Question: Double Pendulum Linearization

I'm confused by the state equations resulting from linearizing the DoublePendulum example model.

The state equations are:

diff(x[1](t),t) = x[2](t),

diff(x[2](t),t) = 0,

diff(x[3](t),t) = x[4](t),

diff(x[4](t),t) = 0


x[1](t) = R1_theta(t),

x[2](t) = diff(R1_theta(t),t),

x[3](t) = R2_theta(t),

x[4](t) = diff(R2_theta(t),t)

Am I understanding correctly that the state equations say that the angular acceleration of both revolute joints is zero? This is clearly not the case when you simulate the model and view the plots for angular acceleration. Can someone explain to me what's going on? Model with worksheet attached.

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