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Permalink Reply by Michael Pursifull on June 10, 2012 at 6:38pm

Odd. As a test, try creating a longer telemetry cable and placing the telemetry model farther away from the APM1. See if that makes any difference. 

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Permalink Reply by Rigel on June 10, 2012 at 9:26pm

No luck.  I've tried placing the XBee module as far as possible without touching the props and I still have the same problem.  The fix I had mentioned earlier with the USB cable doesn't work.  Actually, it only works for a short time and shortly after switching to wireless the system reverts to crazy altitude readings.  I tried flying it in altitude hold mode as well as loiter and the hexa is just jumping all over the place.

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Permalink Reply by Michael Pursifull on June 10, 2012 at 9:29pm

Did you cover the barometer? Do you see this erratic behavior only when the props are spinning?

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Permalink Reply by Rigel on June 10, 2012 at 9:37pm

I've had it covered and uncovered and I get the same problem in both instances.  The problem starts right away, even before the props are spinning.

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Permalink Reply by Michael Pursifull on June 10, 2012 at 9:58pm

well, values of +/- 1.5 are not atypical variations. But there are two things I can think of, other than air buffeting and radio interference, that can cause this which you might test.

Put your quad on your bed, with no one else on the bed, and try again. If it smooths out, then it was caused by vibrations. You might have vibrations up through your floor from a downstairs ceiling fan, or the AC unit, or some other mechanical vibrations. 

The other thing, I think the barometer is an analog sensor, which means it needs a reference voltage. If that reference voltage is not stable, the resulting values may not be stable. If the bed test doesn't settle things down, maybe your power source is not as stable as you might like?

Sorry, these are my best guesses...

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Permalink Reply by Michael Pursifull on June 10, 2012 at 10:03pm

Also, add "target_alt" to your chart, and you'll feel better about it, at least a little, I think. It doesn't change anything, but your line shouldn't look quite so bad ;)

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Permalink Reply by Rigel on June 10, 2012 at 10:57pm

Thanks for all the tips.  However there were no vibrations at all when booting since I've booted on a table outside, and on the ground.  Also if you take a look at the graph, we're not talking about + or -1.5m but closer to + or - 5m or more.  I've checked the input voltage at the board in the past and it looked pretty stable.  I'll double check though.  Any chance  the magnetometer could cause some sort of interference? 

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Permalink Reply by Michael Pursifull on June 10, 2012 at 11:12pm

RX, ESCs, Telemetry, VTX, yes. Sonar, maybe. Magnetometer, GPS, no.

I get +/- 1.5 when I put it on the floor. I get almost 0 when I put it on the bed. 

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Permalink Reply by Rigel on June 11, 2012 at 12:12am

Mike,

Thanks for all the help.  I finally figured it out.  My 900MHz Xbee was the cause of the problem.  I switched it with a 2.4GHz Xbee and VOILA! now it works just fine.

Too weird how the 900MHz would cause such a disturbance.  Too bad too since I'll be loosing range...oh well.

Permalink Reply by Chris Huitema on June 11, 2012 at 12:55am

I have the exact same problem, thats almost identical to my alt graph thismorning. logs show that ots the sonar, wirh very bad noise. seems to be crosses up with thr gyros or something because its relatively quick movements that make it worse, slowly tilting the quad does nothing and tapping the bars with ascrewdriver does nothing. ill post my log files when i get home. 

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Permalink Reply by Michael Pursifull on June 11, 2012 at 1:45am

Yours sounds like RF/magnetic interference too. The movement is moving the source of interference (Xbee radio field) across the conductors (i2c wires?) and this adds to the normal frequency changes, thus making the noise worse.... or that is what I would guess. Move your transmitters farther away, or unplug them (and use the USB) to test the theory. Then look at ways to isolate or shield your sensitive devices, move the antennas away, etc. 

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Permalink Reply by Michael Pursifull on June 11, 2012 at 1:42am

Yeah, you might not have to give up on the 900Mhz entirely. If you want to keep it, look at moving the RX farther way from the Xbee (sometimes these two interact and cause problems together.... I know, an RX is not supposed to transmit, but they do, sometimes, when near a VTX or Xbee.) and look at shielding some of the exposed cables.... it might be your sonar cable, or GPS cable or any cable (did you say you have a magnetometer? I believe that is i2c, and I believe the barometer is i2c also) that is getting a signal from the Xbee and causing fluctuations on the bus used by the barometer. I'd suspect the i2c devices and wires first.... Is your mag soldered to the board, or attached with a cable?

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