human factors and haptic interfaces lynette jones, department of mechanical engineering,...
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Human Factors and Haptic Interfaces
Lynette Jones,Department of Mechanical Engineering, Massachusetts Institute of Technology.
Surface of average sized adult human: 1.8 m2
(1000 times that of retinae)
Density: 1250 kg/m3
Weight: 5 kg
Total number of axons converging on CNS: 1.1*106 Retina: 106 axons, Cochlea: 6*104 axons
Human skin
Haptics and Vision
Information Temporal capacity (bits/sec) acuity
Fingertip 102 5 ms
Ear 104 0.01 ms
Eye 106-109 25 ms
Time delays differ for hand and eye – hand is quicker than the eye
SENSORY7 classes of Mechanoreceptor (17,000 – glabrous skin)2 classes of Thermoreceptor4 classes of Nocioceptors3 classes of Proprioceptor (0-300 in different muscles)
Information Channels in Human Hand
Sensory Receptor Properties and Haptic Display Design
Resolution and sensitivity of the sensors Temporal processing characteristics
(adaptation, summation) Spatial features of processing Delays in processing information (0.4-120
m/s)
Tactile Receptor Types (7)
Cutaneous and subcutaneous Fiber ModalityMechanoreceptors group
Meissner corpuscle A, Stroking, fluttering
Merkel disk receptor A, Pressure, texture
Pacinian corpuscle A, Vibration
Ruffini ending A, Skin stretch
Hair tylotrich, hair-guard A, Stroking, fluttering
Hair-down A Light stroking
Field A , Skin stretch
Tactile Tuning Curves
0.0001
0.001
0.01
0.1
1
0.1 10 1000
Frequency (Hz)
Abs
olut
e th
resh
old
(mm
)
Merkel cell: 5-15 Hz
Meissner’s corpuscles: 20-50 Hz
Pacinian corpuscles: 60-400 Hz
Perceived intensity of vibration varies as a function of frequency as well as amplitude
If the amplitude of vibration is changed while frequency is maintained constant, at certain levels a noticeable change in rate occurs
Perception of Vibrotactile Stimuli – Frequency and Intensity Interact
Equal subjective intensity curves
-5
-4
-3
-2
-1
0
1
20 30 40 50 60 70
PSE
(db
re:
sta
ndar
d)
Comparison frequency (Hz)
Standard 25 Hz
Spatial acuity of the skin Skin movement: 0.1-0.2 mm (Gould et al.,
1979)
Spatial frequency of grating: fingertip can distinguish 40-50 µm in spatial period of 0.7-1.0 mm (Morley et al., 1983)
On smooth glass surface, dot of height 1-3 µm and diameter of 550 µm can be detected by the fingertip (Johansson & LaMotte, 1983)
Temporal variation often improves acuity and spatially extensive stimuli improve performance
(Johnson & Phillips, 1981)
Temporal acuity and touch
Successiveness - 2 stimuli (1 ms duration) must be separated by 5.5 ms to be perceived as 2 at a single locus
Temporal order – 2 successive stimuli at separate sites must be separated by 20 ms in order to determine which site first
20 ms5 ms
Haptic Displays
Surface texture of object Contact in the environmentMoving probe across surface - volumeCompliance/stiffness of objectMass of objectThermal properties of object
Human Perception and Haptic Display Design
Scaling (gain) of information between object and operator
Bandwidth required to present information Mechanical properties of interface Magnitude of delays that are tolerable -
symmetric, modality specific
Force-reflecting Interfaces
Scaling (gain) of force between object and operatorRing finger
Women: 10 NMen: 18 N
Middle fingerWomen: 14 N
Men: 26 N
Index fingerWomen: 19 N
Men: 35 N
Activation force <1 N
Mean force: 0.8-0.9 N
Peak force: 1.8-3.3 N
Weight Discrimination
0
0.04
0.08
0.12
0.16
Active lifting
Reflexlifting
Staticposition
Handpassive
Web
er fr
actio
n
Rapid motion
Fatigue and Perceived Force
0
20
40
60
80
100
0 50 100 150 200 250 300
Time (s)
Per
ceiv
ed fo
rce
(%M
VC
)
0
3
6
9
12
15
Force control - Index finger flexor
2 4 6
Force (N)
Coe
ffic
ien
t of
var
iati
on (
%)
Haptic
Visual and haptic
1
10
100
1000
1 10 100 1000
Dif
fere
nti
al t
hre
shol
d (
N.s
/m)
Reference Viscosity (N.s/m)
Viscosity Discrimination
Discriminability Parameters
Variable Weber fraction ResolutionAmplitude of vibrotactile 25% 0.1m stimulation (20-300 Hz)Frequency of vibrotactile11% 0.4 Hz stimulation (5-200 Hz)Pressure on skin 12% 2 gm/mm2
Force 7% 30 mN
Stiffness/compliance 17%
Viscosity 19%
Temperature (cold) 2% 0.02oC
Discriminability Parameters
Variable Weber fraction Range
Limb position 7% 5-9%
Limb movement 8% 4-19%
Force 7% 5-12%
Stiffness/compliance 17% 7-26%
Viscosity 19% 10-29%
0
5
10
15
20
25
30
35
40
1 100
SD
Pos
itio
n (
mm
)
Reference Viscosity (N.s/m)
Movement Amplitudes
low
high
0
20
40
60
80
100
120
1 100
SD
Vel
ocit
y (m
m/s
)
Reference Viscosity (N.s/m)
Movement Velocities
low
high
Training for the haptically inept
FeedbackOptimal motor strategyImprove discrimination but not detection
Interface Mechanical Properties and Human Operator Performance
150
200
250
300
0 500 1000 1500 2000 2500
Interface Stiffness (N/m)
Del
ay (
ms)
Human Operator Performance
0
0.2
0.4
0.6
0.8
1
0 500 1000 1500 2000 2500
Interface stiffness (N/m)
Gai
n
Human Operator Performance
450
500
550
600
650
700
750
10 1000
Interface Viscosity (N.s/m)
Tim
e to
0.5
pt o
n St
ep
Res
pons
e Fu
nctio
n
Thermal Interfaces and Perception
Thermoreceptor range: 13-45o C Resolution: 0.02-0.05o C (transient) Thermal discriminability: 2% Neutral thermal sensation: 31-36o C Skin temperature correlates well with
perception of cold not warmth Thermoreceptors are NOT thermometers
Thermal thresholds
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10 12 14
Area (cm sq)
Ele
vati
on
in
sk
in t
emp
eratu
re (
deg
C)
Thermal Interfaces
Disadvantages Slow response Poor localization (improves with increases
in temperature – 40° C) Prodigious capacity for adaptation and
summation (intensity and areal extent traded)But could be analogous to color in visual
displays.
Illusions – can they be used to trick the haptic
system?
Size-weight illusions – visual and haptic phenomenon
Saltation effect Use auditory cues to enhance haptic
perception
Size-weight Illusion
0
5
10
15
20
0 200 400 600 800
Weight (g)
Perc
eive
d M
agni
tude
LARGE
SMALL
1.4
1.8
2.2
2.6
3
100Volume (cc)
350 gm
904 gm
Visual Haptic
(Masin & Crestoni, 1988) (Ellis & Lederman, 1993)
Saltation effect
At short and long time intervals (20-30 and > 200 ms) P1 will appear near to or coincident with P2
Warning stimulus (P1) 1 s prior to two taps on skin. Present a stimulus at one location (P2 red) and a second at another (P3 blue).
As time disparity increases above 50 ms the first stimulus travels back to its veridical locus
P1/P2 locus P3 locus
150 ms
110 ms
75 ms
< 25 ms
100 mm