Harmonic noise suppression in electromagnetic flowmeter

In an electromagnetic flowmeter in which fluid to be metered passes through a flow tube to intersect a magnetic field whereby the voltage induced in the fluid is transferred to a pair of electrodes mounted on the tube to yield a flow-induced signal, the field being established by an electromagnet supplied with excitation current through a periodically-operated switch to create a magnetic field which during each operating cycle is alternately turned on and off or reversed in flux direction at a predetermined drive frequency, as a consequence of which the signal includes undesirable noise components formed by even and odd harmonics of the drive frequency;

• a converter responsive to said flow-induced signal to produce an output signal as a function of flow rate that is substantially free of said noise components, said converter comprising:
• A. a pair of synchronous demodulators;
• B. means to apply said flow-induced signal to said demodulators in phase opposition;
• C. an integrator coupled to the outputs of both demodulators;
• and
• D. gating means synchronizing the demodulators with said periodically-operated switch to cause one demodulator to pass its output to said integrator during the first half cycle of the drive frequency only in a first gating interval in which the magnetic field is turned on and is at a constant flux level, and thereafter to cause the other demodulator to pass its output to said integrator during the second half cycle of the drive frequency only in a second gating interval in which the magnetic field is turned off or is at a constant flux level in the reverse direction.