THE HALBACH ARRAY

High-strength neodymium-iron-boron (NIB) permanent magnets, assembled in a Halbach array such as is shown in the upper left figure, form the core of the SPM suspension design. The polarities of individual magnets in the array are arranged such that the fields reinforce each other on the active face, producing a strong magnetic field, and largely cancel each other on the inactive face, leaving virtually no field at all. The Halbach array was invented more than two decades ago and are used in a variety of industrial applications. 

To create a maglev suspension, two Halbach arrays are used as shown in the second figure. The active face of the vehicle array or “ski” points downward, while the active face of the stationary track array points upward. The vehicle array is larger in cross section than the track array to provide sufficient lift force while minimizing track costs.

This suspension design produces more than three times the force of an equivalent mass of magnet material configured as simple opposing dipoles and also confines the field to a more specific area. The vertical and lateral forces produced are illustrated in the third figure at left. With vehicle and track arrays vertically aligned as shown in the graph (offset = 0), vertical force is maximized and lateral force is zero. As the lateral offset increases, the magnitude of the lateral force grows and tends to increase the offset even more; thus the suspension is laterally unstable.

To control lateral instability, electromagnetic voice coil lateral stabilizers are mounted under the vehicle magnets to interact with the track magnets as shown on the lower left. A feedback control system monitors ski to rail alignment and varies electric current in the voice coils to adjust the magnetic fields and keep the vehicle centered. The skis extend for most of the length of the vehicle and provide a levitation gap of three to eight centimeters at all speeds, with no levitation power requirement and essentially no drag. Using “virtual zero power” control, stabilization power is on the order of 100 watts per ton of vehicle weight.

This design also confines magnetic fields almost entirely to the levitation gap, preventing high intensity fields from penetrating the vehicle above and adversely affecting cargo or passengers. The magnetic field inside the carriage is even less than in the Transrapid design, which itself is equal to 1 gauss, or the typical background magnetic field on the surface of the Earth.