Danfoss VLT 5000 Series Manual page 32

2. A closed-loop PID control. The PID controller
monitors the difference between the actual
position and the setpoint position. Based on
this information it calculates a control signal to
minimize the position deviance. Thus the
SyncPos option is able to compensate for
changes in load or friction. The PID controller
is also necessary to compensate for any posi-
tion deviance caused by inaccurate setting of
the open-loop feed-forward controller.
In short: The feed-forward control is used to handle
changes in the setpoint reference (especially im-
portant in synchronization applications), while the
PID control is used to handle changes in load con-
ditions or inaccuracies of the feed-forward control.
Proportional factor KPROP (11)
The Proportional factor Proportional factor Proportional factor Proportional factor is multiplied with the posi- Proportional factor
tion deviance and the result is added to the con-
trol signal (the internal speed-reference to the VLT).
Since the calculated control signal is proportional
to the position deviance (or error) this kind of con-
trol is called proportional control. The behavior of
the proportional control is similar to that of a spring
– the further the spring is extended the stronger
the counter-force it produces.
Influence of the Proportional factor Proportional factor Proportional factor: Proportional factor Proportional factor
KPROP too small large position deviation due to
non-compensatable load and
frictional moment;
KPROP larger quicker reaction, smaller
steady-state deviation, larger
overshoot, lesser damping;
KPROP too great heavy vibrations, instability.
Derivative factor KDER (12)
Derivative factor Derivative factor
The Derivative factor Derivative factor is multiplied with the deriva- Derivative factor
tive of the position deviance (the 'velocity' of the
position deviance) and the result is added to the
control signal. The behavior of the derivative con-
trol is similar to that of an absorber – the faster the
absorber is extended the stronger the counter-
force it produces. Thus using the Derivative factor
increases damping in your system.
Influence of the Derivative factor Derivative factor Derivative factor Derivative factor Derivative factor:
KDER small no effect;
KDER larger better dampening, lesser over-
shoot; if KPROP is increased
simultaneously: faster reaction to
control deviation at the same
level of vibration;
KDER too large heavy vibrations, instability.
32
VLT
Integral factor KINT (13)
The sum of all error is calculated every time the
control signal is updated. The Integral factor
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
levelled 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).
Influence of the Integral factor
KINT very small
KINT larger
KINT too large
Integration limit KILIM (21)
The Integration limit
nal generated by the integral part of the controller.
This helps to prevent the so called "wind-up" pro-
blems which typically occurs in applications where
the overall control signal (the internal speed-refe-
rence) 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 VLT. 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.
Velocity Feed-forward FFVEL (36)
The V V V V V elocity feed-forward
that is multiplied with the derivative of the setpoint
position (the velocity of the setpoint). The result of
this operation is added to the overall control signal.
Derivative factor Derivative factor
Derivative factor Derivative factor
This feature is especially useful in applications
where there is a good correlation between the
control signal (the VLT speed reference), and the
speed of the motor. This is indeed the case with
most applications using an asynchronous motor.
® 5000 SyncPos option
Integral factor Integral factor
Integral factor: Integral factor
steady-state position deviance
is very slowly regulated to zero;
faster regulation towards zero
steady-state position deviance,
larger overshoot;
heavy vibrations, instability.
Integration limit Integration limit Integration limit Integration limit sets a limit for the control sig-
elocity feed-forward elocity feed-forward elocity feed-forward factor is a scaling factor elocity feed-forward
MG.10.J4.02 – VLT is a registered Danfoss trade mark
Integral factor Integral factor is Integral factor
Integral factor