Balança de Precisão com Strain Gages.pdf

8/9/2019 Balana de Preciso com Strain Gages.pdf

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Universidade Federal de Minas GeraisDepartamento de Engenharia Eletrnica

Laboratrio de Controle eAutomao I

Instrumentao

Balana de PrecisoCom Strain Gages

Prof. Ansio R. Braga, CEFET/MGProf. Fbio G. Jota, DELT/UFMG

Prof. Jos Carlos R. de Oliveira, DELT/UFMG

Belo Horizonte, junho de 2002Reviso: maro de 2008


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STRAIN GAGES

Sensor de Deformao

Deformaes e fadiga so geradas em componentes, subsistemas e sistemas,devido a peso, temperatura, presso, vibrao ou foras de deslocamento. Umdos mtodos mais usuais para realizar estas medies atravs do uso deextensmetros metlicos, ou strain gauges ("gages"), conectados em ponte deWheatstone.

O extensmetrobaseia-se no princpio de que, quando um condutor est sujeitoa um esforo de tenso ou compresso, ocorre uma variao de sua resistncia. Aamplitude da variao, relacionada com a resistncia original, proporcional intensidade do esforo aplicado, ou ainda:

L

L

riginalocompriment

omprimentodoiaomaomicrodeformximaEsforoE

===

o

cvar)(

Em aplicaes de extensmetros utiliza-se uma constante de proporcionalidadeconhecida como Fator de Calibrao (Gage Factor), que varia de 2 a 4 para asligas mais usuais na fabricao de extensmetros. Este parmetro baseado navariao da resistncia ocorrida no extensmetro para sua resistncia total,relacionada com a variao no comprimento do condutor para seu comprimentounitrio, ou ainda:

A tenso de sada do amplificador de um medidor de deformao, comextensmetros em ponte de Wheatstone dada por:

GVR

Re exo =

onde Vex a tenso de alimentao da ponte e G o ganho do amplificador de

instrumentao.

Exemplo: Uma ponte de extensmetros com G=2 eL

L =1500E (dados

provenientes de catlogo do fabricante), possui ERR 300015002 == .

Desta maneira, pode-se calcular a tenso de sada do medidor como sendo:

GVe exo = 3000 .

LLRRGF

=


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BALANA DE PRECISO

PREPARAO

1) Ler o Tutorial Strain Gage Technical Data da OMEGA Engineering.

2) Descrever as funes e as principais caractersticas dos circuitos integradosLM723 e INA114, com o auxlio dos datasheets da National Semicondutor e daBurr Brown/Texas Instruments.

3) Descrever o funcionamento do Mdulo Condicionador para Clulas de Carga(Braga, 2002), apresentado na Figura 1 abaixo. A ponte de Wheatstone comstrain gages e a fonte de alimentao so externos ao Mdulo. Este, por suavez, constitudo por quatro elementos:

Regulador de tenso, para excitao da ponte de Wheatstone (tenso Vex); Circuito de balanceamento da ponte; Amplificador de instrumentao, para aumentar o nvel da tenso de

desequilbrio da ponte; Filtro RC passa - baixa, para atenuar rudos de 60 Hz no sinal de sada.

Vof

Regulador de tenso para

a tenso de excitao do

circuito em ponte de

Wheatstone

0

R32.2k

Clula de Carga

R8

15k

C2

33nF0

0

VIN(-)

0

-

+

U2

INA114AP

18

2

3

6

7

4 5

GS1GS2

-

+

OUT

V+

V-

REF

0

RG10k

Filtro

f0=100HzR4R3

R2 R1R=120

Strain-gages

0

R21k

G=1+50k/RG

R4560

C1 33nF

VCC

0

GND

0

VCC

R7560

1234

JP2VCC

VEE VIN(+)

Fonte de Alimentao

VEE

Vex=6V

12VGND

Vo

VCC

Vo

R6

100k

0

+Vex

VEE

-

+

U1

LM723

12

11

10

7 13

4

5

23

69

Vcc+ V

c

OUT

Vcc-

COMP

-

+

CLCS

VrefVz

12V

0

Amplificador de Instrumentao

Balanceamento

R1100

C5.1uF

Balano

R520k

C3.1uF

VCC

VEE

C4.1uF

Figura 1: Mdulo condicionador para clulas de carga.


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PARTE EXPERIMENTAL

1) Verificar a construo fsica da balana e a fixao dos strain-gages da

Ponte de Wheatstone na haste metlica. Analisar os esforos possveissobre os mesmos: quais strain-gages sofrero trao e quais sofrerocompresso?

2) Fazer o ajuste de zero com o prato vazio, com VO= 0,00 Volts.

3) Fazer o ajuste de fundo de escala, de modo que a massa M = 300g de umcorpo padro corresponda tenso de sada VO= +3,00 Volts.

ATENO: utilizar massas de at 300 g para no deformar a balana. Tomarcuidado ao manusear os padres a serem utilizados na calibrao, pois eles

no devem ser tocados com as mos.

4) Levantar pontos (VOx M) da Caracterstica Esttica da Balana, utilizandoos diversos corpos padro. Trabalhar com variaes (intervalos) de 20gramas, sendo 16 pontos com variaes crescentes e 16 com variaesdecrescentes. Observar que, dos 16 pontos, um se refere balana semcarga.

5) Com o auxilio de um Software (p. ex., Planilha Excel ou MATLAB) obter osgrficos VO[Volts] x M [gramas] para os testes crescente e decrescente.- Fazer regresses de primeira ordem nestas curvas experimentais. Existe

histerese significativa entre as duas?- Obter a funo analtica V'O = f(M), isto , a equao de calibrao dotransdutor. Qual o seu ganho?- Calcular e plotar o erro de linearidade, em funo da massa M:

EL(M) = VO(M) V'O(M)

- Para qual faixa de massa M o erro maior? Por qu?- Verificar, com um osciloscpio, o rudo presente na medio e o efeito dafiltragem realizada.

6) Estudar, agora, o Comportamento Dinmico da Balana, identificando afuno de transferncia:

G(s) = VO(s) / M(s)

Para isso, com o auxlio de um osciloscpio digital, fazer a aquisio dosinal VOcom o tempo, tanto para um degrau de massa igual a 200 gramas(deixada cair no prato da balana), quanto para um impulso (aplicado naextremidade da haste metlica, neste caso com e sem a massa de 200 g).Em cada caso, exportar o sinal do osciloscpio para um microcomputador,atravs de comunicao serial.


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7) Encontrar uma funo G(s) para cada teste, medindo o perodo dasoscilaes (intervalo de tempo entre picos), assim como plotar a envoltriaem um grfico semi-logartmico e fazer uma regresso de primeira ordem.

O ganho esttico de G(s) a inclinao da caracterstica esttica j obtida,ou seja, o ganho do transdutor. O modelo pode ser validado para otransitrio completo? Por qu?

8) Analisar o comportamento dinmico da Balana, modelando-a teoricamentepor um sistema massa-mola-amortecedor. Para simplificao, considerarum sistema de translao retilnea vertical, uma vez que o deslocamentoangular da haste bem pequeno. Obter um modelo de segunda ordem everificar analiticamente como a massa, o coeficiente de rigidez (da mola) eo coeficiente de atrito influenciam esta funo de transferncia: osresultados tericos so coerentes com as medies experimentais?

9) O comportamento dinmico da Balana adequado? Por qu? Qual(ais)alterao(es) mecnica(s) e /ou eltrica(s) poderiam ser feitas, de forma aalterar esse comportamento? Comentar as caractersticas estticas edinmicas da balana. Tirar concluses.

Referncias Bibliogrficas

Doebelin, E. O.Measurement Systems Application and Design (Cap. 3)

McGraw-Hill International Editions, 4thEdition, 1990.

OMEGA Engineering, IncThe Pressure, Strain and Force Handbook, Section E, 2000.

www.omega.com/techref/strain-gage.html Strain Gage Technical Data, 2002.

National InstrumentsStrain Gauge Measurement A Tutorial.Application Note 078, 1998.

www.national.comLM723 Voltage RegulatorNational Semiconductor.

www.ti.comINA114 Precision Instrumentation Amplifier.Texas Instruments / Burr-Brown.


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Anexo 1: Strain Gages

Fonte: www.omega.com

Data de acesso:03/03/2008


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Anexo 2: Amplificador INA114

Fonte: www.ti.com



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Anexo 3: Regulador de Tenso LM723

Fonte: www.national.com



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Anexo 4: Tutorial Strain Gage Technical Data

Fonte: www.omega.com/techref/strain-gage.html


THE STRAIN GAGE IS ONE OF THE MOST IMPORTANT TOOLS of the electricalmeasurement technique applied to the measurement of mechanical quantities. Astheir name indicates, they are used for the measurement of strain. As a technicalterm, "strain" consists of tensile and compressive strain, distinguished by a positiveor negative sign. Thus, strain gages can be used to pick up expansion as well ascontraction. The strain of a body is always caused by an external influence or aninternal effect. Strain might be caused by forces, pressures, moments, heat,structural changes of the material and the like. If certain conditions are fulfilled, the

amount or the value of the influencing quantity can be derived from the measuredstrain value. In experimental stress analysis this feature is widely used.Experimental stress analysis uses the strain values measured on the surface of aspecimen or structural part to state the stress in the material and also to predict itssafety and endurance. Special transducers can be designed for the measurementof forces or other derived quantities, e.g., moments, pressures, accelerations, anddisplacements, vibrations and others. The transducer generally contains apressure sensitive diaphragm with strain gages bonded to it.

Strain Gage Measurement

The most universal measuring device for the electrical measurement of mechanicalquantities is the strain gage. Several types of strain gages depend on theproportional variance of electrical resistance to strain: the piezoresistive or semi-conductor gage, the carbon-resistive gage, the bonded metallic wire, and foilresistance gages.

The bonded resistance strain gage is by far the most widely used in experimentalstress analysis. These gages consist of a grid of very fine wire or foil bonded to thebacking or carrier matrix. The electrical resistance of the grid varies linearly withstrain. In use, the carrier matrix is bonded to the surface, force is applied, and the

strain is found by measuring the change in resistance. The bonded resistancestrain gage is low in cost, can be made with a short gage length, is only moderatelyaffected by temperature changes, has small physical size and low mass, and hasfairly high sensitivity to strain.

In a strain gage application, the carrier matrix and the adhesive must work togetherto transmit the strains from the specimen to the grid. In addition, they serve as anelectrical insulator and heat dissipator.


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The three primary factors influencing gage selection are operating temperature,state of strain (gradient, magnitude, and time dependence) and stability required.

Because of its outstanding sensitivity, the Wheatstone bridge circuit is the mostfrequently used circuit for static strain measurements. Ideally, the strain gage is theonly resistor in the circuit that varies and then only due to a change in strain on thesurface.

There are two main methods used to indicate the change in resistance caused bystrain on a gage in a Wheatstone bridge. Often, an indicator will rebalance thebridge, displaying the change in resistance required in micro-strain. the secondmethod installs an indicator, calibrated in micro-strain, that responds to the voltageoutput of the bridge. This method assumes a linear relationship between voltage

out and strain, an initially balanced bridge, and known V in. In reality, the V out-strain relationship is nonlinear, but for strains up to a few thousand micro-strain,the error is not significant.

Potential Error Sources

In a stress analysis application, the entire gage installation cannot be calibrated ascan some pressure transducers. Therefore, it is important to examine potentialerror sources prior to taking data.

Some gages may be damaged during installation. It is important therefore to checkthe resistance of the strain gage prior to stress.

Electrical noise and interference may alter your readings. Shielded leads andadequately insulating coatings may prevent these problems. A value of less than500 M ohms (using an ohmmeter) usually indicates surface contamination.

Thermally induced voltages are caused by thermocouple effects at the junction ofdissimilar metals within the measurement circuit. Magnetically induced voltagesmay occur when the wiring is located in a time varying magnetic field. Magneticinduction can be controlled by using twisted lead wires and forming minimum but

equal loop areas in each side of the bridge.

Temperature effects on gage resistance and gage factor should be compensatedfor as well. This may require measurement of temperature at the gage itself, usingthermocouples, thermistors, or RTDs. Most metallic gage alloys, however, exhibit anearly linear gage factor variation with temperature over a broad range which isless than 1% within 100C.


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Prime Strain Gage Selection Considerations

Gage Length

Number of Gages in Gage Pattern Arrangement of Gages in Gage Pattern Grid Resistance Strain Sensitive Alloy Carrier Material Gage Width Solder Tab Type Configuration of Solder Tab Availability

Strain gage dimensions

The active grid length, in the case of foil gages, is the net grid length without thetabs and comprises the return loops of the wire gages. The carrier, dimensions aredesigned by OMEGA for the optimum function of the strain gage.

Strain gage resistance

The resistance of a strain gage is defined as the electrical resistance measuredbetween the two metal ribbons or contact areas intended for the connection of

measurement cables. The range comprises strain gages with a nominal resistanceof 120, 350, 600, and 700 Ohms.

Gage Factor (Strain Sensitivity)

The strain sensitivity k of a strain gage is the proportionality factor between therelative change of the resistance.

The strain sensitivity is a figure without dimension and is generally called gagefactor.

The gage factor of each production lot is determined by samplemeasurements and is given on each package as the nominal value with itstolerance.

Reference Temperature.

The reference temperature is the ambient temperature for which the technical dataof the strain gages are valid, unless temperature ranges are given. The technicaldata quoted for strain gages are based on a reference temperature of 23C.


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Temperature Characteristic

Temperature dependent changes of the specific strain gage grid resistance occurin the applied gage owing to the linear thermal expansion coefficients of the gridand specimen materials. These resistance changes appear to be mechanical strainin the specimen. The representation of the apparent strain as a function oftemperature is called the temperature characteristic of the strain gage application.In order to keep apparent strain through temperature changes as small aspossible, each strain gage is matched during the production to a certain linearthermal expansion coefficient. OMEGA offers strain gages with temperaturecharacteristics matched to ferritic steel and aluminum.

Service Temperature Range

The service temperature range is the range of ambient temperature where the useof the strain gages is permitted without permanent changes of the measurementproperties. Service temperature ranges are different whether static or dynamicvalues are to be sensed.

Maximum Permitted RMS Bridge Energizing Voltage

The maximum values quoted are only permitted for appropriate application onmaterials with good heat conduction (e.g., steel of sufficient thickness) if room

temperature is not exceeded. In other cases temperature rise in the measuring gridarea may lead to measurement errors. Measurements plastics and other materialswith bad heat conduction require the reduction of the energizing voltage or the dutycycle (pulsed operation).

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Balança de Precisão com Strain Gages.pdf