M. Zareinejad 1

2 Outline # Sensors –––– Sensor types Sensor examples #Actuators Actuator types Actuator examples ––––

3 The Haptic System Human

4 Passive & Active joint

5 Sensor Applications Eye tracking Head tracking Body tracking Hand tracking – Most important for typical haptic interfaces

6 Sensor types Magnetic Optical Acoustic Inertial Mechanical – Most important for typical haptic interfaces

7 Mechanical Trackers Ground-based linkages most commonly used Position Sensors –––– digital: optical encoders analog: Hall-effect (magnetic)

8 Optical Encoders How do they work? –––– A focused beam of light aimed at a matched photodetector is interrupted periodically by a coded pattern on a disk Produces a number of pulses per revolution (Lots of pulses = high cost) Quantization problems at low speeds Absolute vs. Incremental Emitter Detector

9 Optical Encoders Absolute vs. Incremental Resolution?

10 Optical Encoders Phase-quadrature encoder 2 channels, 90° out of phase – allows sensing of direction of rotation

11 Encoder States & Decoding

12 Hall-Effect Sensors How do they work? – A small transverse voltage is generated across a current-carrying conductor in the presence of a magnetic field (Discovery made in 1879, but not useful until the advent of semiconductor technology.)

13 Hall-Effect Sensors Amount of voltage output related to the strength of magnetic field passing through. Linear over small range of motion – Need to be calibrated Affected by temperature, other magnetic objects in the environments R h IB t Vh Vh  V h  Hall voltage R h  Hall coefficient I  Current B  Magnetic flux density t  Element thickness

14 Hall-Effect Sensors R h IB t Vh Vh  Vh = Hall voltage Rh = Hall coefficient I = Current B = Magnetic flux density t = Element thickness  The voltage varies sinusoidally with rotation angle Resolution?

15 Potentiometers

16 Potentiometers Resolution?

17 Acoustic Tracker SpeakerMicrophone

18 Acoustic Tracker

19 Magnetic Tracker

20 Magnetic Tracker

21 Optical Tracker  Inside-Looking-Out  Outside-Looking-In

22 Optical Tracker

23 Data gloves

24 Data gloves

25 Measuring Velocity Differentiate position –––– advantage: use same sensor as position sensor disadvantage: get noise signal Alternative – for encoders, measure time between ticks

26 Digital differentiation Many different methods Simple Example: –––––– Average 20 readings = P1 Average next 20 readings = P2 where t is the the period of the servo loop Differentiation Increases noise P1- P2 t V V 

27 Time-between-ticks use a special chip that measures time between ticks – Fares poorly at high velocities ptpt v v  Time per ticks rather than ticks per time Especially good to do at slow speeds –

28 Actuator Types Electric motors  DC (direct current)  Brushed  PM (permanent magnet) Pneumatic Actuators

29 PM DC brushed motors How do they work? –––– Rotating armature with coil windings is caused to rotate relative to a permanent magnet current is transmitted through brushes to armature, and is constantly switched so that the armature magnetic field remains fixed.

30 DC motor components

31 DC motor components

32 DC motor terms Cogging – Tendency for torque output to ripple as the brushes transfer power Friction/damping – Caused by bearings and eddy currents Stall torque – Max torque delivered by motor when operated continuously without cooling

33 Motor Equations Torque constant, K T T = K T I Dynamic equation dddd + RI + EV = LV = L

34 Pneumatic Actuators How do they work? – Compressed air pressure is used to transfer energy from the power source to haptic interface. Many different types Concerns are friction and bandwidth

35 Some Terms AD/DA –––– analog to digital digital to analog Interrupt routine Servo Loop Servo rate – Usually needs to be >500 Hz

36 DecimalBinaryHexadecimal A B C D E F D/A and A/D Converts between voltages and counts Computer stores information digitally, and communicates with the outside world using +/- 5V signals MSB LSB

37 D/A and A/D Converts voltages to counts and vice versa A 12-bit card: – 2 12 decimal numbers (4096) Decimal (base 10): Binary (base 2): Hexadecimal (base 16): B B2=BB2 2994