Open-Hardware Magnetic Tracker - Call to Arms

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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by jestersdream »

Have you guys seen this video yet?

[youtube-hd]http://www.youtube.com/watch?v=TEbwgDel6lk[/youtube-hd]
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by WiredEarp »

@ Fredz: that might explain it. Definitely accuracy drops off quite significantly once you start moving towards the limits of the Hydras range. You'd think this would be ok with the base positioned close however, although then you'd be coming up against the refresh rate/latency limitations...
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by cybereality »

jestersdream wrote:Have you guys seen this video yet?
I think that might be the prototype that pre-dated the Razer Hydra. I heard some developers got a hold of the initial wireless prototypes from Sixense but then Razer decided to go wired on the commercial unit. Honestly not sure they have sold enough of them for the product to be considered a success, so I'm not holding my breath for a future wireless version. I could be wrong though...
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by bobv5 »

Oculus could be using Razer for tracking in the future. It was mentioned that they were in dicussions with peripheral makers.
"If you have a diabolical mind, the first thing that probably came to mind is that it will make an excellent trap: how do you get off a functional omni-directional treadmill?"
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by WiredEarp »

I know what you mean CyberReality, I dont know how much of a success the Hydra has been either. If it had been wireless, I think they would have sold a lot more of them. Its a shame as the tech is so cheap compared to everything else out there magnetic tracker wise.
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by keithoc »

Hey, been lurking for a while so figured I'd finally join in. I'm a game developer who's had a great interest in VR since doing some related research in my college days. Seeing (and backing) the Rift Kickstarter sucked me back into that research, since I'm now in a position to actually make VR games instead of just dreaming about them! Not just regular games with stereo 3D & head tracking, but proper VR games - and that means VR input.

Although I'm a software guy, I've done some electronics tinkering and would love to put something like this together myself, since there doesn't seem to be any (reasonably priced) commerical solution for what I'm looking for. I'm sure there will be eventually, but in the mean time I want to do some prototyping. Magnetic tracking seems to have much more potential than optical or other methods, for certain applications anyway.

Did anything come of this project? What are my chances of putting something like this together myself? Seems like all the information is out there in some form, just wondering if brantlew came up against any roadblocks I should know of?
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by brantlew »

It's a good idea but none of us (as far as I know) have actually started down this path. I suspect that "the devil is in the details" with this project. You could probably get something working "reasonably" within a few months concentrated effort, but getting high performance might be a significant task. I would love for someone to try and publish their efforts.
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by virror »

Very interesting post!
Maybe it would help if we could structure it up a bit, like list all the difficulties. Maybe that would help visualize things better and ppl could so what needs to be done and pick a task?
If there is any hardware that needs to be built later i can offer my services there but im not much of a software guy compared : p

Edit: Found this very old article describing how magnetic tracking works, i guess its still relevant even though its old.
http://resenv.media.mit.edu/classes/MAS ... rticle.pdf

and

http://www.cedrat-technologies.com/file ... R_2010.pdf

and this:
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by MSat »

I was starting to respond to this thread yesterday, but then I deleted it as I discovered flaws in my logic from hoping to take a simple route. This is mainly because my math skills are not very strong (to put it lightly). If you're up for the challenge, then I might be able to get you started down at least one path. The following is based on a system using 3-axis magnetometers to measure the vectors of magnetic fields in free space generated by 3 DC electromagnets (not pulsed-DC), and using that information to triangulate position. I'm not sure if using sensing coils would be significantly different (other than having to use pulsed-DC electromagnets)


The major challenge lies in the fact that you need to have an accurate model of the magnetic fields and then determine where the 3 vectors should meet up in free space. Consider the difficulty due to the fact that the direction of the field vectors vary around the magnet, so it's not even straightforward triangulation (not even sure if you could call it that). To show you my point, look at the following image:
magneticDipole.jpg
I marked 3 areas of interest with a red, green and yellow dot. Lets say your right hand was a sensor with your palm facing down (lets call down the Z axis), your thumb pointing to the left along the x axis - we'll ignore the Y axis for this example. If your hand was centered at the red dot, the field vector would be pointing strictly along the Z axis. However, if you moved it to the right so that it was now centered at the green dot, the direction of the vector (as well as the magnitude) would have changed from being strictly along the z axis, to now being strictly along the x axis. If you then moved your hand to the yellow dot, the direction of the vector would be the same as the green, but the magnitude would have changed again. Lets say that the magnitude of the vector at the red and yellow point are the same. If you placed your hand at the red point and tilted it so that your thumb was now facing down, the vector detected by your hand would be the same as if you held it flat at the yellow point. This should shed some light on the challenges posed by this system.


Obviously, we're not just using one magnet, so we can extrapolate all the information we need by using 3 if them as has been proven by several commercial products. However, I don't believe the common cross-axis dipoles (i.e. Hydra) are actually necessary, or if they would make the calculations any easier than a parallel triplet arranged in a triangle. Regardless, I do believe that using an accelerometer to determine the node's orientation relative to gravity would simplify the calculations somewhat, especially in my tilted hand example.

Here are some pages that are somewhat relevant:

http://www.netdenizen.com/emagnet/offaxis/mmoffaxis.htm

http://www.acasper.org/2012/02/19/3d-ma ... alization/

http://www.physicsinsights.org/dipole_field_1.html (derived from the site above)

I would love to see this happen, so if you or anyone thinks they can figure out the software, the hardware should be relatively easy.
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by virror »

I think trying to copy the project brantlew posted about in his first post would be the easiest first step as they have both hardware and software descriptions:
http://www.na-mic.org/Wiki/index.php/20 ... penIGTLink
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by cadcoke5 »

I have not worked with magnetic tracking stuff, but from my limited exposure to the topic during my electronics training, I know it is just weird. The environment has an effect on it, the stuff inside the circuit has an effect on it, etc.

Vision processing is certainly a booming area, but I think the real area where things are happening is in the process of sensor integration.

Keithoc, if your skills are primarily software, then I suggest that it is best to take advantage of that, rather than start down what may be a frustrating road.

I wonder if a tracking camera with integrated on-board hardware processing is a good way to maximize your software skills, while developing something that seems to be needed in the world of VR. Another area might be to work on integrating a variety of existing sensors onto a processor that can crunch your code fast enough to reduce the lag time significantly.

Joe Dunfee
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by MSat »

virror wrote:I think trying to copy the project brantlew posted about in his first post would be the easiest first step as they have both hardware and software descriptions:
http://www.na-mic.org/Wiki/index.php/20 ... penIGTLink

Hmmm.. There's some good info there that I missed the first time around.
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by virror »

Seems like a good basic design, but i think we need a filter and amplifying stage before the A/D converter if we want to get a good range.
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by keithoc »

Some interesting information, thanks guys.

I agree Joe that optical tracking would play more to my strengths, but part of my reason for looking into this is to get more familiar with electronics. You're right though that I might be just in for a whole load of frustration, but I've learned a bunch already just doing some research, and I'm really excited about this topic which I think counts for a lot.

MSat, I've been looking at some magnetometer-based stuff too as it seems to have the potential to be much simpler hardware-wise. What I haven't been able to see though is how quickly they can respond to rapid changes in orientation with respect to the magnet - every example video I've seen respond too slowly to be of much use to VR. Those videos have only been basic compass apps though, so I'd be interested if anyone here has experience with them.

For the moment though there's more information out there about the coil inductance method to learn from, and there have been proven implementations too so I think I'm more likely to have some measure of success trying that way. Definitely keep us updated with anything new you come across though.
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by MSat »

keithoc wrote:
MSat, I've been looking at some magnetometer-based stuff too as it seems to have the potential to be much simpler hardware-wise. What I haven't been able to see though is how quickly they can respond to rapid changes in orientation with respect to the magnet - every example video I've seen respond too slowly to be of much use to VR. Those videos have only been basic compass apps though, so I'd be interested if anyone here has experience with them.

For the moment though there's more information out there about the coil inductance method to learn from, and there have been proven implementations too so I think I'm more likely to have some measure of success trying that way. Definitely keep us updated with anything new you come across though.

Most magnetometers tend to be slow - I believe the one in the iPhone4(S?) has an update rate of only 8Hz. However, Anistrophic Magnetoresistance (AMR http://en.wikipedia.org/wiki/Magnetores ... _.28AMR.29) which is the underlying tech used in most ICs is not inherently slow. In fact, it has the capability of having very fast response time and high accuracy. The thing is, most devices that require the use of a magnetometer (such as electronic compasses, phones) need energy efficiency rather than speed. The fastest integrated device that I have found is the Freescale FXOS8700CQ (http://www.freescale.com/webapp/sps/sit ... FXOS8700CQ) which has a max sampling rate of 800Hz @16-bit depth, so it should be sufficient for magnetic tracking. Besides the benefit of having the sensing elements, amps and ADCs in a single package, the digital output values can easily be scaled to Tesla or Gauss, whereas if I'm not mistaking, the output from a coil->amp->ADC chain would be non-linear. While I can't say for sure, I have a hunch that using magnetometers would not just be simpler to implement than coils in terms of hardware, but also software.
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by virror »

Only issue i see with this solution is how to handle three different magnetic fields? A magnetometer can only measure one field at a time right?
With the original approach you can separate them by frequency but how to do that with magnetometers?
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by MSat »

virror wrote:Only issue i see with this solution is how to handle three different magnetic fields? A magnetometer can only measure one field at a time right?
With the original approach you can separate them by frequency but how to do that with magnetometers?
Whether you use coils or magnetometers, the three electromagnets are activated in a sequence. Two or more electromagnets are never active at the same time. The difference between using coils or magnetometer sensing elements are this:

With coils, you must use an AC or pulsed-DC driving signals as no currents would be induced in sensing coils if the fields are static. Magnetometers on the other hand sense the instantaneous magnitude of a field when it is being sampled, meaning you want the electromagnet fully on at the time of sample - the only tricky part of using a magnetometer IC is if sampling moment can't be determined.

Also, since magnetometers sense static magnetic fields (such as the Earth's), after sampling the 3 electromagnet's fields you have to take a sample with all of them off in order to be able to separate any background magnetic fields from the 3 previous samples.
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by keithoc »

Can magnetometers detect a magnetic field in any direction? There are ones I've looked at that have tilt compensation, but even then they seem to have a maximum tilt before the magnetometer readings start becoming incorrect. Surely a magnetometer-based tracker would need to be able to rotate the magnetometers to any orientation? Others that have tilt compensation don't mention any maximum tilt. Does anyone have experience actually using 3-axis magnetometers in practice?

The datasheet for the FXOS8700CQ says "The angle at which the device no longer detects the orientation change is referred to as the 'Z-lockout angle'. The device operates down to 29° from the flat position. All angles are accurate to ±2°." - I don't know if this is related to the magnetic field reading though, or a separate orientation feature of the chip.
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by MSat »

3-axis magnetometers will sense fields as a scalar (up to +/- max field strength sensitivity) in each each axis which can be used to create a 3DOF vector - just like coils in an XYZ configuration do for varying fields. It's helpful to think of 3-axis magnetometers as a triplet of coils, with the exception that they can detect static (instantaneous) magnetic fields.

The Z-lockout which you referred to is an optional embedded function for use in devices such as cellphones and tablets to limit the angle at which portrait and landscape mode can be toggled - so if you hold it flat and rotate it, it will not toggle (this is strictly a function of the accelerometer). It's irrelevant for the purpose of a magnetic tracker, and is not a limit of the device's accelerometer or magnetometer angular sensitivity.
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by keithoc »

Ah awesome, ok thanks for the info. I guess the next step is to order a magnetometer!

I think I'll get a LSM303DLMTR to test first and see how it goes, as there's an available Arduino library in development which seems like a good place to start. I also found an evaluation board for the FXOS8700CQ, but it's more expensive so I'll see how the cheaper one goes. You're right though, I haven't been able to find anywhere that sells the chip itself in quantities of less than 1000... but first things first!
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by MSat »

Yeah, I had contacted Freescale several weeks back seeing if they would just sell the sensor board itself as I was looking to order 5, but didn't need the USB interface board. Unsurprisingly, the answer was "Nope".


That particular sparkfun board looks like a good starting point. It has a reasonable update rate of 220Hz, but more importantly, it has a mag data ready pin and data register overwrite lockout so it should be easy to synchronise with the field generating coils.


I'd love to help in any way that I can, but I probably won't be of much use when it comes to the algorithms, though I can probably help find relevant information, and give suggestions on the hardware side. :)

I'm glad someone is picking up the torch on this. Keep us updated!
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by V8Griff »

All gone quiet in here.

My personal opinion, having been exposed to most levels of professional tracking by Polhemus and Ascension, is that this is the simplest and potentially most accurate way to achieve positional tacking for both head and hands as demonstrated by the Hydra, although the low power of that device and posts I've read suggest it's not accurate or fast enough for head tracking. (Not had experience with a Hydra yet although I have ordered one)

I have several bits and pieces in my collection but the most interesting to me are the InsideTraks I have from Polhemus, the issue with those of course is they're on an ISA bus and I have no experience of modern motherboards running these buses.

So questions:

1. Anyone have any experience with systems that have ISA bus slots?
2. Would some pictures of the cards be of any use to the technically minded of you?
3. (Probably the most difficult to answer.) Is the technology still protected under patent, or will it now have expired as these cards date from 1993?
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by WiredEarp »

My personal opinion, having been exposed to most levels of professional tracking by Polhemus and Ascension, is that this is the simplest and potentially most accurate way to achieve positional tacking for both head and hands
I totally agree. I've been a long fan of magnetic tracking, despite its limitations. Probably why I've been talking about the Hydra for so long, its good to finally see it get the recognition it deserves.

I've got an Ascension Nest of Birds myself, keep meaning to take photos of the boards to upload here. Unfortunately no transmitter yet (got one coming supposedly).

Not sure re running old ISA cards in modern equipment, but surely you could just get an old ISA supporting PC, and use it solely to run the InsideTraks, using VRPN to communicate back to your 'real' PC?
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Re: Open-Hardware Magnetic Tracker - Call to Arms

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V8Griff wrote:... I have several bits and pieces in my collection but the most interesting to me are the InsideTraks I have from Polhemus, the issue with those of course is they're on an ISA bus and I have no experience of modern motherboards running these buses. ...
You can plug ISA bus cards into a USB port with a USB 2.0 to ISA bus adapter:

Image
http://www.arstech.com/item-USB-2-0-to-ISA-card-ROHS-usb2isar.html wrote:usb2isa-r - USB 2.0 to ISA card ROHS

If you need to use your ISA card, but your new computer or laptop lacks ISA slots, use our usb2isa card to convert your USB port to a virtual ISA slot.
...
ARS Technologies’ USB2ISA® allows plugging and handling of any standard or custom ISA card. Our usb2isa provides simple migration from ISA form factor to USB, full access to memory and I/O space of the ISA card, and handling of IRQ and DMA events.

The usb2isa functions on these Operating Systems:

Windows - 98, ME, 2000, XP, Server 2003, Vista, Server 2008, 7
Linux - 2.6.xx and 3.0.xx kernels
Mac OS X 10.5, 10.6, 10.7 (for developers)
Installation Software
...
Pre-order Considerations
1. Powering needs. Our power supply module provides -5V, +12V, and -12V for using the usb2isa family of products. To find whether you need a Power Supply Module, please read ISA card - finding the voltages used.

2. Software. The UNIVERSAL SOFTWARE LAYER should provide all the software you would need to get our usbisa card to work with your isa card(s). We offer an sdkbus for developers. If you have access to the source code, you may be interested in our SDKBUS product. Otherwise, our hardware comes with all the software and drivers you will need.
...
3. Enclosures. ARS Technologies provides plastic enclosures/boxes for mounting a usb2isa and an ISA card. Please check the dimensions on the enclosures before ordering to ensure that the boxes will fit your ISA card using the ISA-B1 or ISA-B2.

Price - in US Dollars: $149US each
...
A bit pricey, but worth the money if you really need it. I have seen similar products for a lot less money from Chinese eBay stores...

EDIT: Nothing on eBay now, but for a $169, ARS has 3-slot USB to ISA adapter:
http://www.arstech.com/g/usb2isa-x3-us.html
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by MSat »

I love that kind of hardware. I had a feeling someone made a USB<->ISA interface. However, I'm guessing that getting drivers to work for speciality hardware would be a bit tricky.
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by V8Griff »

geekmaster wrote:
V8Griff wrote:... I have several bits and pieces in my collection but the most interesting to me are the InsideTraks I have from Polhemus, the issue with those of course is they're on an ISA bus and I have no experience of modern motherboards running these buses. ...
You can plug ISA bus cards into a USB port with a USB 2.0 to ISA bus adapter:
Good find thanks. I couldn't find anything other than prototypes when I searched.
MSat wrote:I love that kind of hardware. I had a feeling someone made a USB<->ISA interface. However, I'm guessing that getting drivers to work for speciality hardware would be a bit tricky.
I discovered in my archives that I do have some C++ source code for the hardware dating back to when I was working with the cards in the late '90s early '00s. I hadn't realised and I only searched for it when Polhemus were reluctant to let me have anything when I wrote to them last year.

The readme in the text suggests it works with NT so I'm assuming I could give that to a good programmer to write some drivers?
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Re: Open-Hardware Magnetic Tracker - Call to Arms

Post by geekmaster »

MSat wrote:I love that kind of hardware. I had a feeling someone made a USB<->ISA interface. However, I'm guessing that getting drivers to work for speciality hardware would be a bit tricky.
Notice that the quoted description says their USB driver handles IRQ and DMA events, so perhaps it would not be too difficult. It might even "just work". But really "old school" hardware often came with device driver source code and/or protocol description, so the included SDK would be handy in difficult cases. These adapters are popular in astronomy and medicine, where a modern replacement for their still functional ISA card may cost over $10,000USD.

These days, companies hold onto their source code (even the OLD stuff that used to be included with the product), as though it were made of gold...
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