Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 280)

Publication 1784-UM003A-EN-P – June 2003
272 Motion Object Attributes
Command Position Command Position is the desired or commanded position of a physical axis, in
the configured Position Units of that axis, as generated by the controller in
response to any previous motion Position Control instruction. Command
Position data is transferred by the ControlLogix Processor to a physical axis as
part of an ongoing synchronous data transfer process which results in a delay
of one coarse update period. Thus, the Command Position value that is
obtained is the command position that will be acted upon by the physical servo
axis one coarse update period from now.
The figure below shows the relationship between Actual Position, Command
Position, and Position Error for an axis with an active servo loop. Actual
Position is the current position of the axis as measured by the feedback device
(e.g., encoder). Position error is the difference between the Command and
Actual Positions of the servo loop, and is used to drive the motor to make the
actual position equal to the command position.
Figure 13.1 Actual and Command Position Error Relationship
Command position is useful when performing motion calculations and
incremental moves based on the current position of the axis while the axis is
moving. Using command position rather than actual position avoids the
introduction of cumulative errors due to the position error of the axis at the
time the calculation is performed.
Strobe Position Strobe Actual Position, and Strobe Command Position are used to
simultaneously store a snap-shot of the actual, command position and master
offset position of an axis when the MGSP (Motion Group Strobe Position)
instruction is executed. The values are stored in the configured Position Units
of the axis. Refer to the AC Motion Instruction Specification for a detailed
description of the MGSP instruction.
GSV/SSV Access Attribute Name Data Type Values
GSV Command Position REAL Position Units
GSV/SSV Access Attribute Name Data Type Values
GSV Strobe Actual Position REAL Position Units
GSV Strobe Command Position REAL Position Units