Control circuitry for viscosity sensors

• US 5,054,313 A
• Filed: 07/17/1990
• Issued: 10/08/1991
• Est. Priority Date: 07/17/1990
• Status: Expired due to Term

• Current Assignee: Galvanic Applied Sciences U.S.A.
• Original Assignee: National Metal Refining Company
• Inventors: Stone, John, Dougherty, Edmond, Batton, John, Fitzgerald, John V., Matusik, Frank J., Walsh, Teresa M.
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US07/554,285
• Time in Patent Office: 448 Days
• Field of Search: 73/54, 73/59, 73/60
• US Class Current: 73/54.27
• CPC Class Codes: G01N 11/16 by measuring damping effect upon oscillatory body
• CPC Class Codes: G01N 2011/0006 Calibrating, controlling or cleaning viscometers

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.

Claims

1. 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.

2. The control circuit according to claim 1, wherein said RMS to DC converter has a characteristic such that the varying DC signal at the output terminal thereof varies at a slow rate in comparison with the frequency of oscillation of the sensor.

3. The control circuit according to claim 1, wherein said integrator has the maximum time constant with which said sensor can oscillate at the natural resonance frequency thereof with an amplitude corresponding to the value of said reference signal.

4. The control circuit according to claim 1, wherein said integrator comprises an operational amplifier with an input resistor and a feedback capacitor selected so that their product is in the range of 0.5 to 2 times the period of mechanical oscillation of the sensor.

5. The control circuit according to claim 4, wherein said input resistor and feedback capacitor are selected to that their product is approximately 0.7 times the period of mechanical oscillation of the sensor.

6. The control circuit according to claim 1, further comprising computer means for generating a series of viscosity-density product sample signals at predetermined time intervals, corresponding to the power supplied to said drive means.

7. The control circuit according to claim 6, further comprising computer means for repetitively averaging a predetermined number of the most recently generated sample signals, to provide a running average viscosity-density product signal.

8. A control circuit for an oscillatory viscometer having a transducer assembly including a sensor having a portion adapted for immersion in a fluid a viscous property of which is to be determined, drive means responsive to a drive signal for causing said sensor to mechanically oscillate, 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;

AC to DC conversion means coupled to said detector means for generating at an output terminal thereof a varying DC signal having a value corresponding to the amplitude of mechanical oscillation of said sensor;

comparator means coupled to the output terminal of said conversion means 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;

integrator means coupled to said comparator means for integrating said difference signal to generate said gain control signal; and

means for coupling said gain control signal from said integrator means to said gain control terminal of said variable gain amplifier,

said integrator means responding to changes in said difference signal sufficiently rapidly so that said sensor is caused to oscillate with an amplitude corresponding to the value of said reference signal.