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Design, make, test and evaluate a strain gauge sensor.

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Introduction

Sean R Wheeler        Page         08/05/2007

Physics Coursework:Strain Gauge

Aim:

To design, make, test and evaluate a strain gauge sensor. The sensor has to detect how the change in strain on a wire affects the voltage across the wire.

Reasoning behind aim:

I have chosen the strain gauge because it is Interesting as well as challenging. The strain gauge presents many different variables and complications, which will help me to understand the meaning of strain and stress.

Theory:

        I have used various resources, including books, CD-ROM’s and the Internet to come to this conclusion of the Wheatstone bridge.

image00.png

The Wheatstone bridge circuit in its simplest form (Diagram shown) consists of four resistive elements, or bridge arms (R1, R2, R3, R4), connected in a series-parallel arrangement, and a voltage source(E). The electrical connections where pairs of bridge arms are joined to the wires from the voltage source are referred to as input corners of the bridge. In my experiment R4 will be a weak wire strand and R1, R2, R3 123Ωresistors. A differential output voltage (eo) is measured at the two remaining bridge corners, referred to as output or signal corners.

With Mathematical proof, it can be shown that if the arms resistance’s are chosen such that the bridge is resistively symmetrical about an imaginary line drawn through the bridge output corners e.g. between [R1, R2 and R3, R4]. Therefore differential output voltage (eo) will be identically zero regardless of the value of the supply voltage.

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Middle

350

7.5

10.2

11.3

11.7

400

8

11.5

11.7

11.3

450

6.3

13.3

11

10.7

500

7.8

10.2

10

9.8

        These are my preliminary results that helped me to understand the physics and approach needed to make my experiment successful. In exp1, 3 and 4 I used the same wire and exp2 was a fresh wire. Clearly u can see the set of results obtained in exp2 were different to the other sets of data. After a wire stretches the wire Changes State and becomes a harder material, meaning it will stretch less the next time u apply a strain. Therefore in my real experiment I used fresh wire for each experiment.

Trend In Graphs:

From the graphs u can see a general trend formed in exp2, as weight is applied the voltage across the rest of the circuit decreases, therefore increasing across the wire. In the other three experiments u can see weight has little or no effect on voltage, thus showing the wire has already been stretched. This can be seen in ‘Chart 1’.

Results

I have used ‘Ohm’s law’ which is a formula, to get a resistance for my results. R= V/I

Experiment one

Experiment 2

Experiment 3

Length of Constantine wire (cm)

Voltage (v)

Resistance (ohms)

Voltage (v)

Resistance (ohms)

Voltage (v)

Resistance (ohms)

10

0.29

1.45

0.24

1.2

0.23

1.15

20

0.41

2.05

0.4

2

0.43

2.15

30

0.6

3

0.6

3

0.59

2.95

40

0.74

3.7

0.73

3.65

0.73

3.65

50

0.86

4.3

0.88

4.4

0.94

4.7

60

1.1

5.5

1.06

5.3

1.1

5.5

70

1.23

6.15

1.22

6.1

1.29

6.45

80

1.37

6.85

1.47

7.35

1.46

7.3

90

1.59

7.95

1.58

7.9

1.56

7.8

100

1.8

9

1.73

8.65

1.8

9

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Conclusion

        Chart 8 shows the general pattern between voltage and current and how resistance increases. Between point 6 and 7 on the X-axis the wire has deformed and resistance has greatly increased. The wire has stretched 45mm, which also increase resistance.

Second Experiment:

                There is one major anomaly in experiment 2 and that is when 100grams were placed on. The reading of power/voltage and current are much too high. On Chart 5 and 6 I have draw a pencil line which I believe should have been the results gained.

The general trend again shows that as u stretch the wire the resistance across it increases. Chart 7 shows the general pattern between voltage and current and how resistance increases.

Evaluation:

        A strain gauge can predict when a wire or something with applied stress or stain will deform or even break. When the voltage and current increases in a wire it must mean resistance has also increased.

Improvements:

        If I were to do the experiment again, I could make some improvements to make the level of accuracy better.  For example, I could make sure that the joints/connections were jointed properly and would have to make sure I use the same equipment every time. I could also get a machine to measure the length of the fine wire.

I would also take more than two sets of results, which would eliminate any factors of anomalies and would therefore make the experiment a lot more reliable.

...read more.

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