ME 322: InstrumentationLecture 18

March 2, 2015Professor Miles Greiner

TC signal conditioner, Computer data acquisition introduction, Lab 7, myDAQ, LabVIEW program

Announcements/Reminders• HW 7 due Friday• Lab 6 (wind tunnel) this week

– See WebCampus for schedule – Please be on time. – Bring Excel from HW 6 and use it to process the data

you acquire. – This will help check the data as you take it and allow you to

complete the data acquisition phase of the lab in one hour

Thermocouples

• Consist of two dissimilar metal wires, A(+) & B(-) connected to a digital voltmeter (copper terminals C)

• Output voltage VOUT depends on the sensing junction temperature TS – However, VOUT’s sensitivity S = 20–50 mV/°C is very small– Moreover, VOUT also depends on (undesired inputs)

• Terminal temperature TT • High Frequency Radio Frequency Noise• Ground loop currents

TT

TS VOUT+

-

• Type J (Iron/Constantan) thermocouples – + Iron (white insulation) – - Constantan (red or white with red stripe insulation)

• Omega DRE–TC-J TC Signal Conditioner ; web– http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Labs/Lab%2007%20Boiling%20Water%20Temperature/Lab7%20Index.htm – Wiring:

• Iron (white insulation) goes to +Tc; Constantan (red stripe) goes to (-Tc) • Output Voltage, VSC: Com = -; VOUT = +• Power: 110 VAC (top)

In ME 322 Labs

Constantan ( )

+

-

Transfer Function (Type-J-TC/DRE–TC-J TC)

• For TS < 400C– (linear)

• ; = 500• For boiling water , For ice ?

– To use: Inverted transfer function: TS = (40°C/V)*VSC

• Conditioner Provides– Reference Junction Compensation ( not sensitive to TT)– Amplification (Allows normal DVM or computer acquisition to be used) – Low Pass Filtration (Rejects high frequency RF noise) – Linearization (Easy to convert voltage to temperature)– Galvanic Isolation (TC can be used in water environments)

ReadingVSC [V]

Measurand, T [°C]00

400

10? Out of

rangeTransferFunction

𝑆𝑆𝐶=𝜕𝑉 𝑆𝐶

𝜕𝑇

Computer Data Acquisition (DAQ)

• Sensors detect measurands and produce signals– Voltages, currents, resistances, pulses,…

• Conditioners convert those output signals to easily measureable voltages• A multiplexer (MUX) sweeps channel to channel and feeds individual signals, at

different times, to the Analog-to-Digital (A/D) converter• The A/D converter rapidly samples real signals (7.674337…V) and converts them to

integers (0, 1, 2,…) that the digital computer can work with.• Computer programs store and/or process the data

– In ME 322 we use LabVIEW and DAQmx drivers (in ECC)

Lab 7 Boiling Water Temperature in Reno

• Elevation 4500 ft, PATM = 86 kPa (101.3 kPa at sea level)• Beaker, water, immersion heater• Type-J TC, DRE–TC-J TC Signal Conditioner • National Instruments myDAQ (USB to computer)

National Instruments myDAQ

• $179 on Studica– http://

www.studica.com/us/en/NI-Hardware-Only/student-ni-mydaq/781325-01_2.html?utm_source=google&utm_medium=ppc&kpid=734446&gclid=CJXw956H8LwCFYlafgodb3wAMg

• Reads TC Signal Conditioner output (or other device) – Connect to Analog Input (AI) Channel 0 (could use channel 1)

• COM to 0- and AGND (negative analog input and ground)• Vout to 0+ (positive analog input)• This is a Differential Input

– Difference between two channels, gives better common-mode rejection

Check Connections

• Open NI–MySystem-Measurement & Automation Explorer–This program comes with LabView

• MySystem (upper left corner)–Device & Interfaces (Double Click)

• MyDAQ (Right Click)– Device Pin outs– Test Panel

Lab 7 LabVIEW Data Acquisition Program

• Called a VI – Virtual Instrument– Front Panel – Indications and Controls– Block Diagram – Devices, connections and logic

• Lab 7 Objectives– Measure VOUT from AI channel 0 for 3 seconds

• Sample rate 1000 times per second• Convert to TS = (40°C/V)VOUT

– Plot Temperature versus time on front panel• Write data to an Exel file

– Find average temperature, TAVG

LabVIEW Five Main Acquisition StepsMeasurement I/O, NI DAQmx1) Create a channel2) Timing3) Start Process4) Read Data analog waveform

– 1 Channel– N-Samples– Output voltage – convert to ̊ C

5) Clear the testProgramming; Dialog and User Interface

simple error handler

You will need to download and install DAQmx to on your computer (after installing LabVIEW) find these icons.In this class we give and modify example LabVIEW programs to help you quickly learn to perform data acquisition. However, we don’t deal with structured programming.

LabVIEW program (block diagram)

LabVIEW Front Panel

MUX

Lab 7 VI Objectives• Measure atmospheric pressure PATM

– Find TSAT at that pressure

• Allow water in beaker to reach a full, rolling boil• Measure VOUT for 3 sec

– Sample 1000 times per second– Convert to TS = (40°C/V)VOUT

• Plot Temperature versus time– Write to a file

• Find average temperature, TAVG • Repeat for ice/water mixture and room temperature water• Compare to liquid in glass thermometer• Is TAVG,Boil ± wTavg ~ TSAT • Determine effect of salt in the water