a typical motor system

1.system equation

$$ J\ddot\theta=u(t)+d(t) $$

​ where $J$ is rotary inertia; $\theta(t)$ represents position instruction; $u(t)$ respresents coantrol input; $d(t)$

represents external disturbance.

​ more precise, it can describe as: $$ \ddot\theta=-f(\theta,t)+hu(t)+d(t) $$ ​ $f(\theta,t)$ is 粘滞阻力，或者时变变量。