Pid Factors - Danfoss MCO 305 Operating Instructions Manual

MCO 305 Operating Instructions
In short: The feed-forward control is used to handle changes in the setpoint reference (especially important
in synchronization applications), while the PID control is used to handle changes in load conditions or
inaccuracies of the feed-forward control.

PID Factors

Proportional Factor: KPROP
The par. 32-60 Proportional Factor is multiplied
with the position deviance and the result is added
to the control signal (the internal speed-reference
to the FC 300). Since the calculated control signal
is proportional to the position deviance (or error)
this kind of control is called proportional control.
The behavior of the proportional control is similar
to that of a spring – the further the spring is exten-
ded the stronger the counter-force it produces.
Derivative Value for PID Control: KDER
The par. 32-61 Derivative Value is multiplied with
the derivative of the position deviance (the
'velocity' of the position deviance) and the result is
added to the control signal. The behavior of the
derivative control is similar to that of an absorber –
the faster the absorber is extended the stronger
the counter-force it produces. Thus using the
Derivative Value increases damping in your
system.
Integral Factor: KINT
The sum of all error is calculated every time the control signal is updated. The par. 32-62 Integral Factor is
then multiplied with the sum of all positioning errors and added to the overall control signal.
Thus in case that steady-state position errors
occurs in your application, make sure you use the
integral part of the controller. Steady-state errors
will be leveled out as the summed error increases
over time until the control signal eventually
matches the load.
It is possible to set a limit for the control signal
generated by the integral part of the controller
(anti-windup).
Limit Value for Integral Sum: KILIM
The par. 32-63 Limit Value for Integral Sum sets a limit for the control signal generated by the integral part
of the controller. This helps to prevent the so called "wind-up" problem which typically occurs in applications
where the overall control signal (the internal speed-reference) is maxed out for long periods of time.
This feature is also very helpful in applications where the power to the motor is turned off and on while the
option card is controlling the FC 300. Cutting of power to the motor (by setting terminal 27 low) while little
positioning deviance is present in the controller, could result in an enormous control signal being generated
once the power is turned back on.
__ Optimizing the PID Controller __
Influence of the Proportional Factor:
KPROP too small
KPROP larger
KPROP too great
Influence of the Derivative Value:
KDER small
KDER larger
KDER too large
Influence of the Integral factor:
KINT very small
KINT larger
KINT too large
MG.33.K2.02 – VLT is a registered Danfoss trademark
large position deviation due to
non-compensatable load and
frictional moment
quicker reaction, smaller
steady-state deviation, larger
overshoot, lesser damping
heavy vibrations, instability
no effect
better dampening, lesser over-
shoot; if KPROP is increased
simultaneously: faster reaction
to control deviation at the same
level of vibration;
heavy vibrations, instability
steady-state position deviance
is very slowly regulated to zero
faster regulation towards zero
steady-state position deviance,
larger overshoot
heavy vibrations, instability
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