For my sensing project I decided to make a Digital Micrometer.

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Instrumentation Task                Physics

PHYSICS Advanced Subsidiary GCE

UNIT 2862 – (a) Instrumentation Task

Aim:

        For my sensing project I decided to make a Digital Micrometer. I use analogue micrometers quite often in design technology but I always find it a long and time consuming process to read off the scale and find the reading that the micrometer gives. I am planning to make the micrometer accurate to microns as all micrometers should be, however this will require a great deal to calibrating and will require highly accurate test equipment. The project will measure distances using an accurate linear potentiometer. Therefore in short my project is to calibrate a linear potentiometer so you can reliably measure changes as small as a micron.

Plan:

        In order to set up my micrometer I will need the linear potentiometer which the physics department already has. This potentiometer is mounted on a stand with the measuring arm sticking straight out into a large screw thread with a low pitch. With this setup I will be able to push the potentiometer in with the screw thread, therefore giving a simple yet reliable way of getting precise positions on the potentiometer. With these precise known distances (which will become thicknesses) I can take a reading through the potentiometer which will be proportional to the position of it. To take the readings I will have to connect up the potentiometer in a circuit. Because it is a potentiometer there are three terminals. With these terminals voltage is fed into one terminal at 5V and 0V into another. The third terminal is the output and will be connected to the 0V rail through a voltmeter which will give the readings required. The circuit will be set up as follows:

        With this circuit the voltage flowing through the voltmeter will vary from 0-5V depending on how far in the linear potentiometer is pushed therefore the system can be easily tested to see what voltages the different thicknesses give. The Voltmeter needs to be both accurate and reliable in order to calibrate the project properly. The system will be set up using a 5V regulated power supply a multimeter and four 4mm banana plugs. This simplification in the design will hopefully reduce any errors, and make problems easy to spot.

        The results taken at regular intervals of 1mm can then be analysed and using statistical formulae the equation of the graph can be calculated along with the mean square error. The equation gained from the results and calibration can then be used to work out the thickness of the test sample in mm from a voltage readout. Once the micrometer has been constructed and calibrated it can be tested using slip gauges which are highly accurate thicknesses of metal which you slip together to make different thicknesses.

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Practical:

When it came to construction it became apparent that getting 1mm thicknesses was going to be very difficult as there was an absence of larger slip gauges. The sets only went up to .6mm and were rather rusty, therefore a better way of changing the thickness was needed. The screw thread was screwed in until it met the potentiometer and then screwed in 2 full turns to create a safe starting point. The screw handle has a large cross painted on the end of it separating the head into quarters. This cross was aligned with the thickest ...

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