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8-2
8.1 PID Control
8.1.1 Operation of PID control
PID is a control method widely used in the instrumentation field involving feedback control of process
quantities such as temperature, pressure, flow, and fluid level.
- Proportional operation
Proportional operation generates an output which is proportional to the input.
The amount of control is held constant.
An offset (steady-state deviation) remains.
Proportional control grows stronger as “Kp” is
increased.
Kp: Proportional gain
- Integral operation
Integral operation generates an output which is proportional to the integral time of the input.
In combination with proportional operation or
proportional-derivative operation, integral
operation removes the offset produced by these
methods.
Integral operation grows stronger as the integral
time “Ti” is shortened.
Ti: Integral time
- Derivative operation
Derivative operation generates an output which is proportional to the derivative time of the input.
The advancing characteristic of derivative
operation alleviates the adverse effect that the
delaying characteristic of the process exerts on
control.
Derivative control grows stronger as the
derivative time “Td” is increased.
In the case of pure derivative operation, control
can temporarily become ineffective if noise is
input, and this can have an adverse effect on the
process being controlled. For this reason,
incomplete differential operation is executed.
Td: Derivative time
- PID operation
PID operation is a combination of proportional, integral, and derivative operations.
If the parameters are set to the optimum values,
PID control can quickly bring the amount of
control to the target value and maintain it there.
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