What’s left?

  • Refine the u-blox NEO-M8T to use the “pedestian” model algorithm which is more accurate than the current “portable” model used.
  • Test the yaw code I’ve been working on in the background so ‘H’ always points in the way she’s going.
  • Stop swearing at Microsoft for the Spectre and Meltdown updates they installed this morning which has crippled my PC just loading web pages!

Sixty seconds sanity check

A 60 seconds hover in preparations for my next GPS tracking flight on Wednesday when the sun will be out:

Weather conditions are poor today: light contrast is low due to thick clouds and the temperature is about 3°C.  The code is using the butterworth filter to extract gravity from the accelerometer, and the hover height is set higher at 1.5m to avoid the longer grass in the field next door on Wednesday: takeoff at 50cm/s for 3 seconds to get away from the ground quickly; descent is at 0.25cm/s for 6 seconds for a gentle landing.  The aim here is to move the down-facing LiDAR into a zone where it’ll be more accurate / stable vertically, while checking the down-facing video still provides accurate horizontal stability at this higher height, lower contrast lawn surface.

The check has passed well, boding well for Wednesdays GPS field flight!

Worth a Betta Bitta Butter

I suspect the low temperature increasingly unstable GPS tracking flights are partly due to the LiPo power (fixed with the hand / pocket digital warmers), and partly the accelerometer drift over time / temperature.  The problem with the latter is that gravity is recorded at the start of the flight and hence is fixed, meaning that accelerometer drift during the flight results in velocity and distance drift as the integration of (accelerometer – gravity) grows .  This is currently overcome by having second sources of velocity and distance (down-facing LiDAR + video) fused with the drifting IMU integrated (accelerometer – gravity) values.  The instability sets in over time as the IMU velocity and distance values drift increasingly over time but the LiDAR and video values don’t.  Ultimately these sources increasingly diverge and the instability ensues.

I remembered using a Butterworth IIR filter to extract gravity from the accelerometer dynamically 3 years ago to account for the accelerometer drift in cold temperatures, but it either failed or I got distracted by a better solution.

Yesterday, I tried again, but this time with a little better understanding: I now have a way to prime the filter without taking many seconds to do so.

Here’s the result: a stable hover without drift despite the LiDAR / video disabled – only the IMU status is in use:

Once the windy weather’s gone next week, I’ll be heading out into the next-door field again to fly with more GPS-waypoint intermediate targets, and this time, hopefully I’ll be able to complete the flight without the increasingly instability seen so far.

Here we go round the Mulberry bush.

Yesterday it was a cold and frosty morning, but more critically, sunny with only a light breeze:

28/12/2017 BBC weather forecast

Thursday’s weather

So I managed to squeeze in another test flight, where the plan was to fly ‘H’ between 3 GPS wayspoint frisbee-markers from a takeoff in between all three.  As you can see, reality deviated significantly from the plan.

I’d made a minor change to the code such that the maximum horizontal speed was 1 m/s reducing proportionally as ‘H’ got less than 5 meters from the target e.g at 3m from the target, the speed is set to 0.66cm/s.  That worked well approaching the first red frisbee-marker after takeoff.  However the next phase, although heading in the right direction between red and orange frisbee-markers, was very unstable and ultimately overshot the orange frisbee-marker, so I killed the flight.  Here’s what the flight controller saw:

GPS TARGET 6m -69o
GPS TARGET 6m -69o
GPS TARGET 5m -70o
GPS TARGET 4m -71o
GPS TARGET 4m -73o
GPS TARGET 3m -75o
GPS TARGET 2m -78o
GPS TARGET 2m -82o
GPS TARGET 1m -86o
GPS TARGET 1m -90o
GPS TARGET 2m -117o
Flight time 20.242111

The green text is from takeoff to the red frisbee-marker.  The yellow section shows a good heading towards the second frisbee-marker.  It started at the right speed of 1m/s while more than 5m away, but the red lines show velocity increasing to 2, 2 and 4 m/s as H got closer to the orange frisbee-marker.  On the plus side, she knew she’d overshot the target and had been told to double back at the point I killed the flight -check the angles at the end of the red lines.

Time to go bug hunting – it’s hopefully just a stupid crass typo on my part.  Luckily, the kids go back to school on Tuesday, and the weather forecast so far is looking good that morning:

Tuesday's weather forecast

Tuesday’s weather forecast

I hope by Tuesday I’ll also have worked out how to get better video quality from the Mavic!

P.S. No crass bug found, so my second best guestimation is that the LiPo cooled to below optimal performance temperature; this has the same effect as seen, and I had set the heater elements on the lowest level. The plan for the next flight is identical to before, but with the heaters on high and with full logging enabled in case this also fails and I need to diagnose in detail the source of the problem from the lateral velocity targets.

Perfect, nearly!

With a minor tweak to the u-blox NEO-M8T GPS receiver configuration, ‘H’ headed off in the right direction, making subtle changes of direction as she got closer to the pre-recorded GPS target point, and when within one meter, hovered briefly before landing. Only down side was the LiPo was at under 40% by then (it started at 48%), and that seems to be the point there’s not enough power for a stable descent, hence the very chaotic landing.

This is probably the last test for 2017 due both to the weather forecast and the chaos of Christmas.  See you all in the New Year – have a great holiday break!

P.S. The latest code has been updated on GitHub.

Same shᴉt, different day.

I went to the neighbouring field that the farmer isn’t using because they are digging for gravel in 90% of it, still leaving a sizeable 10% right next to my house; again tried GPS tracking, and again overshat (overshotted? overshooted? overshot?) significantly.  She started 13 meters away from the orange frisbee based on the GPS position of both, correlating nicely with the video.  She’s facing almost exactly away from the target as passed from the autopilot and logged by the motion process:

 GPS TARGET 13m 173o

The target is in a NNE direction by gut feel and confirmed by the GPS log stats:

GPS tracking

GPS tracking

GPS thought it had travelled about 5.3 meters when in fact this was more than 16 meters, based both on the video and on the GPS itself showing 33 samples at 1Hz with ‘H’ programmed to fly at 0.5 m/s.

Now what’s interesting here are the spacing between the dots on the graph. Since each dot is one second apart, this give the speed the GPS thought it was moving:

GPS speed

GPS speed

As already mentioned, ‘H’ is flying at a constant and stable 0.5m/s, confirmed by the video, but GPS ‘speed’ climbed and has not yet reached the stable 0.5m/s.

As a result of all the above, I finally I have a clue of why ‘H’ keeps overshooting her GPS target: it’s like the NEO-M8T algorithm uses some form of low pass filter, which gives brilliant accuracy for a stable location (i.e. the waypoints and at takeoff), but when moving, each new reading is fused with historic readings, causing significant lag.

The NEO-M8T has a vast amount of config parameters accessible via its u-center app.  Time for me to explore the options.

P.S. A post to the u-blox forum quickly yielded the solution: the UBX-CFG-NAV5 can be set via the u-center app (or other means, I’m sure).  By default, the NEO-M8T algorithm uses a “stationary” model, but there are many other options:



I’ll try “portable” model first based on their descriptions of the different models in their NEO-M8T spec, section 7, followed by “pedestrian” and finally “Automotive”.

P.P.S. The “portable” model worked perfectly as shown by the next post.  There was one additional config change to make which is in the “CFG” section: changes need to be saved in all possible options so the update to “portable” survived after reboot.

Winter wonderland?

Yes, based upon my walk to the next village shop along a bridleway this morning:

Winter wonderland 1

Winter wonderland 1

Winter wonderland 2

Winter wonderland 2

Sadly no, based on flying Hermione in the park shortly after I got back from my walk:

GPS gave 15 samples corresponding well with the lateral flight time.  However the stats showed it still had 4 meters to reach the first waypoint when I aborted the flight.  I’m not clear as to the cause of this; perhaps how GPS prerecorded the waypoints verses how it then flew towards them?

On the plus point, she

  • flew well at 1m/s despite the reduced contrast for the down-facing video
  • was heading in the right direction.

On the downside, due to weather, start of kids’ school holidays, and Christmas, this may turn out to be my last flight in 2017!

Best lawn mower ever!

Note, best watched full screen:

Hermione starts to the right of the video; the yellow frisbee (upper left) identifies the first GPS waypoint she is to reach, the red one (lower left) the second and last where she should land. And she was doing so well right until she reached the first waypoint and flipped over, left mowing the park grass!

Best guess, main battery was running low, she clipped the ground, and flipped.  No real damage done; she’s just outside at the moment drying, so I can brush the grass cuttings off her!

Park life

Hermione and I went to the play park to test the GPS against a preconfigured 5m square flight.

Video isn’t so great due to looking directly into the low midday sun.  Only real thing of note was that despite the grass having been recently mown (and hence featureless), Hermione did the square beautifully, landing just a few centimetres from where she took off.

More interesting to me was that the ublox NEO-M8T ran through the flight and produced this:

NEO-M8T live tracking

NEO-M8T live tracking

This is well within the bounds of GPS accuracy needed for flight control even though the number of satellites was only 8 at best; 2 samples showed 7 in the 121s flight.

It’s 2pm now and the sun is already setting, but from the above results, I have no choice tomorrow but to do a complete GPS driven flight i.e. a flight which tracks through a set of predefined waypoints.

P.S. I forgot to mention Hermione was pointing roughly south on take off.

P.P.S. It’s now “tomorrow” I referred to above, and the breeze has increased to about 15mph; this is fine in a sheltered area like my garden or drive, but not in the exposed park area required for the GPS tracking flight; also, it’s sub-zero outside, and although the batteries have heaters to keep their chemical ions flowing freely, I don’t.  Testing will resume when the breeze drops and the temperature rises.

2m square

While waiting for half-decent weather, I’ve been tinkering and tweaking to make sure everything is as good as possible before moving on to the GPS tracking code. This is the result: a 2m square.

As a result, I’ve updated the code on GitHub.