APM and Compass Calibration
Each time I've reconfigured my TRex500 with APM - equipment changes, different position for the APM, comapss and GPS etc., I've of course redone the compass calibration. But each time, although MP seemed satisfied with the new offsets calculated from rotating the heli, pointing the heli in different directions did not give very accurate headings on HUD. Far from it. Today I did a calibration on a new setup, and got the following offsets: -90 -3 -5
However, standing the heli on a small table mounted on a photo tripod and then rotating the heli to the 4 compass points gave:
N = 351.8
W = 275.9
S = 184.3
E = 80.0
So I once again resorted to some manual tweaking of the offsets, first I changed the Y-axis offset and measured the heading when the heli was pointed to magnetic N & S:
offset Y -3 -13 -20 -21
N= 351.8 354.5 356.7 357.1
S= 184.3 180.1 177.5 177.1
At this point - since both readings were now about 3° LESS than the true heading - I physically adjusted the compass on the boom - I mounted it on a platform which can be rotated a few degrees, then clamped. I rotated the compass a couple of degrees CW, and then readjusted the Y-offset to -27 which gave me N = 359.5 and S = 179.8
East and West headings were then very close, but changing the X-offset to -92 gave me E = 91 and W 270.8
I then inclined the heli 30° forward tilt whereupon W = 283.8
Changing the Z-offset to 70 gave the much better reading of 269.6
Previously with this method I then re-tweaked the settings by going through the procedure a second time, also inclining the heli on roll, to get even a slightly better accuracy. It seems to me the auto-calibration process can't even come close to these results. Comments?
Replies
Peter, Do you have a picture of where you compass is mounted? When the heli is on your small table is it in a house, porch, or are you in a open field. I only ask this because I have seen variations before from rebar in a concrete porch and in my house I get different headings depending on where in the house I am. I have a friends back yard that I take off and land from that causes a 15 degree error but the only flat area in his yard is close to the pool deck. I assume it is all the metal reinforcements.
Have you flown it on the original offsets?
Regards,
David R. Boulanger
Compass is about halfway out on the boom, CF plate, brass screws, Rx antenna there too but it is nonmagnetic. In previous trials I had compass in a couple of different spots as well, but altho the offsets were different, always the same deal: autocalibrate gives offsets that are not nearly as good for givi9ng correct headings as my method. The (aluminum, no magnetic parts) tripod with small table is outside the shop, quite in the open - the shop is a 200 yr old stone building, no rebar in those days! Even inside the shop a meter from a big bandsaw the readings aren't too bad once good offsets are found. I did the auto-calibrate in exactly the same spot as my tripod tweaks.