Rockwell Automation 1398-DDM-xxx USE MNL/ULTRA 200 DIG.SERVO.DR - Manual (Page 172)

Publication 1398-5.0 – October 1998
9-2 Tuning
Mechanical Resonance
Mechanical resonance between the motor and the load occurs when
the motor and load are oscillating with the same frequency but
opposite phase: when the motor is moving clockwise the load is
moving counter clockwise. The amplitude of the motor and load
oscillations is such that the total momentum of the oscillating system
is zero. In the case of a high load to motor inertia ratio this means that
the motor may be moving quite a lot while the load is not moving
nearly as much. Mechanical resonance occurs as a result of
compliance (springiness) between the motor inertia and load inertia. It
may result from belts, flexible couplings or the finite torsional
stiffness of shafts. In general, the stiffer the couplings, the higher the
resonant frequency and lower the amplitude. If the motor shaft is
directly coupled to the load, a mechanically resonating system usually
emits a buzz or squeal at the motor.
There are several ways of dealing with this problem but they fall into
two groups: change the mechanical system or change the servo-motor
response. Changing the mechanical system might involve reducing
the inertia ratio via gearboxes or pulleys, or by increasing the stiffness
of the couplings. For very high performance systems and systems with
low resonance frequencies the mechanics may require changing to
effectively deal with the resonance.
The second way of dealing with mechanical resonance is by changing
the servo-motor response. This may be done by using a negative D-
gain value and by reducing the P-gain, I-gain, velocity loop update
rate or low-pass filter value. The D-term of the PID velocity regulator
(see the velocity and torque current conditioning structure) subtracts
(or adds) a proportion of the motor acceleration from the velocity
error. The D-gain has the effect of increasing the acceleration current
if the motor is accelerating in the wrong direction, but reducing the
acceleration current if the motor is already accelerating in the right
direction. When used in this way the D-gain dampens an oscillating or
ringing system. In the case of motor-load mechanical resonance, a
positive D-gain actually worsens the situation. When a negative D-
gain value is used in a mechanically resonating system it may be
thought of as subtracting the load acceleration (the opposite sign of
the motor acceleration since the system is resonating). This tends to
bring the motor and load back into phase with each other and therefore
reduces or eliminates mechanical resonance.
Reducing the value of the P-gain, low-pass filter frequency and the
update frequency all have the effect of reducing the servo-motor
bandwidth. As long as the resonating frequency is fairly high this will
likely be acceptable, but if the resonating frequency is low it may be
necessary to modify the mechanics of the system.