RE: [Paparazzi-devel] distance measurement for landing.

[gisela.noci]

What 'Baro' sensor?

The maths does not lie.

We cannot generalize when categorizing sensors in this class of performance.

 The picture I painted below is typical of a fairly good quality MEMS type
airdata sensor. For example, the Honeywell ASDX015 is a 0 to 103.4kPa
sensor, fully temp compensated from 0 to 85degC with 4V output span, and
2%/V accuracy. This means that 4V output covers the full 10,000m altitude
pressure range range and that means 4/10000 = 400uV per meter. To achieve
14cm, you need to be able to sample around 50uV. If you have ever tried this
it is not easy, esp in the noisy environment of RF modems and video
transmitters, etc

In addition, the inherent sensor accuracy is typically in the order of 1 to
2% per volt, ie, at sea level, lets say at a sensor o/p of 1volt  the error
could be 10mv, or 25 meters!!   

One can amplify the sensors output, and offset the output, to generate a
large sensor voltage change for a smaller altitude change, thereby reducing
the number of sampling bits of the A/D, althought then the sensor range is
limited. This is often done, since most of our applications do not go to
10,000meters,(I suspect this is what they have done in the sensor in the
quadrotor) .....But, there is no free lunch here - you have to use
amplifiers that have very low input and output noise, very low DC offset,
and good temp stability. If you have access to a high accuracy pressure
calibration system, you could put the sensor pack in the environmental
chamber, and cycle the temp from 0 to whatever, and vary the pressure to the
sensor at each temp, and generate a calibration curve to eliminate all the
errors,ie, temp, offset, drift, etc but the cost.....(takes many hours to
do) 

In essence, obtaining repeatability, high accuracy, and temp stability, to
achieve centimetric pressure alt accuracy, is what $10k sensors are all
about. 

We do this type of work at our company ( we are a UAV manufacturer) and all
this come from painfull experience..

Have fun all
Joe
PS - I could not access the sensor referenced by Eldon - seems the only one
I find is a 1meter 'accuracy', and 400m or so range??


--Original Message-----
From: address@hidden
[mailto:address@hidden
] On Behalf Of antoine drouin
Sent: Monday, November 09, 2009 10:48 PM
To: address@hidden
Subject: Re: [Paparazzi-devel] distance measurement for landing.

this is done with a baro and a 10 bits ADC

http://poinix.org/video/booz2_vert_ctl_v0.2_perturb.mp4

On Mon, Nov 9, 2009 at 9:31 PM, gisela.noci
<address@hidden> wrote:
> Baro_altitude sensors can certainly measure to that kind of accuracy. The
> difficulty arises in digitizing the output ( via an A/D convertor) and
> ensuring that the noise on the signal is very low. Some typical sensors
give
> a 0-4v output for a pressure variation from 1013millibar (close to sea
> level) to around 200millibar ( a difference of 813mb, 81kPa, about
> 8000meters alt change). This means to achieve 14cm resolution, you need
> 8000m/0.014m = 57000 bits, or increments. This implies at least a 16bit
A/D,
> and then remember that each bit is 4v/57000 = 70uV !!!   Couple this with
> the sensor output variation over temp ( not insignificant!) and the noise
on
> the signal, you will be VERY lucky to get 1meter accuracy, and that at a
> specific temp.
>
> Ask any real pilot - land with your eyes, not your altimeter!!!
>
> Ground ranging (Radio-Altimeter, radar, ultrasound, etc is the only way to
> go.
>
> Joe
>
> -----Original Message-----
> From: address@hidden
>
[mailto:address@hidden
> ] On Behalf Of Elden Crom
> Sent: Monday, November 09, 2009 7:57 PM
> To: address@hidden
> Subject: RE: [Paparazzi-devel] distance measurement for landing.
>
>
> I was considering the pressure/altitude route
>
>
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6776&Prod
> uct_Name=WS_HowHigh_Altimeter_Feet_&_Meter_(New_Version)
>
> But I think this may require one on the ground station as well to deal
with
> a cold front coming in and changing the barometric pressure.
>
> Supposedly, this thing can measure altitude accurately to 14cm.
> Does anybody believe that? (I have on one order to see if it's actually
that
> accurate)
> I wonder how to mount it such that it is not affected by wind speed.
>
>
>
>
> -----Original Message-----
> From: Dirk-Willem van Gulik [mailto:address@hidden
> Sent: Fri 11/6/2009 1:04 PM
> To: address@hidden
> Subject: Re: [Paparazzi-devel] distance measurement for landing.
>
> Roman Krashanitsa wrote:
>
>> The http://www.maxbotix.com/ founder did some comparison for performance
>> of his sensors with Sharp sensors you are linking to. As far as I
>> remember, there are some plots in the FAQ section and in "Preformance
>> Data" section that might be useful for you.
>
> I found that even very simple DIY shop ultrasonic meters; like below:
>
>
http://www.sella.co.nz/general/building-renovation/tools/other-tools/503tx7/
>
> which can be had for 5 to 10 euro's at the local DIY market - are easily
> hacked and not that unreliable.
>
>
> Thanks,
>
> Dw
>
>
>
>
>
>
>
>
> _______________________________________________
> Paparazzi-devel mailing list
> address@hidden
> http://lists.nongnu.org/mailman/listinfo/paparazzi-devel
>


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