A fuel injector system (10) calculates a first derivative filtered signal
and a second derivative filtered signal from an unpowered spool valve coil
signal. When the spool valve (16) reaches the end of travel, either open
or closed position, a flat slope f is generated on the falling edge of the
bell shaped signal. The second derivative provides a readily apparent
determination of when the steep slope occurs. A comparator (62) thereby
need only determine when the second derivative is zero to identify that
the spool valve has reached the end of travel.
