What does « ultimate gain Ku » in Ziegler–Nichols method mean?

This is a good and simple explanation on ultimate gain Ku which is required for the Z-N PID tuning method.

http://electronics.stackexchange.com/questions/286858/pid-what-does-ultimate-gain-ku-in-ziegler-nichols-method-mean

PID-controller and Ziegler-Nichols Method: How to get oscillation period?

This is a set of good illustration and instructions on how to attain the Ku (ultimate gain) which is important for using Zielger-Nichols tuning method.

Strategies for dealing with Gyro drift

Powerpoint to various gyro drift compensation strategies. Very clear layout of the approaches.

Kalman vs Complementary Filter

Useful link to comparison of Kalman versus Complementary filter. Good simple pseudo code include with a good explanation of why complementary is easier to use but yet performs well.

Reading a IMU Without Kalman: The Complementary Filter

Thanks to Pieter Jan for summarizing the use of complementary filter against the IMU readings.