rotary encoder.pdf

7/28/2019 Rotary Encoder.pdf

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

PRESENTATION

ROTARY ENCODERSOn

Course Coordinator Dr. Sameer Saraswati


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Definition What is an Encoder?

An encoder is a device that converts motion into a sequence of digitalpulses.

A rotary encoder, also called a shaft encoder, is an electro-mechanicaldevice that converts the angular position or motion of a shaft or axle toan analog or digital code.

By counting a single bit or by decoding a set of bits, the pulses can beconverted to relative or absolute position measurements.


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TYPES OF ROTARY ENCODER

Absolute Encoders - The output of absolute encoders indicatesthe current position of the shaft, making them angle transducers.

Incremental Encoders - The output of incremental encodersprovides information about the motion of the shaft.


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ABSOLUTE ENCODER

An absolute encoder maintains position information when power is removed from the system.

Produces a separate and unique coded word for each shaftposition.

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 abinary code that distinguishes N distinct positions of the shaft.


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ABSOLUTE ENCODER WORKING

A disc or a plate containing opaque andtransparent segments passes between alight source (such an LED) and detector tointerrupt a light beam.

The electronic signals that are generatedare then fed into the controller whereposition and velocity information iscalculated based upon the signals received.


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

Each partitioned area of the matrix correspond to a bit of data.Transparent area=1,Opaque area=0;

Each track has a pick off sensors arranged on radial line facingtrack on one side of the disk it is illuminated by a light sourcefrom 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 andbinary codes.The gray code is used so that only one track (one bit) willchange state for each count transition, unlike the binary codewhere multiple tracks (bits) change at certain count transitions.


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Absolute Optical Encoder

In the fig word size of data is 4bits.Outermost Element is LSB.

Innermost Element is MSB.The Angular position is given as=360/2 4.


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Comparison between Binary Code & Gray Code

Decimal cod e 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

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 atcertain count transitions.


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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.


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Angular Position and Resolution

To determine we need the count of the pulse, nmaximum count M, corresponding to highest displacement max

= n max / M If we use a digital counter of resolution n bits

M = 2 n-1 (zero count included)M= 2 n-1 1 (zero count not incl uded)

ResolutionPhysical Resolution=2/N(If one pulse signal is used)

where, N=no. of lines.


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APPLICATIONS Motor Speed / RPM Readout

In this type of application, the encoder is mounteddirectly to the end of a motor via a shaft.

As a rotary feedback devices.

Scroll wheel of optical mice.

Multi- axis control

Radar antenna, robotic arms.

Position measurement etc.


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THANK YOU

rotary encoder.pdf

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