Best method to generate and read resistance based on distance

Thread Starter

reverendalc

Joined Sep 17, 2023
20
I have a metal tube which fills with 30 discs which are dispensed from the top similar to a pez dispenser. The tube is (painted) steel and the discs are brass. There is a large spring with a plastic follower which pushes the discs up as they’re dispensed.

I need to be able to track how many discs are still in the tube, but there is no visibility. There is a channel of about 8mm up the side of the tube which serves as a guide or key.

my original plan was to place 30 contacts up the channel with various resistors and a lead on the follower to report back a unique resistance for each “stopping point” of the follower, however space constraints make this unlikely without outsourcing the job.

another idea is to run a strip of nichrome up the channel, but I’m not sure if the resistance is great enough to quantify each dispensement at about 4mm per disc. Perhaps there’s another more resistive material.

better yet, there’s something entirely different I’m not seeing as I’m a mechanical engineer not an electrical one.

I apologize if this is in the wrong place. Thanks in advance for your insight!
 

Thread Starter

reverendalc

Joined Sep 17, 2023
20
Thanks for the reply!

I forgot to mention that I have very little space/access to the dispensing head as it's inside a piece of equipment, and these tubes get moved around between jobs. the ideal would be that these hoppers would provide their quantity rather than the tool recording how many discs were dispensed.
 

Jon Chandler

Joined Jun 12, 2008
1,099
Depending on the travel needed, a string pot may be an idea solution. A string pot works by having a string turn a potentiometer as it's pulled out by whatever is being measured and turning the pot the other direction as the object moves back and a spring mechanism pulls the string back, turning the pot in the opposite direction. Commercial units achieve high accuracy over a 10' travel distance or more. But the cost is high.

Instuckables has a low cost DIY version using a badge reel retainer as the return spring.
 

panic mode

Joined Oct 10, 2011
2,887
30x 4mm = 120mm. this is a pretty short magazine.
what is the environment (air, water, oil, high temp, high vibrations, pressure...?)

i was going to suggest string potentiometer. or even slide pot since travel is so short
what is the spring exactly? if it is an air cylinder, you can use version that already has built in potentiometer.
 

Thread Starter

reverendalc

Joined Sep 17, 2023
20
Thanks for the ideas! The discs are fed into a pneumatic tool. There is some physical shock involved so I’m a little concerned about moving parts. The tool isn’t used in egregiously cold temperature, hopefully never wet, and while heat *may* be generated, there’s no appreciable amount.

I’m not sure of the composition of the spring, it’s just a pretty standard coil spring. My father told me about some apparatus that can measure the tension of a spring, but(while I admit I know nothing of it) it sounds like 4mm decompressions would require a very special and/or expensive spring.

between my controller, lcd display, and some other incidentals, I’m already at about $12/hopper. Since I have between 150-200 to equip, I’d really like to keep them under $20. I probably could’ve done way better than a raspberry pico, but again I’m a little out of my element here.

I’ll have a look at these string/slide pots!

am I way off, thinking about measuring resistance across a highly impedant piece of metal? I’ve already bought the picos.

thank you all so much
 

Thread Starter

reverendalc

Joined Sep 17, 2023
20
I’ll have a look at these string/slide pots!
string pots are way too expensive. Slide pots look fantastic, but I can’t seem to find any that are the correct or even approximate length. 100mm is too short and the next I found is 400mm. Also, the top 1.5” insert into the tool, so I don’t think I could fit one. Darn.
 

Ya’akov

Joined Jan 27, 2019
9,267
Welcome to AAC.

Could you provide a drawing of the mechanism? Literally working blind means guessing over and over which can be frustrating.
 
Run a strip of nichrome up the channel, but I’m not sure if the resistance is great enough to quantify each dispensement at about 4mm per disc.
I personally think you could make this solution work. I calculate the resistance of 12cm of 0.1mm diameter nichrome wire would be about 23 ohms. A phosphor bronze wiper contact mounted on the follower should give a 0.75 ohm resistance between discs. If you want to measure the values continously you should connect the wire across a low enough voltage (by connecting a series resistor) so that the current is low enough not to heat the wire. If you can live with a measurement each time you press a button a higher voltage for a brief time should be okay. If reading the value into a processor low voltage values (where a series resistor is used to keep the current low) could be amplified by an op amp before the analogue to digital conversion.
 

MrChips

Joined Oct 2, 2009
31,099
Nichrome wire in the channel would work. But then you have to make contact with the wire and this could be intermittent.
I would try optical sensor. This would be non-contact. Can you mount anything inside below the plastic plate?
 

Thread Starter

reverendalc

Joined Sep 17, 2023
20
I cannot find a way to edit the original post to include this drawing:
IMG_5288.jpeg

I also have my concerns about regular contact with a very fine nichrome (or otherwise) wire. I found some nice “bands” or strips, several mm wide… I would be very confident with maintaining contact on that, but I’m unsure if the resistance will be too low with such a large amount?
 

Alec_t

Joined Sep 17, 2013
14,417
I was thinking an LVDT might work for this, but of course that would involve a bit of circuitry so could put you over budget.
 

Thread Starter

reverendalc

Joined Sep 17, 2023
20
The LVDT while functionally viable does seem overly sophisticated and budget unfriendly. Thanks for the idea though!

if the nichrome wire isn't feasible, I could always do something like this:
1695055453354.png

and just drag a contact up the left side of the resistor stack? I'm not sure if the resistors would survive that, or for how long. I could always via the resistors, install the board facing away with some kapton tape, and pad the back side like this:
1695055620294.png

I do have access to PCB, copper tape, and resistors... but it would be an enormous time investment to hand-build these.
 
I still think 0.1mm nichrome wire glued to a (bare) PCB with a phosphor bronze contact sliding along it would work well but I also like the idea of surface mount resistors connected in series. You'd need a PCB a bit like strip board except with strip spacing of 4mm instead of 2.5mm with a resistor soldered between each track. The contact would brush along the adjacent PCB tracks, not the resistors. I don't know how many of these you need to make, but getting an etched prototype board made with a lot of these on it is not so expensive and getting the resistors soldered, by hand for small quantity, automated placement and reflow for larger quantity, might give you a robust solution.
 

Thread Starter

reverendalc

Joined Sep 17, 2023
20
I will order some nichrome wire. I like this idea the best as it seems the easiest to implement, I just have zero experience with the conductivity (resistance) of it. If I can expect a detectible and reproducible change in resistance every 4mm, that's perfect. I could also 3d print some inserts to hold the wire rather than using PCB material. I am slightly concerned though, I expect these changes in resistance to be incredibly minute (is that right?). since these will employ contacts to relay the resistance to the pico, I might introduce confounding resistances due to poor/improper contact. again, I'm just thinking ahead with my limited electrical knowledge.

In many ways I prefer the idea of the resistors, but it seems awfully more involved to complete at scale... at least the PCB portion. I could easily paste & place the resistors and set them in my shop oven for a bit?
 

Thread Starter

reverendalc

Joined Sep 17, 2023
20
You can coil it up around an insulating rod to increase the resistance per unit length of rod.
This had occurred to me, but I'll freely admit that I'm not confident that I can wrap this wire 150-200 times with equivalent results. I did find some pre-wrapped options which appear to be tailored to DIY vapers.
 

Jon Chandler

Joined Jun 12, 2008
1,099
I presume the spring's diameter is close to the inner diameter of the tube? If so, positioning some kind of contact to ride on the spring is going to need difficult.
 
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