Techniques for the automatic optimisation of active shields

The present work is about the development of automatic techniques for the o ptimisation of active shields for stationary magnetic fields. An active mag netic shield is basically made by a number of coils, fed with suitably chos en currents. In this way a magnetic field as equal as possible to the distu rbing one is generated. The resulting effect is the reduction of the distur bing field in the area of interest. To achieve reasonable results a stochas tic optimisation procedure has been used to optimise the non linear part of the problem (i.e. the geometry of shielding coils); the linear part can be optimised "on the fly" in a much more straightforward way through the solu tion of a Last squares problem. The stochastic optimiser used is based on t he very fast simulated reannaling (VFSR), allowing to get a good optimum wi th a much reduced sampling of the objective function. It is used combining it with a deterministic optimiser (Nelder-Mead simplex method), to get a fa ster optimisation process as soon as the valley of the global optimum is lo cated Basically the VFSR has a different Point generating function and a di fferent cooling schedule with respect to the standard Boltzmann annealing, but the concept is clearly the same.