AC. frequency monitoring method

The method includes steps: sampling AC current to be monitored; based on sampled values, looking up crossover points of the sampled waveform; calculating period T of AC current to be monitored based on crossover points; calculating frequency of AC current monitored. Crossover points are searched through software directly so as to reduce hardware spending. Averaging results of multiple times of calculation obtains final outcome in high precision. Anti-interference ability is high since questionable data including too large and too small data are thrown off. Advantages are: simple and reasonable, accurate and reliable data, and high practicability.