Is this idea feasible??

[Mario Arpino]

I'm working on a project at the moment that requires a low cost linear motion potentiometer with a linear displacement of about 10mm giving 0>10k. The problem I'm coming across is that manufacturer's of these items have a very different view of what is low cost, as their market is typically aerospace industries and the like. The cheapest I've found so far was about $60 and I need to get the price down to below $10. Precision and linearity are not critical for my application but a high duty cycle is required (>1000,000 full cycles and 50,000,000 dithers). I found one with a duty cycle of >100,000,000 but the price is over $150!!
Would it be possible to achieve the same result with an electronic circuit measuring capacitance of a moving rod? This way the the moving part would be non contact and would give me the duty cycle and reliability I'm looking for. Would it also be possible to set jumpers on the circuit board to change the full scale linear displacement to different values. e.g. 0>1k, 0>2k, 0>5k, 0>10k ?
Sorry if this seems a little vague, I'm good with mechanical bits but not really an electronics person. Can anyone out there help me?

Mario

 

[dknguyen]

If you need such a high component life, probably no mechanical device (like a potentiometer) can fit the bill. I didn't even know they had pots that could handle 100 million cyckes,. Since you seem open to alternative methods, I would think that just getting a rotary optical encoder (they look just like rotary pots) would be easier than measuring capacitance (seeing as how you say you are not so electrically minded anyways). You could always use mechanical methods to convert linear motion to rotary. They are still kind of expensive though...$20-$40 if I remember right, at least for the ones I saw.

Changing the linear scale and or measuring capacitance or both, will definately not be very pretty if you aren't so electrically minded. All the things you suggested so far (except for the potentiometer) won't be very pretty to the mechanically minded.

Why do you need it to cost $10 with a lifetime of 100 million cycles? If it's $10, they can just chuck it when it gets worn out and get another. Lifetime and price are two things that don't go together well. I hope you realize how high 100 million cycles is- that's 70 days of continuous run time for something spinning at 1000RPM or 17RPS. 100 million cycles is an insane amount for the vast majority of applications, so it's going to cost a lot.

 

[john1]

You could try one of those tuning inductors.
The ones from old car radios move a bit more than a centimetre,
if you fed a long wave one with a suitable frequency, you could
get a decent impedance change with the movement of the insert.

Or maybe one of those little "Led and Sensor" units which have an
Led and a sensor in a little plastic unit with a gap, like they used to
use on car distributors ... i reckon you could make some piece of
card or plastic to fit the gap, with maybe a slot that widens with
the movement you want.

Some simple potentiometers will handle the usage and life that you
propose, the type that are fitted to some motorcycles, i'm afraid i
don't know much about them other than they have extremely high
use-age handling.

I will try to find out what i can about those potentiometers,
i don't think they are very expensive.

John

 

[john1]

Hi,

Maybe not a hundred million, but definately a few million ....
I managed to find a reference to this strange and magic pot here:



I am sorry i cannot give any more details other than the thread
which contains the tale of this unbelievable pot, which is:

https://www.electro-tech-online.com/threads/dc-amplifier-12vdc-to-5vdc.8502/

Its an old thread from over a year ago, i did ask for details,
but none came.


From "dansoarr"

First off thanks for the help, the pot in question is used on an output shaft that has a 120° movement and is in constantly moving for 6 hours a day and has been in service for 5 years. I belive that the pot orgionally came from a kawasaki motorcycle.

If you can find any details about this pot i would be very interested
indeed. There are things i would like to make which i could make with
much less hassle using a pot like this.

Regards, John

 

[Hero999]

electronic circuit measuring capacitance of a moving rod?

Try a PCB with two 10mm long and traces, cover them with insulation tape, when a brass bar slides over them the capacitance will change, so you could use a high frequency oscillator and measure the impedance.

 

[The Mad Professor]

Given the high duty cycle required I would advise against the capacitive approach. An induction coil or perhaps a hall sensor might do the job.
A lot depends upon the specific details of your project, you have told us a high duty cycle is involved but can you tell us if a high accuracy is needed for the position sensor? It may be posibble to solve this optically either by using reflected energy and equating the strength of this to distance or for a more accurate solution we can go digital and use a diffraction grating and count the pulses this produces.

 

[hjames]

I thought LVDT's were the "standard" solution for these types of problems - extrememly rugged, no contact and hard to disturb - capacitive types are going to have a whole pile of enviromental issues - dampness being the #1 problem, grounding and isolation being a close second.

https://www.sensorland.com/HowPage006.html