Apply Small Perturbations to Simscape Multibody Rotational/Translational States for Model Linearization

I want to linearize a nonlinear aircraft model i made using Simscape Multibody. To do so, i considered the 'linearize'-command from the Control Design Toolbox to be useful. To recieve the entries of the A,B,C and D matrices relating to selected states (Euler-angles, rotational rates, velocities...) i want to set linear analysis points with the 'linio'-command ('inputs' & 'outputs'). The relevant part of the model consists of a world frame connected to body frame by a 6-DOF-Joint. With the 'Transform Sensor' it is possible to place analysis points for sensing state perturbations. However, i have not found a possibility to apply perturbations to a Simscape frame to manipulate e.g. pitch angle or pitch rate.

The documentation suggests that analysis points only can be applied to Simulink signals but even with a Simulink-PS Converter i only can use external forces (wich are proportional to the body accelerations only) to manipulate the body/frame. Furthermore the 6-DOF-Joint does not allow to specify Motion Inputs for the spherical primitive, thus this is not an option either. I can and will work with operating points/simulation snapshots later on, however, i can not use 'trim' command because the aircraft is an ornithopter and has no steady states.

How do i apply perturbations in translational and rotational states to a body or frame, respectively?

Answer 1

[sloppydisk]

You can specify motion to the 6-DOF joint by going to actuation and setting the force to be automatically computed and the motion to be specified by input.

[Philipp Hansmann]

That's right, but it does not work for the Spherical Primitive

[sloppydisk]

Ah sorry, I misread. In that case you can use the bushing joint instead, it has three rotational primitives.

[Philipp Hansmann]

Sorry for the late answer. I tried different approaches but none of them were really satisfying.

Anyway, Bushing joint was a good idea.

Unfortunately, if i choose 'Provided by input' i have to define a trajectory for my body because the forces acting on the body will not be computed automatically of course. So, i could define a trajectory by recording the body's movement in one simulation and just copy the trajectory for my linearization run. In this case, however, it is still not possible to add a perturbation to e.g. the x-position because 1.: i technically cant find out how to perturb a signal build with the signal builder and 2.: if i define a position vs time signal, i implicitly define the velocity and acceleration, thus i can't add a perturbation to these states independently (same with the Euler angles and so on).