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Definitions

Count (N): The number of bar and window pairs or counts

per revolution (CPR) of the codewheel. Or the number of

lines per inch of the codestrip (LPI)

1 shaft rotation = 360 degrees

= N cycles

1 cycle (c)

= 360 electrical degree, equivalent to 1

bar and window pair.

Direction of Rotation: When the codewheel rotates in the

counter-clockwise direction (as viewed from the encoder

end of the motor), channel A will lead channel B. If the

codewheel rotates in the clockwise direction, channel B

will lead channel A.

Line Density: The number of window and bar pair per

unit length, express in either lines per inch (LPI) or lines

per mm (LPmm)

Optical Radius (Rop): The distance from the codewheel’s

center of rotation to the optical center (O.C) of the

encoder module.

Gap (G): The distance from surface of the encoder to the

surface of codewheel or codestrip

center of rotation to center of Alignment Tab receiving

hole.

rotary motion mechanical displacement in the radial and

tangential directions relative to the nominal alignment

of the sensor in relation to then tangential direction.

This applies for both rotary and linear motion.of elec-

trical degrees that an output is high during one cycle,

nominally 180°e or 1/2 a cycle.

Theory of Operation

The HEDS-97EX is a C-shaped emitter/detector module.

Coupled with a codewheel/codestrip, it translates rotary

motion into a two-channel analog output.

The module contains a single Light Emitting Diode (LED)

as its light source. The light is collimated into a parallel

beam by means of a single lens located directly over the

LED. Opposite the emitter is the integrated detector

circuit. This IC consists of multiple sets of photodetectors

and the signal processing circuitry necessary to produce

the analog waveforms.

The codewheel/codestrip moves between the emitter

and detector, causing the light beam to be interrupted

by the pattern of spaces and bars on the codewheel/

codestrip.

The photodiodes, which detect these interruptions, are

arranged in a pattern that corresponds to the radius and

count density of the codewheel/codestrip.

These detectors are also spaced such that a light period

on one pair of detectors corresponds to a dark period on

the adjacent pairs of detectors. The photodiode outputs

are fed through the signal processing circuitry, which

produce the final outputs for Channel A and Channel

B. Due to this integrated phasing technique, the analog

output of Channel A is in quadrature with Channel B (90

degrees out of phase).