Robot Sensors Introduction to Robotics Lecture Handout September 20, H. Harry Asada Massachusetts Institute of Technology

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1 Robot Sensors 2.12 Introduction to Robotics Lecture Handout September 20, 2004 H. Harry Asada Massachusetts Institute of Technology

2 Touch Sensor CCD Camera Vision System Ultrasonic Sensor Photo removed for copyright reasons: Sony Aibo dog robot. Products/aibo/. Wrist Force Sensor Tension Sensor Photo removed for copyright reasons. Robot hand with finger and thumb. Torque Sensor Tactile Sensor Touch Sensors Ultrasonic Sensor Infrared Photo removed for copyright reasons. Transition Technology meal delivery robot. Bumper Sensor Distance Sensor Slip Sensors

3 Internal and External Sensors Reference Command Actuators Environment Internal Sensors External Sensors

4 Robot sensor taxonomy Internal sensors Position sensor Velocity sensors Torque and acceleration sensors External sensors Tactile sensors Force and torques sensors Proximity sensors Range sensors Vision sensors Others

5 Internal Position Sensors Potentiometer Optical shaft encoder DC Motor Gearing Arm Links Joint Axis Reference Command Feedback Law Actuator & Load Position Sensors

6 Optical Shaft Encoder Opaque Disk with grid pattern Light source: LED Translucent Shaft Photodetector As the shaft rotates, a pulse train is generated. Counting the number pulses gives the angle of rotation.

7 Distinguishing clockwise and counter-clockwise rotations Track A Track B Photodetectors Clockwise rotation Counter-clockwise rotation A A B B +90 o Track A is 90 degrees ahead of track B. -90 o Track A is 90 degrees behind.

8 Using an up-down counter, CW and CCW rotations are correctly counted. A Phase B Phase Up-Down Counter Most sensitive bit n-bit parallel Clear/Initialization Least sensitive bit

9 Initialization of the Up-Down Counter Z B A Home Position When the joint is brought to a home position, the z-phase pulse is generated for initializing the updown counter.

10 Absolute Encoder Parallel Data No initialization needed

11 Internal Velocity Sensors Velocity Measurement Using an Encoder ~ Velocity = Pulse Frequency Counter clear Sampling period T = Pulse counting interval

12 The drawback of pulse frequency measurement As the angular velocity gets slower, only a few pulses are observed in the fixed time interval: discretization error increases. T t int A better alternative is to measure the interval between adjacent pulses tint, and take the reciprocal for estimating the velocity. 1 ω tint

13 Hybrid velocity counter Measuring the time interval between adjacent pulses: More accurate in slower speed t int Low speed counter T High speed counter Counting the number of pulses in a fixed time interval: More accurate in higher speed

14 Sensor as a System Signal Source Multi-stage Signal transduction and modulation Key Transducer Active Modulation Low-level Filtering High-level Filtering A sensor is often a system consisting of multiple stages of signal transduction and modulation, low-level and high-level filters, and a modulation unit. It comprises a communication and a power unit as well.

15 The shaft encoder for measuring both position and velocity Shaft rotation Signal Source Disc rotation LED-PD Threshold : binary signals A-B phase detection and counting Key Transducer Low-level Filtering Active Modulation Light intensity modulation Hybrid velocity estimation High-level Filtering

16 Internal Torque Sensors τ m = Torque Measurement Via armature current measurement: K t i The current is measured by inserting a small resister (2~3 Ω) in series and measuring the voltage drop across the resister. i R t V t = R t i

17 Torque Measurement Via measurement of the torsion/strain of the shaft: τ k θ m = a Strain Gauges

18 Strain Gauge Force/Torque Sensors Strain Gauges V out Temperature-Compensated Wheatstone Bridge

19 Wrist Force/Torque Sensors 6-Axis Wrist Force/Torque Sensor Fx Nx Strain Gauges Fy Ny Fz Nz Strain Gauges Outputs: y1, y2,, ym

20 Strain Gauges Outputs: y1, y2,, ym 6-Axis Wrist Force/Torque Sensor = = = = z z y x m z m x m x z y x sensor sensor N F F F y y N y F y F y N y F y F y M M L L M O M O L F y J y J F, # Fy Ny Fz Nz Fx Nx

21 Wrist Force/Torque Sensor Hybrid Position/Force Control Position Feedback + Position Reference Inputs Force Reference Inputs + Position/velocity control compensator + Robot + Force/torque control compensator Task Environment Force Feedback

impregnated with conductive dopants Force/Pressure P Other

22 Tactile Sensors Tactile Pad Principle Force/Pressure P Electrodes Resistance R Conductive Rubber: Media impregnated with conductive dopants Force/Pressure P Other Methods: Capacitive Optical Piezoelectric Magneto-resistive Magneto-strictive

23 Sensor Pad: 2-D sensor array Pressure distribution Technical Issues: How to process the 2-D data of pressure distribution How to reduce wires

24 A tactile sensor = Measuring 2-D pressure distribution A Matrix Wiring Structure for Reducing Cables N Switches Sensor Element N Switches

25 B.E. Robertson and A.J. Walkden, x 16, 256 elements Photo and diagrams removed for copyright reasons. Integrating sensor elements with signal processing circuitry on a VLSI chip

26 Integrating sensor elements with signal processing circuitry on a VLSI chip

27 Pressure patterns (a) Intensity (b) Contour Two photos removed for copyright reasons. VLSI tactile array sensor with 20 x 20 grids of 75 um separation by M.H. Raibert and J.E. Tanner, 1982

28 Distance Sensors Transmitted Sonic Pulse Ultrasonic Sensors Object Reflected Echo Transmitted pulse t Echo pulse Measuring the distance d to an object by the time interval between the transmitted and reflected sonic pulses. 2 d = v t v = speed of sound, t= time interval

29 PiezoelectricTransducer for Transmitting and Receiving Acoustic Signals Actuator Sensor Pressure Wave F x V F = d 33 q Piezoelectric Material V = d 33 x (Force) = proportional to (Charge) (Voltage) = proportional to (Displacement)

30 Dielectric Capacitance Transducer for Transmitting and Receiving Acoustic Signals Figure by MIT OCW.

31 Swept frequency method Machine Filter Process Receive Amplifier Rx Transducer Human Processor Auditory Link (Earphones) Hearing Process Variable Frequency Generator Drive Amplifier Tx Transducer CWFM Sonar for Blind Persons and Machines Figure by MIT OCW.

32 Proximity Sensors Basic Principle Transmitter + Receiver Transmitter + Receiver The approach of an object changes: Back pressure: pneumatic sensor Inductance: inductive/magnetic sensors Capacitance: capacitive sensor

33 Pneumatic Proximity Sensor Supply Pressure Distance Object Back Pressure Output Pressure Threshold Distance

34 Glass Tube Leads Magnetic Lead Switches Filled with Inert Gas S Permanent OFF ON Magnet N N S S N For extending the detectable range Magnetic Lead Switch Permanent Magnet Magnetic Material Magnetic Material: A Fake Mine

35 Optical Proximity Sensors LED Photo Detector 3V R2 R1 Photodiode Vout LED Photo Detector

36 Imaging Sensors Photo-electric effect Photon CCD Camera Photo Current oc Illumination Intensity Absorbs photon energy to move electrons to a higher energy level Energy = (Plank s Constant) x (Speed of Light)/(Wavelength) CCD (Charge Coupled Device) Light beam Photo Sensitive Array Scanned pixel by pixel

37 Image Processing A. Two-dimensional Images

38 Structured Lighting Figure by MIT OCW.

39 Camera Slit Light Figure by MIT OCW.

40 Structured Lighting Slight light scanned over 3D objects Photos removed for copyright reasons.

41 3D range Finders 3D Range Finders M Laser Range Finder ω Known distance between the mirror and the detector B S O L 2 ω T X D N Rotational Mirror ω M α T Laser Light Source S O N α L Detector D 2 ω B Figure by MIT OCW.

42 Simultaneous Location And Mapping (SLAM) Sensor Data Map Building Image courtesy of JPL. Robot Control Location Estimation Planet Exploration Robotics

43 Human Sensors Measuring human motion for controlling Remote manipulators Hazardous environment Human Operator Joystick Manipulator Figure by MIT OCW.

44 Surgical Robot System Virtual Reality Robot Control Photo removed for copyright reasons. Surgical robot with virtual reality workstation (left) and robot (right)

45 Data Glove Measurement of hand posture and touch force Fingernail Sensors: Measurement of Fingertip Touch Force and Posture through Nail Color Change Photos removed for copyright reasons. Free-Fingered Glove

46 Instrumented Fingernails Micro LED & Photo Detectors Nail B ed Nail Nail Matrix No sensor pad is needed Pressure: vein occlusion Figure by MIT OCW. After Spence, Basic Human Anatomy.

47 Measurement Principle Transmitted Light Reflected Light Vout 3V R2 R1 P h otodio d e 660 nm Red LED Vout 0 Contact Pressure

48 Applications Virtual Switches Task and Skill Monitoring Photos removed for copyright reasons.

Electronic Instrumentation and Measurements

Electronic Instrumentation and Measurements A fundamental part of many electromechanical systems is a measurement system that composed of four basic parts: Sensors Signal Conditioning Analog-to-Digital-Conversion