# yaw control.

Currently, yaw angle PID target stays set at zero throughout the flight; the piDrone always faces the same direction regardless of the way it’s travelling.  Next step on the way to ultimately tracking between GPS points is to have her point in the direction she’s travelling; it’s not actually necessary for my girls; the Mavic Pro does it as the forward facing camera video is streamed back to it’s remote control so I get an FPV (first-person view – i.e. as though I was a pilot sitting on it).

My intention is that the yaw angle tracks the earth frame flight plan velocity vector, so the piDrone points the way it _should_ be going.  This is roughly what the Mavic does.

I though it would be trivial, and added the one new line of code, and then I realised the gotchas which led to me blogging the details.  There are 3 problems.

• Conversion of the lateral velocity vector to an angle.  tangent only covers ±90°; this means the 1/1 and -1/-1 vectors both come out as 45° angles.  Also 1/0 would throw a exception rather than 90°.  Luckily math.atan2(x, y) resolves this spitting out angles of ±180°.  Thats my yaw angle PID target resolved.
• Alignment of the integrated gyro input in the same scale as above.  If the piDrone flies 2.25 clockwise loops around the circle, the integrated gyro will read 810° when it needs to read 90° – doing yaw = yaw % 180° should sort this out.  That’s the yaw angle PID input sorted
• Finally if the yaw PID input is 179° and the yaw PID target is -179°, the PID (target – input) needs to come out at +2° not -358° i.e. the angle must always be <= |180°|.  I’ve sorted this out in the code by adding a custom subclass overriding the default error = (target – input):
```####################################################################################################
#
# PID algorithm subclass to cope with the yaw angles error calculations.
#
####################################################################################################
class YAW_PID(PID):

def Error(self, input, target):
#-------------------------------------------------------------------------------------------
# An example in degrees is the best way to explain this.  If the target is -179 degrees
# and the input is +179 degrees, the standard PID output would be -358 degrees leading to
# a very high yaw rotation rate to correct the -358 degrees error.  However, +2 degrees
# achieves the same result, with a much lower rotation rate to fix the error.
#-------------------------------------------------------------------------------------------
error = target - input
return (error if abs(error) < math.pi else (error - 2 * math.pi * error / abs(error)))
```

Now I just need the spring wind and torrential rain showers to ease up for an hour.

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