While discussing fundamentals of process control, we come across two important and fundamental concepts: (i) Open Loop Control System and (ii) Closed Loop Control System. Open loop control system is also known as feedforward control. And, the closed loop control system is also called feedback control.
Closed loop or feedback control is more commonly used than open loop or feedforward control. Closed loop control is designed to achieve and maintain the desired process condition by comparing the actual process condition with the desired condition, the setpoint value (SP), to get an error value (ERR).
Open loop control
In open loop control, the control action (controller output signal OP) is not a function of the process variable (PV). The open loop control does not self-correct when the PV drifts, and this may result in large deviations from the optimum value of the PV.
Also read: Process control loop components – PV, MV & Setpoint
Use of open loop control
This control is often based on measured disturbances to the inputs to the system. The most common type of open loop control is feedforward control. In this technique, the control action is based on the state of a disturbance input without reference to the actual system condition. i.e. the system output has no effect on the control action, and the input variables are manipulated to compensate for the impact of the process disturbances.
Function of open loop or feedforward control
Feedforward control results in a much faster correction than feedback control but requires considerably more information about the effects of the disturbance on the system, and greater operator skill (Figure 1).
Examples of open loop control
A common domestic application that illustrates open loop control is a washing machine.
The system is pre-set and operates on a time basis, going through cycles of wash, rinse and spin as programed. In this case, the control action is the manual operator assessing the size and dirtiness of the load and setting the machine accordingly.
The machine does not measure the output signal, which is the cleanliness of the clothes, so the accuracy of the process, or success of the wash, will depend on the calibration of the system.
An open loop control system is poorly equipped to handle disturbances which will reduce or destroy its ability to complete the desired task. Any control system operating on a time base is an open loop.
Another example of this is traffic signals.
It is difficult to implement open loop control in a pure form in most process control applications, due to the difficulty in accurately measuring disturbances and in foreseeing all possible disturbances to which the process may be subjected.
As the models used and input measurements are not perfectly accurate, pure open loop control will accumulate errors and eventually, the control will be inadequate.
Closed loop control
In closed loop control, the objective of control, the PV, is used to determine the control action. The concept of this is shown in Figure 2 and the principle is shown in Figure 3.
This is also known as feedback control and is more commonly used than feedforward control.
Closed loop control is designed to achieve and maintain the desired process condition by comparing it with the desired condition, the setpoint value (SP), to get an error value (ERR).