In a general aspect, a position detection system includes a magnetic field generator configured to generate a time-varying magnetic field. The magnetic field generator includes a carrier assembly that defines a first axis of rotation and comprises a permanent magnet having a center of mass. The magnetic field generator also includes a drive assembly that is coupled to the carrier assembly and configured to act on the carrier assembly to rotate the permanent magnet simultaneously about the first axis of rotation and a second axis of rotation. The second axis of rotation intersects the first axis of rotation at an intersection that is offset from the center of mass of the permanent magnet. The position detection system additionally includes a computer device configured to determine a position of a sensor based on magnetic field measurements obtained by the sensor in the time-varying magnetic field.