I am currently implementing an autonomous quadcopter which I recently got flying and which was stable, but is unable to correct itself in the presence of significant external disturbances. I assume this is because of insufficiently tuned PID gains which have to be further tweaked inflight.
- I ruled out a barometer since the scope of my research is only indoor flight and the barometer has a deviation of +-5 meters according to my colleague.
- I am currently using an ultrasonic sensor (HC-SR04) for the altitude estimation which has a resolution of 0.3cm. However I found that the ultrasonic sensor's refresh rate of 20Hz is too slow to get a fast enough response for altitude correction.
- I tried to use the accelerations on the Z axis from the accelerometer to get height data by integrating the acceleration to get velocity to be used for the rate PID in a cascaded pid controller scheme. The current implementation for the altitude PID controller is a single loop pid controller using a P controller with the position input from the ultrasonic sensor.
- I had taken into account the negative acceleration measurements due to gravity but no matter how much I compute the offset, there is still the existence of a negative acceleration (eg. -0.0034). I computed the gravitational offset by setting the quadcopter to be still on a flat surface then collecting 20,000 samples from the accelerometer z axis to be averaged to get the "offset" which is stored as a constant variable. This variable is then subtracted from the accelerometer z-axis output to remove the offset and get it to "zero" if it is not accelerating. As said in the question, there is still the existence of a negative acceleration (eg. -0.0034). My quad then proceeds to just constantly climb in altitude. With only the ultrasonic sensor P controller, my quad oscillates by 50 cm.
How can this consistent negative acceleration reading be effectively dealt with?
Possible Solution: I am planning to do a cascading PID contoller for the altitude hold with the innerloop (PID controller) using the accelerometer and the outer loop (P controller) using the sonar sensor. My adviser said that even a single loop P controller is enough to make the quadcopter hold its altitude even with a slow sensor. Is this enough? I noticed that with only the P gain, the quadcopter would overshoot its altitude.
Leaky Integrator: I found this article explaining how he dealt with the negative accelerations using a leaky integrator however I have a bit of trouble understanding why would it work since I think the negative error would just turn to a positive error not solving the problem. I'm not quite sure. http://diydrones.com/forum/topics/multi-rotors-the-altitude-yoyo-effect-and-how-to-deal-with-it
Single loop PD controller with the ultrasonic sensor only: Is this feasible using feedback from a slow sensor?
LSM303DLHC Datasheet: http://www.st.com/web/en/resource/technical/document/datasheet/DM00027543.pdf