Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Reexamination Certificate
2002-05-06
2003-06-10
Noori, Max (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
Reexamination Certificate
active
06575044
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to a method for measuring the flow rate of a fluid whereby the propagation times of ultrasonic signals transmitted through the fluid are detected to determine flow rate. The invention further relates to improving the precision of measurement and its, reliability.
2. Background
Ultrasonic transit-time flow sensors, also known as “time-of-flight” ultrasonic flow sensors, detect the acoustic propagation time difference between upstream and downstream ultrasonic transmissions. This time difference, which results from the movement of the flowing fluid, is processed to derive a fluid flow rate. Examples of such flow sensors are found in my U.S. Pat. No. 6,178,827 and U.S. Pat. No. 6,370,963 the disclosures of which are herein incorporated by reference. The sensors described in those references use transit-time difference detection means that phase compare signals at the acoustic propagation frequencies, where the phase angle difference is proportional to the flow rate of the fluid, the distance between the transducers along the flow path, and the frequency. These phase measurement approaches, which may comprise an Exclusive-OR phase detector, are relatively simple and cost-effective but impose a performance limitation on the sensor. For example, the Exclusive-OR phase detector is unambiguous for signal phase shifts only within the range of 0+/−90 degrees. This limitation requires that an acoustic signal frequency and/or distance between the transducers be selected so that the maximum fluid flow rates never produce a greater phase shifts.
Thus, although the measurement precision of the sensor is nominally enhanced by operating at high acoustic frequencies and/or with large distances between the transducers, a clear limitation of the prior art is that of generating an ambiguous flow rate output if either too high a frequency or too great a distance is selected. This is a particular problem in applications in which the maximum flow rate of a range of flow rates is not known in advance of installation so that a measurement system designer is driven to select a lower acoustic propagating frequency or a shorter distance between the transducers than is actually necessary. Using a lower frequency or a smaller transducer spacing disadvantageously uses less of the modular phase response range of the phase detector and thereby reduces the phase shift detection sensitivity, which degrades the measurement precision of the sensor.
Ultrasonic transit-time flow sensors typically operate at acoustic propagation frequencies of several megahertz in order to be able to detect the relatively small time differences produced by flowing fluids. At these frequencies, scale or other forms of fouling on wetted transducer surfaces can severely degrade the efficiency of coupling the acoustic energy between the transducer and the fluid. This degrades the quality of measurement of the sensor or, in extreme cases, causes it to cease functioning altogether. Hence, other problems with prior art time-of-flight flow meters are an inability to detect fouling-induced performance degradation and the lack of a backup measurement capability that can be used until corrective maintenance can be performed.
SUMMARY OF THE INVENTION
A preferred embodiment of an improved time-of-flight flow sensor of the invention selectively operates at two distinct frequencies. Operation at a first, relatively low propagation frequency, yields a first phase difference signal that is within the modular response range of a phase detector and is hence unambiguously representative of any flow rate within a range of specified flow rates, but that is characterized by a first measurement error that is larger than desired. The first phase difference signal is preferably used by a signal processor to determine a compensation term that can be combined with a second phase difference signal, measured at a second, higher, frequency to resolve any phase shift detector ambiguity which may occur at high fluid flow rates. The operating modes are preferably time shared and flow rate output signals corresponding to both modes are provided to the signal processor which normally uses the output signal from the lower frequency mode of operation to determine whether the phase detector output from the higher frequency mode is ambiguous or inverted and if so, make the necessary corrections to the flow rate output signal.
As noted above, over a period of operation of a time-of-flight sensor, accumulations of scale or other fouling can cause deterioration of performance and eventual complete failure by degrading the acoustic coupling between the transducer and fluid. This degradation generally increases with frequency, as does the acoustic loss through the fluid. Hence, sensor operation at the lower of the two frequencies is less prone to fouling-induced degradation. In a preferred mode of operation, a flow sensor of the invention detects the amplitude of the received transducer signal in the high frequency mode and, if that amplitude is below a selected level, uses only the flow rate signal from the lower frequency mode as the basis for the sensor flow rate output signal. In addition, when the lower precision flow rate signal has been selected, a preferred sensor of the invention supplies an additional output indicative of a need for maintenance.
A preferred time-of-flight flow meter of the invention comprises two transducers spaced out along a direction in which fluid flows when the device is in use. This preferred apparatus comprises at least one oscillator for supplying transducer-driving signals at each of at least two distinct frequencies so as to cause each of the two transducers to propagate a respective acoustic signal through whatever fluid is present between them. In some embodiments the sensor comprises a switching circuit for sequentially applying only one of the transducer-driving signals at a time to each of the two transducers. It will be understood, of course, that one could also elect to use separate oscillators for each frequency. In addition, the preferred apparatus comprises at least one receiver circuit that may be time shared for receiving the transmitted acoustic signals at each of the at least two frequencies and at least one phase detector for detecting a respective phase difference between the acoustic signals transmitted in the opposite directions at each of the at least two frequencies. Again, it will be understood that although the preferred apparatus is configured to operate in a switched, time shared mode, one could also elect to use greater numbers of some of the components. The preferred apparatus also comprises a signal processor for comparing the phase differences and selecting one of them for use in determining the rate at which the fluid flows.
In a particular preferred embodiment, the transducers comprise piezoelectric elements mechanically configured and sized to resonate at both of the two selected frequencies. This arrangement provides for efficient coupling of the acoustic energy to the fluid at each frequency.
In preferred methods of operation, the two selected frequencies are integral multiples of each other. As will be made clear in the following detailed description, this is a matter of convenience. All that is required is that the two selected operating frequencies be distinct in the sense of being sufficiently different that for a given transducer-to-transducer spacing and a given maximum directly measurable phase difference, there is at least one choice of fluid flow rate within a selected operating range that leads to a situation in which a measured flow-induced phase difference at the higher of the two operating frequencies corresponds to one of several possible flow rates, while the phase difference measured at the lower frequency yields an unambiguous flow rate.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and wishes to practice the invention, it will be rec
Kiewit David
Miller Takisha S
Noori Max
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