1. Field of the Invention
The present invention relates to an ultrasonic flow velocity meter for measuring a flow velocity and a flow rate of a fluid and, more specifically, to an ultrasonic flow velocity meter and an ultrasonic transducer thereof in which Doppler shift and the difference of propagation time are utilized. Since measurement of the flow rate is possible by multiplying the flow velocity measured by the ultrasonic flow velocity meter by a cross-sectional area of a flow path, the ultrasonic flow velocity meter includes a concept of an ultrasonic flow rate meter.
2. Description of the Related Art
In the related art, an ultrasonic flow velocity meter has been used as a device for measuring the flow velocity and flow rate of a fluid. The ultrasonic flow velocity meter includes a pair of ultrasonic transducers opposed to each other and arranged along a conduit line at a distance from each other, and emits ultrasonic waves from one of the pair of ultrasonic transducers into the fluid flowing in the conduit line and causing the other transducer to receive the ultrasonic waves propagated in the fluid. Then, the flow velocity of the fluid is obtained from the difference between the amount of time that the ultrasonic wave is propagated upstream and the amount of time that the ultrasonic wave is propagated downstream, and the flow rate is measured by multiplying the flow velocity by the cross-sectional area of the flow path.
In the ultrasonic sensor of a time of flight system on the basis of the amount of time required for propagation described above, if the fluid to be measured contains a large amount of particles such as air bubbles or contaminants, there arises a problem for measurement such that the ultrasonic wave emitted in the fluid may be reflected or dispersed by such particles and hence a level of receiving the transmitted component is lowered. For such a reflective fluid, Doppler system utilizing Doppler shift is suitable. Doppler system is a method receiving an ultrasonic wave reflected in the fluid and detecting the amount of Doppler shift occurred in frequency thereof so that the velocity of reflective substances that caused reflection, such as particles, that is, the flow velocity of the fluid is measured. The known flow velocity meter used in Doppler system includes the one for industrial use (JP-A-60-262015) or the one for medical use that measures the blood flow velocity (JP-A-11-137554, JP-A-2002-143161, JP-A-2002-143162).
The flow velocity meter of Doppler system includes Pulse Doppler system in which an ultrasonic wave is emitted in pulses (WO2005/083372) and Continuous Wave Doppler system in which the ultrasonic waves are continuously emitted. Pulse Doppler system has a high spatial resolution and hence has an advantage such that a distribution of flow velocity of a fluid can be obtained. In contrast, since Pulse Doppler system is based on a principle such that the flow velocity is detected from extremely short signal, it has a disadvantage such that an SN ratio is low, and hence it is necessary to average measurement values obtained through repetition of a number of times of measurement. On the other hand, Continuous Wave Doppler system has an advantage such that information on an entire irradiated range can be obtained at once and has a disadvantage such that the high spatial resolution cannot be achieved. The both methods have advantages and disadvantages.
FIG. 9 shows an example of arrangement of an ultrasonic transducer of an ultrasonic flow velocity meter of Doppler system. In other words, the ultrasonic flow velocity meter 101 in the related art shown in FIG. 9 is configured in such a manner that a sending transducer 102a and a receiving transducer 103a are mounted to both sides of a conduit line 111 on the outside thereof via substantially triangular mounting bases 102, 103 in an opposed state, so that an axis of sending of the ultrasonic wave of the sending transducer 102a intersects with an axis of reception of the ultrasonic wave of the receiving transducer 103a at a center portion of the conduit line 111. An ultrasonic wave emitted from the sending transducer 102a passes through the mounting base 102 and a wall of the conduit line 111 and is emitted in the fluid, and then is reflected by air bubbles or particles S in the fluid. Part of the reflected wave passes through the wall of the conduit line 111 and the mounting base 103, and then is received by the receiving transducer 103a. 
FIG. 10 is a cross-sectional view of another example of the arrangement of the ultrasonic transducer of the ultrasonic flow velocity meter of Doppler system. An ultrasonic flow velocity meter 106 shown in FIG. 10 is configured in such a manner that a sending transducer 107a and a receiving transducer 108a are mounted to one side of the conduit line 111 on the outside thereof in adjacent to each other via substantially triangular mounting bases 107, 108, so that an axis of sending of the ultrasonic wave of the sending transducer 107a and an axis of reception of the ultrasonic wave of the receiving transducer 108a extend in parallel with each other. The ultrasonic wave emitted from the sending transducer 107a passes through the mounting base 107 and the wall of the conduit line 111 and is emitted in the fluid, and then is reflected by the air bubbles or the particles S in the fluid. Part of the reflected wave passes through the wall of the conduit line 111 and the mounting base 108, and then is received by the receiving transducer 108a. 
The ultrasonic flow velocity meter 101 of Doppler system shown in FIG. 9 has an advantage such that it is apparent that the flow velocity at the center portion of the flow path is measured, since the axis of sending and the axis of reception of the ultrasonic waves intersects at the center portion of the flow path. In addition, since the propagation route of the ultrasonic wave is apparent, calculation is facilitated and the accuracy is improved. However, in this arrangement, since a long propagation route of the ultrasonic wave that transverses the flow path is formed, the amount of attenuation of propagation due to reflection or dispersion of the ultrasonic wave caused by the air bubbles or the particles increases. Therefore, it has a disadvantage such that if the degree of contamination of the fluid increases, the signal level of the signal received by the receiving transducer 103a and the SN ratio (signal/noise ratio) are lowered, and hence there arise disadvantages such that the accuracy of measurement is lowered, or measurement is impossible for a highly contaminated fluid.
The ultrasonic flow velocity meter shown in FIG. 10 has an advantage such that it can be applied to measurement of the highly contaminated fluid since the propagation route of the ultrasonic wave formed in the fluid is short. However, since the axis of sending and the axis of reception of the ultrasonic wave do not intersect with each other, the area from which the wave is reflected is not clear, and hence the accuracy is deteriorated. Since the original area of the reflected wave is not clear, it is difficult to estimate the flow rate from the result of measurement of the flow velocity and calculation is complicated.
The ultrasonic flow velocity meter in conjunction with FIG. 9 and FIG. 10 also has a disadvantage such that when the degree of contamination of the fluid to be measured is changed, the arrangement of the sending transducer and the receiving transducer must be changed in order to achieve measurement suitable for the degree of contamination. The both ultrasonic flow velocity meters have a common disadvantage such that the level of reception of the reflected wave is low in the case of a clear fluid in which the density of the air bubbles or particles that generate the reflected waves is low, so that measurement is disabled. In the configurations of the ultrasonic flow velocity meters shown in FIG. 9 and FIG. 10, the entry angle of the ultrasonic wave from the sending transducer into the fluid must be coincided with that of the receiving transducer for calculation. Therefore, there may be a case in which these entry angles are physically shifted, and hence the measurement accuracy is deteriorated. In the configurations of the both ultrasonic flow velocity meters shown in FIG. 9 and FIG. 10, the transmitted signal and the reflected signal (received signal) are mixed in a Doppler shift detection circuit for calculating the flow velocity of the fluid. Since the transmitted signal and the received signal are mixed to derive a signal of torsional component (the difference frequency component) (heterodyne method) for calculating the flow velocity, a circuit for mixing the signal is required.