Transit-time flow sensor combining high resolution and wide dynamic range

• US 6,575,044 B1
• Filed: 05/06/2002
• Issued: 06/10/2003
• Est. Priority Date: 05/06/2002
• Status: Expired due to Fees

• Current Assignee: ONICON, Inc. (TASI Holdings, Inc.)
• Original Assignee: Murray F. Feller
• Inventors: Feller, Murray F.
• Primary Examiner(s): Noori, Max
• Assistant Examiner(s): Miller, Takisha S
• Application Number:
  US10/139,528
• Time in Patent Office: 400 Days
• Field of Search: 738/612.7, 738/612.3, 738/612.8, 738/612.9
• US Class Current: 73/861.27
• CPC Class Codes: G01F 1/662 Constructional details
• CPC Class Codes: G01F 1/667 Arrangements of transducers for ultrasonic flowmeters; Circuits for operating ultrasonic flowmeters
• CPC Class Codes: G01F 7/00 Volume-flow measuring devices with two or more measuring ranges; Compound meters

Abstract

A time-of-flight flow sensor, of the type in which a measured phase difference between upstream and downstream acoustic propagations is representative of fluid flow rates, is operated at two distinct frequencies. Operation at a relatively low propagation frequency yields a first phase difference signal that is unambiguously representative of the rate of flow but that has a larger than desired measurement error. Operation at the higher frequency provides a lower measurement error, but may be ambiguous because of the modular nature of phase detectors. The low frequency phase difference signal can be used by a signal processor to determine a compensation term that can be combined with the higher frequency phase difference signal to remove the phase detector ambiguity, if one is present.

Claims

1. An acoustic time-of-flight flow sensor for determining a flow rate, within a range of flow rates, at which a selected fluid flows along a line, the sensor comprising two transducers spaced apart along the line and at least one phase detector for measuring at least two phase differences between acoustic signals transmitted in opposite directions along the line between the two transducers, the phase detector having a modular response range characterized by a modulus, the flow sensor comprising:

at least one oscillator for supplying transducer-driving signals at each of at least two distinct frequencies selected so that when the sensor is operated the at least one phase detector measures at least a first phase difference at a lower of the at least two frequencies, the lower frequency lying within the modular response range for all flow rates within the range of flow rates, and a second phase difference at the higher of the at least two frequencies, the second phase difference exceeding the modulus; and

a signal processor for determining, from the phase difference measured at the lower of the at least two distinct frequencies, a correction term which, when combined with the second phase difference measured at the higher of the at least two frequencies, provides a corrected phase difference representative of the rate at which the fluid flows.

2. The flow sensor of claim 1 wherein at least one of the two transducers comprises a single piezoelectric element configured to resonate at both of the at least two frequencies.

3. The flow sensor of claim 1 wherein the signal processor comprises a logic circuit having the phase difference measured at the lower of the two frequencies as an input, the logic circuit having a plurality of outputs, a combination of which determines the correction term.

4. The flow sensor of claim 1 wherein the signal processor comprises a microprocessor operating under control of a stored program to determine the correction term from the phase difference measured at the lower frequency.

5. A method of operating a time-of-flight flow sensor for measuring a rate at which a fluid flows along a path which two transducers are spaced apart, the time-of-flight flow sensor comprising a phase detector having a modular range of response, the method comprising the steps of:

operating the two transducers at a first, relatively low, selected acoustic frequency to obtain a first phase difference signal within the modular range of response and thereby unambiguously representative of the rate of flow, the first phase difference signal comprising a first measurement error;

determining, from the first phase difference and the first selected frequency, a correction term representative of a modular ambiguity in a second phase difference signal obtained at a second selected frequency that is higher than the first selected frequency;

operating the two transducers at the second selected frequency to obtain the second phase difference signal comprising the modular ambiguity and a second measurement error smaller than the first measurement error;

combining the second phase difference signal with the correction term to remove the modular ambiguity therefrom and to thereby determine the rate of flow.

6. The method of claim 5 wherein each of the transducers comprises a single piezoelectric clement resonant at both the first and the second selected frequencies.

7. The method of claim 5 wherein the modular range comprises a phase range of three hundred sixty degrees.

8. The method of claim 5 wherein the correction term comprises an integer representative of a number of moduli of offset and the step of combining the second phase difference signal with the correction term comprises addition.

9. The method of claim 5 wherein the higher selected frequency is an integral multiple of the lower selected frequency.

10. The method of claim 5 wherein the steps of operating the two transducers at the first and at the second frequencies are carried out simultaneously.

11. The method of claim 5 wherein the steps of operating the two transducers at the two frequencies are carried out in a time-shared fashion.

12. A method of detecting fouling in a time-of-flight flow sensor comprising two transducers spaced out along a fluid flow path, the method comprising the steps of:

operating the two transducers at a first selected acoustic frequency to obtain a first phase difference signal unambiguously representative of a rate of flow;

operating the two transducers at a second selected frequency that is higher than the first selected frequency to obtain a second phase difference signal also representative of the rate of flow;

comparing an amplitude of the second phase difference signal to a selected level;

if the amplitude is greater than the selected level, using the second phase difference signal for determining the rate of flow; and

if the amplitude is not greater than the stored value, using the first phase difference signal for determining the rate of flow.

13. The method of claim 12 further comprising a step of supplying an output indicative of the presence of the fouling if the amplitude is not greater than the selected level.

14. The method of claim 12 wherein the steps of operating the two transducers at the first and at the second frequencies are carried out simultaneously.

15. The method of claim 12 wherein the steps of operating the two transducers at the two frequencies are carried out in a time-shared fashion.

16. The method of claim 12 wherein at least one of the transducers comprises a single piezoelectric element resonant at both the first and the second selected frequencies.