Instrumentation Overview Spring 2012 The laboratory is a controlled environment where we can measure isolated physical phenomena with a view to eventual understanding them. The suite of tools we use in this endeavor is typically called “instrumentation.” Scott Russell and Roger Gans 1

Lab Instruments Various, selected to measure what you want. Off-the-Shelf or Scratch Built. Must be calibrated. Used with a PC or “stand alone.” Responds to phenomena with some “signal.” The signal is typically a voltage or current. Instruments have varied response times, accuracy, and precision. Here’s a cartoon of the process 2

the lab transducer A/D conversion computer pressure, e. g. voltage binary number LabVIEW: hardware/software 3 Data acquisition (the measurement part) Most of your data acquisition will be done using LabVIEW on a PC. You will learn how to do this early on.

4 Some issues to be addressed: (I’ll look at some of these in the context of temperature measurement another day. First I’d like to look at some ideas in general.) accuracy and precision (system resolution) instrument response in time (transients, lag time) Measuring time-dependent phenomena noise!

From the lab to your dorm I (some language) Sample rate refers to the number of data sets that can be processed (converted or decoded) per second. If, for example, the sample rate is 1000 Hertz, then we are gathering 1000 snapshots per second of information (may be multidimensional) from the real world into the PC. Bit resolution describes the precision of that sample, or how much discrete detail (significant figures) is captured in each snapshot. The voltage signal from the instrument is converted to a binary number. The number of bits in the number determines the possible precision. Resolution is therefore the SMALLEST DETECTABLE increment of measurement. For PC-based DAQ, resolution is limited to the number of bits used to quantize or translate the analog to the digital signal. 5

6 We’ll look at some real noise once we’ve played with the other stuff

From the lab to your dorm II (resolution) DAQ devices are specified in terms of resolution — the number of bits in the converted binary number. Consider a 14 bit instrument measuring a voltage between -1 and +1: 14-bits of resolution over a minimum range of ± 1 V. What does this mean in terms of signal capture quality???? The voltage resolution is (1 - (-1))V / (2 14 ) = 2 / (16384) V = 122 µV. This is excellent for a voltage of 0.5, but useless for a voltage below 122 µV. It also cannot distinguish between 0.5 and ; whether this is important depends on the problem Let’s look at this graphically (for 0 < V < +1). 7

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8 bit = bit = bit =

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Read data Analyze Data (?) Store data Take a look Take a look LabVIEW Overview 12

13 The analysis part on the previous slide is not the final analysis It is often useful to do some preliminary analysis in LabVIEW BUT...

14 The LabVIEW software can store data in Excel format You can take it away on a flash drive to analyze at your leisure From the lab to your dorm III (transport)

15 ON NOISE The air is full of EM signals Your instrument can pick them up range=±10 v range = ±10 mv

16 Change the red to ± 10 mv

17 FILTERS cutoff = 10 Hzcutoff = 1 Hz

18 A quick introduction to LabVIEW All Programs —> Engineering —> National instrumentsLabVIEW (or something close to this) Select New Blank VI

19 You will get a Front Panel and a Block Diagram Each will have a palette

20 run run continuously stop controls palette FRONT PANEL

21 BLOCK DIAGRAM functions palette much of what we want is here useful analytical tools are here

22 CALIBRATION VIRTUAL INSTRUMENT dial in the independent variable, x read the dependent variable, y write the result to a file you will build one of these tomorrow

23 SIMPLE FRONT PANEL input, x value, y allows writing to file file name

24 SIMPLE BLOCK DIAGRAM acquires data combines x and y in a “vector” writes x and y to a file

25 DATA FILE THE HEADER

26 THE DATA

27 DATA PLOT WITH REGRESSION LINE

28 I can make LabVIEW do more or less what I want it to do, but Scott is a LabVIEW expert If Scott “helps” you too much, you will wind up with a LabVIEW program I cannot help you with!

29 About Gavett 244 Your ID should get you into the room Your UserID should allow you access to the machines You may have to change the password; they made me do that It takes close to two minutes for the log on process DO THAT FIRST

30 They wipe the machines overnight so bring a flash drive to take your stuff home with you