The Dilemma of the String Potentiometer
For one part of the control of our project we need to use string potentiometers. They are basically an automatically retracting string or wire connected via a gearbox to a potentiometer in a box. When the string is pulled, it outputs a value of resistance depending on the string position. These are fairly specialised components which are used primarily in robotics to measure distance covered by arms or other things.
http://cdn.instructables.com/FCA/BQ6F/HX0DOZDG/FCABQ6FHX0DOZDG.MEDIUM.jpg
As you can see in this image, the wire is connected to an arm and it can measure the distance.
So we went researching where to get these devices and found it was quite difficult to obtain them without buying wholesale. We eventually found one which we thought suited our needs
Perfect! Low cost and small enough. It also had a useable range for our project. We proceeded to try to find where to purchase them from and to receive pricing information.
I’m sorry could you repeat that please?
Hmm, so, in the famous words of the internet:
This didn’t seem so ideal any more, especially as we potentially need 5 or more of these things. So we went back to researching and came up with nothing. There are cheaper ones out there but they are still ridiculously expensive.
So back to the drawing board.
We was as home and had a brainwave to use retractable keyrings. They have a string and they retract. And most importantly, theyre cheap and easy to get hold of.
Hmm 4 stars, its a gamble. But we are men who live life on the ragged edge so we went balls to the wall and ordered a bunch.
But now for problem number 2. How could the long range (and many rotations of the internal wheel) be converted into a useable distance for our project? And how could the low power of the keyring’s spring be turned into something with more torque?
The next idea was born. Lego. We proceeded to create a gearbox out of lego which turned the large motion and many rotations of a keyring into a smaller and useable distance (3/4 of a turn ideally). Lego is cheap and easy to build. And while it may not be ideal for industrial robotics, it proved to be useable for this project.
In the next post we’re going to show the building process of the lego parts.
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