CONTROL SYSTEM
Control: The word control means regulate, manipulate, command or adjust. Let us consider the following examples:
A container is to be filled with water from a tap. Once the water fills the container the valve is closed (that is spilling of water is avoided) by observation from a human being who senses the filling and based on the observation closes the valve.
The driver applies the brake for the vehicle when he observes red traffic light. The traffic personnel regulate the traffic in order to maintain smooth flow of traffic.
In all the above cases, the human senses the requirement or necessity and based on it he or she acts, controls or regulates the system.
The temperature of our body is maintained constant immaterial of whether it i s summer or winter unless we are unwell.
It is impossible to control earth quakes, thunder, intensity of sun's heat etc., by human being.
Definition of control system
A group of devices or units or elements which maintain the required output based on the preset or predefined level or quantity by controlling or manipulating the parameters responsible for the output constitutes a control system. Generally the output of a measurement system will be the input to the control system. Fig. 1.5 illustrates a few simple examples of control system.
Si milarly the speed of a motor depends on the power supplied to the motor. In an open loop system the speed of the shaft rotation is solely determined by the position of speed selection knob. Any changes on the shaft load will result in change in speed and the change in speed will not be compensated for maintaining a constant speed. Fig. 1.7.
Advantages of Open-loop System
a. Relatively simple
b. Low cost
c. Good reliability
d. Easy maintainability
e. Inherently stable
power. The amount of power that can be input to the heater is say 1 kW or 2 kW. If a person just switch on the 1 kW position of the switch, the room will heatup and reach a particular temperature. If there are any changes in the condition such as windows being opened, then there is no heat adjustment to compensate for this variation. There is no information fedback to the heating system. Fig. 1.7(a).
Classification of Control System:
Control systems are broadly classified into two types. They are
a. Open loop control system, b. Closed loop control system (Feed back control system).
Open loop control system :
A control system in which the output is dependent on the input, but input or controlling action is independent of output or change in output, is called an open loop control system.
Examples:
A switch is used to off or on an electric lamp. This is an open-loop system. Fig. 1.6.