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

[gisela.noci]

Hi Eldon.

Ok, had a look on the web site. Pity they do not show the other side of the
PCB....Anyway, as I intimated, the only way they can achieve 140mm
resolution is by amplifying the sensor, and limiting the range( as they say,
from 50 to approx 2000meters)  to squash more bits per meter in. They
certainly cannot have a 16 to 18bit A/D on that little PCB! The lower 4 to
5bits would be in the noise... And, most important, they talk of resolution,
which is fine for maintaining an 'altitude' whatever that might be , but
they do not mention the units accuracy and that is what is important in
trying to use it for AGL determination for autoland. There is simply no unit
accurate enough, and repeatable enough, to be able to use for autoland. In
addition, the atmospheric model used for pressure- altitude calculation
varies all over the world...

That is why Pilots are told by control tower to set their altimeter to a
specific 'altitude' in that towers controlled airspace. It might not be an
accurate altitude, but at least all other aircraft in the same airspace are
referencing the same 'inaccurate' altitude. Resolution is important, to
insure you do not drift up or down without knowing it, but accuracy is so
poor, that it is not used in absolute terms.

It seems that sensor is 'read' by observing the led flash count, in which
case the output rate is going to be VERY slow - seconds - rendering it
useless even for altitude control.  

Our autopilot flies to pressure altitude, which is augmented in a kalman
filter against GPS Alt to get some semblance of reasonable altitude. The
sensor sample rate is at 60Hz, while the GPS is at 4Hz. The pressure Alt is
used to determine a climb/descent rate, to feed the pitch/altitude loops.
So, sampling at a rapid rate is essential for good altitude control
response. 

However, as to what sample rate is best for landing by means of pressure alt
sensor, all I can say is - Don't.... The only way to land with a pressure
sensor as your only reference is to sample  at least at a 50Hz rate, and to
use that to derive a descent rate and then to approach from a long way,
descend to  a 'safe' altitude, say 30 or 40 meters AGL at least, and then
just keep wings level, speed constant at 3 or 4meters/second above stall,
and fly in at a controlled descent rate of around 1 meter/second, and wait
till you reach terra-firma. It works, I have done this a number of times,
but you need space, lots of it. For example, at 16meter/second approach,
with a 1m/sec descent rate you will cover 40X16 meters = 640 meters. If the
accuracy of the 'safe' altitude was such that is was actually 50 meters AGL,
then you will cover 800meters......No magic bullet here, I fear. A 1/4 mile
is to short for the above scenario; no room for error. If your approach
speed is 5meters/sec then maybe, but at that speed, just catch it in your
hand as it passes by......

I am not sure what you mean by not wanting to use 'radar' because of sloping
terrain; that is in fact the best reason to use sonar/radar, and then to fly
at a fixed height above ground,ie, terrain following flight, and then
descend when at the correct point, reducing height AGL at 1meter/second,
flying an actual trajectory to the touchdown point. That is what we do, and
we touch down at 20m/s ( a HEAVY plane!!!), within plus/minus 15meters each
time of the designated touchdown point. That requires better than 1 second
accuracy in descent rate, speed and pitch control.

We do the same on a much bigger plane, 6meter wingspan and 145kg all up
weight.. 

And if you want to use radar to detect the height AGL of the flat patch,
just use it to land with......

I have watched a $300K UAV with a infrared camera in the nose try to land
3meters below ground level, at 4meters/sec descent rate - no more landing
gear and droopy wing resulted.......

But, don’t give up. If you really want to try to do landing with a pressure
sensor reference, I would suggest that you use a sensor such as the
Honeywell ASDX015, a 4volt output for 100 to 1013millibar sensor that is
temp compensated pretty well, amplify and offset the output to give you good
resolution for your desired altitude range, make 2 identical units, keep one
on ground and use any change on that one to correct the airborne one. This
should give you a short term accuracy in the 1 to 3 meters range, and then
land by approaching to 5 or so meters AGL, and then wings level, kill
throttle and glide in. Works (almost) every time ..

Happy landings

 Joe

-----Original Message-----
From: address@hidden
[mailto:address@hidden
] On Behalf Of Elden Crom
Sent: Wednesday, November 11, 2009 7:44 PM
To: address@hidden
Subject: RE: [Paparazzi-devel] distance measurement for landing.



Thanks Joe, 

The link must have clipped, the long way is 
http://www.hobbycity.com
On the left side choose DataRecording
Then ShadowWing
Then How High

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=6776&Prod
uct_Name=WS_HowHigh_Altimeter_Feet_&_Meter_(New_Version)
 
I just go it in the mail, but I haven't had time to play with it much yet.
The output is already digital and they taught me something.
(off topic) LEDs work in reverse! (same band gaps as a solar cell, I should
have already know this) The output led also a light sensor.  Try it
yourself, just hook up a digital meter (ie HiZ) and shine a light at a
random LED in your junk drawer, I got about 0.5 volt with a red LED and a
flash light.

Anyway, they also have a 'reader', so I'm hoping to have TWOG read the
already digitized data (from what I'm hearing, it will need to be as farway
from the antennas as possible!)

Another thing I don't know is how slow the reading will be. (I'm guessing
that after internal filtering it will be slowish, like 1 sec or worse, I've
got my fingers crossed for 100milli-sec)

Any idea what sampling rate I must have to be able to use it as landing
data?  If I have a long approach (1/4 mile) does this sound plausable?  My
issue for not using a radar/sonar range finder is very slopped terrain with
a smallish flat spot to land on.


My worst case may be a radar/sonar first pass over flat spot to adjust the
altemeter and a second pass to actually land.


-----Original Message-----
From: gisela.noci [mailto:address@hidden 
Sent: Tuesday, November 10, 2009 12:27 AM
To: address@hidden
Subject: RE: [Paparazzi-devel] distance measurement for landing.

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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