Method of detecting absolute rotational position

A method of detecting absolute rotational position using a two-pole absolute-value encoder and a multi-pole absolute-value encoder to detect absolute rotational positions of a rotating shaft within one rotation, the multi-pole absolute-value encoder having Pp pairs of magnetic poles (where Pp is an integer of 2 or greater);

• comprising:
• the two-pole absolute-value encoder having a bipolarly magnetized two-pole magnet rotating integrally with the rotating shaft, and also having a pair of magnetic detecting elements whereby sinusoidal signals having a phase difference of 90° are output as one wave period per rotation of the rotating shaft in accompaniment with the rotation of the two-pole magnet;
• and
• the multi-pole absolute-value encoder having a multi-pole magnet magnetized so as to have Pp pairs of magnetic poles, the multi-pole magnet rotating integrally with the rotating shaft, and also having a pair of magnetic detecting elements whereby sinusoidal signals having a phase difference of 90° are output as Pp wave periods per rotation of the rotating shaft in accompaniment with the rotation of the multi-pole magnet;
• wherein, in advance of an operation for detecting the rotational position of the rotating shaft, the rotating shaft is caused to rotate, absolute values Belt of the multi-pole absolute-value encoder are measured and assigned to respective absolute values θt of the two-pole absolute-value encoder, and temporary pole-pair numbers Nx of the multi-pole magnet are assigned to the respective absolute values θt of the two-pole absolute-value encoder;
• and
• wherein, when detection of the rotational position of the rotating shaft is started, the absolute value θti of the rotating shaft according to the two-pole absolute-value encoder is measured;
• the absolute value θelr of the rotating shaft according to the multi-pole absolute-value encoder is measured;
• the temporary pole-pair number Nx assigned to the absolute value θti is corrected and a pole-pair number Nr is calculated on the basis of the absolute value θelt assigned to the measured absolute value θti and on the basis of the measured absolute value θelr;
• and
• a mechanical angular absolute position Gabs of the rotating shaft within one rotation is calculated according to the following equation using a mechanical angle θelp that corresponds to an electrical angle of one period of an output signal of the multi-pole absolute-value encoder.
• θabs=(Nr×θelp+θelr)/Pp