Blogs

Seeing Andrew Tridgell's post on the SF02 LRF used for landings on APM, I thought I would post some flight logs of our landings using the same rangefinder.

NOTE - Log sample rates are 120milliseconds - each vertical line in the graph is 2.4seconds from the next.

The top chart shows Baro height (ASL..) . Our SurVoyeur A/C landing commences with a spiral down to 80meters AGL, and then lines up with a computed descent point, down a parabolic curve in a circle, to exit the circle at a point approx 20meters above ground, and lined up with the touchdown point.

Point - 1 above is the beginning of the spiral from 80M AGL down to the 20meter spiral exit point.

Ignore point -2 - that is where the datalink power is switched from 300mW to 20mW - short link loss during this period.

Point-4 is where the LRF is enabled - measures 38meters AGL - we switch to laser height here from ( always erroneous) Baro height. Pressure Alt is always in error - 0.5mbar is around 4 meters, so that bad for the touchdown. In a half hour flight, baro pressure can easily change by 0.5mbar...Temp compensation of static pressure sensors to give better than 2meter error is not simple, and even 2 meters error is not good for touchgdown.

Point-6 is the 20meter AGL spiral exit, lining the A/C onto the straight in landing approach, aiming at a GPS point.

A throttle from airspeed error and throttle from height error control loop manage throttle in this part, till touchdown.

In the top graph, the RED line is the commanded height to fly down - the green is the actual A/C height from Baro pressure.

In the lower graph the RED line is the LRF range height, and the green line from point 6 is the height line we wish to follow onto the flare to ground. This green line is computed form a desired descent rate, and manages the A/C height accordingly. Lots of gain scheduling in the height, speed and throttle loops happen from point-4 to point-5, the touchdown point. At point 5, the A/C flares in the final touchdown, with a descent rate of around 0,25m/s

In the graphs below, Point-1 is the start of the spiral down from 80meters AGL, 2 is the link power change point, and 3 is the flare in the pitch angle for the flare to touchdown - all works smoothly.

The oscillation in the A/C pitch angle in the lower graph (RED) at the touchdown, is from A/C pitching as it rolls over the rough terrain - a gravel plain with golfball size gravel...in the desert here. A/C is a tail dragger, so resting pitch is plus 12 deg, as seen in the final pitch angle below.

The lower graph - GREEN is the commanded pitch angle, RED is the A/C actual pitch angle.

The SF02 Laser Rangfinder works VERY well indeed - it does not compromise on range , nor does it have any limitations related to the type of surface from which it will work thanks to some smart and innovative design. ( ok, a mirror might not work..).

We used to use a SESCOMP ultrasonic rangefinder - which is a very good product, accurate, no noticeable wind disturbance effects, etc. However, the reliable working range is limited to 6meters max, and if you have a pressure ALT error of say 0.6mbar either way, an error of around 5meters, you introduce major pitch disturbances into the final flare due to a major step change between pressure alt and Ultrasound height when ultrasound is detected. This is at the worst point, where you are close to the ground, etc. The laser fixes all this!   

There is a new laser from LightWare.co.za, the SF10/a, with serial, I2C and analogue outputs, its smaller , and very neat. We intend to fit these as standard now on all our autoland airframes.

I would beware of options with smaller lenses and lower power emitters - they will not work reliably on 'all' surfaces' at expected ranges - from costly experience...

See my previous blog on the LRF for more info on its fitment to SurVoyeur, etc

http://diydrones.com/profiles/blogs/laser-rangefinder-flights-tests-in-namibia-1

From the NamPilot..

Below the Ultrasound and Laser rangefinders                                                        Below the new SF10/A