hui2007 2 tactile print

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Haptic User Interfaces Fall 2007

J ukka Raisamo 1

TACTILE SENSING & FEEDBACK

Jukka Raisamo

Multimodal Interaction Research Group

Tampere Unit for Computer-Human Interaction

Department of Computer Sciences

University of Tampere, Finland

Contents

Tactile sensing in detail

Tactile feedback

Feedback technologies & displays

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Tactile sensing

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Tactile sensing

Theres two different types of receptorsresponsible for tactile sensing found in the skin

free nerve endings

encapsulated nerve endings, i.e., mechanoreceptors

Most tactile information is delivered viamechanoreceptors but, e.g., hair receptors alsoaffect the sensations

Bent hair

RAreceptor

Indentedskin

RAreceptor

Indentedskin

Sustainedpressure

SA receptor

Skin

Bent hair

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Mechanoreceptors 1/3

Mechanoreceptors are sensitive to mechanicalpressure or deformation of the skin

four types: Meissners corpuscles, Pacinian corpuscles,Merkels disks and Ruffini endings

differ in size, receptive fields, rate of adaptation,location in the skin, and physiological properties

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Thresholds of different receptors overlap

perceptual qualities of touch are determined bythe combined inputs from different types ofreceptors

operating range for the perception of vibrationabout 0.04 to 500 Hz

frequencies over 500 Hz are felt more astextures, not vibration

skin surface temperature affects perceivingtactile sensations

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Mechanoreceptors are generally specialized tocertain stimuli

contact forces are detected by Merkels discs and Ruffiniendings

vibration primarily stimulates the Meissners corpusclesand Pacinian corpuscles

>20 mm

35 mm

>10 mm

23 mm

Receptivefield

Deep

High

Deep

High

Location

Unlocalized high frequencyvibration; tool use

80400 HzPC (RA-II)Paciniancorpuscles

Local skin deformation, lowfrequency vibratory sensations

1060 HzRA-IMeissnerscorpuscles

Directional skin stretch,tension

015 HzSA-IIRuffiniendings

Pressure; edges and intensity030 HzSA-IMerkelsdisks

FunctionStimulusfrequencyRate ofadaptationReceptor

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Hairy vs. hairless skin

Hairy skin is generally less sensitive tovibration compared to glabrous skin

there seems to be no PC receptors in the hairy

skin, however, they are present in the deeperunderlying tissue surrounding joints and bones

Hairy skin has poorer absolute threshold forboth vibration & pressure

still about the same capacity for discriminatingvibrotactile frequencies

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Tactile dimensions

Tactile acuity (vibration & pressure)

Spatial acuity

Temporal acuity

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Tactile acuity for vibration

Vibration primarily stimulates the Paciniancorpuscles and Meissners corpuscles

pacinian channel (high frequency, from about 60Hz)

non-pacinian channel (low frequency, below 60Hz)

Human thresholds for detecting vibration:

sensitivity for mechanical vibration increasesabove 100 Hz and decreases above 320 Hz (250

Hz being optimum)

The spatial acuity and pattern perception isbetter for skin deformation compared tovibrotactile stimuli

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Tactile acuity for pressure

Sensitivity for pressure is largely dependanton the area of stimulation

discrimination has higher resolution at those partsof the body with a low threshold (e.g. fingertips)

Discrimination is not constant throughout theentire intensity scale, as with vision andauditory senses

amplitude indentation discrimination is low at lowintensities

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Tactile acuity

Threshold responses for

pressure (bars) and

vibration (dots) for 15body sites

human body is highlysensitive for vibration

thresholds correlate with

the density of cutaneousmechanoreceptors

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Deterioration of tactile acuity

There appears to be no significantreduction in vibrotactile detection at thefingertips in older subjects.

reflects either the high receptor density of the

area, or the functional importance of

vibrotactile sensibility of the fingertips (orsome combination of both of these factors)

Pressure sensitivity reduces as a function ofage

Training can be used to improve sensimotorperformance

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Spatial acuity 1/3

Fingertips are the most sensitive part of thehuman hand in texture & vibrotactile perception corresponding to the largest density of PC receptors

the more spatially distant two stimuli are, the moredifficult it is to discriminate them

Tactile texture perception is mediated byvibrational cues for fine textures, and by spatialcues for coarse textures

discrimination of spatial information is considerably moreaccurate than their temporal interval

when using hand, exploration of spatially varying surfacesis done with the entire fingertip (increased sensitivity byactive touch)

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Spatial acuity 2/3

Threshold = the point at which an effect begins tobe produced

detection threshold (the smallest detectable level ofstimulus; a.k.a absolute threshold)

difference threshold (the smallest detectable differencebetween stimuli; a.k.a just noticeable difference (jnd))

Successful method to reduce the detectionthreshold is either to increase the duration of the

tactile stimulation, or the interval of twoconsecutive stimuli

Why do people do better with gratings than two-point discrimination? active vs. passive touch

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Spatial acuity 3/3

Spatial dimension for touch

2-point discrimination (1 mm at fingertips, 30-70 mm in the back)

localization

texture detection (depends on the surface)

grating discrimination (detectable distancebetween two gratings)

pressure sensitivity

temporal discrimination

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Spatial acuity for pressure 1/2

Spatial acuity for

two-point thresholds

(bars) and errors of

localization (dots) for14 body sites

smallest resolution infacial area & hands

differences due to

both task demands &neural activity

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Spatial acuity for pressure 2/2

Variation in pressure

threshold over thebody

smallest in facial area

fingers have about

the same acuity astrunk

the right side seems

to have slightly betteracuity on average

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Temporal acuity

Resolution of temporal frequencydiscrimination is finer at lower frequencies

Thresholds for tactile sensations are

lowered with increased duration andinterval

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Thermotactile interactions

Eventhough being separate modalities,

temperature and touch have interactions thermal adaptation

cooling degrades tactile sensitivity

warming sometimes enhances

thermal intensification

cold objects feel heavier

warm objects feel heavier but less than cold ones

thermal sharpening

the warmer or colder the two points are, the easier theyare to discriminate

Thermal cues are very important in theidentification of textures

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Touch is not an absolute sense

Several factors affect the sensitivity

age

individual differences, habits

attention, fatigue, mood, stress

diseases, disabilities

training

...

scalability is important factor for tactileinterfaces

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Tactile feedback technologies

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Methods for tactile stimulation

Types of skin sensory stimulation:

skin deformation

vibration

electric stimulation

skin stretch

friction (micro skin-stretch)

heat

Possible actuator configurations:

single element

multiple elements (array)

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Tactile actuators

Some technologies used in tactileinterfaces

vibrating motors

linear motors

solenoids

piezoelectric actuators

pneumatic systems

shape memory alloys

electrorheological fluids

thermoelectric elements

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Actuators: vibrating motors

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Vibrating motors

How they work:

provides relatively small-amplitude vibration(linear or rotary)

applies motion either directly to the skin orthrough mediating structure

used singly or in arrays

Most common types

DC-motors with eccentric rotating mass

voice coils

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Vibrating motors: eccentric rotating mass

DC-motor rotates an off-center spinning mass

inexpensive & exsistingtechnology

poor resolution: it takestime to start and stop

Frequency control only(amplitude = freq2)

amplitude fixed by thesize & the weight of therotating mass

Used in various devices

mobile phones, pagers,gaming devices, etc.

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Vibrating motors: voice coils

Voice coil basics

current driven through themovable coil

created magnetic field

interacts with the field of the

permanent magnet (one-waymovement)

vibrations created byswitching the current on/off

Both frequency and amplitudecan be controlled somewhatindependently

however, the motor has

always peak at certainfrequencies (e.g. 250 Hz)

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Vibrating motors: overview

Advantages:

simple, existing technology

relatively inexpensive

easily powered and controlled

quite small power consumption

Disadvantages:

not very expressive feedback

vibration can be irritating

sometimes hard to miniaturize efficiently

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Example: vibrotactile devices

Logitech iFeel mouse &

Kensington Orbit 3Dtrackball

These ones use the

TouchSense technology

by Immersion Corporation(http://www.immersion.com)

Have a small rotating DC-

motor inside the device

which applies the

vibration through thestructure

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Actuators: linear motors

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Linear motors: pin displays

How they work:

pins in an array are actuated independently

the actuated pins contact the surface of the skin

Advantages:

simple, readily available

continuously positionable

versatile: static pressure, vibration; shapes or forcedisplay

relatively fast

Disadvantages:

very difficult to pack tightly

relatively high cost (lots of motors/device)

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Example: tactile array

Mimics complex tactilesensations

Exeter arrays stimulate the fingertips

each pin haspiezoelectric actuator

Array 1: 100 pins over 1cm2, frequency range25-400 Hz

Array 2: 24 pins with 2mm spacing, 25-500Hz

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Example: tactile arrays in a mouse

Allows the user to scan the ofan image

the pins rise and falldynamically delivering atactile stimuli to thefingertips

can be used to code patternsand colours into tactile data

VTMouse (2001) three 4x8 matrix (32 pins)put in the place of thebuttons

VTPlayer (2003) two 4x4 matrix with 16 pins

(http://www.virtouch2.com/)

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Actuators: solenoids

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Solenoids

Multi-modal mouse by

Akamatsu & MacKenzie(1996)

solenoid driven pin

under the right index

finger that rises andfalls

Haptic Pen by Lee etal. (2004)

solenoid shakes the pen

by moving up and downin top of the pen

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Example: solenoids in Braille displays

Braille = tactilelanguage for sensorysubstitution

Traditionally Braille

displays use solenoids

to push up the pins

(nowadays mostly

piezoelectric actuatorsare used)

Solenoids have poorpower consumption

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Actuators: piezoelectric actuators

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Piezoelectric actuators 1/2

How they work:

single or multilayer ceramic elements

an element expands/bends when voltage isapplied

multiple layers can be used to amplify the effect

Properties: very large forces but small motions

one element typically around 0.2-1.0 mm thick

resolution for frequencies ~0.01 Hz

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Piezoelectric actuators 2/2

Electromechanical device

that converts electrical

energy into mechanicalmotion

Typically very compact

as only few components

are used in a completesystem

actuator itself can be verysmall

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Example: STReSS & Virtual Braille Display

2D tactile display with

an array of miniatureactuators

stimulate the fingertip atabout 1 cm2 in area

elements can be bendedin two directions to

increase the forcesapplied to the fingertip

(Hayward et al.)

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Example: Tactile Handheld MiniatureBidirectional (THMB)

THMB is an improved

version of VBD miniaturizedto fit inside a PDA-size case

The handheld device

comprises an LCD screen

that allows combiningtactile and visual feedback

THMB stimulates the user's

thumb and is mounted on a

vertical slider so that it can

be dragged up and down

along the left side of thecase

(http://www.laterotactile.com/)41


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Piezoelectric actuators: overview

Advantages: small in size

potentially inexpensive in large volumes

high frequency and static modes

very fast response time

low power consumption

Disadvantages:

dynamics: small displacements require accurateamplification

high voltage

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Actuators: pneumatic systems

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Pneumatic systems

Two possible output modes based on skinindentation (and vibration)

suction

air-pressure

How it works:

technologies: fillable air-pockets, air jets,suction holes

vibratory rates: typically 20-300 Hz

static pressure with sealed pockets

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Pneumatic systems: suction

Draws air from a suction

hole creating an illusionthat the skin is pushed

Very low spatial resolution

(only appropriate for thepalm)

two basic patterns ofstimulation (large holes andsmall holes)

Need for regulation of air

pressure (=lots ofequipment)

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DataGlove bandwidth of 5 Hz, amplitude & frequency modulated

Teletact II 29+1 air pockets (40 tubes to control the air-pressure)

object slippage (fingers) + force feedback (palm)

Pneumatic systems: air-pressure

DataGlove with pneumatics

(Sato et al., 1991)

Teletact II (Stone, 1992)

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Pneumatic systems: overview Advantages:

tubing make it possibly to take the bulky part away frompoint of application

pressure can be more appropriate for some applicationsthan pins or vibrating motors

can mimic skin-slip (with multiple adjacent inflatedpockets)

Disadvantages:

requires bulky parts (air compressor or motor-drivenpistons)

not really portable

can be very noisy

difficult to display sharp edges or discontinuities

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Actuators: shape-memory alloys

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Shape-memory alloys

Metals that "remembers" their geometry restores its original geometry when heated

usually temperature change of about 10C is

necessary to initiate the phase change

How it works:

expands (and heats up) when current runsthrough it

contracts when cools down

stimulates the skin when vibrates (expand-contract cycles)

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Shape memory alloys

Wearable Tactile Displays (MIT Touchlab)

Tactile Display based onShape Memory Alloy

Tactile Display based onElastomer Actuators

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Actuators: electrorheological fluids

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Electrorheological fluids

Liquid which viscosity changes

into semi-solid when electriccurrent is applied

change in viscosity feels asmore resistive surface

usually packed in 2-3mmbubbles

can change from liquid to gel,and back, within milliseconds

The change in viscosity is

proportional to the appliedcurrent

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Electrorheological fluids: overview

Advantages: low power consumption

no moving parts

controlled electrically

very compact

performance improves as size decreases

Disadvantages: high voltage required

cant control force, only viscosity

sharp edges and discontinuities difficult torender

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Tactile displays: skin stretch

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Skin-stretch

Two main methods: rotational skin stretch

lateral skin stretch

What happens:

forces are applied to skinfor displacement

contact forces are

perceived as stretching ofthe skin

Applying skin stretch is

being investigated as an

alternative method tovibrotactile feedback

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Friction: skin-slip display

Micro skin-stretch

motor driven smoothcylinder strapped againstfinger

when rotates, stimulates themechanoreceptors

Felt as a sensation of slip

grasp simulations: causesthe user to increase gripforce

often used to append forcefeedback displays

(Chen and Marcus, 1994)

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Tactile displays: electrotactilestimulation

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Electrotactile stimulation

Electrical stimulation is not widelyaccepted to consumer use

often sudden bursts give an "invasive"impression

square waves can be easily felt as too strongstimuli and they keep tickling the nerves

the sensitivity to electrical stimulation varies

greatly between and within individuals (e.g.,sweating & pressure affect the sensation)

Used mostly in research prototypes and forrehabilitation purposes

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Example: SmartTouch

Tactile display to presentrealistic skin sensation

a thin electrotactile display anda sensor mounted

Two layers top layer: 4x4 array of

stimulating electrodes

bottom layer: optical sensors

Visual information is captured bythe sensors and displayed throughelectrical stimulation e.g. the black stripes are

perceived as bumps

(http://www.star.t.u-tokyo.ac.jp/projects/smarttouch/)59


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Example: Electric mouse

Array of small electrodesplaced to fit fingertip

64 electrodes, 1mm indiameter

Pressure sensor located

under the electrodes tomeasure finger pressure.

electrical current is

controlled as a function ofpressure

creates more stable

vibratory sensations

compared to traditionaldisplays

(http://www.star.t.u-tokyo.ac.jp/projects/tactile-display/ )60

Example: Bioforce by Mad Catz (2001)

A game controller that

delivers mild cramps tothe user

An electrical shock isdelivered by wired padsattached to the forearm

3x1.5V batteries provide 16mA shocks

similar to the shocks used

for years by physicaltherapists

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hui2007 2 tactile print

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