Types Of Control System

The Engineering Concepts

3 years ago

Types Of Control System

Control System:

Control system is a system of devices which is used to manage, commands, directs and regulates the behaviour of the other devices to accomplish a desired level. In other words, it is a system which controls other device system to achieve a desired result. Control systems are present almost everywhere. For eg. Control system is used in a single home heating controller using a thermostat as a controller for domestic boiler to a large scale industrial system.

Aricle written By : Pallavi Wankhede

Types of Control System:

There are two main types of control system: Open loop control system and closed loop control system.

Open Loop Control System

In open loop control system, the input control action from the controller is independent of the system output or process variable. In other words, when there is a variation in input variable, it doesn’t make changes in the output variable.
This type of control system is also referred as non-feedback control loop system. As the control response between input and output are not compared.
For an instance, an immersion rod is used for heating the water. When the input is provided by supplying external electric current to the rod, the rod starts to heat the water. While there is no feedback for the sensing of required heating level for rod.

***Figure 1: Open Loop Control System Block diagram***

2. Closed Loop Control System

In a closed loop control system, the input control action from the controller is dependent of the system output or process variable. When there is a variation in input of the controller, the output or process variable will change.
Closed loop control system has feedback loop which helps to ensure the controller exerts a control action to control a process variable or output at the same value as the setpoint.
Thus, closed loop control system is also called as feedback controllers.

For an instance, an air conditioner is used for cooling the room. Initially, the actual temperature of the room is measured once you switch on the conditioner. During regulating a specific temperature as per requirement, air conditioner uses a thermostat which gives feedback to the control system.

***Figure 2 : Closed Loop Control System Block diagram***

Other than these two control systems, the types may vary depending on the application requirement. These are as follow:

3. Feedback Control System: In this type of control system, sensors, control algorithms, and actuators are involved to regulate the process variable at a setpoint. Feedback Control System tries to achieve a desired result with the help of feedback and can change the output at some extent when conditions vary. The closed loop control systems are the example of feedback control system. (see closed loop control system block diagram fig. 2)

4. Feedforward Control System : In this type of control system

No corrective action is taken until after a deviation in the controlled variable occurs. Thus, perfect control, where the controlled variable does not deviate from the set point during disturbance or set-point changes, is theoretically impossible.
Corrective action taken before the actual disturbance occurs on the output.
Feedback control does not provide predictive control action to compensate for the effects of known or measurable disturbances.
It may not be satisfactory for processes with large time constants and/or long time delays. If large and frequent disturbances occur, the process may operate continuously in a transient state and never attain the desired steady state.
In some situations, the controlled variable cannot be measured on-line, and, consequently, feedback control is not feasible.

Figure 3 : Feed Forward Control loop

5. Linear and Nonlinear Control System: Linear Control System use negative feedback to produce a control signal to maintain the controlled Process Variable (PV) at the desired Setpoint (SP). It also follows homogeneity and additivity. While nonlinear control system does not follow homogeneity.

***Figure 4 : Linear Control System with negative feedback loop***

***Figure 5: Nonlinear Control System***

6. Continuous time and discrete time control system: This type of control system is based on the type of signal used. In continuous time control system, all the signals are continuous in time. In discrete time control system, one or more signals are discrete time signals.

***Figure 6 : Continuous time and discrete time control system***

7. SISO and MIMO Control System: Based on the number of input and output, control system can be classified as SISO (Single Input Single Output) and MIMO (Multiple Input Multiple Output). SISO stands for Single Input and Single Output, the name SISO itself suggest that control system has one input and one output. On the other hand, MIMO stands for Multiple Inputs and Multiple Outputs. MIMO has more than one input and one output.