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d)
Time the period of oscillation (in minutes). This is a measure of the response time of
the system.
e)
Set I to a value approximately equal to the response time. Then increase the P setting
to a point where oscillation just ceases. Note the value of P at this point, then set it to
approximately double this value. This gives a good starting point for the P and I
control terms.
f)
Test how the system responds to step changes in the SET point and modify the P and I
settings for a reasonably fast response without excessive overshoot.
g)
If overshoot remains a problem following a large step change in SET, try the effect of
adding some DERIVATIVE action. A good initial setting is half to one third of the
system response time measured above. This will probably require P to be re-optimised
for best results.
h)
When optimising P, I and D the aim should be to achieve the lowest values of all three
terms, consistent with no oscillation and an acceptably small amount of overshoot. This
will give the fastest response for the system.
When adjusting the control terms remember that reducing P increases the controller gain.
This can cause some confusion when the concept of PID control is first encountered.
