Method of analysis of an excitation system

A method of analysing a non-contact excitation system for a spin rig, the spin rig comprising a plurality of test piece elements for attachment to a test piece for rotation about an axis of rotation (X) of the spin rig, and a plurality of exciter elements arranged around the axis of rotation;

• the method comprising the steps of:
• (i) providing the test piece elements and the exciter elements formed from magnetically attractive material;
• (if) providing either or both of the test piece elements and the exciter elements as comprising magnets;
• (ii) arranging each exciter element as being moveable between an active position, in which a magnetic force is provided between each exciter element and at least one of the test piece elements, and an inactive position, in which the magnetic force between the exciter elements and the at least one test piece element is reduced;
• (iv) calculating a magnetic field per test piece element;
• (v) integrating the magnetic field over the test piece element surface to get a force per test piece element;
• (vi) repeating step (iv) for different test piece element positions;
• (vii) performing a Fourier analysis of the forces acting on the test piece element as it moves past a periodic array of exciter elements;
• (viii) calculating a damping added to the test piece element due to magnetic induction;
• (ix) calculating a vibration response of the test piece element as it moves past a stationary array of exciter elements;
• (x) calculating a transient response of the test piece element where the stationary array of exciter elements is rapidly moved away from the test piece elements;
• and
• (xi) using the calculated transient response of the test piece element to calculate a required electromagnetic holding force for the array of exciter elements.