data acquistion 1.ppt

7/25/2019 data acquistion 1.ppt

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DATA ACQUISITION

Todays Topics

Define DAQ and DAQ systems

Signals (digital and analogue types)

Transducers Signal Conditioning

- Importance of grounding

- Differential, eferenced and nonferenced single

ended signals- A to D con!ersion considerations (resolution,de!ice and signal range, sampling rate)

In Class "#ample $ Signal input range calculation

Assign %ome&or' and egin in class


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DATA ACQUISITION - DEFINED

DATA ACQISITI*+(DAQ) is t%e measurementor generation of p%ysical signals

DAQ systems typically consists of fi!e elements

. Signals

/ Transducers

0 Signal-conditioning %ard&are

1 DAQ de!ice or module2 Application soft&are


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TYPICAL DAQ SYSTEM ELEMENTS


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SIGNALS

Signals are p%ysical 3uantities t%at are functionsof an independent !ariale (suc% as time) and

contain information aout a natural

p%enomenon

T&o types of signals may e defined4 digital andanalogue

- Digital signals pro!ide information regarding

t%e !oltage state (typically %i or lo&) and5or t%e rate of c%ange of t%ese states

- An analogue signal typically pro!ides !oltage

le!el, s%ape or fre3uency content information


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Signal types and classes

ADC - analogue to digital con!erter, &%ic% con!erts t%e analogue signal

into a digital signal &%ic% can e read y a computer DAC - digital to analogue con!erter, &%ic% con!erts a digital signal to an

analogue signal TT6 - transistor to transistor logic

T&o types of Digital signals

- on-off - pulse train, T%ree types of analogue

- DC - AC - fre3uency domainTotal of 2 signal classes


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A transducer is a de!ice &%ic% con!erts a p%ysical

p%enomenon into a measurale electrical signal

TRANSDUCERS


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SIGNAL CONDITIONING

Transducer output is usually computer ready and must econditioned, eit%er using %ard&are or soft&are

Transducer excitatin$ use of an e#ternal !oltage orcurrent to e#cite t%e transducer (e# strain gauges)

Lineari!atin$ creation of a linear relations%ip et&eent%e transducer output and t%e p%ysical p%enomenon eing

measuredIs"atin- isolation of t%e transducer signal from t%ecomputer and ot%er transducers

Fi"terin#- conditioning of a signal to reduce un&antedcomponents (ie, noise, %ig% or lo& fre3uency components,etc)A$%"i&catin- increace of t%e output signal of t%etransducer to increace accuracy and signal to noise ratio


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LA'(IE) SC*ISIGNAL CONDITIONING

SC7I (Signal Conditioning 7tensions forInstrumentation) is a %ard&are de!icedesigned to condition lo& le!el signals in a

noisy en!ironment &it%in an e#ternalc%assis located near t%e sensor

Some signal conditioning, suc% as

lineari8ation and filtering, can often eperformed using soft&are, and 6a9I":pro!ides se!eral 9Is for t%ese purposes


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SIGNAL GROUNDING

T&o types of signal sources

- #runded signals &%ic% are referenced to a system ground

e#..;9 outlets, signal generators, po&er

supplies

- +atin#, signals &%ic% are not connected

to an asolute referencee# attery po&ered sources, t%ermocouples

transformers


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NOTES ONMEASUREMENT SYSTEMS

A measurement system can e placed into on of t%ree categories

. Di/erentia"

- +eit%er t%e positi!e or negati!e terminal of t%e source ortransducer is connected to ground- T%e system only measures t%e difference et&een t%e terminals,and re


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ANALOGUE TO DIGITALCON(ERSION CONSIDERATIONS

T%e 3uality of t%e analogue to digital con!ersion is

dependent on t%e follo&ing four parameters

. esolution

/ De!ice range

0 Signal input range

1 Sampling rate

T%ese &ill e set using %ard&are (oard s&itc%es)or soft&are (=easurement and Automation

"#plorer in 6a!ie&)


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. RESOLUTION

Res"utin T%e numer of its used torepresent t%e analogue signal

T%e ao!e e#ample s%o&s t%e difference

et&een 0 (/0>? di!isions) and .@ it (/.@>

@2,20@ di!isions) resolution

.@-it


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2. DE(ICE RANGE

de6ice ran#e,minimum and ma#imumanalogue signal le!els t%at t%e ADC can

con!ert

T0e de6ice ran#e s0u"d 7e $atc0edt t0e ran#e 1 t0e ana"#ue in%utsi#na" t 7est ta8e ad6anta#e 1 t0e

a6ai"a7"e res"utin9


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2. DE(ICE RANGE 3cnt:s4

"#ample If a 0-it ADC (%a!ing ? di!isions) is used o!era range of ; to .; !olts, !oltage c%anges of ./29 can e

measured

o&e!er, if t%e range is increased to -.; to .;9, t%en t%e

smallest !oltage c%ange &%ic% can e measured rises to

/29


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5. SIGNAL INPUT RANGE

si#na" in%ut ran#e, t%e ma#imum and minimum !alueof t%e signal eing measured

T%e closer t%e signal input range is to t%e incoming

analogue signal ma# and min, t%e more digital di!isions

&ill e a!ailale to t%e ADC to represent t%e signal


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SMALLEST DETECTA'LE(OLTAGE C;ANGE

Determined y t%e resolution and range of a DAQde!ice and t%e signal input range

T%is c%ange in !oltage represents . "eastsi#ni&cant 7it(6SB) of t%e digital !alue and isoften called t%e cde


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>. SAMPLING RATE sa$%"in# rate,t%e rate at &%ic% t%e DAQ de!ice samples

an incoming analogue signal

Determines %o& often an analogue to digital con!ersionta'es place

Computing t%e proper sampling rate re3uires 'no&ledge oft%e ma# fre3uency of t%e incoming signal and t%e accuracyre3uired for digital representation

In general, a fast sampling rate ac3uires more points perunit time and results in a etter representation of t%eincoming signal

data acquistion 1.ppt

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