operating manual ver 1 - welcome to cgi bharatpur | cgi …cgibp.com/data/lab_manual/strain gauge...

Strain Gauge Trainer ST2304

Operating Manual Ver 1.1

An ISO 9001 : 2000 company

94-101, Electronic Complex Pardesipura, Indore- 452010, India Tel : 91-731- 2570301/02, 4211100 Fax: 91- 731- 2555643 e mail : [email protected] Website : www.scientech.bz Toll free : 1800-103-5050

mailto:[email protected]

http://www.scientech.bz

ST2304

Scientech Technologies Pvt. Ltd. 2

ST2304


RoHS Compliance

Scientech Products are RoHS Complied. RoHS Directive concerns with the restrictive use of Hazardous substances (Pb, Cd, Cr, Hg, Br compounds) in electric and electronic equipments. Scientech products are “Lead Free” and “Environment Friendly”. It is mandatory that service engineers use lead free solder wire and use the soldering irons upto (25 W) that reach a temperature of 450°C at the tip as the melting temperature of the unleaded solder is higher than the leaded solder.

Strain Gauge Trainer ST2304

Table of Contents

1. Introduction 4

2. Features 4

3. Technical Specifications 5

4. Operating Instructions & Panel Control Description 6 5. Functional Description of Blocks 7

6. Theory 10 7. Experiments

• Experiment 1 15 Study of Strain Measurement using Strain Gauges and Cantilever assembly

• Experiment 2 18 Determination of Linear Range of operation of Strain Measurement

• Experiment 3 20 Determination Sensitivity of Trainer

8. Glossary 21

9. Warranty 22

10. List of Accessories 22

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Introduction

Strain Gauge Trainer ST2304 provides a study of the Strain Gauge and its application for the measurement of strain. It helps to study bridge configuration of Strain Gauge and the signal conditioning circuits required to measure strain. It uses cantilever to produce strain on Strain Gauges. The Strain Gauges are firmly cemented to the cantilever at the point where the strain is to be measured. Weights are placed on the free end of the cantilever. Strain developed changes the resistance of Strain Gauge which is detected by full bridge configuration.

The seven-segment LED display shows strain in micro strain units. Different weights are provided to perform linearity and sensitivity experiments. Model ST2304 is fully covered self contained single box with USB interface and easy to use design detailed experiment manual is supplied with interactive real time software. The manual includes theory of the subject and experiments block description, operating instruction etc.

Strain Gauge Trainer ST2304 can be interfaced with PC using real time software. *Note : USB interface is optional.

Features

• Self-contained and easy to operate.

• Sensitive, Linear, Stable & Accurate.

• Functional blocks indicated on board Mimic.

• Test-points to observe the Input Output of each block.

• Onboard Gain Adjustment.

• Onboard Offset null Adjustment.

• Built-in DC Power Supplies.

• 3½ digits LED display.

• Onboard Cantilever Arrangement.

• High Repeatability and Reliability.

• *USB interface with Real-time Software (PC) for step by step approach. * only in USB interface model.

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Technical Specifications

Strain Gauge (350Ω)

Gauge Factor Maximum Bearable Weight Cantilever Material Cantilever Width Cantilever Thickness Cantilever Length Bridge Voltage Bridge Configuration Display Test Points Power Requirement Dimensions (mm) Weight *Baud Rate

: 4 Nos.

: 2:1 : 500 gms.

: Stainless Steel : 2.5 cm

: 0.16 cm : 20 cm

: +8 V DC : Full Bridge

: 3½ Digit LED : 8 in numbers

: 230V ±10%, 50 Hz : W420× H100× D255 : 3.5 Kgs. (approximately)

: 4800 *ADC Resolution : 8 Bit

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Operating Instructions and Panel Control Description

The ST2304 Strain Gauge Trainer is equipped with a built in DC-Power supply. Attach the three pin mains cord to the three pin socket and connect the other end to a stable 230V AC supply. A fuse (100mA/250V) rating is connected in series with mains supply. When the ‘On’/‘Off’ switch of the trainer is turned ‘On’ the power LED indication will glow, indicating that the trainer is ‘On’.

The Maximum weight that can be placed over the cantilever is 500 gms. Strain due to weights as low as 5 gms can be detected. Do not press the cantilever by hand or by other means as it may damage spring action of the cantilever. After placing the weight, make the cantilever stable by hand to get steady reading on the display. Place weights at the centre of the tray to get exact strain values. Test points are given on the mimic board to measure the input output voltages of different stages of instrumentation system of the strain gauge. Real time software is installed to make the operation of the trainer more interactive. Ground of the Strain Gauge Bridge is isolated from the circuit ground. So whenever voltages of the bridge arms are to be measured, the bridge ground should be used.

Gain of instrumentation amplifier can be adjusted with the Gain Adjust preset given in instrumentation amplifier block. Don't disturb the preset as it is factory calibrated otherwise display reading will not be accurate. Offset Null Adjust preset is used to make the display to read 000, when no weight is placed on the cantilever. Gain Adjust preset of Low Noise Amplifier can be used to increase the span limit i.e. to increase the display reading for maximum weight 500 gms. 3½ Digit LED display will indicate strain in µ-strain. Note : Do not disturb the preset settings except offset null adjust prior to initiating strain measurement. Offset null is to be done every time before strain measurement. For lower weights like 5, 10, 15 gm subtract No Load display reading (in case display reading is not 000) from the display reading for 5, 10, 15 gm to get the result close to theoretical strain. The LSB of display may fluctuate for weight placed on the cantilever. For accurate strain measurement, take middle value of fluctuating LSB.

For software installation, open the CD provided with the trainer. Open Setup2304 folder, double click on “setup.exe”, and follow the instruction. Click on check box of install USB driver option and click on finish.

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Functional Description of Blocks 1. Cantilever :

One end of the cantilever is fixed while the other end is free to move. Weights are placed on free end which causes bending of the cantilever and strain is produced on the fixed end. Four strain gauges are pasted on fixed end to measure strain. Two strain gauges are pasted above the cantilever and other two are pasted below the cantilever. Weights up to 500 gms can be placed on the cantilever. Strain due to weights as low as 5 gms can be detected.

Figure 1

Figure 2

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Figure 3 2. Strain Gauge Bridge :

Four strain gauges are connected in the four arms of the Wheatstone bridge. +8 V DC is used to excite the bridge and output is taken from the remaining arm. When no weight is placed on the cantilever, strain developed is zero. Resistances of all four strain gauges are equal so bridge is balanced and output of bridge is zero. Whenever weight is placed, strain developed at fixed end causes resistance of strain gauge to vary which disturbs the balanced condition of bridge and output is produced which is amplified by instrumentation amplifier and low noise amplifier and given to display.

3. Instrumentation Amplifier : It consists of two stages. The First stage is the buffer stage and the second stage is the differential amplifier. Buffer stage is used to provide high input impedance to amplifier. Differential amplifier amplifies weak difference signal and rejects common mode signals such as 50 Hz hum and noise. The output of

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this block is single ended. Instrumentation amplifier is used to amplify weak signals and have a high value of CMRR, voltage gain and low value of noise, offset voltage, offset drift etc. Gain of instrumentation amplifier is set by Gain Adjust preset.

4. Low Noise Amplifier : It is a low noise, low drift amplifier. This gives additional current gain to the output of the Instrumentation amplifier. The auxiliary output is in mV as indicated by the display. The output can be used as input to some recording stage to record the data.

5. Display : It is 3½ Digit LED display. It shows strain developed at the fixed end of the cantilever in µ-strain.

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Theory Strain Gauge : If a metal conductor is stretched or compressed, its resistance changes on account of the fact that both the length and diameter of the conductor change. There is also a change in the value of resistivity of the conductor when it is strained and this property is called piezoresistive effect. This is the principle of strain gauge. Strain gauge is a device the electrical resistance of which varies in proportion to the amount of strain in the device. The most widely used gauge is the bonded metallic strain gauge.

A strain gauge of length L, area A, and diameter D when unstrained has resistance R = (ρL)/ A When a gauge is subjected to positive strain, its length increases while its area of cross section decreases, resistance of gauge increases with positive strain.

Poisson ratio = strain allongitudin

strain lateral = LLDD

//

−∂−

ε = strain = ∆L/L

Gauge Factor = LLRR

//

∆∆

Strain definition :

Strain is the amount of deformation of a body due to an applied force. More specifically, strain (ε) is defined as the fractional change in length, as shown below.

Figure 4

ε = LL∆

Strain can be positive (tensile) or negative (compressive). Although dimensionless, strain is sometimes expressed in units such as in/in or mm/mm. In practice, the magnitude of measured strain is very small. Therefore, strain is often expressed as micro strain (µ-strain), which is ε x 10-6.

Types of Strain gauges : 1. Unbonded metal strain gauges. 2. Bonded metal wire strain gauges. 3. Bonded metal foil strain gauges. 4. Vacuum deposited thin metal film strain gauges. 5. Sputter deposited thin film metal strain gauges.

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6. Bonded semiconductor strain gauges. 7. Diffused metal strain gauge

Bonded metallic (Foil type) strain gauges are commonly used due to their advantages over other strain gauges thus it is discussed in detail below.

The metallic strain gauge consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction as shown below. The cross sectional area of the grid is minimized to reduce the effect of Shear strain and Poisson Strain. The grid is bonded to a thin backing, called the carrier, which is attached directly to the test specimen. Therefore, the strain experienced by the test specimen is transferred directly to the strain gauge, which responds with a linear change in electrical resistance. Strain gauges are available commercially with nominal resistance values from 30 to 3000Ω, with 120, 350 and 1000Ω being the most common values.

Bonded Metallic Strain Gauge

Figure 5 It is very important that the strain gauge be properly mounted on to the test specimen so that the strain is accurately transferred from the test specimen, through the adhesive and strain gauge backing, to the foil itself. A fundamental parameter of the strain gauge is its sensitivity to strain, expressed quantitatively as the gauge factor (GF). Gauge factor is defined as the ratio of fractional change in electrical resistance to the fractional change in length.

GF = ε

∆=

∆∆ RR

LLRR /

//

The Gauge Factor for metallic strain gauges is typically around 2.

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Strain Measurement Techniques : In practice, the strain measurements rarely involve quantities larger than a few milli-strain (ε × 10-3). Therefore, to measure the strain requires accurate measurement of very small changes in resistance. To measure such small changes in resistance, strain gauges are almost always used in a bridge configuration with a voltage excitation source. The general Wheatstone bridge, illustrated below, consists of four resistive arms with an excitation voltage, VEX, that is applied across the bridge.

Figure 6

The output voltage of the bridge, Vo, will be equal to :

Vo =

+

−+ 21

2

43

3

RRR

RRR

• VEX

From this equation, it is apparent that when R1/R2 = R4/R3, the voltage output Vo will be zero. Under these conditions, the bridge is said to be balanced. Any change in resistance in any arm of the bridge will result in a nonzero output voltage. Therefore, if we replace R4 in Figure shown above with an active strain gauge, any changes in the strain gauge resistance will unbalance the bridge and produce a nonzero output voltage. If the nominal resistance of the strain gauge is designated as RG, then the strain-induced change in resistance, DR, can be expressed as DR = RG.*F.*ε. Assuming that R1 = R2 and R3 = RG, the bridge equation above can be rewritten to express Vo/ VEX as a function of strain as shown below. Note the presence of the 1/(1+GF⋅ε/2) term that indicates the nonlinearity of the quarter bridge output with respect to strain (ε = strain).

Figure 7

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ε•+

ε•−=

2GF1

14

GFVV

EX

O

Ideally, it is required that the resistance of the strain gauge to change only in response to applied strain. However, strain gauge material, as well as the specimen material to which the gauge is applied, will also respond to changes in temperature. Strain gauge manufacturers attempt to minimize sensitivity to temperature by processing the gauge material to compensate for the thermal expansion of the specimen material for which the gauge is intended. While compensated gauges reduce the thermal sensitivity, they do not totally remove it. The sensitivity of the bridge to strain can be doubled by making both gauges active in a half-bridge configuration. It can be further increased by making all four resistances of the arms of the bridge by active strain gauges in a full-bridge configuration. The full-bridge circuit is shown below.

Figure 8

ε•−= GFVV

EX

O

When no strain is applied the output of the Wheatstone bridge circuit is zero. In practice however, resistance tolerances and strain induced by gauge application will generate some initial offset voltage. This initial offset voltage is typically handled in two ways. First, a special offset-nulling or balancing circuit to adjust the resistance in the bridge to rebalance the bridge to zero output can be used. Alternatively, the initial unstrained output of the circuit can be measured and compensate in final measurement.

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Signal conditioning stages for strain gauge : Amplification : Strain gauges typically provide small signal levels. It is therefore important to have accurate instrumentation to amplify the signal before it is given to next stage i.e ADC, display etc. Excitation : Strain gauges require voltage excitation to generate a voltage representing strain. This voltage source should be constant and at a level recommended by the strain gauge manufacturer. Bridge Completion : Strain gauges are offered in several different configurations: quarter-bridge, half-bridge, and full bridge. For quarter and half-bridge strain gauges, instrumentation should provide bridge completion, adding the necessary resistors to complete a Wheatstone bridge. 1. Full-bridge strain gauge : The entire Wheatstone bridge is provided with the

strain gauge. Instrumentation only needs to provide the excitation inputs. 2. Half-bridge strain gauge : Half of the Wheatstone bridge is provided with the

strain gauge. Instrumentation needs to provide two of the four resistors to complete the Wheatstone bridge. This is known as half-bridge completion.

3. Quarter-bridge strain gauge : Quarter of the Wheatstone bridge is provided with the strain gauge. Instrumentation needs to provide three of the four resistors to complete the Wheatstone bridge. This is known as quarter-bridge completion.

Linearization/Strain Gauge Conversion : While strain gauges are close to linear, they do stray from linear at large strains. In addition, it will need some hardware or software to convert the voltage output of the strain gauge into a strain measurement. The conversion formula depends on the type of strain gauge used. Half and full-bridge strain gauges offer more accurate conversion formulas. Offset Nulling Circuitry : A strain gauge application will have some position that will be identified as the rest position (a reference position). The strain gauge should produce 0 volts at this position. Offset nulling circuitry is used to produce 0V at rest position. Formulas : Ohm's Law : Voltage = Current x Resistance (written V = IR) Resistance = Resistivity x Length / Cross-Sectional Surface Area R = ρ L / A Stress = Strain x Modulus of Elasticity (written s = E × ε) Strain = (6 x Force (N) x Length) / (Width x Thickness2 x Young's modulus) Modulus of Elasticity of stainless Steel = 10 x 106 psi Young's Modulus E = 200 × 109 N/m2 for steel

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Experiment 1 Objective : Study of Strain measurement using strain gauges and cantilever assembly Procedure : 1. Switch ‘On’ the trainer.

2. Observe reading of the display. It should be 000.

3. If the display reading is not 000 then adjust offset null. 4. *Connect USB cable between ST2304 Trainer and PC.

5. *Open the Real time software and select port where the USB cable is connected. If USB port connects beyond com10, it will not be showing in drop down list. Go on Device manager, change its property, and assign USB port between com 2 to com 9.

6. *Click on start button. 7. *Place a weight of 400 gms on the cantilever's free end.

8. *Match Calibrated value ‘C’ with reference Value ‘R’ by using + and – button. 9. *Press set Button.

10. Place a weight of 50 gms on the cantilever's free end and observe the display reading. It indicates strain developed on cantilever in µ strain

11. Calculate theoretical strain by the formula.

(6 × F × L) / (W × T2 ×Y) F = mg

Where, F = force (N), m = mass (Kg),

g = acceleration due to gravity (9.8m/Sq. sec),

L = Length (m), w = width (m),

T = thickness (m),

Y = Young's modulus (N/Sq. m) = 200×109 N/m2 for stainless steel. Or

12 *Select the weight (which you place on the cantilever) on real time software, it will show theoretical strain (µStrain) and then click on get button to compare theoretical and practical strains. There will be 3-4% variation between theoretical strain and practical strain on software window.

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13 Repeat the above maintained for different weights as shown in the following

table and complete the table.

Sr. No. Weight (gm) Theoretical

Strain (ε ×10-6) Display

Reading (ε × l0-6)

1 50 2 100 3 150 4 200 5 250 6 300 7 350 8 400 9 450

10 500

14. *After taking readings, click on Arrange Button.

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15. *Select theoretical Value and click on plot to get the plot between Theoretical Strain and Weight it will appear to be linear.

16. *Select Practical Value and click on plot to get the plot between Practical Strain and Weight.

17. *To analyze the difference between Practical strain and theoretical strain select both and click on plot button.

For lower weights like 5, 10, 15 gm subtract no load display reading (in case display reading is not 000) from the display reading for 5, 10, 15 gm to get the result close to theoretical strain. The LSB of display may fluctuate for any weight placed on the cantilever. For accurate strain measurement, take middle value of fluctuating LSB. Real time software works only for weight above 50 gms.

* only for usb interface model.

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Experiment 2 Objective : Determining Linear Range of operation Procedure : 1. Switch ‘On’ the trainer. 2. Observe display reading. It should be 000.

3. If display reading is not 000, then adjust Offset Null Adjust preset slowly. 4. Place weight of 50 gms on cantilever's free end.

5. Note the display reading. 6. Calculate theoretical strain by the formula.

(6 × F × L) / (W × T2 ×Y) F = mg

Where, F = force (N), m =mass (Kg), g = acceleration due to gravity (9.8 m/Sq. see), L = Length (m), w = width (m), T = thickness (m), Y = Young's modulus (N/ Sq. m) = 200 × 109 N/m2 for stainless steel.

7. Complete the following table by repeating steps 4, 5 and 6 for weights shown in the table.

Sr. No. Weight (gm) Theoretical Strain

(ε ×10-6) Display Reading

(ε × l0-6)

1 50 2 100 3 150 4 200 5 250 6 300 7 350 8 400 9 450

10 500

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8. Plot the graph between weight (gm) and display reading (µ-strain).

Figure 9

9. Observe the graph. and point out the linear portion of the graph. This is the linear range of operation.

10. Calculate :

% Linearity = gm 500for strain lTheoretica

100 strain al) theoreticfrom readingdisplay ofdeviation (Maximum ∗

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Experiment 3 Objective : Determining sensitivity of Trainer Theory : Sensitivity : The ratio of the change in auxillary output to a change in the value of the measurand (strain). Sensitivity is the smallest change in strain, which the trainer is able to detect. Strain is directly proportional to weight.

Sensitivity S = Weight

Output Auxillary mV /gm

Procedure : 1. Switch ‘On’ the trainer.

2. Measure the auxillary output.

3. Adjust Offset Null Adjust preset slowly to get 0 mV at auxillary output terminal. 4. Place weight of 5 gm on cantilever and measure the auxillary output voltage by

multimeter in 200 mV range. 5. Repeat the above step by placing the weights of 10gm, 20 gms etc.

6. Calculate :

S = Weight

Output Auxillary for above specified weights.

= ……………. mV/gm 7. Compare value of sensitivity for different weights.

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Glossary End Points : The outputs at the specified upper and lower limits of the range. Unless otherwise specified, end points are averaged during anyone calibration. Excitation : The external electrical voltage and/or current applied to a transducer for its proper operation. Usually expressed as range(s) of voltage and/or current values. Linearity : The closeness of a calibration curve to a specified straight line. Linearity is expressed as the maximum deviation of any calibration point on a specified straight line, during anyone calibration cycle. It is expressed as "within +/percent of full range output." Measurand : A physical quantity, property or condition which is measurand. The term measurand is preferred to “input,” “parameter to be measured,” “physical phenomenon,” “stimulus,” and “variable.” Range : The measurand values, over which a transducer is intended to measure, specified by their upper and lower limits. Repeatability : The ability of a transducer to reproduce output readings when the same measurand value is applied to it consecutively, under the same conditions, and in the same direction. Repeatability is expressed as the maximum difference between output readings; it is expressed as "within percent of full-scale output." Two calibration cycles are used to determine repeatability unless otherwise specified. Resolution : The magnitude of output step changes as the measurand is continuously varied over the range. Resolution is best specified as average and maximum resolution; it is usually expressed in percent of full-scale output. In the sense of the smallest detectable change in measurand, use threshold. Sensitivity : The ratio of the change in transducer output to a change in the value of the measurand. In the sense of the smallest detectable change in measurand, threshold is used. Span : The algebraic difference between the limits of the range. Transducer : A device which provides a usable output in response to a specified measurand. The term transducer is usually preferred to sensor.

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Warranty 1. We guarantee the product against all manufacturing defects for 24 months from

the date of sale by us or through our dealers. Consumables like dry cell etc. are not covered under warranty.

2. The guarantee will become void, if

a) The product is not operated as per the instruction given in the operating manual.

b) The agreed payment terms and other conditions of sale are not followed.

c) The customer resells the instrument to another party. d) Any attempt is made to service and modify the instrument.

3. The non-working of the product is to be communicated to us immediately giving full details of the complaints and defects noticed specifically mentioning the type, serial number of the product and date of purchase etc.

4. The repair work will be carried out, provided the product is dispatched securely packed and insured. The transportation charges shall be borne by the customer.

List of Accessories

1. Weight 5 gm ........................................................................................ 2 Nos. 2. Weight 10 gm ....................................................................................... 2 Nos.

3. Weight 20 gm ....................................................................................... 1 No.

4. Weight 50 gm ....................................................................................... 3 Nos.

5. Weight 100 gm ..................................................................................... 2 Nos. 6. Weight 200 gm ..................................................................................... 1 No.

7. Mains Cord ........................................................................................... 1 No. 8. e-Manual (PC Software inclusive)......................................................... 1 No.

9. Aligner-911 .......................................................................................... 1 No. 10. * USB Cable ......................................................................................... 1 No.

* Only in USB interface model.

Updated 01-07-2009

operating manual ver 1 - welcome to cgi bharatpur | cgi …cgibp.com/data/lab_manual/strain gauge...

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