The acceleration stems from an inverted pendulum actuated with sinusoidal motion, and is in the translational x direction. That frame being measured has its z-axis in the rotational direction and sits on top of the pendulum. The reference frame is the global one, with x-axis directed in the same direction as the measured frame... The pendulum swings back and forth with an amplitude of 15 deg and unity radian frequency. I would expect sinusoidal-looking acceleration...
The modeling is done using Simscape Multibody and the motion is sensed using a Transform Sensor. The plot shown is from a simulation using the variable-step auto(ode45) solver. Another simulation using the ode23s solver gives a totally different response, although not at all sinusoidal.
BTW, the pendulum represents a non-inertial base which to append an electro-mechanical system.
The model can be found at https://www.dropbox.com/s/7udnhog2i0fvkba/forum.slx?dl=0