What is a Control System?

Before we can answer this question, let us understand what a system is?  A system is a set of objects placed together to complete a specific task. When stimulated by a machine with an input, a particular response is obtained.

A physical interconnection of devices that influences the behavior of other devices or systems is known as an electrical control system. An input, a mechanism, and an output make up a simple electronic device. Signals are integrated into the system's input and output variables.

Sensors and other input devices collect and respond to data and monitor a physical process with electrical energy in the form of an output action.  The input signal is the cause of the change in process or device operation, while the output signal is the 'effect,' or outcome of the cause.

A control system is a system that controls the output to generate the desired response. In simple language, a control system is nothing but a system that controls other systems. As we move towards a modernized world, the demand for automation is increased. Automation needs control over systems of interrelating devices.

Control system design is a special type of engineering design with the objective of control engineering design to determine the specification, configuration, and understanding of critical parameters of a system to satisfy a real-world requirement. A controller's choice is a summing amplifier that compares the desired and actual responses and then sends the error-measurement signal to an amplifier. Adjusting the system's parameters to achieve the desired result is the final step in the design process.

Examples of systems are

  • A Resistor
  • A Capacitor
  • An air conditioner, and
  • A vehicle

The diagram below depicts the input-output configuration of a process that needs to be performed in a specific manner.

“The image that shows a simple block diagram of a system”

Control System Definition

A Control System is a system in which the input is varied and the corresponding output is measured.  

“The image that shows a flow diagram of a control system”

In a system where the output quantity is regulated by varying the input quantity, it is referred to as the control system. Thus, the amount of output is called the controlled response variable, and the amount of input is called the command signal or excitation.

Generally, control systems are classified as

  1. Natural control system: Typically, biological processes are natural systems. e. g. The human being's respiratory system and temperature.
  2. Man-made control system: System developed and created by people, e. g. Cars, different gadgets.
  3. Combinational control system: Having natural and man-made combinations, e. g. Vehicle driving drivers, etc.
  4. From an engineering point of view, control systems are classified as given below

Time-Varying and Time-Invariant Systems

  • In time-varying systems, systems parameters are varying with time. A space ship whose mass decreases when it leaves earth over time.
  • In the time-invariant system, the input and output of the system may be a time function, but the parameters are time-independent. For example, network elements-resistance, inductance, capacitance.

Linear and Nonlinear Systems

  • Linear systems adopt the theory of superposition, additive property, and homogeneous property. In the absence of input, the output of the linear system is zero.
  • In the case of a non-linear system theory of superposition theorem, additive properties and homogeneous properties cannot be applied.

The continuous-time and discontinuous time or Analog and Discrete-time control time

  • In a continuous-time control system, all system variables are a function of continuous-time.
  • A discrete-time control system involves one or more variables that are known only at a discrete instant time. For example, the following figure shows the signal amplitude and its equivalent discrete signal amplitude.
“The image that shows Continuous Signal Amplitude and its equivalent Discrete Signal Amplitude”

Deterministic and Stochastic System

  • A Control system is said to be deterministic if the response to input is predictable and repeatable.
  • A Control system is said to be stochastic if the input response is neither predictable nor repeatable.

Lumped Parameter and Distributed Parameter

  • The value of the parameter is assumed to be concentrated at a given point. Control system an ordinary differential equation can describe with constant coefficient,e.g., Network element with R, L, C component.
  • Distributed parameter control systems are those that partial differential equations may describe. This value of the parameter is assumed to be distributed over the entire length. The example includes transmission line (containing resistor, inductor and capacitor) analysis.

SISO and MIMO systems

  • Single Input Single Output (SISO): This is the simplest and most common type, in which one output is controlled by one control signal. An example is an audio system, in which the control input is the input audio signal, and the output is the sound waves from the speaker.
  • MIMO System: Systems may have multiple types of inputs and multiple outputs. Those are called multiple inputs and output systems.

Open Loop Control System

Any system which does not automatically correct the variation in its output or control system in which the output quantity does not affect the input quantity is called an open-loop control system, and this means that output is not fed back to the input for correction. In addition, any control system that operates on a time base is called an open loop.

e.g., Sprinklers used in gardens, toaster , traffic light controller, washing machine, coffee server, electric lift, fan regulator, room heater, etc.

“The image that shows Block Diagram of an Open Loop Control System”

Advantages of Open-Loop Control System

  1. Simplicity and stability: The layout is simple, economical, and stable.

2. Open-loop systems are economical.
3.Easy to make.
4. Generally, these systems are stable.

Disadvantages of Open-Loop Control System

1. Accuracy and Reliability: As these systems do not have a feedback mechanism, they are very inaccurate result output, and hence they are unreliable.

2. Output is varied by external changes. These are not corrected automatically.

Closed-Loop Control System

Control systems in which the output has an effect upon the input quantity in such a manner as to maintain the desired output values are called closed-loop systems. Closed-loop systems or automatic control systems are also referred to as feedback control systems. Feedback control methods are used to minimize system error in this system.

“The image that shows Block Diagram of a Closed Loop Control System”

The output is compared to the reference input in a closed-loop system, and an error signal is generated. To minimize this error and get the desired output, the controller is fed this error signal. This is an iterative method.

Advantages of Closed-Loop control system

  1. Systems are accurate
  2. Less noise affected.
  3. External and internal disruptions are less affected.

Disadvantages of Closed-Loop control system

  1. Complex and costlier
  2. May lead to an oscillatory response
  3. Overall gain is less (Feedback reduces gain)
  4. Stability is a major problem.

Examples include

  1. Automatic electric iron.
  2. Servo voltage stabilizer.
  3. An air conditioner.

Feedback in Control System

Feedback occurs when the output, or a portion of the output, is restored to the input side and used as part of the system input. When it comes to improving the efficiency of control systems, feedback is essential. There are two forms of feedback: positive and negative.

Open-loop and closed-loop systems are differentiated by feedback. It can also be defined as transforming an open-loop control system into a closed-loop system. By using sensors/transducers, feedback feeds a portion of the output back to the input.  Such feedback can be added to or subtracted from the input, and therefore it can be positive or negative feedback.

Positive feedback happens when feedback is applied to the input, and the error signal never decreases. As a result, positive feedback is used in only a few fields, such as signal generators and oscillators. Negative feedback occurs when the feedback is subtracted from the input, and the error signal gradually decreases until the device achieves the desired output. Negative feedback is used in the vast majority of control systems.

“The image that shows Block Diagram of a Feedback Control System”

Different sections in the above image are as follows

  • Reference Input /Command Input: This is the input stage.
  •  Input transducer: Used to convert the input signal to usable form for controller.
  • The output transducer: Used to measure the output and to convert the signal to usable form for the controller.
  •  Feedback Path: This gives a portion of output back to the summing junction and error detector.
  • Error detector: This receives the feedback and compares it with input. The difference between the two signals produces the error signal  (actuating signal).
  • Control Element/controller: Controls and regulates the output.

Effect of Feedback on System Gain

  • Feedback reduces the system gain.
  • By using Positive feedback, we can increase the system gain.
  • System gain also depends on the frequency of operation.
  • Feedback could increase the system gain in one frequency range and decrease it in another frequency range.

Effect of Feedback on Sensitivity

  • The effect of a good control system should always be insensitive to the parameter variations, and it should be sensitive to commands.
  • Hence feedback can increase or decrease the sensitivity of the system.

Feedback on Bandwidth

For the open-loop configuration, the frequency response is seen in the following figure,

“The image that shows Frequency Response for Open Loop Configuration”

For the closed-loop configuration, the frequency response is seen in the following figure,

 “The image that shows Frequency Response for Close Loop Configuration”
  • From the above frequency responses, it is clear that the bandwidth is improved by integrating feedback.
  • Also, when there is positive feedback, the bandwidth decreases.

Effect of Feedback on Stability

The input in a system can boost reliability and, if not correctly implemented, it can also be harmful.

System Response Time

Apart from these, feedback is also known to reduce the system response time. With every cycle of feedback, the output is forced towards the desired output, and this will reduce the time the system takes to attain the desired output.

Difference between an open-loop system and a closed-loop system

Sr.NoOpen Loop systemClosed-Loop system
1These are easier to build.These are difficult to build.
2These systems are not reliable.These systems are reliable.
3These systems are slow.These systems are faster.
4These systems are generally more stable.These systems are less stable.
5Optimization is not possible.Optimization is possible.
6Examples - Hand dryer, washing machine.Servo voltage stabilizer, air conditioner.

Common Mistakes

Students get confused in detecting the type of given control system

  • In an Open-loop control system, the control action is independent of the desired output signal.
  • In a Closed-loop control system, control action is dependent on the desired output.
  • Students should understand the main function of a control system

Context & Applications

Control systems can be used

  • To reduce the high labor content of manufactured goods
  • To handle smaller production runs of various goods
  • To increase manufacturing accuracy
  • To provide sophisticated tests of products during the manufacturing phase
  • Manufacturing Industries
  • Engineering Field

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