Understanding Control Systems: Proportional, Integral, Derivative & More
Understanding Control Systems
Proportional Control
A proportional (P) control system responds to an error signal, providing a symmetrical correction proportional to the error. A constant error signal results in a constant corrective action. Sudden changes in the error signal lead to a fast response from the controller.
Proportional-Integral Control
In proportional-integral (PI) control, the error signal causes a growing response due to the integral action of the controller. This type of control is suitable for processes with large, slow changes. However, the integral component’s response time means that changes should be relatively slow to avoid oscillations.
Differences Between P, I, and PI Controllers
- Proportional (P) Controller: Reacts quickly and symmetrically to the error.
- Integral (I) Controller: Increases its action based on the accumulated error over time until the value stabilizes.
- Proportional-Integral (PI) Controller: Combines the benefits of both P and I controllers, providing a quick response while eliminating offset.
Proportional-Integral-Derivative (PID) Control
PID control combines three control actions:
- Proportional: Eliminates absolute error.
- Integral: Eliminates offset error.
- Derivative: Reduces delays.
PID controllers are fast and do not exhibit offset. However, they can be slow in general terms and relatively expensive.
Cascade Control
Cascade control is used when the controlled variable cannot be maintained at the setpoint due to disturbances. This structure uses measurements of internal variables to quickly detect disturbances and initiate corrective action. It involves multiple nested feedback loops, often used to control flow or energy. Flow detection is performed with a measurement of an internal variable.
Override Control (Selective or Constraint Control)
Override control calculates and predicts when a constraint will be met and adjusts the system as it approaches the limit. It keeps the unit operating at its constraint limit, employing maximization and minimization strategies.
Feedforward Control
Feedforward control compensates for disturbances affecting the process. It measures a secondary variable that has a predictable effect on the controlled variable and takes corrective action on the manipulated variable.
Ratio Control
Ratio control calculates parameters involving various relationships within a process. It controls one process variable in relation to another. A common application is maintaining the correct air/fuel ratio in a steam boiler’s combustion process.
Split-Range Control
In split-range control, one manipulated variable takes precedence over others. Valve positioners are used to control valves to achieve the signal partitioning.
Ramp Control
Ramp control allows the operator to specify the rate of change for process variables in engineering units per unit time. Large setpoint changes are implemented as small, periodic adjustments, reducing repeated alterations and aiding in implementation and shutdowns. This control method is used to automatically ramp a stream or temperature up or down.
Selective Control
Selective control uses a single manipulated variable to control several output variables.
Deadband Control
Deadband control’s aggressiveness depends on the width of the deadband limits. Narrow limits result in aggressive control, while wide limits lead to a near-total lack of control.