Rotational Spring
=================

A rotational spring/damper induces an attractive moment between two frames about the rotation axis corresponding to the relative orientation of the frames. The moment magnitude is

    M = k θ + c θ'
    
where k is the spring stiffness, θ is the angle between the two frames, c is the damping constant, and θ' is the differential angular speed between the two frames.

**Keyword Arguments**

:param str ref_point_key: Key for the point where the rotational spring is visualized.

:param str first_frame_key: Key for the first frame.

:param str second_frame_key: Key for the second frame.

:param k: Parameter corresponding to the spring stiffness constant.
:type k: str or numeric

:param c: Parameter corresponding to the damping constant.
:type c: str or numeric

:return str: Message describing the created spring