ECE5320 Mechatronics

1. ECE5320 Mechatronics • Prepared by: • Vipin Varghese • Dept. of Electrical and Computer Engineering • Utah State University • Email: vipin@cc.usu.edu • Tel: (435)881-6159 Assignment#01: Literature Survey on Sensors and Actuators Topic: TACTILE SENSORS ECE5320-Mechatronics

2. Reference list. • To probe further. • Basic definition of tactile sensors. • Classification of tactile sensors. • Major applications. • Basic Working Principle. • A typical sample configuration in application: Robotics • Major specifications. • Advantages and Limitations • Where to buy. Outline ECE5320-Mechatronics

3. Reference list • http://www.soton.ac.uk/~rmc1/robotics/artactile.htm • http://library.thinkquest.org/C0126120/tactile.htm • http://www.engineering.unl.edu/academicunits/chemicalengineering/research/MesoscaleEngineeringLab/ElectronicSkin.shtml • http://robotics.eecs.berkeley.edu/~ronf/tactile-const.html • http://mass.micro.uiuc.edu/research/current/advsensors/image_tactilesensor.html ECE5320-Mechatronics

4. http://www.neuroinformatik.ruhr-uni-bochum.de/ VDM/research/robotics/Hardware/TactileSensor/ contents.html • http://www.araa.asn.au/acra/acra2002/Papers/Hellard-Russell.pdf • http://www.star.t.tokyo.ac.jp/projects/tactile-display/index.html Reference list ECE5320-Mechatronics

5. To explore further:(survival pointers of web references) http://www.media.mit.edu/resenv/classes/MAS965/readings/piezoarray.pdf http://mic.unituebingen.de/mic/index.php?id=95&lang=eng http://ieeexplore.ieee.org/iel3/3952/11434/00525871.pdf?tp=&arnumber=525871&isnumber=11434 ECE5320-Mechatronics

6. What are Tactile Sensors? Simple Definition Tactile sensor are devices which measures the parameters of a contact between the sensor and an object. Scientific Definition Tactile Sensing is the detection and measurement of the spatial distribution of forces perpendicular to a predetermined sensory area, and the subsequent interpretation of the spatial information. Courtesy:IEEE Paper: Tactile Sensor Based on Piezoelectric Resonanceby G. Murali Krishna and K. Rajanna http://mass.micro.uiuc.edu/research/current/advsensors/2006-focus intro/slide7.html ECE5320-Mechatronics

7. Classification of Tactile Sensors • Tactile sensors can be broadly classified into three categories: • Whiskers • Touch • Pressure • Whiskers Tactile Sensors: Whiskers tactile sensors are based on the property of whiskers in an animal like rats. It contains a long bending wire or beam which on movement from its original position indicates the existence of obstruction or presence of object in the path. ECE5320-Mechatronics

8. Touch Tactile Sensor: Touch Tactile sensors works on the principle of touch sensing. Touch sensing is basically the process of detection and measurement of a contact force at a defined point. A touch sensor is generally restricted to binary information, namely touch and no touch. Courtesy: IEEE Paper: Tactile Sensor Based on Piezoelectric Resonance by G. Murali Krishna and K. Rajanna Courtesy: http://www.essd.com Pressure Tactile Sensor: In a pressure tactile sensor, the pressure applied on the surface of the sensor changes either the electric property (piezoelectricity) or resistance of the material, thereby exhibiting the characteristics of touch to the sensor. Courtesy: http://www.rm.mce.uec.ac.jp/study-shmj/ TactileSensor/ ECE5320-Mechatronics

9. Tactile sensor technology using various physical principles • Mechanical Based:Force applied to the sensor is directly applied to a conventional mechanical micro-switch to form a binary sensor. (i.e. On/Off) Other approaches are based on a mechanical movement activating a secondary device such as a potentiometer or displacement transducer. • Resistive Based: The basic principle of this type of sensor is the measurement of the resistance of a conductive elastomer or foam between two points. As the resistance of the elastomer changes with the application of force, it results in the deformation of the elastomer altering the particle density and hence the overall resistance. ECE5320-Mechatronics

10. Force sensing resistor:A force sensing resistor is a piezoresistivity conductive polymer, which changes resistance in a predictable manner following application of force to its surface. It is normally supplied as a polymer sheet which has had the sensing film applied by screen printing. The sensing film consists of both electrically conducting and non-conducting particles suspended in matrix. Applying a force to the surface of a the sensing film causes particles to touch the conducting electrodes, changing the resistance of the film. • Capacitive based:A capacitive touch sensor relies on the applied force either by changing the distance between the plates or the effective surface area of the capacitor. In such a sensor, the two conductive plates of the sensor are separated by a dielectric medium, which is also used as the elastomer to give the sensor its force-to capacitance characteristics. Courtesy: http://www.soton.ac.uk/~rmc1/robotics/artactile.htm ECE5320-Mechatronics

11. To measure the change in capacitance, a number of techniques can be used. The most popular is based on the use of a precision current source. A second approach is to use the sensor as part of a tuned or L.C. circuit, and measure the frequency response. • Magnetic based sensor:There are two approaches to the design of these sensors based on magnetic transduction. Firstly, the movement of a small magnet by an applied force will cause the flux density at the point of measurement to change. The flux measurement can be made by either a Hall effect or a magnetoresistive device. Second, the core of the transformer or inductor can be manufactured from a magnetoelastic material that will deform under pressure and cause the magnetic coupling between transformer windings, or a coil’s inductance to change. • Piezoelectric sensors:Here polymeric materials that exhibit piezoelectric properties such as polyvinylidene fluoride (PVDF) are used. A thin layer of metallization is applied to both sides of the sheet to collect the charge and permit electrical connections to be made. ECE5320-Mechatronics

12. Strain Gauges in Tactile sensor:A strain gauge when attached to a surface will detect the change in length of the material as it is subjected to external forces. When applied to robotic touch applications, the strain gauge is normally used in two configurations: as a load cell, where the stress is measured directly at the point of contact, or with the strain gauge positioned within the structure of the end effector. • Optical fibre based sensors:Here the tactile sensors are constructed with optical fibers. The property ofinternal-state micro bending of optical fibers is used to find the degree of attenuation with respect to radius of curvature and spatial wavelength of the bend. • Silicon based sensors:This sensor is based onthe use ofmicromachining techniques to develop very small, efficient sensors to remove expensive calibration procedures generally used in sensor development technology. The use of MEMS technology in sensors have resulted in SOC’s (System-on-chip). ECE5320-Mechatronics

13. Optical Tactile Sensors:The operating principles of optical-based tactile sensors are divided into two classes: • Intrinsic, where the optical phase, intensity, or polarization of transmitted light are modulated without interrupting the optical path • Extrinsic, where the physical stimulus interacts with the light external to the primary light path. • Intrinsic and extrinsic optical sensors can be used for touch, torque, and force sensing. • The two common principles used for tactile sensing using optical characteristics are: • Modulating the intensity of light by moving an obstruction into the light path. As shown, above the fibre is a layer of clear elastomer topped with a reflective silicon rubber layer. The amount of light reflected to the receiver is determined by applied force, that changes the thickness of the clear elastomer. ECE5320-Mechatronics

14. Photoelasticity: Here light is passed through the photoelastic medium. As the medium is stressed, the photoelastic medium effectively rotates the plane of polarization and hence the intensity of the light at the detector changes as a function of the applied force. • Multi-stimuli Touch Sensors and smart sensors:In multi-stimuli touch sensor for object recognition between materials that have different thermal conductivity, e.g., between a metal and a polymer, both force sensing and temperature sensing is employed. By the use of two or more force-sensitive layers on the sensor, which have different characteristics (e.g., resistive elastomer and PVDF), it is possible to simulate the epidermal and dermal layers of human skin. Smart sensors are designed by taking into consideration the sensing object and also the corresponding signal conditioning require to give a faithful output.Generally,signal processing is done in the same interface as the sensing object by use of technologies like VLSI and MEMS. ECE5320-Mechatronics

15. Major Applications • Agriculture • Food Processing • Biomedical Devices • Health Care • Robotics • Biometrics • Military Applications • Smart Electronics • Automotive -------------------------------The list is endless--------------------------------- ECE5320-Mechatronics

16. Basic Working Principle A tactile sensor consists of an discrete array of touch sensitive sites or cells called “texels”. They are arranged in homogeneous matrices, capable of measuring more than one property like force,temperature,pressure or detecting an applied load profile. The contact forces measured by the sensor cells conveys a large amount of information about the state of a grip, texture, slip, impact and other contact conditions. This information can be used to generate force and position signatures, that can be used to identify the state of a manipulation. ECE5320-Mechatronics

17. The changes in the output can be in many forms like resistance change, change in the intensity of light, proportionate voltage change, movement, change in current, etc. For data acquisition, the measurement converter is connected to a local intelligence, a sensor controller, digitizing the sensor signals and preprocessing them. A host system processes the data made available by the controller and extracts characteristics. The data can then be used e.g. for reactive control of a robot. Courtesy: IEEE Paper on:-”The Working Principle of Resistive Tactile Sensor Cells”-Karsten Weib and Heinz Worn ECE5320-Mechatronics

18. A typical sample configuration in application: Robotics Here a tactile sensor is used to control a manipulator for grasping an unknown object based on tactile feedback. The tactile sensor placed on each jaw of the manipulator is a new dynamic sensor, which is based on a capacitive sensor connected with fibers for the tactile feedback. These new tactile hair sensors are very sensitive, they even react to vibrations of the robot during the movement or to streams of air. Such application can be used in development of advance appendages for the handicapped individuals,braile learning for the blind,etc. Courtesy: http://www.neuroinformatik.ruhr-uni-bochum.de/VDM/research/robotics/Hardware/TactileSensor/contents.html ECE5320-Mechatronics

19. Video showing the application in detail If the video doesn’t work, kindly play it manually (robotics) from the folder ECE5320-Mechatronics

20. Another Application - Biomedical High resolution tactile Nanosensor: If the video doesn’t work, kindly play it manually (bio) from the folder Courtesy:http://www.engineering.unl.edu/academicunits/chemical-engineering/ research/ MesoscaleEngineeringLab / ElectronicSkin. shtml ECE5320-Mechatronics

21. Major specifications • A tactile sensor should ideally be a single-point contact, though the sensory area can be any size. In practice, an area of 1-2 mm2 is considered a satisfactory compromise between the difficulty of fabricating a sub-miniature sensing element and the coarseness of a large sensing element. • The sensitivity of the tactile sensor is dependent on a number of variables determined by the sensor's basic physical characteristic. In addition the sensitivity may also be the application, in particular any physical barrier between the sensor and the object. A sensitivity within the range 0.4 to 10N, together with an allowance for accidental mechanical overload, is considered satisfactory for most industrial applications. • A minimum sensor bandwidth of 100 Hz. ECE5320-Mechatronics

22. Major specifications • The sensor’s characteristics must be stable and repeatable with low hysteresis. A linear response is not absolutely necessary, as information processing techniques can be used to compensate for any moderate non-linearities. • If the tactile sensor is to be used in an industrial application, it will need to be robust and protected from environmental damage. • If a tactile array is being considered, the majority of application can be undertaken by an array 10-20 sensors square, with a spatial resolution of 1-2 mm. ECE5320-Mechatronics

23. Major Advantages • Explicit Three- Dimensional Information: The data from a tactile sensor can represent explicit three dimensional information about a contacting stimulus. • No Specularity:Specularity can be a problem in vision systems, particularly if there are physical constraints on the positioning of the illumination sources. Tactile sensors do not have this problem. since they are not based on direct optical reflection from the object. • No Shadows: Tactile sensors by their nature do not suffer from problems of shadows in the image. The intensity readings obtained being derived solely through contact between sensor and stimulus. • Direct Measurements of Stimulus: There is a direct mapping between stimulus shape and image, since non-background intensities are present in the tactile image when there is actual contact between sensor and stimulus. • Generally lightweight ECE5320-Mechatronics

24. Major Advantages • No Perspective Distortions: Tactile measurements are direct, and do not introduce any distortion into the image. Vision sensing can have difficulties arising from distortion of the objects in the image due to perspective. • Ease of Separation of Stimuli from Background: The separation of features in a tactile image from the image background is a relatively straightforward task compared to that in some vision scenarios. • Tactile Sensors Provide Local Views: The view of the environment provided by tactile sensors is very much localized, because such sensors only give responses to stimuli which are actually touching the sensor surface. • Sensors can be of any size necessary • Low cost relative to other option. ECE5320-Mechatronics

25. Major Disadvantages/Limitations • Tactile Sensing May Produce Physical Distortion:By its very nature, tactile sensing will involve physical interaction with a sensed object. This interaction can result in the physical deformation of the object under examination, or perhaps its movement in space. • Using tactile sensors to navigate an unknown space could be very difficult. • Most accurate in a clean environment. • Accompanying hardware is bulky. • Small range of measurement. Courtesy: Overton, K.J.: The acquisition, processing, and use of tactile sensor data in robot control. I.-niv.Massachusetts at Amherst. Dept. Computer and Info. Science report COINS 84-08. 1984. Tactile Sensing for Robotics by Howard R. Nicholls ECE5320-Mechatronics

26. Where to Buy? ECE5320-Mechatronics