Abstract: In a general aspect, a system for selectively positioning a work piece includes multiple bodies. A first body includes a first exterior surface and a boss extending from the first exterior surface along a boss axis. The boss includes a boss end surface that terminates the boss. Boss perimeter surfaces define a perimeter of the boss and extend between the first exterior surface and the boss end surface. A second body includes a second exterior surface and a pocket extending from the second exterior surface into the second body along a pocket axis. The pocket is shaped to receive the boss in a discrete number of positions when the boss axis and the pocket axis are aligned. Pocket perimeter surfaces define a perimeter of the pocket and extend into the second body from the second exterior surface.

Abstract: In a general aspect, a position tracking system includes a sphere magnet that produces a magnetic field in a space. A magnetic field control system is configured to change the magnetic field by rotating the sphere magnet through a time-series of rotations over a time period. A position marker includes a magnetic field sensor that is configured to measure the magnetic field in the space during the time period. A data processing apparatus is configured to determine the position of the position marker based on magnetic field measurements obtained by the magnetic field sensor in the space during the time period.

Abstract: 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.

Abstract: In a general aspect, a position tracking system includes a sphere magnet that produces a magnetic field in a space. A magnetic field control system is configured to change the magnetic field by rotating the sphere magnet through a time-series of rotations over a time period. A position marker includes a magnetic field sensor that is configured to measure the magnetic field in the space during the time period. A data processing apparatus is configured to determine the position of the position marker based on magnetic field measurements obtained by the magnetic field sensor in the space during the time period.