PID CONTROLLER TUNING
If the controller is withdrawn from the control panel face, further adjustments are available which are used to tunethe controller to the process.
When a control loop is commissioned, the controller settings are adjusted to correspond to those, which have been specified during the design of the control system. If a large section of process is to be commissioned; possibly a mathematical model of the process will have been developed from which the optimum controller settings can be calculated for efficient and stable operation. It is these values which are set into the controller before start-up and, if calculated correctly, no further adjustment will be required.
In some cases it will be necessary to tune a controller without having the benefit of knowing what the settings should be. It must always be remembered that the adjustments cover a very wide range of sensitivity and response. If adjusted haphazardly, the process may shut down and damage to equipment and lost production may occur.
The task of controller tuning is usually left to an instrument technician with experience in the cause and effect of process reaction and controller adjustments.
There are many trial and error methods of controller tuning which do not involve mathematical analysis and should be demonstrated by an experienced person, otherwise shutdowns may occur.
The first adjustment, which would normally be made, would be to set forward or reverse action as required.
A forward acting controller has increasing output in response to an increasing measured variable.
A reverse acting controller has decreasing output in response to an increasing measured variable.
Empirical Tuning Method
Proportional only controller :
Proportional plus integral action
Proportional plus derivative action
Proportional plus integral plus derivative action
A three-term controller is therefore adjusted as for a P + D controller and the integral value simply related to thederivative value. In many cases, the setting procedure may be shortened by omitting settings, which are outside the probable range.
The process should then respond to set point or load changes, where integral action removes offset and the second overshoot of set point is approximately 1/4 the amplitude of the first.
This is commonly referred to asthe 1/4 decay method and is generally agreed to be the optimum controller setting for a P + I controller.
The above method is only used when no other controller setting data is available and must be practised with care.
Optimum Settings (Ultimate Method)
The closed loop or ultimate method involves finding the point where the system becomes unstable and using this as a basis to calculate the optimum settings.
The following steps may be used to determine ultimate PB and period:
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