Rather than mounting it on top of the primary, it is placed to the side on a slightly smaller radius. By then adding additional magnets of variable field strengths, a two track system quasi-positioning system can be incorporated with little additional effort. The position indicating magnets are Samarium Cobalt rod shaped magnets, which produce a magnetic field intensity of approximately one kilogauss. Intensity will be controlled by adjusting the depth of the mounting hole. Being a rotary system there is no need for limit switches or hard stops to prevent excess travel.
All linear elements move back and forth on lead screws over a finite travel distance.
Encoding scheme for linear positioned elements. Coarse position is determined by measuring the flux between a North-South pair of magnets. The stage is designed so that it is always operated in the linear region, providing a useful although not extremely accurate encoding of the position stage. Note that any changes in the sensitivity of the sensor electronics or DC offset can affect this encoding scheme. This is adequate, because the encoding scheme needs only to position the stage datum to within one turn of the main gear.