Magnetic Methods of Sensing Shielded Part Motion 820201
A commercially available magnetic reluctance sensor is used to determine the angular velocity of turbocharger impeller blades from outside the aluminum housing. Eddy currents are induced in the aluminum blades by blade motion through the magnetic field projected by an externally mounted Samarium-Cobalt permanent magnet. Test results show that a circuit designed to track the blade signal gives an analog voltage output proportional to the frequency of blade passage over the operating range of the Cummins VT-903 and Detroit 8V-71T turbocharged diesel engines.
Cam shaft rotational frequency is measured on Cummins and Detroit deisel engines by sensing the motion of ferromagnetic rocker arm parts through a conducting and possibly ferromagnetic valve cover shield. A strong rare earth Samarium-Cobalt magnet and a wire sensing coil are placed outside the valve cover above the rocker arm. Changing magnetic flux induces a voltage in the coil that is filtered and shaped to give a pulse train output at the frequency of the rocker arm.