Heat Manager Pro User Manual V1.5 Page 72
B.6
C
OMMON PITFALLS
A system that is oscillating around its set value, or reacting too slow to process variations, is
almost always caused by a PID controller that needs fine-tuning.
Beware when it takes many minutes of time for a process value to change after step-wise
change of the set value. This is caused by a so-called dead-time in the process and is very
difficult to overcome with PID settings in 1 PID loop alone.
The most common solutions for these kind of process issues are:
Measure more directly the actual product process value (bring sensor nearer to product).
Make use of a so-called cascaded control system where we make use of a master-slave PID
loop where the slave loop controls the heating to the process (with its separate sensor near the
heater as process value input]. The set value for the slave comes from output of the master PID
controller that measures the actual desired process value of the product.
When you change a P-action, you also influence the I and D action responses in some kind of
way.
The same goes when you change the I or D action, everything is connected together more or
less. Don’t start to move down in spiraling circles and change only 1 parameter at a time and
verify very well if this improves the situation or not.
A good idea is to always write down carefully what you are doing, so you begin to see a
“pattern” in your workflow and take a few steps back, if desired.
Don’t set the D action too high, since this very easily results in an unstable process control. Use
this only for fine tuning after you set the P and I factors correctly.
When you change an action you need to wait for the process to respond to the new situation.
This can take many minutes and depends on the inertia of the process. Watch the recorder out-
put of the process value to determine if it stabilizes or not.
Generally speaking it is important to keep the ratio between heating power versus mass of the
system the same, after determining the correct PID settings for your system. You can then use
the same settings for different situations.
B.7
M
ORE INFORMATION
Consult the internet on more information about this, there is an excellent Wikipedia page with
much more information on this subject.
See:
http://en.wikipedia.org/wiki/PID_controller
Parts of this description comes from the website of RKC instruments,
See:
http://www.rkcinst.co.jp/english/control_01.htm
