Optical Encoder Disk; Qep Encoder Output Signal For Forward/Reverse Movement - Texas Instruments Concerto F28M35 Series Technical Reference Manual

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Introduction

A single track of slots patterns the periphery of an incremental encoder disk, as shown in

These slots create an alternating pattern of dark and light lines. The disk count is defined as the number

of dark/light line pairs that occur per revolution (lines per revolution). As a rule, a second track is added to

generate a signal that occurs once per revolution (index signal: QEPI), which can be used to indicate an

absolute position. Encoder manufacturers identify the index pulse using different terms such as index,

marker, home position, and zero reference

To derive direction information, the lines on the disk are read out by two different photo-elements that

"look" at the disk pattern with a mechanical shift of 1/4 the pitch of a line pair between them. This shift is

realized with a reticle or mask that restricts the view of the photo-element to the desired part of the disk

lines. As the disk rotates, the two photo-elements generate signals that are shifted 90° out of phase from

each other. These are commonly called the quadrature QEPA and QEPB signals. The clockwise direction

for most encoders is defined as the QEPA channel going positive before the QEPB channel and vise

versa as shown in

Figure 9-2. QEP Encoder Output Signal for Forward/Reverse Movement

Legend: N = lines per revolution

The encoder wheel typically makes one revolution for every revolution of the motor or the wheel may be at

a geared rotation ratio with respect to the motor. Therefore, the frequency of the digital signal coming from

the QEPA and QEPB outputs varies proportionally with the velocity of the motor. For example, a 2000-line

encoder directly coupled to a motor running at 5000 revolutions per minute (rpm) results in a frequency of

166.6 KHz, so by measuring the frequency of either the QEPA or QEPB output, the processor can

determine the velocity of the motor.

SPRUH22I – April 2012 – Revised November 2019

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Figure 9-1. Optical Encoder Disk

Clockwise shaft rotation/forward movement

Anti-clockwise shaft rotation/reverse movement

0 N−1 N−2 N−3 N−4 N−5 N−6 N−7

N−6 N−5 N−4 N−3 N−2 N−1

C28 Enhanced QEP (eQEP) Module

Introduction

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