•
Time t=2s
: output = 20
•
…
•
Time t=10s
: output = 100
The example above shows that even a small error may cause large offset (correction) value if it lasts
for longer time. In practice we deal not with seconds but with fraction of seconds because the PID
controllers work from a few hundred to several thousand times per second.
Matching the ’P’ and ‘I’ terms we get a controller which immediately reacts on large error values (P),
and some other deviations it corrects with small delay (I). So it all starts to work pretty well.
9.2.3
The Derivative term – D
After reading descriptions of ‘P’ and ‘I’ terms we can come to conclusion, that we do not need
nothing more. In many situations that is true and very often in practice the derivative gain ‘D’ is set
to 0, what cause it is skipped.
However sometimes there is necessity to add some stabilization element as "sharply" tuned ‘P’ and
‘I’ may cause unwanted vibrations near the set point. Here the derivative term ‘D’ will be helpful
which works as a dumper - prevents sudden, rapid movements. The ‘D’ reacts the stronger the faster
a position error changes. If the error increases fast, the ‘D’ control will react towards ‘-‘strongly. If the
error decreases quickly then the ‘D’ control will strongly react towards „+”.
9.2.4
The „sixth” sense – the mysterious K
VFF
parameter
Where this parameter comes from if we described all the terms of PID controller? Actually it is not
another PID controller term it is rather an element which makes PID controller work easier.
Looking carefully at all the three PID terms descriptions we can see that outputs of each term depend
on a position error. So the controller does not work if the position error is zero. The problem is that
we want the error to be as small as possible, the best if it would be zero because it means the best
accuracy of work we do.
Here we have the K
VFF
parameter which reacts in advance, before the error occurs. Of course working
in advance base on prediction and prediction is never 100% sure, so some error occur for sure and
PID has got then work to do but in practice well-matched K
VFF
gain is able to decrease temporary
position errors even up to ten times.
48
simDrive™ - AC Servo Drive User Guide
