M4 Ladybrid Position Hold and PH controller Questions

[bn999]

Mr OlliW,

Looks to me you are one to not give up in pursuit of answers to questions. This is familiar to me as I tend to work the same way. First thing you need to know, is that there are no hard specifications. When you ask how much deviation is allowed, the answer is simply "the less, the better." Perhaps not what you want to hear, but it is the truth.

Watching your video, I'm fairly certain you do not have the GPS reception necessary to perform a tight position hold. This first shows up with vertical position uncertainty. And, since vertical position is mostly determined by satellites at a perpendicular angle to your local vertical, positioning yourself between two houses only works against you.

Next, the antenna that you are using is not going to be anywhere as effective as an active ceramic patch. I know for a small machine, carrying such a burden is a problem, but it is necessary for any sort of precision. I'm not saying that you can not get the dipole antenna to work to some degree, but it is far from ideal. A 35mm, active ceramic patch antenna with the proper ground plane will allow an M4 FC to perform to 10cm precision. Yes, just the antenna would weigh more than the entire gross weight of the craft, but that's the problem. I've used 10mm active ceramic patches with fairly good results, but far from perfect.

Then, to truly diagnose the situation, you are going to need a log of the flight. Due to its small size, the M4 does not have an onboard uSD interface. You would need an expansion board attached to achieve this. The logged data should paint a picture which show exactly what is going on. You show a <0.5m HACC while static (which I believe) but I don't know what is being reported during the course of the actual flight.

For the altitude hold, there are now two modes automatically switched between depending on the reported GPS accuracy. The primary is dominated by the GPS's reported vertical velocities. The second is a backup used when there is "far less than ideal" GPS data and relies entirely on the barometer, ACCs and estimated attitude - mainly used for indoors.

As to the questions about the MAG sensors, if you have run through the onboard calibration routines, your MAGs are probably calibrated well enough to resolve to better than 5 degrees from actual. If you have good GPS signal, the system will refine this as time goes on. In your video, I do not see any evidence that your estimated heading is causing you problems.

Let's all keep in mind that the M4 in a LadyBird configuration is less than 40 grams. It is amazing to me that such a setup is able to have any autonomous capabilities, much less precision. Please do not let this tarnish the true strength of the AQ firmware which with proper configuration, can perform incredibly well, even with the tiny M4 FC. The M4 in a native configuration is extremely fun to fly, but other than the novelty of the fact that it has an autonomous mode, is really just a toy because the necessary antennas and shielding simply weight too much. The idea is that this "toy" is able to be upgraded and used in a serious application without compromise.

[OlliW]

[bn999]

[OlliW]

[bn999]

[OlliW]

now things start to clear up significantly in my head, pieces fall together and the observed behavior starts to make sense ... hopefully weather will allow some tests tomorrow

there is then obviously room for improvement in the code, instead of a hysteresis a smooth crossover function would likely do a much better job ... I guess I should see if I can get the code compiled LOL

thanks again very much, this was very helpful
I'll report my new findings

[bn999]

What has not been completely clear to me is how much our traditional GPS shield acting as a ground plane vs acting as a EMI shield contributes to its success. In the case of a passive dipole, you have neither.

An interesting test would be to surround the entire M4 board in tin foil, leaving only the dipole sticking through. Of course, you would need to cover it in something non-conductive first. This should cut down on the radiated EMI significantly. If something like that works, it might suggest that there would be a benefit to some sort of light weight RF opaque canopy or housing for the board.

[Kisssys]

Hi Olliw,

I wrote a long post today and when I took a phone call it all dropped out of the editor so I'll try again. I would doubt if anyone as spent more time than I have on the M4 and it's use on dozen's of different frames. From my experience this is what I've found IMHO.

It will in fact do exactly what everyone has described and all of their fixes are dead on. But if we've led you to believe that with that dipole antenna you are always going to get good gps hold we have been amiss IMHO. If you went out to a open field with full view of the sky, and if you allow the M4 to get it's accuracy down to less than 1 meter, and if you arrive at a good compass heading before entering position hold, and if you are flying during the time of day with optimum satellite position you will get outstanding position hold with that very very very poor antenna. Lot's of if's.

The dipole is used because it's light enough for the Ladybird to carry and let's people play with position hold which is fun on a small machine. The antenna does not have circular polarization and is therefore more susceptible to multipath signals and it is as able to pick up signals that come from the M4, BT or reflecting off of the ground as easily as it picks up signals from the sky. It is very hard to tune an antenna at those frequency's so it's probably not on frequency. Just the plastic or glue one uses can throw it off. The little patch antenna's that we sometimes use can be bought in packs of 10 or 20 each with a slightly different frequency. The idea being you try each one to see which one shifts to the correct frequency in the environment that it has around it. The point being is it's hard to make a good antenna installation even if you have the best equipment to work with. The fact the dipole or little patch or whatever you use on a small machine works at all is amazing and is a statement as to how good the onboard gps receiver is.

It would be absolutely impossible for you to judge what the M4 can do without giving it a good antenna system and flying it in a suitable location. I would encourage you to compile the code and make contributions but I would suggest flying it on a full size machine with a good ground plane and patch antenna before making judgement.

Cheers
Steve

[kinderkram]

Yah, Steve has done a lot of tests with ALL kinds of GPS modules and antennas, groundplanes, shields, hoods, whatever.

And he had proven that you´ll be getting a perfect Hacc of 10cm with an active 35mm patch antenna.
So it´s not the M8Q module - it´s the best we ever had.

GPS antennas is still in discussion and Larry has posted an interesting one here:
viewtopic.php?f=40&t=3975&start=10#p29434

[OlliW]

thanks a lot for these usefull thoughts!

I'm not sure at which point but somewhere a slight missunderstanding seems to have creeped in. I'm not doubting the M4's cababilities in any way, I in fact believed and continue to believe that this is just an amazing piece of hardware. In whatever I'm saying I'm referring ONLY to it installed on a ladybird with a dipole antenna.

I'm following the GPS antenna thread with much interest