For a while now I’ve wanted to replace the crappy, noisy pots that I’d used in my Jentron MK2 gimbal that was in the 109F/X #0 cockpit. They were so bad that it was impossible to use the gimbal for flight at all.
The best way to replace a mechanical potentiometer is to use a hall effect device. This is essentially a sensor that will output a 0 to 5v signal based on the position of a magnet. I’m using the Allegro 1302 for this project. It works very, very well and can be a direct replacement for any three wire pot installation.
My design uses a 7/8″ (22mm actually) bearing with an 8mm center bore. The center bore allows you to use a “traditional” Bic pen body as an input shaft. You can press the body segment into the bearing and it won’t be coming out any time soon! You can purchase the bearings here: http://www.vxb.com/page/bearings/PROD/SB/Kit1063
I installed the pen body into the bearing and then glued a pair of 1/4″ square neodymium magnets (oriented NS-pen-NS) to either side of the pen body with some thick Cyanoacrylate glue. Works great!
In order to be a direct replacement for the pots, I needed to add similar control arms to them. I did this by laser cutting a press-fit back plate that I threaded for #4-40 screws.
Here’s the end result so far:
I then bolted it all together and attached the control arms with screws:
The tiny circuit board holds the A1302 hall effect sensor so that it’s in the exact center of the pen body and aligned with the two magnets.
After cutting out the old pots and adding a couple of terminal blocks, the new control inputs were installed:
When connected to the pots, the joystick calibration screen in Windows would show a constant jitter that ran +/- 200-400 around the center point. With the new A1302 based inputs installed the jitter was ZERO. I’m really looking forward to getting these installed into the rudder pedals, toe brakes and throttles!
Here’s a more detailed how-to on building these for yourself:
Here’s the parts you need:
The parts shown are a 22MM bearing (just a little shy of 7/8″ diameter), two 1/4 x 1/4 neodymium magnets, an Allegro A1302 hall effect sensor and a Bic pen body.
Using a vice, carefully press in a short segment of pen body into the bearing – do NOT hammer it! The pen body plastic is very brittle. Steady force using a vice will “drift” in the pen body quite easily.
Next, glue the two neodymium magnets to opposite sides of the pen body – the magnets should be oriented N-S (they’d come together if the pen wasn’t in the way). The way I did mine was to rest the magnet on the inner race of the bearing with a business card keeping it from physically touching the race. I then used “thick” CA (cyanoacrylate) to glue the magnets in place – have some kicker handy to speed up the cure process.
You should end up with something that looks like this:
Now you need to build the board that will hold the sensor in place. I used a small slice of copper clad perf-board. This holds the 3 pin connector and the A1302 very well.
Next, you want to get some 1/2″ or 3/8″ plywood and bore a 7/8″ diameter hole in it using a forstner bit. Dont’ bore the hole completely through – leave about 1/8″ of material. Then drill a 1/4″ hole using the same center as the 7/8″ hole. I use a set-screw to hold the bearing in, but you could easily hot-glue it in if you’re careful to keep the glue out of the bearing races. Tape the bearing in for now so you can align the hall sensor properly. A properly aligned sensor will fit in the center of the pen body with the magnets on either side of it.
You should end up having something that looks like this:
Assembled, it should look like this:
You can connect any kind of pushrod arm, gear, or whatever to the pen body – just don’t apply too much side force to it. Too much side stress will crack the plastic. This setup gives 180 degrees of usable travel and works very well! The bearing size that you get should have an inner diameter just a tiny bit less than the widest point on the hexagonal Bic pen body. This allows for a very tight friction fit that doesn’t require any adhesive to hold it in place.
You can easily custom design your own bracket for this – this was a simple prototype that proved out the technique and a refined version will end up in my ’109 as well as a replacement for the pitch axis sensor I screwed up in my BRFS gimbal build.