DIY string potentiometer

[dsm2002]

While I am convalescing from an injury, I have a small project to measure shock absorber travel using a diy string pot made from monofilament string, multi-turn potentiometers, spools/pulleys and spiral springs. I have some cheap ebay 10K 10 turn pots, odds and sods to make a few prototypes just to see how they work. I was contemplating a Dataq DI-149 data acquisition kit.

Would linearity and accuracy be best served using say a 10 turn pot with a smaller pulley or larger pulley with a pot with fewer turns?

 

[john monks]

Can't say. I can say that generally a ten turn pot is designed for greater accuracy than a one turn pot. But if this is to be used in a moving vehicle I can see that a lot of rotational inertia would be a big problem with a small pulley. How do you intend to use this?

 

[KMoffett]

If you are looking at recording the extension of the shock during actual driving, as john said it won't work. It can't respond fast enough. An LVDT (linear variable differential transformer) would more appropriate...but a bit more complete to implement with a DI-149.

Ken

 

[(*steve*)]

The problem with most potentiometers is that they're not made for continuous movement. This is even more true of trimpots.

It is likely that it would fail relatively quickly. In a hostile environment (dust, moisture, heat, vibration) the process would likely accelerate.

Some form of rotary encoder would probably be more suitable.

 

[QuantumCheese]

I don't think, for the reasons already stated above, a POT it going to work well/if at all.

I would suggest an optical solution. depending on the resolution & accuracy required you can use a simple slot type sensor as used in old-fashioned mice (the ones with balls!) the benefit being if your already geared up for a pot type solution, you could possibly mod a pot and substitute it's innards with the bits required.
If you want greater accuracy, use a shaft encoder.

either way, then you just need to count the pulses to know the amount of travel

 

[weird_dave]

I'd go with the LVDT, it'll limit the amount of mechanical parts which will most likely fail or have too much slack.
Here's a pretty good page about them:
http://www.mikesflightdeck.com/lvdts/lvdts.html