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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 14:53:41 +0200

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:
Hi Tilman,

It's the same sensor as the ardupilot classic - it's not great.
http://cache.freescale.com/files/sensors/doc/data_sheet/MPXV7002.pdf?pspll=1

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.


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Héctor



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