Touch and tactile perception for robots Václav Hlaváč Czech Technical University in Prague (ČVUT) Czech Institute of Informatics, Robotics, and Cybernetics (CIIRC) Prague 6, Jugoslávských partyzánů 1580/3 Czech Republic [email protected] http://people.ciirc.cvut.cz/hlavac/ 1 Human sense of touch Czech Institute of Informatics, Robotics and Cybernetics 2 Related human vocabulary •Tactile •Haptic • Perceptible to the sense • Of or relating to the sense of touch. of touch; tactile. • From Latin tactilis (“that • From Ancient Greek may be touched, ἁπτικός (haptikos, “able tangible”). to come in contact with”), ἅπτω (haptō, “I touch”). • From French tactile. •Haptics •Touch • (in medicine) The study of • Make physical contact the sense of touch. with. • (in computing) The study • From e.g. French of user interfaces that use toucher. the sense of touch. Source: Wiktionary Czech Institute of Informatics, Robotics and Cybernetics 3 A greater picture, a somatosensory system Czech Institute of Informatics, Robotics and Cybernetics 4 Somatosensory system • The touch impression uses several modalities. • Somatosensory system comprising the receptors and processing centers to perceive touch, temperature, proprioception (body position from stimuli inside the body), and nociception (pain). • Cutaneous sensations obtains inputs from the receptors embedded in the skin (examples: temperature, pressure, pain). • Kinesthetic sensations gets inputs from the receptors within muscles, tendons and joints (examples: body position, movement, weight, equilibrium). Czech Institute of Informatics, Robotics and Cybernetics 5 Towards robot tactile sensors •Receptors in humans cover the skin and epithelia, skeletal muscles, bones and joints, internal organs, and the cardiovascular system. •Tactile sensors in robotics ≈ cutaneous sensory receptors in humans. Czech Institute of Informatics, Robotics and Cybernetics 6 Human sensory physiology Afferent CNS Stimulus Receptors pathway integration Internal Sense organs External Transducers Energy source Czech Institute of Informatics, Robotics and Cybernetics 7 External stimuli, special senses Czech Institute of Informatics, Robotics and Cybernetics 8 Neurophysiological view Somatosensory system Cortical homunuculus comprises of 3 parts: . Visualization of the •Exteroceptive point-to-point mapping cutaneous system. between body surfaces (and function) to the •Proprioception system brain surface. (monitors body position). •Interoceptive system (monitors conditions within the body as blood pressure). Czech Institute of Informatics, Robotics and Cybernetics 9 Somatosensory map Czech Institute of Informatics, Robotics and Cybernetics 10 Sensory modality Czech Institute of Informatics, Robotics and Cybernetics 11 Various receptors in the skin Czech Institute of Informatics, Robotics and Cybernetics 12 Human touch signals • FAI; Meissner corpuscules; Fast Adapting type I; Respond to skin deformation only. • SAI; Merkel disc; Slow Adapting type I; dynamically sensitive and exhibit a response linked to the strength of maintained skin deformation. • FAII; Pacini corpuscules; Fast Adapting type I; Respond to changes in skin deformation and vibrations. • SAII; Ruffini receptors; Slow Adapting type II; Dynamically sensitive and exhibit a response linked to the strength of maintained skin deformation. Czech Institute of Informatics, Robotics and Cybernetics 13 Touch reception in animals •Touch reception (called also tangoreception) is a perception in an animal when in contact with a (solid) object. •Two types of receptors are common: •Tactile hairs (in many animals from worms, birds to mammals). Some can be very specialized as, e.g., cat whiskers. •Subcutaneous receptors, which lie in “the skin”. Czech Institute of Informatics, Robotics and Cybernetics 14 Whiskers In the nature: • comparable to finger tips • motion detection of In robotics, so far: distant objects • limited (binary, strain • navigation in the dark sensors, bending angles) • rich shape and texture information • neural processing model system for somatosensory processing Czech Institute of Informatics, Robotics and Cybernetics 15 Tactile sensing vs. haptics in robotics and/or computing Tactile sensing Haptics • What is sensed? • Haptics explores human Deformation of bodies touch sense as a (strain). channel. • Through deformation • The counterforce and its measure change of dynamics stimulates parameters, and find: touch, compliance, • Static texture, local compliance, vibrations, etc. or local shape. • ≈ 1 kHz loop needed. • Force (normal and/or shear) (indirect). • Two main devices: • Pressure. • Force feedback devices. • Slippage. • Haptic displays and rendering algorithms. Czech Institute of Informatics, Robotics and Cybernetics 16 Haptics, ideas • Haptics provides a • Haptics expands the human an additional bidirectional communi- communication channel cation by providing to sight and sound in sensory feedback that (computer) applications. simulates physical • Traditionally, the properties and force. bidirectional • Machine part of the communication is often haptic interface exerts secured by a keyboard forces to simulate and a mouse only. contact with a virtual object. Czech Institute of Informatics, Robotics and Cybernetics 17 Haptic devices • Virtual reality / telerobotics: • Exoskeletons. • Gloves. • Feedback devices: • Force feedback devices. • Tactile display devices. Czech Institute of Informatics, Robotics and Cybernetics 18 Haptics has many applications • Blind Persons • Entertainment • Programmable Braille • Arcade (steering wheels) • Access to GUIs • Home (game controllers) • Training • Automotive • Medical Procedures • BMW “iDrive” • Astronauts • Haptic Touchscreens • Education • Mobile Phones • Computer-Aided Design • Immersion “Vibetonz” • Assembly-Disassembly • Material Handling • Human Factors • Virtual Surfaces • Art / Animation / Modeling Czech Institute of Informatics, Robotics and Cybernetics 19 Pneumatic / magnetic tactile display • The inverse problem: When the collected data is to be presented directly to human as touch, force feedback… • UC Berkeley’s tactile display: • 5 x 5 array of pneumatic pins • 0.3 N per element, 3 dB point of 8 Hz, and 3 bits of force resolution 20 Czech Institute of Informatics, Robotics and Cybernetics Piezoelectric display for the blind • Display with 256 tactile dots on an area of 4 x 4 cm. • Displays characters instead of Braille cells. • Piezoelectric actuators. http://www.abtim.com/home__e_/home__e_.html Czech Institute of Informatics, Robotics and Cybernetics 21 Principles of tactile sensors • Mechanical – micro • Tactile element (tactel) switch. • A grid of tactels • Resistive – elastomer or foam. • Capacitive. • Magnetic (Hall effect). • Piezoresistive, etc. Czech Institute of Informatics, Robotics and Cybernetics 22 Mechanical sensor • One-directional reed switch • Omni-directional reed switch • Roller contact switch • Strain gauge (tensometer) • Etc. Czech Institute of Informatics, Robotics and Cybernetics 23 Strain gauge tactile sensor • Measures also the shear force . • Double Octagon Tactile Sensor (DOTS) Application in a gripper Czech Institute of Informatics, Robotics and Cybernetics 24 Resistive sensor • The basic principle is the measurement of the resistance of a conductive elastomer or foam between two points. • The majority of the sensors use an elastomer that consists of a carbon doped rubber. Czech Institute of Informatics, Robotics and Cybernetics 25 Disadvantages, resistive sensors • An elastomer has a long • If the elastomer becomes nonlinear time constant, permanently deformed different for applying and then a fatigue leading to releasing force. sensor malfunction. • Highly nonlinear transfer • This will give the sensor a function. poor long-term stability • Cyclic application of and will require its forces causes resistive replacement after an medium migration within extended period of use. the elastomer in time. Czech Institute of Informatics, Robotics and Cybernetics 26 Common package and pricing Price ranges from a few dollars to a few tens of dollars. Czech Institute of Informatics, Robotics and Cybernetics 27 Force-Sensitive-Resistor sensor • FSR = Force-Sensitive-Resistor • Used also for touch keyboards. Czech Institute of Informatics, Robotics and Cybernetics 28 Resistive touchscreen • Two flexible resistive layers are separated by a grid of spacers. • When the two layers are pressed together the resistance can be measured between several points. • This determines where the two resistive layers contacted. Czech Institute of Informatics, Robotics and Cybernetics 29 Capacitive force sensor (1) • Capacitance between two parallel plates = , where • is the permittivity of the dielectric medium, • is the plate area, • is the distance between plates, • The elastomer gives force-to-capacitance characteristic. Czech Institute of Informatics, Robotics and Cybernetics 30 Capacitive force sensor (2) • As the size is reduced to increase the spatial resolution, the sensor’s absolute capacitance will decrease. • To maximize the change in capacitance as force is applied, it is preferable to use a high permittivity, dielectric in a coaxial capacitor design. • The use of a highly dielectric polymer such as poly vinylidene fluoride maximizes the change capacitance. Czech Institute of Informatics, Robotics and Cybernetics 31 Capacitive touchscreen • A conductive layer is covered with a dielectric layer. • The finger = the other plate of the capacitor. • A few kHz signal is transmitted