Device for measuring the frequency of an electrical current

Abstract

A device to make a measurement of a variable quantity such as the speed of rotation of a first shaft or the speed of operation of an ignition system of an engine, wherein electrical pulses are produced at a frequency which is a function of the speed. The device comprises fixed first and second coils having axes in a common plane, and the planes of the coils are disposed at right-angles. Square waves corresponding to the pulses energize the first and second coils alternately, and each successive energizing of each coil is in an alternate direction, thus the magnetic field produced by the coils rotates continuously and rotates a disc magnet within the coils about an axis in the planes of the coils at an angular velocity which is a function of the speed. A rotor of conducting material in the form of a cylindrical cap over and about the magnet is integral with a second shaft coaxial with the magnet. Eddy currents induced in the rotor by the rotating magnet result in the rotor rotating in the magnetic field of the coils to rotate the shaft against arresting spring force proportional to the driving torque of the rotor which is a function of the angular velocity of the magnet. Thus a pointer on the second shaft registers a value on a dial corresponding to the speed of the first shaft or ignition system.