ME 322: Instrumentation
Lecture 35
April 18, 2014
Professor Miles Greiner
Announcements/Reminders
• HW 11 is due now,
• HW 12 Due Friday, 4/25/2014
• Don’t start L12PP until next week (revising)
• Next 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%20K
armon%20Vortex/Lab%20Index.htm
• Extra-Credit LabVIEW Workshop
– Today, 2-4 PM, Jot Travis Room 125D
– Make sure you sign-in to get credit
• Lab Practicum Final
– Guidelines, Schedule
• http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Tests/Index.htm
–
Practice Periods
• May 2-4, 2014
Fry Pan Controller
Increase TSP
Decrease TSP
• Bi-metallic strip deforms as its temperature changes
• Opens switch (turns heater off) when it gets to hot, and closes
it (turn heater on) when too cool
• Dial physically moves strip and sets desired or “set-point”
temperature TSP (at which heater turns off)
• Feedback Control
• Measures temperature and adjusts corrective action
• Full on/off control
• “Bang/Bang” control
• Would not work for a cruise control
On/Off Control
TSP
T
T
Heater off
Error
e=T-TSP
Heater on
• The sensor and heater are not at the same location
– By the time the sensor reaches the set-point temperature TSP and turns off the heater, the
heater is above TSP
– The sensor temperature continues to rise as energy from the heater diffuses it.
– Eventually the sensor temperature decreases below TSP and the controller turns on the
heater
– There is a delay before the sensor detects a temperature rise
• Even though the sensor is very accurate and turns the heat on/off at TSP the delayed
response of sensor to heater causes on/off control to exhibit oscillations.
– Oscillations might be smaller if we did not use full on/off control
– We would like the error e = T-TSP to be zero.
Desired Characteristics
• Reach desired temperature quickly
• Minimize error e = T – TSP
• Robust to changes in the environment
– Such as wind and external temperature
• Be able to follow time-dependent set point
TSP(t)
Controller Examples
• Thermostat
• Oven
• Motor speed controller
– Garage door opener, fan
• Car cruise control (not full on/off)
• Unmanned Autonomous Systems (UAS)
– Direction, speed, altitude, level
• Missile or rocket guidance
– Correct for wind conditions
• Self-driving cars
– Sense distance between cars and maintain it
• In each case, sense variable to be controled, compare to
desired value, and take corrective action
Lab 12 Temperature Feedback Control
• Measure temperature in a beaker of water, T
– Thermocouple, signal conditioner, myDAQ, VI
• You’ve done this already
• Is the water temperature uniform? What is T?
• Control power to heater to bring water to TSP
– Before: the heater was on 100% of the time so the water
boiled
– Now: Actively turn the heater on/off according to different
control logic structures
• i.e. On/Off, Proportional, Integral…
• Use myDAQ analog output to control a digital relay that turns heater
on/off
• If TSP = TEnvironment is there a need for control?
• What if TSP is > 100°C?
Lab 12 Setup
• myDAQ has two analog output (AO) channels
– V = ±2 and ±10 volt ranges, N = 16 (216 = 65,536),
– Low current (2 mA, can’t power heater)
– http://www.ni.com/pdf/manuals/373060e.pdf (page 36)
• Solid State Relay = voltage-controlled switch
– Switch is on (closes) when V > 3 volt; Off when V < 1 volt
– http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Labs/Lab
%2012%20Thermal%20Control/Lab%20Index.htm
Schematic
Heater
TC Signal
Conditioner
Analog Input
±10 and ±2 Volt,16 bit
TC
myDAQ
Solid State Relay
Tyco SSRT-2400-10
Power
Switch
Analog Output
±10 and ±2 Volt,16 bit
Input
+
Ground
Turn light on/off
• NI Measurement and Automation explorer
– Analog Output
– Update
• LabVIEW VI
– Create Channel (Digital Output)
– Write Data
– While Loop
VI to turn light on/off
• Block Diagram and Front panel
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
• Starting Point VI
– Temperature versus time from earlier labs
– http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation
/Labs/Lab%2012%20Thermal%20Control/Lab%20Index.htm
Full On/Off Temperature Control
Front Panel
Next time
• Review program construction/logic
• Consider proportional control
– Heater Power is proportional to error e = T-TSP
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
T = TSP – DT
3) T > TSP
f=0
f=1
FTO = 0
Strobe Light VI