Rockwell Automation 1395 Digital DC Drive FRN 5-10.10/9.30 - Manual (Page 102)

Chapter 5
Functional Description
5-16
Current Tune
The Autotune Current Tune calculates the maximum current loop
bandwidth and current loop gains. The KP and KI Armature Loop gains
(Parameters 735 and 736) are based on the maximum discontinuous current
(Parameter 734), desired Current Loop Bandwidth (Parameter 741) and
Damping Factor (Parameter 743). Parameter 734 is used by autotuning to
calculate the current loop gains because Parameter 734 is inversely
proportional to the armature inductance. Autotuning does not look at the
actual current loop response to determine the gains or verify the actual
bandwidth. Therefore, the desired bandwidth should be used as a measure
of relative performance and not absolute performance. For example, if
maximum performance is desired, then the desired bandwidth should be set
equal to the maximum bandwidth.
The current processor limits the desired bandwidth that the user has entered
to the maximum bandwidth calculated by the drive, which cannot be
modified by the user. Whenever the user enters or reads the desired
bandwidth, the current processor recalculates the maximum bandwidth
(Parameter 742) and limits the desired bandwidth. Next, the current loop
gains are calculated based on the damping factor, desired bandwidth,
maximum discontinuous current and the AC line frequency which is
measured by the current processor.
When Autotune Current Tune is executed, the present values for the
desired bandwidth and damping factor specify the desired dynamic
behavior of the current loop. If the user wants to tune the current loop with
a different value of damping factor, the parameter has to be updated before
the tune is executed. The current loop will be more responsive and
reproduce the current reference more accurately if the bandwidth is
increased. However, the current may exhibit more noise and overshoot as
the current loop bandwidth is increased. Typically, the bandwidth should
be set to about 90% of maximum to provide fast performance yet minimize
the effects of noise and possible excessive overshoot. The damping factor
influences the amount of overshoot the current loop will exhibit during a
transient. The current will typically exhibit more overshoot and become
oscillatory (underdamped) as the damping factor is reduced below one.
For a damping factor above one, armature current should not exhibit much
overshoot and have a slower rise time for a given current loop bandwidth.
Velocity Loop Tuning The velocity loop tuning functions enable you to calibrate the drive to the
motor and the system connected to the motor. The velocity loop function
can effect the following parameters:
Parameter No. Description
613
Motor inertia
659 KI Velocity Loop
660 KP Velocity Loop
700 Velocity Loop Desired Bandwidth
701 Velocity Loop Maximum Bandwidth
703 System Inertia