New hall sensor design

Posted By: Brandano

New hall sensor design - 08/09/10 04:37 PM

The sensor shaft on my rudder pedals snapped a few days ago (it was a nylon shaft from a potentiometer, and I had carved a groove in it for a small clip retainer... bad idea) so I made a replacement for it using another pot with an aluminium shaft. I tried out a new concept to see if I could get the same sort of performance using a single magnet... well the results are mixed. Here is a comparison of the new and the old sensors side by side, in a blurry cellphone picture:

The old sensor is on the right, and it's the classic "sensor between magnets" setup, with a small metallic strip acting as a flux concentrator to increase the magnetic field between the magnets and provide some magnetic shielding.
The new sensor is on the left. On both sensors the potentiometer tracks were filed down to break the electric contact, and neither respects the original potentiometer pinout, so be careful if you try to replicate this. The actual hall sensor is buried in a slot between two metal flux concentrators, cut out from three washers stacked and glued with CA glue. The magnet is in the center. The problem with this approach, or at least with this iteration is that the full range is over 180 degrees of swing rather than over 90 degrees, that would suite me better for my direct drive setup. The advantage is that it does use a single magnet and is slightly more compact, at least in thickness. I bet that the sensing angle can be made smaller by using shorter arcs for the flux rings, and that the linearity can be adjusted by careful shaping of the flux concentrators. Does anyone know if there is software available to run simulations of magnetic flux?

[edit] To be more precise, "new" means it's the first time I try this, not the first time this has been done. A quick search on Google patents shows that the concept is not new in any way.
Posted By: Ltfransky

Re: New hall sensor design - 08/09/10 05:48 PM

That's pretty cool. It'd be cool to see some clearer pictures.
Posted By: Brandano

Re: New hall sensor design - 08/09/10 10:46 PM

I took a few more pictures of both the old and new sensors, still poor but hopefully a bit clearer. Here is the old setup:

And this is the new hall effect sensor arrangement, already assembled on the pedals:

The scratched disk under the flux concentrators is made out of two layers from a CD jewel case that I glued together and milled on a mini-drill to the shape of the washers, and acts as a spacer. I scratched the surface to give more bite surface for the CA glue to hold the flux concentrators in place.

The hall sensor used is the Allegro A1302, not the most sensitive one but apparently it's good enough. I have a rougher (essentially the magnets are attached to a short shaped shaft through the C shaped piece of tin, while the hall sensor is attached to a plastic bracket holding it between the magnets) equivalent of the old arrangement in the throttle, pitch and roll axes of my Thrustmaster Hotas Cougar and I get fine control through the whole range. Probably the circuitry of the Cougar and its automatic calibration play a large part in this.
While I was at it, I have added a few more pictures of my rudder pedals, with the new footrests salvaged from a pair of CH (non PRO) pedals, which gives me toe brakes, and the new "soft" centering system. I have also added some end stops. The shape of the levers already ensures that the pedals can't exceed a maximum travel, but this gives more positive and repeatable rudder limits

Posted By: Mike Powell

Re: New hall sensor design - 08/10/10 04:00 AM

Neat work and interesting reading. A few comments:

There is indeed software for calculating field distributions. Unfortunately, I can't find the links. As I recall it's payware, but there may have been a trial or student version.

Your double magnet approach will offer the best results if you keep some distance between the sides of each magnet and the C-shaped shield. When the magnet is too close, the shield "short circuits" a significant amount of the flux and the field distribution in the gap is distorted.

The single magnet approach works best when you use low remanence material for the flux concentrators. It looks like they are made from mild steel washers. The problem is that they magnetize slightly and add hysteresis to the system. If you turn the shaft to the right and then to center you get one voltage out. If you then turn the shaft to the left and back to the same center position you get a slightly different output voltage.

It's nice to see more work done in this area. Hall devices are under utilized by hobbyists.
Posted By: Brandano

Re: New hall sensor design - 08/10/10 06:35 AM

I had not considered the hysteresis, that's a good point. I suppose the best material to use would be that used in electric motor rotors and transformer cores? I have not measured the readings from the new sensor, but the joystick output seems to give me repeatable centering. Assuming the rudder is deflected on average each way about 50% of the time, the centering should work over time. However this this does not hold true, especially when flying helicopters, so this new approach will drift over time. I think I will recover the sensor from the older setup and rebuild it with a new flux concentrator, this time in the shape of a ring running around the whole assembly, and still fixed to the magnets.
Posted By: f15sim

Re: New hall sensor design - 08/10/10 01:53 PM

I'll second that, great job Brandano!

Posted By: Ltfransky

Re: New hall sensor design - 08/10/10 05:48 PM

Thanks for the pics
Posted By: NamelessPFG

Re: New hall sensor design - 08/10/10 06:41 PM

Your use of the original CH pedals gave me an interesting idea about modding toebrakes into my RCS.

Since they're old gameport ones, just pivot, are usually spaced extremely close together (and the Pro Pedals were already too close for comfort), and generally not in high demand, I could probably pick up a set for not too much, remove them from their original base, plop them on my RCS, and wire the pots up accordingly (which will be much easier since I have a Hall sensor kit installed, which already has the necessary wire leads exposed for just that sort of thing).

Oh, and nice Hall sensors. I might just consider plopping a couple into my Strike Force 3D (which uses Cougar-style mini-pots) in the process of modifying the gimbals, but I'm concerned about the force-feedback motors possibly interfering with their operation. (Then again, if the G940 packs a tri-axis Hall sensor for the stick along with similarly large FFB motors...)
Posted By: Brandano

Re: New hall sensor design - 08/10/10 09:51 PM

Cutting away the base of the CH pedals is a long and tedious task. Try to get a hold of an electric reciprocating saw if you can, and be very careful during the cut. You have to remove the entire pedal beforehand, or it will get in the way of the cut. If you are also widening the pedal base make sure that both pedals are well supported, perhaps adding small rollers underneath the pantograph arms. In my pedals the drawer slides tend to bend under the weight and catch on the fixing screws if turning while braking fully. I have "fixed" this by filing the slides end stops partially away, but it would have been better if I had arranged for the load to be applied on them sideways.

The way I added the sensors in my joystick means that the original gimbals are untouched by the mod. With a bit of care you can just try one sensor and see if the joystick can handle it properly. All modern joysticks use a voltage divider arrangement, so it really ought to work as a drop in replacement. If you plan to build your own sensors, the 2 magnet approach seems to work the best as Mike pointed out. Hopefully I will have an updated version of that to test soon, using a flux concentrator that should interfere less with the flux between the magnets. That said, it seems to be already strong enough that the hall sensor is saturated at 45 degrees deflection, so I might have to use smaller magnets if I want to get a greater range of movement.
Posted By: julian265

Re: New hall sensor design - 08/11/10 01:21 PM

Nice work - I like your lateral thinking with the new pot. A drop-in replacement like this (even if mechanically, but not electrically) for the average pot could turn a desire into reality for many people.

Extending what Mike said, have you tried mounting the magnets on a non-ferrous material, in the same way? Sure you'd need either glue or some kind of non-ferrous strap to hold them in place - but the linearity and sensitivity should be good.
Posted By: Brandano

Re: New hall sensor design - 08/11/10 03:43 PM

Only the new design allows for a correction of response linearity, but even then it's not a trivial task as you'd have to adapt the shape of the flux concentrators and it's hard to predict what the actual result would be. Essentially you could obtain a linear response working by attempts, building new concentrators and measuring each new shape. However, as Mike pointed out, this approach introduces new drawbacks that require better, more expensive materials. There's "drop in" replacement hall sensors, normally used in the car industry, that will cost around 30 US$ each, so if you factor in the time and the bill of materials eventually you are better off buying the rotary hall sensors already available. There's also quadrature chips that offer you perfect linearity, since they compare the readings from sensors placed at 90 degrees rather than reading an absolute. I suspect that these have a digital output, but there might be some that offer an analogue output instead. The older arrangement cannot be linear, it will by necessity read the central portion of a sinusoidal signal. the magnets can be mounted on a non-ferrous material, the classic setup uses a portion of a Bic pen with the magnets glued on the outside and the sensor in the middle. What I am trying to do is to increase the flux by providing a shorter magnetic path for the magnet's free poles.
Posted By: Ltfransky

Re: New hall sensor design - 08/11/10 04:58 PM

Allegro has a quadrature chip called the A1230. I may try one out on my new build.
Posted By: Mike Powell

Re: New hall sensor design - 08/11/10 06:12 PM

Originally Posted By: Brandano
I had not considered the hysteresis, that's a good point. I suppose the best material to use would be that used in electric motor rotors and transformer cores?

Transformers and AC electric motors use a high silicon content steel. It's not your best choice for flux concentrators. Its composition is a tradeoff between magnetic properties and increased electrical resistance to reduce eddy current losses.

The usual material for these applications is a high nickel content steel like permalloy. It has good permiability, but doesn't magnetize. It tends to be expensive.

A better, non-exotic choice is annealed mild steel. Just heat the steel washer to red heat and let it cool slowly.

The bottom line is really how things work for you. If hysteresis isn't causing you any heartburn, don't worry about it.
Posted By: Brandano

Re: New hall sensor design - 08/23/10 01:16 AM

new iteration, this time with the old two magnet approach, but with a flux ring surrounding them:

Both the oldest and this version of the sensor give me a greater sweep over the raw values read from the joystick, but this might be just the result of using two magnets. I tried to make the ring out of the steel spring from a windscreen wiper, but I realized that the specific steel didn't have much permeability. And annealing it brought its magnetic permeability down to near zero! Looking things up on Wikipedia I found that the magnetic properties of annealed steel vary a lot between different steel grades. The ideal material would be mu-metal, a steel that contains a certain percentage of Bronze and whose permeability increases when annealed
Posted By: f15sim

Re: New hall sensor design - 08/23/10 01:36 PM

Brandano, a good source of mu-metal is RF shields in consumer electronics. If you've got a Commodore 64 laying around, you can steal the shield that covers the whole motherboard - it's all mu-metal.

Same goes for the C-128. smile

Posted By: Brandano

Re: New hall sensor design - 08/23/10 04:34 PM

Thanks for the suggestion, but I am a ZX Spectrum zealot. The spectrum had practically no shielding at all. In any case, I am not going to gut my Speccy, and if I had a Commodore 64 I'd probably leave it alone as well smile
I bet that the casing from old PC PSU's will do just as well, though it probably is a lower grade of mild steel. However with this latest design hysteresis is a non-issue, so annealing shouldn't be needed anyway.
Posted By: uckfaouya

Re: New hall sensor design - 09/30/12 01:01 AM

i use the a1302 to turn my joystick pots into hall sensors too.
Posted By: f15sim

Re: New hall sensor design - 09/30/12 06:01 PM

It's kind of hard to surmise how it's used out of context. smile Got any more pics?

Posted By: uckfaouya

Re: New hall sensor design - 10/04/12 08:14 PM

Posted By: Brandano

Re: New hall sensor design - 10/04/12 08:46 PM

Very neat design, it might even be possible to seal it with little effort. What is the source of the shielding/flux concentrator ring?
Posted By: uckfaouya

Re: New hall sensor design - 10/04/12 10:28 PM

old golf club handle
Posted By: Brandano

Re: New hall sensor design - 10/05/12 11:36 AM

won't that be made out of aluminium? For optimal performance it should be made out of mild steel or another magnetically permeable metal.
Posted By: uckfaouya

Re: New hall sensor design - 10/06/12 04:56 AM

it steel, the only reason i used it is because it was the right diameter.
this is a new design with a single magnet hopefully the degree of rotation between 0 - 5 volts wont be to much
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