Conair Thermolator TW-S User Manual page 114

B-2
I A p p e n d i x
PID Pa ram e te rs
In t e gr a l
Using only proportional control will cause the Thermolator to have steady-state error (it
will never exactly reach setpoint). Integral response is used to eliminate this undesirable
condition.
Integral logic introduces the awareness of the passage of time into the logic by looking
into the past—and observing how far the process has been from the setpoint over time.
The farther away the process is from setpoint for a longer and longer time, the more it
causes the Thermolator to produce a stronger counter-response. Integral action is inter-
nally disabled whenever the Thermolator is far (further than the proportional band) from
the setpoint because it has no merit under this condition.
A smaller number will produce a more aggressive integral response. "0" will completely
turn off integral response.
If the process temperature is approaching the setpoint too slowly, a stronger integral
response (a smaller parameter number) can be used to remedy the situation. Too much
integral response can cause the Thermolator temperature to severely oscillate. Typical
values would range from 30 to 400.
D er i va t i ve
Derivative response is used to eliminate overshoot. It is also used to compensate for the
slow-responding floating valve option. Like integral logic, it is aware of the passage of
time—it looks forward into the future and anticipates if the machine will be overshooting
the setpoint at some point in the future, based on current trends.
Derivative action is disabled whenever the Thermolator is far (further than twice the
proportional band) from the setpoint.
A larger number will produce a more aggressive derivative response.
If the system temperature is overshooting the setpoint, try a more aggressive derivative
response. If the system stutters or temporarily reverses temperature direction as approach-
ing setpoint, your derivative response is too aggressive. If overshoot is not a concern, or
you have a very large system, derivative control can be completely turned off by setting
the parameter to "0" without negative consequences.
M a nu a l Tu ni n g Pr ocedur e
If you find yourself in a situation where the Thermolator is responding in an unpredictable
manner, follow the procedure below to simplify the control loop and pick appropriate PID
parameters.
PREREQUISITES:
r Your cooling water must be at a reasonably stable temperature and pressure.
r Your external heat load on the Thermolator must be reasonably constant.
r Select a setpoint for tuning that is similar to a typical setpoint for the process.
r You must have sufficient time to run your system through several thermal cycles
in order to perform a full tuning.
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