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Haptics and Virtual Reality

M. Zareinejad

Lecture 3:

Sensors & Actuators

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Outline

# Sensors

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Sensor types

Sensor examples

#ActuatorsActuator types

Actuator examples

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The Haptic System

Hum

an

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Passive & Active joint

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Sensor Applications

Eye tracking

Head tracking

Body tracking

Hand tracking– Most important for typical haptic interfaces

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Sensor types

Magnetic

Optical

Acoustic

Inertial

Mechanical– Most important for typical haptic interfaces

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Mechanical Trackers

Ground-based linkages most commonly used

Position Sensors–

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digital: optical encoders

analog: Hall-effect (magnetic)

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Optical Encoders

How do they work?–

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A focused beam of light aimed at a matchedphotodetector is interrupted periodically by a codedpattern on a disk

Produces a number of pulses per revolution (Lots ofpulses = high cost)

Quantization problems at low speeds

Absolute vs. Incremental

EmitterDetector

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Optical Encoders

Absolute vs. Incremental

Resolution?

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Optical Encoders

Phase-quadrature encoder

2 channels, 90° out of phase– allows sensing of direction of rotation

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Encoder States & Decoding

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Hall-Effect Sensors

How do they work?– A small transverse voltage is generated across a

current-carrying conductor in the presence of amagnetic field

(Discovery made in1879, but not usefuluntil the advent ofsemiconductortechnology.)

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Hall-Effect Sensors

Amount of voltage output related to thestrength of magnetic field passing through.

Linear over small range of motion– Need to be calibrated

Affected by temperature, other magneticobjects in the environments

Rh IBt

Vh =

Vh = Hall voltage

Rh = Hall coefficient

I = Current

B = Magnetic flux density

t = Element thickness

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Hall-Effect Sensors

Rh IB

tVh =

Vh = Hall voltage

Rh = Hall coefficient

I = Current

B = Magnetic flux density

t = Element thickness

The voltage varies sinusoidally with rotation angle

Resolution?

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Potentiometers

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Potentiometers

Resolution?

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Acoustic Tracker

Speaker Microphone

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Acoustic Tracker

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Magnetic Tracker

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Magnetic Tracker

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Optical Tracker

Inside-Looking-Out Outside-Looking-In

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Optical Tracker

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Data gloves

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Data gloves

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Measuring Velocity

Differentiate position–

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advantage: use same sensor as position sensor

disadvantage: get noise signal

Alternative– for encoders, measure time between ticks

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Digital differentiation

Many different methods

Simple Example:–

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Average 20 readings = P1

Average next 20 readings = P2

where t is the the period of the servo loop

DifferentiationIncreases noise

P1- P2t

V =

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Time-between-ticks

use a special chip that measures time between ticks

– Fares poorly at high velocities

pt

v =

Time per ticks rather than ticks per time

Especially good to do at slow speeds–

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Actuator Types

Electric motors DC (direct current) Brushed PM (permanent magnet)

Pneumatic Actuators

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PM DC brushed motors

How do theywork?–

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Rotating armaturewith coil windingsis caused to rotaterelative to apermanent magnet

current is transmitted through brushes toarmature, and is constantly switched so that thearmature magnetic field remains fixed.

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DC motor components

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DC motor components

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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

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Motor Equations

Torque constant, KT

T = KT IDynamic equation

dIdt

+ RI + EV = L

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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

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Some Terms

AD/DA–

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analog to digital

digital to analog

Interrupt routine

Servo Loop

Servo rate– Usually needs to be >500 Hz

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Decimal Binary Hexadecimal

0 0000 0

1 0001 1

2 0010 2

3 0011 3

4 0100 4

5 0101 5

6 0110 6

7 0111 7

8 1000 8

9 1001 9

10 1010 A

11 1011 B

12 1100 C

13 1101 D

14 1110 E

15 1111 F

D/A and A/D

Converts betweenvoltages and counts

Computer storesinformation digitally,and communicateswith the outsideworld using +/- 5Vsignals

101010101

MSB LSB

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D/A and A/D

Converts voltages to counts and vice versa

A 12-bit card:– 212 decimal numbers (4096)

1 10 01 0 0 11 1 0 1

1 10 01 0 0 11 1 0 1

Decimal (base 10):

Binary (base 2):

Hexadecimal (base 16):

B B 2 = BB2

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