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Calibrating a Potentiometer sensor.

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Introduction

Sensor Coursework - Calibrating a Potentiometer sensor. Introduction Sensor: A device capable of detecting and responding to physical stimuli such as movement, light or heat. All sensors have an input, which can be any physical variable, Common examples include, temperature, position, light intensity, and sound. This input is then converted into a readable output, normally an electrical signal. Sensors are extensively used in the modern world, and are important components in many household electrical appliances such as ovens, computers, and televisions. The purpose of this experiment is to calibrate a sensor, which involves establishing and marking units on a measuring device. In electronic sensors, the calibration is achieved by the production of a graph with input plotted against output. From the resultant curve, expected output can by interpolated for a given input, and vice versa. Aim The aim of this investigation was to calibrate, and investigate the properties, of a sensor that finds the weight of objects. Preliminary Work What sensor should I use? The sensor to be used should have input, varied by applying different weights, to give a reasonable range of outputs. I've decided to look a few possible sensors. Sensor What it measures (input) Why used / not used Pressure The force of objects pushing down on the sensor. I'm not going to use this sensor as it's an air filled ball, when a force is applied the air is squeezed out this displaced air is then measured. The sensor is unstable and a heavy weight or large object would easily fall off. ...read more.

Middle

This resistance is printed upon the POT during manufacture. This formula only applies to the POT sensor while the springs attached to the board are obeying HOOKES LAW. Hookes Law is the rule which shows how a spring stretches as you apply an increasing force to it, it states that force is proportional to extension. So, force = spring constant extension F = ke This can be shown clearly in the diagram below: When a great enough force is applied to the springs, they reach their elastic limit, this it the point at which they become permanently deformed, and their stretching no longer obeys Hookes law, they don't stretch in proportion to load applied. This means the mechano arm doesn't move in proportion to loads applied, so the POT's arm is also not turned in proportion to load applied and thus the output voltage equation does not apply. IT ONLY works while the springs are obeying Hookes law, so during the course of my experiment All the loads need to remain below the mass needed to reach the springs elastic limit. I tested my springs to show that they follow Hooke's law: Mass (Kg) Extension (m) 0 0.04 0.1 0.078 0.2 0.162 0.4 0.31 Method Equipment * POT Sensor * Weights (0.05kg) * Voltmeter * Clamps (2) * Springs (4) * Powerpack * Wooden board with four hooks in corners * Mechano arm * Small bolt and blue tack Diagram 1) Set up the experiment as shown above. Making sure wooden board is horizontally straight, and supported by all four springs, adjust springs and clamps if not. ...read more.

Conclusion

The output voltage would therefor be proportional to the mass added, giving me the straight line from my calibration curve. Overall I think this experiment was very successful and I fulfilled all the aims. The calibration curve produced was accurate as were the sensor properties of the POT I found. The method I used was a sensible and pertinent choice for the available materials and the time we had. There were a few improvements, which I would make to the experiment if I had the time and resources; these would include using a more sensitive voltmeter so my resolution would be higher. Making sure the board was perfectly straight at all times; this could possibly be done my using a spirit level, and adjusting the springs throughout the experiment, although this may have implications on the results. A different set up could have been used, perhaps using different equipment; one idea would be to use a pivoting arm. The arm could be attached to the POT, which would be attached to a vertical bar; a spring could then be attached to one end and secured. Weights would be added to one side, making the arm turn as one side went down, thus turning the POT. There are many more improvements, which could have given me slightly more accurate results, but as my line of best fit was a straight line already, these improvements would have altered the final calibration graph very little. This type of calibration curve would be useful to manufactures of electronic scales, who may wish to use it to programme their products, it could also be used to compare the input against outputs in industry, for example checking electronic equipment. Paul Bevan ...read more.

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