labview instrumentoinnissa, 55492,...
TRANSCRIPT
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Course contents
1. Labview basics– virtual instruments, data flow, palettes
2. Structures– for, while, case, ...– editing techniques
3. Controls&Indicators– arrays, clusters, charts, graphs
4. Additional lecture– State machines, SubVIs, MainCluster
5. Modular programming + recording– SubVIs– File I/O– Analysis– Signal processing– Communication between loops
6. Instrument control– DAQ , Data collection, GPIB, Serial
7. Additional lecture– Data Acquisition, Instrument control
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Most common signal conditioning
• Amplification
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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
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Signal Sources
• Floating signal– not referenced to any common ground
– for example batteries, thermocouples
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Measurement systems
• Differential measurement– measuring with respect to floating
ground
– neither of the inputs tied to fixed reference (building ground)
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Measurement system
• Referenced single-ended– measurement with respect to building ground
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Measurement system
• Nonreferenced single-ended– all measurement with respect to a common reference
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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 ground-referenced signal sources (ground loops!)
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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)
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Connections
• See the user manuals for more information– e.g. USB-6210
http://www.ni.com/pdf/manuals/371931f.pdf
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• Differential • Referenced single-ended
• Non-referenced single-ended
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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
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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
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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.
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Analog input circuitry
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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
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Analog-to-Digital Converter (ADC)
• Device Range– minimum and maximum analog signal levels the device can digitize
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Analog-to-Digital Converter (ADC)
• Code Width– smallest detectable change in the signal, i.e. resolution
–
– 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
(bits) resolution2
range device width code
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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
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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
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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
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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
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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 DAQ-device to generate this word
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Wirings
• USB-6008
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Instrument Control
• GPIB
• Serial port
• Image Acquisition
• USB
• Ethernet
• Parallel port
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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
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GPIB
• GPIB configurations– you can have multiple devices
connected to the same computer
• Device groups– Talker
– Listener
– Controller
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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)
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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
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Serial Port Communication
• Before communication you need to define– 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
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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
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Instrument Drivers
• Download from ni.com
• Help >> Find Instrument Drivers– requires login
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Instrument Drivers
• After installation the drivers can be found from functions palette
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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
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