Some of you may have noticed that your multi-copter will lose from 1.5 to 3 or 4 meters at the end of a horizontal run when in any of the Altitude Stabilized modes, like Alt Hold, Loiter (and PosHold for 3.2 candidates) or Auto.
Several heated debates have considered the reasons and possible causes, until finally a more demonstrative test was undertaken, using the 3DR X8 above with Pixhawk
But first some ground data:
The following graph shows how the altitude influence is directly affected by velocity (especially forward velocity), but to a lesser extent latteral velocity.
A general conclusion is that a "pressure bubble" forms in front of a moving object:
This is an image of a pressure gradient around a sphere moving towards the left of the image. This gradient is greater or less depending on the physical configuration of the vehicle. The vehicles that are most effected by the altitude drop tend to be less streamlined, with greater frontal area, and therefore a greater pressure buildup.
As the vehicle accelerates, the pressure starts to rise slowly and the forward tilt of the vehicle moves the higher pressure area more directly into contact with the altitude sensor. Then with sudden slowing, quick pitch changes and dropping pressure simulates that the vehicle is rising. Altitude controllers react to counter the effect and the vehicle drops..... sometimes like a rock.
So whereas the debate postulated that software should be able to resolve this problem the problem remains that, if it's really true that the cause of the problem is the "pressure bubble", then the software can only react to what it "sees and feels". If you make the assumption that it is possible to generate a countering force at the end of forward travel, you may end up sending the vehicle into space if the wind is from behind. The processor doesn't know any more than it can "feel".
So I built the testbed above to test the theory. You'll see a tube coming out of the front of a Pixhawk, just about where the altitude sensor is. There's a hole in the Pixhawk cover, and the altitude sensor is sealed with Plastecine, so that the outside air pressure from the "static sensor" is fed directly into the altitude sensor.
The white plastic rectangle on the other end of the tube has a tiny hole (not very visible in this photo, but about 1.5mm diameter) drilled from side to side, and then another hole of the same diameter leads from those holes to the tube. In this way, forward movement does not exert any pressure into the tube, furthermore, as the vehicle pitches forward, the static sensor holes move down into a lower pressure area outside the bubble. Pitching back up, if anything the static holes move into a slightly higher pressure area. the end result is ZERO DROP in altitude.
Lateral movement has no effect, as the side to side holes let flow pass though unrestricted and no pressure gain is seen.
This baby flies as flat as a pancake!
Comments
This baro is sensitive to light. It's better to cover it with some black material instead of leaving as it is..
I put a small piece of sponge inside my tube, in an attempt to help smooth out any "noise". Don't know if it does anything or not, haven't tried it without.
Looks good JoeBob!
"Yes, F over your sensor on rancesco, Phanivyas"
should be
"Yes, Francesco, Phanivyas"... :p
Yes, F over your sensor on rancesco, Phanivyas.
It's more the repositioning of the sensor point outside the pressure bubble. In Phanivyas' case, He moved the whole autopilot. The foam helps as a "higher frequency" filter, but just putting the foam over your sensor on the autopilot without changing the position of the autopilot won't do too much.
One person tried combining a dome and foam. Neither one worked by itself, he had to do both.
From my personal experience mounting the flight controller on top of the frame gave me good flight performance rather than mounting it inside. As far as the foam is concerned I have used 3 layers of black double sided tape cut into a square shape 1"x1" with a small hole in the middle & stick it over baro and placed some cotton into that hole. I threw away the piece of foam which came along with APM (such a dumb thing to do) so I'm using this currently.
@Randy, works well in ALL directions.
The "sensor interface plate" exposed to the outside air has a small hole drilled side to side, the idea being that rather than build "ram" pressure with latteral movement, the air passes freely from one side to the other. The static tube takes a pressure point off of the center of the small hole/tube running between the two holes.
This is a top view of the "sensor". left is front.
I have already attempted the same thing with my APM 2.6 quad 2 months ago. I have placed a silicone fuel tube (used for rc glow engines) on top of baro sensor and sealed it completely with hot glue so that the airflow to the baro is from the tube only. The APM was inside a plastic case installed inside F450 frame and other end of the tube was installed onto the GPS mast with zip tie. I have attached a sample image (see yellow arrow in pic). I could notice the difference, the copter was holding altitude good but there is altitude drop when large amount of roll or pitch is applied. Finally I have moved APM to the top plate (removed all tubing & hot glue, covered baro with foam), now it works well than before.
Apologies for the low quality image, it was taken from a crappy mobile phone.
Does it perform ok when rolling as well?
I guess with a smart phone I could show what it's doing now. I'd have to see how the vibrations/jello are. Visually the sink isn't as obvious from the camara as it is watching from outside.
I'll see if I can find a piece of video that shows the sink at the end of a run from my bigger octo. I might have one where it drops about 3 meters and comes quite close to the ground.
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