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

[gisela.noci]

Hi Chris.

No, I have not tried the Xl series, but speaking from experience, I do not
believe that they will do the job. Having said that I guess I should
quantify a little...

I designed a ultrasonic ranger using dual sensors, with very good signal
processing. The Tx transducer is also driven at 40v Peak to Peak. All the
16mm diameter open face transducers typically have a max AC drive allowable
of 40V PtP. This is simply not enough for UAV type applications. I reliably
range 3meters max, flying at 14meter/s to 28meter/s, and ranging over desert
sand and gravel. My autoland descent rate is around 3meters/s, flaring out
once I range the ground. 3m range height is to low. The energy in the 40v
PtP pulse is largely absorbed by the poor ground, and increasing the RX
transducer gain starts to cause problems with wind noise ( which is in the
ultrasonic range) and other vibration.

A good, safe autoland really requires 6 to 8 meters ground ranging, and the
way I have achieved that is to use 4 transducers on the TX side, pulsed in
phase @ 40v PtP, with a single RX transducer. I achieve 15meters targing
hard surfaces. I can go to 22meters easily, but I limit the range to allow
fast sample times for rapid altitude updates during the land. The sample
time is 50milliseconds, ie, a range of 8.5meters. ( time to target and back
=50ms,ie, 25ms to target, = 8.5meters)Targeting soft sand and gravel, as
well as short grass (100mm or less), I achieve the 8meters nicely. You need
to remember that the aircraft rolls (and pitches a little) on approach, and
so the beam is also reflected away from the receiving transducer when this
happens ( angle of incidence and reflection..) Using 4 TX transducers allows
me to 'misalign' the 4 a little, ie not mounted exactly on boresight axis,
so that during the roll, as one sensor goes out of 'optical' alignment,
another becomes a little more aligned.

Lots of trick needed to get truly reliable range data, with no mis-ranging
at the critical landing phase, which is only a second or so.

Happy Landings..
Joe

-----Original Message-----
From: address@hidden
[mailto:address@hidden
] On Behalf Of Chris
Sent: Tuesday, November 10, 2009 9:09 PM
To: address@hidden
Subject: [Paparazzi-devel] RE: distance measurement for landing. (Chris)

Hi.
I can also verify what has been said.
When i was using a baro for altitude in an another good airborne 
telemetry unit i got tired from altitude drift.
0 meters during take off were becoming ~5 meters after 30 minutes of flying.
Joe what do you think about the MAXSONAR's XL series (not the LV)?
Have you tried them also?
They state that the XL units use 40v and produce about 1/2 W of AF power.
Chris



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