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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 - PowerPoint PPT Presentation
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ME 322: InstrumentationLecture 37
April 22, 2015
Professor Miles Greiner
Proportional control characteristics, Shift register and integral control program
Announcements/Reminders• HW 12 Due Friday
• X2 (write a Proportional Control VI)
• HW 13 Due Monday• L12PP (on/off, proportional and integral control)
• HW 14 Due Wednesday• X3
• Review for final (Next Wed. & Fri.)• Open Lab Practice (Next Saturday and Sunday)• Lab Practicum Finals (Schedule on WebCampus)
– Guidelines• http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Tests/Index.htm
• This week: Lab 11 Unsteady Karmon Vortex Speed• One-hour periods with your partner• How is it going?
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
Proportional Control
• Fraction of time on (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 control is the same as full power On/Off control• Corrective Heat input:
– Q = QMAX*FTO = – QMAX= V2/R
CurrentTemperature
Proportional Control• VI construction (start at midpoint)
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
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
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
Unsteadiness and Error versus DT
• Unsteadiness TRMS decreases as DT increases
– And as TSP decreases
– Want this to be zero
• The average steady-state error e = TSS-TSP – Is positive for DT = 0, but decreases as DT increases– Magnitude increases as DT increases– Want this to be zero
𝑇 𝑅𝑀𝑆
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?
Energy Balance
• So far we haven’t done much quantitative analysis• Proportional Control
– Q = QMAX*FTO = – At , Q = 0,
• The steady state temperature will always be below TSP (so that QIN balances QOUT)
• Under steady-state conditions, QIN = QOUT, we want T = TSP
– FTO*QMAX = hA(TSP-TENV)
• But we don’t (want to) know h or and they may be changing
• What is another scheme to find FTP?–
TQIN = FTO(QMAX) QOUT = hA(T-TENV)
TENV
20
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40
50
60
70
80
90
0 10 20 30 40 50 60 70 80 90
Te
mp
era
tu
re
, T
[C
]
Time, t [minutes]
T TSP
TSP - DT
Integrate Error
• Integrate error–
• Corrective Action from integration (integral fraction of time on)–
• FTOi will – Increase with time when )– Decrease with time when )– Stay constant when
• How to choose DTi?– Q will be too responsive if DTi is small (or not responsive enough if DTi is too
large). In ME 410 Control, you will learn how to choose DTi. – Wait for temperature to be steady before turning on integral control (Decreasing
DTi)
If T-TSP > 0, thenDecrease FTO
If T-TSP < 0, thenIncrease FTO
How to implement this in LabVIEW
• Need to calculate at each time step– Then sum at each step
• Within While Loop– Use Shift Register to pass data from one step to the next
• Modify Proportional Controller to include integration
Figure 2 VI Block Diagram
Write To Measurement File File Format: Microsoft Excel (.xlsx) File Path:C:\Users\Miles Greiner\Documents\LabVIEW Data\test.xlsx Mode: Save to one file Ask user to choose file: False If a file already exists: Use next available filename X value(time) columns: One column only Description:
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
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)
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