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- Basic Elements of Control Systems -

FREDERICK UNIVERSITY CYPRUS

AMEM 326

Lecturer: Dr. Sotiris Omirou

Basic Elements of an open-loop system

Control element

Correction element

Process

input output

controller

signal expected to produce required output

controlled variable

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Example (1) of an open-loop system

An electric fire used to heat a room

Controlled variable:

Control element:

Correction element:

Process:

room temperaturethe operator

the switch and the fire

the room

Identify the subsystems in an open-loop controlled speed motor

Controlled variable:

Control element:

Correction element:

Process:

motor speedthe operator

the switch

the motor

Example (2) of an open-loop system

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Basic Elements of a closed-loop system

Control element

Correction element

Process

inputoutputcontroller

reference value controlled variable

measurement element

error signal

+-

feedback

comparison element

Comparison element

Control element

Correction element

Process

inputoutputcontroller

reference value controlled variable

measurement element

error signal

+-

feedback

comparison element

Compares the reference value with the measured value and produces an error signal

Error = reference – measured value

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Control element

Control element

Correction element

Process

inputoutputcontroller

reference value controlled variable

measurement element

error signal

+-

feedback

comparison element

This element decides what action to take when it receives an error signal

Correction element

Control element

Correction element

Process

inputoutputcontroller

reference value controlled variable

measurement element

error signal

+-

feedback

comparison element

This element is used to produce a change in the process to remove the error and is often called an actuator

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Process element

Control element

Correction element

Process

inputoutputcontroller

reference value controlled variable

measurement element

error signal

+-

feedback

comparison element

The process or plant is the system of which a variable is being controlled

Measurement element

Control element

Correction element

Process

inputoutputcontroller

reference value controlled variable

measurement element

error signal

+-

feedback

comparison element

This produces a signal related to the variable being controlled and provides the signal fed back to the comparison element to determine if there is an error

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Feedback

Control element

Correction element

Process

inputoutputcontroller

reference value controlled variable

measurement element

error signal+_

feedback

comparison element

The feedback can be either positive or negative:

Error signal = reference value + feedback

Error signal = reference value - feedback

Example (1) of a closed-loop system

An electric fire used to heat a room with a thermo-meter indicating the current room temperature

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Example (1) of a closed-loop system (cont…)

Controlled variable:

Reference value:

Comparison element:

Error signal:

Control element:

Correction element:

Process:

Measuring device:

the room temperature

the operatorthe difference between measured and required temperature

the operator

the required room temperature

the hand of the operator

the room

the thermometer

Example (2) of a closed-loop system

The automatic control of water level in a tank

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Example (2) of a closed-loop system (cont…)

Controlled variable:

Reference value:

Comparison element:

Error signal:

Control element:

Correction element:

Process:

Measuring device:

Water level in tank

the leverthe difference between the actual and initial setting of the lever positions

the pivoted lever

Initial setting of lever position

the flap opening or closing the water supply

water in the tank

the floating ball and lever

Example (4) of a closed-loop system

Driving a car

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Example (4) of a closed-loop system (cont…)

Controlled variable:

Reference value:

Comparison element:

Error signal:

Control element:

Correction element:

Process:

Measuring device:

direction of the car along the road

the driverthe difference between the actual and required directions

the driver

the required direction along the road

the hands on the steering wheel

the vehicle dynamics

visual observation

Control of direction

Example (4) of a closed-loop system (cont…)

Controlled variable:

Reference value:

Comparison element:

Error signal:

Control element:

Correction element:

Process:

Measuring device:

speed of the car along the road

the driverthe difference between the actual and required speeds

the driver

the required speed along the road

the foot on the pedals

the vehicle dynamics

visual observation

Control of speed

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Example (5) of a closed-loop system

Maintenance of the liquid level in a container

Example (5) of a closed-loop system (cont…)

Controlled variable:

Reference value:

Comparison element:

Error signal:

Control element:

Correction element:

Process:

Measuring device:

level of liquid in the tank

the workmanthe difference between the actual and required levels

the workman

the required level

the valve

the water in the container

visual observation of the gauge glass