Types of Automatic Control System

Types of Automatic Control System

The automatic control systems are classified into two types they are:

1) Open loop (or) Unmonitored system

2) Closed loop (or) monitored system 

1. Open-loop or unmonitored system: When the input to a system is independent of the output from the system, then the system is called an open-loop or unmonitored system. It is also called calibrated system. Most measuring instruments are open-loop control systems, as for the same input signal, the readings will depend upon things ambient temperature and pressure. Following are the examples of open-loop system:

• A simple Bourdon tube pressure gauge commonly used for measuring pressure.
• A simple carburattor in which the air-fuel ratio adjusted through venturi remains same irrespective of load conditions.
• In traffic lights system, the timing of lights is present irrespective of intensity of traffic.

2. Closed-loop or monitored system: When output of a system is measured and is continuously compared with the required value, then it is known as closed-loop or monitored system. In this system, the output is measured and through a feedback transducer, it is sent to an error detector which detects any error in the output from the required value thus adjusting the input in a way to get the required output. Following are the examples of a closed-loop system:

(a) In a traffic control system, if the flow of traffic is measured either by counting the number of vehicles by a person or by counting the impulses due to the vehicles passing over a pressure pad and then setting the time of signal lights.

(b) In a thermostatically controlled water heater, whenever the temperature of water heater rises above the required point, the thermostat senses it and switches the water heater off so as to bring the temperature down to the required point. Similarly, when the temperature falls below the required point, the thermostat switches on the water heater to raise the temperature of water to the required point.

Thermostatically controlled water heater

Block Diagrams

Block Diagrams

Fig. Block diagram of a single Carburettor.

          The block diagrams are used to study the automatic control system in simplified way. In this, the functioning of a system is explained by the inter connected blocks where each block represents a labelled rectangle and is thought of as a block box with a definite function. These blocks are connected to other blocks by lines with arrow marks in order to indicate the sequence of events that are taking place. It may also show how the system operates, what are its inputs and outputs at various stages, and how the energy, information, and/or materials flow through it. Above Fig., shows the diagram of a simple carburattor. The reduction of a control system to a block diagram greatly facilitates the analysis of the system performance or response.

Lag in Response

Lag in Response

          We know that response is the subsequent result of the system to the command. In any control system, there is a delay in response (output) due to some inherent cause and it becomes difficult to measure the input and output simultaneously. This delay in response is termed as lag in response. For example, in steam turbine, with the sudden decrease in load, the hydraulic relay moves in the direction to close the valve. But unless the piston valve ports are made with literally zero overlap, there would be some lag in operation, since the first movement of the piston valve would not be sufficient to open the ports. This lag increases the probability of unstable operation.