Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 403)

Publication 1784-UM003A-EN-P – June 2003
Motion Object Attributes 395
In general, modern velocity servo systems typically run with a unit gain
bandwidth of ~40 Hertz. The typical value for the Velocity Proportional Gain
is ~250 Sec
-1
.
Maximum Bandwidth
There are limitations to the maximum bandwidth that can be achieved for the
velocity loop based on the dynamics of the inner torque loop of the system
and the desired damping of the system, Z. These limitations may be expressed
as follows:
Bandwidth (Velocity) = 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.
Based on this number the corresponding gains for the loop can be computed.
Note that the bandwidth of the torque loop includes feedback sampling delay
and filter time constant.
Velocity Integral Gain When configured for a torque (current) loop servo drive, every servo update
the current Velocity Error is also accumulated in variable called the Velocity
Integral Error. This value is multiplied by the Velocity Integral Gain to
produce a component to the Torque Command that attempts to correct for
the velocity error. The characteristic of Vel I Gain correction, however, is that
any non-zero Velocity Error will accumulate in time to generate enough force
to make the correction. This attribute of Vel I Gain makes it invaluable in
applications where velocity accuracy is critical. The higher the Vel I Gain value
the faster the axis is driven to the zero Velocity Error condition. Unfortunately,
I Gain control is intrinsically unstable. Too much I Gain will result in axis
oscillation and servo instability.
In certain cases, Vel I Gain control is disabled. One such case is when the
servo output to the axis’ drive is saturated. Continuing integral control
behavior in this case would only exacerbate the situation. Another common
case is when performing certain motion. When the Integrator Hold Enable
attribute is set, the servo loop automatically disables the integrator during
commanded motion.
Due to the destabilizing nature of Integral Gain, it is recommended that
Position Integral Gain and Velocity Integral Gain be considered mutually
exclusive. If Integral Gain is needed for the application use one or the other,
but not both. In general, where static positioning accuracy is required, Position
Integral Gain is the better choice.
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Velocity Integral Gain REAL 1/mSec