386 Rockwell Automation Publication MOTION-RM003I-EN-P - February 2018
Position Unwind
Usage Access Data
Type
Default Min Max Semantics of Values
Required - E
Set/
SSV#
DINT
Default Motion
Resolution
1 10
9
Counts/Cycle
# Indicates the attribute cannot be set while the tracking command (Tracking Command bit in CIP Axis Status is true).
If the axis is configured for cyclic Travel Mode, a value for the Position Unwind
attribute is required. This is the value used to perform electronic unwind of the
cyclic axis' position. Electronic unwind operation provides infinite position range
for cyclic axes by subtracting the position unwind value from both the actual and
command position every time the axis completes a machine cycle. To avoid
accumulated error due to round-off with irrational conversion constants, the
unwind value is expressed as an integer number of feedback counts per cycle.
See also
Motion Resolution Value Examples on page 386
Motion Resolution is one of the Motion Scaling attributes. These examples
demonstrate how the Motion Resolution value may be used together with the
Conversion Constant to handle various machine applications.
Direct-Drive Rotary Shear Application
In this mechanical configuration a rotary motor is directly driving a rotary shear
drum equipped with three knives to cut a product to specified length, thus
producing three products per revolution of the output shaft. Because the default
Motion Resolution value is 1,000,000 Motion Counts/Motor Rev and the user's
Position Unit is, say, Products, the Conversion Constant would be 1,000,000/3
Motion Counts/Product. This is particularly problematic when configured to
perform an Unwind operation every product cut cycle where a 1/3 count error
would accumulate with every cycle. But, because Motion Resolution is
configurable, you can simply set the Motion Resolution to 300,000 Motion
Counts/Motor Rev and the Conversion Constant could then be set to 100,000
Motion Counts/Motor Rev, and a Rotary Unwind value of 100,000 Motion
Counts/Cycle.
Because the Conversion Constant is now a clean rational number, this system
scales without any loss of mechanical precision, for example, a move of three
Products would move the output shaft exactly one revolution. By setting the
Travel Mode to Cyclic, entering Position Scaling of three Products per Motor
Rev, and a Position Unwind value of one Product per Cycle, appropriate values for
scaling factors, Motion Resolution, Conversion Constant, and Rotary Unwind are
automatically calculated.
Motion Resolution Value Examples
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