Control System
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• Control is the process of causing a system variable to conform to some desired value.
• Manual control Automatic control (involving machines only).
• A control system is an interconnection of components forming a system configuration that will provide a desired system
response.
Control System
Output Signal Input
Signal
Energy Source
BMT 437
Control System
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(1) Measurement(M).
Measuring the variable to be controlled
(2) Decision(D). Based on the measurement, the controller decides what to do to maintain the variable at its desired value.
(3) Action(A). As a controller’s decision, the system must take an action. This is usually accomplished by the final control element.
Three basic operations:
Fig. 1-2 Bioreactor control loop
TT TC
SP
Controler Final control
element
Cooling water
Transmitter
Sensor;
T(f) Feed
Ti(f)
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i. Process
– To be controlled ii. Sensors
– Provides measurement of the system output iii. Actuators / transmitter
– Converts the control signal (measurement) to a power signal iv. Controller
- The brain of the control system.
- It decides what to do to maintain the variable at its desired value v. Final control element
- receive the decision from controller and do the action i. Reference input
– Represents the desired output after action was taken
BMT 437
Control System Components
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• Open-Loop
• Systems in which the output quantity has no effect upon the process input quantity
• Simple system
• Must be closely monitored
• Closed-Loop (feedback)
• Systems in which the output has an effect upon the process input quantity in such a manner as to maintain the desired output value
• Uses the output of the process to modify the process to produce the desired result
• Continually adjusts the process
Control System Classification
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Control System Classification
• An open-loop control system utilizes an actuating device to control the process directly without using feedback.
• A closed-loop feedback control system uses a measurement of the output and feedback of the output signal to compare it with the desired output or reference.
Actuating
Device Process Output
Desired Output Response
Desired Output Response
Measurement
Output Controller Process
Comparison
Single Input Single Output (SISO) System
Desired Output Response
Measurement
Output Variables
Controller Process
Multi Input Multi Output (MIMO) SystemNur Istianah-THP-FTP-UB-2014
• Application: CD player, computer disk drive
• Requirement: Constant speed of rotation
• Open loop control system:
• Block diagram representation:
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• Closed-loop control system:
• Block diagram representation:
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Types of control strategies
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Feedback (FB) control
Feed Forward (FF) control
Cascade (FB-FF)
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Feedback control
Fig. 1-2 Bioreactor Feedback control loop
TT TC
SP
Controler Final control
element
Cooling water
Transmitter
Sensor;
T(f) Feed
Ti(f)
# Measurement of controlled variable (output) is used to adjust the desired value of that variable. Disturbance variable is not measured
1. Simple technique
2. Overcome all disturbances
3. The feedback loop does not know, nor does it care, which disturbance enters the process. It tries only to maintain the controlled variable at set point and in so doing compensates for all disturbances.
4. The controller changes its input in such a way as to return the controlled variable to the set point.
5. Feedback control is the most common control strategy in the process industries
The advantage of feedback control:
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1. It can resolve a disturbance only after the controlled variable has deviated from set point.
2. That is, the disturbance must enter to the process before the feedback control scheme can initiate action to resolve it.
The disadvantage of feedback control
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Feed forward control
Fig. 1-2 Bioreactor Feed forwardcontrol loop
TT
TC SP
Controler Final control
element
Cooling water Transmitter
T(f) Feed
Sensor;
Ti(f)
# Measurement of disturbance variable (input) is used as a set point to adjust the desired value of that controlled variable. Controlled variable is not measured
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• The objective of feed forward control is to measure the
disturbances and resolve for them before the controlled variable deviates from set point.
• Suppose that in bioreactor example the major disturbance is
inlet temperature. To implement feed forward control the disturbance first (Ti) must be measured and then a decision is be made how to manipulate cooling water flow rate to resolve for this change.
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Criteria for the measured variable (disturbance) in FF control:
it must indicate the occurrence of an important disturbance
the control variable must not affect the disturbance
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The advantage of feed forward control:
1. Corrective action is taken before the controlled variable deviates (not same) with the desired value (set point)
2. Corrective action resolve the disturbance effects on the controlled variable so that controlled variable will be constant (not be disturb)
3. If applied correctly, the controlled variable deviation would be minimum.
Wit h F ee d f or w ar d con tr ol
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The disadvantage of feed forward control:
The disturbance variable must be measured
No corrective action is taken for unmeasued disturbance variable
A process model is requested
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• Cascade Control Systems are needed if a single loop control system (FF/FB) are not enough. Disturbance causes poor control of desired variable.
• It contain integrated sets of control loops
Primary Loop: Monitors the controlled variable and uses deviation from its set point to provide an output to the secondary loop. FF control
Secondary Loop: Receives its set point from the primary loop and controls the reference variable accordingly. FB control
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Cascade control system
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Feedforward control with feedback control
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Feed forward control now resolve for the “major” disturbance; feedback control maintained controlled variable and resolve for all other disturbances.
Fig 1-6 Feed forward control with feedback control
TT TC
SP
FB Controler
Final control element
Cooling water
Transmitter
Sensor;
T(f) Sensor;
N (rpm) FF
Controler
Transmitter ST
SC
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The advantage of cascade control:
Effectively accounts for external disturbances
Reduces dead time in variable response
Compatible with other
Control Systems
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The disadvantage of cascade control:
• Multiple control loops make physical and computational architecture more complex
• Additional controllers and sensors can be costly
• The disturbance dynamics must not be
significantly faster than the main input-
output dynamics
THANKS FOR YOUR ATTENTION
The best person is one give something useful always
Nur Istianah-THP-FTP-UB-2014
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