ME 322: Instrumentation Lecture 36

1. ME 322: InstrumentationLecture 36 April 21, 2014 Professor Miles Greiner

2. Announcements/Reminders • HW 12 Due Friday, 4/25/2014 • Don’t start L12PP (revising) • This week: Lab 11 Unsteady Karmon Vortex Speed • 1.5-hour periods with your partner • Schedule (please be on time and come prepared) • http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Labs/Lab%2011%20Karmon%20Vortex/Lab%20Index.htm • How was the Extra-Credit LabVIEW Workshop? • Certification • Lab Practicum Final • Guidelines, Schedule • http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Tests/Index.htm • Practice Periods • May 2-4, 2014

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

4. Full on/off Control • LabVIEWVI “logic” • Measure thermocouple temperature for 1 sec • Average, T, display • Compare to TSP (compare and select icons) • Turn 200 W heater on/off if T is below/above TSP • Waveform Chart • T and TSP versus time • e = T-TSP versus time • Repeat • Constructed last lecture • http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Labs/Lab%2012%20Thermal%20Control/Lab%20Index.htm

5. Full On/Off Temperature Control

6. Front Panel

7. On/Off Control Temperature Response • Full On/off control • Reaches TSP after ~3 minutes • Gives oscillatory response • Average temperature TAvg > TSP • Maximum error is roughly 2.5°C • Want heater power to be high to reach TSP quickly • Would oscillations decrease if power decreased near T ~ TSP?

8. How to reduce heater power using a relay? • Reduce the Fraction of Time the heater is On (FTO) • Maximum heater power QMax = V2/R • Reduce FTO to decrease heater power • Heater Q = (FTO)(QMax) • How to implement this in LabVIEW? FTO = 0.9 FTO = 0.1 FTO = 0.5

9. Strobe Light VI • Stacked sequence loop • Milliseconds to Wait • Vary cycle time and FTO

10. Proportional Control • Reduce heater power (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 • What is Q when

11. How to construct a Proportional-Control VI • Stacked sequence loop • Indicate FTP using a bar or dial indicator • Write to a Measurement File VI • Segment Headings (No Headers) • X value (time) Column (one column only) • Starting Point

12. Proportional Control

13. Proportional-Control Temp versus Time On/Off • TSP = 65°C and TSP= 85°C • As DT is increases (control becomes more proportional) • Oscillatory amplitude decreases • Temperature eventually becomes steady • The “steady-state” average temperature decreases • Error magnitude increases with DT and Proportional Proportional

14. Average Temperature Error and Unsteadiness versus DT and TSP • The average temperature error • Is positive for DT = 0, but decreases and becomes negative as DT increases. • Decreases as TSP increases • TRMS (same as standard deviation) is and indication of thermocouple temperature unsteadiness • Unsteadiness decreases as DT increases, and as TSP decreases.

15. Proportional-Control Questions • Why do temperature oscillations disappear as DT gets larger? • Why is the steady temperature below the set-point (desired) value? • Is there another control technique that eliminates the steady state error?

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

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

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

21. 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