7.1 Introduction
An incremental encoder disk is patterned with a track of slots along its periphery, as shown in
. These
slots create an alternating pattern of dark and light lines. The disk count is defined as the number of dark and
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
QEPA
QEPB
QEPI
Figure 7-1. Optical Encoder Disk
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 detected 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 conversely, as shown in
T
0
0
1
2
3
4
5
6
7
N−6 N−5 N−4 N−3 N−2 N−1
0
QEPA
QEPB
QEPI
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
6
5
4
3
2
1
0
N−1 N−2
QEPA
QEPB
QEPI
T
0
Legend:
N = lines per revolution
Figure 7-2. QEP Encoder Output Signal for Forward/Reverse Movement
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
Enhanced Quadrature Encoder Pulse (eQEP)
462
TMS320x2806x Microcontrollers
SPRUH18I – JANUARY 2011 – REVISED JUNE 2022
Copyright © 2022 Texas Instruments Incorporated
Содержание TMS320 2806 Series
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