Rockwell Automation 21G PowerFlex 750-Series AC Drives Reference Manual - Manual (Page 55)

Rockwell Automation Publication 750-RM001F-EN-P - February 2012 55
General Device and Feedback Monitoring Configuration Chapter 5
Dual Encoders
If the P27 [Fbk Mode] parameter is set to 2 Encoders, each encoder input is
processed by a single channel and cross-checked in a 1oo2 architecture.
Discrepancy checking is performed between the two encoders. After the
discrepancy checks have passed, the speed, direction, and stopped status are
derived from encoder 1.
Speed and direction checks are affected by these parameters:
• Dual Feedback Speed Ratio, P39 [Fbk Speed Ratio]
• Dual Feedback Position Tolerance, P41 [Fbk Pos Tol]
• Dual Feedback Speed Discrepancy Tolerance, P40 [Fbk Speed Tol]
Dual Feedback Speed Ratio
The Dual Feedback Speed Ratio, P39 [Fbk Speed Ratio] parameter, is defined as
the ratio of the expected speed of encoder 2 divided by the expected speed of
encoder 1. This parameter configures the anticipated gearing between encoder 1
and encoder 2.
If P27 [Fbk Mode] equals 0 (1 encoder), the only legal value for P39 [Fbk Speed
Ratio] parameter is 0.0.
If P27 [Fbk Mode] is greater than 0, the range of legal values for P39 [Fbk Speed
Ratio] is from 0.0001…10,000.0.
For example, if encoder 2’s speed is expected to be 1000 revolutions per second
while encoder 1’s speed is expected to be 100 revolutions per second, then the
P39 [Fbk Speed Ratio] should be configured as 10.0.
The units used to measure encoder speed are configurable as either rotary (rev) or
linear (mm) units. Any combination of rotary and linear units for the two
encoders is allowed.
IMPORTANT All monitoring functions are based on the speed of encoder 1. The encoder 2
signal is used for fault diagnostics.
Dual Feedback Speed Ratio
P39 [Fdk Speed Ratio] Parameter
=
Expected Speed of Encoder 2
Expected Speed of Encoder 1