Haptic User Interfaces Fall Contents TACTILE SENSING & FEEDBACK. Tactile sensing. Tactile sensing. Mechanoreceptors 2/3. Mechanoreceptors 1/3

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1 Contents TACTILE SENSING & FEEDBACK Jukka Raisamo Multimodal Interaction Research Group Tampere Unit for Computer Human Interaction Department of Computer Sciences University of Tampere, Finland Tactile sensing in detail Tactile feedback Feedback technologies & displays 1 Tactile sensing Tactile sensing There s two different types of receptors responsible for tactile sensing found in the skin free nerve endings encapsulated nerve endings (i.e., mechanoreceptors) Most tactile information is delivered via mechanoreceptors but, e.g., hair receptors also affect the sensations Bent hair Skin Indented skin Bent hair RA receptor Indented skin RA receptor Sustained pressure SA receptor Remember from Lecture 1: RA = rapidly adapting SA = slowly adapting 2 3 Mechanoreceptors 1/3 Mechanoreceptors 2/3 Mechanoreceptors are sensitive to mechanical pressure or deformation of the skin differ in size, receptive fields, rate of adaptation, location in the skin, and physiological properties four types: Meissner s corpuscles, Pacinian corpuscles, Merkel s disks and Ruffini endings Thresholds of different receptors overlap in the brains the sensation is determined by the combined inputs from different types of receptors operating range for the perception of vibration about 0.04 to 500 Hz (for hearing about Hz) frequencies over 500 Hz are felt more as textures, not vibration skin surface temperature affects perceiving tactile sensations (inhibites or excites individual receptors) 4 5 Jukka Raisamo 1

2 Mechanoreceptors 3/3 Hairy vs. hairless skin Receptor Merkel s disks Ruffini endings Meissner s corpuscles Pacinian corpuscles Rate of Location Receptive adaptation field SA I Shallow 2 3 mm SA II RA I PC (RA II) Deep Shallow Deep >10 mm 3 5 mm >20 mm Stimulus frequency 0 30 Hz Function Pressure; edges and intensity 0 15 Hz Directional skin stretch, tension Hz Local skin deformation, low frequency vibratory sensations Hz Unlocalized high frequency vibration; tool use Mechanoreceptors are generally specialized to certain stimuli contact forces are detected by Merkel s discs and Ruffini endings vibration primarily stimulates the Meissner s corpuscles and Pacinian corpuscles 6 Hairy skin is generally less sensitive to vibration compared to glabrous skin there seems to be no Pacinian receptors in the hairy skin (however, they are present in the deeper underlying tissue surrounding joints and bones) Hairy skin is poorer to detect both vibration & pressure 7 yet has about the same capacity for discriminating vibrotactile frequencies Tactile dimensions Tactile acuity (vibration & pressure) Spatial acuity Temporal acuity About thresholds Threshold = the point at which an effect is consciously experienced detection threshold (the smallest detectable level of stimulus; a.k.a. absolute threshold) difference threshold (the smallest detectable difference between stimuli; a.k.a. just noticeable difference (JND)) To reduce the detection threshold: increase the duration of the tactile stimulation increase the area of stimulation increase the temporal interval of two consecutive stimuli (within certain limits) 8 9 Tactile acuity for vibration Vibration primarily stimulates Pacinian corpuscles and Meissner s corpuscles pacinian channel (high frequency, from about 60Hz) non pacinian channel (low frequency, below 60Hz) Human sensitivity for vibration: sensitivity for mechanical vibration increases above 100 Hz and decreases above 320 Hz (250 Hz said to be the optimum) An absolute threshold of 0.2 m (2/1000 of a millimetre) in amplitude has been reported on the palm of a hand for 250 Hz vibration Tactile acuity for pressure Pressure primarily stimulates the Merkel s disks Sensitivity for pressure is largely dependant on the location of stimulation discrimination has higher resolution at those parts of the body with a low threshold (e.g., fingertips) The face is being reported to have the smallest detection threshold of about 5 mg (5/1000 of a gram) in weight (equals dropping a wing of a fly from 3 cm onto the skin) Jukka Raisamo 2

3 Tactile acuity Age and tactile acuity Threshold responses for pressure (bars) and 200 Hz vibration (dots) for 15 body sites Noteworthy: human body is highly sensitive for vibration vibration thresholds correlate with the density of cutaneous mechanoreceptors There appears to be no significant reduction in vibrotactile detection at the fingertips in older subjects. Pressure sensitivity reduces as a function of age Training can be used to improve sensimotor performance Spatial acuity 1/2 Spatial acuity 2/2 Fingertips are the most sensitive part of the human hand in texture & vibrotactile perception reported to have the largest density of PC receptors the more there is spatial distance between two stimuli, the more difficult it is to discriminate them Tactile texture perception is mediated more by vibrational cues for fine textures, and by spatial cues for coarse textures discrimination of spatial interval is considerably more accurate than temporal interval when using hand, exploration of spatially varying surfaces is done with larger area of skin (increased sensitivity by active touch) Spatial dimensions for touch two point discrimination (two simultaneous points of stimulation) point localization (two consecutive point of stimulation) grating discrimination (detectable difference between two gratings) Why do people do better with gratings than two point discrimination? active vs. passive touch Spatial acuity for pressure 1/2 Spatial acuity for pressure 2/2 16 Spatial thresholds (in mm) for two point discrimination (bars) and point localization (dots) for 14 body sites Noteworthy: smallest threshold in facial area & hands threshold for point localization lower throughout the body 17 Pressure thresholds (in mg) for two point discrimination applied on the left (dots) and right (bars) side of the body Noteworthy: no major difference between the sides of the body smallest in facial area, fingers have about the same acuity as trunk Jukka Raisamo 3

4 Temporal acuity Temporal resolution for touch with two successive stimuli is about 5 ms To compare: for audition: about 0.01 ms for clicks for vision: about 25 ms Resolution for tactile numerosity is reported to lie between 1 to 5 pulses (with intervals of 20 and 100 ms) at brief intervals two pulses presented to the same location may mask one another 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 they are to discriminate Thermal cues are very important in the identification of objects Touch is not an absolute sense Several factors affect the sensitivity age sex individual differences attention, fatigue, mood, stress diseases, disabilities training... Tactile feedback technologies scalability is important factor for tactile interfaces Methods for tactile stimulation The methods include: skin deformation vibration electric stimulation skin stretch friction (micro skin stretch) temperature Tactile actuators There are several different technologies used in tactile interfaces vibrating motors linear motors solenoids piezoelectric actuators pneumatic systems...whatever causes an effect can be used Possible actuator configurations single element multiple elements (an array/matrix) Jukka Raisamo 4

5 Vibrating motors Actuators: vibrating motors How they work: provides relatively small amplitude vibration (linear or rotary) applies motion either directly to the skin or through mediating structure used singly or in arrays Most common types DC motors with eccentric rotating mass voice coils Vibrating motors: eccentric rotating mass Vibrating motors: voice coils DC motor rotates an offcenter spinning mass inexpensive & exsisting technology poor resolution: it takes time to start and stop Frequency control only (amplitude ~= freq 2 ) amplitude fixed by the size & the weight of the rotating mass and the speed of rotation Used in various devices mobile phones, pagers, gaming devices, etc. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Voice coil basics current driven through the movable coil created magnetic field interacts with the field of the permanent magnet (one way movement) vibrations created by switching the current on/off Both frequency and amplitude can be controlled somewhat independently however, the motor has always a peak at certain frequencies (e.g., 250 Hz) The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again 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 Actuators: linear motors Jukka Raisamo 5

6 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 force display relatively fast Disadvantages: very difficult to pack tightly relatively high cost (lots of motors/device) Example: tactile array Mimics complex tactile sensations stimulate the fingertips each pin has piezoelectric actuator Array 1: 100 pins over 1 cm 2, frequency range Hz Array 2: 24 pins with 2 mm spacing, Hz Example: Braille displays Example: tactile arrays in a mouse Braille = tactile language for sensory substitution Traditionally Braille displays used solenoids to push up the pins (nowadays mostly piezoelectric actuators are used) 32 ( Allows the user to scan the of an image the pins rise and fall dynamically delivering a tactile stimuli to the fingertips can be used to code patterns and colours into tactile data VTMouse (2001) three 4x8 matrix (32 pins) put in the place of the buttons VTPlayer (2003) two 4x4 matrix with 16 pins 33 Solenoids Actuators: solenoids Multi modal mouse by Akamatsu & MacKenzie (1996) solenoid driven pin under the left index finger that moves up & down to generate vibration Haptic Pen by Lee et al. (2004) solenoid shakes the pen by moving up and down at top of the pen Jukka Raisamo 6

7 Piezoelectric actuators 1/2 Actuators: piezoelectric actuators How they work: single or multilayer ceramic elements an element expands/bends when voltage is applied multiple layers can be used to amplify the effect Properties: very large forces but small motions one element typically around mm thick resolution for frequencies ~0.01 Hz Piezoelectric actuators 2/2 Example: STReSS & Virtual Braille Display Electromechanical device that converts electrical energy into mechanical motion Typically very compact as only few components are used in a complete system actuator itself can be very small 2D tactile display with an array of miniature actuators stimulate the fingertip at about 1 cm 2 in area elements can be bended in two directions to increase the forces applied to the fingertip ( Example: Tactile Handheld Miniature Bidirectional (THMB) ( THMB is an improved version of VBD miniaturized to fit inside a PDA size case The handheld device comprises an LCD screen that allows combining tactile 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 the case 40 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 accurate amplification high driving voltage 41 Jukka Raisamo 7

8 Pneumatic systems Actuators: pneumatic systems Two possible output modes based on skin indentation (and vibration) suction air pressure How it works: technologies: fillable air pockets, air jets, suction holes vibratory rates: typically Hz static pressure with sealed pockets Pneumatic systems: suction Pneumatic systems: air pressure Draws air from a suction hole creating an illusion that the skin is pushed Very low spatial resolution (only appropriate for the palm) two basic patterns of stimulation (large holes and small holes) Need for regulation of air pressure (=lots of equipment) 44 DataGlove with pneumatics (Sato et al., 1991) Teletact II (Stone, 1992) 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) 45 Pneumatic systems: overview Advantages: tubing make it possibly to take the bulky part away from point of application pressure can be more appropriate for some applications than pins or vibrating motors can mimic skin slip (with multiple adjacent inflated pockets) Disadvantages: requires bulky parts (air compressor or motor driven pistons) not really portable can be very noisy difficult to display sharp edges or discontinuities Actuators: shape memory alloys Jukka Raisamo 8

9 Shape memory alloys Shape memory alloys Metals that "remembers" their geometry restores its original geometry when heated usually temperature change of about 10 C is necessary to initiate the phase change How it works: expands (and heats up) when current runs through it contracts when cools down stimulates the skin when vibrates (expandcontract cycles) Wearable Tactile Displays (MIT Touchlab) Tactile Display based on Elastomer Actuators Tactile Display based on Shape Memory Alloy Skin stretch Tactile displays: skin stretch 50 Two main methods: rotational skin stretch lateral skin stretch What happens: forces are applied to skin for displacement contact forces are perceived as stretching of the skin Applying skin stretch is being investigated as an alternative method to vibrotactile feedback 51 Friction: skin slip display (Chen and Marcus, 1994) Micro skin stretch motor driven smooth cylinder strapped against finger when rotates, stimulates the mechanoreceptors Felt as a sensation of slip grasp simulations: causes the user to increase grip force often used to append force feedback displays Tactile displays: electrotactile stimulation Jukka Raisamo 9

10 Electrotactile stimulation Electrical stimulation is not widely accepted to consumer use often sudden bursts give an "invasive" impression square waves can be easily felt as too strong stimuli 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 for rehabilitation purposes Example: SmartTouch Tactile display to present realistic skin sensation a thin electrotactile display and a sensor mounted Two layers top layer: 4x4 array of stimulating electrodes bottom layer: optical sensors Visual information is captured by the sensors and displayed through electrical stimulation e.g., the black stripes on a paper are perceived as bumps 54 ( tokyo.ac.jp/projects/smarttouch/) 55 Example: Dielectric polymer Tactile displays: Dielectric elastomer actuators Uses a dielectric polymer film (c) between two electrodes (b & d) voltage causes the electrodes to attract each other the film contracts in thickness and expands in area Runs at around 1000 V (DC) at very low current require less power compared to traditional vibration motors and piezo actuators Example: Ultrasonic transducer Actuators: Ultrasonic transducers Based on acoustic radiation pressure a prototype consists of 324 airborne ultrasound transducers controlled individually the feedback can be felt about 20 cm over the surface Although the produced force is weak to feel constant pressure, it was sufficient for vibratory sensation Jukka Raisamo 10

11 Example: Electrorheological fluids Actuators: Electrorheological fluids (a) (b) Liquid which viscosity changes into semi solid when electric current is applied (pic. a b) change in viscosity feels as more resistive surface can change from liquid to gel, and back, within milliseconds The change in viscosity is proportional to the applied current Can be used to simulate different surface frictions Jukka Raisamo 11