration .................................................................... 54

4.2 68HCll EVB Port Assignment .......................................................... 58

4.2.1 Port B of the 68HCll EVB ................................................... 58

4.2.2 Port C of the 68HCll EVB ................................................... 58

4.2.2 Port E of the 68HCll EVB .................................................... 59

SOFTWARE DESCRIPTION AND SYSTEM OPERATION ................... 60

5.1 Initialization ......................................................................................... 60

5.1.1 Power Up Initialization .......................................................... 60

5.1.2 SCI Initialization ...................................................................... 60

5.1.3 A/D Converter Initialization ................................................ 60

5.2 Software Location on Memory ......................................................... 61

5.3 Communication Software between Host and 68HCll EVB ....... 61

5.3.1 Host Communication Software ......................................... 62

5.3.2 68HCll EVB Communication Software .......................... 63

5.4 Software Developed to Control the Tactile Sensor ...................... 63

5.4.1 Software Developed for the 68HCll ................................. 64

5.4.1.1 Sensor Commands ................................................ 64

5.4.1.1.1 tacScanAll Command ......................... 64

5.4.1.1.2 tacElementOne Command ................ 65

5.4.1.2 A/D Converter Operation .................................... 65

5.4.2 Software Developed for the Host Computer .................. 66

5.5 Data Acquisition Example .................................................................. 67

DISCUSSION .................................................................................................... 70

CONCLUSIONS ............................................................................................... 75

iii
Page

LITERATURE CITED .................................................................................................. 77

BIBLIOGRAPHY ........................................................................................................... 78

iv

LIST OF TABLES

Table 2.1

Table 3.1

Table 3.2

Table 3.3

Table 3.4

Table 3.5

Table 3.6

Table 3.7

Table 3.8

Table 3.9

Table 3.10

Table 3.11

Table 3.12

Table 3.13

Table 3.14

Table 3.15

Table 3.16

Table 3.17

Table 4.1

Table 4.2

Page

Connections From the Sensor Pad to the Analog Board ............ 9

Forces Recorded on Comer #1 of the Sensor Pad ......................... 18

Forces Recorded on Comer #2 of the Sensor Pad ......................... 19

Forces Recorded on Comer #3 of the Sensor Pad ......................... 20

Forces Recorded on Comer #4 of the Sensor Pad ......................... 21

Force Interpolation Measurements for Test Point #1 .................. 23

Force Interpolation Measurements for Test Point #2 .................. 25

Force Interpolation Measurements for Test Point #3 .................. 27

Force Interpolation Measurements for Test Point #4 .................. 29

Hysteresis Data Taken at Test Point #1 for Experiment #1 ......... 33

Hysteresis Data Taken at Test Point #2 for Experiment #1 ......... 35

Hysteresis Data Taken at Test Point #3 for Experiment #1 ......... 37

Hysteresis Data Taken at Test Point #1 for Experiment #2 ......... 39

Hysteresis Data Taken at Test Point #2 for Experiment #2 ......... 41

Hysteresis Data Taken at Test Point #3 for Experiment #2 ......... 43

Hysteresis Data Taken at Test Point #1 for Experiment #3 ......... 45

Hysteresis Data Taken at Test Point #2 for Experiment #3 ......... 46

Hysteresis Data Taken at Test Point #3 for Experiment #3 ......... 47

Connections between Analog Board and 68HCll EVB ............... 55

List of Diodes on the Sensor Pad and Configuration for

Port C so they can be Activated ......................................................... 56

V
Page

Table 4.3

Table 5.1

List of Transistors on the Sensor Pad and Configuration

for Port B so they can be Activated ................................................... 57

Analog Input to 8-Bit Result Translation Table ............................ 66

vi
Figure 2.1

Figure 2.2

Figure 2.3

Figure 2.4

Figure 2.5

Figure 2.6

Figure 3.1

Figure 3.2

Figure 3.3

Figure 3.4

Figure 3.5

Figure 3.6

Figure 3.7

Figure 3.8

Figure 3.9

LIST OF FIGURES

Page

Diagram of the Existing Sites in the Sensor Pad ........................... 6

Optoelectronic Layout inside the Tactile Sensor Pad ................... 7

Diode-Transistor Pair Configuration ............................................... 8

Analog Board Hardware ..................................................................... 10

Voltage Map Circuit ............................................................................ 13

Digital Gain Chip and its Connections ............................................ 14

Setup used to perform the experiments ......................................... 16

Graph Corresponding to the Linear Regression Analysis

Done on Test Point #1 ........................................................................ 24

Graph Corresponding to the Linear Regression Analysis

Done on Test Point #2 ........................................................................ 26

Graph Corresponding to the Linear Regression Analysis

Done on Test Point #3 ........................................................................ 28

Graph Corresponding to the Linear Regression Analysis

Done on Test Point #4 ........................................................................ 30

Graph Corresponding to Experiment #1 using Test Point

#1 ........................................ ..................................................................... 34

Graph Corresponding to Experiment #1 using Test Point

#2 ............................................................................................................. 36

Graph Corresponding to Experiment #1 using Test Point

#3 ............................................................................................................. 38

Graph for Experiment #2 using Test Point #1 ............................... 40

vii
Page

Figure 3.10

Figure 3.11

Figure 3.12

Figure 3.13

Figure 3.14

Figure 3.15

Figure 3.16

Figure 3.17

Figure 3.18

Figure 3.19

Figure 3.20

Figure 4.1

Figure 5.1

Graph for Experiment #2 using Test Point #2 ............................... 42

Graph for Experiment #2 using Test Point #3 ............................... 44

Data Gathered for Experiment #3 using Test Point #1.

Runs 5, 6, and 7 of the Experiment were Plotted .......................... 46

Data gathered for Experiment #3 using Test Point #2.

Runs 2, 3, and 4 of the Experiment were Plotted .......................... 47

Data Gathered for Experiment #3 using Test Point #3.

Runs 2, 3, and 4 of the Experiment were Plotted .......................... 48

Contour Plot of the Sensor Pad Showing an Applied

Diagonal Force ...................................................................................... 50

Mesh Plot of the Sensor Pad Showing an Applied

Diagonal Force ...................................................................................... 50

Contour Plot of the Sensor Pad Showing a Circular Object

Pressed Against the Left Side of the Pad ......................................... 51

Mesh Plot of the Sensor Pad Showing a Circular Object

Pressed Against the Left Side of the Pad ......................................... 51

Contour Plot of the Sensor Pad Showing a Circular Object

Pressed Against the Right Side of the Pad ...................................... 52

Mesh Plot of the Sensor Pad After a Circular Object Pressed

Against the Right Side of the Pad ..................................................... 52

Hardware Configuration for the Tactile Sensing System ........... 54

Host - Slave Software Configuration ............................................... 62

viii
Page

Figure 5.2

Figure 5.3

Figure 5.4

Figure 5.5

Contour Plot of the Sensor Pad Before a Circular Force is

Applied ................................................................................................... 68

Mesh Plot of the Sensor Pad Before a Circular Force is

Applied ................................................................................................... 68

Contour Plot of the Sensor Pad After a Circular Force is

Applied ................................................................................................... 69

Mesh Plot of the Sensor Pad After a Circular Force is

Applied ................................................................................................... 69

ix

ACKNOWLEDGEMENT

I would like to express my gratitude to my thesis advisor, Dr. Robert

Kelley, for his advice and guidance during the course of my research. I would

also like to thank Brian Romansky, Jodi Tsai, and all the people that were

there to answer my questions whenever I needed help.

Next, I want to thank my family and friends for their love and

support this past year. I would like to specially thank my fiancG Jose A. Faria,

for all his help while I was finishing up this project. Finally, I would like to

thank the GEM Consortium for awarding me a fellowship that allowed me to

pursue this Master's degree.

X
ABSTRACT

The main purpose of this thesis is to explore active (dynamic) tactile

sensing using a commercially available tactile array sensor. This task requires

the redesign of the sensor interface and a full understanding of the old sensor

hardware implementation. There were different stages to this research; the

first stage involved the reverse engineering of the old tactile sensor. The

second stage had to do with the exploration of the characteristics and behavior

of the tactile sensor pad. The next stage dealt with the redesign of the sensor

interface using the knowledge gained from the previous two stages. Finally,

in the last stage, software to control the tactile sensor was developed to aid in

the data acquisition process.

xi
CHAPTER 1

INTRODUCTION AND HISTORICAL REVIEW

Many robotic tasks are attempted without sensing, thereby assuming

a simplified and static world. An example could be the typical "pick and

place" task of an assembly line, where the origin and destination of the object

is known. As you may already know, this scenario is not very realistic. The

robotic systems of the future need the ability to use sensory feedback to

understand their environment. That is why sensor research is needed.

Most of the research in object recognition has been in the computer

vision area. However, sometimes it is very hard to determine the

characteristics of an object by just using one type of sensor. An interesting

approach is to be able to supplement the visual information by another type

of sensory input. This input could be a tactile sensor, which is the topic of

this thesis. By adding this type of sensor, the vision information could be

combined with the tactile sensor readings to produce a more robust and error

free description of the objects in the environment.

While adding a tactile sensor to a robotic system can potentially

produce more accurate results, it will probably add more complexity to the

system. This complexity lies in how. to integrate all the sensor components,

and how to control and coordinate them at the same time.

There are several types of tactile sensors: piezoresistive, magnetic and

electromagnetic, capacitive, electro-optic, and ferro-electric polymer sensors

[1]. The piezoresistive sensors are based on materials or elements whose

electrical conductivity varies as pressure is applied. Magnetic and
electromagnetic sensors use changes in their internal magnetic moment,

changes in the mechanical forces applied to or by it, or changes in its physical

length to sense objects in its surroundings. The next type, capacitive sensors,

use capacitance changes to sense forces. Electro-optic sensors, on the other

hand, rely on the modulation of a light source by the mechanical deformation

of a flexible material. Lastly, ferro-electric polymer sensors use their

piezoelectric and pyroelectric properties to sense applied forces.

The tactile sensor that is the subject of this thesis, falls into the

electro-optic category. This category is divided into two subclasses: Total

Internal Reflection (TIR), and Non-TIR [2]. The underlying principle of

operation of the TIR sensors is the frustration of total internal reflection

within a planar or curved waveguide caused by contact of an elastomeric

membrane against the waveguide surface. The tactile sensor used in this

research falls into the second subclass. In this type of sensor, the deflection of

elements protruding from a touch surface (an elastomer) reduces the light

transmitted to detectors by physically blocking the light. This deflection is

measured and then processedby a control system in order to be converted to a

signal appropriate for the system.

While raw data acquisition from the tactile sensor takes a reasonable

amount of time, the conversion from deflection readings to force readings

would take too long to be performed concurrently by the same

microprocessor. Therefore, the solution lies in leaving the data processing

part to a host computer, and only use the controlling microprocessor to

perform the data acquisition.
The previous reasoning is the motivation for this research. A sensor

manufactured by the Lord Corporation is used as the subject of investigation

of this project. The primary advantage of this type of sensor is that a high

degree of mechanical isolation could be maintained between the sensor

surface and the transduction elements, thereby permitting the sensor to be

very rugged [3]. One of the drawbacks of this sensor is that the shear force at

each individual site can not be measured unless a special module is attached

[3].

The tactile array sensor was studied and modified by replacing its

original microprocessor and leaving the data processing part to a host

computer. This task was accomplished by first reverse engineering the tactile

sensor array, and determining which parts of the tactile sensor would still be

useful after modifications to the hardware were done. Also, the

characteristics and properties of this sensor were studied and a formula to

convert the existing output of the sensing system, a voltage, into a force

reflecting the state of the sensor was derived. Next, a new interface using the

68HCll EVB was designed in order to use any of the Sun workstations as a

host computer. Finally, software following the guidelines of the functionality

implemented by the old tactile sensor was developed.

The contents of this thesis are structured such that the chronological

order of the tasks is observed. Chapter 2 discusses the inner workings of the

tactile array sensor and the analog board that was part of the original design.

Also some issues that were encountered while studying the analog board are

explained.
Chapter 3 is devoted to the investigation of the characteristics of the

tactile sensor array. Four major issues are addressed at this stage. The first

issue is whether some force reading could be interpolated from the sensor

output and, if it was possible, how it could be done. Secondly, the hysteresis

effects on the output readings are explored. Next, the issue of tactile pad

sensitivity is addressed. Finally, the effects of an applied force at the

surrounding neighbors of a chosen point are investigated.

The hardware interface between the 68HCll board and the tactile

sensor board is discussed in Chapter 4. In Chapter 5, the software written to

control the tactile sensor is explained. Chapter 6 gives a discussion of the

information and data gathered during the course of this research. Finally,

Chapter 7 ends this thesis with closing remarks, and suggestions for future

uses of this tactile sensor.
CHAPTER 2

REVERSE ENGINEERING

The tactile sensor, which is the subject of this research, is the LTS-200

manufactured by Lord Corporation. This sensor incorporates two forms of

tactile sensing, but only one was kept after the original sensor was modified;

this form of sensing is the tactile array sensor. The other form of tactile

sensing was a gross load sensor and, as a result of not being able to take apart

the tactile pad, it could not be incorporated into the modified sensor.

Nevertheless, even if the tactile pad could be opened up, there was no

guarantee that the pad could be put back together and still function properly.

2.1 Electrical Description of the Tactile Array Pad

The tactile array sensor is composed of sensitive sites embedded in an

elastomeric touch surface. The sites are organized as a 10 x 16 orthogonal

array with 0.071 inches between sites, as shown in Figure 2.1. In Figure 2.2,

the overall internal configuration of the sensor pad is presented. Each site is

composed of an emitter and a detector, as is shown in Figure 2.3. The emitter

is a diode, which produces a light of a certain intensity that is proportional to

the voltage through the diode. The detector is a phototransistor that receives

the light emitted by the diode and amplifies it by the current gain of the

phototransistor. Usually this type of configuration is used in applications

where greater sensitivity is required.

5
O

16 32 48 64 80 96 112 128 144 160

15 31 47 63 79 95 111 127 143 159

14 30 46 62 78 94 110 126 142 158

13 29 45 61 7'7 93 109 125 141 157

12 28 44 60 76 92 108 124 140 156

11 27 43 59 75 91 107 123 139 155

10 26 42 58 74 90 106 122 138 154

9

8

7

6

5

4

3

2

1

25 41 57 73 89 105 121 137 153

24 40 56 72 88 104 120 136 152

23 39 55 71 87 103 119 135 151

22 38 54 70 86 102 118 134 150

21 37 53 69 85 101 117 133 149

20 36 52 68 84 100 116 132 148

19 35 51 67 83 99 115 131 147

18 34 50 66 82 98 114 130 146

17 33 49 65 81 97 113 129 145

Columns

Figure 2.1 Diagram of the Existing Sites in the Sensor Pad.
7

,.z

E

(This side connections to the analog board)

5 Volts

(Columns)

© © Q

Figure 2.2

5 Volts

(Columns)

O- Emitter (Diode) - Detector (Transistor)

Optoelectronic Layout inside the Tactile Sensor Pad
Ea_.chsite, using the previously described co_iguration, morutors the

deflection of the small portion of the elastomeric touch surface it occupies.

The result _om this action is a voltage, which could be translated into an

applied force. _ere are 40 diodes and 54 emi_ers on the tactile pad, w_ch

confi_red in the previously described fashion, produce 160 sensible sites.

_e location of these sites was previously shown in Fi_e 2.1.

2.2 Hardware Description

The first task to be accomplished was to produce sc_hematics for the

tactile sensor hardware. _is need arose based on the fact that no hardware

diagrams were part of the original documenta_on provided with the Lord

Co_ora_on tactile sensor.

© ©

_Y-_'-20milliamp

current six

Fibre 2.3 Diode-Transistor P_r Confi_ra_on
Table 2.1 Connections From the Sensor Pad to the Analog Board

Connector Pin # Signal Name Signal Definition Chip number

13 +GEX Voltage Supply 4057

14 R9-D emitter 4057

15 R8-D emitter 4057

16 R7-D emitter 4057

17 R6-D emitter 4057

18 R5-D emitter 4057

19 R4-D emitter 4057

20 R3-D emitter 4057

21 R2-D emitter 4057

Chip label Component

U15 transistor

U15 transistor

U15 transistor

U15 transistor

U15 transistor

U15 transistor

U15 transistor

37

38

39

40

41

C5+E Anode 4051

C4+E Anode ' 4051

C3+E Anode 4051

C2+E Anode 4051

C1 ÷E Anode 4051

34 R3-E cathode 4051

35 R2-E cathode 4051

36 R1-E cathode 4051

29 RS-E cathode 4051

30 R7-E cathode 4051

31 R6-E cathode 4051

32 R5-E cathode 4051

33 R4-E cathode 4051

26

27

28

24

25 C4+D collector 4051

C3+D collector 4051

C2+D collector 4051

CI+D collector 4051

U14 transistor

U14 transistor

U14 transistor

U14 transistor

U18 diode

U18 diode

U18 diode

U18 diode

U18 diode

U18 diode

U18 diode

U18 diode

U19 diode

U19 diode

U19 diode

U19 diode

U19 diode

22 R1-D emitter 4057 U15 transistor

23 C6+D collector 4051 U14' transistor

C5+D collector 4051 U14 transistor

U15 transistor

U15 transistor
10

Tactile sensor

board output

68HCll

2S 29 30 31

<2)o so

E U14 sl

$2

Q0

E U15

Tactile

Sensor

Board

so

S1

S2

$3

323334

mI-_ ÷,'v

$1

U19 s2

Figure 2.4 Analog Board Hardware
11

The hardware for the original sensor was composed of two boards:

the first one contained the analog circuitry and the other one contained the

digital circuitry. Since one of the objectives of this research is to replace the

6502 microprocessor of the original sensor with a 68HCll microprocessor, it

was decided not to use the digital board at all and only concentrate on how

the analog board worked.

2.2.1 Description of the Analog Board Components

In Figure 2.4, devices belonging to the analog board are shown. These

4 devices are labeled as U14, U15, U18, and U19. All of them are analog

multiplexers: U14, U18, and U19 are 8 channel analog multiplexers, and U15

is a 16 channel analog multiplexer. The function of these devices is to

activate the diodes (U18 and U19) and transistors (U14 and U15) inside the

sensor pad.

The other relevant component located on the analog board is U17,

the digital gain chip. This device has as an input the voltage generated when

a diode and a transitor are activated inside the sensor pad. Inside this device,

the gain of the signal is increased by 32.

2.2.2 Selecting a Site in the Tactile Array Pad

The analog board contains the appropriate circuitry which takes a

reading from one of 160 sites (Figure 2.1), and converts that into a voltage

which could be later be transformed into a force measurement. There are
12

certain steps that have to be followed to select a given site in the tactile array.

First, determine the appropriate diode and transistor pair corresponding to

the desired site by looking at Figure 2.1. Then, find the corresponding row

and column in the optoelectronic layout using Figure 2.2. After this, using

Table 2.1, get the corresponding number for the connection with the tactile

sensor pad. Finally, select those connections on the multiplexers shown on

Figure 2.4. The multiplexers labeled as U15 and U14 correspond to the

activation of the transistors. U18 and U19 correspond to the selection of a

particular diode. After selecting the desired diode-transistor pair, a deflection

for the desired site could be measured. This deflection is output in the form

of a voltage, which is later fed to the voltage map circuit, so the A/D

converter inside the 68HCll EVB could be used.

2.2.3 Voltage Map Circuit

Before the voltage coming from the tactile sensor is fed to the

68HCll EVB, there is some signal amplification that takes place within the

analog board. The resulting amplification yields an output of a +12V to -12V

range. In the original board, further processing of this signal took place. The

voltage was mapped into a 0V to 5V range by using some combination of opamps and switches. Instead of using their scheme, which is practically

impossible to trace in the analog board, I decided to design my own voltage

map circuit. In my scheme, I used a buffer op-amp which is directly

connected to a 10 kf2 potentiometer. This potentiometer is used to adjust the

gain of the circuit. The other side of the potentiometer is connected to
13

another 10 kf2 potentiometer which is used to adjust the offset of the output

signal. The resulting output from this circuit yields the desired voltage range

of 0V to 5V. Before this signal is connected to the input of the A/D, a

noninverting op-amp is used to lower the loading effects caused when the

connection between the 68HCll EVB and this circuit are made. This voltage

range of 0V to 5V is needed so the A/D converter supplied within the 68HCll

EVB can be used.

+12V

Input coming _,. _:_

from tactile_

-12V

+12V

-12V

+I2V

Output going to A/D

Figure 2.5 Voltage Map Circuit
14

Z--

Z--c

Gain to

the op-amps

GND

CS

U17

13006

out

input

Dig_In 1

Dig__In 2

Dig_In 3

Analog Jumper

Pin 8

Input from sensor

array pad

Figure 2.6 Digital Gain Chip and its Connections

2.2.4 Digital Gain for the Signal's Amplification

The analog board uses a digital gain chip in order to adjust the gain of

the op-amps by means of a microprocessor. The input to this device is the

signal coming from the sensor pad. At the stage where this signal enters this

gain chip, it has already gone through several stages of manipulation. A

detailed description of this amplification is very difficult to trace on the

analog board; therefore, only the digital gain chip is shown in this thesis.

After some experiments, I decided to preset this gain to a value where

the tactile array sensor will be most sensitive. The other choices in gains will

yield readings only after enormous pressures are applied to the sensor.

Taking into consideration that a large applied force can damage the diodetransistor pair at each site, I decided on a choice of gain (32) that will detect an
15

applied force within a safe range of applied pressures. In the next chapter, I

will explain in detail how the safe force range was chosen.
CHAPTER 3

CHARACTERISTICS OF THE TACTILE SENSOR ARRAY

There were four major issues that had to be investigated when

experiments to observe the behavior of the tactile sensor were done. The first

issue is how the output voltage of the tactile sensor could be mapped into a

force that could accurately represent the load on a specific site of the sensor.

The second issue deals with the effect of hysteresis on the tactile pad, and how

that affects the precision of the force measurements. The question of how

sensitive is the tactile pad, and how precise are its results is the subject of the

third issue tackled at this time. Finally, the fourth issue explores the effect on

the surrounding neighbors when a force is applied to a given site.

lever arm

surface

connection to

the

hardware

balance block

Figure 3.1 Setup used to perform the experiments

16
17

3.1 Experimental Setup

Measurements throughout the tactile pad were taken using the setup

shown in Figure 3.1. This setup consists of a lever arm and a balance block. A

set of weights was used to apply different forces to each site of the tactile

sensor.

3.2 Protecting the Tactile Pad

In the course of the experiments, some deterioration of the tactile pad

was observed. It seemed that as more continuous pressure was exerted on a

point, some physical deformation was occurring to the pad. This effect is

highly undesirable for two reasons: first, if deformation occurs, the force

readings would not be very accurate, and second, there is no other tactile pad

available if the current one ceases to function.

There were several approaches taken to alleviate this problem. The

first step taken was to file the object used to apply pressure on the pad, in this

case a nail, until a rounder end point was obtained. While this approach

relieved some of the symptoms of deformation, it still did not completely

eliminate the problem. The tactile pad is made of an elastomeric material,

very rubber like, so I decided that by covering the pad with a similar material,

the original characteristics of the sensor will not be lost. A typical rubber

balloon seemed to be the obvious choice. It seemed to work very well, and as

far as I can see there shouldn't be any difference on the measurement results

with and without the balloon covering the sensor. The only problem with
18

using a balloon is that every couple of weeks it needs to be replaced due to

wear and tear.

3.3 Force Around the Tactile Pad Borders

Several sets of measurements were taken at each one of the corners of

the tactile sensor. Tables 3.1, 3.2, 3.3, and 3.4 show the results of this

experiment. The experiment consisted of applying a constant force at a point,

and then immediately recording the voltage reading.

Table 3.1 Forces Recorded on Corner #1 of the Sensor Pad

Distance Voltage Actual Force Approximated Force

(cm) (Volts) (N) (N)

9.5 8.74 0.08 0.07

9 7.54 0.11 0.11

8.5 6.05 0.15 0.16

8 3.56 0.19 0.24

7.5 0.73 0.23 0.34

7 0.76 0.26 0.33

6.5 -1.79 0.30 0.42

6 -2.23 0.34 0.43

5.5 -2.45 0.38 0.44

5 -2.60 0.41 0.45

4.5 -2.85 0.45 0.46

4 -3.28 0.49 0.47

3.5 -3.46 0.53 0.48

3 -3.62 0.57 0.48

2.5 -3.71 0.60 0.48

2 -3.79 0.64 0.49

Diode: 1 Transistor: 1 Mass Used: 100 g
19

Table 3.2 Forces Recorded on Comer #2 of the Sensor Pad

Distance Voltage Actual Force Approximated Force

(cm) (Volts) (N) (N)

10 9.93 0.02 0.03

9.5 8.87 0.04 0.06

9 6.56 0.06 0.14

8.5 4.66 0.08 0.20

8 2.98 0.09 0.26

7.5 1.34 0.11 0.32

7 0.31 0.13 0.35

6.5 -0.49 0.15 0.38

6 -1.04 0.17 0.40

5.5 -1.41 0.19

5 -1.75 0.21

4.5 -1.93 0.23

0.41

0.42

0.42

4 -2.12 0.25 0.43

3.5 -2.25 0.26 0.44

3 -2.37 0.28

2.5 -2.52 0.30

2 -2.66 0.32

1.5 -2.87 0.34

1 -2.98 0.36

0.5 -3.16 0.38

0.44

0.44

0.45

0.46

0.46

0.47

Diode: 8 Transistor: 9 Mass used: 50g
20

Table 3.3 Forces Recorded on Comer #3 of the Sensor Pad

Distance Voltage Actual Force Approximated Force

(cm) (Volts) (N) (N)

9 8.80 0.06 0.07

8.5 5.71 0.08 0.17

8

7.5

7

6.5

6

5.5

5

4.5

4

3.5

3

3.60

2.02

1.01

0.49

0.25

0.10

0.02

-0.04

-0.07

-0.14

-0.34

0.09

0.11

0.13

0.15

0.17

0.19

0.21

0.23

0.25

0.26

0.28

0.24

0.29

0.33

0.34

0.35

0.36

0.36

0.36

0.36

0.36

0.37

2.5 -0.41 0.30 0.37

2 -0.55 0.32 0.38

1.5 -0.64 0.34 0.38

1 -0.73 0.36 0.38

Diode: 40 Tr0nsistor: 54 Mass used: 50g
21

Table 3.4 Forces Recorded on Comer #4 of the Sensor Pad

Distance

(cm)

9.5

9

8.5

8

7.5

7

6.5

5.5

5

4.5

4

3.5

3

2.5

2

1.5

1

Voltage ActuaJ Force

(Volts) (N)

5.07 0.04

4.28 0.06

3.63 0.08

3.01 0.09

2.44 0.11

Approximated Force

(N)

0.04

0.07

0.09

0.11

0.13

2.01 0.13 0.15

1.48 0.15 0.16

1.04 0.17

0.79 0.19

0.47 0.21

0.24 0.23

0.17 0.25

0.00 0.26

-0.10 0.28

-0.15 0.30

-0.23 0.32

-0.27 0.34

-0.29 0.36

0.18

0.19

0.20

0.21

0.21

0.21

0.22

0.22

0.22

0.22

0.22

Diode: 33 Tronsistor: 46 Mass used: 50g

The setup used to carry out this task was explained in the previous

section. For the first comer, results using a 100g mass are shown. The results

presented for the rest of the comers use a 50g mass. A 100g mass was used

only on the first corner because this site seemed to be less sensitive than the

other corners.

As you can observe from the tables 3.1-3.4, when the approximated

force applied to a site is calculated (according to the methods explained in the
22

following section), there is a some deviation on most of the readings from

what the actual force at that point should be. There could be several

explanations for this behavior. But the most likely answer for this

phenomenon might be that being on the sensor pad's border means less

protective pad covering the inner mechanical parts of the sensor. This can

translate into a higher probability of malfunction for the sites along the

borders.

Also, as a direct result of this situation, there is less precision of

readings on any site which is located on the borders of the sensor. For this

reason, the data taken around the corners was not used when trying to

interpolate a force from the voltage measures given as an output from the

tactile sensor.

3.4 Force Measurement Interpolation

By choosing points at random, measurements were taken throughout

the rest of the tactile sensor. All the data sets were plotted to find out the

characteristics of the applied force on each one of the selected sites. Several

things were discovered in the course of these experiments. The first

observation had to do with the maximum voltage at each site. Most of the

sites surveyed had a maximum voltage between 10V and 7.5V. There were

other sites, not many, where this maximum voltage was much less that 7.5V.

This characteristic of the sensor pad indicated a non-uniformity between

components inside the pad. It also indicated that the force reading to be
23

extrapolated from the sensor output voltage is not the same function at each

one of the sites.

Table 3.5 Force Interpolation Measurements for Test Point #1

Distance Voltage Force Approx. force

(cm) (Volts) (N) (N)

10 9.69 0.01 0.03

9.5 9.63 0.02 0.03

9 9.51 0.03 0.04

8.5 9.4 0.04 0.04

8 9.27 0.05 0.04

7.5 9.08 0.06 0.05

7

6.5

6

5.5

5

4.5

4

3.5

3

2.5

2

1.5

1

0.5

8.89 0.07 0.06

8.67 0.07 0.06

8.44 0.08 0.07

8.19 0.09 0.08

7.91 0.10 0.09

7.56 0.11 0.10

7.13 0.12 0.11

6.8 0.13 0.13

6.42 0.14 0.14

6.06 0.15 0.15

5.75 0.16 0.16

5.41 0.17 0.17

5.13 0.18 0.18

4.87 0.19 0.19

Diode: 10 Transistor: 11 Mi_ssused: 20g
24

A

_O

;>---

Linear Ree'ressionv Analysis For Test Point #1

10

9 m m a

8

7

6

4 , , I ,, i

0.0 0.1 0.2

FORCE (N)

y = 10.57 - 29.23x

Figure 3.2 Graph Corresponding to the Linear Regression Analysis Done

on Test Point #1
25

Table 3.6 Force Interpolation Measurements for Test Point #2

Distance Voltage

(cm) (Volts)

10 1.43

9.5 1.21

9 0.93

8.5 0.69

Force Approx. force

(N) (N)

0.01 0.03

0.02 0.04

0.03 0.05

0.04 0.06

8 0.46 0.05 0.07

7.5 0.22 0.06 0.07

7 -0.04 0.07 0.08

6.5 -0.37 0.07 0.09

6 -0.57 0.08 0.10

5.5 -0.87 0.09 0.11

5 -1.19 0.10 0.12

4.5 -1.47 0.11 0.13

4 -1.73 0.12 0.14

3.5 -2.02 0.13 0.15

3 -2.31 0.14 0.16

2.5 -2.62 0.15 0.17

2 -2.91 0.16 0.18

1.5 -3.18 0.17 0.19

1 -3.34 0.18 0.19

0.5 -3.72 0.19 0.20

Diode: 26 Transistor: 38 Mass used: 20g
26

b_

O_

Linear Re_ression Analysis For Test Point #2

2

1

0

-1

-2

-3

-4

0.0 0.1 0.2

FORCE (N)

y = 1.82 - 29.32x

Figure 3.3 Graph Corresponding to the Linear Regression Analysis Done

on Test Point #2
27

Table 3.7 Force Interpolation Measurements for Test Point #3

Distance Voltage Force Approx. force

(cm) (Volts) (N) (N)

10 3.75 0.01 0.04

9.5 3.61 0.02 0.04

9 3.45 0.03 0.05

8.5 3.23 0.04 0.06

8 3.07 0.05 0.06

7.5 2.84 0.06 0.07

7

6.5

2.61 0.07

2.37 0.07

6 2.1 0.08

5.5 1.82 0.09

5

4.5

4

3.5

1.56 0.10

1.19 0.11

0.71 0.12

0.25 0.13

3 -0.2 0.14

2.5 -0.67 0.15

2 -1.15 0.16

1.5 -1.6 0.17

1 -2.09 0.18

0.5 -2.56 0.19

0.08

0.08

0.09

0.10

0.11

0.12

0.14

0.16

0.17

0.19

0.20

0.22

0.23

0.25

]2]_o_.d__: 29 Tiansistor: 41 Mass used: 20g
28

Linear Re_ressionv Analysis For Test Point #3

4

[]

2 _ mm [] mm mm

0

-2

0.0 0.1 0.2

FORCE (N)

y = 4.77 - 36.10x

Figure 3.4 Graph Corresponding to the Linear Regression Analysis Done

on Test Point #3
29

Table 3.8 Force Interpolation Measurements for Test Point #4

Distance Voltage Force Approx. force

(cm) (Volts) (N) (N)

8.5 10.79 0.04 0.0003

8 10.73 0.05 0.0023

7.5 10.31 0.06 0.02

7 9.94 0.07 0.03

6.5 9.51 0.07 0.04

6 9.19 0.08 0.05

5.5 8.6 0.09 0.07

5 8.05 0.10 0.09

4.5 7.74 0.11 0.10

4 7.04 0.12 0.13

3.5 6.56 0.13 0.14

3 5.97 0.14 0.16

2.5 5.51 0.15 0.18

2 4.92 0.16 0.20

1.5 4.2 0.17 0.22

1 4.04 0.18 0.23

0.5 2.89 0.19 0.26

Diode: 25 Transistor: 38 Mass used: 20g
30

12

10

Linear Re_rression Analysis For Test Point #4

2 , I

0.0 0.1 0,2

FORCE (N)

y = 13.37 - 53.03x

Figure 3.5 Graph Corresponding to the Linear Regression Analysis Done

on Test Point #4

The other characteristic of the pad observed was that each one of the

sites had a linear region which was consistent throughout most of the data

points used. It was also noted that following this linear region, there was a

saturation region, where force readings were completely unreliable. After
31

comparing, all the plots for the data sets taken, a linear region that could be

applied to each one of the tactile sensor sites was chosen. This region is also

considered the safe region for the tactile sensor array. Forces that don't fall

within this range could damage the diode-transistor pairs at each of the sites.

Before any further analysis took place, I decided that for the rest of the

experiments only masses that yield force readings within this linear region

should be used. As you can see, this is the case for the test points shown on

Figure 3.2, 3.3, 3.4, and 3.5.

After several experiments were done (using only masses that would

result in forces within the linear range), a regression analysis was performed.

Due to the fact that the voltage when no force is applied in the pad is not

uniform throughout the pad, an exponential or higher order regression

analysis was not done. In Tables 3.5-3.8 and Figures 3.2-3.5 the results for the

linear regression analysis can be observed. Most of the sites on the pad fall

within the first or second graphs, having the third and fourth being a special

case. From the previously described analysis, an equation which converts a

voltage to an applied force was produced. The equation for the applied force

has the form:

y=mx+b

where m is the slope of the equation and b is the y-axis intercept. The b for

each one of the sites represents the maximum voltage when no force is

applied. The m was carefully chosen using as a guideline the slopes obtained
32

as a resu!t in the linear regression plots.

voltage to force equation was -30.

The final chosen slope for the

3.5 Hysteresis Analysis

Hysteresis analysis was also performed to find how reliable the force

measurements from the tactile array sensor are. There were three different

sets of experiments done on each one of the sites that were studied. Out of all

the sites studied, only three will be presented.

The first set of experiments consisted of gradually applying more

pressure to a site until reaching the maximum force that could be applied

with the chosen mass (see description of the experimental setup). When this

maximum force is reach, then gradually unload the sensor until force is no

longer applied to the sensor.

From Figures 3.6, 3.7, and 3.8 two observations can be made. The first

one is that there exists a discrepancy between the readings taken while

loading the sensor (labeled as Voltage 1), and readings taken while unloading

the sensor (labeled as Voltage 2). Also noted is that this phenomenon occurs

only in some sites. This is the first indication of greater hysteresis effects than

were anticipated. As a demonstration that all the sensor sites might not

behave the same, the recorded data for test point #1 is shown (Figure 3.6). It

seems that this particular sites is extremely sensitive to pressure, therefore

creating a narrow range of readings. Before any conclusions were made,

another set of tests had to be conducted.
33

Table 3.9 Hysteresis Data Taken at Test Point #1 for Experiment #1.

Distance Force Voltage I Voltage 2

(cm) (N) (V) (V)

10 0.02 -9.89 -9.97

9.5 0.05 -9.98 -10.12

9 0.07 -10.09 -10.26

8.5 0.09 -10.21 -10.38

8 0.12 -10.34 -10.56

7.5 0.14 -10.58 -10.72

7 0.16 -10.67 -10.89

6.5 0.19 -10.85 -10.98

6 0.21 -10.96 -11

5.5 0.23 -11 -11.03

5 0.26 -11.02 -11.03

4.5 0.28 -11.03 -11.04

4 0.30 -11.04 -11.04

3.5 0.33 -11.04 -11.05

3 0.35 -11.05 -11.05

2.5 0.37 -ll.05 -11.05

2 0.40 -11.05 -11.06

1.5 0.42 -11.06 -11.06

1 0.44 -11.06 -11.06

0.5 0.47 -11.06 -11.06

Voltage 1:

Diode: 11

Loading the sensor

Transistor: 13

Unloading the sensor

Mass used: 50g
34

Experiment #1 Using Test Point #1

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

-9.8 t

v -10

O -10.2

L -10.4 t

AT -10.6 _-

G -10.8 t

E -11

-11.2 I

FORCE (N)

0.50

I

Figure 3.6 Graph Corresponding to Experiment #1 using Test Point #1
35

Table 3.10

Distance

(cm)

10

9.5

8.5

7.5

6.5

5.5

4.5

4

3.5

3

2.5

2

Hysteresis Data Taken at Test Point #2 for Experiment #1

(N)

0.02

0.05

0.07

0.09

0.12

0.14

0.16

0.19

0.21

0.23

0.26

0.28

0.30

0.33

0.35

0.37

0.40

Voltage 1

(V)

10.8

10.67

7.44

4.64

2.15

-0.57

-2.75

-4.86

-6.31

-7.84

-8.95

-9.97

-10.98

-11.04

-11.05

-11.05

-11.06

1.5 0.42 -11.06

1 0.44 -11.07

Voltage 1; Loading the sensor

Diode: 22 Transistor: 25

Voltage 2:

v

Voltage 2

(V)

10.8

8.76

1.8

-1.01

-3.27

-5.53

-6.91

-8.24

-9.44

-10.6

-11

-11.04

-11.04

-11.05

-ll.06

-11.06

Unloading the sensor

Mass used: 50g
36

Experiment #1 Using Test Point #2

v

o

L

T

A

G

E

15

10

5

0

_50 _0

-10

-15

l!'_'m I I I I I 1 I

0.05 0.10__O.20 0.25 0.30 0.35 0.40 0.45

FORCE (N)

""Voltage1 O-Voltage 2 i

Figure 3.7 Graph Corresponding to Experiment #1 using Test Point #2
37

Table 3.11 Hysteresis Data Taken at Test Point #3 for Experiment #1.

Distance

(cm)

10

9.5

9

8.5

7.5

7

6.5

5.5

4.5

3.5

3

2.5

1.5

Force

(N)

0.02

0.05

0.07

0.09

0.12

0.14

0.16

0.19

0.21

0.23

0.26

0.28

0.30

0.33

0.35

0.37

0.40

0.42

Voltage 1

(v)

10.83

9.84

7.52

5.15

3.16

1.52

-0.25

-2.19

-3.84

-5.54

-6.84

-8.51

-9.83

-10.19

-11

-11.04

-11.05

-11.05

1 0.44 -11.05

0.5 0.47 -11.06

Voltage 2

(v)

10.83

8.59

6

3.91

0.13

-1.81

-3.75

-5.46

-7.01

-8.33

-9.49

-10.33

-11

-11.03

-11.05

-11.05

-11.05

Voltage 1: Loading the sensor Voltage 2: Unloading the sensor

Diode: 28 Transistor: 32 Mass used: 50g
38

Experiment #1 Using Test Point #3

v

O

L

T

A

G

E

15

10

5

0

-10

-15

0.05 ., I I I I I I I

)0 0.10 0.1 b'__ 0.25 0.30 0.35 0.40 0.45 0.50

FORCE (N)

""Voltage1 C_-Voltage 2 [

Figure 3.8 Graph Corresponding to Experiment #1 using Test Point #3
39

The second set of experiments consisted of taking measurements by

first loading the sensor applying a force. Then, before applying further

pressure to get the next voltage reading, I completely unloaded the sensor

until it reached the original reading when no force was applied.

Table 3.12 Hysteresis Data Taken at Test Point #I for Experiment #2.

Distance Force Voltage 1 Voltage 2

(cm) (N) (V) (V)

10 0.02 -9.94 -9.95

9.5 0.05 -9.98

9 0.07 -10.04

8.5 0.09 -10.11

8 0.12 -10.2

7.5 0.14 -10.25

7 0.16 -10.36

-10

-10.04

-10.12

-10.21

-10.28

-10.37

6.5 0.19 -10.43 -10.46

6 0.21 -10.55 -10.55

5.5 0.23 -10.64 -10.67

5 0.26 -10.71 -10.75

4.5 0.28 -10.85 -10.87

4 0.30 -10.95 -10.96

3.5 0.33 -10.98 -10.99

3 0.35 -10.99 -11

2.5 0.37 -11 -11.03

2 0.40 -11.03 -11.03

1.5 0.42 -11.03 -11.03

1 0.44 -11.04 -11.04

0.5 0.47 -11.04 -11.05

Diode: 11 Transistor: 13 Mass used: 50g
4O

Experiment #2 Using Test Point #1

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

-9.8 [ I I I

v -10

O -10.2

L -10.4 t

AT -10.6 t

G -10.8 t

E -11 _[

-11.2

FORCE (N)

i "_"Voltagel el-Voltage 2 I

0.50

I

Figure 3.9 Graph for Experiment #2 using Test Point #1
41

Table 3.13 Hysteresis Data Taken at Test Point #2 for Experiment #2.

Distance Force Voltage 1

(cm) (N) (V)

10 0.02 10.82

9.5 0.05 10.08

8.5

7.5

6.5

5.5

0.07

0.09

0.12

0.14

0.16

0.19

0.21

0.23

6.47

3.66

1.13

-1.23

-3.59

-5.41

-6.91

-8.11

Voltage 2

(V)

10.6

7.52

4.3

1.86

-0.56

-2.82

-4.77

-6.31

-7.58

-8.74

5 0.26 -9.15 -9.78

4.5 0.28 -10.19 -10.82

4 0.30 -10.92

3.5 0.33 -11.03

3 0.35 -11.04

2.5 0.37 -11.05

2 0.40 -11.06

1.5 0.42 -11.06

1 0.44 -11.06

-11.03

-11.04

-11.05

-11.06

-11.06

-11.07

-11.07

Diode: 22 Transistor: 25 Mass used: 50g
42

Ex-periment #2 Using Test Point #2

v

O

L

T

A

G

E

15

10

5

0

4

-10

-15

)0

I I I I I I

• o,o_I_:__ . 'oo_

FORCE (N}

I _J'Voltagel _'Voltage 2 I

Figure 3.10 Graph for Experiment #2 using Test Point #2
43

Table 3.14 Hysteresis Data Taken at Test Point #3 for Experiment #2.

Distance Force Voltage 1 Voltage 2

(cm) (N) (V) (V)

10 0.02 10.82 10.78

9.5 0.05 10.66 9.77

9 0.07 9.25 8.45

8.5 0.09 7.91 7.28

8 0.12 6.61 5.99

7.5 0.14 5.33 4.93

7 0.16 4.12 3.78

6.5 0.19 2.76 2.61

6 0.21 1.81 1.63

5.5 0.23 0.61 0.6

5 0.26

4.5 0.28

4 0.30

3.5 0.33

3 0.35

2.5 0.37

2 0.40

1.5 0.42

1 0.44

0.5 0.47

-0.24 -0.41

-1.32 -1.37

-2.36 -2.52

-3.31 -3.52

-4.25 -4.59

-5.32 -5.3

-6.03 -6.23

-7.09 -7.21

-7.73 -7.86

-8.11 -8.46

Diode: 28 Transistor: 32 Mass used: 50g
44

V

O

L

T

A

G

E

15

10

5

0

_50

-10

-15

Experiment #2 Using Test Point #3

0.05 0.10 0.20 0.25 0.30 0.35 0.40 0.45

FORCE (N)

_-Voltagel 4::l'Voltage 2 ]

Figure 3.11 Graph for Experiment #2 using Test Point #3

Figures 3.9-3.11 belong to this set of experiments. In these figures,

Voltage 1 refers to the voltage reading taken using the method explained

above, and Voltage 2 refers to the readings taken by slowly applying more

force without first unloading the sensor. Again, the same problems observed

in the previous test are present on this one. These problems are the

discrepancy between readings for the same force, and also the fact that this is

not true for all the points.

Finally, a third set of experiments was done. Measurements for these

tests were taken by first choosing an initial applied force to the sensor and

then loading and unloading the sensor about that force. Here an important

observation was made: when the range of loadings/unloadings about a force
45

is large (more than + 0.08 N of the chosen force), the sensor readings fluctuate

enough to be unreliable. However, if the range is smaller, then the force

readings stay within a 5% of what they are supposed to be.

Table 3.15

distance

(cm)

10

9.5

9

8.5

8

7.5

7

6.5

6

5.5

5

4.5

4

3.5

3

2.5

2

1.5

1

Diode: 11

Hysteresis Data Taken at Test Point #1 for Experiment #3.

Force

(N)

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.07

RUNS OF THE EXPERIMENT

1 2 3 4 5 6 7 8 9 10

-9.95 -9.97

-9.97 -9.97 -9.98

-9.99 -9.98 -999 -9.99

-9.99 -10.01 -10 -10.01 -10.02

-10.01 -10.01 -10.02 -10.02 -10.02 -10.03

-10.03 -10.03 -10.03 -10.04 -10.04 -10.04 -10.04

-10.07 -10.06 -10.06 -10.06 -10.07 -10.08 -10.08 -10.08

-10.07 -10.09 -10.08 -10.09 -10.09 -10.09 -10.1 -10.1 -10.09

0.08 -10.11 -10.1 -10.11 -10.1 -10.11 -10.11 -10.11 -10.12 -10.11 -10.11

0.09 -10.14 -10.12 -10.11 -10.12 -10.12 -10.12 -10.12 -10.13 -10.13 -10.13

0.10 -10.16 -10.13 -10.15 -10.14 -10.14 -10.14 -10.14 -10.15 -10.16 -10.16

0.11

0. I2

0.13

0.14

0.15

0.16

0.17

0.18

Transistor: 13

-10.18 -10.19 -10.18 -10.18 -10.16 -10.19 -10.19 -10.21 -10.2

-1021 -10.21 -10.21 -10.22 -10.22 -10.22 -10.23 -10.23

-10_23 -10,24 -10.25 -t0.25 -10.25 -10.26 -10.26

-10.27 -102.8 -10.27 -10.28 -10.28 -10.29

-10.32 -10.32 -10.32 -10.33 -10.32

-10.34 -10.35 -10.35 -10.35

-10.38 -10.38 -10.38

-10.44 -10.42

Mass used: 20g
46

V

O

L

T

A

G

E

0.00

-9.95

-10

-10.05

-10.1

-10.15

-10.2

-10.25

-10.3

-10.35

Experiment #3 Using Test Point #1

0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

I I I I I I I I

FORCE (N)

m

_'5

X6

o- 7

Figure 3.12 Data Gathered for Experiment #3 using Test Point #1. Runs 5,

6, and 7 of the Experiment were Plotted

Table 3.16

distance

(cm)

7

6.5

6

5.5

5

4.5

4

Diode: 22

Hysteresis Data Taken at Test Point #2 for Experiment #3.

Force 1 2

(N)

0.07

0.07 3.46

0.08 2.68 2.52

0.09 2.01 1.61

0.10 0.84 0.71

0.11 0.24

0.12

Transistor:. 25

RUNS OF THE EXPER/MENT

3 4 5

4.63 •4.44

3.37 3.5 3.54

2.71 2.41 2.61

1.62 1.71 1.81

0.55 0.66 0.7

-0.27 -0.4 -0.2

-1.13 -1.21

6 7 8 9 10

3.49 3.59 3.43 3.56 3.46

2.51 2.42 2.47 2.63 2.39

1.72 1.66 1.56 1.68 1.62

0.83 0.63 0.54 0.58 0.58

-0.23 -0.27 -0.24 -0.36 -0.33

Mass used: 20g
47

Experiment #3 Using Test Point #2

V

O

L

T

A

G

E

5

4

3

2

1

0

0_6

-1

-2

!

0.07 0.08 0.09 0.10 0_

I

0.13

FORCE (N)

•X 2

43-3

-_4

Figure 3.13 Data gathered for Experiment #3 using Test Point #2. Runs 2,

3, and 4 of the Experiment were Plotted

Table 3.17 Hysteresis Data Taken at Test Point #3 for Experiment #3.

RUNS OF THE EXPERIMENT

distance Force 1 2 3 4 5 6 7 8 9 10

(cm) (N)

6.5 0.07 4.03 4 3.97 3.97 4.01 3.94 3.88 3.98 3.97

6 0.08 3.25 3.07 2.96 3.12 3.04 3.04 3.12 3.11 3.01 2.89

5.5 0.09 2.61 2.23 2.23 2.64 2.1 2.33 2.14 2.29 2.32 2.7

5 0.10 1.51 1.54 1.62 1.41 1.48 1.78 1.48 1.32 1 1.63

4.5 0.11 0.95 0.49 0.69 0.52 0.63 0.54 0.55 0.46 0.43

Diode: 28 Transistor: 32 Ma_s used: 20g
48

Experiment #3 Using Test Point #3

0.07 0.08 0.08 0.09 0.09 0.10 0.10 0.11 0.11 0.12

FORCE (N)

•X 2

"_3

"¢'°4

Figure 3.14 Data Gathered for Experiment #3 using Test Point #3. Runs

2, 3, and 4 of the Experiment were Plotted.

3.6 Sensitivity Analysis

The next set of experiments were aimed to discover how sensitive

the tactile sensor pad is. In order to perform these experiments, the control

and communication software written for the tactile sensor was used.

Two objects, at different times, were pressed against the surface of the

sensor pad: a circular object and a rectangular object. Before each one of these

tests took place, a reading of the unloaded sensor pad was taken. Then, an
49

object was applied to the pad surface, and immediately another scan of the

sensor was done. In order to check whether the pressed object was actually

sensed by the pad, the data for the unloaded sensor was subtracted from the

data for the loaded sensor.

The results of these experiments are shown on Figure 3.15 to Figure

3.20, and for each one of the objects applied, a contour and a mesh graph of

the sensor pad were drawn. In Figure 3.16, a rectangular object pressed

diagonally on the sensor pad surface can be seen. In the previous figure (Fig.

3.15), a contour graph of the same experiment is plotted. In this graph you

can notice that where the object is not applied, sometimes there are some

small voltage drop readings. This is due to some error introduced to the

system by the A/D converter. Nevertheless, this voltage drop reading has a

magnitude of 0.094 V (in its worst case), introducing only a 1.56% of error to

the sensor sensitivity. You can notice this same phenomenon in each one of

the contour graphs drawn for the rest of the experiments.

The other two experiments performed on the sensor pad consisted

on applying a circular object to the pad surface. In one case the circular object

was applied to the left, and in the other case it was applied to the right. As it

can be seen from the mesh graphs (Figure 3.18 and 3.20), the sensor pad is

deformed according to the shape of the applied force.

On and off during the course of these experiments some change in

force (A) appears on the diode-transistor pairs (1,1) and (17,19), even though

no force was applied at this moment. It seems that on a previous project

where this tactile sensor was also used, the behavior of the first diodetransistor pair (1,1) was noticed but not of the second diode-transistor pair.
50

8

7-

6

5

4

3

2

1

2

Figure 3.15

4 6 8 10 12 I4 16

Contour Plot of the Sensor Pad Showing an Applied Diagonal

Force.

Figure 3.16 Mesh Plot of the Sensor Pad Showing an Applied Diagonal

Force.
51

10

7

6

3

2

i

2 4 6 8 10 12 14 I6

Figure 3.17 Contour Plot of the Sensor Pad Showing a Circular Object

Pressed Against the Left Side of the Pad.

Figure 3.18 Mesh Plot of the Sensor Pad Showing a Circular Object Pressed

Against the Left Side of the Pad.
52

10

9

8

4

3

1

2 4 6 8 10 12 14 16

Figure 3.19 Contour Plot of the Sensor Pad Showing a Circular Object

Pressed Against the Right Side of the Pad.

Figure 3.20 Mesh Plot of the Sensor Pad After a Circular Object Pressed

Against the Right Side of the Pad.
53

(17,19) [4]. The A for the first diode transistor pair every time it occurs is the

_.

same; its magnitude is 4.63 V. The same behavior occurs in the other diodetransistor pair mentioned, but in this one the magnitude of the A is 1.70V.

This phenomenon is probably due to some malfunction of these diodetransistor pairs inside the pad at this location.

3.7 Effects of an Applied Force on its Neighbors

Another aspect of the tactile sensor that was studied dealt with how

an applied force to a site affected its nearest neighbors. It was observed that as

long the applied force to a site was less than 0.857 Newtons, there was no force

reading recorded on the nearest neighbors. If the applied force to a point was

greater than this, then force readings would be sensed at the nearest neighbors

and probably some type of damage to the selected diode-transistor pair could

result.
CHAPTER 4

SENSOR INTERFACE WITH THE 68HCll EVALUATION BOARD

A 68HCtl EVB was chosen as the controlling microprocessor for this

sensor system. The reasons behind this decision are the low cost and ease of

use of this microprocessor.

I Tactile Pad ] 41/ /

Analog Circuit Board

-_[ Voltage Map Circuit

I 68HCll EVB I--_,_ 1/

Lq--RS-232 cable

!

Sun Workstation

Figure 4.1 Hardware Configuration for the Tactile Sensing System

4.1 Hardware Configuration

The final hardware interface, as seen in Fig 4.1, consists of the tactile

sensor pad connected to the analog board of the old sensor. The cable used for

this connection is the same one that was used in the old sensor.

54
55

Table 4.1 Connections between Analog Board and 68HCll EVB

Analog Board Jumper Description

5V

5V

GND

68HCll EVB

Connections

26

26

Description

Vdd

Vdd

GND

4 GND 1 GND

5 Pin 6-U18 1 GND

6 Pin 3-VR2 1 GND

7 1 GND

Not Connected

Analog GND

Pin 3-$1, Pin 7-U17 Not Connected

15

9 Pin 1-U16, Pin 2-U16 Not Used Not Used

10 Pin 10-U17 26 Vdd

11 Pin 9-U17 1 GND

12 Pin 8-U17 26 Vdd

13 Pin1 and 2-$2 26 Vdd

14 Not Connected Not Connected Not Connected

Not Connected

16

17

18

19

2O

21

22

23

24

25

Not Connected

Not Connected

Not Connected

Pin 11-U18

Pin 10-U18

Pin 9-U18

Pin ll-U19

Pin 10-U19

Pin 9-U19

Not Connected

Pin 8-$1

Not Connected

Not Connected

Pin 10-U15

Pin 11-U15

Pin 14-U15

Pin 13-U15

Pin 11-U14

26

27

28

29

Not Connected

Not Connected

Not Connected

12

13

14

I0

11

Not Connected

Not Connected

Not Connected

39

38

37

36

42

30

31

32

Not Connected

Not Connected

PC3

PC4

PC5

PC0

33 Pin 10-U14 41

34 Pin 9-U14 40 PB2

35 -12V Not Used Not Used

36 -12V Not Used Not Used

37 1 GND

38

39

Analog GND

Analog GND

+12V Not Used

40 +12V Not Used

PC1

PC2

Not Connected

GND

Not Connected

Not Connected

PB3

PB4

PB5

PB6

PB0

PB1

GND

Not Used

Not Used
56

The analog board is connected to the 68HCll by means of their respective

jumpers. In Table 4.1 the connections between the analog board and the

68HCll EVB are detailed. Finally, the 68HCll EVB is connected to a Sun

workstation by a RS-232 cable.

Table 4.2 List of Diodes on the Sensor Pad and Configuration for Port C

so they can be Activated.

DIODE PORT C CONFIG.

1 '$04'

2 'SOD'

3 '$14'

4 '$1C'

5 '$24'

6 '$2C'

7 '$34'

8 '$3C'

9 '$03'

10 'SOB'

11 '$13'

12 '$1B'

13 '$23'

14 '$2B'

15 '$33'

16 '$3B'

17 '$04'

18 ;$0A'

19 '$12'

20 '$1A'

DIODE PORT C CONFIG,

21 '$22'

22 '$2A'

23 '$32'

24 '$3A'

25 '$01'

26 '$09'

27 '$11'

28 '$19'

29 '$21'

30 '$29'

31 '$31'

32 '$39'

33 '$00'

34 '$08'

35 '$10'

36 '$18'

37 '$20'

38 '$28'

39 '$30'

40 '$38'
57

Table 4.3 List of Transistors on the Sensor Pad and Configuration for

Port B so they can be Activated.

TRANSISTOR PORT B CONFIG.

1 '$05'

2 'SOD'

3 '$15'

4 '$1D'

5 '$25'

6 '$2D'

7 '$35'

8 '$3D'

9 '$45'

10 '$04'

11 '$0C'

12 '$14'

13 '$1C'

14 '$24'

15 '$2C'

16 '$34'

17 '$3D'

18 '$44'

19 '$03'

2O 'SOB'

21 '$13'

22 '$1B'

23 '$23'

24

25

26

'$2B'

'$33'

'$3B'

27 '$43'

TRANSISTOR

28

29

3O

31

32

PORT B CONFIG,

'$02'

'$0A'

'$12'

'$1A'

'$22'

33 '$2A'

34 '$32'

35 '$3A'

36 '$42'

37 '$01'

38 '$09'

39 '$11'

40 '$19'

41 '$21'

42 '$29'

43 '$31'

44 '$39'

45 '$45'

46 '$06'

47 '$08'

48 '$10'

49 '$18'

5O

51

52

53

54

'$20'

'$28'

'$30'

'$38'

'$40'
58

4.2 68HCll EVB Port Assignment

Out of the 4 existing parallel I/O ports in the 68HCll EVB, only three

are used. These ports are B, C, and E. Port C selects the desired diode, and

port B the transistor. Each port controls two multiplexers on the analog

board, one corresponding to the column and the other to the row where the

desired component is located. The last port used, port E, is the input port for

the built-in A/D converter.

4.2.1 Port B of the 68HCll EVB

As it was mentioned before, Port B selects which transistor is going to

be activated. The higher 4 bits (PB3-PB6) are used to select the row, and the

lower 3 bits (PB0-PB2) select the column. Table 4.2 shows a list of all the

transistors in the tactile pad, and how Port B should be configured in order to

activate a specific transistor.

4.2.2 Port C of the 68HCll EVB

Port C selects which diode is going to be activated. The higher 3 bits

(PC3-PC5) are used to select the row, and the lower 3 bits (PC0-PC2) select the

column. Table 4.3 shows a list of all the diodes on the pad and how Port C

should be configured so a desired diode can be activated.
59

4.2.2 Port E of the 68HCll EVB

Port E of the 68HCll EVB has a dual role. Its serves as an I/O parallel

port and also as the input for the built-in A/D converter. There are 8 input

lines to port E but only one channel was needed. The assigned bit was bit PE1.

Bit PE0 of this port could not be used as the input to the A/D because upon

reset, the monitor program in the 68HCll checks the state of PE0. If a low

state is detected, the monitor program is executed and the prompt displayed.

If a high state is detected, the monitor will automatically jump to the

EEPROM and execute the users code. If the output of the voltage map circuit

is connected to PEO, upon reset it might confuse the monitor program and the

program might jump to an unknown location.
CHAPTER 5

SOFTWARE DESCRIPTION AND SYSTEM OPERATION

5.1 Initialization

5.1.1 Power Up Initialization

Upon power up, the 68HCll EVB starts executing the instructions

starting at $E000. Two registers, the OPTION and the TMSK2, have to be

initialize within 64 bus-cycles after power-up. If this period of time expires

before these registers are configured, some of the bits in these registers become

read-only and they can not be change.

5.1.2 SCI Initialization

The SCI is the sei'ial port used for communication with the host

computer. At this time, this serial port is configured to 1 start bit, 8 data bits,

and 1 stop bit. Also the baud rate, 9600, and the clock speed, 8 Mhz EXTAL are

set. Note that there is another port, the terminal I/O port, which uses the

ACIA and is not used in this project.

5.1.3 A/D Converter Initialization

When the OPTION register is initialize at power up, two of its bits

(ADPU and CSEL), have to be set properly. If this bits are not

6o
61

correctly initialized, the A/D converter will not be operational. The A/D

converter is by default not activated, therefore the ADPU bit has to be set in

order to enable it.

The CSEL bit has to be set to zero in order to minimize the

conversion errors if the alternate clock source is selected during the A/D

conversion.

5.2 Software Location on Memory

The communication and data acquisition programs are located

starting at address $E000 on the 68HCll EVB Monitor EEPROM. This

assembler code is designed to replace the Buffalo monitor located originally at

this location.

5.3 Communication Software between Host and 68HCll EVB

Before any code was written, a communication protocol between the

68HCll EVB and the host computer (Sun workstation) had to be defined.

Having only two functions to implement, simplifies the final design. Each

time one of the functions is invoked, the host communicates the desired

operation to the EVB and it initiates execution. Before the EVB begins

execution, an acknowledgment of the requested command is sent to the host.

This is implemented as a safety feature. When the end of the desired

information is reached, a delimiter indicating end of transmission is sent.
62

5.3.1 Host Communication Software

The host in this system is a Sun workstation which is connected to a

68HCll EVB using the serial port. This port is labeled as Host I/O in the EVB

Manual, and the cable used for communication is a regular modem cable [5].

The software developed for the host consists mainly of two parts:

transmitting or receiving data using Port B of the workstation, and changing

the received ASCII characters into a 16-bit integer format. Instead of sending

integers through the communication line, ASCII characters are used because

that is the only type the host can read or write.

Before transmitting or receiving any data, the port had to be

initialized and configured. A pointer to the I/O port had to be set up before

any communication took place, and the data word characteristics had to be

l 68HCllslave J_..,

IPort jumper

P3 is the

communication port

Host

Sun Workstation

Port B is the

communication port

Figure 5.1 Host - Slave Software Configuration
63

determined. Usually the Sun workstations are configured as 1 start bit, 7 data

bits, and 1 stop bit, but the 68HCll required a different configuration: 1 start

bit, 8 data bits, and I stop bit.

The data coming from the microprocessor is made up of two ASCII

characters that represent one byte. In order to be able to use this data, they had

to be changed into the 16-bit integers which the combination of ASCII

characters represented.

5.3.2 68HCll EVB Communication Software

While the host computer is performing other tasks, the 68HCll will

sit and wait until some information is requested from the host computer. As

soon as this happens, it will immediately start performing the desired task.

Delimiters, indicating the start and end of the transmission, are implemented

so the host can verify that the microprocessor has acknowledged the request

for a specific task.

5.4 Software Developed to Control the Tactile Sensor

In order to write the software to control the tactile sensor, it was first

necessary to decide which tasks were going to be performed by the

microprocessor and which tasks were the host's responsibility. As is

mentioned throughout this thesis, one of the main objectives was to speed up

the original tactile sensor by performing the least number of operations in the

microprocessor. Having this in mind, I decided to use the microprocessor to
64

perform the data acquisition, and leave the data manipulation to the host

computer.

5.4.1 Software Developed for the 68HCll

5.4.1.1 Sensor Commands

The software developed to control the data acquisition, from the

68HCll side, has a similar functionality as the one used in the old sensor

design. Out of all the options offered in that software, there were only two

commands that applied to the redesigned sensor. These two commands are:

tacScartAll, and tacElementOne.

5.4.1.1.1 tacScanAU Command

The tacScanAll command polls each one of the 160 sites and gets the

state of the sensor. This command is useful due to the non-uniformity of the

readings at each one of the sites. Deformation, temperature, continued use,

aging, etc. causes the zero force voltage at each one of the sites to fluctuate.

Therefore, before any force is applied to the sensor pad, this command must

be invoked. Also immediately after the force is applied, this command

should be call again to find out the nature of the pressure applied.

There are two arguments for this function: the first one is a pointer

to a software array of size 160 where the state of the sensor will be stored. The

second argument is the I/O port used on the host computer.
65

5.4.1.1.2 tacElementOne Command

This command is used whenever there is a need to explore the force

applied to only one point of the tactile sensor. Before this command is

invoked and before any pressure is exerted on the sensor pad, the tacScanAll

command must be called to get the current state of the sensor before any force

is applied.

There are five arguments that have to be defined when this function

is call. The first two arguments are two characters that correspond to the

hexadecimal number that activates the desired diode. The third and fourth

arguments correspond to the transistor that is going to be activated. The last

argument corresponds to the I/O port used by the host computer. This

function returns the state of the sensor at the chosen point.

5.4.1.2 AdD Converter Operation

There were two choices as how the A/D could be configured. In the

first choice, the selected channel is converted 4 consecutive times with the

first result being stored in A/D result register 1 (ADR1) and the fourth result

being stored in register ADR4. After the fourth convertion is complete, all

the convertion activity is halted until the user decides to continue the

process. In the second choice, instead of stopping on the fourth convertion,

the converter continues convertion storing the fifth convertion in register

ADR1, the sixth convert'ion in register ADR2, and so on. Based on this I

decided to configure the A/D to operate in the first described manner.
66

For each one of the sites, this convert-ion process is repeated twice to

be able to average 8 sampled readings.

5.4.2 Software Developed for the Host Computer

Upon reaching the host, the data is converted from an ASCII

representation to an integer representation. Now the data is in the form of a

voltage, and ranges from 0V to 5V.

The first hexadecimal character corresponds to the left nibble of a byte,

and the second corresponds to right nibble of the byte. Using Table 5.1, the two

hex characters can be combined and translated into a voltage ranging from 0

V to 5V.

Table 5.1 Analog Input to 8-Bit Result Translation Table

1

%

iVolts 2

bit 7 bit 6 bit 5 bit 4 bit 3 bit 2 bit I bit 0

50% 25% 12.5_, 62_5_, 3.12V, 1.56_ 0.78_, 0.79_

2.5 12.5 0.625 0.3125 0.1562 0.0781 0.0391 0.0195

(1) % of VRH- VRL

(2) Volts for VRL = 0 V; VRH = 5.0 V
67

5.5 Data Acquisition Example

In order to verify that the previously described software works, a

simple data acquisition program was written. In order to ensure best

operation of the tactile sensor, a 20 minutes warm-up period after power-up is

necessary. Before any pressure was applied to the sensor pad, the tacScanAll

command was executed to obtained the state of the pad while it was

unloaded. Then, the program was executed again but now applying pressure

to the pad. Several runs of the experiment were performed but only one of

them is shown. In the case shown, a two forces are applied to the sensor pad.

The data gathered from the programs was converted into a Matlab file

and then plotted. Two kinds of plots were obtained for each experiment, a

mesh plot and a contour plot. The mesh plots used as x and y points the

position of a site in the sensor, whereas z corresponds to the voltage at that

particular point. The contour plots show with more clarity where the

pressures are being applied in the sensor. In Figures 5.1 and 5.2, a contour

plot and a mesh plot, show the state of the sensor before any pressure is

applied to it. Figures 5.3 and 5.4 are plotted after pressure is applied to the pad

in the indicated fashion.
68

10

7

6

5

4

L

Figure 5.2

J

2 4 6 8 10 12 14 16

Contour Plot of the Sensor Pad Before a Circular Force is

Applied.

Figure 5.3 Mesh Plot of the Sensor Pad Before a Circular Force is Applied.
69

2 4 6 8 10 12 14

Figure 5.4 Contour Plot of the SensorPad After a Circular Force is

Applied.

16

Figure 5.5 Mesh Plot of the SensorPad After a Circular Force is

Applied.
CHAPTER 6

DISCUSSION

The improvement of a tactile sensor array made by Lord Corporation

was the main objective of this research. This objective meant speeding up the

sensor's response time, and redesigning its interface so the sensor could be

controlled from any workstation. In order to accomplish this task, several

steps were taken: first, an understanding of the tactile sensor hardware was

needed. After this, exploratory work on the behavior and characteristics of

the tactile sensor was performed. When enough information about the

sensor was achieved, its old microprocessor, a 6502, was replaced with a

68HCll EVB. Finally, control software was developed in order imitate some

relevant functions of the old tactile sensor.

As it was mentioned in Chapter 2, the behavior of the tactile sensor

hardware was studied and specifications for the analog board, belonging to the

original tactile sensor, were produced. It was also decided at this stage that

this analog board would remain as part of the modified tactile sensor. The

other board, which contained the digital circuitry, was discarded as the

68HCll EVB took the place of the controlling microprocessor. At this time,

an issue which was taken care of in the original digital board, was addressed:

the restriction in the output range and its accuracy.

After studying the manipulation done by the tactile sensor board on

the output signal, I decided to design the final stage of amplification to the

signal. This final stage involved the mapping of a signal ranging from +12V

to -12V to a range between 0V to 5V. The 0V to 5V range was needed in

7O
71

order to u_se the A/D converter inside the 68HCll, and the voltage map

circuitry implementation shown in Chapter 4 performed this task.

Extensive research on the behavior of the tactile sensor pad was

performed. The experiments were simple, but important enough so that

relevant information about the characteristics of this tactile pad could be

gathered. The tactile pad is made of a type of elastomeric material, which

according to some literature has a very low hysteresis [6]. The first set of

experiments done to the pad had as a goal to map, in an accurate fashion, the

output voltage from the tactile sensor to a useful type of measurement, a

force reading. After it was established that the desired output was a force in

the z direction (force perpendicular to the pad surface), several readings

throughout the pad were taken using the setup previously described in

Chapter 3. The results were plotted, and a linear regression analysis for these

points was conducted. The results of the regression analysis for several points

in the array were compared. At this point, I had to decide in what

neighborhood of applied forces I desired a better mapping. After a careful

consideration, I decided that most of the pressures exerted on the surface will

not be extremely large. Therefore, I strived for better mapping at lower forces.

The second set of experiments performed on the tactile pad had the

goal of finding the precision of the Voltage readings coming from the sensor

pad. In order to obtain the answer to this question, hysteresis experiments

were conducted on the pad. As you may gather from the plots explained

previously in Chapter 3, the claim by some authors that the elastomeric pad

on this sensor had a very low hysteresis is not true [3]. According to the

hysteresis plots shown in Chapter 3, any force reading from the sensor will
72

not be very accurate. Nevertheless, in the linear region, where force readings

are matched to the expected force readings, the effects of hysteresis are

considerable but tolerable.

The topic of this sensor pad sensitivity was the subject of the third set

of experiments. At this stage it was proven that the tactile pad is very accurate

within a +1.56% margin of error. It was noticed that there is also some error

introduced by the A/D when the conversion takes places. This problem can

be seen in places where no force is applied. When the data for the unloaded

sensor is subtracted from the data for the loaded sensor, at some places there

is a discrepancy of 0.0195 V. This error is consistent with these error that this

A/D converter is expected to give.

The other problem noticed at this point is probably due to some

malfunction of the diode-transistor pairs (1,1) and (17,19). There is voltage

drop recorded at this point when in reality no force is applied. The solution

for this problem is probably to compensate the reading at this places by means

of software.

The final set of experiments performed to the pad had to do with the

influence caused to the neighbors where a force is applied to a site. By doing

this experiment, the maximum force that could be applied to a point before

damage occurs could be derived. After choosing a couple of points and

watching the behavior of the neighboring sites, I concluded that as long as the

force is less than 0.857 Newtons, no voltage readings on the neighbors would

be recorded. This is a good characteristic of this pad because, if the sensor is

used in an application where a binary response is needed and this response

has to be in real-time, it could be quickly deciphered whether an object is
73

present or_not. All of the previously described experiments were conducted

using a force applied to one point, instead of the whole sensor pad.

The previously described experiments were performed at the

hardware level, using a voltmeter and logic probes instead of using software.

The reason for this decision was simple; developing software to calibrate the

tactile sensor would have meant putting all the necessary hardware together,

and then if problems arose, to try and figure out if it was the hardware's fault

or the software's fault. In my approach, after gaining knowledge of the

workings of the old tactile sensor, I put together a test board where I would

manually select each point of the array and study its characteristics. By doing

this, my theories about how the board worked were confirmed, and the

necessary steps to study the behavior of the board were simpler. Then, after

the hardware was fully understood and the investigation about the behavior

of the tactile sensor was enough for this project's scope, I felt it was

appropriate to move on to the next stage, the interface between the analog

board with the 68HCll board, and the development of control software.

The only issues that arose when the tactile sensor board was

interfaced to the 68HCll evaluation board were which ports to use and what

was the desired function of the A/D converter. I chose ports B and C as the

controlling ports of the multiplexers. The input coming from the sensor was

fed into port E, the A/D port input. There were two choices as how to

configure the A/D. The A/D could convert the input signals continuously or

convert four readings and then stop. I chose the latter scheme becausein this

one I would know for certain that the sampled voltages are from the current

site instead of belonging to the previous site.
74

The last stage of this project consisted of developing the software to

communicate and control the tactile sensor. A couple of problems arose at

this part; the Sun workstation is normally configured with 7 data bits while

the 68HCll only accepts 8 or 9 data bits. As a result, the configuration of the

workstation had to be changed and, if any other workstations are going use to

control the sensor, it should be kept in mind that a reconfiguration of the I/O

port is needed. The only thing needed is a small function that is called at the

beginning of the program (supplied with the library).

The second problem encountered had to do with properly

configuring the 68HCll. At power-up, the default I/O port is the ACIA in the

68HCll. For this project's purposes, there was a need to use the SCI port,

thereby a special configuration was needed. In order to enable this port, a

logic one has to be written to any address from $4000-$5FFF.

The previous paragraphs give a description of all the issues addressed

during the course of my thesis. In the next section, the conclusions of this

research will be presented, and suggestions for further improvements on the

tactile sensor will be given.
CHAPTER 7

CONCLUSIONS

A tactile sensor made by Lord Corporation was the subject of this

research. One of the problems of the old sensor was its speed. I tried to make

the new design faster by just performing the data acquisition in the

microprocessor, and leaving the manipulation of data to the host computer.

Another problem with this sensor was its lack of precision. After some study

of the hardware, I reached the conclusion that in order for the design to be

simpler, the designers at Lord Corporation sacrificed the sensor's precision.

Finally, the A/D converter used in the old design had a data width of 4 bits

which would result in a large error after the digital conversion. In my design,

the 8-bit A/D supplied with the 68HCI1 was used, introducing less error in

the digital conversion.

There is still room for improvement for this sensor. Some possible

suggestions are: adding another digital gain chip that could make the sensor

even more sensitive, and using a different A/D that could accept voltage

ranges from -12 V to +12V so that the errors introduced by mapping this

range into 0V to 5V are eliminated.

This modified tactile sensor will yield better results in binary

applications where there is a need to know only whether there is an object

present and where. The error in this type of operation is small, 1.56%. In

applications where an applied force is needed, the formula described in

Chapter 3 can be used. This formula will probably not yield extremely

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76

accurate results, but it will provide the user with a good notion of the

magnitude of the applied force.
LITERATURE CITED

[1] Dario, P., Bergamasco, M. and Fiorillo, A.S., Force and Tactile Sensing for

Robots, Springer-Verlang, New York, NY, 1988.

[2] Bergej, Stefan, "Planar and Finger-shaped Optical Tactile Sensors for

Robotic Applications," IEEE Journal of Robotics and Automation,

Vol.4, No.5, pp. 472- 484, October 1988.

[3] Preliminary LTS-200 Operations Manual, Rev 0.7, Lord Corporation,

Cary, NC, 1984.

[4] Sekelsky, Stephen M., The Usefulness of the LTS-200 Tactile Sensor,

Senior Project Final Report, ECSE, July 1989.

[6] Motorola Inc, M68HC11EVB Evaluation Board User's Manual, 1986.

[5] Rebman, J. and Morris, K.A., A Tactile Sensor with Electrooptical

Transduction, Springer-Verlag, New York, NY, 1987.

77
BIBLIOGRAPHY

Allen, Peter K., Robotic Object Recognition Using Vision and Touch,

Kluwer Academic Publishers, Norwell, MA, 1987.

CoIIey, William C., 68HCll Cross-Assembler (Portable) Version 0.I, 1987.

Lipovski, G.J., Single and Multiple-Chip Microcomputer Interfacing,

Prentice-Hall Inc., Englewood Cliffs, New Jersey, 1988.

Luo, Ren C. and Kay, Michael G., "Multisensor Integration and Fusion

in Intelligent Systems," IEEE Transactions on Systems, Man, and

Cybernetics, Vo1.19, No.5, pp. 901-931, September/October 1989.

Mazda, F.F, Electronic Engineer's Reference Book, 5th Edition,

Butterworths, London, England, 1986.

Motorola Inc, M68HC11 Reference Manual, Prentice-Hall Inc,

Englewood Cliffs, New Jersey, 1989.

Ritchie, Dennis M and Kernighan, Brian K., The C Programming

Language, Prentice-Hall Software Series, Englewood Cliffs, New

Jersey, 1978.

Roach, John W., Paripati, Praveen K. and Wade, Michael, "Model-based

Object Recognition Using a Large-Field Passive Tactile Sensor," IEEE

Transactions on Systems, Man, and Cybernetics, Vol. 19, No. 4, pp.

846-853, July/August 1989.

Tsai, Jodi, M68HC11 Gripper Controller Software , Master's Thesis,

May 1991, ECSE, Rensselaer Polytechnic Institute.

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