Quadcopter multiple ESC angles glitch

Question

I'm developing my fligth controller board on Tiva Launchpad for quadcoper and while calibrating PID I discovered an unexpected behaviour: sometimes quadcopter seems to experience random angle errors. While trying to investigate it, I've figured out that my code if fairly trying to compensate tham, as soon as they appear - but do not cause them. Even more - i've discovered that such behaviour appears only when two (or more) motors are adjusted, while one motor system shows pretty good stabilisation.

Here is code for PMW output for different motors:

torque[0] = (int16_t)(+ angles_correction.pitch - angles_correction.roll) + torque_set;
torque[1] = (int16_t)(+ angles_correction.pitch + angles_correction.roll) + torque_set;
torque[2] = (int16_t)(- angles_correction.pitch + angles_correction.roll) + torque_set;
torque[3] = (int16_t)(- angles_correction.pitch - angles_correction.roll) + torque_set;

and here is recorded angles for system with one motor and two motors:

To be sure that it's not the algorithm problem, while recording this angles only Integral part of PID was non-zero, so angles were not even stabilised.

My question is - could esc noise each other (in my quad they are quite close to each other - just few sentimeters away) to cause such behaviour? Thanks a lot!

Answer 1

I don't know that there's enough information here to diagnose anything.

How can you get any angle on a quadcopter with only one or two motors running?

What do you mean, "while recording [these] angles only the integral part of PID was non-zero"?? Did you manually set the proportional and derivative errors or gains to zero?

My guesses would be that:

(1) you are supplying power to the motors as though they were a quadcopter when you have the physical configuration in something other than a quadcopter - you can't tune a stabilization algorithm with half of the stabilizers (motors) offline.

(2) don't monkey with the PID controller by turning off random portions and expecting performance to be the same.

(3) in general, I would avoid using the derivative term of a PID controller if at all possible (that is, use a PI controller instead). If noise causes a spike in your sensor reading for one sample then it doesn't add a lot to the P or I terms, but it causes a huge spike in the D term. Sensor noise may also play a part in your issues, but you can filter the sensor to check.

Answer 2

To me, the straightforward explanation seems that you are applying a desired torque, but on multiple motors -- when you use 1 motor you get 1× torque, and when you use two motors you are getting 2× torque.

I'd imagine that your code should look more like this:

motors_active = 2
c_p = angles_correction.pitch / motors_active
c_r = angles_correction.roll / motors_active
torque[0] = (int16_t)(+ c_p - c_r) + torque_set;
torque[1] = (int16_t)(+ c_p + c_r) + torque_set;
torque[2] = (int16_t)(- c_p + c_r) + torque_set;
torque[3] = (int16_t)(- c_p - c_r) + torque_set;

I'm unsure why all 4 motors are contributing to both pitch and roll (I would have expected pitch and roll axes to be aligned on the quadrotor axes, but technically things will still work if they are 45° rotated). If that's the case, I assume you've corrected for the rotation in the calculation of angles_correction.

Answer 3

It looks like your PID is oscillating, have you tried turning down the P gains in your controllers? Most PIDs are tuned P first by turning all terms to zero and turning up the P to just above the point where it oscillates then for the I gain adjust it till it "snaps" into position, the D gain can be trickier to get but you should start there. Here is a good reference for tuning PIDs on a Multicopter.