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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
