US 5,054,313 AGranted

Control circuitry for viscosity sensors

AssigneeNational Metal and Refining Company, Ltd.

InventorsJohn V. Fitzgerald, Teresa M. Walsh, Frank J. Matusik, John Stone, Edmond Dougherty, John Batton

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Abstract

Electronic control circuitry for obtaining precise inline process control data as to viscosity of fluids over a wide viscosity range, with high tolerance of ambient noise and vibration. An electromechanical transducer with an oscillating sensor is immersed in a flowing liquid, with the power required to sustain predetermined oscillation parameters being a measure of viscosity-density product. The gain of a variable gain amplifier which provides positive feedback to sustain oscillation is controlled in response to the integrated error signal output of a comparator which compares a DC value corresponding to the RMS amplitude of mechanical oscillation with a DC reference value. The monitoring of RMS amplitude rather than peak amplitude (as is done by a phase-sensitive sample-and-hold arrangement in the prior art), coupled integration of the error signal, results in a great improvement in immunity to ambient noise and vibration. Other features includes automatic calibration and temperature compensation whereby viscosity at a desired temperature can be determined even though the measurement is made at different temperature.

A control circuit for an oscillatory viscometer having a transducer assembly including a sensor having a portion adapted for immersion in a fluid the viscosity or viscosity-density product of which is to be determined, drive means responsive to a drive signal for causing said sensor to mechanically oscillate at the natural resonance frequency thereof, and detector means for generating a sensor movement signal corresponding to the movement of said sensor, said circuit comprising:

a variable gain amplifier having an input terminal coupled to said detector means for receiving said sensor movement signal, and an output terminal coupled to said drive means for providing said drive signal thereto, the gain of said variable gain amplifier having a value determined by a gain control signal applied to a gain control terminal thereof;
said transducer assembly and variable gain amplifier forming a primary oscillator loop for causing said sensor to mechanically oscillate;
feedback control loop amplifier means having an input terminal coupled to said detector means for receiving said sensor movement signal, and an output terminal;
an RMS to DC converter having an input terminal coupled to the output terminal of said amplifier means, for generating at an output terminal thereof a varying DC signal having a value corresponding to the RMS amplitude of mechanical oscillation of said sensor;
comparator means coupled to the output terminal of said RMS to DC converter for generating a difference signal corresponding to the difference between the value of said varying DC signal and a reference signal having a value corresponding to a desired amplitude of mechanical oscillation of said sensor;
an analog integrator coupled to said comparator means for integrating said difference signal to generate said gain control signal;
means coupled to said integrator for phase shifting said gain control signal;
and
means for coupling said gain control signal from said integrator to said gain control terminal of said variable gain amplifier,
said integrator having a time constant sufficiently low so that said sensor is caused to oscillate at the natural resonance frequency thereof with an amplitude corresponding to the value of said reference signal.

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Abstract

Claims8