ME 322: Instrumentation Lecture 36 April 20, 2015 Professor Miles Greiner Proportional Control

Announcements/Reminders HW 12 Due Friday This week: Lab 11 Unsteady Karmon Vortex Speed One-hour periods with your partner Schedule on WebCampus – Please be on time and come prepared! Lab Practicum Final – Guidelines, Schedule – Schedule On WebCampus Please let me know if there are conflicts with other finals – Practice Periods May 2-3, 2014

Lab 12 Setup Measure beaker water temperature using a thermocouple/conditioner/myDAQ/VI Use myDAQ analog output (AO) to operate a digital relay that turns heater on/off to control the water temperature

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 Constructed last lecture – /Labs/Lab%2012%20Thermal%20Control/Lab%20Index.htm /Labs/Lab%2012%20Thermal%20Control/Lab%20Index.htm

Full On/Off Temperature Control

Front Panel

On/Off Control Temperature Response Full On/off control – Reaches T SP after ~3 minutes – Gives oscillatory response – Average temperature T Avg > T SP – Maximum error is roughly 2.5 ° C Want heater power to be high to reach T SP quickly Would oscillations decrease if power decreased near T ~ T SP ?

How to reduce heater power using a relay? Reduce the Fraction of Time the heater is On (FTO) – Maximum heater power Q Max = V 2 /R Reduce FTO to decrease heater power – Heater Q = (FTO)(Q Max ) How to implement this in LabVIEW? FTO = 0.1FTO = 0.5 FTO = 0.9

Strobe Light VI Stacked sequence loop Milliseconds to Wait Vary cycle time and FTO

Proportional Control

How to construct a Proportional-Control VI Calculate FTO – Indicate FTP using a bar, dial and/or numeric indicator Use stacked sequence loop to turn heater on and off Write to a Measurement File VI – Segment Headings (No Headers) – X value (time) Column (one column only) Starting Point Current Temperature

Proportional Control

Proportional-Control Temp versus Time On/Off Proportional

Average Temperature Error and Unsteadiness versus DT and T SP

Proportional-Control Questions Why is the steady temperature below the set-point (desired) value? Why do temperature oscillations disappear as DT gets larger? Is there another control technique that eliminates the steady state error?

Steady State Temperature Error