ME262
Introduction to Microprocessor & Digital Logic (Sensor and Sensor Technology) Summer 2008
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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What is sensor? A measuring device to measure a physical quantity. It is also known as a feedback control unit.
+ ∑
Controller
Power Electronics
D/A
Actuators
PC, Control Unit
A/D
Signal Conditioner
Sensors
Close-loop Process Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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Type of Sensors Variables
Sensors
Sound
Microphone
Temperature
Thermocouple, Radiation Thermometer
Light
Phototransistor
Force/torque
Strain gauge Piezoelectric Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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Variables
Sensors
Position/angle
Encoder, LVDT, Capacitors, Potentiometer, Hall Sensors
Velocity
Tachometer, Encoder
Acceleration
Accelerometer, Piezoelectric, indirectly from position sensors
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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Position and Velocity Sensor In this course, we study three type of sensors: Electrical Tachometer, Potentiometers, and optical Encoder. The emphasize is on encoders.
Electrical Tachometer A permanent Magnet DC motor is used to measure the velocity.
Output Voltage
Permanent Magnet DC Motor
Shaft Speed
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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Potentiometer Potentiometers or variable resistors can be used for position measurements. They can be used for translational and rotational velocity. They are temperature dependant and they are not useful for velocity measurement. They are susceptible to wear and noise.
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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Optical Encoder Two type of encoders are available in large: 1. Absolute encoder 2. Incremental encoder These two types can be rotary or linear. A digital optical encoder is a device that converts motion into a sequence of digital pulses. By counting a single bit or by decoding a set of bits, the pulses can be converted to relative or absolute position measurements. Most encoders are composed of a glass or plastic code part with a photographically deposited radial or linear pattern organized in tracks. As radial/linear lines in each track interrupt the beam between a photoemitterdetector pair, digital pulses are produced. Before we explain the structure of these encoders, we define a special coding called “Gray Coding”. Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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Gray Code An ordering of 2 n binary numbers such that only one bit changes from one entry to the next. Gray codes for 4 or more bits are not unique.
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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Absolute Encoder The optical disk of the absolute encoder is designed to produce a digital word that distinguishes N distinct positions of the shaft. For example, if there are 8 tracks, the encoder is capable of producing 256 distinct positions or an angular resolution of 1.406 (360/256) degrees. The most common types of numerical encoding used in the absolute encoder are gray and binary codes. The linear patterns and associated timing diagrams are what the photo-detectors sense as the code disk circular with the shaft. Disadvantage: Needs a larger disk or strip for higher resolution
Binary Absolute Encoder
Resolution of 360/8 = 45 Deg.
http://www.kavlico.com/index_home.html
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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Gray Absolute Encoder
http://www.heidenhain.com/ phaise2/posps.html
Resolution of 360/210 = 0.350
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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4-bit Gray code absolute encoder disk track patterns
4-bit Binary code absolute encoder disk track patterns
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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4-Bit gray and natural binary codes Decimal code
Rotation range (deg.)
Binary code
Gray code
0
0-22.5
0000
0000
1
22.5-45
0001
0001
2
45-67.5
0010
0011
3
67.5-90
0011
0010
4
90-112.5
0100
0110
5
112.5-135
0101
0111
6
135-157.5
0110
0101
7
15.75-180
0111
0100
8
180-202.5
1000
1100
9
202.5-225
1001
1101
10
225-247.5
1010
1111
11
247.5-270
1011
1110
12
270-292.5
1100
1010
13
292.5-315
1101
1011
14
315-337.5
1110
1001
15
337.5-360
1111
1000
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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Incremental Encoder
Incremental encoders operate by means of a grating moving between a light source and a detector. They need a reference for position measurement. Higher resolution can be obtained easier. Needs a decoder to detect direction and position/velocity.
http://www.kavlico.com/index_home.html
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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Decoding 01
01
CU
A B
CU
A
CD 11
B
00
00
11 CD
A
CD CU
B
10
10 A
2X decoding
1X decoding B 01
A
CU
B
CU
CD
CD
00
11
Counter Clockwise Rotation CD
CD
CU
A
CU 10
B Clockwise Rotation
4X decoding
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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Incremental encoder disk patterns
Results of X1, X2, X4 decoding Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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The quadrature signals A and B can be decoded to yield the direction of rotation as shown in last page figure. Decoding transitions of A and B by using sequential logic circuits in different ways can provide three different resolutions of the output pulses: 1X, 2X, 4X. 1X resolution only provides a single pulse for each cycle in one of the signals A or B, 4X resolution provides a pulse at every edge transition in the two signals A and B providing four times the 1X resolution. The direction of rotation (clockwise or counter-clockwise) is determined by the level of one signal during an edge transition of the second signal.
Introduction to Microprocessor and Digital Logic, ME262, University of Waterloo, S'08
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