# Compass

The MPU-9250 has a built in magnetometer which up to now I’ve been ignoring.  However while I’m stuck waiting for stuff in the post, I’ve given it more than a seconds thought and it could be useful.

The magnetometer is reading the strength and direction of the local magnetic field, primarily from the Earth’s magnetic north pole.  So it can be used for knowing the direction a flight is taking which is useful if the flight is tracking between a series of GPS positions.  That aspect is of no interest to me at the moment, but there’s another trick.

Because the output of the magnetometer is a vector of the local magnetic field, it can be used to calculate the absolute quad-frame pitch, roll and yaw angles since take-off – exactly what the integrated gyro is doing short term.  But because the magnetometer gives absolute angles based upon an absolute fixed source, the magnetometer angles can be fused with short-term integrated gyro readings to keep them very tightly on the straight and narrow.

Use of a magnetometer as a backup a yaw sensor is common, but I’ve not come across it being used as an absolute pitch, roll and yaw sensor.

There is only one problem I can see, and that’s that metals in the flight area distort the earth’s magnetic field, so some care is needed fusing the readings to ignore any rapid changes that don’t match the gyro integration.

For now though, I’ll just be adding the magnetometer readings into the diagnostics, polling it perhaps once a second just to check the validity of the readings compared to the integrated gyro readings.  If that holds out, then this could lead to much longer zero drift flights.

# Blind, deaf, disorientated, lost and scared of heights…

and yet, amazing.

So in that last video of a ~10s flight, Phoebe drifted about 1m when she shouldn’t have.  To me, that’s amazing; to you, that may be underwhelming.  So here’s why you should be amazed.  Basic school math(s):

distance = ½ x acceleration x time² ⇒ plugging in the numbers above says she was accelerating at 0.02m/s² (2cm/s²) instead of zero to travel 1 meter in 10 seconds.

Gravity is roughly 10m/s².  The accelerometer is set to read up to ±4g.  So that’s a range of 8g or roughly 80m/s²

So 0.02/80 * 100 = 0.025% error or a ±8 error value in the sensor range of ±32768.

Now the critical part – the sensors are only rated at 2% (±655) accuracy, and yet I’m getting 0.025% or 80 times that degree of accuracy.

And that’s why I don’t think I can get things very much better, whatever I do.

There is a slight chance that when the A2 is released (sounds like that’s shifted to next year now), I may be able to run the sampling rate at 1kHz without missing samples (I’ve had to drop to 500Hz to ensure I don’t miss samples at the moment).

Other than that though, she needs more sensors:

• camera for motion tracking (!blind)
• ultrasonic sensors for range finding (!deaf)
• compass (!disorientated)
• GPS (!lost)
• altimeter (!scared of heights).

but they’ll also need an A2 for the extra processing.  So this is most definitely it for now.

# My aversion to more sensors

The PC World mention, along with some recent comments got me thinking about why I lack interest in adding an altimeter / magnetometer / GPS / remote control to HoG.  After all, it’s the obvious next step.

I have the sensors already, and none of the above would add much in the way of overheads to the code processing – perhaps only 1Hz polls of the GPS, compass and altimeter fused with the existing data from the accelerometer and gyro about orientation, direction and speed feeding into the existing flight targets.  All relatively straightforward.

Autonomy vs. remote control was purely a personal decision based upon my ineptness at controlling machines with joysticks.  I stopped computer gaming with Half-Life2 2 when I was within seconds of the end and yet  lacked the hand-eye coordination to win that final battle to stop the launch of the missile / rocket / bomb.

It’s a combination of time and testing that is the biggest problem.  Up to now, all testing happens in the back garden – it now takes me less than 5 minutes to run a flight, and get back indoors to analyze the diagnostics.  Even when the weather is terrible, those 5 minute slots exist virtually every day.  But with GPS movement tracking resolution of 10m, the garden is nowhere near big enough to test autonomous path tracking – just too many high stone walls to crash into.  I could move testing to the village play park a hundred meters down the road, but it’s called a kids play park for a good reason.  I could move into the fields a couple of hundred meters away, but that’s just far enough away that time steps in – to-ing and fro-ing between the fields and the “engineering lab” (home office) would just be too inefficient for the limited time available due to a full-time job and two kids under 7.  Oh, and the farmers probably wouldn’t appreciate HoG scything down their crops or sheering their sheep!

So I settled on short term autonomous flights within the bounds of the back garden.

Having said all that, I’m off to the kids’ play park tomorrow during school time just to see how long HoG can maintain a stable hover with limited drift, and perhaps add some horizontal movement to her flight plan if all goes well.  Weather forecast is good, sunshine and only a gentle breeze so hopefully I’ll be able to get a longer video of what she can do!

# Sub-Zero

Every time I threaten to retire Phoebe, she throws a spanner in the works.

I took her out for some horizontal velocity PID tuning this morning.  It’s below freezing point outside and my expectations weren’t high, but her thermostat code still brought her sensors up to the 40° calibrated temperature.  Probably 5 flights consisting of 3 seconds take-off at 0.5m/s, 5 seconds hover, and 3 seconds descent at 0.5m/s.

Of those 5 flights, one was, well, there’s no other word for it, perfect.  A perfect pass of the Ronseal test.  Don’t get me wrong, the other did well wrt drift management and getting to approximate hover over the take-off point but this particular flight just worked: take-off from a sloping ground stabilized up to 1.5m, completely static hover for 5s, and then an elegant descent back to the original take-off point.

The cause: gravity calibration in the Z axis.  Every flight starts with Phoebe peacefully sitting on the ground, measuring gravity in her X, Y, and Z axis.  That then produces a set of Euler angles used to rotate gravity back to earth axes where gravity should read 0, 0, 1 in the X, Y and Z axes respectively.  X and Y always measure zero to 8 decimal places.  Z seems to have a variability of 0.2% still though despite the thermostat – gravity could read between 0.998 and 1.002g and it seems entirely random.

But this particular flight, the sensors said earth gravity was 0, 0, 1.000009g!  That’s 0.001% error in the Z-axis or 0.1mms-2 acceleration error.  That correspond to a maximum drift velocity over the course of the 11s flight of 0.55mms-1!

That has multiple effects, the key two being angles and velocity calculation: with 0.001% error in the Z axes, then all angles and velocities were essentially right for the duration of the flight, hence the fact she rose to the right height and hovered over her takeoff position.

I’d love to know the cause of the random variance of accelerometer values from the Z axis – is it sensor tolerance, or ground vibrations or something else I’ve missed (the motors aren’t running at this stage of initialized before you ask)?

A barometer / altimeter + magnetometer / compass may fix it entirely:

•  the barometer / altimeter will provide an alternate measure of vertical velocity @ ±10cm resolution
• the magnetometer with provide orientation, and critically pitch, roll and yaw compared to the earth magnetic core.

A fusion of these sensor readings with their current accelerometer source could well improve matters – “fusion” just being the merging of the sources through a complementary or Kalman filter.

So why don’t I just get on with it?  Because still I cannot find a sensor that meets critical physical properties – none of them provide a double row of pins with 0.1″ pitch in both directions to allow connection to a breadboard or prototype PCB.

I think I need to put my politest head on and see whether DroTek have fixed their pin pitch spacing for their 10 DOF IMU yet.  Wish me luck!