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Having been introduced to the basic principles of feedback control modes, we now discuss how these modes can be applied to real life control problems.
It should now be clear that using proportional, integral or derivative mode alone cannot achieve the desired control objective. What works is a combination of these modes.
By using all the three control modes together, we can:
(1) Hold the process near set point without major fluctuations with proportional control.
(2) Eliminate offset with integral control.
(3) Achieve rapid response to major disturbances with derivative control.
PID Relationships:
(1) P-Control is the principle method of control and should do most of the work
(2) I-Control is added carefully just to remove the offset left behind by P-control.
(3) D-Control is there for stability only. It should be set up so that its stabilizing effect is larger than the destabilizing effect of I-control. D-Control is never used in a system that has a lot of noise.
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| Block Diagram of a feedback control system with a PID controller |
In the diagram above:
SP = controller set point
ERR = SP – PV
KC = Controller gain
OP = Controller output
MV = Manipulated Variable
PV = Process Variable
P = Proportional Control
I = Integral Control
D = Derivative Control
Enabling/Disabling Integral and Derivative Control in a Controller:
In an actual controller:
(1) Set TD (derivative time constant) to zero, if no derivative action is wanted.
(2) Set TI (integral time constant) to a large value (say 900 minutes, for example), if no integral action is wanted.
Applications of Proportional, Integral and Derivative control modes:
Proportional mode (P)
This is the most basic form of control. This can be used if the resultant offset or error in the output is constant and acceptable. It can be varied by the controller gain KC.
Proportional and integral mode (PI)
Integral control can be added to the proportional control to remove the offset from the output. This can be used if there are no stability problems such as in a tight flow control loop.
Proportional, Integral and Derivative mode (PID)
This is a full 3-term controller, used where there is instability caused by the integral mode being used. The derivative function amplifies noise and this must be considered when using the full three terms.
Proportional and derivative mode (PD)
This mode is used when there are excessive lag or inertia problems in the process.
Integral mode (I)
This mode is used almost exclusively in the primary controller in a cascaded configuration. This is to prevent the primary controllers output from performing a “step change” in the event of the controller’s set point being moved.
Common Types of Control Loops
The table below shows common types of control loops and which types of control modes are typically used:
| Controlled Variable |
Proportional Control
|
PI Control
|
PID Control
|
Flow
|
Yes
|
Yes
|
No
|
Level
|
Yes
|
Yes
|
Rare
|
Pressure
|
Yes
|
Yes
|
Rare
|
Temperature
|
Yes
|
Yes
|
Yes
|
Analytical
|
Yes
|
Yes
|
Rare
|
