How well the process parameters are controlled depends on the control strategy implemented for that process.
Basic control strategies used in most of the industries are
- ON – OFF Control
- Open-Loop Control
- Feed- Forward Control
- Closed-Loop control
ON – OFF Control:
It is also called as two- position control system that has only two states fully on and fully off. It is the oldest type of control strategy that gives simple on- off mechanism for the set limits.
When the process variable (PV) or measurement is below the set limit, then the controller is switched ON and the manipulated variable (MV) changes to maximum value. Similarly, if the PV is above the set limit, then the controller gets turned OFF and the MV changes to minimum value.
But, practically ideal on-off controller is not recommended since it is subject to process disturbances and other electrical interferences. To prevent this, on –off hysteresis or differential is added to the control function.
In this control structure, control action is not a function of process variable or any load changes. This is also not a self-correcting one. This control scheme independently calculates control output needed to the actuator to produce the desired response.
But this system never knows the status of actuator output or process condition and also controller doesn’t consider any disturbance inputs in the process. That’s why this is limited for most of the applications.
This control scheme is implemented to reduce the effect of input disturbance on process. A sensor in this system detects the disturbances or any load changes in the process, and then controller calculates the necessary signal to the actuator.
This allows the controller to take corrective action before the disturbance effect the process. But this control scheme is more expensive and complicated and requires a better understanding of the process to implement control logic.
Closed-loop control scheme is widely implemented in many of the industrial control applications. It uses the output measurement as feedback signal; and, it is compared with desired or set-point value. The difference between these actual and desired output is called error.
Thus, the controller gives the control input to the actuator till the error gets minimized so that the output is maintained at the desired value as shown in the figure. Most of the closed loop control systems are implemented with PID controller for accurate and reliable control.