Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 281)

Publication 1784-UM003A-EN-P – June 2003
Motion Object Attributes 273
Since the MGSP instruction simultaneously stores the actual and command
positions for all axes in the specified group of axes, the resultant Strobe Actual
Position and Strobe Command Position values for different axes can be used
to perform real time calculations. For example, the Strobe Actual Positions can
be compared between two axis to provide a form of “slip compensation” in
web handling applications.
Start Position Whenever a new motion planner instruction starts for an axis (for example,
using a MAM instruction), the value of the axis command position and actual
position is stored at the precise instant the motion begins. These values are
stored as the Start Command Position and Start Actual Position respectively in
the configured Position Units of the axis.
Start Positions are useful to correct for any motion occurring between the
detection of an event and the action initiated by the event. For instance, in coil
winding applications, Start Command Positions can be used in an expression
to compensate for overshooting the end of the bobbin before the gearing
direction is reversed. If you know the position of the coil when the gearing
direction was supposed to change, and the position at which it actually changed
(the Start Command Position), you can calculate the amount of overshoot, and
use it to correct the position of the wire guide relative to the bobbin.
Average Velocity Average Velocity is the current speed of an axis in the configured Position
Units per second of the axis. Unlike the Actual Velocity attribute value, it is
calculated by averaging the actual velocity of the axis over the configured
Average Velocity Timebase for that axis. Average velocity is a signed value with
the sign indicating the direction the axis is currently moving.
The resolution of the Average Velocity variable is determined by the current
value of the Averaged Velocity Timebase parameter, and the configured
Conversion Constant (feedback counts per Position Unit) for the axis. The
greater the Average Velocity Timebase value, the better the speed resolution,
but the slower the response to changes in speed.
The Average Velocity resolution in Position Units per second may be
calculated using the equation below.
GSV/SSV Access Attribute Name Data Type Values
GSV Start Actual Position REAL Position Units
GSV Start Command Position REAL Position Units
GSV/SSV Access Attribute Name Data Type Values
GSV Average Velocity REAL Position Units / Sec