# To the voltage source are referred to

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.

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 123? resistors. A differential output voltage (eo) is measured at the two remaining bridge corners, referred to as output or signal corners.

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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. In this condition, the bridge is resistively balanced. If the bridge is not in balance, a differential voltage will be present at the output corners of the bridge, and the magnitude of this output voltage will be proportional to the amount of unbalance. Hypothesis:

As the wire is stretched due to increasing weight applied, the resistance will increase proportionally. As the resistance increases the voltage and amperage across the wire will also increase. Justification: As the length of the wire increases, so do the chances for the electrons to collide with a positive ion. The chances increase because there is a bigger distance for the electrons to travel. A rule is that energy is always given out from a collision in the form of heat. Therefore the voltage and amperage will have to increase to provide the electrons enough energy to pass through the wire.

List Of Required Equipment:  Wires  3x 123? resistors 1metre length of very fine wire i?? amp meter i?? volt meter  12volt battery pack  pulley  weights (up to 700 grams) Why did I choose the Wheatstone Bridge? Mainly I believe out of all the circuits I have researched, Wheatstone Bridge and the Potential Divider Circuit, I found the Potential Divider circuit was not sensitive enough. The Potential Divider circuit couldn’t detect the strain on the fine wire well enough to predict the effects. With any sensor system u need a big enough signal given out so an instrument which u have can record it.

There are two problems 1. The signal has to be the right size for the detector to detect it 2. Is the resistance too big for the current being passed into the circuit? Point 2 was the case in the Potential Divider circuit, and therefore I chose the Wheatstone Bridge circuit. Method: Firstly I set up the equipment as shown below in my diagram. I have placed three resistors in my experiment to see if a differential voltage will be present across the wire. The three 123ohm resistors connected across the circuit as shown below.

As my theory shows the resistance of the wire must be the 123ohms also, but as we didn’t have sufficient equipment we could only achieve 122. 8ohms. To show relationship between stress and strain this was accurate enough to obtain good results. The Resistance of R3 and R4 shouldn’t change therefore showing me a change in resistance across the wire as it stretches. I have connected a 12volt-battery pack providing enough voltage to obtain accurate results, rather than a 6volt, which was too weak to obtain accurate results. I’m using 1m of very fine wire so I can measure the strain on the wire more accurately.

I found the best way to attach the fine wire to the circuit was to use crocodile clips rather than rapping it round. I have placed a voltmeter and ampmeter across the circuit to measure the voltage and amperage change of the fine wire. I will adjust the voltmeter and ampmeter to 0 when the weight is 0 to show how these variables change to increase in strain. To insure we connected the circuit up correctly we used an A1 sheet of paper to the worktop and drew the circuit on it. This helped us with construct the circuit with ease and also mark how the wire stretched when weight was added.

Diagram: Risk Assessment/Safety: There are very few risks and safety aspects involved, but I must be sure to wear safety glasses in case the wire snaps. I must also be sure to connect the circuit up correctly. I must check the wires can withstand the amount of voltage being passed through them, so they do not catch alight. Variable Table: Variable Value or range How Measured Dependant Variables Length of wire Measured in millimetres Ruler (mm) Output Voltage Measured in i?? Volts Millivoltmeter Accurate to 1 i?? V Independent Variables Force(discrete variable)

Increasing from 0-700 up in 50N Pre-set weight values 100g-700g Control Variables Resistance of the 3 Resistors 123? each Ohms (? ) Gravitational Pull The pull on an object dependent on it’s mass and shape. Atmospheres (earth is 1 atmosphere) Preliminary Results: Weight(grams) Millivolts Millivolts Millivolts Millivolts exp1 exp2 exp3 exp4.