Rockwell Automation GMLC Reference Manual - Manual (Page 224)

Publication GMLC-5.2 - November 1999
Redefine Position 187
Absolute and Relative selections have the same effect when the axis is not
moving. When the axis is moving, however, absolute mode introduces a
position error equal to the motion of the axis during the time it takes to
execute the Redefine Position block and assign the new position. Relative
mode does not introduce this error and guarantees an exact correction
independent of axis speed or position.
Relative Mode
When you select Relative from the Mode menu, the new position value
offsets the current position of the axis. No motion occurs—the current
axis position (actual or command) is simply redefined, and becomes the
sum of the current position unit value plus the specified new position unit
value.
In relative mode, this block redefines axis position in such a way that no
position errors are introduced if the axis is moving. This block is
particularly useful for unwinding axis position under program control,
rather than using the built-in rotary axis feature.
Absolute and Relative mode Redefine Position blocks have the same
effect when the axis is not moving. When the axis is moving, however,
absolute mode introduces a position error equal to the motion of the axis
during the time it takes to execute the Redefine Position block and assign
the new position. Relative mode does not introduce this error and
guarantees an exact correction independent of axis speed or position.
Actual Position
When you select Actual from the Position menu, the motion controller
directly applies the new position value to the actual position of the
physical or virtual axis. Because the imaginary axis does not have an
actual position, you can select only Command from the Position menu for
the imaginary axis. The motion controller also adjusts the command
position of servo axes to reflect the new actual position and to preserve
any position error that exists. This ensures that there is no unexpected axis
motion when the positions are redefined. See Command Position for more
information on command position, actual position, and position error.