SELF SYNCHRONOUS NOISE REJECTION CIRCUIT FOR FLUID VELOCITY METER

• US 3,855,858 A
• Filed: 08/01/1973
• Issued: 12/24/1974
• Est. Priority Date: 08/01/1973
• Status: Expired due to Term

• Current Assignee: Vincent J. Cushing
• Original Assignee: Vincent J. Cushing
• Inventors: Cushing, Vincent J.
• Primary Examiner(s): Ruehl, Charles A.
• Application Number:
  US05/384,770
• Time in Patent Office: 510 Days
• Field of Search: 73/194EM,181
• US Class Current: 73/861.17
• CPC Class Codes: G01F 1/60 Circuits therefor

Abstract

A noise rejection circuit for use with electromagnetic fluid flow meters. An electromagnetic fluid flow meter with an electromagnet to induce a magnetic flux of periodically changing polarity. Sensors are positioned so as to receive the voltages induced therein by reason of flow of fluid cutting the flux lines. The low level signal of the sensors requires extremely efficient noise elimination circuits. After eliminating noise in conventional manners, the oscillator which drives the electromagnet is synchronized to the noise. In one embodiment the signal voltages are periodically sampled through a switch controlled by the oscillator output. Since the oscillator is synchronized to the noise voltages, so is the sampling switch. Consequently, the noise signal appears as a DC level which is eliminated by passing the signals of interest through an AC coupled amplifier. In another embodiment the signals are fed through a phase sensitive demodulator to eliminate the noise components of the signal.

Claims

1. An electromagnetic flow meter comprising an oscillator driving an electromagnet to produce electromagnetic flux periodically reversing in polarity, elecrodes in which a signal is induced related to fluid flow and an amplifier to amplify said signal induced in said electrodes, the improvement comprising a self-synchronous niose rejection circuit including, (a) circuit means to sense coherent noise signals in the metered fluid, (b) second circuit means to phase lock said oscillator to said coherent noise, and (c) means connected to said amplifier to detect, synchronously with said oscillator, said signal, (d) whereby said coherent noise is substantially rejected.

2. The apparatus of claim 1 wherein said means connected to said amplifier includes a sampling switch connected between said electrodes and said amplifier, said sampling switch being operated synchronously with said oscillator.

3. The apparatus of claim 2 wherein dividing circuit means is connected between said oscillator and said sampling switch.

4. The apparatus of claim 1 wherein dividing circuit means is connected to said oscillator output and periodically connects said second circuit means to said oscillator.

5. The apparatus of claim 1 wherein said means connected to said amplifier includes phase sensitive demodulator means.

6. The apparatus of claim 5 wherein dividing circuit means, connected to said oscillator output, is connected to and controls said phase sensitive demodulator.

7. The apparatus of claim 1 where said electromagnet operates at a frequency which is chosen as an even sub-harmonic frequency of expected coherent noise.

8. The apparatus of claim 7 wherein said means connected to said amplifier includes a sampling switch connected between said electrodes and said amplifier, said sampling switch being operated synchronously with said oscillator.

9. The apparatus of claim 8 wherein dividing circuit means is connected between said oscillator and said sampling switch.

10. The apparatus of claim 7 wherein dividing circuit means is connected to said oscillator output and periodically connects said second circuit means to said oscillator.

11. The apparatus of claim 7 wherein said means connected to said amplifier includes phase sensitive demodulator means.

12. The apparatus of claim 11 wherein dividing circuit means, connected to said oscillator output, is connected to and controls said phase sensitive demodulator.