Position sensing

Thread Starter

geoffers

Joined Oct 25, 2010
306
Hi all,
I need to sense the position of a rod, it always returns to 0.

Travel needs to be around 12"/30cm, I had initially thought about a potentiometer and a/d on a pic, however ones that length are expensive!
Reducing travel might be tricky.

Latest idea is to threaded bar and something like a crankshaft position sensor?

Any suggestion as to what sensor I can use? Cheaper and more cheerful the better! If it can run on 5v even better.

Cheers Geoff
 

Reloadron

Joined Jan 15, 2015
4,518
Depending on constraints of space a resistive linear pot could be used and to reduce cost run it on a pivot arm. Rather than for example 12" pot travel you could use 3 or 4 inches. Then there are string pots or yo yo pots depending on what you want to call them. Really per shortbus more information is needed and a good definition of inexpensive verse expensive would help. Things like how accurately or to what uncertainty do you want to measure?

Ron
 

BR-549

Joined Sep 22, 2013
4,936
If the rod's full travel is 12 inches...........what minimum amount of movement do you need to detect?

What kind of physical area is around rod while moving?
 
Or a toothed bar driving a gear on a 10 turn potentiometer or optical encoder?

Printers sometimes detect the print head position by using a transparent band with fine lines on it and an optical detector to detect the lines combined with a position zero optical detector

Or if the bar is driven, use a stepper motor combined with a position zero detector
 

Thread Starter

geoffers

Joined Oct 25, 2010
306
Thanks everyone,
I posted last night and as I nodded off thought, that's not enough info!

The rod will be moving through a tube, I was thinking of 10-12mm bar as a starting point, then if I use 1.0mm pitch I would have 1.0mm resolution.

The movement is from it being placed on a object the the rod dropping down onto the ground to measure the height, so not too fast.

I like the idea of the grayscale though I will have a search for that, optical would be ideal with minimum moving parts.
Cheers Geoff
 

shortbus

Joined Sep 30, 2009
6,988
Thanks everyone,
I posted last night and as I nodded off thought, that's not enough info!

The rod will be moving through a tube, I was thinking of 10-12mm bar as a starting point, then if I use 1.0mm pitch I would have 1.0mm resolution.

The movement is from it being placed on a object the the rod dropping down onto the ground to measure the height, so not too fast.

I like the idea of the grayscale though I will have a search for that, optical would be ideal with minimum moving parts.
Cheers Geoff
So your just measuring penetration of the rod into the ground? This could be done with a mechanical not electronic solution. Just have shallow grooves around the rod at 1.0mm intervals. Drop the rod through the tube, then remove the tube and note how many grooves are embedded into the ground.

Your first post sounded like you needed to stop the rod at the same place every time, not measuring a depth of penetration.
 

mcgyvr

Joined Oct 15, 2009
5,394
We are still detail poor here.. Feed us man.. feed us..

So far a tape measure is a viable solution to the details as posted..
 

Thread Starter

geoffers

Joined Oct 25, 2010
306
Sorry,
Your all right , lack of info...

What I need to do is take lots of measurements, in that range and average them, I can do the averaging etc with a pic, that's no problem.

Its the best way to do the measuring I'm after, if its a linear pot I can use the a/d or count pulses from a sensor.

Its going to be used outside, needs to be robust and repeatable, I've had some electronic calipers, cheap ones, and they normally fail after abit, especially if its damp, the one you look at is OK!

Cheers Geoff
 
If the rod has to fall freely under gravity you ideally need a frictionless measurement, so that would be optical, capacitive or inductive. I don't know if it would be possible to make any of those precise and robust enough to detect the threads on a rod reliably though (cam shaft position sensors are inductive). The calipers we've mentioned use a capacitive system & as you say they're very sensitive to getting wet.

I reckon if you tried an inductive approach you'd need an inductive ring with a fine slot in it aligned with the thread direction. You could try an old fashioned tape head used as an inductor in an oscillator circuit & detect the frequency shift. (Does anyone remember magnetic tape).I don't know if there's any off the shelf detectors available. But you'd have to keep the rod exactly the same distance from the head and very close to the head as it fell. If you used optical detection you could magnify the thread by fixing a point source of light close to the rod to create a diverging beam of light which you could project onto a sensor the other side of the rod, creating a shadow of the threads on the side of the rod. For further magniication you could put a lens between the rod and the sensor
 
You could make a capacitive thread detector by running the rod inside a snug fitting plastic walled tube & melting 2 pins into the wall of the tube at right angles to the rod, one above the other separated by the distance of the thread pitch, so they remain entirely within the plastic wall, going through the wall at a tangent. The capacitance between the pins will vary as the threads go by. Encase the outside of the pins & associated electronics in a big gob of silicone. You could probably increase sensitivity by having more pairs of pins
 

strantor

Joined Oct 3, 2010
5,033
Since its going thru a tube, spiral a wire around the tube and measure the inductance change as the rod moves through it. TI's LDC1000 makes this easy.
 
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