Apparatus and method of lensing an ultrasonic beam for an ultrasonic flow meter

A method for sensing flow within a pipe having an internal passage disposed between a first wall portion and a second wall portion is provided, comprising the steps of: 1) providing a flow meter having at least one ultrasonic sensor unit that includes an ultrasonic transmitter attached to the first wall portion and an ultrasonic receiver attached to the second wall portion and aligned to receive ultrasonic signals transmitted from the transmitter; 2) selectively operating the ultrasonic transmitter to transmit a beam of ultrasonic signal, which beam has a focal point such that within the pipe, the beam is either colliminated, divergent or convergent; and 3) receiving the ultrasonic signals within the beam using the ultrasonic receiver. An apparatus operable to perform the aforesaid method is also provided.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to methods and apparatus for improving the signal-to-noise ratio of ultrasonic sensors utilized for sensing fluid flow within a pipe in general, and to methods and apparatus for lensing an ultrasonic beam or signal to optimize the signal properties of the transmitted ultrasonic beam passing through a fluid flow having entrained particles or bubbles.

2. Background Information

Multiphase fluid flows pose certain issues for flow meters utilizing ultrasonic sensors. In addition to other factors, such as absorption and reflections, particles and/or bubbles entrained within a fluid flow will attenuate an ultrasonic signal or beam propagating through the fluid. The attenuation of the signal may result in a low signal-to-noise ratio, which in turn can make it more difficult to accurately detect and measure the transmitted ultrasonic signal.

For those ultrasonic flow meters that utilize transmitter/receiver sensor units disposed outside the flow pipe the signal-to-noise ratio of the device can also be negatively affected by undesirable “secondary” signals that are portions of the original signal that have been refracted or reflected along a different path through the pipe instead of through the fluid flow. These secondary signals often possess sufficient strength to reach the receiver transducer of the sensor unit, and may interfere with the desired signal passing through the fluid flow. Examples of undesirable secondary signals include “ring-around” signals that travel through the pipe wall.

FIG. 12diagrammatically illustrates an ultrasonic sensor arrangement where ring-around signals and through-transmitted signals are generated in a sensor unit having an ultrasonic transmitting sensor40and the ultrasonic receiving sensor42of a sensing unit attached to the exterior surface of a pipe14. The sensors40,42are arranged such that the generated ultrasonic signal15is injected normal to the pipe14(and the flow path through the pipe), with the intent to have the injected signal travel through the center of the fluid within the pipe14. As the ultrasonic signal15bisects the pipe14, bubbles17and other matter within the pipe14will scatter and attenuate the signal before it fully traverses the pipe14and is detected by the receiving transducer42. At the same time, ring-around signals19, created through reflection and diffraction between the ultrasonic transmitter and the wall, travel within the pipe wall and are sensed by the receiver42along with any desired signals. The noise created by the secondary signals (e.g., ring-around signals) coupled with the significant attenuation of the through-transmitted signal, creates an undesirable signal-to-noise ratio that makes it difficult to accurately detect and measure the desired through-transmitted ultrasonic signal.

What is needed is a method and apparatus operable to optimize the signal properties of an ultrasonic signal to improve the detection of the ultrasonic signal under various flow conditions related to particles and bubbles entrained within the fluid flow, and one which thereby diminishes the effects of undesirable noise resulting from secondary signals.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a method and apparatus operable to optimize the signal properties of an ultrasonic signal to improve the detection of the ultrasonic signal under various flow conditions related to particles and bubbles entrained within the fluid flow, which method and apparatus thereby diminishes the effects of undesirable noise resulting from secondary signals.

According to the present invention, an ultrasonic signal sensing apparatus for sensing flow within a cylindrical pipe having an internal passage is provided. The apparatus includes at least one ultrasonic signal transmitter attached to a first wall section of the pipe, at least one ultrasonic signal receiver attached to a second wall portion of the pipe, and a lens disposed between the first wall section of the pipe and the transmitter. The transmitter is operable to transmit ultrasonic signals into the pipe. The receiver is aligned with the transmitter to receive transmitted ultrasonic signals. The lens is disposed between the first wall section of the pipe and the transmitter, and aligned with the transmitter such that ultrasonic signals transmitted by the transmitter must pass through the lens prior to entering the internal passage of the pipe. The lens changes the focal point of a signal beam of the ultrasonic signal such that within the pipe, the signal beam is either colliminated, divergent or convergent to thereby optimize the signal properties of the transmitted ultrasonic signal to improve the detection of the ultrasonic signal by the ultrasonic signal receiver under various flow conditions related to particles and bubbles entrained within the fluid flow. The lens may be formed of a single material or multiple materials, each material having a selected speed of sound and shape to provide the desired focal point of the signal beam.

According to the invention, an ultrasonic sensing device for sensing flow within a pipe is provided. The apparatus includes at least one sensor mounted to the pipe and a lens. The sensor has a transmitter mounted on a first wall portion of the pipe for transmitting an ultrasonic signal into the pipe and a receiver mounted on a second wall portion of the pipe for receiving the transmitted ultrasonic signal. The lens is disposed between the transmitter and the first wall portion of the pipe. The lens is formed of one or more materials and is operable to shape the transmitted ultrasonic signals into a beam having a selected focal point such that the beam is either colliminated, divergent or convergent within the pipe.

According to the present invention, a method for sensing flow within a pipe having an internal passage disposed between a first wall portion and a second wall portion is provided, comprising the steps of: 1) providing a flow meter having at least one ultrasonic sensor unit that includes an ultrasonic transmitter attached to the first wall portion and an ultrasonic receiver attached to the second wall portion and aligned to receive ultrasonic signals transmitted from the transmitter; 2) selectively operating the ultrasonic transmitter to transmit a beam of ultrasonic signal, which beam has a focal point such that within the pipe, the beam is either colliminated, divergent or convergent; and 3) receiving the ultrasonic signals within the beam using the ultrasonic receiver.

According to the present invention, a method for sensing flow within a pipe having an internal passage disposed between a first wall portion and a second wall portion is provided, comprising the steps of: 1) providing a flow meter having at least one ultrasonic sensor unit that includes an ultrasonic transmitter attached to the first wall portion and an ultrasonic receiver attached to the second wall portion and aligned to receive ultrasonic signals transmitted from the transmitter; 2) selectively transmitting ultrasonic signals from the transmitter; 3) shaping the transmitted ultrasonic signals into a beam that is either colliminated, divergent or convergent within the pipe; and 4) receiving the ultrasonic signals within the beam using the ultrasonic receiver.

An advantage of the present invention method and apparatus is that a fluid flow having bubbles and/or particles entrained within the fluid flow can more accurately be sensed. Depending upon the specific flow conditions, the lens can be used to colliminate, diverge or converge the ultrasonic signal within the pipe to thereby optimize the signal characteristics of the ultrasonic signal at the receiver. For example, an ultrasonic signal beam that is colliminated within the fluid flow is less susceptible to being completely blocked by the bubbles and particles within the fluid flow. A narrow beam width associated with a convergence point makes the beam more susceptible to being completely blocked by bubbles or particles, but also provides very high power transmission of the ultrasonic signal and signal to noise at the receiver when the convergent beam is not blocked by a particle or bubble.

DETAILED DESCRIPTION OF THE INVENTION

The invention teaches methods to optimize the signal properties of a transmitted ultrasonic beam passing through a fluid flow having entrained particles or bubbles, the ultrasonic beam being provided by a clamp-on flow ultrasonic flow meter, similar to that described in U.S. patent application Ser. No. 10/756,977, filed Jan. 13, 2004, which is incorporated herein by reference.

FIGS. 1 and 2diagrammatically depict an ultrasonic clamp-on flow meter10, an example of which is described in U.S. patent application Ser. No. 10/756,977. The ultrasonic flow meter10includes an array of ultrasonic sensors16having a plurality of ultrasonic sensor units18-21disposed axially along the length of the pipe14. Each ultrasonic sensor unit18-21comprises a transmitter40and a receiver42. The transmitter40provides an ultrasonic signal to the corresponding receiver42, wherein the ultrasonic signal is orthogonal to the direction of the fluid flow12. While this embodiment of the present clamp-on ultrasonic meter10is described, one will appreciate that the present invention is applicable to the other embodiments described and taught in U.S. patent application Ser. No. 10/756,977, including embodiments in non-orthogonal ultrasonic signals, pitch and catch configurations, pulse echo configurations, and combined transmitter/receiver ultrasonic sensors, as shown respectively inFIGS. 3-6.

The signals S1(t)-SN(t) received from each ultrasonic sensor unit18-21are processed by an ultrasonic signal processor37and a signal processor24(having FFTs30-33and an array processor36) for determining the velocity of the fluid flow and/or volumetric flow rate of the fluid flow. One should appreciate that the present invention is applicable to at least all the configurations of an ultrasonic flow meter considered hereinbefore.

Multiple techniques may be used to process the transmitted ultrasonic signals in order to provide the flow rate of the fluid flow12. For example, the time it takes for the ultrasonic signal to arrive at the receiver42for each sensor unit18-21is calculated and fed to signal processing algorithms (in the array processor36) where the flow rate is calculated. Alternatively, the characteristics of each of the signals S1(t)-SN(t) received from each ultrasonic sensor unit18-21are processed by an ultrasonic signal processor37and a signal processor24and compared in a cross-correlation algorithm or Sonar algorithm to provide the volumetric flow rate or fluid velocity. This algorithm works well when a good signal-to-noise ratio is observed at the receiver. When bubbles or particles intersect the signal path between the transmitter and receiver, however, significant attenuation can occur that can result in severe degradation of the received signal quality. The amount of attenuation will vary depending on the bubble and particle characteristics such as size and density.

The transmitting ultrasonic transducer array is periodically pulsed to create the ultrasonic signal that bisects the pipe. Each transducer will have a fundamental oscillation frequency, which when pulsed will emit a short ultrasonic burst signal.FIG. 7diagrammatically shows an ultrasonic signal created by a 1 MHz ultrasonic transducer when pulsed with a 10 nS width pulse created in the flow meter10. In typical applications the receiving ultrasonic receiver42, located on the opposite side of a pipe14, will receive this signal once it has bisected the pipe. In addition to the primary through-transmitted signal, however, other unwanted secondary signals will also be detected. As indicated above, secondary signals include portions of the original signal that have been refracted or reflected along a different path through the pipe than the preferred direct bisection.FIG. 8shows an example of a received ultrasonic signal along with an unwanted ‘ring-around’ secondary signal. The arrow inFIG. 8indicates the location of the through-transmitted pulse in relation to the large ‘ring-around’ signal. Contrast this to the clean ultrasonic signal seen inFIG. 7.

The quality of any flow measurement, independent of the technology, is dependent upon the signal-to-noise ratio. Noise, in this case, may be defined as any portion of the measured signal that contains no flow information. It is desirable to maximize the signal-to-noise ratio to obtain optimum sensor performance. This can be done by decreasing the attenuation of the transmitted ultrasonic signal.

Through-transmitted ultrasonic signals emitted by the transmitters40may be viewed as a “beam”. The ultrasonic signal beam may be shaped and redirected by using materials with desirable impedances disposed within the acoustic path. The material or “lens” within the path of the ultrasonic signal effectively changes the focal point of the ultrasonic signal, thereby changing the beam shape of the ultrasonic signal. For example, a convergent beam that comes to a focus point inside the pipe or at the location of the receiver could potentially be blocked by a small obstruction, such as a bubble or particle, whereas a collimated or divergently shaped beam would not be as likely to be blocked. In a cross correlation algorithm, it may be desirable to utilize a convergent focused beam (where the beam is focused at or near the receiver42) such that the bubbles or particles passing by the pairs of ultrasonic transmitters and receivers may be correlated. In this type of cross-correlation application, it may be desirable to tailor the beam so the maximum power is placed on the receiver sensor42where the obstructions, such as bubbles and particles, can modulate the beam amplitude. Alternatively, in a transit time configuration, it may be desirable to provide a beam having a distant or infinite focal point such that the beam is colliminated or divergent within the pipe. In this case, only a portion of the beam may be blocked by any one bubble or particle. In general, it may be desirable to vary the statistic of the beam modulation; i.e., vary the amount of the flow captured by the beam, and therefore statistically how much of the entrained bubbles, particles, or other aspect of the flow is exposed to the beam. Varying the beam shape is one way to accomplish this.

A ray trace analysis was conducted to better understand the lensing impact of the curved interfaces traversed by the acoustic beam; i.e., curved interfaces like those encountered by signals emitted from an ultrasonic sensor externally mounted on a pipe. The results of this study showed the potential to improve the signal robustness to accommodate entrained air and/or particulates within the flow stream. For example,FIG. 9diagrammatically illustrates an ultrasonic signal beam path44created by using a metal shoe or lens46as the interface between the pipe14and the transmitter sensor40. The metal shoe has a sound speed of about 5850 m/s and a density of about 7800 kg/m3. In this configuration the acoustic beam44is focused at a point located near to the transmitter sensor40, and potentially within the process flow12.FIG. 10diagrammatically illustrates an ultrasonic signal beam44created by using a plastic lens46as an interface between the pipe14and transmitter sensor40. The plastic shoe has a sound speed of about 2400 m/sec and a density of about 1200 kg/m3. In this case, the focal point48of the ultrasonic beam44shifts to the wall of the pipe14opposite the wall where the transmitter sensor40is mounted. In a transit time application, this configuration would be susceptible to bubbles and contaminants. However, as discussed above, for a cross correlation application, this configuration may be desirable.FIG. 11diagrammatically illustrates an ultrasonic signal beam44created by using a lens46comprising a low sound speed material such as an RTV polymer as an interface between the pipe14and transmitter sensor40. The RTV polymer has a sound speed of about 1300 m/s and a density of about 1200 kg/m3(water has a sound speed of about 1480 m/s). The elimination of the ultrasonic beam focus within the fluid flow12by improved collimation improves the signal robustness in the presence of entrained air and particulate flow; e.g., the more collimated beam44does not narrow to a focal point within the flow, which focal point could coincide with a bubble or a particle traveling within the flow for a period of time thereby potentially causing a loss of signal. The broader signal beam44is less apt to be completely blocked.

Although an RTV polymer is an example of an acceptable lens material, the present invention contemplates that the lens46may comprise any material having a sound speed that provides the desired focal point of the ultrasonic beam, and therefore the desired ultrasonic beam shape. In addition, the present invention contemplates that one or more materials may be used to form the lens46to thereby provide the desired focal point and corresponding beam shape. Additionally, the shape of the lens material may be varied to provide the desired focal point and corresponding beam shape. The invention also contemplates that other methods known in the art and in the field of optics to collimate, converge or diverge the ultrasonic signal may be used.

As indicated above, the quality of any flow measurement, independent of the technology, is dependent upon the signal-to-noise ratio. The present invention of focusing the signal beam improves the signal-to-noise ratio by decreasing the amount of through transmitted signal attenuation, thereby increasing the amount of desirable signal relative to the level of noise present in the system; i.e., the signal-to-noise ratio.

It should be understood that any of the features, characteristics, alternatives or modifications described regarding a particular embodiment herein may also be applied, used, or incorporated with any other embodiment described herein.

Although the invention has been described and illustrated with respect to exemplary embodiments thereof, the foregoing and various other additions and omissions may be made therein and thereto without departing from the spirit and scope of the present invention. It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous other modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention, and the appended claims are intended to cover such modifications and arrangements.