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ME 322: Instrumentation Lecture 19 March 4, 2015 Professor Miles Greiner LabVIEW program, A/D converter characteristics, actual measured grounded output, Input resolution error

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Announcements/Reminders Please fully participate in each lab and complete the Lab Preparation Problems For the final you will repeat one of the last three labs, solo, including performing the measurements, and writing Excel, LabVIEW and PowerPoint. The labs help prepare you for the final HW 7 due Friday Lab 6 (wind tunnel) this week Please see schedule on WebCampus and be on time Bring Excel from HW 6 How are things going in lab this week so far?

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LabVIEW LabVIEW is available in the Engineering Computer Center (ECC) You can buy LabVIEW on the web for around $20, but you dont have to http://www.studica.com/us/en/National-Instruments-students-ni-labview-mydaq/labview- student-edition/779252- 02.html?utm_source=google&utm_medium=ppc&kpid=371806&gclid=COPM1Pym- bwCFdBqfgodUF4A4A http://www.studica.com/us/en/National-Instruments-students-ni-labview-mydaq/labview- student-edition/779252- 02.html?utm_source=google&utm_medium=ppc&kpid=371806&gclid=COPM1Pym- bwCFdBqfgodUF4A4A If you purchases it, you will need to download and install DAQmx after installing LabVIEW to use the Measurement I/O icons we use in class http://www.ni.com/dataacquisition/nidaqmx http://www.ni.com/dataacquisition/nidaqmx

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LabVIEW Five Main Acquisition Steps Measurement I/O, NI DAQmx 1)Create a channel 2)Timing 3)Start Process 4)Read Data analog waveform 1 Channel N-Samples Output voltage convert to C 5)Clear the test Programming; Dialog and User Interface simple error handler In this class we give and modify example LabVIEW programs. The intent is to help you quickly learn to perform data acquisition programing. However, we dont deal with structured programming.

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LabVIEW program

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Computer Data Acquisition (DAQ) Sensors detect measurands and produce signals Voltages, currents, resistances, pulses, Conditioners convert those output signals to moderately large voltages Multiplexer (MUX) sweeps channel-to-channel and feeds individual signals, at different times, to the Analog-to-Digital (A/D) converter A/D converter samples real voltages (7.674337V) 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: LabVIEW and DAQmx drivers

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How could an A/D Converters work? One method: Saw Tooth Compare (not really used) Function generator produces ramps from V RL to V RU within period T S Converter break T S into M (= 2 N, N = integer) equal sub-steps I OUT for each time step is the first sub-step when V ST V MEASURED To interpret I OUT V Digitized = V RL + (I OUT +1/2)[(V RU -V RL )/M] (1/2)(V RU -V RL )/M Uncertainty decreases as M = 2 N increase, and/or FS = V RU -V RL decreases Measurement is associated with center of time period Time uncertainty: T S /2 V V RL V RU t 0 TSTS 2T S 3T S I OUT = 1I OUT = 2 I OUT = 5 0 1 2 3 4 5 6 7 V Measured

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Characteristics of A/D Converters Full-scale range V RL V V RU FS = V RU - V RL For myDAQ the user can chose between two full-scale ranges 2 V, 10 V for Lab 7, 0 to 2.5 V, which range is used? Number of Bits (in its digital word) N The A/D converter breaks the full scale range into 2 N sub- ranges For myDAQ, N = 16, 2 16 = 65,536 What does this mean? For example, a 2 bit word __ __, in which each bit can be 0 or 1 Has 2 2 = 4 combinations: 00, 01, 10, 11 These are the digital signals (words) the A/D converter passes to the computer

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Sampling Rate Sampling Frequency f S = samples converted to digital per second [Hz] Sampling Period T S = 1/ f S ; timed required to find I OUT myDAQ (f S ) max = 200,000 Hz, T S = 0.000,005 sec = 5 sec User can chose lower rates If both channels are used, then (f S ) max = ? Hz

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myDAQ Absolute Voltage Uncertainty More information myDAQ user guide, page 36-38 http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Labs/Lab%2007%20Boiling %20Water%20Temperature/Lab7%20Index.htm http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Labs/Lab%2007%20Boiling %20Water%20Temperature/Lab7%20Index.htm Demonstration Short myDAQ Analog Input 1 and observe signal What should the reading be when shorted? In my office: 10 V range: V ~ -0.0008 to -0.0026 V = 1.7 0.9 mV (0.009% FS) 2 V range: V ~ -0.0003 to -0.0009 V = 0.6 0.3 mV (0.02% FS) Is it larger at higher voltages? Same in class? (0.12%FS) (0.11%FS) (0.19%FS) (0.22%FS)

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Example A/D Converter Transfer Function

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Input Resolution Error

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A/D Converter Characteristics Full-scale range V RL V V RU FS = V RU - V RL For myDAQ the user can chose between two ranges 10 V, 2 V (FS = 4 or 20 V) Number of Bits N Resolves full-scale range into 2 N sub-ranges Smallest voltage change a conditioner can detect: V = FS/2 N For myDAQ, N = 16, 2 16 = 65,536 10 V scale: V = 0.000,31 V = 0.31 mV = 310 V 2 V scale: V = 0.000,076V = 0.076mV = 76 V Sampling Rate f S = 1/T S For myDAQ, (f S ) MAX = 200,000 Hz, T S = 5 sec

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Input Resolution Error

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myDAQ Uncertainties What are these? AA: Maximum error of the voltage measurement reported by the manufacturer for all voltage levels At different temperatures MSVE: Maximum error measured at V = 0V for one device IRE: Random error due to digitization process Which one do you think characterizes voltage uncertainty?

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Example (cont)

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Transfer Function First Order: Generic Where

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Example

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Second Order damping ration n natural frequency without damping

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Example

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Second Order