Rotary Encoder.pdf 5th20

PRESENTATION On

ROTARY ENCODERS Course Coordinator Dr. Sameer Saraswati

Submitted by: RAHUL SOREN (2012CC12) M.TECH.(CAD/CAM)

Definition – What is an Encoder? • An encoder is a device that converts motion into a sequence of digital pulses. • A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. • By counting a single bit or by decoding a set of bits, the pulses can be converted to relative or absolute position measurements.

TYPES OF ROTARY ENCODER

• Absolute Encoders - The output of absolute encoders indicates the current position of the shaft, making them angle transducers. • Incremental Encoders - The output of incremental encoders provides information about the motion of the shaft.

ABSOLUTE ENCODER • An absolute encoder maintains position information when power is removed from the system. • Produces a separate and unique coded word for each shaft position. • Every reading independent of the preceding one. Major Elements:1. A multiple track light source. 2. A multiple track light receiver. 3. A multiple track rotary disk.

• The optical disk of the absolute encoder is designed to produce a binary code that distinguishes N distinct positions of the shaft.

ABSOLUTE ENCODER WORKING • A disc or a plate containing opaque and transparent segments es between a light source (such an LED) and detector to interrupt a light beam. • The electronic signals that are generated are then fed into the controller where position and velocity information is calculated based upon the signals received.

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The number of tracks is ‘n’ in this case. The disk is divided into ‘2n’ Sectors.

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Each partitioned area of the matrix correspond to a bit of data.

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Transparent area=1,Opaque area=0; 

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Each track has a pick off sensors arranged on radial line facing track on one side of the disk it is illuminated by a light source from the other side of the disk. As Disk rotates bank of pick up sensor generates a set of pulses. Numerical encoding used in the absolute encoder are gray and binary codes. The gray code is used so that only one track (one bit) will change state for each count transition, unlike the binary code where multiple tracks (bits) change at certain count transitions.

Absolute Optical Encoder

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In the fig word size of data is 4bits. Outermost Element is LSB. Innermost Element is MSB. The Angular position is given as=360/24.

Comparison between Binary Code & Gray Code Decimal code

Rotation range (deg.)

Binary code

Gray code

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0-22.5 22.5-45 45-67.5 67.5-90 90-112.5 112.5-135 135-157.5 15.75-180 180-202.5 202.5-225 225-247.5 247.5-270 270-292.5 292.5-315 315-337.5 337.5-360

0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111

0000 0001 0011 0010 0110 0111 0101 0100 1100 1101 1111 1110 1010 1011 1001 1000

Gray code is designed so that only one track (one bit) will change state for each count transition whereas in binary code multiple tracks (bits) change at certain count transitions.

INCREMENTAL ENCODER • An incremental encoder records changes in position. • Incremental encoders emit pulses which determine how far the device has rotated. • Consists of two tracks and two sensors whose outputs are called channels A and B. • Channel A gives the information about the rotation. • Channel B provides the sense of rotation. • Another channel Z or INDEX gives the absolute position of shaft.

• The key specification is the number of pulses per revolution (PPR) or pulses per inch (or centimetre). PPR of 250, 512, 1000, 1024, or even up to 100,000 pulses per revolution are available.

Angular Position θ and Resolution • To determine θ we need the count of the pulse, n maximum count M, corresponding to highest displacement θmax θ = n θmax / M • If we use a digital counter of resolution n bits M = 2 n-1 (zero count included) M= 2n-1–1 (zero count not included)

Resolution

Physical Resolution=2π/N (If one pulse signal is used) where, N=no. of lines.

APPLICATIONS • Motor Speed / RPM Readout In this type of application, the encoder is mounted directly to the end of a motor via a shaft. • As a rotary devices. • Scroll wheel of optical mice.

• Multi- axis control Radar antenna, robotic arms. • Position measurement etc.

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