Rockwell Automation 1784-PM16SE SoftLogix Motion Card Setup and Configuration Manual - Manual (Page 415)

Publication 1784-UM003A-EN-P – June 2003
Motion Object Attributes 407
Damping Factor The Damping Factor attribute value is used in calculating the maximum
Position Servo Bandwidth (see below) during execution of the MRAT (Motion
Run Axis Tune) instruction. In general the Damping Factor attribute controls
the dynamic response of the drive axis. When gains are tuned using a small
damping factor (like 0.7), a step response test performed on the axis would
demonstrate under-damped behavior with velocity overshoot. A gain set
generated using a larger damping factor, like 1.0, would produce a system step
response that have no overshoot but have a significantly lower servo
bandwidth. The default value for the Damping Factor of 0.8 should work fine
for most applications.
Drive Model Time Constant The value for the Drive Model Time Constant represents lumped model time
constant for the drive’s current loop used by the MRAT instruction to
calculate the Maximum Velocity and Position Servo Bandwidth values. The
Drive Model Time Constant is the sum of the drive’s current loop time
constant, the feedback sample period, and the time constant associated with
the velocity feedback filter. This value is set to a default value when the axis is
configured based on the specific drive amplifier and motor feedback selection.
Since the bandwidth of the velocity feedback filter is determined by the
resolution of the feedback device, the value for the Drive Model Time
Constant will be smaller when high resolution feedback devices are selected.
Velocity Servo Bandwidth The value for the Velocity Servo Bandwidth represents the unity gain
bandwidth that is to be used to calculate the gains for a subsequent MAAT
(Motion Apply Axis Tune) instruction. The unity gain bandwidth is the
frequency beyond which the velocity servo is unable to provide any significant
position disturbance correction. In general, within the constraints of a stable
servo system, the higher the Velocity Servo Bandwidth is the better the
dynamic performance of the system. A maximum value for the Velocity Servo
Bandwidth is generated by the MRAT (Motion Run Axis Tune) instruction.
Computing gains based on this maximum value via the MAAT instruction
results in dynamic response in keeping with the current value of the Damping
Factor described above. . Alternatively, the responsiveness of the system can be
“softened” by reducing the value of the Velocity Servo Bandwidth before
executing the MAAT instruction.
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Damping Factor REAL
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Drive Model Time Constant REAL Sec
GSV/SSV Access Attribute Name Data Type Values
SSV/GSV Velocity Servo Bandwidth REAL Hertz