5 replies to this topic

[OlivierD.]

I have been thinking a bit on the AHRS development and recalled Jordi's approach at DIYDrones for the Ardu Pilot Mega.

I really like the idea of having the magnetometer separate from the main AHRS board, sort of like this:

http://store.diydron...-hmc5843-01.htm

On fixed wing aircraft or rotary aircraft, the AHRS should ideally be placed near the center of gravity. The trouble is, the magnetometer ends up being too close to objects containing metal and the readings can be affected by the proximity of batteries, servos, wire bundles and in the case of a twin engine aircraft, between 2 magnetic fields since the magnetometer ends up somewhere between the two motors..

A modular magnetometer could be placed farther out where the magnetic disturbance is less therefore increasing its reliability.

Just my two cents worth. Any thoughts?

[peabody124]

OlivierD., on 24 July 2010 - 05:24 PM, said:

On fixed wing aircraft or rotary aircraft, the AHRS should ideally be placed near the center of gravity. The trouble is, the magnetometer ends up being too close to objects containing metal a

[...]

A modular magnetometer could be placed farther out where the magnetic disturbance is less therefore increasing its reliability.

The nice thing about the open design of this project is this would be an easy extension for anyone to make.  The AHRS has a magnetometer, but it wouldn't be much work to make it take that data from a different source (and then make this an easily configurable option for end users).  I would connect the secondary magnetometer to the auxiliary USART/SPI port, and then route that data over to the AHRS for the sensor fusion algorithm.  

That's a good suggestion though.

[dankers]

Don't worry, this was thought of as well with the AHRS design, I'll get to that in a bit. First, I would first suggest you look up hard iron calibration and take a think about the structure of a quad as well. The motors are the worst source of interference and the centre of the Quad is furthest away from all the motors.

Additionally our AHRS will be used in many different aircraft (and likely other projects also), a self contained unit makes the most sense. Just like MikroKopter, the mag was an add-on for ArduPilot project which is why it is separate, their software did not (still might not, not sure) support mags. This is because DCM as used by ArduPilot uses GPS track to work out heading and not a magnetometer, of course useless for a VTOL ot anything that hovers. One of the options we looked at was to add mag support to DCM for OpenPilot as mentioned on the podcast we did with them, this was going to be called the Full Mahony in reference to the Australian professor who's work DCM is based off. Simply, our AHRS has the Mag built in because it was an integral designed in feature of the platform right from the start, Ardu's is separate because theirs was not.

But saying all this, we did think of that and have an external I2C bus on the AHRS by remapping the pins on the serial port, so we can already do as you want if that is your choice, plus we can add other I2C peripherals to the AHRS as well. The OpenPilot hardware design and thought that went in to it is first class and done by industry veterans with decades of experience.  

If you go this way it will also not cost you anything over their IMU + Mag either, our AHRS is less than their Oilpan and ours includes the Mag they charge $45 for! Just disable the onboard mag and use their external mag if you think it is required, our AHRS fully supports it and we even have the 5V supply on the same header as I2C. Of course you also get a powerful STM32 on our AHRS as well.

I keep saying this but the OpenPilot project has spent a great deal of time planning and testing, it is a key asset of the project and a differentiator from most other projects, we are not just flying by the seat of our pants and churning out revision after revision. All this hard work has been internal to the project which has the unfortunate side effect of people not seeing how truly advanced a platform this is.

  

OlivierD., on 24 July 2010 - 05:24 PM, said:

I have been thinking a bit on the AHRS development and recalled Jordi's approach at DIYDrones for the Ardu Pilot Mega.

I really like the idea of having the magnetometer separate from the main AHRS board, sort of like this:

http://store.diydron...-hmc5843-01.htm

On fixed wing aircraft or rotary aircraft, the AHRS should ideally be placed near the center of gravity. The trouble is, the magnetometer ends up being too close to objects containing metal and the readings can be affected by the proximity of batteries, servos, wire bundles and in the case of a twin engine aircraft, between 2 magnetic fields since the magnetometer ends up somewhere between the two motors..

A modular magnetometer could be placed farther out where the magnetic disturbance is less therefore increasing its reliability.

Just my two cents worth. Any thoughts?

[OlivierD.]

Dankers, no worries, I have decided months ago when we talked about my CHIMU AHRS that I will not spend any more money on my UAV project until the Openpilot is available for purchase. (BTW, any update on the time frame for availability?)

For a while, I considered getting and ArduMega with all the bells and whistles but that's only because I wanted to have something to play with this summer. In the end, I decided to hold on to my cash and wait for what looks to be the most promising and flexible platform out there.

As far as the DCM is concerned, William Premerlani flew with a modified code that picks up heading info from a modular mag sensor and reported things went well, I imagine the same code will be ported over to the ArduMega at some point. Regardless, I keep an eye on developments elsewhere just to see what's out there and William is a great guy, together with Paul Bizzard, he was the first to come up with a viable AHRS stabilization solution for RC aircraft. His idea was to have something that would make flying an unstable aircraft easier for the less experienced pilots. And it worked very well. Few people realize how big an achievement that is.

I am looking forward to the release date so I can start playing with the openpilot. Who knows, maybe the built in mag sensors will work as is and I don't have to get a remote mag sensor, it's nice to know the option is there if needed. I think that's why this project is so cool, it's completely open and can be modified in multiple ways. That's what makes it a clear winner when compared to other projects out there.

You guys are doing a great job and I am keeping an eye on the forums on a daily basis, I am very impressed with the amount of talent this team has gathered in one place.

[dankers]

Quote

William is a great guy, together with Paul Bizzard, he was the first to  come up with a viable AHRS stabilization solution for RC aircraft

As much as I admire the work of both Paul and William, I very much doubt they would claim that themselves as it is simply not true. Paparazzi had both a KF and an EKF as part of the Booz project long before DCM was flying.

Quote

Who knows, maybe the built in mag sensors will work as is

I know, it will work perfectly, the people here actually know what they are doing and did a lot of research and testing, research and testing that has been used to bring the whole small UAV community forward and we have not even released anything yet. It would be good to look at the Booz IMU which also has an integrated Mag and has been flying Quads for over two years, there are likely others as well. I am very sure the separate Mag on the Ardu was a commercial rather than technical choice, especially when the guy you mentioned took a year and half to do an Xbee breakout board and was proud of this

If you want something close to comparable to the OpenPilot AHRS that is available now, you should look at the Vector Nav VN-100 apart from they use a lower Spec Micro-controller than us, use ST Gyros that don't work with vibration, is closed source and costs $500. They are also a company that is keeping a close eye on OpenPilot, I'm not surprised as a non-profit UAV platform is very disruptive but of course in a good way for end users.

[OlivierD.]

Great stuff, that's reassuring, less wires to worry about too. Thanks all for the feedback.