Re: [Paparazzi-devel] airspeed_adc with Ardupilot airspeed sensor
From:
Hector Garcia de Marina
Subject:
Re: [Paparazzi-devel] airspeed_adc with Ardupilot airspeed sensor
Date:
Wed, 11 Jul 2012 15:52:46 +0200
On Wed, Jul 11, 2012 at 3:39 PM, Tilman Baumann <address@hidden> wrote:
Please allow me to take this idea and
run with it towards the direction my vivid fantasy has been
occupied with recently...
AOA does not seem to be a information that is really used in ppz,
except for some interesting experiments.
Instead of mechanical fins, AOA information can be gained by using
pressure tubes pointing in different direction. 3D pitod tube in a
way. :)
This is not really a trivial task, a FADS system is really complicated. However, AoA is always an important variable and let you
fly in an optimal way (max lift min drag), prevent stall, etc.
Agree. There are from Honeywell really nice sensors for dynamic pressure at low speeds, 3.3V, small package, analog or digital interface... but be prepare for spending about 80€.
And to address the terrible wiring situation we probably all have.
How about decentralized actor/sensor boards?
I know, the wiring is quite manageable, but your wiring harness
still gets messy very quickly if you for example want your wings
detachable. (I have the airspeed sensor in the wing. Also servos)
And the latent lack of ADC inputs. At least with Lisa/m.
How about running a CAN link to little sensor/actor boards to
where the action is. This way we would never run out of IO and the
wiring would be a nice bus. Imagine only having to unplug RJ45
plugs.
Can is message based. We can probably reuse the telemetry
format...
As always, depends on the size of your plane :P. But yes, some times, having the magnetometer away from any power source is a challenge, specially when it is mounted with the rest of sensors.
I know I took that ball and threw it way out of the ball park.
Especially considering that I still have to do a single successful
flight and so many more important problems.
But that wiring and how the plugs are organized on Lisa is a
constant annoyance to me. I see a need there.
As for baro board. Static pressure seems to be well solved so far.
Or do some autopilot or IMU boards not have one of those tiny ones
on board?
For dynamic pressure, yea, we could do something just like the
eagletree board. With digital output. I like that.
The eagletree board is too big for me to fit it into the wing.
I do not think it is well solved, employing a sensor which can measure more than 40km of altitude, when most of flights are below 5km, means that
we are wasting a lot of potential information. But it is the price that you have to pay for using digital sensors, you can not fit the analog signal according
to your requirements.
Héctor
On 11/07/12 14:02, Hector Garcia de Marina wrote:
I was always thinking about a dedicated board for
pressure.
We usually (I would say almost all of us) fly at low
air-speed (less than 150km/h?) and at low altitude (less than
5km?).
So it makes no sense using the whole range of the static
pressure sensors (up to 100m Bars?) or the whole dynamic
pressure (up to 5 kPa?).
Taking advantage of that, the board can have a dynamic
virtual ground for static pressure (more resolution depending on
your altitude and can be controlled by the DAC of the micro,
lets say each 500m, accuracy of 0.1m),
also, we can use this signal and make the derivative in an
analog way (climb rate sensor), and the same can be made for the
dynamic pressure.
Any ideas of what would be necessary in this kind of
"pressure sensor board"?
Héctor
On Wed, Jul 11, 2012 at 2:53 PM, Hector
Garcia de Marina <address@hidden> wrote:
Hello
Tilman,
you do not have to worry so much about it. Do you know
something about instrumentation?
You can always attach an operational amplifier and play
with the virtual ground (in this case at 2.5V) and a
static feedback gain for increasing the resolution in your
area of interest (low air speed).
I hooked up the oscilloscope and gave it a test.
The ripple is low, not more than I had on the
power supply. Noise seems fine to me. But I'm not
a expert.
The first really weird thing is, that zero is at
2.5V. Negative pressure is from 2.5 to 0 Volt.
(WTF?) And positive windspeed is 2.5 to 5V.
That is bad in two ways. First, for the ADC I
would rather have 3.3V[max]. And what is the deal
with negative pressure?
And also, there is very little response to actual
wind. My relatively powerful desk fan does not
even create a recognizable response. Blowing at it
does, but only very little.
I would guess that you need insane speeds to
saturate this sensor.
To increase accuracy I would have to remove that
2.5V offset. Which would also move me in the range
of a 3.3V ADC.
I could hook up a op-amp with one leg on a 2.5V
reference. But my wiring is already insane, don't
want to make it worse.
I would have to sacrifice LED to get a usable ADC
already. I'm beginning to really dislike that
whole idea...
Damn. The Eagletree sensor was not that much more
expensive. And digital. And calibrated.
Kids, don't try this at home I would say. It's not
worth it.