Control Systems

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A control system is a type of system that controls the output in order to provide the desired response. 

It is a group of either electronic or mechanical devices which utilise control loops in order to control other systems or devices.  Control systems are automated with the use of computers.

It is a vital part of the automation industry.  Some types of control loops are as follows:

  • ICS – Industrial Control Systems
  • SCADA – Supervisory Control and Data Acquisition
  • DCS – Distributed Control Systems

Elements of Control Loops

  • Control server
  • IED – Intelligent Electronic Device
  • PAC – programmable automation controller
  • PLC – programmable logic controller
  • RTU – remote terminal unit
  • Sensor

These elements of control loops are also part of control systems.

Control systems are used in the following areas for improved production, efficiency and safety:

  • Agriculture
  • Boiler controls and power plant
  • Chemical plants
  • Environmental control
  • Food and food processing
  • Metal and mines
  • Nuclear power plants
  • Pharmaceutical manufacturing
  • Pulp and paper mills
  • Quality control
  • Sewage treatment plants
  • Sugar refining plants
  • Water treatment plants

Control system addresses the following needs:

  • 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 manufacture phase

Examples of Control System

  • Traffic Lights Control System – An order of input signal is entered into the control system, and the resultant output is that one of the three lights will be turned on for a duration of time, while the other two lights will be turned off.  The time duration of turning on and off the lights are based on a traffic study at a specific intersection.  The input signal determines the output.  The traffic lights control system works based on time. 
  • Automobile Steering Control System – The system utilises the variance between the actual direction of travel and the desired direction of travel to result in a controlled fine-tuning of steering wheel.
  • Aircraft Flight Path Control System using GPS – The demand for air traffic control systems has been increasing in accordance with the increased amounts of airplane traffic busy airports.  Air traffic control systems and collision avoidance systems are being developed by using GPS (Global Positioning System) navigation satellites.  GPS lets each aircraft know its exact location in the airspace landing corridor and helps to prevent collisions. 

Control System Classification

  • Continuous Time vs. Discrete Time
    • Continuous Time – all signals are continuous time signals.
    • Discrete Time – one or more signals are discrete time signals.
  • SISO vs MIMO
    • SISO (Single Input and Single Output) – one input and output.
    • MIMO (Multiple Inputs and Multiple Outputs) – two or more inputs and outputs.
  • Open Loop vs. Closed Loop
    • Open Loop – output is not sent back to the input.  The desired output does not affect the control action.  This requires human intervention.
    • Closed Loop – output is sent back to the input.  The desired output affects the control action.  This is fully automatic, no need for human interaction. 

Some loops can be converted from open to closed modes and vice versa.  It is manually controlled when open and fully automated when closed.

Differences between Open Loop and Closed Loop Control Systems

Open Loop Control SystemClosed Loop Control System
Independent control actionDependent control action
No feedback pathFeedback path
Non-feedback control systemFeedback control system
Can be designed easilyCannot be designed easily
Manually controlledFully automated
EconomicalExpensive
InaccurateAccurate

If the output or a part of the output is sent back to the input and used as part of the system input, then it is called feedback.  Feedback helps to improve control system performance substantially, and is an important part of any system’s design. 

Types of Feedback

  • Positive Feedback – the reference input and feedback output are added.
  • Negative Feedback – the error between the reference input and system output is reduced.