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SEMESTER 1 ( 2013-2014) 1
PROCESS CONTROL SYSTEM
(KNC3213)
PROCESS CONTROL SYSTEM
(KNC3213)FEEDFORWARD AND RATIO
CONTROLFEEDFORWARD AND RATIO
CONTROL
MOHAMED AFIZAL BIN MOHAMED AMINFACULTY OF ENGINEERING, UNIMAS
MOHAMED AFIZAL BIN MOHAMED AMINFACULTY OF ENGINEERING, UNIMAS
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CONTENTCONTENT1. INTRODUCTION
2. FEEDFORWARD CONTROL
3. RATIO CONTROL
4. FEEDFORWARD CONTROLLER DESIGN BASED ON STEADY STATE MODEL
5. FEEDFORWARD CONTROLLER DESIGN BASED ON DYNAMIC MODELS
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INTRODUCTIONINTRODUCTION
Main Advantages of feedback control1. Corrective action occurs as soon as the
controlled variable deviates from the set point, regardless of the source
2. Feedback control requires minimal knowledge about the process to be controlled; in particular a mathematical model of the process is no required, although it can be very useful for control system design
3. The ubiquitous PID controller is both versatile and robust. If process conditions change, re-tuning the controller usually produces satisfactory control
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INTRODUCTIONINTRODUCTION
Disadvantages of feedback (FB) control1. No corrective action is taken until after a deviation in
the controlled variable occurs. Thus, perfect control, where the control variables does not deviate from the set point during disturbance or set-point changes, is theoretically impossible.
2. It does not provide predictive control action to compensate for the effects of known or measurable disturbances
3. It may not be satisfactory for processes with large time constants and/or long time delays. If large and frequent disturbances occur, the process mat operate continuously in a transient state and never attain the desired steady state.
4. In some situations, the controlled varaible cannot be measured on-line and consequently feedback control is not feasible
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Feedforward ControlFeedforward Control
• The basic concept of Feedforward control is to measure important disturbance variables and take corrective action before they upset the process
• In contrast, a feedback controller does not take corrective action until after the disturbance has upset the process and generated a non-zero error signal.
• Figure 1 shows the simplified block diagrams for feedback and feedforward.
Figure 1: Simplified block diagrams for feedforward and feedback control
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Feedforward ControlFeedforward Control
Control ObjectiveMaintain Y at its set point, Ysp, despite disturbances.
Feedback Control• Measure Y, compare it to Ysp, adjust U so as to maintain Y at Ysp.• Widely used (e.g., PID controllers)• Feedback is a fundamental concept
Feedforward Control• Measure D, adjust U so as to maintain Y at Ysp.• Note that the controlled variable Y is not measured.
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Feedforward ControlFeedforward Control
Advantages of Feedforward (FF) Control1. Takes corrective action before the process is upset
(cf. FB control.)2. Theoretically capable of "perfect control"3. Does not affect system stabilityDisadvantages of Feedforward (FF) Control4. Disturbance must be measured on-line (capital,
operating costs)5. Requires more knowledge of the process to be
controlled (depends on the accuracy of the process model). We need to know how the controlled variable responds to changes in both the disturbance and manipulated variables
6. Ideal controllers that result in "perfect control”: may be physically unrealizable.
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Feedforward ControlFeedforward Control
• For example, a boiler drum with a feedback control system, Figure 2(a)
• The level of the boiling liquid is measured and used to adjust the feedwater flowrate
• This control system tends to be quite sensitive to rapid changes in the disturbance variable (Steam flowrate) – when liquid capacity of the boiler drum become lower.
• Rapid disturbance changes are produced by steam demands.
• Large controller gains cannot be used because level measurements exhibit rapid fluctuations for boiling liquids and lead to amplify the measurement noise and produce unacceptable variations in the feedwater flow rate.
Figure 2(a): Feedback control of the liquid level in a boiler
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Feedforward ControlFeedforward Control
• The feedforward control scheme can provide better control of the liquid level by measured the steam flowrate and adjusts the feedwater flowrate in order to balance the steam demand.
• In this control scheme, the controlled variable (liquid level) is not measured.
• As an alternative, steam pressure could be measured instead of stem flowrate.
• Figure 2(b) shows a feedforward control system for boiler drum
Figure 2(b): Feedforward control of the liquid level in a boiler
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Feedforward ControlFeedforward Control
• In practical applications, feedforward control is normally used in combination with feedback control.
• Feedforward control is used to reduce the effects of measurable disturbances, while feedback trim compensates for inaccuracies in the process model, measurement errors and unmeasured disturbances.
• For this configuration the outputs are added together and the combined signal is sent to the control valveFigure 2(c): Feedforward-
feedback control of the liquid level in a boiler
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Ratio ControlRatio Control
• Special type of feedforward control
• The objective is to maintain ratio of two process variables at a specified value.
• The 2 variables are usually flow rates, a manipulated variable, and a disturbance variable
• The Ratio is
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Ratio ControlRatio Control
• Typical applications of ratio control:Specifiying the relative amounts of
components in blending operationsMaintaining a stoichiometric ratio
of reactants to a reactorKeeping specified reflux ratio for a
distillation columnHolding the fuel-air ratio to a
furnace at the optimum value
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Ratio ControlRatio Control
• Ratio control can be implemented in two basic schemes.
METHOD I• The flowrates for both the
disturbance stream and the manipulated stream are measured and calculated (Figure 3(a))
• The measured ratio
• The output of the divider element is sent to a Ratio Controller (RC) and adjusts the manipulated flowrate accordingly.
• Process gain for this controller Figure 3(a): Ratio Control Method I
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Ratio ControlRatio Control
METHOD II• The flowrate of the disturbance stream is
measured and transmitted to the ratio station (RS), which multiplies this signal by an adjustable gain, , whose value is the desired ratio
Where is the desired ratio. and are the spans of the flow transmitters for the manipulated and disturbance streams
• The output signal from the ratio station is then used as the set point, for the flow controller, which adjusts the flow rate of the manipulated stream,
• Note that disturbance varaible is measured in both Methods I and II. Thus ratio control is a simple type of feedforward control
Figure 3(b): Ratio Control Method II
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Feedforward Controller Design Based on Steady State Model
Feedforward Controller Design Based on Steady State Model
ObjectiveTo design feedforward control scheme to maintain exit composition at a constant set point despite disturbances in inlet composition, Constant VariablesInlet flowrate , and composition of inlet stream 2, Measured VariableComposition of inlet stream 1, Manipulated VariableInlet flowrate for stream 2, Input signal• The measurement • Set point for the exit composition
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Figure 4: Feedforward control of exit composition in the blending system
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Feedforward Controller Design Based on Steady State Model
Feedforward Controller Design Based on Steady State Model
Steady state mass and component balance
Bar over the variable denotes a steady-state value
Solve for
To derive a feedforward control law, replace : by by by
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Eqn. 14.11
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Feedforward Controller Design Based on Steady State Model
Feedforward Controller Design Based on Steady State Model
For Actual implementation• The actual value of is not available but it is
measured • The controller output signal is rather than inlet
flowrate Composition measurement for Refer Section 8.1 (Control System and Instrumentation). For transmitter
Where Gain is the span (measurement range) of the instrument
Eqn. 14.12
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Feedforward Controller Design Based on Steady State Model
Feedforward Controller Design Based on Steady State Model
Control valve & Current-Pressure (P/I)Transducer
Input-output relationship for current-to-pressure transducer and the control valve can be written as
Where and are the steady-state gains for the control valve and I/P transducer, respectively is the flowrate that corresponds to the minimum controller output signal of 4 mA
Eqn. 14.15
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Feedforward Controller Design Based on Steady State Model
Feedforward Controller Design Based on Steady State Model
Rearranging Equation 14.12
Substitute Eqn. 14.14 and 14.15 into 14.11 and rearranging the resulting equation provides a feedforward control law
Where
Eqn. 14.15
Feedforward Controller Design Based on Dynamic Models
Feedforward Controller Design Based on Dynamic Models
• Consider above block diagram for feedback but with additional signal path through and .Where is transfer function for disturbance transmitter and is feedforward controller
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Feedforward Controller Design Based on Dynamic Models
Feedforward Controller Design Based on Dynamic Models
• The output of the feedforward and feedback controllers are added together and the sum is sent to the control valve.
• The closed-loop transfer function for disturbance changes is
• To solve , set the numerator equal to zero
• Based on Eqn. 14.21 and the block diagram figure, we can interpret that via disturbance transfer function it can upset the process. However a corrective action is generated via the path through
Eqn. 14.21
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