GPS Logging

Having enabled the POI functionality, I wanted to properly log the GPS co-ordinates of the 'copter. I had another demonstration to perform to another group of NYSF students, so I figured I would try to get logging working and see if I could record anything. I didn't have time to do much before the demo at 11, so I just quickly popped it up in the air for them and then back down again:

The flight was between 11.08 and 11.09 am WST in the courtyard, and it was just a tad windier than yesterday. Unfortunately the POI was still too far to the south, so I was again not directly pointing at the group of kids!

After this I researched a bit more about how to log the GPS. I found that the MX-20 transmitter can perform logging, but more of the telemetry, like temperatures, control stick settings etc. It also produces very large files which have to be opened using a proprietary program from Graupner. I tested it out and it wasn't very user-friendly, mostly in German and with many dialogues and help boxes empty or containing filler text. As it can only log what it is sent by the 'copter, I realised it probably wouldn't help us with the main effort, to accurately measure the 'copter's position, any more than just recording directly. In the interests of completeness though, here is how you set up some auto-logging that begins when your flight starts:

• Go to 'control switches'
• Move the throttle joystick
• Set the threshold to just past off, e.g. -88%
• Go to 'timers'
• Set that control switch as starting the 'Flt' (flight) time

If there's a micro-SD card in the transmitter, this will start logging when you bring the throttle up after starting the engines. Unfortunately it currently won't stop until you turn the transmitter off.

To log on the 'copter, it's a lot easier - just put a micro-SD card in the Navi-Ctrl board. 2GB is more than enough space as it generates very small files. It creates both a GPX file and a KML file: The KML file has  simple entries like

+115.8894829,-32.0038644, 1.789

showing the longitude, latitude and altitude, and it logs these every second.

The GPX files show a lot more data: 34 different data objects like roll angle and heading, and more information on the 'copter status, like available motor power and the motor temperatures. These points are logged every 2.5 seconds.

Now that I had a way of logging the position of the 'copter, I wanted to try a flight path that resembled the a possible flight plan for the MWA tile measurement: a curved flight with varying altitude, so as to eliminate the effect of taking a straight cut through a spherical beam. I came up with a simple series of waypoints using a sin curve, since I didn't need to be exact, just test the functionality.

When we were in the field, I uploaded the POI as the first waypoint, as I wanted to fly the 'copter over to its exact start position and land it there. I left the POI in the file, a mistake!

Flight start 16:33

I started the 'copter in CF mode and took it to 10m, then engaged 'come home' to send it to the POI. It reached it, but then behaviour I had not accounted for kicked in. Hovering directly over the POI, the 'copter was straining to tilt the camera mount at the point directly beneath it, and was slewing around in a clockwise arc trying to get to an impossible angle. The landing gear was also disrupting the servos on the camera mount. I immediately put it in PH/AH and began to land it, but it was difficult as the 'copter continued to slew around. I had seen warnings about this on the forums, but I didn't realise that it would be so difficult to control. After a few clockwise turns it lost a propeller! I brought it down and we managed to find both the prop and the washer, but lost the nut holding the propeller on. I realised afterward that I should have turned off CF mode, as then the 'copter would not have been trying to point at the POI, since that functionality is then disabled.

Flight end 16:34

Flight start 16:53

Having lost one propeller, I still hoped to see if the curved waypoints would work, so we brought the 'copter back to around 10m east of WP2, and deleted WP1 from the log. I brought it up and started the waypoints, but the 'copter rose poorly and slewed around, spending a lot of time trying to get to each waypoint even though I had told it to spend zero seconds at each one. After a minute I brought it down again, as it was clear it was not going to be a good test of the functionality.

Flight end 16:34

I looked at the logs and they were difficult to understand at-a-glance. I tried putting them into Google Earth but it runs very slowly on my computer and GE's handling of altitude is terrible. Then I found this excellent little online app: http://www.gpsvisualizer.com/ which can produce a nice png of each flight, or transform it into files for use in other programs.

Clockwise spirals while hovering directly over POI.

Poorly-executed curved flight path - missing a propeller.

Lessons:

• Never hover directly over a POI; the 'copter becomes extremely erratic as it tries to get the camera pointing directly at it. If you do accidentally end up doing this, turn off CF mode, turn on PH/AH and land immediately, then reprogram.
• Flying manually is ok with seven propellers, and it can execute simple waypoints, but it can't make fine adjustments well enough to perform a densely-waypointed flight path.
• The propellers have a tendency to come loose when you yaw clockwise - probably due to the unreversed threads on half of the motors.