Six-axis positioning sensor suggestions

[jgannon]

Does anyone have suggestions for an inexpensive six-axis positioning sensor (three accelerometers and three gyros mounted orthogonally)? I don't need anything fancy like Bluetooth or USB... analog outputs would be fine. I recall seeing one recently, but I can't remember the website. Any help would be much appreciated.

[Jared Russell]

Finding a pre-built six-axis INS that is under $200 and still reasonably accurate is nigh on impossible.

Why not buy three gyros and three accelerometers if you are going this route, and assembling them into your own custom unit?

[jgannon]

Originally Posted by Abwehr

Why not buy three gyros and three accelerometers if you are going this route, and assembling them into your own custom unit?

I don't know that I have the mechanical skill to build something as accurately constructed as something that is predesigned.

[DjAlamose]

It all depends on what you are going to be using this for, there are other ways that you can do the same thing but with different data and sensors. And a six axis positioning sensor under $200 would be VERY hard to find.

[jgannon]

Originally Posted by DjAlamose

It all depends on what you are going to be using this for, there are other ways that you can do the same thing but with different data and sensors. And a six axis positioning sensor under $200 would be VERY hard to find.

I'm trying to construct an absolute positioning system for a robot I'm working on. I would want it to be able to work on a variety of surfaces, on ramps, in elevators... just about anything. I don't think that mere encoders will cut it. I'm open to suggestions, but I'm leaning toward a six-axis system.

[Joe Johnson]

It is not what you asked for but it may be helpful just the same.

http://www.sparkfun.com/commerce/pro...roducts_id=244

A 3 Axis magnetometer.

Check it out.

Joe J.

[jgannon]

Originally Posted by Joe Johnson

It is not what you asked for but it may be helpful just the same.

http://www.sparkfun.com/commerce/pro...roducts_id=244

A 3 Axis magnetometer.

Check it out.

Joe J.

Okay, I read through the data sheet, but I'm not quite sure how this helps with positioning. Could you please explain, Dr. Joe?

[Joel J]

Don't use it. You can factor out the magnetic field induced by your robot, and you can calibrate for the offset caused by being indoors (these two things take work, however), but you can't account for random ferrous (would stick to a magnet) bodies in motion.

GPS would be a major help to you, if you were operating primarily outdoors.

Joe J. has a good point, though: you need some sort of stable reference to "zero out" your position error every once in a while.

[BrianBSL]

Originally Posted by jgannon

Okay, I read through the data sheet, but I'm not quite sure how this helps with positioning. Could you please explain, Dr. Joe?

Something like the Sparkfun 6-axis IMU would work, which is $300 when you drop the bluetooth module ( http://www.sparkfun.com/commerce/pro...roducts_id=411 ).

I don't think you are going to find an off-the-shelf 6-axis sensor for $200. I think you need to rethink your requirements, as anything in the "affordable" range isn't going to be sufficient to use for navigation. I hope this isn't going to be the only source of navigation information for whatever project you are going to use this on - as gyro's and accelerometers both have drift issues.

You can pick up three ADXRS300EB's from analog devices for like $50 each, plus accelerometer(s) by Freescale, Analog, etc and build one yourself, simply on PCB or something. Shouldn't require that much mechanical skill. (Sparkfun sells the Freescale 3-axis thats on the IMU board for like $40 IIRC).

[Joe Johnson]

Originally Posted by jgannon

Okay, I read through the data sheet, but I'm not quite sure how this helps with positioning. Could you please explain, Dr. Joe?

accelerometers and angular rate sensors do not give you position either. You need to do some processing to figure things out.

You are going to have to have reduntant sensors to make this work no matter how you slice it.

I would guess that you will have to have wheel encoders plus some other info that will let you know when not to trust your encoders (due to an incline or wheels slipping or whatever). Part of the other info could be provided by this micromag sensor.

It is a complex problem that is sometimes call "sensor fusion."

Your brain does this all the time:

• Sometimes you use your eyes
• Sometimes you trust your inner ear
• Sometimes you trust your sense of touch
• Sometimes you barf your guts out because your sensors don't agree with eachother and your brain doesn't know WHAT to do ;-)

The MicroMag sensor is not a solution. It may be part of the solution.

Good luck keeping your robot from barfing!

Joe J.