Important note for APM 2.5 users: if you're not using an enclosure, cover your baro sensor!

One of the differences between APM 2.0 and APM 2.5 is that on APM 2.5 there is no daughterboard covering the barometric pressure sensor. So what? Well, if you've got your APM 2.5 in an enclosure of some sort, it's no big deal (the official enclosures will be out in a few weeks and will be included with APM 2.5 orders then; people who have received an APM 2.5 already are entitled to a free enclosure with their next order). But if you don't have have your APM 2.5 in an enclosure, the lack of daughterboard protecting the baro sensor from the windstream can wreck havoc with your altitude measurement!

I learned this the hard way this morning, when I mounted my new APM 2.5 on the top of my quad, out in the breeze. When I switched on Altitude Hold mode, it decided to head straight for the skies. A quick switch to manual to land, then an upload of logs to the dev team revealed the problem (thanks to Jason Short and Pat Hickey).  The vortex from my props was generating crazy readings with baro sensor. (This hadn't caused a problem on APM 2.0 because the daughterboard served as a wind shield.) 

Solution: a bit of cotton padding, taped on top of the sensor, as shown above. Back to air and all was well-rock-solid Alt Hold. Whew.  

So if you've got your APM 2.5 mounted out in the open, like I did, tape on a bit of cotton padding or a bit of paper tissue for now, until you get a proper enclosure. And remember: pressure sensors are, well, sensitive to pressure ;-) That includes wind of any sort. Shield your sensors!

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Comment by Stephen Carlson on August 30, 2012 at 12:58am

A related note: Some Baro Sensors have the MEMs element open to ambient light. Just having the light levels change, as in more or less sun pouring into the sensor port, is enough to cause errors of several meters. This may not be so much an issue now, but I know that in 2005 when Procerus was just getting started, they spent many hours trying to figure out why the plane would rise and fall through an orbit with pretty fair consistency. They found that the light striking the element inside was causing a charge buildup, thus throwing off the readings as the plane turned into and then away from the sun.

Comment by UAS_Pilot on August 30, 2012 at 6:15am

This answers so many questions. My quad kept trying to fly away to when swapped to Alt hold. Thanks for the information.

Comment by R. D. Starwalt on August 30, 2012 at 6:25am

I've got quad frame envy Chris, please tell me which one your 2.5 is on.

3D Robotics
Comment by Chris Anderson on August 30, 2012 at 7:53am

Nope, just the $17 Hobbyking frame. (Flamewheel clone) Works great!

Comment by LanMark on August 30, 2012 at 8:43am

Chris.. why is your quad missing screws where it connects the arms?

3D Robotics
Comment by Chris Anderson on August 30, 2012 at 9:26am

Mark: because the Hobbyking frame is poorly made and half the screw holes don't line up. But it works fine with just half of the screws. It's a pretty solid design. 

Comment by Marooned on August 30, 2012 at 10:36am

I also have Q450 frame but without built in PDB and all screws fit very well. One issue is that screws are made from soft metal and has small hex socket. Because of that it's easy to smooth this hex into "circle" making them unable to be removed. Be delicate to those screws.

Comment by Jack Crossfire on August 30, 2012 at 5:39pm

Not unlike this guy, who went overboard with it, 3 years ago:

Hanging the barometer under the fuselage, out of sunlight helped, but really a better position sensor is needed.

Comment by Rob_Lefebvre on August 31, 2012 at 6:23am

Holy cow, the screws don't go into the holes?!  Could you just drill the holes in the board out a bit?

3D Robotics
Comment by Chris Anderson on August 31, 2012 at 6:39am

Robert: No, they're threaded. But it's no big deal. The frame is rigid enough with just half the screws used. 


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