@michinyon
What do you mean? z is the measurement of the state - in this case the measurement is the angle measured using the accelerometer. The x in your equation has nothing to do with the estimate of the state. It is the state of the system which is not observable.
@aziz-qessate_arduino
As I have already wrote to you in my email. I don't have the code for the self stabilizing platform, as I didn't write it. But you could try to contact him here at the forum or youtube and ask for the code: Arduino Forum
You can find some of his code here: Self balancing waiter tray. (or almost it) - #4 by Rivello - Exhibition / Gallery - Arduino Forum