Open Loop and Closed Loop Control System

Block Diagram Illustrating a System:

Control System:

Control systems can be classified into two types depending on whether the output (controlled variable) effects the actuating signal or not. They are:

1. Open loop control system

2. Closed loop control system

(a) The Open Loop Control System:

A block diagram of system is shown an open loop system is shown in Figure. An open loop control system is that in which the control action is independent of the system input. Therefore an open loop system in utilizes a controller or controlling device to control the system process. The controller controls in such a way as to obtain the desired output without considering the actual system input.

Block Diagram of  Open Loop System

The controller uses an external power source to provide proper control action to the controlled system process. The open loop control system is characterized by definite relationship between the desired system output and actual system output without monitoring the actual system.

(b) The Closed Loop Control System:

In a closed loop control system the control action is dependent on the system output. In a closed loop control system the actual system output will be measured. This output is compared with the input. The resulting error signal is used for controlling the system output to obtain the desired value. The sequence of operations in a closed loop system is illustrated in Figure.

Sequence of operations in a Closed Loop System

(c) Features of the Open Loop Control System:

The outstanding features of the open loop system are

1. Moderate accuracy

2. Sensitive to surrounding conditions like vibrations voltage aging etc.

3. Slow response to input command and changes there by making the system slow and sluggish.

(d) Factors for the Accuracy of Open Loop Control System:

1. Accuracy of the designed input output relationship

2. The performance of the calibration over long intervals of time

3. The varying effects of environmental conditions

(e) Advantages of Open Loop System:

The following are the advantages of open loop system

1. Simplicity

2. Low cost.       

(f) Performance and Characteristics of the Closed Loop System:

A block diagram of the closed loop control system is shown in Figure. The operations in the closed loop system are performed by output measuring devices, comparators, amplifiers, controllers and system plant. The measuring devices produce signal proportional to the actual system output.

Block Diagram of a Closed Loop System

This signal is compared with the reference input. An error signal is generated. This error signal is amplified to produce an actuating signal. The actuating system operates the controller applying the control signal to the system plant. This changes the controlled system output to the desired value.

High accuracy fast responses to input signals and relatively independent to operating conditions are the characteristics of the closed loop control system.

(g) Accuracy of Closed Loop System

The accuracy of the closed loop control system is governed by the following factors:

1. Accuracy of the closed loop system.

2. Accuracy of the comparing device.

3. Accuracy of control element and its sensitivity.