US Patent 7584551 Absolute position measuring apparatus

An absolute position measuring apparatus includes a first rotary encoder that detects a rotation of a spindle as a phase signal which varies in a first cycle and a second rotary encoder that detects the rotation of the spindle as another phase signal which varies in a second cycle. A rotation of a first rotor of the first rotary encoder is transmitted to a second rotor via a relay gear that is meshed with a first gear provided on an outer circumference of a first rotary cylinder and a second gear provided on an outer circumference of a second rotary cylinder. Thus, an absolute position of the spindle is calculated on the basis of two phase signals that are different in cycle. Further, it is not necessary to provide a conventionally-known spiral key groove so that the apparatus can be easily downsized. Manufacturing costs can be also reduced.