| 摘要 |
PURPOSE:To detect the frequency with higher accuracy than linear approximation during a half cycle detection period by calculating the actual detection time of half cycle using a sampling phase time difference operated according to an inverse trigonometric function and then determining the frequency based on the reciprocal. CONSTITUTION:An AC amount Vi is sampled at a constant period DELTAT(e.g. 30 deg. of electric angle) and inputted while counting Ki. A current Vi is multiplied by a previous Vi-1 and the sign is determined. If the sign is negative, a zero point of AC is present and the time difference DELTAti between the zero point and current sampling point is calculated and stored. The time Ts of detecting section (between zero points) is then calculated based on the previous phase time difference DELTAti(i) to be stored, the number of sampling times KXDELTAT, and the current DELTAti. The detection frequency is determined as one half of the reciprocal of the time Ts and then the K is cleared thus waiting next input. Since the detecting section can be extended to a multiple of half cycle while sustaining the half cycle detection period, the accuracy can be enhanced. |
