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An indirect speed sensor for an electric motor, such as an electric motor of the universal type, provides for a resistive element in series with the motor windings so that current pertubations that occur as a consequence of armature rotation are manifested as speed-dependent voltage signals across t

An indirect speed sensor for an electric motor, such as an electric motor of the universal type, provides for a resistive element in series with the motor windings so that current pertubations that occur as a consequence of armature rotation are manifested as speed-dependent voltage signals across the resistive element. The voltage signals are amplified and filtered to remove frequency components above and below the expected signal spectrum and then provided to an analog-to-digital converter that converts the filtered analog signal to digital values that are stored in a memory. A stored-program controlled microprocessor analyzes the stored digital information by performing a fast Fourier transform to provide an equivalent amplitude spectrum. The maximum amplitude spectral component is then identified to determine the speed of the motor.

대표청구항 ▼

An apparatus for sensing the operating speed of an electric motor of the type that causes recurring current variations through its windings as a function of its speed, said apparatus comprising: impedance means for connection to an electric motor circuit for providing an electrical signal output tha

An apparatus for sensing the operating speed of an electric motor of the type that causes recurring current variations through its windings as a function of its speed, said apparatus comprising: impedance means for connection to an electric motor circuit for providing an electrical signal output that varies with speed-dependent variations of the current in the electric motor circuit; converting means coupled to said impedance means for converting said electrical signal output to digital values representative of the corresponding value of the electrical signal; stored-program controlled means coupled to said converting means for performing a Fourier transform upon said digital values to obtain an amplitude spectrum of said speed-dependent electrical signal and for identifying a maximum amplitude spectral component therein; utilizing means connected to the output of said stored-program controlled means for utilizing the so-identified maximum amplitude spectral component as representative of the motor\s speed; filter means interposed in between said impedance means and said converting means for passing frequencies in a pass band expected for the range of motor speeds; and said filter means comprising a plurality of cascaded switched capacitor filters.