Torque-free coils are installed in a certain geometric structure through several sets of coils, and through a certain number of turns of each set of coils and a certain current direction, the magnetic field outside the coil is rapidly attenuated, and the magnetic moment of the coil is zero. Torque-free coils have very prominent advantages: when working, they will not produce magnetic interference to the environment at a certain distance, which is very important for multiple systems to work at the same time; in addition, the torque-free coils will not magnetize nearby magnetic materials. , So as not to bring additional errors in this respect to the momentless coil.