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Re: [Paparazzi-devel] airspeed_adc with Ardupilot airspeed sensor
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From: |
Hector Garcia de Marina |
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Subject: |
Re: [Paparazzi-devel] airspeed_adc with Ardupilot airspeed sensor |
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Date: |
Wed, 11 Jul 2012 15:02:32 +0200 |
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).
Here is a useful link for that:
www.ti.com/lit/an/sboa092a/sboa092a.pdf
On Wed, Jul 11, 2012 at 2:37 PM, Tilman Baumann
<address@hidden> wrote:
On 02/07/12 15:56, Gareth Roberts wrote:
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.
--
Héctor
--
Héctor