full state: $$ x=\left [\begin{matrix} x\ z\ \dot{x}\ \dot{z}\ \theta\ \dot{r}\ \dot{\theta} \end{matrix}\right ] $$
flight phase state: $$ x=\left [\begin{matrix} x\ z\ \dot{x}\ \dot{z}\ \end{matrix}\right ] $$ stance phase state: $$ x=\left [ \begin{matrix}r\ \theta\ \dot{r}\ \dot{\theta}\end{matrix}\right] $$ flight phase dynamics: $$ \frac{d}{dt}x=\left[\begin{matrix}\dot{x}\ \dot{z}\ 0\ -g\end{matrix} \right] $$
stance phase dynamics: $$ \frac{d}{dt}x=\left[ \begin{matrix}\dot{r}\ \dot{\theta}\ \frac{k(l_0-r)-mgcos\theta+mr\dot{\theta}^2}{m}\ \frac{-2r\dot{r}\dot{\theta}+grsin\theta}{r^2}\end{matrix}\right] $$
flight->stance: $$ r=l_0\ \theta=u\ \dot{r}=-sin\theta \dot{x}+cos\theta \dot{z}\ \dot{\theta}=-\frac{1}{l_0}(cos\theta\dot{x}+sin\theta\dot{z}) $$ stance->flight(ignore)
raibert style controller:
neutral point front the foot:
$$ sum_{vel}=\sum(\dot{x}-\dot{x}_d)\ x_f=\frac{\dot{x}T_s}{2}+k_d(\dot{x}-\dot{x}d)+k_isum{vel} $$ Ts is stance time
control input: $$ u=asin(x_f/l_0) $$