Magnetic type sensor consisting of a magnet and theHall elements is used to measure the position precisely.When the permanent magnet and the Hall elements travelabove the slots, the Hall elements experience variation ofmagnetic flux because magnetic reluctance varies.The Hall elements measure the variation of themagnetic flux density which is produced by the slotsgrooved on the surface of the cylinder in magnetic typesensor. The permanent magnet provides the constantmagnetic field around the slots. The slots are usuallycoated with ceramic material to make the surface of thecylinder smooth and to prevent erosion. The accuracy ofthis sensor is heavily dependent on the performance ofthe Hall elements. Fig. 4 shows the structure of amagnetic type sensor.