How to capture linear motion on the right scale?

I’m working on some custom gear for a flight simulator, and I want to build a particular control which is a lever you can slide through a good 30cm or so. I have slide pots but of course they are much smaller; is there a clever way to capture this linear motion from 0-300mm without using gears and mapping onto a potentiometer? A rotary encoder would be another way (with a belt) but this doesn’t feel like the smartest solution. I’m not a very competent mechanical engineer, I assume there is a simple answer for this.

Anyone mechanically minded: What’s the assembly that’s sliding on the rail here? I don’t even know what to google.

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Hi Colin.
Not too hard. A 10 turn potentiometer with a pulley and cord system or even a rack and pinion gear system should not be too difficult. 10 turn pots are not too hard to get. For a flight sim a value of 10kΩ would be good.
Cheers Bob

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You could make your own 30cm long slide pot using two parallel Nichrome wires and the slider has a piece of sprung copper that bridges the wires. The resistance is measured at one end and will be quite low (0 to 8 ohm if using AWG28 which seems to be a common size), but there are plenty of ways to measure a small voltage drop when supplied with a few milliamps.

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Hi Alan, Colin
It will need to be about 10kΩ to feed into the analog input of an ADC. The pot does not have to go right to the ends as the flight sim allows calibration by operating the control over the full travel and then the sim knows where the ends are. The cord and pulley I mentioned above could be prone to slipping so cal will be lost. The rack and pinion would be better. Not too hard. With a 10 turn pot the rack needs to have 10 times the number of teeth than the pinion.
Cheers Bob

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I do like this idea. Do you think a 3D-printed rack and pinion would be stable enough?

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Hi Colin
I have no idea. I know pretty much nothing about these things and I have no space for one or I might know a bit more. One of the more experienced people might help here. I think this sort of thing is available commercially but I have never had the need to look. There may be some sort of toothed belt arrangement available too but once again I don’t know.
Cheers Bob

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I’d say a toothed belt setup would be the way to go. The small GT2 type belts used by 3D printers and other CNC machines are available quite cheaply. It’s probably what you can see in that video you linked.

That said, I’ve got a Saitek Flight Yoke which I’ve opened up previously for a feasibility study :wink: (it’s on the to do list to decrease the deadband at the neutral/home position). They just use regular 3/4 turn pots via a clever mechanical linkage. You can see it here in this video:

I believe the Pitch/Throttle/Mixture levers actually also use standard off the shelf pots…

Indeed they do, see here:

No reason you couldn’t 3D print something similar, especially if you’ve got a resin printer. Come to think of it, I’d be very surprised if there aren’t already some open source designs for one. I’m sure you’re bound to find something with a bit of googling.

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Hi Oliver
Saitek yolk. After viewing several videos on this I believe Saitek have a fair bit of dead band built in to their controller. I have followed others and used a Leo Bodnar board to get rid of it. I have also transferred some of the mouse and button switch functions to 10 rotary encoders. They work well but I haven’t hat time ti evaluate the Roll and Pitch without the dead band yet. I am not expecting any problems

Not really. I had a defunct one once so dismantled it. The lever only rotates 90º but the pot continues the full 360º so if you remove it you have to be careful to replace with the rotating bit in the correct position. That is the slot can be 180º out. The resistance track inside seems to be arranged so the full range is over 90º and the outer parts to the wire contact points are very thick and almost zero resistance.

With his time lapse video the bloke fixing this makes it look easy but If you have done this I think you will agree in is not quite so. For a start those wires are very thin and would break easy so one needs to exercise some care.

All the other pots I have seen in this set up including rudder pedals seem to be pretty standard.
Cheers Bob

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Hi Colin
Google “rack and pinion linear motion” and you should get 5,900,000 hits (I did). Should be some ideas there.
Cheers Bob

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Thanks for this. Great ideas there. It’s things like knowing to search for “GT2 belt” - I’ve got plenty to learn here, but keen to give it a go!

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Hi Colin,

Very interesting project, I think Bob and Oliver have nailed it with the belts and rotary pots.
That would be amazing to see Alan, it reminds me of the bigger potentiometers/rheostats used in science classes:

To 3D print your own you’d have to fiddle around with the tolerances a bit but nothing extravagant I’d imagine, check out this guide for getting more accurate prints with a larger nozzle diameter:

Baking in your own way to tension the belts would be essential - the Ender 3 V2 has pretty elegant way of doing so: Ender 3 V2 First Impressions / XY Belt Tensioners and more! - YouTube

I’m keen to see your progress!
Liam.

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Thanks for the suggestions in this thread, you guys are awesome. I learned a lot, sometimes in my ignorance I don’t even know what to google!

Here’s where the project got to: I decided to go with the 10-turn pot, and I’m using a length of V-slot and related components to hold it all together, which I have been learning a bit about. I’m waiting for the gantry and timing belt to show up, but here’s the general idea:

Just an idler pulley at one end and a timer pulley at the other, turning the shaft of the pot. I think it will work well except we’ll see how the sliding gantry bit works and what sort of a handle I can rig up for it.

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Hi Colin,

Great work so far on that linear-to-rotary system!

I’ve recently found out about DROs for lathes and mills, which use (usually) magnetic encoders to work out precisely how far something has moved on an axis.

If kits can be found cheaply, I’d say you’d be able to find an encoder with a usable output format

We sell magnetic strips that use a similar pattern to DRO encoders, you’d just need to bring a hall effect sensor or similar:

Your method is likely going to be cheaper and easier, but I thought I’d chuck another suggestion into the ring just in case it isn’t

-James

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That’s very interesting James - good to have this knowledge in the bank. It’s more flexible in both senses of the word, obviating the need for pulleys hanging off the end. Will think about trying this.

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Not that anyone was waiting with bated breath or anything, but here’s how things turned out, based on the feedback I got here. The motion, tension, length and so on are all perfect and its easy to mount as well.

V-slot extrusion, 10-turn pot to capture the motion, and there’s a snap-action switch at one end as well. I found some little bits, I have no idea what they are called, and put one in the rail with a set screw so that at the end it has a satisfying clicking into place. All that’s left is a couple of cosmetic things and to wire on a proper connector so the unit can plug into a base with the microcontroller.


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Great job Colin.
That’s exactly what I think we were all getting at. Simple and effective.
It looks like a Bourns Pot you have used. If so a good choice. Quality is right up there and you will find repeatability very good. I have a test one mounted up with a Bourns turns counter and have found the pot will return to almost exactly (give or take a bit of meter error. To 1 decimal place it would be Exact) the same resistance every time. Linearity is also good. I think you will be happy with that set up.
Cheers Bob

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