I'm looking at this chip: HMC5983-TR, along with its friends and relations.
Anyone have any mileage with these? Are any ones particularly better or worse? Are any suppliers particularly better or worse?
Mostly, I'm interested in knowing the direction of a rapidly moving platform with respect the Earth's magnetic field, as an aid to moment-to- moment navigation. So both accuracy and high bandwidths are big pluses to me.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
I recently researched sensors, for a system that would measure ambient fields and apply a correction to an instrument. There was a medium-big thread on that here, a couple months back.
I didn't like any of the Hall or AMR/GMR type chips for my application. They are usually slow, inaccurate, and often weird. I decided to use a fluxgate. Autonnic has some nice stuff, coils and boards. Ditto Speake & Co. Both British.
-- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
The only one I've used was already in a can, along with MEMS gyros and accelerometers and a GPS receiver, to create an integrated inertial nav box. I don't know which specific magnetic-compass device it used.
From playing with it on the bench, they weren't fooling about not using steel bolts or otherwise having steel, iron, etc close to it. There are calibration procedures that can get around some of the effects of nearby iron, but you have to run them.
In the finished product, I'm pretty sure I ended up ignoring the magnetic heading data, and just using the inertial and GPS stuff. I couldn't get out of having steel, and motors drawing non-trivial current, relatively close to the can.
One other observation is that pretty much every one of these I've ever seen in a complete system, including ones where the user is expected to be clueless (automotive), have some kind of end-user-accessible calibration procedure. Usually you push the button and then rotate the compass sensor through 360 or 720 degrees and that's it.
Depending on where you are on the planet, the Earth's magnetic field can do strange things. For instance, the nominal magnetic declination in Oregon ranges from roughly 16 to 19 East, but there are some local spots where it can be 4 East or 24 East - see
Matt Roberds
Ugh. I'd kind of thought about the whole magnetic disturbance thing, but not about magnitudes.
I think maybe I'll just leave the compass out, and let the board get smaller. It's another sensor to fuse in a sensor fusion application, but it's only worthwhile if the data is good.
-- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
How slow is slow? It needs to be teeny -- a 4mm square chip and its associated traces and bypass caps takes a huge bite out of my board space budget.
I'm thinking maybe it's not worth the effort and board space.
-- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
I'd like about a KHz of bandwidth, and most of the compass-type chips can't do that. But if that's all the space you have for a 3-axis sensor, a fluxgate is out.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
That's an interesting map. I didn't know there could so much local variation. I guess all that subterranean iron in motion can cause huge magnetic disturbances. Add to that the fact that the magnetic poles wander over time, it's even more complicated.
My personal experience with magnetometers is that we tried to prototype the use of one in a device that included a pair of speakers (think permanent magnet). I can say confidently that such an arrangement is doomed to fail. The market forces of miniaturization make it worst.
JJS
In the application I was working on, it didn't matter so much, because the device was always used in a defined area that was maybe 100 m on a side, there was plenty of time to do calibration passes, and we almost always had GPS as well. If we could have beat the problems with steel and motor currents, the compass would have been a useful additional reference.
The main things the compass seems to be good for is an absolute heading at zero or low speeds, before you switch to GPS, or an absolute heading if you don't have GPS for some reason.
Matt Roberds
GPS won't give you your heading - when you are moving it will give you your track. The earth's magnetic field gives you heading, albeit with the significant ambiguities noted by others.
To understand the difference, consider facing north, i.e. the heading is north. Now step sideways to the left while still facing north - the track is to the west.
The threat indicator called FLARM has, necessarily, a significant well understood non-ideality: it indicates the threat relative to your track - which causes pilots to start looking relative to the aircraft's heading, until they consciously correct.
How much is a heading off from a track in an airplane? The difference would only be due to wind?
I used to geocache a lot and tried teaching some friends how to use the GPS. Often they just couldn't get used to the idea that this wasn't a compass and if you turned your body (and the GPS with it) the GPS didn't indicate that. As you say, it only knows which way you have moved and even then only once your movement was significant, 10 or 20 feet to get a decent "track" as you call it. But when I would use the GPS every week I got very used to it.
-- Rick
When landing, you sometimes sideslip to lose height without gaining speed. See a 45degree track/heading difference in
Apart from that, ...As much as you want, if there is a strong crosswind. Consider a glider flying at 40kt at
5000ft with a 60kt crosswind component! It is most easily visible when landing with a crosswind or when ridge running - where you may have a 30kt perpendicular to a ridge and be travelling at 60kt along the ridge.Basically yes, but if you really want to get into it you'll need to consider the difference between true north and magnetic north, yes.
All sounds right!
GPS + IMU + some lateral vehicle acceleration will give you your heading.
As soon as the kalman filter sees some lateral acceleration on the GPS that it can correlate with accelerometer readings, it knows which way you're pointed.
(Assuming you're in a gravity field)
-- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
Almost once a week I wish that my car GPS had a built-in compass. The usual situation is I want to drive to some address, so I enter it into the GPS while the car is parked. The GPS figures out the route from where I am, but because it doesn't know which way the car is facing, it can't tell me which way to start driving. Garmin made some hiking GPS's with built-in compasses but I've never seen a car GPS with one. I'd buy it if they made it.
Regarding sensors:
Steer one direction with the rudder and the opposite with the elevators and you can get a dead-air heading several degrees off straight, and off vertically too.
-- umop apisdn --- news://freenews.netfront.net/ - complaints: news@netfront.net ---
True, but you don't need to sideslip. Try ridge flying in a glider :) Or even just uncoordinated flying, using the rudder without ailerons :) Or just a strong 90 degree crosswind approx the same as your airspeed :)
I question the accuracy without a compass; the alternative is to determine the angle by integrating the angular rotation. That is, of course, highly sensitive to noise, drift and zero offset.
Solid measurements would be appreciated.
Gravity isn't required - you need angular rotation.
Interesting to see how you would negate the field distortion due to being surrounded by a large hunk of iron! Even then it wouldn't be accurate near Kiirunavarra, even neglecting the magnetic dip at high latitudes
Vaguely what kind of measured performance were you able to achieve? Did you rely on sensor fusion, i.e. use other instruments as well?
Car compasses correct for that. When you first install the compass, you press a calibrate button, then drive the car around in a circle. The changing part of the field is due to the earth, and the fixed part is due to the car. So it can subtract out the fixed part. I've had some compasses that did that (Precision Navigation Wayfinder), and they worked pretty well.
Of course with a compass and gps in the same box, it could know that you were near Kiirunavarra and make appropriate corrections.
OK, thanks.
I very much doubt there the device contains sufficient localised knowledge. Supposedly Kiirunavarra was discovered when someone hiking put his knife down, and found it surprisingly difficult to pick up. The mountain is now being systematically removed, and IIRC will eventually turn into a very big hole!
What I don't understand is given that the heavier elements were all formed in supernovae, how come there is a concentration of them? Surely they should be uniformly distributed.
It is not a uniform distribution, but a random distribution, which by its very nature has "concentrations" (clusters) and "holes". It is like a PRNG returning 2 five times in a row. It is improbable, but it still may happen.
-- Nils M Holm < n m h @ t 3 x . o r g > www.t3x.org
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