Alternator speed estimation using spectrum analysis

A method for determining a speed of a vehicle alternator, said method comprising:

• measuring a current or voltage of a vehicle battery, where the current or voltage includes a ripple current frequency produced as a result of an AC-to-DC conversion in the alternator;
• filtering the current or voltage measurement signal to remove known harmonics using a digital notch filter;
• performing a Fast Fourier Transform (FFT) operation having a limited number of data points to identify a frequency spectrum signal of the filtered current or voltage measurement signal;
• identifying a peak detection search range within the frequency spectrum signal within which the ripple current frequency is known to exist that includes adding and subtracting a predetermined value to a previous ripple current frequency to identify the search range;
• identifying a highest peak frequency within the search range and interpolating between the highest peak frequency and an adjacent frequency to identify the ripple current frequency;
• and
• converting the ripple current frequency to the alternator speed.

A system for determining a speed of an alternator, said system comprising:

• means for determining an output current or voltage signal of the alternator, where the output current or voltage signal includes a ripple current frequency as a result of an AC-to-DC conversion;
• means for filtering the current or voltage signal to remove known harmonic frequencies;
• means for performing a Fast Fourier Transform (FFT) operation to identify a frequency spectrum signal of the filtered current or voltage signal;
• means for identifying a peak detection search range within the frequency spectrum signal within which the ripple current frequency is known to exist;
• means for identifying a highest peak frequency within the search range as the ripple current frequency;
• and
• means for converting the highest peak frequency to the alternator speed.