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Perceptual Issues in Haptic Interface Design
Lynette Jones,Department of Mechanical Engineering, Massachusetts Institute of Technology.
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Overview• Tactile, thermal and kinesthetic sensors• What can we sense?• Field of view – is more better?• Haptic interface configuration
- performance and perception• Tactile illusions and inter-sensory
interactions – perceptual enhancement?• Conclusions
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7 classes of Mechanoreceptor17,000 – glabrous skin of hand
2 classes of Thermoreceptorwarm and cold receptors
4 classes of Nociceptorsmechanical, thermal, chemical, polymodal
3 classes of Proprioceptormuscle spindle receptors (Group Ia and Group II)Golgi tendon organs
Information Channels in Human Hand
(Visual Encyclopedia, 1999)
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Tactile ReceptorsMechanoreceptors ModalityMeissner corpuscle Stroking, fluttering
Merkel disk receptor Pressure, texture
Pacinian corpuscle Vibration
Ruffini ending Skin stretch
Hair tylotrich, hair-guard Stroking, fluttering
Hair-down Light stroking
Field Skin stretch(Johansson & Vallbo, 1983)
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Thermal Sensors
ObjectThermal
interaction
Thermal perceptionThermal
sensation
Change in skin temperature
• Thermoreceptor range: 13-45 °C• Cold receptors respond to decreases in temperature, warm
receptors to increases in temperature• Resolution: 0.02-0.05 °C (transient)• Thermal discriminability: 2%• Neutral thermal sensation: 31-36 °C• Thermoreceptors are NOT thermometers
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Proprioceptors
• Muscle spindles– Afferent neurons
• Ia (primary)• II (secondary)
– Efferent neurons• γ-static motor
neurons• γ-dynamic motor
neurons– An actively
controlled sensor• Golgi tendon organs
– Afferent neurons• Ib
(www.blackwellpublishing.com)
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Why an active sensor?1. Intrafusal muscle fibers keep the spindle in its sensitive range2. Muscle spindle efferents also change the sensor gain
(http://fig.cox.miami.edu/~lfarmer/BIL265/locomotion.html)
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What do we sense? HapticTactile
• Surface texture- Roughness- Hard/soft
• Orientation• Shape• Size• Pressure• Curvature• Motion
• Mass/weight• Force• Stiffness/compliance• Viscosity
Thermal
• Cooling• Warming
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Tactile Spatial Acuity
(Johnson & Phillips, 1981)
Grating-orientation discrimination thresholds
(Revised from Weinstein, 1968) (Vega-Bermudez & Johnson, 2001)
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Tactile Spatial Acuity
On smooth glass surface, dot of height 1-3 µm and diameter of 550 µm can be detected by the fingertip
Spatial frequency of grating: fingertip can distinguish 40-50 µm in spatial period of 0.7-1.0 mm
35 mm
(Eye – 1 min arc, ear – 1°)(Morley et al., 1983) (Johansson & LaMotte, 1983)
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Tactile Temporal Acuity
Successiveness Two stimuli (1 ms duration) must be separated by 5.5 ms to be perceived as two at a single locus
Temporal orderTwo successive stimuli at separate sites must be separated by 20 ms in order to determine which site first
20 ms
5 ms
(Ear - 0.01 ms, eye -25 ms)
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Vibrotactile stimuli• Common in tactile
and haptic displays• Convey tactile
sensations in VE and teleoperation
• Medium of communication
BrailleNote BT
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Vibrotactile stimuli• Frequency • Amplitude• Waveform• Locus - Localization (12 sites on torso),
Optacon (24 by 6 pin array)
• Duration- Burst duration, pulse repetition
rate, number of pulses
Frequency discrimination
Amplitude discrimination
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Vibration• Frequency
Range: 0.4-1000 HzDifference Threshold: 4-50%
• AmplitudeThreshold: 0.01µm at 200 HzDifference Threshold:11%BUT
Frequency and amplitudeinteract (Brisben et al., 1999)
0.01
0.1
1
1 10 100 1000
Frequency (Hz)
Diff
eren
ce th
resh
old
(∆F/
F)
finger
forearm
hand
finger
-5
-4
-3
-2
-1
0
1
20 30 40 50 60 70
PSE
(db
re: s
tand
ard)
Comparison frequency (Hz)
Standard 25 Hz
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Vibration ThresholdsGlabrous skin
0.01
0.1
1
10
100
1000
1 10 100 1000
Disp
lace
men
t (µ
m)
Frequency (Hz)
Hairy skin
(Bolanowski et al., 1994)(Brisben et al., 1999)
DorsalPalmar
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Field of view: Spatial and temporal summation
• Vibrotactile sensitivity- Increases with contact area on hand (above
40 Hz)- Increases with stimulus duration or number of
stimuli in a sequence• Perceived roughness- Increases when surface explored with two
fingers as compared to one• Perceived heaviness- Increases when adjacent fingers lift weights
simultaneously• Haptic object recognition- Whole hand is faster and more accurate than
manual exploration with a single finger
BUTTactile processing of spatial charactersNo improvement when characters are displayed across two adjacent fingers
(Brisben et al., 1999)
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1
10
100
1000
1 10 100 1000
Diff
eren
tial t
hres
hold
(N.s/
m)
Reference Viscosity (N.s/m)
19%
0.01
0.1
1
10
0.1 10 1000
Force (N)
Diff
eren
tial t
hres
hold
(N)
6%
Force
ViscosityHaptic Thresholds
Stiffness
0
200
400
600
800
1000
0 900 1800 2700 3600
Stiffness (N/m)
Diff
eren
tial t
hres
hold
(N/m
)
17%
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Mechanics of Contact and PerceptionMechanical properties of interface:
mass, stiffness, viscosity.Workspace
Joystick/Mouse/KnobStylus/ProbeThimble
PHANTOM™ PHANTOM™
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Interface Design: Control and SensingWhat is optimal?
Keyboard - activation force - 1 N - tactile feedback - 0.04-0.25 N Joystick- thumb control- grasp control
StylusThimbleTrackball - force discrimination
Glove- position resolution
Exoskeleton
Pantograph
Cyberforce™Immersion Corp.
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Performance Issues in Haptic Displays
150
200
250
300
0 500 1000 1500 2000 2500
Interface Stiffness (N/m)
Del
ay (m
s)
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
Mechanical properties of the display (mass, stiffness, viscosity)Limb mechanical properties and magnitude of changes during tasksDelays associated with processing – models of neuromuscular, sensory, and cognitive systems
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Thermal sensing•Thermal cues are useful for identifying objects haptically
– Thermal characteristics of the object– Thermal sensation associated with contact
Bar
rier
Haptic display
Thermal Feedback
Virtual objectObject at remote place
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Thermal Model
ObjectThermal
interaction
Thermal perceptionThermal
sensation
Change in skin temperature
T
t
t
Material Skin
Tskin,i
material, i T
2/12/1
2/1,
2/1,
)()()()(
skinmaterial
skiniskinmaterialimaterials ckck
ckTckTT
ρρρρ
++
=
2/1,"
)()(
tTTk
qskin
iskinsskinskin πα
−=
q”
Ts
Heat transfer process
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Thermal Display
Thermistor 1
Thermistor 2
A/D
D/ABipolar Power Supply
&Voltage AmplifierPI Control
Temperature Curve
20
25
30
35
40
0 2 4 6 8 10 12Time (s)
Tem
pera
ture
(C) Peltier device
Index finger
Thermistor 1
Thermistor 2
A/D
D/ABipolar Power Supply
&Voltage AmplifierPI Control
20
25
30
35
40
0 2 4 6 8 10 12Time (s)
Peltier device
Index finger
Real Simulated
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0
0.2
0.4
0.6
0.8
1
Copper SS Granite ABS Foam
Material
Prop
ortio
n of
cor
rect
resp
onse
s
RealSimulated
Threshold for reliable identification
0
0.2
0.4
0.6
0.8
1
Copper-SS Copper-ABS SS-ABS
Material Combination
Prop
ortio
n of
Cor
rect
Res
pons
e
Threshold for discrimination
R-R
R-S
Thermal sensing: Real vs simulated
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Model Evaluation: Physiological Results
Changes in skin temperature during contact
0
1
2
3
4
5
6
7
0.00 4.00 8.00 12.00
Contact pressure (kPa)
Dec
reas
e in
ski
nte
mpe
ratu
re d
urin
gco
ntac
t (o C
)
Measured dataModel prediction
5.9 kPa 11 kPa
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Thermal Field of View
0
0.2
0.4
0.6
0.8
1
Urethan
e
Woo
d
Glass SS
Copper
Prop
ortio
n of
cor
rect
res
pons
es
Improvement in identifying materials when cues are presented to three fingers but no further improvement with five fingers.
13
5
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Tactile Illusions:Space-time interactions
Saltation
(Geldard, 1975)
Illusory displacement of tactile stimuli
(Cholewiak & Collins, 2000)
(Geldard & Sherrick, 1972)
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Haptic Illusions
0
5
10
15
20
0 200 400 600 800
Weight (g)
Perc
eive
d W
eigh
t
LARGE
SMALL
1.4
1.8
2.2
2.6
3
100 10000Volume (cc)
350 gm
904 gm
Size-weight illusion - visual(Masin & Crestoni, 1988)
Texture-force interaction(Flangan et al., 1995)
Size-weight illusion - haptic(Ellis & Lederman, 1993)
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Thermal Illusions
Thermal referral Temperature-force illusions(Stevens & Hopper, 1982)
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Conclusions• Properties of the somatosensory
system provide a framework for the design of tactile and haptic interfaces
• Psychophysical studies have revealed how humans respond to changes in mechanical and thermal stimuli, and the interactions that occur between these stimuli
• Design of interfaces –prehension, gesture, single digit? What is optimal?