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Proportional Control in Instrumentation Systems: Understanding and Implementation

Explore the principles of proportional control in instrumentation systems, including techniques for temperature regulation and error correction. Learn how to integrate error and select control parameters effectively in LabVIEW.

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Proportional Control in Instrumentation Systems: Understanding and Implementation

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  1. ME 322: InstrumentationLecture 37 April 23, 2012 Professor Miles Greiner

  2. Announcements/Reminders • HW 12 Due Friday, 4/25/2014 • X2 (write Proportional Control VI) • HW 13 Due Monday, 4/28/2014 • L12PP (Draft on web (proportional/integral control) • HW 14 Due Wednesday, 4/30/2014 • X3 (post soon) • Review (4/30 and 5/2) • Open Lab Practice (May 2-4) • Lab Practicum Finals (May 6-13) • Guidelines, Schedule • http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Tests/Index.htm • This week: Lab 11 Unsteady Karmon Vortex Speed • 1.5-hour periods with your partner • LabVIEW Certification • If 75 people are interested then NI will offer a free exam

  3. Lab 12 Setup • Measure the beaker water temperature using a thermocouple/conditioner/myDAQ/VI • Use myDAQ analog output (AO) to turn heater on/off to control the water temperature • Use Fraction of Time On (FTO) to control heater power

  4. Proportional Control • FTO when T is within a small increment DT of TSP • Define • Three temperature zones: • For , f > 1FTO = 1 • For , 1 > f >0 • For , f < 0FTO = 0 • For DT = 0, Proportional is same as full power On/Off • Corrective Heat input: • Q = QMAX*FTO = • QMAX= V2/R

  5. Proportional Control • Continue Constructing VI (last lecture)

  6. Set-Point, Lower-Control, and Measured Temperatures vs Time • Two set point temperatures (65°C and 85°C), • Increasing DT = 0, 5, 10°C decreases unsteadiness but reduces the average steady state temperature TA below TSP • (same as standard deviation) measures unsteadiness • eSS = TAVG-TSP measures steady-state error

  7. Unsteadiness and Error versus DT • Unsteadiness TRMS decreases as DT increases • And as TSP decreases • The average steady-state error e = TSS-TSP • Is positive for DT = 0, but decreases as DT increases • Magnitude increases as DT increases

  8. Proportional Control is Flawed • Proportional control is able to eliminate unsteadiness. • But, we found that if DT is large enough to make the temperature steady, then the steady-state temperature is below the desired set-point value • What should Q (and FTO) be?

  9. Energy Balance TENV QIN = FTO(QMAX) • Proportional Control • Q = QMAX*FTO = • At , Q = 0, • The steady state temperature must be below TSP so that QIN balances QOUT • We want T = TSPunder steady-state conditions • QIN = QOUT • FTO*QMAX = hA(TSP-TENV) • But we don’t know h or and they may be changing • What is another scheme to find FTP? QOUT = hA(T-TENV) T

  10. Integrate Error When T-TSP > 0 Decrease FTO • Integrate error • Corrective Action from integration • This will • increase FTO when • Decrease FTO when • Leave FTO unchanged when • How to choose DTi? • Q will be too responsive (or not responsive enough) if DTi is small (or large) When T-TSP < 0 Increase FTO

  11. How to implement in LabVIEW • Need to calculate at each time step and sum • Within While Loop • Use Shift Register to pass data from one step to the next • Modify Proportional Controller to include integration

  12. Figure 2 VI Block Diagram

  13. Figure 1 VI Front Panel • Plots help the user monitor the measure and set-point temperatures T and TSP, temperature error T–TSP, and control parameters

  14. Figure 3 Measured, Set-Point, Lower-Control Temperatures and DTi versus Time • Data was acquired for 40 minutes with a set-point temperature of 85°C. • The time-dependent water temperature is shown with different values of the control parameters DT and DTi. • Proportional control is off when DT = 0 • Integral control is effectively off when DTi = 107 (10log(DTI) = 70)

  15. Proportional Control Behavior • Why do temperature oscillations disappear as DT gets larger? • Why is the steady temperature below the set-point (desired) value?

  16. Steady State Temperature Error • Let be the temperature under steady state conditions • Magnitude increases with and

  17. Proportional Control Only need control if TSP > T∞ At steady state

  18. only if only if If ? Integrate error How to make Need to calculate TSP during each cycle. Only when

  19. Fractional Time On (FTO) If DT = 0 then full on/off If DT > 0 then proportional 3 Temp Domains • 3) T < TSP – DT FTO = 1 • 2) (TSP – DT) < T < TSP T = TSP f = 0 T = TSP – DT f = 1 • 3) T > TSP FTO = 0

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