Control System | Closed Loop Open Loop Control System

When a number of elements are combined together to form a system to produce desired output then the system is referred to as control system. As this system controls the output, it is so referred. Each element connected to the system has its own effect on the output.

Definition of Control System

A control system is a system of devices or set of devices, that manages, commands, directs or regulates the behavior of other devices or systems to achieve desired results. In other words, the definition of a control system can be simplified as a system, which controls other systems. As the human civilization is being modernized day by day the demand for automation is increasing accordingly. Automation highly requires control of devices.

In recent years, control systems have played a central role in the development and advancement of modern technology and civilization. Practically every aspect of our day-to-day life is affected less or more by some control system. A bathroom toilet tank, a refrigerator, an air conditioner, a geezer, an automatic iron, an automobile all are control system. These systems are also used in industrial processes for more output. We find control systems in the quality control of products, weapons system, transportation systems, power system, space technology, robotics and many more. The principles of control theory are applicable to both engineering and nonengineering field. You can learn more about control systems by studying our control system MCQs.

Features of a Control System

The main feature of a control system is that there should be a clear mathematical relationship between input and output of the system. When the relation between input and output of the system can be represented by a linear proportionality, the system is called a linear control system. Again when the relationship between input and output cannot be represented by single linear proportionality, rather the input and output are related by some non-linear relation, the system is referred to as a non-linear control system.

Requirements of a Good Control System

Accuracy: Accuracy is the measurement tolerance of the instrument and defines the limits of the errors made when the instrument is used in normal operating conditions. Accuracy can be improved by using feedback elements. To increase the accuracy of any control system error detector should be present in the control system.

Sensitivity: The parameters of a control system are always changing with the change in surrounding conditions, internal disturbance or any other parameters. This change can be expressed in terms of sensitivity. Any control system should be insensitive to such parameters but sensitive to input signals only.

Noise: An undesired input signal is known as noise. A good control system should be able to reduce the noise effect for better performance.

Stability: It is an important characteristic of the control system. For the bounded input signal, the output must be bounded and if the input is zero then output must be zero then such a control system is said to be a stable system.

Bandwidth: An operating frequency range decides the bandwidth of the control system. Bandwidth should be as large as possible for the frequency response of good control system.

Speed: It is the time taken by the control system to achieve its stable output. A good control system possesses high speed. The transient period for such system is very small.

Oscillation: A small numbers of oscillation or constant oscillation of output tend to indicate the system to be stable.

Types of Control Systems

There are various types of control system but all of them are created to control outputs. The system used for controlling the position, velocity, acceleration, temperature, pressure, voltage and current etc. are examples of control systems.

Let us take an example of the simple temperature controller of the room, to clear the concept. Suppose there is a simple heating element, which is heated up as long as the electric power supply is switched on. As long as the power supply switch of the heater is on the temperature of the room rises and after achieving the desired temperature of the room, the power supply is switched off. Again due to ambient temperature, the room temperature falls and then manually the heater element is switched on to achieve the desired room temperature again. In this way, one can manually control the room temperature at the desired level. This is an example of manual control system.

This system can further be improved by using timer switching arrangement of the power supply where the supply to the heating element is switched on and off in a predetermined interval to achieve desired temperature level of the room. There is another improved way of controlling the temperature of the room. Here one sensor measures the difference between actual temperature and desired temperature. If there is any difference between them, the heating element functions to reduce the difference and when the difference becomes lower than a predetermined level, the heating elements stop functioning.

Both forms of the system are automatic control system. In former one, the input of the system is entirely independent of the output of the system. The temperature of the room (output) increases as long as the power supply switch is kept on. That means heating element produces heat as long as the power supply is kept on and final room temperature does not have any control over the input power supply of the system. This system is referred as open loop control system.
But in the latter case, the heating elements of the system function, depending upon the difference between, actual temperature and desired temperature. This difference is called the error of the system. This error signal is fed back to the system to control the input. As the input to the output path and the error feedback path create a closed loop, this type of control system is referred to as a closed loop control system.
Hence, there are two main types of control system. They are as follow

  1. Open loop control system
  2. Closed loop control system

Open Loop Control System

A control system in which the control action is totally independent of output of the system then it is called open loop control system. A manual control system is also an open loop control system. The figure below shows a control system block diagram of an open loop control system in which process output is totally independent of the controller action.
control system

Practical Examples of Open Loop Control System

  1. Electric Hand Drier – Hot air (output) comes out as long as you keep your hand under the machine, irrespective of how much your hand is dried.
  2. Automatic Washing Machine – This machine runs according to the pre-set time irrespective of washing is completed or not.
  3. Bread Toaster – This machine runs as per adjusted time irrespective of toasting is completed or not.
  4. Automatic Tea/Coffee Maker – These machines also function for pre adjusted time only.
  5. Timer Based Clothes Drier – This machine dries wet clothes for pre-adjusted time, it does not matter how much the clothes are dried.
  6. Light Switch – Lamps glow whenever light switch is on irrespective of light is required or not.
  7. Volume on Stereo System – Volume is adjusted manually irrespective of output volume level.

Advantages of Open Loop Control System

  1. Simple in construction and design.
  2. Economical.
  3. Easy to maintain.
  4. Generally stable.
  5. Convenient to use as output is difficult to measure.

Disadvantages of Open Loop Control System

  1. They are inaccurate.
  2. They are unreliable.
  3. Any change in output cannot be corrected automatically.

Closed Loop Control System

Control system in which the output has an effect on the input quantity in such a manner that the input quantity will adjust itself based on the output generated is called closed loop control system. Open loop control system can be converted in to closed loop control system by providing a feedback. This feedback automatically makes the suitable changes in the output due to external disturbance. In this way closed loop control system is called automatic control system. Figure below shows the block diagram of closed loop control system in which feedback is taken from output and fed in to input.
closed loop control system

Practical Examples of Closed Loop Control System

  1. Automatic Electric Iron – Heating elements are controlled by output temperature of the iron.
  2. Servo Voltage Stabilizer – Voltage controller operates depending upon output voltage of the system.
  3. Water Level Controller – Input water is controlled by water level of the reservoir.
  4. Missile Launched and Auto Tracked by Radar – The direction of missile is controlled by comparing the target and position of the missile.
  5. An Air Conditioner – An air conditioner functions depending upon the temperature of the room.
  6. Cooling System in Car – It operates depending upon the temperature which it controls.

Advantages of Closed Loop Control System

  1. Closed loop control systems are more accurate even in the presence of non-linearity.
  2. Highly accurate as any error arising is corrected due to presence of feedback signal.
  3. Bandwidth range is large.
  4. Facilitates automation.
  5. The sensitivity of system may be made small to make system more stable.
  6. This system is less affected by noise.

Disadvantages of Closed Loop Control System

  1. They are costlier.
  2. They are complicated to design.
  3. Required more maintenance.
  4. Feedback leads to oscillatory response.
  5. Overall gain is reduced due to presence of feedback.
  6. Stability is the major problem and more care is needed to design a stable closed loop system.

Comparison of Closed Loop And Open Loop Control System

Sr. No.Open loop control systemClosed loop control system
1The feedback element is absent.The feedback element is always present.
2An error detector is not present.An error detector is always present.
3It is stable one.It may become unstable.
4Easy to construct.Complicated construction.
5It is an economical.It is costly.
6Having small bandwidth.Having large bandwidth.
7It is inaccurate.It is accurate.
8Less maintenance.More maintenance.
9It is unreliable.It is reliable.
10Examples: Hand drier, tea makerExamples: Servo voltage stabilizer, perspiration

Feedback Loop of Control System

A feedback is a common and powerful tool when designing a control system. Feedback loop is the tool which take the system output into consideration and enables the system to adjust its performance to meet a desired result of system.
In any control system, the output is affected due to change in environmental condition or any kind of disturbance. So one signal is taken from the output and is fed back to the input. This signal is compared with a reference input and the error signal is generated. This error signal is applied to controller and output is corrected. Such a system is called feedback system. The figure below shows the block diagram of a feedback system.
feedback control system
When the feedback signal is positive then system called positive feedback system. For positive feedback system, the error signal is the addition of reference input signal and a feedback signal. When the feedback signal is negative then the system is called negative feedback system. For negative feedback system, the error signal is given by the difference of reference input signal and the feedback signal.

Effect of Feedback

Refer figure beside, which represents feedback system where
R = Input signal
E = Error signal
G = Forward path gain
H = Feedback
C = Output signal
B = Feedback signal
block diagram of control system

  1. Error between system input and system output is reduced.
  2. System gain is reduced by a factor 1/(1±GH).
  3. Improvement in sensitivity.
  4. Stability may be affected.
  5. Improve the speed of response.

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