Presentation on theme: "Chapter 2 Traditional Advanced Control Approaches – Feedforward, Cascade and Selected Control."— Presentation transcript:
1 Chapter 2Traditional Advanced Control Approaches – Feedforward, Cascade and Selected Control
2 2-1 Feed Forward Control (FFC) Block DiagramDesign of FFC controllersExamplesApplications
3 Why Feedforward ? Advantages of Feedback Control Disadvantages Corrective action is independent of sources of disturbancesNo knowledge of process (process model) is requiredVersatile and robustDisadvantagesNo corrective action until disturbance has affected the output. Perfect control is impossible.Nothing can be done about known process disturbanceIf disturbances occur at a frequency comparable to the settling time of the process. Then process may never settle down.
4 Feedforward Control Feedforward Controller Disturbance Output Process ManipulatedVariable
5 Feedforward Control Advantages Disadvantages Corrective action is taken as soon as disturbances arrives.Controlled variable need not be measured.Does not affect the stability of the processesDisadvantagesLoad variable must be measuredA process model is requiredErrors in modeling can result in poor control
6 EXAMPLES LI FB Boiler Feed control steam FI LI FFC steam FI Feedback controlFeedforward controlLIsteamFIFFCFBΣCombined feedforward-feedback control
7 Design Procedures (Block diagram Method) FFControllerGL(s)Load transfer functionGF(s)LoadManipulated VariableGp(s)ProcessX2COutputLM∑
9 Examples Example 1 Example 2 Let Gp(s)=Kp/τps+1, GL(s)=KL/τLs+1 Then, GF(s)=-(KL/Kp)(τps+1)/(τLs+1)Therefore, feedforward controller is a “lead-lag” unit.Example 2Let Gp(s)=Kpe-Dps/τps+1, GL(s)=KLe-DLs/τLs+1Then, GF(s)=-(KL/Kp)(τps+1)/(τLs+1)e(-DL+DP)sIf -DL+DP is positive, then this controller is unrealizable. However, an approximation would be to neglect the delay terms, and readjusting the time constants. In this case, perfect FF compensation is impossible.
10 Tuning feedforward controllers LetThis has three adjustable constants, K, τ1, τ2Tuning K, K is selected so that for a persistent disturbance, there is no steady state error in output.Adjustingτ1, τ2 can be obtained from transfer functions. Fine tune τ1, τ2 such that for a step disturbance, the response is somewhat symmetrical about the set point.
11 Example: A simulated disturbed plant Disturbed flow rateDVWaste water treatmentChemicalsMVBOD(CV)
14 Example: Distillation Column Mass Balance: F=D+BFz=Dy+BxD=F(z-x)/(y-x)In practiceFor example: If light key increase in feed, increase distillate rate.
15 Design of Feedforward Control Using Material and Energy Balances Consider the hear exchangerEnergy Balance yields Q=WC(T2-T1)=WsλWhere λ=hear of vaporization Ws=WC(T2-T1)/λThis equation tells us the current stream demand based on (1) current flow rate, W, (2) current inlet temperature, T1, (3) desired value of outlet temperature T2.WsSteamw, T1T2Condensate
16 Control Law and Design Implementation: Note no dynamics are incorporatedΣKXmeasuredTsetGainWswT1-+
17 When to use Feedforward ? Feedback control is unsatisfactoryDisturbance can be measured and compensated forFrequency of disturbance variations are comparable to frequency of oscillation of the systemOutput variable cannot be measured.There are large time delays in the system
23 Principal Advantages and Disadvantages Disturbances in the secondary loop are corrected by secondary controllersResponse of the secondary loop is improved, thus increasing the speed of response of the primary loopGain variations in secondary loop are compensated by secondary loopDisadvantagesIncreased cost of instrumentationNeed to tune two loops instead of oneSecondary variable must be measured
24 Design Considerations Secondary loop must be fast responding otherwise system will not settleTime constant in the secondary loop must be smaller than primary loopSince secondary loop is fast, proportional action alone is sufficient, offset is not a problem in secondary loopOnly disturbances within the secondary loop are compensated by the secondary loop. Hence, cascading improves the response to these disturbances
25 Applications: 1. Valve Position Control Valve MotorDesiredpositionSecondary loopValve positionAir Pressure to Valve MotorValve motion is affected by friction and pressure drop in the line. Friction causes dead band. High pressure drop also causes hysteresis in the valve responseUseful in most loops except flow and pressure
28 θGc12Σ+-PrimaryGC2SecondaryG2(S)G3(S)For a cascade system(open-loop)Without cascade controlθc
29 Illustrative Example: Steam Jacket – Continued – Cascade Case Wu = 0.53Mag = 20*log10(AR) = -30 (dB) AR =
30 Illustrative Example: Steam Jacket – Continued – No Cascade Case Wu = 0.25Mag = 20*log10(AR) = 0 (dB) AR = 1
31 Illustrative Example: Steam Jacket – Continued – No Cascade Case Ku = 1;wu = 0.25;Pu = 2*Pi / wu = Kc = Ku/1.7 = Taui = Pu / 2 = Taud = Pu /8 =
32 Illustrative Example: Steam Jacket – Continued – Cascade Case Ku = 20;wu = 0.53;Pu = 2*Pi / wu = 12 Kc = Ku/1.7 = 11.8 Taui = Pu / 2 = 6
33 2-3 Selective Control Systems Override ControlAuctioneering ControlRatio ControlChange from one controlled (CV) or manipulated variables (MV) to another
34 1. Override Control – Example Boiler Control LTLCLSSPCNormalloopwatersteamLSS: Low Selective Switch – Output a lower of two inputsPrevents: 1. Level from going too low, 2. Pressure fromexceeding limit (lower)
35 Example: Compressor Surge Control motorSCHSSPCFCGas outGas inNormal loop
36 Example: Steam Distribution System High Pressure LineLow Pressure LinePCHSS
37 2. Auctioneering Control Systems Length of reactorTemperateT1T2Hot spotTemperature profiles in a tubular reactor
38 Auctioneering Control Systems TTHSSCooling flowTC
39 Temperature ControlSplit Range Control: More than one manipulated variable isadjusted by the controllerTCBypassExchangerT2Steam
40 Example: Steam Header: Pressure Control Boiler 2Steam HeaderPC
41 3. Ratio Control – Type of feedforward control Wild streamFAFTDisadvantage:Ratio may goTo erraticDesiredRatioεGcDriverFTFBControlled StreamBHowever, one stream in proportion to another. Use if the ratiomust be measured and displayed
42 Another implementation of Ratio Control Wild streamFAFTMultiplierDesired Ratio+FCε-FTFBControlled stream