What is a Control System?

Introduction

A control system is an interconnection of various components or elements forming a system and thereby provides the desired system response.

A control system can control its output(s) to a particular value or perform a sequence of events or perform an event if the specified conditions are satisfied based on the input(s) given.

Examples

Control of a variable: In an automobile, the speed can be controlled by adjusting the accelerator to the desired level.

Control a sequence of events: In an automatic washing machine, a number of options are available like ‘normal’, ‘heavy’, ‘whites’ etc. The machine undergoes a sequence of washing cycle appropriate to the washing option chosen.

Control whether an event occurs or not: The automatic washing machine has a safety lock on the door so that the machine does not operate if the door is open. Here, the condition is that the door is to be closed for the machine to operate.

Major Types of Control Systems
There are two major types of control systems. They are open-loop control system and closed-loop control system.
Read: Differences Between Open-Loop and Closed-Loop Control System
Open-loop Control System
An open-loop control system uses a controller and(or) actuator to control the control the process directly without any feedback. The control action is generated based on the reference input only. The actual value of the controlled variable has no influence on control action. The open loop control is simple and cheap as there is no requirement of sensors and transmitters. The presence of nonlinear elements, disturbances or process parameter variations causes the output to deviate from the desired response. There is no scope for correction in open-loop control systems.
Example: Open-loop speed control of a turntable

(a) Open-loop speed control of a turntable (b) Corresponding Block Diagram

Modern devices like a CD player, a computer disk drive use a turntable to rotate a disk at a constant speed. The control system can be designed to be an open-loop. To make the turntable to rotate at a constant speed, a DC motor is used as an actuator because it provides a speed proportional to the applied input voltage. A DC amplifier is used to supply required power to the DC motor from a battery. In case of any motor wear or any variations in components make the actual speed of rotation deviate from the desired speed. This deviation cannot be corrected in an open-loop control system.

Closed-Loop Control System
The closed-loop control system employs the measurement of actual output and modification of the input to the process through feedback of actual output and thereby maintain the output at a required value. Here, the difference between the actual output of the process and the reference input is amplified and used to control the process so that the difference is continually reduced. It is also known as the Feedback Control System. 
Example: Closed-loop control of a turntable

(a) Closed-loop control of a turntable (b) Corresponding Block Diagram

The closed-loop feedback control system requires a sensor. Tachometer that provides an output voltage proportional to the speed of the shaft is utilized as the sensor. The error voltage is generated between the input voltage and the tachometer voltage. The amplifier parameters are adjusted based on the error voltage so that the DC motor rotates at the desired speed. The deviations occurring in the output is sensed and it is corrected continuously.

Computer Control System

Multivariable Control System

The increase in complexity of the system under control and the interest to achieve optimum control has led to the development of control system engineering. As the system complexity increases, the input and output variables of the system is increased. When the number of input and output variables involved is more than one, then such systems are called Multivariable Control System. The interrelationship between the controlled variables is also to be considered for effective control of the process. The control scheme employed is no longer the simple open-loop or closed-loop control. Various control schemes like Feedback Control, Feedforward Control, Cascade Control, Override Control, Ratio Control, Adaptive Control, Inferential Control are used in process control according to the requirements. The controller is no longer a physical component. The controller is the computer which is programmed with the complex control schemes. This is known as the Computer Control System.
Example: Boiler-Turbine-Generator System

Computer Control System for a Boiler-Turbine-Generator

The above figure shows the simplified model of the Boiler-Turbine-Generator System with a computer as the control device. The simplified model shows only the important three (oxygen, temperature, and pressure) of the several control variables of the system. The measurement values of the oxygen, temperature, and pressure are provided to the computer for control calculations. The input variables flow of feed water, fuel, air, and turbine speed are varied in accordance with the control output.