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1 ChE / MET Apr 12

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Feedback Controller Tuning: (General Approaches) 1)Simple criteria; i.e QAD via ZN I, t r, etc easy, simple, do on existing process multiple solutions 2)Time integral performance criteria ISEintegral square error IAEintegral absolute value error ITAEintegral time weighted average error 3)Semi-empirical rules FOPDT (ZN II) Cohen-Coon 4)ATV, or Autotuning 5)Trial and error 6)Rules of thumb 2

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Select the tuning criterion for the control loop. Apply filtering to the sensor reading Determine if the control system is fast or slow responding. –For fast responding, field tune (trail-and-error) –For slow responding, apply ATV-based tuning Trial and Error (field tuning)* * J.B. Riggs, & M.N. Karim Chemical and Bio-Process Control, 3 rd ed. (2006) 3 Turn off integral and derivative action. Make initial estimate of K c based on process knowledge. Using setpoint changes, increase K c until tuning criterion is met

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Decrease K c by 10%. Make initial estimate of I (i.e., I =5 p ). Reduce I until offset is eliminated Check that proper amount of K c and I are used. 4 Trial and Error (field tuning)* * J.B. Riggs, & M.N. Karim Chemical and Bio-Process Control, 3 rd ed. (2006)

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Kc I 5 K c and I levels good?

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Feedback Controller Tuning: (General Approaches) 1)Simple criteria; i.e QAD via ZN I, t r, etc easy, simple, do on existing process multiple solutions 2)Time integral performance criteria ISEintegral square error IAEintegral absolute value error ITAEintegral time weighted average error 3)Semi-empirical rules FOPDT (ZN II) Cohen-Coon 4)ATV, or Autotuning 5)Trial and error 6)Rules of thumb 6

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7 Rules of Thumb Flow Loops: typically PI controllers; PB ~ 150; Level Loops: PI for tight control; P for multiple tanks in series; Pressure Loops: can be fast or slow (like P control by controlling condenser) Temperature Loops: typically moderately slow; typically might use PID controller; PB fairly low (depends on gains); integral time on order of process time constant, with faster process derivative time ~ ¼ the process time constant. * D.A.Coggan, ed., Fundamentals of Industrial Control, 2 nd ed., ISA, NC (2005) * ** W.L.Luyben, Process Modeling, Simulation and Control for Chemical Engineers, 2 nd ed., McGraw-Hill (1990) **

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8 Higher Order Process

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Feedback Controller Tuning: (General Approaches) 1)Simple criteria; i.e QAD via ZN I, t r, etc easy, simple, do on existing process multiple solutions 2)Time integral performance criteria ISEintegral square error IAEintegral absolute value error ITAEintegral time weighted average error 3)Semi-empirical rules FOPDT (ZN II) Cohen-Coon 4)ATV, or Autotuning 5)Trial and error 6)Rules of thumb 9

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Feedback Control Design Disturbances: Load Setpoint Questions: Type of controller to use? How select best adjustable parameters? Performance criteria? Guidelines: Define performance. Obtain best parameters, for Select controller with best performance. 10

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Feed Back Control PID controllers Proportional: Accelerates response Offset Integral: Eliminates offset Sluggish responses If increase Kc, more oscillations -> unstable? Derivative: Anticipates future error Stabilizing effect Noise problem Controller with best performance. P – only if can PI – eliminate offset PID – speed up response of sluggish systems (T, comp, control; multi-capacity systems) 11

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Examples: 12

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17 Controllers: P-Only: P-I Controller: P-I-D Controller: Derivative (rate) time [=] time Chapter 5 ~ p 183

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18 Derivative Action: P-I-D Controller: t A t A

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19 Derivative Action: Another potential problem: noise t A

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Derivative action: PID Reduces overshoot Reduces oscillations Recommended for slow/sluggish processes (speed up control) Advantages: Susceptible to noise Filtering (or averaging PV) introduces delay 3 rd tuning parameter Disadvantages: 20

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PID Control PID Tuning Tune for PI Derivative: Add in D Minimum response time D initial = Tu/8 Adjust Kc and I by same factor (%) Check response has correct level of integral action 21 PS: Try PID for HE process on Loop Pro Developer

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22 Rules of Thumb Flow Loops: typically PI controllers; PB ~ 150; Level Loops: PI for tight control; P for multiple tanks in series; Pressure Loops: can be fast or slow (like P control by controlling condenser) Temperature Loops: typically moderately slow; typically might use PID controller; PB fairly low (depends on gains); integral time on order of process time constant, with faster process derivative time ~ ¼ the process time constant. * D.A.Coggan, ed., Fundamentals of Industrial Control, 2 nd ed., ISA, NC (2005) * ** W.L.Luyben, Process Modeling, Simulation and Control for Chemical Engineers, 2 nd ed., McGraw-Hill (1990) **

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23 ChE / MET 433

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