Transcript overview ppt

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.”
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Scott Russell and Roger Gans
Lab Instruments
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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
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Data acquisition (the measurement part)
the
lab
pressure, e. g.
Most of your data acquisition
will be done using LabVIEW on a PC.
You will learn how to do this early on.
transducer
voltage
LabVIEW: hardware/software
A/D
conversion
binary number
computer
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Some issues to be addressed:
instrument response in time (transients, lag time)
Measuring time-dependent phenomena
accuracy and precision (system resolution)
noise!
(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.)
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From the lab to your dorm I
(some language)
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Sample rate refers to the number of data sets that can be processed
(converted or decoded) per second.
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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.
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Bit resolution describes the precision of that sample, or how much discrete
detail (significant figures) is captured in each snapshot .
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The voltage signal from the instrument is converted to a binary number. The
number of bits in the number determines the possible precision.
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Resolution is therefore the SMALLEST DETECTABLE increment of
measurement.
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For PC-based DAQ, resolution is limited to the number of bits used to
quantize or translate the analog to the digital signal.
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is just unwanted signal
We’ll look at some real noise once we’ve played with the other stuff
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From the lab to your dorm II
(resolution)
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DAQ devices are specified in terms of resolution — the number of bits
in the converted binary number.
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Consider a 14 bit instrument measuring a voltage between -1 and +1:
14-bits of resolution over a minimum range of ± 1 V.
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What does this mean in terms of signal capture quality????
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The voltage resolution is (1 - (-1))V / (214) = 2 / (16384) V = 122 µV.
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This is excellent for a voltage of 0.5, but useless for a voltage below
122 µV.
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It also cannot distinguish between 0.5 and 0.5001; whether this is
important depends on the problem
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Let’s look at this graphically (for 0 < V < +1).
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Bit Contributions
0.5
0.45
0.4
contribution
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
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term
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Conversion of 0.4
0.3
8 bit = 0.3984375
contribution
0.25
12 bit = 0.399902344
0.2
16 bit = 0.399993896
0.15
0.1
0.05
0
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term
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Midrange Digitization
0.525
digitized signal
0.52
0.515
8 bit
0.51
16 bit
0.505
0.5
0.495
0.495
0.5
0.505
0.51
0.515
0.52
0.525
true signal
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Periodic Conversion
0.015
0.01
signal
0.005
true signal
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8 bit
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102 203 304 405 506 607 708 809 910
16 bit
-0.005
-0.01
-0.015
time
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LabVIEW Overview
Take
a
look
Read
data
Take
a
look
Analyze
Data
(?)
Store
data
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The analysis part on the previous slide is not the final analysis
It is often useful to do some preliminary analysis in LabVIEW
BUT . . .
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From the lab to your dorm III
(transport)
The LabVIEW software can store data in Excel format
You can take it away on a flash drive to analyze at your leisure
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ON NOISE
The air is full of EM signals
range=±10 v
Your instrument can pick them up
range = ±10 mv
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Change the red to ± 10 mv
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FILTERS
cutoff = 10 Hz
cutoff = 1 Hz
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A quick introduction to LabVIEW
All Programs —> Engineering —> National instrumentsLabVIEW
(or something close to this)
Select New Blank VI
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You will get a Front Panel
and a Block Diagram
Each will have a palette
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FRONT PANEL
run
stop
controls palette
run continuously
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BLOCK DIAGRAM
functions palette
useful analytical tools
are here
much of what we want
is here
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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
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SIMPLE FRONT PANEL
input, x
allows writing to file
value, y
file name
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SIMPLE BLOCK DIAGRAM
writes x and y to a file
acquires data
combines x and y in a “vector”
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DATA FILE
THE HEADER
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THE DATA
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DATA PLOT WITH REGRESSION LINE
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y = 1.0461x - 0.0395
R² = 0.9997
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Series1
Linear (Series1)
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0
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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!
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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
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They wipe the machines overnight
so
bring a flash drive to take your stuff home with you
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