Appendix C: Tuning Your System
89
and feed forward terms must be doubled. This means that a gain of 10 must be changed to a gain
of 20. This also means that a gain of 9.5 which before could not have been entered can now be
entered as 19. In effect, the shift parameter allows the gains and feed forwards to be finely
tuned.
C.2.9 Friction Feed Forward
The friction feed forward parameter simply adds a constant value to the DAC output when the
command velocity is non-zero. The sign of the value applied to the DAC is equal to the sign of
the command velocity multiplied by the friction feed forward term. The friction feed forward
term is 16 bits and has a range from -32,768 to 32,767. Torque controlled motion systems with
constant friction can benefit most from friction feed forward.
C.3 Tuning Closed-Loop Servos
The following steps provide a quick method for tuning for a stable system with minimal position
errors:
Step 1:
Set Proportional Gain
Step 2:
Set Derivative Gain
Step 3:
Iterate steps 1 and 2
Step 4:
Set Integral Gain
Step 5:
Set Velocity and Acceleration Feed-Forward
For new systems this sequence of steps should be performed twice; once with no motor load to
provide a stable set of starting terms, and once with the motor loaded to fine-tune the initial
parameters.
C.3.1 Step 1: Set Proportional Gain (K
p
)
Start with all of the gains except the offset (K
p
, K
d
, K
i
, K
v
, and K
a
) at 0. The motor should not
turn and the shaft should be free. If the shaft turns the amplifier offset should be adjusted to
reduce the motor torque to zero. Set the Proportional Gain (K
p
) to 1. Watch the position error on
the F2 window as you change the gain. The position error should reduce to a few hundred
counts.
If the motor runs away or the shaft still turns freely, verify the wiring using the procedure
described in Installation and Setup Manual. Increase the gain by factors of 2 until the system
begins to hum or oscillate. Reduce the Proportional Gain to 1/2 the value that first produces
oscillation.
C.3.2 Step 2: Set the Derivative Gain
Start with a K
d
of 100. Increase the value of K
d
by factors of 2. Set the K
d
value to the smallest
value which produces no overshoot in the step response.
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