ME 322: InstrumentationLecture 36

April 20, 2015Professor Miles Greiner

Proportional Control

Announcements/Reminders• HW 12 Due Friday• This week: Lab 11 Unsteady Karmon Vortex Speed• One-hour periods with your partner• Schedule on WebCampus

– Please be on time and come prepared!

• Lab Practicum Final – Guidelines, Schedule

• http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Tests/Index.htm

– Schedule• On WebCampus• Please let me know if there are conflicts with other finals

– Practice Periods• May 2-3, 2014

Lab 12 Setup

• Measure beaker water temperature using a thermocouple/conditioner/myDAQ/VI

• Use myDAQ analog output (AO) to operate a digital relay that turns heater on/off to control the water temperature

Full on/off Control• LabVIEW VI “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

Full On/Off Temperature Control

Front Panel

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?

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 = 0.1 FTO = 0.5 FTO = 0.9

Strobe Light VI

• Stacked sequence loop• Milliseconds to Wait • Vary cycle time and FTO

Proportional Control

• Reduce heater power (FTO) when T is within a small increment DT of TSP – Define

• Three temperature zones:– For , f > 1 FTO = 1– For , 1 > f >0– For , f < 0 FTO = 0

• For DT = 0, Proportional is same as full power On/Off• What is Q when

– Why isn’t that good?

How to construct a Proportional-Control VI

• Calculate FTO– Indicate FTP using a bar, dial and/or numeric indicator

• Use stacked sequence loop to turn heater on and off• Write to a Measurement File VI– Segment Headings (No Headers) – X value (time) Column (one column only)

• Starting Point

CurrentTemperature

Proportional Control

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80 90

Tem

pera

ture

, T [C

]

Time, t [minutes]

T TSP

TSP - DT

Proportional-Control Temp versus Time

• 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

On/Off

Proportional

Proportional

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 1 2 3 4 5 6 7 8 9 10

T RM

S[C

]

DT [C]

TSP = 65°C

TSP = 85°C

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0 1 2 3 4 5 6 7 8 9 10

T A-T

SP[°C

]

DT [°C]

TSP = 65°C

TSP = 85°C

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.

Proportional-Control Questions

• Why is the steady temperature below the set-point (desired) value?

• Why do temperature oscillations disappear as DT gets larger?

• Is there another control technique that eliminates the steady state error?

Steady State Temperature Error

• Let be the temperature under steady state conditions

– Magnitude increases with and