ME 322: Instrumentation
Lecture 35
April 15, 2016
Professor Miles Greiner
On/off feedback control, Lab 12 setup, Analog Output,
Strobe light vi, On/off water temperature control vi
Senior Exit Survey
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Announcements/Reminders
• HW 11 is due now (will accept on Monday)
• Joseph will give tutorial Sunday at 4 pm in SEM 321
• HW 12 due next Friday
• Next week: Lab 11 Unsteady Karmon Vortex Speed
• Sign up for 45 minute periods with your partner
• You cannot perform the experiment until you attend lab demonstration
• Please be on time and come prepared!
• Lab Practicum Final
– Guidelines,
http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Tests/Index.htm
– Will publish schedule soon
• If you want to change your time, please trade with someone else,
both send emails to Marissa and me, and get confirmation.
– Lab Practice Period
• Saturday and Sunday, April 30, May 1, 2016
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 to it.
– Eventually the sensor temperature decreases, and goes below TSP and the controller turns
on the heater
– There is a delay before the sensor receives heat and 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 the 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)
• Note: The controller regulates the sensor
temperature, which is not necessarily the
temperature of object being controlled
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•
•
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Controller Examples
Thermostat
Hot-film anemometer
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 the variable to be controlled,
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
– In Labs 7 and 9: the heater was on 100% of the time so the
water boiled
– Lab 12: 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 38)
• 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
Strobe Light VI
• Stacked sequence loop
• Milliseconds to Wait
• Vary cycle time and FTO
End 2016
• Could do full on/off control, but would not
have time to see it operate.
• May be better to keep it in next lecture and due
strobe VI this lecture
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
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