Rockwell Automation 7000 PowerFlex Medium Voltage AC Drive (B Frame) - ForGe Control (PanelView 550) - Manual (Page 138)

138 Rockwell Automation Publication 7000-UM151E-EN-P - January 2013
Chapter 3 Component Definition and Maintenance
Positional Encoder Guidelines
When selecting a positional encoder, follow these guidelines for optimal
performance.
1. Code Selection: Purchase absolute encoders with either Gray code or
Binary output format. Gray code is a form of binary code where only a
single bit changes at a time for each sequential number or position. The
fact that only a single bit changes at a time make it easier for the Universal
Encoder Interface to read valid positional data and not ambiguous data. If
we compare the Natural Binary code to Gray code for the transition from
255 to 2556, we see:
All nine bits changed in the Binary Code while only the MSB of the Gray
code changed. In the Universal Encoder Interface the frequency filter
components and input hysteresis create delays. Differences in these delays
could cause errors due to reading a bit as ON when switching to OFF or
vice versa. In the case of Gray code, since only one bit ever changes, the
ambiguity error is never more than one count. For this reason and to
reduce inrush currents, Rockwell Automation recommends using Gray
code Positional Encoders.
2. Data Polarity: Absolute encoders typically have a High True output. If the
encoder model does not have a High/True (or Non Inverted/Inverted)
option you should assume it to be High True. In the case of a 10bit High
True encoder the zero position is represented by 0000000000. Whereas a
Low/True encoder the zero position is 1111111111. On the Universal
Encoder Interface the position data is inverted in hardware. That is a ‘1’
will turn on an Optocoupler producing a ‘0’. Therefore a High True
encoder would produce 1111111111 for the zero position. With the
POL_QRDNT jumper you can control the polarity of the input. With the
jumper installed (factory default) it is setup to accept High True encoders
and an extra inversion is done in the Universal Encoder Interface. If you are
using a Low True encoder then this jumper needs to be removed so that the
zero position is inverted by the optocouplers alone.
The other role of the POL_QRDNT jumper is to correct the data in the
event the encoder was mounted so that a CCW rotation produced
decreasing counts. If this is the case the POL_QRDNT jumper should be
configured to the opposite of what it should normally be for the data
polarity. For example if the Universal Encoder Interface is configured to
operate with High True encoders (POL_QRDNT installed), remove it to
correct for encoder mounting.
Binary Code Gray Code
255 011111111 010000000
256 100000000 110000000