The Basic Structure Of Closed-Loop Systems

A closed-loop system consists of the following four main components:
i) plant/plant

ii) Input transducer

iii) controller

iv) output transducer/sensor

The disadvantages of open-loop systems, namely sensitivity to disturbances and inability to correct for these disturbances, may be overcome in closed-loop systems. The generic architecture of a closed-loop system is shown in Figure 1 below.

Figure 1: Block diagram  of closed-loop systems.

The input transducer converts the form of the input to the form used by the controller. An output transducer, or sensor, measures the output response and converts it into the form used by the controller.

The first summing junction algebraically adds the signal from the input to the signal from the output, which arrives via the feedback path, the return path from the output to the summing junction. In Figure 1, the output signal is subtracted from the input signal. The result is generally called the actuating signal. However, in systems where both the input and output transducers have unity gain (that is, the transducer amplifies its input by 1), the actuating signal's value is equal to the actual difference between the input and the output. Under this condition, the actuating signal is called the error.

The closed-loop system compensates for disturbances by measuring the output response, feeding that measurement back through a feedback path, and comparing that response to the input at the summing junction. If there is any difference between the two responses, the system drives the plant, via the actuating signal, to make a correction. If there is no difference, the system does not drive the plant, since the plant's response is already the desired response.

Advantages of closed-loop systems:-


i) Less sensitive to noise, disturbances and changes in the environment.
ii) Transient response and steady-state error can be controlled more conveniently and with greater flexibility in closed-loop systems, often by a simple adjustment of gain (amplification) in the loop and sometimes by redesigning the controller.


Disadvantages of closed-loop systems:


i) More complex than open-loop systems
ii) More expensive than open-loop systems.

Example open loop system

Figure 1: Operator filling a tank with water.

An example of a typical open-loop system in industry would be an operator filling a tank with water. Figure shows an example of this type of system and notice that when the operator manually opens a valve, water starts to fill the tank. When it's determined the water is at the correct depth, the operator turns the valve off. If additional water was needed, the operator would turn the water on again and put more water in the tank. This system is called an open-loop system for several reasons. First, the operator must manually open the water valve if more water is needed in the tank, and close the water valve when the proper amount of water has been added. Second, the operator looks into the tank to determine if more or less water is needed, and then opens or closes the water valve rather than the controller comparing the sensor signal to the setpoint. Open-loop systems tend to be very simple and used frequently in industrial applications because they are inexpensive to operate.

Definition of Control System

Control  System Definition

A control system consists of subsystems and processes (or plants) aassembled for the purpose of controlling the outputs of the processes. For example, a furnace produces heat as a result of the flow of fuel. In this process, subsystems called fuel valves and fuel-valve actuators are used to regulate the temperature of a room by controlling the heatoutput from the furnace. Other subsystems, such as thermostats, which act as sensors, measure the room temperature. In its simplest form, a control system provides an output or response for a given input or stimulus, as shown in Figure 1.1

Figure 1.1 Simplified description of a control system

Input and Output

A control system provides an output or response for a given input or stimulus. The input represents a desired response while the output is the actual response.


Two Control System configurations

1. Open-Loop Systems
2. Closed-Loop Systems

Open-Loop Systems

It Starts with a subsystem called an input transducer, which converts the form of the input to that used by the controller. The controller drives a process or a plant. The input is sometimes called the reference, while the output can be called the controlled variable. Other signals, such as disturbances are shown added to the controller and process outputs via summing junctions, which yield the algebraic sum of their input signals using associated signs.

The distinguishing characteristic of an open-loop system is that it cannot compensate for any disturbances that add to the controller's driving signal. The output of an open-loop system is corrupted not only by signals that add to the controller's commands but also by disturbances at the output (Disturbance 2 in Figure 1.2). The system cannot correct for these disturbances, either.

Figure 1.2: Open-loop system block diagram.

Examples of open-loop systems are:
1. Toaster
2. Mechanical systems consisting of mass, spring and damper with a constant force positioning the mass.
3. Operator Filling a tank with water
4. etc

Disadvantage of Open Loop System

The disadvantages of open-loop systems are sensitive to disturbances and inability to correct for these disturbances.