Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 399)

Publication 1784-UM003A-EN-P – June 2003
Motion Object Attributes 391
In general, modern position servo systems typically run with a unit gain
bandwidth of ~16 Hertz. The typical value for the Position Proportional Gain
is ~100 Sec
-1
.
Maximum Bandwidth
There are limitations to the maximum bandwidth that can be achieved for the
position loop based on the dynamics of the inner velocity and torque loops of
the system and the desired damping of the system, Z. These limitations may be
expressed as follows:
Bandwidth (Pos) = 0.25 * 1/Z
2
* Bandwidth (Vel) = 0.25 * 1/Z
2
*
Bandwidth (Torque)
For example, if the bandwidth of the drive’s torque loop is 100 Hz and the
damping factor, Z, is 0.8, the velocity bandwidth is approximately 40 Hz and
the position bandwidth is 16 Hz. Based on these numbers the corresponding
proportional gains for the loops can be computed. Note that the bandwidth of
the torque loop includes feedback sampling delay and filter time constant.
Position Integral Gain Position Integral Gain, or Pos I Gain, improves the steady-state positioning
performance of the system. By using Position Integral Gain, it is possible to
achieve accurate axis positioning despite the presence of such disturbances as
static friction or gravity. Increasing the integral gain generally increases the
ultimate positioning accuracy of the system. Excessive integral gain, however,
results in system instability. .
Every servo update the current Position Error is accumulated in a variable
called the Position Integral Error. This value is multiplied by the Position
Integral Gain to produce a component to the Velocity Command that
attempts to correct for the position error. The characteristic of Pos I Gain
correction, however, is that any non-zero Position Error will accumulate in
time to generate enough force to make the correction. This attribute of Pos I
Gain makes it invaluable in applications where positioning accuracy or tracking
accuracy is critical. The higher the Pos I Gain value the faster the axis is driven
to the zero Position Error condition. Unfortunately, Pos I Gain control is
intrinsically unstable. Too much Pos I Gain will result in axis oscillation and
servo instability.
If the axis is configured for an external velocity loop servo drive, the Pos I
Gain should be zero–most analog velocity loop servo amplifiers have integral
gain of their own and will not tolerate any amount of Pos I Gain in the position
loop without producing severe oscillations. If Pos I Gain is necessary for the
application, the velocity integrator in the drive must be disabled.
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Position Integral Gain REAL 1/mSec-Sec