Patent ID: 7202658

Claim:
A piston velocity detector to determine the velocity of a magnet carrying piston relative to a cylinder in which the piston is reciprocally movable, the detector comprising: a plurality of magnetic field sensors capable of sensing magnetic field and providing output signals corresponding to the strength of magnetic field, each of the sensors disposed in array fixedly spaced from other of the sensors, electric circuitry which determines, from the output signals from each sensor on movement of the piston past the detector, the relative time when a magnet carried by the piston passes each sensor, and calculates therefrom an estimated velocity of the piston based on the relative positions of the sensors in the array, wherein the magnet carrying piston has its north and south poles displaced in the direction of movement of the piston. a determination is made as to the time when the output signal from each sensor is zero, and for each sensor the electric circuit determines when the output signal is first zero after the output signal has increased beyond a pre-determined absolute threshold value.