Danfoss MCO 305 Operating Instructions Manual page 24

Optimizing the PID controll...
View the velocity profiles: If during acceleration
the actual velocity is constantly lower than the
reference velocity profile, then set a higher value
of Acceleration Feed-forward and → Start the
Testrun again.
Run successive test runs until the two velocity
profiles shown in the Testrun graph have similar
rampup and ramp-down curves.
Acceleration Feed-forward is now optimized, save
the current value.
32-6* PID-Controller
32-60 Proportional factor
32-61 Derivative factor
32-62 Integral factor
32-65 Velocity Feed-Forward
32-66 Acceleration Feed-Forward
5. Next step is finding the maximum stable value of
the Proportional Factor in the PID controller.
Execute a Testrun with KPROP=0, KDER=0,
KINT=0. Set FFVEL and FFACC at the optimized
values found above.
View the velocity profile. If the velocity profile is
not oscillating then increase the Proportional
Factor. Run successive testruns until the actual
velocity profile is oscillating mildly.
Decrease this "mildly" unstable Proportional
Factor value to about 70 %. Save this new value.
32-6* PID-Controller
32-60 Proportional factor
32-61 Derivative factor
32-62 Integral factor
32-65 Velocity Feed-Forward
32-66 Acceleration Feed-Forward
6. In order to dampen the oscillations created by
the Proportional part of the controller, the
Derivative Value should now be optimized. Start a
Testrun with KINT=0 and KDER=200. Set FFVEL,
FFACC and KPROP at the optimized values found
above.
Run successive test runs with increasing values of
the Derivative Value. At first the oscillations will
gradually reduce. Stop increasing Derivative Value
when the oscillations begin to increase.
Save the last value of Derivative.
32-6* PID-Controller
32-60 Proportional factor
32-61 Derivative factor
32-62 Integral factor
32-65 Velocity Feed-Forward
32-66 Acceleration Feed-Forward
7. In any system that requires a zero steady-state
error, the Integral part of the controller must be
MCO 305 Operating Instructions
0
0
0
result of step 3
10
KPROP 0
KDER 0
KINT 0
FFVEL result of step 3
FFACC result of step 4
KPROP result of step 5
KDER 200
KINT 0
FFVEL result of step 3
FFACC result of step 4
®
MG.33.K3.02 - VLT is a registered Danfoss trademark
used. Setting this parameter though is a trade-off
between achieving zero steady-state error fast
(which is good) and increasing overshoot and
oscillations in the system (which is bad).
If you are using the Integral part of the PID
controller, remember to reduce the 32-63 Limit
Value for Integral Sum as much as possible
(without losing the Integral Factor effect of
course) in order to reduce oscillations and
overshoot as much as possible.
8. Reduce the 32-64 PID Bandwidth as much as
possible. With a properly optimized open-loop
control Bandwidth could be reduced to as little
as 6 or 12 % (60 – 120).
9. Set the 32-67 Max. Tolerated Position Error back to
normal e.g. 20,000.
Once the Testrun is concluded, then → Save the
10.
new parameters as the user parameters. Thus,
these parameters are saved in the controller and
in the future will be used for all programs.
What to do if....
....there is a tendency towards instability?
In the event of a strong tendency towards instability
reduce 32-60 Proportional factor and 32-61 Derivative factor
again, or reset 32-62 Integral factor.
....stationary precision is required?
If stationary precision is required, increase 32-62 Integral
factor.
....the tolerated position error is exceeded?
If the test run is constantly interrupted with the message
"position error" set 32-67 Max. Tolerated Position Error –
within the tolerable limits – as large as possible.
If the position error occurs during the acceleration phase
that suggests that the set acceleration cannot be achieved
under the existing load conditions. Increase 32-67 Max.
Tolerated Position Error or determine a Maximum
Acceleration suitable for the entire system.
If position errors do not occur until after the acceleration
phase and they can be delayed but not eliminated by
increasing 32-67 Max. Tolerated Position Error, this suggests
that the Maximum Velocity (RPM) chosen is too high.
Determine a maximum velocity suitable for the entire
system.
....the maximum acceleration is not achieved?
In general, the technical data for a frequency converter are
only valid for a freely rotating axle end. If the frequency
converter is carrying a load the maximum acceleration is
reduced.
The theoretical maximum acceleration will also not be
achieved if, for example, the PID controller output is too
small, or the FC 300/motor is not sized correctly and
therefore does not provide enough energy for peak
consumption during acceleration.
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