Enhanced Quadrature Encoder Pulse (eQEP) Module
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15.4 Enhanced Quadrature Encoder Pulse (eQEP) Module
15.4.1 Introduction
A single track of slots patterns the periphery of an incremental encoder disk, as shown in Figure 15-129.
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.
Figure 15-129. 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
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 degrees 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 15-130.
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.
1650
Pulse-Width Modulation Subsystem (PWMSS) SPRUH73H–October 2011–Revised April 2013
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