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Before going to study in details about control system lets have an overview of System first.
A system is a combination of a functional unit that acts together in such a manner as to achieve the desired objective.
The term “System” is used to name a machine, set of rules or any collection of entities that produces an output signal in response to input signals.
This broad definition allows anything to be considered as System.
The above figure shows that the Block diagram of a representation of the system. It produces an output corresponding to a given input signal in accordance with some rule. There can be any number of input and output signals.
A system can be Electrical, Mechanical, Hydraulic, pneumatic, Chemical, Analog and digital or combination.
It comprises the following:
- The material in solid, liquid or gaseous state.
- The flow of material.
- Interaction between material leading to physical and chemical changes and reactions.
- Process variable such as temperature, pressure, Humidity, Speed so on.
All these dynamic variables together with the dynamic changes taking place in the system constitute a process. Whenever these dynamic variables are controlled manually or automatically, we term as process control.
Automation has made the control of a process simpler, more precise, faster and more reliable.
Air Conditioner: Temperature and humidity are inputs and controlled air is an output.
Doorbell: The press of a button is input and the Sound is output.
The above figure shows the block diagram of the electronic measuring system. It consists of the following four functional units:
- Transducer or Primary Sensor
- Signal Conditioner
- Display device
- Regulated power supply
Transducer or Primary Sensor:
The transducer or primary sensor sens the quantity under measurement and change it to a proportionate electrical signal. The input quantity in most of the instrumentation system is non-electrical (such as mechanical, chemical, optical, thermal, etc). This input quantity ultimately converted into an electrical signal because the electrical signal can easily be amplified, attenuated, filtered, detected, analyzed, modulated, transmitted, recorded and so on.
The signal conditioner converts the transducer output into an electrical quantity suitable for control, recording, and display. It is here that the signal is amplified, filtered or otherwise modified acceptable to the display device.
The readout or display device may be a simple indicating meter, an oscilloscope or a chart recorder.
The information about the measurement is displayed in analog or digital format. Analog signals converted into digital form for automatic analysis, recording or process control.
Conversion of analog signals into digital format is done through the analog to digital converter (ADC).
Regulated power supply:
The electronically regulated power supply provides the required excitation to the transducer and the necessary electrical power to the signal conditioner and display device.
A control system consists of the following three components.
- Objective of control
- Control system control
- Result or Output
The relationship between these three components is shown in the above figure. The objective can be identified with inputs or actuating signals. The result also called output or controlled variables. The goal of the control system is to control the output in some prescribed manner by the inputs through the elements of the control system.
- Material handling
- Biomedical, Surgical and endoscopic
- Space industries
- Smart transportation system on systems such as drive by wire system and driver assist system.
- Steering control of an automobile
- Idle-speed of an automobile
- Sun tracking control of solar collectors.
- OPEN LOOP CONTROL SYSTEM
- CLOSED LOOP CONTROL SYSTEM
OPEN LOOP CONTROL SYSTEM:
An open loop control system is one in which the control action is independent of the output. As the above figure has shown the elements of an open loop control system can be divided into the following two parts:
- Controller and
- Controlled process
A reference input signal is applied to the controller, whose output access actuating the signal. The actuating signal then controls the control process so that the control variable will perform according to some prescribed standards.
Consider an electric room heater operated by a switch. If a person turns on the switch, the room will heat up and reach the temperature which is only determined by the wattage rating electric heater. The heat output cannot be adjusted and constant if there are changed in weather condition because no information is fed back to the heating element.
The conventional electric washing machine is an example of an open loop control system because the wash time is set by the estimation of the human operator, but not on the basis of whether the clothes are clean properly.
No information is fed back cleanliness off the clothes.
Advantages / Merits:
- The input command is the sole factor responsible for providing the control system.
- It is an ability to perform accurately is determined by its calibration.
- It is easier to build.
- It is not troubled with the problem of instability.
Disadvantages / De-merits:
- Presence of non-linearities causes malfunctioning.
CLOSED LOOP CONTROL SYSTEM:
Feedback from the output to the input is missing in the open loop control system. To obtain More accurate Control, the controlled variable should be fed back and compared with the reference input. An Actuating signal proportional to the difference of the input and output is used to correct the error. A System with one or more feedback path is called a closed loop system.
A closed loop control system is shown in the above figure.
It consists of the following five elements.
1. Comparison element: Comparison element gives the difference between the reference Input and feedback signal.
Error signal= (Reference value signal-Measured value signal)
2. Control element: Control element decides what action to take(for example, switch on or off when it receives an error signal)
3. Correction element: The correction element produces a change in the process to correct or change the controlled condition. Thus it might be a switch which switches on a heater and so increase the temperature of the process or a valve which opens and allow more liquid to enter the process. The term actuator is for the element of a correction unit that provides the power to carry out the control action.
4. Process element: The process being controlled could be a room in a house with its temperature being control or a tank of the water with its level being controlled.
5. Measuring element: The measuring element produces a signal related to the control variable(output). For example, it might be a Thermocouple which gives and EMF proportional to the output temperature.
Advantages / Merits:
- High accuracy
Disadvantages / De-merits:
- Complicated and costly.
- The system has a tendency to oscillate.
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