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ME 322: Instrumentation Lecture 37

ME 322: Instrumentation Lecture 37. April 23, 2012 Professor Miles Greiner. 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

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ME 322: Instrumentation Lecture 37

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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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