Dynamics (matrix form, fully expanded)
\(\mathbf{q}=\begin{bmatrix} \alpha & \beta & \gamma \end{bmatrix}^{\!T}\) (rad); \(a,b\) (m); \(g\) (m/s²); \(k_1,k_2\) (N/m); masses (kg); inertias (kg·m²).
\[ \begin{bmatrix} J_1+m_1\,a^2+J_3+m_3\,(a+b)^2 & 0 & 0 \\[8pt] 0 & J_2+m_2\,b^2+m_4\,(a+b)^2 & 0 \\[8pt] 0 & 0 & J_4 \end{bmatrix} \begin{bmatrix} \ddot{\alpha} \\[4pt] \ddot{\beta} \\[4pt] \ddot{\gamma} \end{bmatrix} \;+\; \begin{bmatrix} (k_1+k_2)\,a^2-m_1\,g\,a-m_3\,g\,(a+b) & -k_2\,a\,b & 0 \\[8pt] -k_2\,a\,b & k_2\,b^2+m_2\,g\,b+m_4\,g\,(a+b) & 0 \\[8pt] 0 & 0 & 0 \end{bmatrix} \begin{bmatrix} \alpha \\[4pt] \beta \\[4pt] \gamma \end{bmatrix} \;=\; \begin{bmatrix} 0 \\[4pt] 0 \\[4pt] 0 \end{bmatrix} \]Scalar rows (same model)
\[ \begin{aligned} \bigl(J_1+m_1 a^2+J_3+m_3(a+b)^2\bigr)\ddot{\alpha} &= -k_1 a^2\alpha -k_2(\alpha a-\beta b)a + m_1 g a\,\alpha + m_3 g(a+b)\alpha,\\ \bigl(J_2+m_2 b^2+m_4(a+b)^2\bigr)\ddot{\beta} &= -k_2(\beta b-\alpha a)b - m_2 g b\,\beta - m_4 g(a+b)\beta,\\ J_4\,\ddot{\gamma} &= 0. \end{aligned} \]