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

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Introduction

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. ...read more.

Middle

All random error would become apparent from the graph, but any systematic error caused by poor setting of the home point would be harder to track down however, the graph should cross at 0,0 so if it does not, then systematic error should have caused that. When the experiment was carried out the following results were noted down, the first sheet is from the original 5V system and the second set from the more complex 15V data logged set, the third sheet shows the results obtained using the multimeter, which gives a higher number of decimal places and is more sensitive, but it is less accurate and less reliable. Analysis of results: The results from measuring the pitch of the screw thread appear to be satisfyingly accurate. The pitch is accurate to within �0.02 mm however this tolerance is 20 microns, which will make the micrometer accurate to that. Therefore the project of a distance measurer cannot strictly be called a micrometer as the inaccuracy will increase with each turn until you get �0.1mm after 10 turns, however all of the 'micrometers' which we have in engineering are only accurate to 10 microns. Expensive micrometers have an additional vernier scale to make them more accurate. The original calibration table was small, inaccurate and unreliable with only 11 readings each accurate to 0.01V with this data plotted on a graph an equation could be found but it would ...read more.

Conclusion

appear correct to 10 microns but the 50 and 40 slip gauges were not so good as they are at best accurate to 200 microns, due to the rust on the gauges, sensitivity of the voltmeter and accuracy of the equation these thicknesses are not perfect, The 0.4mm thick gauges are really very thin and therefore the effect of the rust on the gauges is very strong. There is a great deal of random error on these results due to the inaccuracy of readings. Some of the readings are highly accurate and some are much less accurate. This random error could be minimised by grinding the surfaces of the sensor and screw thread in order to give an accurate datum. Also the slip gauges should be newer and free of grease and rust. With these problems addressed the project could be highly accurate, but with the equipment available in school this project will unfortunately only be reliably accurate to 0.05mm, which happens to be the same accuracy as the vernier calliper gauge used to measure the thread in the original stages. Therefore this sensor is useful as it is far easier to read than the vernier scale on a calliper gauge and will save time with a simple digital readout on the voltmeter and a simple equation in Microsoft excel which will give the calculated thickness all in a matter of seconds. Instrumentation Task Physics Ben Crundwell ...read more.

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