On Thu, Nov 12, 2009 at 9:27 PM, Elden Crom <
address@hidden> wrote:
>
> Joe, your bumming me out on the altimeter route to precision landing....... Painfully you make a convencing argument....
>
> Perhaps another route. No cammera, only light intensities needed.
>
> Do you know how a/c land on an aircraft carrier (old style anyway)?
> One of the systems that they use is to have a set of light and Fresnel lens on the deck of the carrier.
> The pilot looks at the lights (the ball), if they are on the correct glide slope it appears green, too low or too high, left or right, it appears to them as a diffeent color. (not exactly right but for my purposes....)
> (For a better explaination
http://everything2.com/title/How+to+land+a+jet+plane+on+an+aircraft+carrier
> ,
http://answers.yahoo.com/question/index?qid=20071105145150AAYcwq8)
>
> Anyway,
> Perhaps a simmilar system with a box of lights on the ground and 6 light sensors on the A/C. Three colors for up and down. Threee colors for left and right. (when I say 'colors of lights' I proabably mean different frequencies of infrared.)
>
> Therory of Operation (only describing up and down for now,left and right will be similar):
> The box of lights shines 3 beams of light. The beams diverge at ~3degrees.
> The too high light is red.
> The too low light is blue.
> The just right light is green.
> X RRRRRRRR
> X RRRRRRRR
> X AC RRRRRRRR
> X GGGGGG RRRRRRRR
> X GGGGGG RRRRRRRR
> X GGGGGG RRRRRRRR
> X BBBBBBBBB GGGGGG RRRRRRRR
> X BBBBBBBBB GGGGGG RRRR[----Box---]
> X BBBBBBBBB GGGGGG[----of----]
> X BBBBBBBBB[--Lights--]
> The A/C approachs the landing site via GPS (+/-20meters=GPS + flight error?) at ~1 kilometer out the A/C sees the green only.
> The navigation system swithes to moth mode (ie follow the lights). If it sees red, more elevator; if it sees blue, less elevator; if it sees green, do nothing. As the thing gets closer to the box, it will see pairs and maybe all three colors because of bleed. So basically just fly the plane such that Red and Blue have the same intensity.
>
> Come to think about it, the desire for the Green is more a human thing...perhaps just Red and Blue and fly the path that keeps them the same intensity. (This will be all sorts of fun getting the PID loop constants adjusted!)
>
> Back into 3D space,
> So that would just require, 4 light sensors with different color filters on them to be on the AC. Less than 20 grams? They would all be aligned the same (downward at about 3 degrees) so they could be flat on one PCB. The box of lights could just be 4 flash lights with lens....maybe even 1 flash lights with a four color lens.
>
> Or even better, one ground light with a lens that is in thirds and only 3 light sensors on the AC.
>
> It might even (maybe, perhaps, questionably?) be possible with just one color, but now the AC would be constantly drifting left/right and up/down trying to cross the highest intensity portion of the beam, kinda like WiMax (WiFi's bigger bother) does in the time domain......but the SW algoithm gets a bit compilcated!
>
> Enough babling for now.....
>
> Elden
>
>
>
>
>
>
> -----Original Message-----
> From: gisela.noci [mailto:
address@hidden]
> Sent: Wednesday, November 11, 2009 12:58 PM
> To:
address@hidden
> Subject: RE: [Paparazzi-devel] distance measurement for landing.
>
> 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: paparazzi-devel-bounces+gisela.noci=
ate-international.com@
nongnu.org
> [mailto:
paparazzi-devel-bounces+gisela.noci=
ate-international.com@
nongnu.org
> ] 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: paparazzi-devel-bounces+gisela.noci=
ate-international.com@
nongnu.org
> [mailto:
paparazzi-devel-bounces+gisela.noci=
ate-international.com@
nongnu.org
> ] 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:
>> paparazzi-devel-bounces+gisela.noci=
ate-international.com@
nongnu.org
>>
> [mailto:
paparazzi-devel-bounces+gisela.noci=
ate-international.com@
nongnu.org
>> ] 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
>>
>>
>>
>>
>>
>>
>>
>>
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