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ME 322: Instrumentation Lecture 35 April 18, 2014 Professor Miles Greiner.

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Presentation on theme: "ME 322: Instrumentation Lecture 35 April 18, 2014 Professor Miles Greiner."— Presentation transcript:

1 ME 322: Instrumentation Lecture 35 April 18, 2014 Professor Miles Greiner

2 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) – armon%20Vortex/Lab%20Index.htm 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 – Practice Periods May 2-4, 2014

3 Fry Pan Controller 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 T SP (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 Decrease T SP Increase T SP

4 On/Off Control The sensor and heater are not at the same location – By the time the sensor reaches the set-point temperature T SP and turns off the heater, the heater is above T SP – The sensor temperature continues to rise as energy from the heater diffuses it. – Eventually the sensor temperature decreases below T SP 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 T SP 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-T SP to be zero. T Error e=T-T SP Heater on Heater off T SP T

5 Desired Characteristics Reach desired temperature quickly Minimize error e = T – T SP Robust to changes in the environment – Such as wind and external temperature Be able to follow time-dependent set point T SP (t)

6 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

7 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 T SP – 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 T SP = T Environment is there a need for control? What if T SP is > 100 ° C?

8 Lab 12 Setup myDAQ has two analog output (AO) channels – V = ±2 and ±10 volt ranges, N = 16 (2 16 = 65,536), – Low current (2 mA, can’t power heater) – (page 36) Solid State Relay = voltage-controlled switch – Switch is on (closes) when V > 3 volt; Off when V < 1 volt – %2012%20Thermal%20Control/Lab%20Index.htm %2012%20Thermal%20Control/Lab%20Index.htm

9 Schematic TC Signal Conditioner TC myDAQ Solid State Relay Tyco SSRT Analog Output ±10 and ±2 Volt,16 bit Analog Input ±10 and ±2 Volt,16 bit Heater Power Switch Input Ground +

10 Turn light on/off NI Measurement and Automation explorer – Analog Output – Update LabVIEW VI – Create Channel (Digital Output) – Write Data – While Loop

11 VI to turn light on/off Block Diagram and Front panel

12 Full on/off Control LabVIEW VI “logic” – Measure thermocouple temperature for 1 sec Average, T, display – Compare to T SP (compare and select icons) – Turn 200 W heater on/off if T is below/above T SP – Waveform Chart T and T SP versus time e = T-T SP versus time – Repeat Starting Point VI – Temperature versus time from earlier labs – /Labs/Lab%2012%20Thermal%20Control/Lab%20Index.htm /Labs/Lab%2012%20Thermal%20Control/Lab%20Index.htm

13 Full On/Off Temperature Control

14 Front Panel

15 Next time Review program construction/logic Consider proportional control – Heater Power is proportional to error e = T-T SP

16 Fractional Time On (FTO) If DT = 0 then full on/off If DT > 0 then proportional 3 Temp Domains 3) T < T SP – DT FTO = 1 2) (T SP – DT) < T < T SP T = T SP f = 0 T = T SP – DT f = 1 3) T > T SP FTO = 0

17 Strobe Light VI


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