Introduction to control system

History of control system:  The first control system probably was used more than two thousands year ago. It was a water clock, which records the time based on the level of water in a container. Curiosity of human beings to automate certain monotonous and hazardous works led to invention and use of control system over the many centuries. From simple control action in hand held electric cloth press to more advance Auto-pilot control system in aircraft, the design and use of control systems has advance tremendously in recent past. The use of miniaturization of electronic circuits (integrated circuits) to process the signal at faster rate has made the design of more complex control system easier and cost effective.

 Control System Basics: A control system tries to achieve a regulated output depending on input and control actions. Control systems can be open loop type or closed loop type. Open loop system is simple but there is no control to regulate output. Example of open loop control system is an Automatic washing machine. We do not have control over cleanliness of clothes. Most of the automatic control systems are closed loop type. Take an example of Air conditioner (AC), a closed loop system. The chilled air in a chamber is controlled with the help of an automatic control system. We preset the chamber temperature. As long as temperature inside the chamber is higher than the preset temperature, compressor of the AC pumps in the chilled air to cool and lower the temperature of the chamber. A temperature sensor continuously compares the chamber temperature and preset temperature and gives output to controller.  When temperature reaches below the preset temperature, Controller detects the changes and cut off the power supply to the compressor and compressor stops. Again compressor starts when the temperature exceeds some predefined value. This way temperature of the chamber is regulated. Basics components of this control system are , feedback element (temperature sensor), Controller (on-off controller), actuator (switch) , Plant (compressor).

Temperature sensor: It senses the temperature and converts it into equivalent electrical signals. Examples are Resistance temperature Detector (RTD), Thermo-couple, IC temperature detector.

Controller:  Controller compares feedback signals with preset signals (input) and generates error signal. Examples are ON-OFF controller, Proportional-Derivative (PD), Proportional Integral (PI), Proportional Integral -derivative (PID) controllers.

Actuator: Actuator is a interfacing component between signal and power. It actuates a high power device to operate to do control task. Examples are Relay, Power amplifiers, electronics switches, etc.

Plant: Plant is the component which does actual work to vary the output. Examples: Compressors in Air conditioner, refrigerators; stepper motors in door control and position control.

Control system Configuration: There are two broad configurations of control systems: Open loop control system and closed look (feedback) control system.

Open loop control system: Output has no control on result of controlled output in Open loop control system. Once input is given, the output changes independently. These systems are cheap and very effective and when there is no proven technology to measure the output. Advantages are low cost, simple design.

Closed loop (feedback) control system: But in feedback control system output and inputs are compared continuously and error signal is generated to reach desired output. Most of the automatic control systems are closed loop type. Advantages are error or disturbance correcting ability, and less human interventions.

Example of control system: House hold items: Refrigerator, Air Conditioner, water geyser, Microwave oven. Industrial:  Automatic control of coal flow in a thermal power station, proximity sensed door, aircraft auto pilot system, Satellite guiding and tracking system, etc.

Control system design: Design of control system revolves around the how well we can fit the control system into a mathematical model using the physical laws governing the components in the control system. Then we utilize the well developed theory of mathematics to solve the control design problem. For single input and single out put control design, we can use time domain design approach such as Root locus or frequency domain design approach such as Naquist plot, Bode plot. In multi input and multi output control system, we invariable use state space analysis.

Simulation software in design and analysis of control system: Various simulation software is used for designing and analysing control system. Such as MATLAB, Octave. Most extensive commercial and student use is MATLAB by Math Works corp. It has rich set of mathematical function library, signal processing function libraries, and almost all mathematical modelling function libraries. It has in-built modelling and simulator called simulink to see how your design model works. These software help greatly to see how your designed system performs before going to component level of system.

Conclusion: Use of control system in every space of human life has improved quality of life and comfort of living. Due to advent of control system, many monotonous works are being performed by highly sophisticated control system called robots. Also human is not heavily exposed to hazardous environments. Human intervention in process control has been reduced. In years to come, we would see much sophisticated control system doing most of the odd works for human beings.

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