Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 398)

Publication 1784-UM003A-EN-P – June 2003
390 Motion Object Attributes
Position Proportional Gain The Position Error is multiplied by the Position Proportional Gain, or Pos P
Gain, to produce a component to the Velocity Command that ultimately
attempts to correct for the position error. Increasing this gain value increases
the bandwidth of the position servo loop and results in greater “static
stiffness” of the axis which is a measure of the corrective force that is applied
to an axis for a given position error. Too little Pos P Gain results in excessively
compliant, or mushy, axis behavior. Too large a Pos P Gain, on the other hand,
can result in axis oscillation due to classical servo instability. .
A well-tuned system moves and stops quickly or "smartly" and exhibits little or
no "ringing" during constant velocity or when the axis stops. If the response
time is poor, or the motion "sloppy" or slow, the proportional gain may need
to be increased. If excessive ringing or overshoot is observed when the motor
stops, the proportional gain may need to be decreased.
While the Pos P Gain is typically established by the automatic servo tuning
procedure, the Pos P gain may also be set manually. Before doing this it must
be stressed that the Torque Scaling factor for the axis must be established for
the drive system. Refer to Torque Scaling attribute description for an
explanation of how the Torque Scaling factor can be calculated. Once this is
done the Pos P Gain can be computed based on either the desired loop gain or
the desired bandwidth of the position servo system.
Loop Gain Method
If you know the desired loop gain in Inches per Minute per mil or millimeters
per minute per mil, use the following formula to calculate the corresponding P
gain.
Pos P Gain = 16.667 * Desired Loop Gain (IPM/mil)
According to an old machine tool rule of thumb, a loop gain of 1 IPM/mil
(Pos P gain = 16.7 Sec
-1
) provides stable positioning for virtually any axis. In
general, however, modern position servo systems typically run much tighter
than this. The typical value for the Position Proportional Gain is ~100 Sec
-1
.
Bandwidth Method
If you know the desired unity gain bandwidth of the position servo in Hertz,
use the following formula to calculate the corresponding P gain.
Pos P Gain = Bandwidth (Hertz) / 6.28
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Position Proportional Gain REAL 1/Sec