ME 322: Instrumentation
Lecture 35
April 17, 2015
Professor Miles Greiner
On/off feedback control, Lab 12 setup, Analog Output,
Strobe light vi, On/off water temperature control vi
Announcements/Reminders
• HW 11 is due now
• HW 12 due next Friday
• Next week: Lab 11 Unsteady Karmon Vortex Speed
• 1-hour periods with your partner
• Schedule on WebCampus (please be on time and come prepared)
• Lab Practicum Final
– Guidelines,
http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Tests/Index.htm
– Schedule on WebCampus
• Please let me know if this schedule conflicts with other finals
• If you want to change your time, please trade with someone else,
both send emails to Marissa and me, and get confirmation.
– Practice Periods
• Saturday and Sunday, May 2-3, 2014
Aeronautical Engineering at UNR
1. The Mechanical Engineering Department currently
offers a course in Aerodynamics. Would you be
interested in taking courses in Airframe Design,
Propulsion Systems, or Aeronautical Component
Manufacture if they were offered?
2. Would you be more interested in attending graduate
school at UNR if it offered advanced training in
Aeronautical Engineering?
3. If there were Aeronautical Engineering jobs in Reno,
would this increase your interest in taking
undergraduate courses or attending graduate school in
that field?
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 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 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)
•
•
•
•
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
– 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