Physics – Rotary Potentiometer

Introduction

I wanted to find out how a rotary potentiometer could be utilized to work as a weighing scale. The specific interest is the accuracy of this method and distinguish the advantages and disadvantages of using digital readings instead of our common everyday analogue spring gauge balance at the local supermarket. This investigation will require a potentiometer device being loaded with a series of masses which are linked to a voltmeter measuring the differences in voltage as more masses are added while a spring is used to hold the weight. The benefit of this system is that it can be calibrated in imperial and metric measurements as the results appear in Volts, whereas modern scales only provide ‘grams’ or ‘kilograms’ as opposed to pounds. Accuracy is vital in this task as there is a challenge to see how effective a potentiometer can be, with most spring scales can be accurate up to + or – 1g.

Apparatus

> 6V Battery pack (4 ‘D-sized’ batteries), for ease of use and more energy efficient that a 12v mains power supply

> Rotary potentiometer, attached to a DIY MDF wheel which rotates. It has a spring attachment hole at one end and a mechano shaft for attaching weights

> Expendable Spring, an attachment unit to keep the rotating apparatus together, and the main unit holding weight of equipment

> Digital Multimeter, for measuring voltage differences

> Wires, for construction of circuit

> Weights, each weighing a mass of 10 grams

> Clamp stand, to ensure the potentiometer is stabilised and attaches the spring

> 360� Protractor, to measure the angle of rotation done by the MDF wheel

Circuit Diagram:

Safety

Working with 6v is relatively safe; the only caution is to ensure wires are making minimal contact to prevent shorting of the circuit. Caution must also be taken when using heavy weights as they have the risk of falling of the table causing potential damage to the floor or the foot. In addition, offloading weights may must be proceeded slowly as a quick unload will mean the instant spring back of the expendable spring may cause an item to strike the eye/face i.e. weight handle. One must also be careful of the spring, as it may unexpectedly snap when on a heavy load.

Measurement Plan

I plan to measure the voltage (potential difference) increase on the multimeter (voltmeter) as weights are loaded on the rotary potentiometer device. The sensitivity of the potentiometer needs to be taken account of as I need to ‘zero’ the readings when setting up the experiment, the expendable spring holding the MDF wheel will no doubt play a part In the resistance of pulling back the weights but this is a vital variable I have already taken account of as I intend to subtract the original voltage (caused by the pullback of the spring) from the final voltage.

Gravity will enforce a pull on the load at 9.81N/kg which is counter-acted by the tension of the spring. I shall also be measuring the angle of change on the wheel as each weight is loaded on, the only alarm is that the weight holding meccano strip may buckle or bend due to the extensive number of weights attached on. Due to the circuit being able to collect data resulting in a push of pull, this can be further improved by creating a fair system induce both forces thus providing negative and positive results, however the lack of time and equipment will prohibit the measurement of pushing forces.

Prediction

I predict that as more masses are added and the weight increases on the potentiometer, it will rotate anti-clockwise causing the potential difference at Vout to increase as more weights are added. However, I expect there will be glitches at several positions due to the spring’s nature of popping off the wheel (not perfectly rounded) influencing the angle of change to be more than usual.

Another item to take in consideration is how accurate the calibration is, after the experiment I intend to use 100g weights to repeat the experiment and compare to see the difference in voltage readings. This will enable to me to see how accurate and consistent the readings are.

Scientific Explanation

A potentiometer is essentially a variable resistor; the angle of rotation is an adjustable method of manipulating the resistance, as the slider moves it alters Vout from a fixed input. The potentiometer I used was a low-power single turn with 3 terminals. It has the ability to control voltage (via resistance induction) and act as a potential divider. On my specific circuit layout where the multimeter is connected through the circuit on the centre terminal and to the centre of the pack of cells, it has the ability to change resistance when twisted clockwise in the forms of negative (-3v) and positive (3v) when anti-clockwise, whereas, it is typical used for controlling a singular direction. Commercially this allows the adjustment of voltage; a potentiometer is commonly used on audio devices which control the volume.

Measurement Methods

Consistency of the apparatus was key to accuracy, due to the constant stress of holding weights the meccano shaft potentiometer and constant resistive losses I had to complete collecting readings within a day to ensure no changes were made to the shaft and spring.

Firstly I shall begin with acknowledging the original voltage as zeroing the potentiometer at 0.00v I deduced would be very difficult. Therefore from that I would add 10g masses on and measure the readings on the voltmeter minus the original voltage. After adding a mass I did not unload the units as it may cause a permanent deformation to the spring and changing the original reading.

After I took the voltage readings, I will user a 360� protractor to measure the angle of rotation by lining up the shaft with the protractor shaft and placing the centre of the protractor on the centre of the rotating wheel.

Precautions/Errors

Due to human imperfections when constructing the apparatus, one did not consider a catch for holding the expansion of the spring when stretched. This denotes that at heavy loads the spring comes loose off the wheel and I am required to physically re-assemble the spring in its original position after each addition of mass is added. The spring also sometimes got caught on the imperfections of the MDF wheel, as a result, there were times when the spring reached a level of load at which it stopped stretching until a significant weight is added. In addition, one must consider the permanent deformation of the spring as it is constantly stretched beyond 5 times its length, going beyond its elastic limit may lead to an easier turning of the rotary device. There was also relative nervousness that if the spring would snap under tension.