Control Systems

In recent years, control systems are playing  important role in the development and advancement of modern civilization and technology. In particular, every aspect of day-to-day activities is affected by some type of control system. A home heating system, automatic washing machine,  refrigerator, air conditioner, and automobile are all examples of control system. In this topic we have to deal with the control of engineering systems that are governed by the laws of physics and therefore called as physical systems.
In control applications the system to be controlled is given various names such as process, plant and controlled system. In process industries there is need to control temperature, flow rate, liquid level, pressure, humidity, chemical composition etc. such applications are called as process control applications.

What is a System?
A system is an abstract object that accepts inputs and produces outputs in response. Systems will also be referred to as plants or processes. Figure below shows the input-output configuration of a process(plant) which are required to behave in some specific fashion.

control_system1

Control System :– An interconnection of components forming a system configuration that will provide a desired response. Process outputs are the response variables. e.g. Circuit, DC Motor, Economic System

What is Control?

The field of control systems deals with applying/choosing the inputs to a given system to achieve a desired output. Control system theory evolved as an engineering discipline. Control theory is playing a vital role in the advance engineering discipline. The automatic control or feedback control has become an integral part of modern manufacturing and industrial processes e. g. NC/CNC machine tool in manufacturing industries, process industry for controlling – pressure, temperature, humidity, viscosity, flow etc.  When a number of elements or components are connected in a sequence to perform a specific function, the group thus formed is called a system. In a system when output quantity is controlled by varying the input quantity. The system is called control system. The output quantity is called controlled variable of response and input quantity is called command signal or excitation.

Classification of Control System:

Broadly control system is classified as –Natural control system, Manmade control system, Combinational control system

  • Biological systems are generally natural systems. e. g. rerspiration system and temperature of human being
  • System designed and manufactured by human being e. g. vehicles, various gadgets
  • Having combination of natural and man made together e. g. Driver driving vehicle etc.

But from engineering analysis control systems are classified as given below

  1. Time Varying and Time Invariant Systems:
    • In time varying systems parameters of systems are varying with time. E. g. space vehicle whose mass decreases with time as it leaves earth.
    • In time invariant system, input and output of system may be function of time but parameters are independent of time. E. g. Network elements- resistances, inductances, capacitances.
  2. Linear and Nonlinear Systems:
  •       Linear systems obey superposition principle, additive property   and  homogeneous property. Also in the absence of the input the output of linear system is zero.
  • In case of non linear system principle of superposition theorem, additive property and homogeneous properties cannot be applied.

3. Continuous time and discontinuous time or Analog and Discrete time control time

  • In continuous time control system, all system variables are function of continuous time ‘t’
  • A discrete time control system involve one or more variables   that are known only at discrete instant time.

4.Deterministic and Stochastic System:

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

5. Lumped Parameter and Distributed Parameter :

  • Value of parameter is assumed to be concentrated at a given point. Control system that can be described by ordinary differential equation with constant coefficiente.g. Network element with R, L, C Component.
    • Distributed parameter control systems are those that may be described by partial differential equations. In this value of parameter is assumed to be distributed over the long length. E. g. transmission line analysis for R, L and C

6. 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. 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.
  • M IMO System : Systems may have multiple type of inputs and multiple outputs those are called multiple input and output system

Open Loop Control System:

Any system which does not automatically correct the variation in its output or control system in which the output quantity has no effect upon the input quantity are called open loop control system, this means that output is not feed back to the input for correction. Any control system that operates on time base is called open loop.

e.g. Sprinkler used to water a lawn, toaster system, traffic light controller, washing machine, coffee server, electric lift , fan regulator, room heater etc.

Control_System2

Advantages :
1. Simplicity and stability: they are simpler in their layout and hence are economical and stable too due to their simplicity
2. Open loop systems are economical.l
3. Construction: Since these are having a simple layout so are easier to construct.
4. Generally these systems are stable

Disadvantages:

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

2. Output changes due to external disturbances which are not corrected automatically

Closed loop control system:

Control system in which the output has an effect upon the input quantity in such a manner as to maintain the desired output value are called closed loop systems. feedback control systems are often referred as closed loop systems or automatic control system. In this systems use of feedback control action is to reduce the system error.

Control_System3

Advantages:

  1. Such systems are accurate
  2. Less affected by noise
  3. Less affected by external and internal disturbances’

Disadvantages:

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

 

Examples of closed loop control system:

  1. Air conditioning system
  2. Example: Basic operation of a car cruise control system:
  • Sensors (speedometers) in the car measure the current speed.
  • The controller in the car uses these measurements to produce control signals (corresponding       to the throttle angle).
  • The control signals affect the speed of the car via actuators (i.e., the engine).

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