Effects of frame design on barometer performance ( and all other non 3.3 specific AC questions)

According to Randy the know issue of altitude loss after a high speed run is at its heart a frame issue.

This seems to me a very worthwhile discussion to have.

Once the majority of us understand this, we will be less likely to muck up other threads.

Take it away Randy...

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Randy,

Any info you can provide on the external baro requirements? Does the EKF fuse the two baro sensors, or do you need to disable one in the code?

Mike,

After a quick check with Tridge, the easiest way would be to:

1. buy an I2C based MS5611 baro (probably like this one)

2. make a cable to plug it into the Pixhawk's I2C port correctly (see this wiki page)

3. load up the "latest" Copter firmware by pressing Ctrl-Q on the MP's install firmware page (sorry, dual baro's not supported in Copter-3.3, but will be in Copter-3.4 which will begin beta testing later this month)

4. set the GND_PRIMARY to "1" to use the external baro (switching it back to "0" will make it use the internal baro which uses SPI by the way).

Ah, you might have to be careful about the voltages supplied to the baro.  The Pixhawk's I2C port provides 5V power although the signal levels are all 3.3V.  So just need to make sure that the external baro board has a regulator that can accept 5V for it's power source.

This is great, thx !.I always got in trouble covering the internal baro w/o covering its "hot" neighbours as well, so this is a fine alternative.

Btw. The board you linked to, is called GY-63 and its coming with a low drop LDO advertised for 3-5V operation.

I thought I had seen a reference to something like that. External baro would be trey cool.

I have yet to prove how well my implementation actually works. CFD on the Gill inlet is coming along. Many irons in the fire right now. None of them getting hot very fast. 

BTW - I'm really liking this dialog. Lots of good ideas and solutions.

I have sort of the same problem on my own boards (used in turbine powered modelplanes). I have a MS5611 on a board, and sometimes I see altitude drop because of pressure buildup inside the airframe (depends on the type of plane etc). I had a friend design a "small thing" that fits over the MS5611/5607 and also MPL3115 so it is easy to attach a 3mm Festo tubing (to "Y" connect it to the static line from the Pitot)

Maybe someone can use the design, included stl file in the zip file so you can have your own made

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OK here is one for you.

I have just been out testing a rig that is sealed. It is actually at positive pressure as there is a powerful fan sucking air in. the air intake is higher then the venting. 

It dawned on me just now. It held altitude well! 

Is the Barometer that sensitive it can still distinguish a change in altitude even when under high pressure.

not sure what the PSI is but it blows the lid off when you undo the catches.

wow that's very interesting. Could you provide a picture of your set up? What did you use for the fan? I have a feeling the size of that opening / fan and it's location is the key. Did you try the same set up with the fan off to compare? Thx

Came across this thread and now finally enlightened as to why my copter suddenly lost altitude on Position Hold mode. But how come Marco Robustini's demo of POS hold doesn't have this problem? He was using a F450 frame I believe. So, was it the frame design of the F450 or some mod Marco did on his FC? Just curious.

Thanks

I finally have *something* to report. I don't yet have flight logs that are worth anything, but I can report that it appears that my current mod is rather inconsistent in performance. While early, non-scientific tests, and early flights looked promising, I can't consistently produce good Alt. Hold performance. I'm going to try some other locations for the inlet and see how that goes.

In other news, I did get some simulation results from the Gill inlet model. The first round was not perfect, but really close. You can see in the pics that the pressure is close to neutral and the velocity is very low at the inlet's center port. I made a couple of tweaks to the model based on educated guesses, and I'll see about getting one or more 3D printed on SLS, with glass-filled nylon. The small geometry and aerodynamic requirements of this part make it a poor candidate for FDM, so it will be a bit more expensive.

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I think we are fundamentally wrong in the idea that solution to the alt hold problem is mechanical. I've just stripped a naza lite with v2 firmware off its casing and used a piece of tape to secure foam over baro, guess what copter flies in a straight line just like with its case on!  We gotta think outside the box with this... There is obviously a software solution to this problem.  

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