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Author Topic: Analog pressure gauge / potentiometer coupling?  (Read 1165 times)
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Hi Guys,

I have a project where I think an Arduino could be put to good use.  It involves reading the hydraulic presure developed in a bottle jack used in a hydraulic press.

The problem is that the pressure developed can be up to 10,000 psi and though sensors in this range are available they are prohibitively expensive for this project.

Gauges are availabe quite reasonably though.

OK, so I have a gauge with a pointer that moves.  I'm thinking if I would couple that to a potentiometer somehow that I could Have the Arduino read the potentiometer.

The problem is how to couple the pot to the gauge pointer.  I've thought of taking a pot apart to loosen it up as much as I can (there are several tutuorials on the net about how to do this - it's kind of common for muscians to want to be able to adjust pots on their instruments with just the brush of a finger) and just JB welding the end of the pot to the gauge pointer base.  I'd have to rig up an anchor for the body of the pot though.

I also have a couple 10K pots with solid shafts (no split in the end, no ridges, etc. they're just aluminum round) and I've considered cutting them shorter and drilling a hole in the end and then removing the pointer and putting some epoxy in the hole and shoving it down on the gauge pointer shaft.  Again, I'd need to rig up an anchor for the pot body.  The problem is the pin shaft is pretty small and drilling a small hole exactly in the center of the pot shaft may be challenging.

I've also wondered if I could just bend the end of the pointer up and then install a flat disc on the pot shaft and put a hole in the outter edge of the disc for the bent portion of the pointer to fit into.  This method may be a little sloppy but would help negate the need for having the pot and the pointer shaft exactly aligned.

In my net search for ideas I ran across a YouTube video that shows how a gauge works.  It appears it's a coiled tube that moves a geared linkage that meshes with gears on the shaft.  The geared linkage isn't exactly straight but it's not overly curved either, so I'm also wondering if I couldn't rig a linkage from it to a straight slide pot.

I'm wondering if I'm not re-inventing the wheel here though and I just don't know the right words to put into the search engines to find what I'm looking for.

Does anyone know of an inexpensive off the shelf device to do this?  Or have an idea for an easily fabricated gauge/pot coupler?

Thanks!

Steve
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Sensors like that are a few hundred dollars/euros.
And a simple mechanical gauge is only a few dollars/euros.
That just how it is, as far as I know.

You could use a hall or optical sensor inside the gauge.
The gauge has a simple construction and it probably wont have enough power to turn a pot.
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What you need is a pressure potentiometer. Just kidding.

There is a company, Wika, that makes a gauge called the Intelligauge that is a mechanical gauge with an auxiliary 4-20mA output. The problem is, I don't know what the cost in a 10,000 psig model. I noticed there was a 160 psig model for sale on ebay for $100. Maybe it would be in your curiosity budget to buy one and disassemble it to find out how it works. Maybe you could contact the seller and see if a higher range is available for the same price.
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Sensors like that are a few hundred dollars/euros.
And a simple mechanical gauge is only a few dollars/euros.
That just how it is, as far as I know.

You could use a hall or optical sensor inside the gauge.
The gauge has a simple construction and it probably wont have enough power to turn a pot.

Yes, most gauges with that pressure range tend to be liquid filled to help dampen the pointer which is gear driven to the bourdon tube pressure sensing assembly, so interfacing to a pot is pretty much out of the question.

 http://www.freepatentsonline.com/6604428-0-large.jpg

Lefty
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You could use a hall or optical sensor inside the gauge.
The gauge has a simple construction and it probably wont have enough power to turn a pot.

A more pro solution would be a magnetic encoder IC (e.g. AS5040). That would then just require a magnet attached to the center pin on the dial.
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I'm amazed this product isn't already out there.  You'd think an injection molded thing that clipped onto the gauge hand and a band thing to hold the pot case would be simple enough to have made and would sell well, I mean if you can get break out boards for surface mount joy stick pots you'd think this would be available as I think as many or more would probably like to be able to inexpensively read a gauge in their projects.

The fact that it's a hydraulic gauge and is oil filled is no biggie, it just makes it a bit harder to disassemble (I just did one of each type this morning).  On plain ones the clear gauge glass is plastic and pops off easily.  On the oil filled ones there's a rubber plug at the top you can pop out and drain the oil from.  Then the clear glass is made of actual glass and it's got a rubber gasket and a crimped metal ring to hold the glass and gasket onto the gauge tightly.  You can roll the criped metal up easily though.

Once inside it's just like any other gauge.  The oil is just a vibration damper, it has no other purpose.  In fact it tends to slow the reaction time of gauges down and it's often recommended that if you need faster reaction times and the equipment doesn't vibrate you should drain the oil.

Oh, the thought the gauge hand doesn't have any significant torque is wrong!  It has plenty!  I doubt I would even need to loosen the pot up, though I would just feel better doing so.

I'll give this some more thought . . . I've actually got a couple more project ahead of the press so I have some time.  I just figured this would be a common thing and someone would've already figured out the easy way and posted it.  If I get it figured out I'll post it!

Take care all,

Steve
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I suspect it will be a challenge. One thing to pay attention to is the spring that is used to keep the gear teeth properly meshed to prevent backlash. You might want to consider mounting the pot from the back of the gauge in order to maintain a visible analog readout should your electronic one fail or go squirrelly. Good luck!
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My thought was to put a tee in with another guage so I have a sensor and an unmolested gauge to reference to for calibration and later accuracy checks.

I took a pot apart and it's really very easy to get them loose but still give reliable readings.  I don't think the gear will be any issue at all.  In fact it's common to bend a needle before stripping the gears.

The challenge is simply how to connect a very small diameter pin to a large diameter pin and how to mount the pot to hold the body stable.  With a standard gauge I would think one could simply remove the needle, cut the pot shaft so it's just short of the gauge face, drill a hole in the end of the shaft slightly larger than the needle shaft and then drill a hole in the plastic glass to mount the pot.  I'd shove some epoxy in the hole in the end and slide it over the needle shaft and let it set up.  Unfortunately with the actual glass gauge glass and crimped on glass seal of the oil filled gauges that's not as easy and I can't find a dry 10,000 psi gauge that's cheap.  I imagine one could just cut a plexiglass circle and epoxy it on to the gauge body.

We'll see - I had time to disassemble a couple gauges and the pot today but I won't have time to hit the shop and cut plexiglass, play with epoxy, etc. for a bit.
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Hmmm, I see SparkFun has "softpot" and "hotpot" membrane potentiometers.  I wound if they couldn't be mounted to the gauge glass (or a piece of plexiglass cut in a circle for the liquid filled gauges) and some sort of wiper hooked to the gauge pointer?
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I don't think the gear will be any issue at all.  In fact it's common to bend a needle before stripping the gears.

It's the backlash you need to be concerned about, a sort of mechanical hysteresis.
You might take a look at the pots they use in servos. They may be looser than normal.
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We'll have to see how it behaves.  I'll be using an air actuated bottle jack and it doesn't move too quickly so the pressure gently builds, it doesn't pop right up there.

Worse comes to worse, maybe put a bare wire on the needle and then mount small springs at the various pressures I'm targeting and then just check if they make contact?

So many projects, so little time!

I used to program a bit years ago but haven't lately, boy what a shock!  Nothing was visual when I was programming!  The Arduino environment seems must more comfortable to me than a visual pc environment!

Steve
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Sounds like a fun project!
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Looking around fleaBay last night I found a digital pressure gauge that goes up to 10,000 psi.  It's about $80 which is more than I want to spend but sometimes expediency is worth the cost.  It displays the pressure in psi on a LCD display.  I would like to use that psi reading, along with my ram diamter, to calculate the pounds of force being delivered by the press.  I imagine it's possible to tap into the data lines to that lcd display and feed them to an Arduino?  Hmmm, it'd probably be worth a look inside the thing to see if one could identify and interface with the pressure sensor directly?  Probably beyond my abilities right now, it's been years since I've programmed or played with things electronic, but something to keep in mind for the future.
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If it doesn't have a dedicated data output, it could be difficult to interface to the Arduino. Many devices like that use a SOC that drives the LCD segments directly.
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I agree with PapaG, it could be hard to do, probably impossible.
Using the sensor part could be possible, but it could be a big disappointment. Some sensors are embedded in the plastic encapsulation and are too simple to be useful.

This is the cheapest I could find: http://www.ebay.com/itm/300456514748
It is a used one, and shipping costs are not included.
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