Rotation angle detection device and rotation angle detection method

A rotation angle detection method comprising:

• a current measuring process of measuring a current flowing in a direct current motor;
• a filtering process of extracting a ripple component from the current;
• a ripple detecting process of detecting a ripple from a signal obtained by the filtering process;
• and
• a rotation angle detecting process of detecting a rotation angle of the direct current motor based on a number of ripples detected by the ripple detecting process,
• wherein the rotation angle detecting process includes
• measuring an observation period based on consecutive ripples detected by the ripple detecting process, the observation period being a period of time between the consecutive ripples,
• calculating, as an estimation period, a true period between the consecutive ripples based on the observation period,
• determining whether the observation period is within a normal range based on the estimation period and the observation period,
• in response to determining that the observation period is within the normal range and an immediately-previous observation period is outside the normal range, calculating a number of correction ripples CRN based on an equation (2) below,
• CRN=TRN−SN (2)
• in which TRN is a value obtained by dividing a sum of observation periods during an abnormal period of time, in which the observation periods are consecutively determined to be outside the normal range, by the estimation period so as to obtain the TRN as an estimated number of ripples during the abnormal period of time, and SN is a number of ripples detected by the ripple detecting process during the abnormal period of time in which the observation periods are consecutively determined to be outside the normal range, and
• correcting the number of the ripples based on the calculated CRN, thereby detecting the rotation angle of the direct current motor.