LabView instrumentoinnissa, 55492, 3op

pribblingchoppedElectronics - Devices

Nov 15, 2013 (3 years and 8 months ago)

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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




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 ground
-
referenced 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 2
3

= 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







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)

resolution
2
range

device

width
code

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 DAQ
-
device to generate this word

Wirings


USB
-
6008

Instrument Control


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


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