LabView instrumentoinnissa, 55492, 3op

Report
Course contents
1.
Labview basics
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2.
Structures
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3.
SubVIs
File I/O
Analysis
Signal processing
Communication between loops
Instrument control
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7.
State machines, SubVIs, MainCluster
Modular programming + recording
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6.
arrays, clusters, charts, graphs
Additional lecture
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5.
for, while, case, ...
editing techniques
Controls&Indicators
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4.
virtual instruments, data flow, palettes
DAQ , Data collection, GPIB, Serial
Additional lecture
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Data Acquisition, Instrument control
Most common signal conditioning
• Amplification
Signal Sources
• Grounded Signal
– referenced to system ground (earth, building ground)
– example: devices that plug into building ground through wall outlets
(e.g. signal generator)
– be aware of ground loops: Two independently grounded signal sources
are generally not at the same potential
Signal Sources
• Floating signal
– not referenced to any common ground
– for example batteries, thermocouples
Measurement systems
• Differential measurement
– measuring with respect to floating
ground
– neither of the inputs tied to fixed
reference (building ground)
Measurement system
• Referenced single-ended
– measurement with respect to building ground
Measurement system
• Nonreferenced single-ended
– all measurement with respect to a common reference
What system to use?
• In general, differential measurement system is preferable
• Differential measurement rejects ground loops and noise
from the environment
• Single-ended measurements allow twice as many channels as
differential
• Use single-ended only if you have all of the following:
– high-level signals (normally, greater than 1V, so that the induced errors
are lower than the required accuracy)
– short or properly-shielded cabling (normally, less than 3 m)
– all signals can share commmon reference signal at the source
• Do not use referenced single-ended connections with groundreferenced signal sources (ground loops!)
What system to use?
• The noise rejection with non-referenced single-ended mode is
better than referenced single-ended
• Differential is better than non-referenced single-ended mode
(AISENSE connection is shared with all channels)
Connections
• See the user manuals
for more information
– e.g. USB-6210
http://www.ni.com/pd
f/manuals/371931f.pdf
• Differential
• Referenced single-ended
• Non-referenced single-ended
Multichannel scanning considerations
• Multiplexer switches from one AI channel to the next
• Instrumentation amplifier has to settle to the new input range
• Settling time: time it takes the amplifier to amplify the input
signal to the desired accuracy before it is sampled
For fast settling times:
• Use low impedance sources
– accumulated charge in multiplexer capasitor leaks through from
previous to the next channel when switching between channels
(ghosting)
• Carefully choose the scanning order
– avoid switching from large to small input range
– scan grounded channel between signal channels: improves settling
time
– even with the same input range selected, if you know the expected
signal levels, group the similar expected ranges together in your
scanning list
– If it’s not necessary to switch between channels, scan for example 100
samples from the first channel and only then switch to second channel
and scan 100 samples
For fast settling times:
• Avoid scanning faster than necessary
– more time to settle
– example: You need to scan 10 channels over a period of 20 ms average
the result. Even if scanning with 250 kS/s gives more samples and
therefore improves the standard error of the mean, scanning with 125
kS/s gives more settling time and can in some cases give more
accurate results.
Analog input circuitry
Analog-to-Digital Converter (ADC)
• Resolution – number of bits in your ADC
– Example: 3-bit ADC divides the measurement range to 23 = 8 divisions
With 16-bits you have 65536 divisions
Analog-to-Digital Converter (ADC)
• Device Range
– minimum and maximum analog signal levels the device can digitize
Analog-to-Digital Converter (ADC)
• Code Width
– smallest detectable change in the signal, i.e. resolution
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code width 
device range
2 resolution (bits)
– for example: 16-bit resolution, range from -10 to +10V
code width = 20 V/2^16 = 305 µV
– Nominal resolution is worse due to the calibration method of the
device
Sampling rate
• How often A/D conversion takes place
• Aliasing is a result of too low sampling rate
• Nyquist theorem
– sampling rate has to be at least twice the measured frequency to
accurately represent the signal
– Nyquist frequency = Sampling frequency/2
Sampling rate
• Example: Sampling rate 100 S/s; signal at 25 Hz is measured
correctly but signals at 70 Hz, 160 Hz and 510 Hz are aliased to
30 Hz, 40 Hz, and 10 Hz
Hardware vs Software timing
• Timing source can be on hardware or on software
– on hardware a clock on the device determines the timing
– on software the program loop determines the timing
• Hardware timing is more accurate and faster
Analog output
• Digital-to-Analog conversion
– generate analog signal from
computer
• Single point update
– software timed generation
– change the output value
everytime the program calls the
VI
• Buffered analog output
– hardware timed generation
– upload a waveform to the device
and set the update rate of the
device to go through the points
Digital I/O
• Two states:
– high and low
• Control digital or finite state devices
– switches, LEDs
• Program devices or communicate between devices
– Example: Digital frequency generator takes 30-bit control word which
defines the generated frequency – use digital output ports of a DAQdevice to generate this word
Wirings
• USB-6008
Instrument Control
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GPIB
Serial port
Image Acquisition
USB
Ethernet
Parallel port
GPIB
• General Purpose Interface Bus (GPIB)
– a.k.a HP-IB, IEEE 488
• GPIB is usually used in stand alone bench top instruments to
control measurements and communicate data
– supported by many instrument manufacturers
• Digital, 24-conductor, 8-bit parallel communication interface
• 16 signal lines, 8 ground return lines
– 8 data lines: data sended in bytes
– 3 handshake lines: control the transfer of messages
– 5 interface management lines
• Data transfer rate typically 1Mbyte/s
• IEEE 488.1 and 488.2 define standards for GPIB
GPIB
• GPIB configurations
– you can have multiple devices
connected to the same
computer
• Device groups
– Talker
– Listener
– Controller
GPIB
• GPIB has one (active) controller that controls the bus
– usually this is the computer
– it connects the talkers to listeners
• Physical requirements
– maximum separation between two devices 4 m (for high-speed use
only 1 m)
– maximum total cable length 20 m
– maximum of 15 devices on a bus (at least 2/3 turned on)
Serial Port Communication
• Communicate with only one device
• No need to buy additional hardware like with GPIB (although
modern computers don’t always have RS-232 port anymore)
• Send data one bit at a time
– you can have long distance between devices
– data transfer rate is low
Serial Port Communication
• Before communication you need to define
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baud rate
number of data bits for a character
parity bit
number of stop bits
• Two voltage stages
– positive > 3V
– negative < -3V
– area between +3V and -3V is designed to absorb noise
Instrument Drivers
• Software to control a particular instrument
• VISA – Virtual Instrument Software Architecture
– library for controlling GPIB, serial, Ethernet, USB, or VXI instruments
• Example: Agilent 34401 Digital Multimeter
Instrument Drivers
• Download from ni.com
• Help >> Find Instrument Drivers
– requires login
Instrument Drivers
• After installation the drivers can be found from functions
palette
Links
• User manual for M-series USB-621x
– http://www.ni.com/pdf/manuals/371931f.pdf
• Labview data-aquisition manual
– www.ni.com/pdf/manuals/320997e.pdf
• Labview Measurement Manual
– http://www.ni.com/pdf/manuals/322661b.pdf
• Understanding Instrument Specifications
– http://zone.ni.com/devzone/cda/tut/p/id/4439#2
• Ghosting in multichannel sampling
– http://digital.ni.com/public.nsf/allkb/73CB0FB296814E2286256FFD00
028DDF?OpenDocument

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