What Is The Open Loop Control System?
In real world applications the control systems can be divided into two main categories: open loop control system and closed loop control system. Open loop control systems have a much simpler structure (and a lower performance) compared to closed loop control systems as elaborated in the following.
In order to understand the concept of an open loop control system consider, for example, the room temperature control problem. Assume that a heater with adjustable power is placed in a room and we need to regulate the room temperature at 25 degrees Celsius where the outdoor temperature equals -10 degrees Celsius.
Evidently, by some trial and error and of course wasting a lot of time (since the time constant of the process is very large) we can adjust the power of heater such that at steady state the room temperature be approximately equal to the desired value. This technique for regulating the room temperature is called the open loop control since the power generated by heater is not automatically and continually changed based on the measured room temperature (it is adjusted just once by user and there is no control loop).
But, one problem with the above open loop control system is that if a person leaves the room door ajar or the outdoor temperature changes, the room temperature at steady state changes as well, which is not desired. More precisely, in this example the cold air flow or any changes in outdoor temperature acts as an input disturbance, and obviously, the open loop control system is unable of rejecting such disturbances and regulating the temperature at the desired value.
The other limitation of the open loop control system is that it is not robust to changes in the process model, which are unavoidable in real world applications. For instance, assume that in the above room temperature control problem after adjusting the power of heater we cover the walls of room with a thermal insulator material. Of course, because of this modification, the room temperature at steady state will be higher than what it was without using the thermal insulator material, and the open loop control system is unable of regulating the temperature at the desired value.
The general block diagram of the open loop control system is shown in Figure 1. In this figure the dynamics of controller ideally equals the inverse of the dynamics of plant. It means that the controller and plant cancel the dynamics of each other and the series connection of these two systems equals unity (i.e. the controller is the inverse system for plant). In this way, in the open loop control system of Figure 1 the output of plant is equal to the setpoint as desired.
But, unfortunately, such an ideal controller which cancels the dynamics of plant cannot be realized in real world applications since the inverse of the plant (as a causal system) is usually non causal and cannot be realized in real time. The other reason is that the exact mathematical model of plant is never known in practice and the controller as an inverse system can never fully cancel the dynamics of plant. Hence, the transfer function of the open loop control system, which is equal to the multiplication of the transfer functions of plant and controller, is not exactly equal to 1.
Fig. 1: block diagram of the open loop control system
To sum up, the open loop control system shown in Figure 1 has two main drawbacks compared to the closed loop control system. First, it cannot reject disturbances enter to the input or output of plant, and second, it is so sensitive to uncertainties in the mathematical model of plant (more precisely, sensitivity of the open loop control system to the plant model is equal to 1).
Note that the open loop control system does not continually measure the output variable and does not change the control input accordingly to remove the steady state error caused by disturbance inputs or uncertainties/changes in the plant model. Moreover, unlike the feedback control systems where PID/PI controllers are very popular, in open loop control systems such controllers are absolutely useless.
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Written by Farshad Merrikh Bayat, last updated on September 6, 2022