336 Rockwell Automation Publication MOTION-RM003I-EN-P - February 2018
Interpolated Actual Position
Usage Access T Data
Type
Default Min Max Semantics of Values
Required - E
Get/
GSV
T REAL - - - Position Units
The Interpolated Actual Position attribute is the interpolation of the actual
position, based on past axis trajectory history, at the time specified by the
Interpolation Time attribute.
Command Position
Usage Access T Data
Type
Default Min Max Semantics of Values
Required - FPV
Get/
GSV
T REAL - - - Position Units
Tag access is supported but the
value is valid only when Auto Tag
Update of the Motion Group
Object is enabled.
The Command Position attribute 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 Logix 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, for example
an 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.
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 errors due to the position error of the axis at the time the
calculation is performed.
Tag access supported but value is valid only when Auto Tag Update of the Motion
Group Object is enabled.
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