An Active Haptic DisplayAnd Psychophysics Experiments

A Thesis

Presented to

The Academic Faculty

by

Martin Young

Thesis Committee:

Dr. Wayne Book, Chair

Dr. Imme Ebert-Uphoff, ME

Dr. Elizabeth Mynatt, COC Tuesday, June 6, 2000

Background

• What is a haptic display?– Greek word meaning “touch”

– Alternate channel for information exchange

• Analogous to a visual display:

• Visual display • Haptic display

Background

• Applications:• “touchable” CAD models

• medicine (e.g., surgery training, tele-surgery, tele-diagnosis)

• human task enhancement (e.g., trajectory guidance)

• Motivation• Active testbed for IMD Laboratory

• Emulate a passive haptic display

• Examine human perceptive abilities

Presentation Outline

• Developing an active haptic display– Commercial robot

– Custom robot

• Evaluation and Experimentation– Proof of concept

– Accuracy testing

– Human psychophysics experiments

System Requirements

• Physical interface (tool)

• Active manipulator (robot arm)

• Force sensor

• Feedback controller

CRS A465 Robot arm• 6 degrees of freedom

• dc servomotors withoptical encoders

• custom gripper added totool flange

C500 Controller• programmable

• closed loop servo control

• force sensor inputs

• force control algorithm

Commercial Robot

2

1

3

4

5

6

A465 Robot Arm

Commercial Robot

ManipulatorCF KA

x

xe

FFd xf

Force Control Diagram

)( FFCx dff −=Position offset:

Force compensation: sKKC Dff +=

Commercial Robot

Stiff Boundary at x = -25 cm

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

0 20 40 60 80 100 120 140

Samples

Pos

ition

& V

eloc

ity (

cm, c

m/s

)

dx/dt

x

Commercial Robot

Collision detection algorithm

• Sampling frequencies

– Servo loop 1000 Hz

– Force sensor 400 Hz

– Force gain updates 2.5 Hz (RAPL-II code)

• Solution: increase sampling rate

– Share processing burden with Windows NT

– ACI protocol -- read/write controller memory

Commercial Robot

Visual Basic application

• successfully removed collision detection andforce gain updates from C500

• doubled sampling rate (6 Hz)

Alternative solutions

• next generation controller from CRS

• design a new controller

Custom Robot

HURBIRT

• two dc servomotors withoptical encoders

• gear reduction of 20:1 and60:1

• safety issues

• parallel linkage kinematicstructure

HURBIRT (Human Robot BilateralResearch Tool)

HURBIRT

Arm kinematics

)sin()cos(

)cos()sin(

2411

2411

qLqLy

qLqLx

+=+=

Forward kinematics

Differential kinematics

−

−=

2

1

2411

2411

)cos()sin(

)sin()cos(

y

x

q

q

qLqL

qLqL

�

�

�

�

qJx �� =

HURBIRTArm dynamics

Fq)(Jg(q)q)qC(q,qB(q) T+=++ ��

Joint space dynamic model:

Parallel linkage simplification:

FJqgqB T+=+ τ)(��

FJqgB T−+= )(γτChoose

Feedback linearization

γ=q��

HURBIRTImpedance control

FKxxBxM =++ ���

x is related to F through a generalized mechanical impedance

B

K

M F

x

}][{ 11 qJxKxBFMJ ttt �

�

� −−−= −−γ

Controlling HURBIRT

Real time extension for Windows NT

– Windows doesn’t support real time interrupts

– Latency period of non-dedicated system

Hyperkernel/STG (CAMotion, Inc.)– PC’s processor is shared

– Priority based scheduling

– Shared memory interface

– User-friendliness of Windows

Control Software

InterruptProcessing

Thread

Windows NT Hyperkernel

Windows NTInterface

SharedMemory

Hardware InterfaceCard

SignalingThread

MainThread

Top-level design

- Achieves hard real time control

- Easy to program/troubleshoot

- Does not require separate processor (DSP)

Control Software

F

v

θ Passive: cos θ > 0

• Dissapative passive device is unidirectional

Passive emulation:

• Can only create a unidirectional stiffness

• Monitor force and velocity, limit torquecommands to prevent violation of passivity

SoftwareWindows Application

• Launch Hyperkernel application

• Interface for operator to set parameters

• real time display of virtual world

Haptic Display

(a) (b) (c)

(e)(d) (f)

* *

* * *

*

• Proof of concept

• Different geometries, physical properties

AB

F

C DE

y1

y2

x1 x2X

Y

Haptic Display

Collision Detection algorithm

• subdivide surface

• current tool point position

• logic checking

• returns physical parameters

Example surface

ApplicationTrajectory Enhancement

Tool point

0 .24

0 .34

0 .44

0 .54

0 .64

0 .74

0 .84

0 .24 0 .34 0 .44 0 .54 0 .64 0 .74 0 .84

0 .25

0 .35

0 .45

0 .55

0 .65

0 .75

0 .85

0 .25 0 .35 0 .45 0 .55 0 .65 0 .75 0 .85

Visual feedback Haptic feedback

EvaluationTesting accuracy of modeled environment

• Compare commanded stiffness with measuredstiffness

• Target impedance models stiffness as a linearspring: F = Kt(x-xsurface)

Experiment

• Move tool point into surface

• Measure force vs. displacement

• Linear regression

Evaluation

0

5

10

15

20

25

30

35

40

45

50

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

Too

l poi

nt fo

rce

(N

)

Spring displacement (m)

Linear fit: R squared of 0.998

Stiffness (slope) = 951 N/m

1000 N/m commanded stiffness

Error less than 5 %

CommandedStiffness (N/m)

ExperimentalStiffness (N/m)

R-Squared Value % error inmeasured value

500 477 0.998 4.61000 951 0.998 4.91500 1433 0.997 4.41575 1526 0.998 3.11650 1608 0.999 2.51800 1752 0.999 2.72200 2133 0.998 3.03000 2967 0.998 1.1

EvaluationHysteresis effect

0

10

20

30

40

50

60

70

80

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

Too

l poi

nt fo

rce

(N

)

Spring displacement (m)

• 12-20 N drop in force

• Same effect for all levels ofcommanded stiffness

• Unmodeled friction in joints

Psychophysics Tests

• Characterize haptic display and human operator

• Guide development of future haptic interfaces

• Perform tests in “passive” mode

• Signal Detection Theory: determine the just noticeabledifference (JND) for discriminating stiffnesses

Decision Axis

• Normal distribution

• Constant varianceµ

SDTf(x|S1)

Psychophysics TestsTheory

“S2” “S1”

S2 hits misses

S1 false alarms correct rejections

Stimulus-response matrixf(x|S1)

f(x|S2)

CorrectRejections False

Alarms

Misses

Hits

k

µ1

µ2

d'

Sensitivity:

d’ = (µ 1- µ 2)/σ

Threshold: d’ = 1

Psychophysics Tests

Experimental Procedure

• vertical wall, 4 cm in front of home position

• reference stiffness of 1500 N/m

• four stiffer surfaces: 5 %, 10 %, 15 %, 20 %

• 64 trials per stiffer surface

• respond “reference” or “stiffer”

Data analysis

0/ KK

d

∆′

=δ 1001

% ×=δ

JND

Psychophysics TestsResults

0

5

10

15

20

25

30

35

S1

S2

S3

S4S5

Average JND% = 15 %

Stif

fnes

s JN

D%

Psychophysics TestsPassive emulation

Stif

fnes

s JN

D%

Average JND % = 12 %

0

5

10

15

20

25

30

S1

S2

S3

S4

S5

Conclusions

• CRS system proved to be ineffective haptic display

• HURBIRT effectively created virtual surfaces

• Surfaces exhibited good rendering of stiffness

• Psychophysics tests revealed a JND of 12%-15 %

Future Work

• Limit hysteresis effect

• Study human performance for multiple tasks

• Model effective impedance of PTER

• Examine “stiffness”