Rockwell Automation GMLC Reference Manual - Manual (Page 863)

Publication GMLC-5.2 - November 1999
826 Merging Different Motion Types
If the speed of the axis lies somewhere between multiples of 15 RPM, the
measured actual velocity varies from update to update such that it
averages out to the correct speed. For example, 2,000 RPM is between
1,995 (15 x 133) and 2,010 (15 x 134) RPM. So, with the axis moving at
2,000 RPM (due to a Gear Axes or Position Lock Cam block), the motion
controller calculates velocities of 1,995 RPM, 1,995 RPM, 2,010 RPM,
1,995 RPM, 1,995 RPM, and 2,010 RPM for successive updates. Taken
together it averages 2,000 RPM.
However, when the move or jog takes over from gearing or camming, the
velocity used is either 1,995 or 2,010 RPM (whichever is the most recent
sample), not exactly 2,000 RPM as expected. If the master’s speed is less
than 15 RPM, sometimes the measured actual velocity is zero. If the most
recent sample, when the merged move or jog is initiated, has measured
zero velocity, the axis does not move as expected. Actually, as far as the
motion controller is concerned, it is moving at zero speed.
There are two solutions to this problem:
• Gear or cam to command position instead of actual position.
• Merge to a move or jog at a programmed speed equal to the
average_velocity variable for the axis.
When gearing or camming to command position, use the command
velocity. This is more precise than the actual velocity, because it is
generated internally as a floating point number (rather than the
accumulation of individual encoder counts). To gear to command
position, select Command Position from the Slave to menu in the Gear
Axes function block.
The Average_Velocity for an axis is calculated by averaging the measured
actual velocities over the averaged velocity timebase (specified in the
motion controller’s application setup menu). Therefore, its resolution is
±1 count per averaged velocity timebase, rather than ±1 count per servo
update. Continuing our example, an averaged velocity timebase of 0.1
seconds results in an average velocity resolution of the following, that is
better by a factor of 100.