Double pendulum cart, but with a good function library

[BystrickyK]

Trig functions of double angle differences can be decomposed into sine and cosine monomials like this:

Using completely clean data, acceleration measurements generated using the analytical model (rather than numerical differentiation)

Discovered models, dotx4 = cart acceleration, dotx5 = first pendulum angular acceleration, dotx6 = second pendulum angular acceleration

Prediction results:

Prediction is nearly perfect, but simulated trajectories decouple immediately; it might be due to a bug in the code though
https://user-images.githubusercontent.com/55796835/117227428-11243b00-ae17-11eb-9df1-95b025d6f23b.mp4