Outline Sensors. EE Sensors. H.I. Bozma. Electric Electronic Engineering Bogazici University. December 13, 2017

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1 Electric Electronic Engineering Bogazici University December 13, 2017

2 Absolute position measurement

3 Outline Motion Odometry Inertial systems Environmental Tactile Proximity Sensing Ground-Based RF Beacons and GPS Cameras and vision Kinect

4 Approaches Dead Reckoning - Determining the present location of based on some previous position through known course and velocity info over a given length of time Displacement along the path of travel - derived from some onboard instrumentation - some type of encoders directly coupled to the motor armatures or wheel axles Odometry Inertial systems

5 Encoders that accurately quantify angular position and velocity Available sensors: Brush encoders. Potentiometers. Synchros. Optical encoders. Magnetic encoders. Inductive encoders. Capacitive encoders

6 Encoders (cont.) Outline Types: Incremental vs Absolute Positive features: Self-contained and always providing an estimate Negative issues: Errors accumulate

7 Incremental Optical Encoders

8 Simplest type - Single-channel tachometer encoder Tradeoff here - resolution versus update rate: Used as velocity feedback sensors in medium- to high-speed control systems, but noise and stability problems at extremely slow velocities due to quantization errors Cannot detect the direction of rotation and thus cannot be used as position sensors. Improved transient response Faster update rate Phase-quadrature incremental encoders: Overcome this via adding a second channel, Incremental nature of the phase-quadrature output signals: Angular position can only be relative to some specific reference, as opposed to absolute.

9 Absolute Optical Encoders Typically used for slower rotational applications when potential loss of reference from power interruption is not acceptable Best suited for slow and/or infrequent rotations such as steering angle encoding, as opposed to high-speed continuous rotations Instead of the serial bit streams of incremental designs, parallel word output with a unique code pattern for each quantized shaft position!

10 Absolute Optical Encoders

11 Gray-Code Encoding Outline

12 Approaches Active beacons: Measuring the direction of incidence of 3 or more active transmitting beacons Artificial landmark recognition: Distinctive artificial landmarks Natural landmarks Model matching

13 Heading Help to compensate accumulation of error Detect orientation errors Gyroscropes & Accelerometers Compasses

14 Gyroscropes & Accelerometers To measure rate of rotation and acceleration Measure the angular velocity of the system in the inertial reference frame. Integrating the angular velocity, compute the system s current orientation Mechanical gyroscopes - ( USD) Piezoelectric gyroscopes - Around 300 USD Optical gyroscopes Geomagnetic sensors Positive features: Self-contained Negative issues: Not suitable for accurate positioning over extended period of time and price

15 Proximity Outline Aimed at detecting presence rather than actual profile! Can operate in rugged environments reliably Magnetic Ultrasonic Optical Inductive Capacitive

16 Optical Proximity Outline

17 Tactile (Haptic) Sensing Outline Last resort indication of collisions Force sensing: Direct physical contact btw sensor and object of interest Sensor Typles Contact closure Magnetic Piezoelectric Photoelectric Magnetoresistive Piezoresistive Ultrasonic

18 Passive Feeler Outline

19 Contact Closure Fleers Passive: A simple wire that when rel. motion occurs, closes a loop Active: Incorporate a search strategy for increased coverage

20 Active Feelers Outline

21 Tactile Bumpers Outline Touch-based microswitches

22 Distributed Surface Array Embedded tactile arrays to provide 2D profile Skin-like sensor array that is placed above the manipulator arm

23 Doppler Outline

24 Doppler Actual ground velocity along path V a Measured doppler velocity V d Angle of inclination α Observed Doppler shift F d Transmitted frequency F t V a = cf d 2F t cosα

25 Doppler - Problems Errors in detecting true ground - Side-lobe interference, Vertical velocity components introduced by vehicle reaction to road surface anomalies, Uncertainties in the actual angle of incidence due to the finite width of the beam. Around 500 USD/sensor

26 TOF Outline Time-of-flight range sensors d = vt Ultrasonic Laser Phase-shift measurement or Frequency modulation Reliability issues: Variations in the speed of propagation, particularly in the case of acoustical systems. Uncertainties in determining the exact time of arrival of the reflected pulse. Inaccuracies in the timing circuitry used to measure the round-trip time of flight. Interaction of the incident wave with the target surface.

27 Laser-Based TOF Laser-based TOF ranging systems, also known as laser radar or lidar, Laser energy - emitted in a rapid sequence of short bursts aimed directly at the object being ranged. Time required for a given pulse to reflect off the object and return is measured and used to calculate distance to the target based on the speed of light. Accuracies - Few centimeters over the range of 1 to 5 meters Price Expensive!

28 Cameras Camera types 1. Analog 2. Digital Interface electronics Required in case of analog cameras

29 Kinect Outline 1. Kinect Sensor: An RGB camera, infrared laser projector combined with a monochrome CMOS sensor and multi-array microphone running proprietary software

30 Typical Configurations Outline Encoders Cameras Targeting camera Laser range finder Tactile sensing

31 References Outline H.R. Everett. for Mobile Robots: Theory and Application. A.K. Peters, J. Borenstein, H.R. Everett and L. Feng. and Methods for Mobile Robot Positioning, Technical Report, The University of Michigan, 1996.