Patent Publication Number: US-2017350739-A1

Title: Measurement apparatus, measurement method, and program

Description:
The contents of the following Japanese patent application are incorporated herein by reference:
         NO. PCT/JP2016/053592 filed on Feb. 5, 2016.       

     BACKGROUND 
     1. Technical Field 
     The present invention relates to a measurement apparatus a measurement method and a program. 
     2. Related Art 
     By measuring a time period in which an ultrasonic wave is propagated in a medium, a velocity at which the medium flows (that is, a flow velocity) or a rate at which the medium flows (that is, a flow rate) can be measured, or components of the medium can be identified. 
     Among measurement methods of flow velocity using an ultrasonic wave, a trigger method and a correlation method have been known (for example, refer to Patent Document 1). With the trigger method, a propagation time period when an ultrasonic wave is propagated in a fluid in a flow direction of the fluid and a propagation time period when the ultrasonic wave is propagated in the opposite direction to the flow direction are measured, and the flow velocity is obtained from a difference between results of these measurements. The trigger method has a disadvantage that although the flow velocity can be accurately measured with the trigger method, in a case where a foreign substance is contained in the fluid, for example, in case where air bubbles are contained in a liquid fluid or in a case where liquid drops are contained in an gas fluid, the measurement precision is significantly lowered due to the scattering of the ultrasonic wave resulted from the foreign substance. With the correlation method, the ultrasonic waves are respectively propagated in a forward direction and a backward direction of the flow in the fluid and are received, and the flow velocity is obtained from a correlation between the respective waveforms. Although with the correlation method the flow velocity can be measured even if a foreign substance with a certain amount is contained in the fluid, the correlation method has a disadvantage that the measurement precision is poor, compared to the trigger method. 
     An identification of medium components by using an ultrasonic wave is performed by measuring a propagation time period of the ultrasonic wave in a medium to identify the medium based on the measurement result. Similar to the trigger method, this method also has a disadvantage that in a case where a foreign substance is contained in the fluid, the identification precision is significantly lowered due to the scattering of the ultrasonic wave and a turbulence of the waveform resulted from the foreign substance. 
     [Patent Document 1] Japanese Patent Application Publication No. 2010-38667 
     [Patent Document 2] Japanese Patent Publication No. 5326697 
     In a case where a foreign substance is contained in the medium, the amplitude of the ultrasonic wave is weakened due to the scattering of the ultrasonic wave resulted from the foreign substance, and the amplitude is partially weakened if the scattering is sparse. Accordingly, a signal-to-noise ratio (S/N ratio) is decreased. Here, for example, a plurality of reception signals of the ultrasonic waves are additionally averaged. Accordingly, the amplitude of a signal component included in the reception signal is strengthened by the addition operation, the amplitude of the noise is weakened by the averaging, and the S/N ratio is improved as a result. However, because the ultrasonic wave is not only propagated in the medium but also transmitted to a pipe in which a medium flows, generally, the noise (referred to as an interference noise) derived from the ultrasonic wave transmitted to the pipe is steadily superimposed on the signal component derived from the ultrasonic wave propagated in the medium. Since the interference noise is derived from the ultrasonic wave, that is, a coherent wave, the amplitude of the interference noise is not weakened even if the additionally averaging is performed, strengthened instead in some cases. Also, since those frequencies are equal to each other because the interference noise is derived from the ultrasonic wave, same as the signal component, the filter processing cannot remove the interference noise only. 
     According to the method described in Patent Document  1 , when a flow rate measurement is to be performed, it is determined whether the trigger method or the correlation method is appropriate according to waveform characteristics of a received waveform, a previously measured measurement value, and the like, and the flow rate is measured by switching to the appropriate method based on the determination result. However, in a case where a foreign substance is contained in the medium and the like, if the reception signal becomes small, the method is switched to the correlation method and the measurement precision becomes lowered. 
     Also, according to the method described in Patent Document 2, before the flow rate measurement, a reverberation component is obtained by using a first component which is not influenced by a reverberation and a second component which is influenced by the reverberation, and when the flow rate measurement is to be performed, the reverberation component is removed from the reception signal to measure the flow rate. However, in a case where a foreign substance is contained in the medium, the reverberation component which is influenced by the scattering due to the foreign substance cannot be appropriately removed from the reception signal. 
     SUMMARY 
     In a first aspect of the present invention, a measurement apparatus is provided, including a measurement unit to propagate a first measurement wave and a second measurement wave in a medium and to receive the first measurement wave and the second measurement wave, a difference calculating section to calculate a difference signal obtained by taking a difference between the received first measurement wave and the received second measurement wave, and an identifying section to identify a reception timing of transmission of a measurement wave based on a target signal component included in the difference signal. 
     In a second aspect of the present invention, a measurement method is provided, including a step of propagating a first measurement wave and a second measurement wave in a medium and receiving the first measurement wave and the second measurement wave, a step of calculating a difference signal obtained by taking a difference between the received first measurement wave and the received second measurement wave, and a step of identifying a reception timing of transmission of a measurement wave based on a target signal component included in the difference signal. 
     In a third aspect of the present invention, a program is provided, the program causing a computer to execute a procedure of propagating a first measurement wave and a second measurement wave in a medium and receiving the first measurement wave and the second measurement wave, a procedure of calculating a difference signal obtained by taking a difference between the received first measurement wave and the received second measurement wave, and a procedure of identifying a reception timing of transmission of a measurement wave based on a target signal component included in the difference signal. 
     The summary clause does not necessarily describe all necessary features of the embodiments of the present invention. The present invention may also be a sub-combination of the features described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a configuration of a measurement apparatus according to the present embodiment. 
         FIG. 2  shows a flow of a measurement method according to the present embodiment. 
         FIG. 3  shows a waveform of a measurement wave. 
         FIG. 4A  shows a waveform of a normal reception signal received with respect to the measurement wave of  FIG. 3 . 
         FIG. 4B  shows a waveform of an abnormal reception signal received with respect to the measurement wave of  FIG. 3 . 
         FIG. 5  shows a waveform of a difference signal obtained by a difference between the reception signal of  FIG. 4A  and the reception signal of  FIG. 4B . 
         FIG. 6  shows one example of a hardware configuration of a computer according to the present embodiment. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, the present invention is described through the embodiments of the invention. However, the following embodiments do not limit the invention according to the claims. Also, all the combinations of the features described in the embodiments are not necessarily essential for solving means the invention. 
       FIG. 1  shows a configuration of a measurement apparatus  100  according to the present embodiment. The measurement apparatus  100  is an apparatus which measures a flow velocity of a medium  98  flowing inside a pipe  99  by measuring a time period (referred to as a propagation time period) in which a measurement wave is propagated in the medium  98  in order to measure the flow velocity precisely even if a foreign substance is contained in the medium  98 . Note that it is supposed that the medium  98  is a liquid such as water and contains a foreign substance such as air bubbles. Also, the medium  98  may be gas such as air and vapor, and may contain a foreign substance such as water drop. Also, for convenience of description, the medium  98  is set to flow inside a pipe in a direction shown by an outline arrow in the drawing, and this direction and a direction opposite to this direction are respectively set as a forward direction and a backward direction with respect to the flow of the medium. The measurement apparatus  100  includes a measurement unit  10  and a calculating section  20 . 
     The measurement unit  10  is a unit which propagates a measurement wave in the medium  98  flowing through the pipe  99 , and receives the measurement wave. In the present embodiment, an ultrasonic wave is used as the measurement wave. The measurement unit  10  includes sensors  11 ,  12 , switches  13 ,  14 , a transmission section  15 , a reception section  16 , and a timing generating section  17 . 
     The sensors  11  and  12  are transceivers which transmit and receive ultrasonic waves. The sensors  11  and  12  are respectively fixed to positions on one side of a diameter direction of an outer surface of the existing pipe  99  (that is, an upper side of the pipe  99  extending in a crosswise direction in the drawing) and on the other side (that is, a lower side), where one of the positions is shifted to one side of a flow direction of the medium  98  (that is, an upstream side) and the other of the positions is shifted to the other side (that is, a downstream side). The sensors  11  and  12  receive ultrasonic waves transmitted from the transmission section  15  and output the ultrasonic waves into the pipe  99 , and also receive ultrasonic waves propagated in the medium  98  inside the pipe  99  and output the ultrasonic waves into the reception section  16 . 
     The switches  13  and  14  are switching switches which respectively connect the sensors  11  and  12  to the transmission section  15  or the reception section  16 . The switch  13  receives a switching signal from the timing generating section  17 ; for example, the switch  13  receives a high-level signal (H signal), and connects the sensor  11  to the transmission section  15 , or receives a low-level signal (L signal) and connects the sensor  11  to the reception section  16 . The switch  14  receives from the timing generating section  17  the high-level signal (H signal) and connects the sensor  12  to the reception section  16 , or receives the low-level signal (L signal) and connects the sensor  12  to the transmission section  15 . 
     The transmission section  15  has an ultrasonic wave source (not shown), by using which the transmission section  15  receives a drive signal (for example, an ON signal) from the timing generating section  17  to generate and output the ultrasonic wave. As the ultrasonic wave source, a piezoelectric element containing a mixture, for example, PZT and the like, can be adopted. 
     The reception section  16  receives a drive signal (for example, an ON signal) from the timing generating section  17  and receives the ultrasonic wave, and outputs, to the calculating section  20 , a voltage signal with a strength corresponding to the amplitude of the received ultrasonic wave as the reception signal. Here, the drive signal transmitted from the timing generating section  17  is synchronized with the drive signal transmitted from the timing generating section  17  to the transmission section  15  by a delay of a typical propagation time period of the ultrasonic wave in the medium  98 , for example, or of a constant time period corresponding to the propagation time period. Accordingly, the waveform of the ultrasonic wave output from the transmission section  15  and propagated in the medium  98  can be captured. 
     The timing generating section  17  generates the switching signal (that is, the H signal and the L signal) and transmits the switching signal to the switches  13  and  14 , and also generates the drive signal (that is, the ON signal) and transmits the drive signal to the transmission section  15  and the reception section  16 . The timing generating section  17  synchronizes the switching signal and the drive signal; for example, the timing generating section  17  repeatedly generates the H signal and the L signal in a constant cycle, generates the ON signal synchronizing with the H signal and the L signal. Accordingly, if the switching signal is the H signal and the drive signal is the ON signal, the ultrasonic wave output from the transmission section  15  is transmitted to the sensor  11  via the switch  13 , output from the sensor  11  to the inside of the pipe  99 , propagated in the medium  98  inside the pipe  99  in the forward direction and received by the sensor  12 , and received by the reception section  16  via the switch  14 . If the switching signal is the L signal and the drive signal is the ON signal, the ultrasonic wave output from the transmission section  15  is transmitted to the sensor  12  via the switch  14 , output from the sensor  12  to the inside of the pipe  99 , propagated in the medium  98  inside the pipe  99  in the backward direction and received by the sensor  11 , and received by the reception section  16  via the switch  13 . 
     The calculating section  20  is a unit which calculates a flow velocity of the medium  98  by processing the reception signal output from the measurement unit  10 . The calculating section  20  includes an AD converter  21 , a memory  22 , and a calculating section  23 . 
     The AD converter  21  is connected to the reception section  16  of the measurement unit  10 , and then sequentially converts the input reception signal (that is, a voltage signal) into a digital signal. Accordingly, the reception signal becomes a sequence of a digital value, representing a signal wave which changes relative to the time. The AD converter  21  transmits the converted reception signal to the memory  22 . 
     Note that an amplifier (not shown) may be provided in a fore stage of the AD converter  21 , via which the reception signal may be amplified and input to the AD converter  21 . 
     The memory  22  is connected to the AD converter  21  and stores the reception signal input from the AD converter  21  along with a signal number. The signal number is, for example, a series number in an order that the signal is input from the reception section  16  or an order that the signal from the AD converter  21  is stored in the memory  22 . 
     The calculating section  23  performs a calculation processing on the reception signal stored in the memory  22 . The calculating section  23  implements a difference calculating section  23   a , a polarity determining section  23   b , an adding section  23   c , an identifying section  23   d , a mode selecting section  23   e , and a flow velocity calculating section  23   f  by executing a controlling program. 
     The difference calculating section  23   a  is connected to the memory  22 , generates a difference signal by calculating a difference in waveforms between at least two different reception signals from among a plurality of reception signals stored in the memory  22 , and outputs the difference signal to the polarity determining section  23   b  in a later stage. 
     The polarity determining section  23   b  is connected to the difference calculating section  23   a , and determines a polarity of the target signal component included in the difference signal input from the difference calculating section  23   a . Since most of the plurality of reception signals are in similar figures, the polarity determining section  23   b  may determine the polarity of the target signal component according to, for example, the polarity of a point at which an absolute value of the difference signal becomes maximum. Note that the polarity of the target signal component may be determined to be equal to the polarity of the point at which the absolute value of the difference signal becomes maximum, that is, may be determined as respectively positive and negative relative to positive and negative values of the point at which the absolute value of the difference signal becomes maximum. The polarity determining section  23   b  outputs, along with the determination result of the polarity, the difference signal to the adding section  23   c  in a later stage. 
     Note that based on the determination result of the polarity, the polarity determining section  23   b  may output the difference signal (by +one time) if the polarity is positive, or invert the difference signal (by −one time) and output the inverted difference signal if the polarity is negative. In this case, the adding section  23   c  described later becomes to add the difference signal only. 
     The adding section  23   c  is connected to the polarity determining section  23   b  and the memory  22 , adds the difference signal input from the polarity determining section  23   b  based on the determination result of the polarity, and also adds the reception signal stored in the memory  22  to output the respective results to the identifying section  23   d  in the latter stage. 
     Note that the adding section  23   c  may be set to respectively add a predetermined arbitrary number of the difference signals and a predetermined arbitrary number of the reception signals and output the result to the identifying section  23   d . The arbitrary number is set to be 1 or more. If the arbitrary number is  1 , the adding section  23   c  becomes to output each difference signal and each reception signal to the identifying section  23   d  without performing the adding processing. 
     The identifying section  23   d  is connected to the adding section  23   c , and identifies a reception timing of transmission of the ultrasonic wave for each of the forward signal and the backward signal based on the target signal component, which is included in the added difference signal input from the adding section  23   c , or the added reception signal. For example, the reception timing may be determined by a timing at which the amplitude of the target signal component, which is included in the difference signal, or of the reception signal exceeds a predetermined threshold. The identifying section  23   d  identifies the reception timing based on the target signal component included in the added difference signal in a first measurement mode, and identifies the reception timing based on the added reception signal in a second measurement mode. 
     The mode selecting section  23   e  is connected to the identifying section  23   d , and selects the measurement mode of the identifying section  23   d  by using the difference signal added by the identifying section  23   d  or the individual difference signal. For example, the mode selecting section  23   e  selects the first measurement mode of the identifying section  23   d  if the amplitude of the added difference signal or the individual difference signal is equal to or more than a reference value, or selects the second measurement mode of the identifying section  23   d  if the amplitude is less than the reference value. In a case where no foreign substance is contained in the medium  98 , since the signal wave component in the plurality of reception signals do not weaken the amplitude, not only the interference noise but also the measurement wave are offset by the difference between two reception signals; accordingly, the target signal component in the difference signal becomes not to have an amplitude sufficient for identifying the reception timing. Here, in this case, if the amplitude of the difference signal is equal to or more than the reference value, that is, in a case of abnormality where a foreign substance is contained in the medium and the amplitudes of the signal waves for the individual reception signals are significantly different from each other, the first measurement mode using the difference signal is set to be selected, and if the amplitude of the difference signal is less than the reference value, that is, in a case of normality where a foreign substance is hardly contained in the medium and the amplitudes of the signal waves for the individual reception signals are approximately constant, the second measurement mode using the reception signal is set to be selected. 
     Note that the mode selecting section  23   e  may select the measurement mode of the identifying section  23   d  in accordance with whether or not the amplitudes of a predetermined number or proportion or more of the difference signals among a plurality of the difference signals generated by the difference calculating section  23   a  are equal to or more than a reference value. For example, the mode selecting section  23   e  selects the first measurement mode if the number or proportion of the difference signals having the amplitudes that exceed the reference value exceeds an expected number or proportion, or selects the second measurement mode if the expected number or proportion is not exceeded. 
     Also, not limited to use the added difference signal or the individual reception signal, the mode selecting section  23   e  may select the measurement mode of the identifying section  23   d  by using the added reception signal or the individual difference signal. For example, the mode selecting section  23   e  selects the second measurement mode of the identifying section  23   d  if the amplitude of the added reception signal or the individual reception signal is equal to or more than the reference value, and selects the first measurement mode of the identifying section  23   d  if the amplitude is less than the reference value. Also, the mode selecting section  23   e  may select the measurement mode of the identifying section  23   d  in accordance with whether or not the amplitude of the predetermined number or proportion or more of the reception signals among the plurality of reception signals are equal to or more than the reference value. For example, the mode selecting section  23   e  selects the second measurement mode if the number or proportion of the reception signals having the amplitudes that exceed the reference value exceeds an expected number or proportion, or selects the first measurement mode if the expected number or proportion is not exceeded. 
     The identifying section  23   d  may identify the reception timing, for example, an average reception timing, based on the target signal component included in each of two or more difference signals, or two or more reception signals. Note that when the reception timing is to be identified by using two or more difference signals, it can be considered that the difference signal is generated by adding each difference signal based on the polarity determined by the polarity determining section  23   b , and the identifying section  23   d  identifies the reception timing of transmission of the measurement wave based on the target component included in the difference signal and the polarity determined by the polarity determining section  23   b . The identifying section  23   d  outputs the identified reception timing to the flow velocity calculating section  23   f.    
     The flow velocity calculating section  23   f  is connected to the identifying section  23   d , and calculates the flow velocity of the medium  98  based on the reception timing of transmission of the ultrasonic wave identified for each of the forward signal and the backward signal input from the identifying section  23   d . The flow velocity of the medium  98  is calculated by obtaining a difference in the reception timings of each of the forward signal and the backward signal (equal to a difference in the propagation time periods when the ultrasonic wave is propagated in the medium  98  in the forward direction and in the backward direction) and using the difference and a separation distance between the sensors  11  and  12 . The flow velocity calculating section  23   f  outputs the calculated flow velocity to the outside. 
     Note that not limited to the flow velocity of the medium  98 , the flow velocity calculating section  23   f  may be set to calculate the flow rate of the medium  98  by multiplying the calculated flow velocity with a cross-sectional area of the pipe  99  and output the flow rate to the outside. 
       FIG. 2  shows a flow of the measurement method by using the measurement apparatus  100  according to the present embodiment. 
     In a step S 1 , according to the measurement unit  10 , an ultrasonic wave is propagated in the medium  98  flowing through the pipe  99  and is received. Here, one example of a waveform of the ultrasonic wave generated by the transmission section  15  of the measurement unit  10  is shown in  FIG. 3 . The ultrasonic wave has a waveform with a basic pulse having a positive pulse with a positive amplitude and a negative pulse with a negative amplitude following the positive pulse, the basic pulse repeated for three times. 
     By repeating, for example, at an interval of 1 to 5 milliseconds, the above-described two operations, that is, the switching between the switches  13  and  14  by the switching signal generated by the timing generating section  17  and the driving of the transmission section  15  and the reception section  16  by the drive signal generated by the timing generating section  17 , the measurement unit  10  repeatedly propagates the ultrasonic wave in the medium  98  inside the pipe  99  from the sensor  11  in the forward direction and receives the ultrasonic wave by the sensor  12 , and also propagates the ultrasonic wave in the medium  98  inside the pipe  99  from the sensor  12  in the backward direction and receives the ultrasonic wave by the sensor  11 . Here, one example of a waveform of the ultrasonic wave received by the reception section  16 , that is, the reception signal, is shown in  FIG. 4A  and  FIG. 4B . 
     The waveform of  FIG. 4A  is the waveform of a received normal reception signal with respect to the ultrasonic wave of the waveform of  FIG. 3 . The reception signal has two components. One of the two components is the signal wave which is propagated in the medium  98  inside the pipe  99  from the sensor  11 ( 12 ) and is received by the sensor  12 ( 11 ). The signal wave vibrates in the same cycle as that of the ultrasonic wave generated by the transmission section  15 , and the amplitude of the envelope of the vibration is also increased or decreased in a longer cycle due to the scattering and the like at the pipe wall within the pipe  99 . The other of the two components is the interference noise which is transmitted from the sensor  11 ( 12 ) to the pipe  99  and is received by the sensor  12  ( 11 ). The interference noise vibrates in a cycle equal to that of the ultrasonic wave generated by the transmission section  15 . The envelope of the vibration has an amplitude sufficiently small with respect to the signal wave and is approximately constant over time. In the reception signal, since these two components are superimposed on each other, the interference noise appears around a time t, and the signal wave appears dominantly in the middle of the time t. 
     The waveform of  FIG. 4B  is a waveform of a received abnormal reception signal with respect to the waveform of the ultrasonic wave of  FIG. 3 . The reception signal has two components, that is, the signal wave and the interference noise, similar to the normal reception signal of  FIG. 4A . Here, in a case of the signal wave, the ultrasonic wave output by the sensor  11  ( 12 ) to the inside of the pipe  99  is scattered due to the foreign substance contained in the medium  98  and the amplitude is significantly attenuated. On the other hand, in a case of the interference noise, since the interference noise is derived from the ultrasonic wave transmitted to the pipe  99 , the interference noise is received with the amplitude which is hardly attenuated. Accordingly, in the reception signal, the amplitude of the signal wave for the interference noise is small, that is, the S/N ratio is low. 
     The reception signal received by the reception section  16  is sequentially transmitted to the calculating section  20  and stored in the memory  22  along with the signal number via the AD converter  21 . In the present embodiment, it is supposed that the reception signal (referred to as the forward signal) derived from the ultrasonic wave which is propagated in the medium  98  inside of the pipe  99  from the sensor  11  in the forward direction and is received by the sensor  12 , and the reception signal (referred to as the backward signal) derived from the ultrasonic wave which is propagated in the medium  98  inside the pipe  99  from the sensor  12  in the backward direction and is received by the sensor  11  are respectively assigned with the signal numbers in a reception order and be stored in the memory  22 . 
     In a Step S 2 , by the difference calculating section  23   a  of the calculating section  20 , a difference in the waveforms of a plurality of reception signals stored in the memory  22  is calculated to generate a difference signal. As one example of the waveform of the difference signal, a difference signal obtained by a difference between the reception signal of the waveform of  FIG. 4A  and the reception signal of the waveform of  FIG. 4B  is shown in  FIG. 5 . The difference signal includes two components included in the reception signal, that is, the signal wave and the interference noise, and since the signal waves in two reception signals have different amplitudes of the envelope of the vibration from each other, the signal wave having the envelope with the finite amplitude also appears in the difference signal. On the other hand, since the interference noises in two reception signals have approximately the same amplitudes of the envelope of the vibration, the interference noises are offset by the difference of the two reception signals and hardly appear in the difference signal. Accordingly, the difference signal becomes to the one corresponding to the amplitude difference of the signal wave component only in the waveform of the two reception signals, and may be set as the target signal component for identifying the reception timing of the ultrasonic wave. 
     The difference calculating section  23   a  calculates the difference in the waveforms of each of two or more sets of two reception signals of the plurality of reception signals for each of the forward signal and the backward signal, and generates the difference signal. For example, the difference calculating section  23   a  calculates the difference in the waveforms of each of two reception signals having signal numbers, one of the signal numbers just before the other of the signal numbers, that is, the difference in the waveforms between the i-th reception signal and the i+1th reception signal (where i=1˜total number of the forward signal or the backward signal−1), and generates the difference signal. Otherwise, the difference calculating section 23 a  calculates the difference in the waveforms of each of two reception signals having signal numbers which are before and after N which is a number of two or more, that is, the difference in the waveforms between the i-th reception signal and the i+Nth reception signal (i=1˜total number of the forward signal or the backward signal−N), and generates the difference signal. Accordingly, in order to secure a time lag between the first measurement wave and the second measurement wave, an effect of easily extracting the difference between the first measurement wave and the second measurement wave is obtained. If the flow velocity of the medium  98  is slow, since the fluctuation of the amplitude of the signal waves in the continuous reception signals is smooth, the value of N may be set to be large, and if the flow velocity of the medium is fast, since the fluctuation of the amplitude of the signal waves in the continuous reception signals is rapid, the value of N may be set to be small. 
     In a Step S 3 , the polarity of the target signal component included in the difference signal generated by the difference calculating section  23  is determined by the polarity determining section  23   b . As one example, the polarity determining section  23   b  determines that the polarity of the target signal component is positive (negative) if a value of a point (a point Sm in  FIG. 5 ) at which an absolute value of the difference signal becomes maximum is positive (negative). 
     In a Step S 4 , the difference signal generated by the difference calculating section  23   a  is added for each of the forward signal and the backward signal by the adding section  23   c . Here, the adding section  23   c  adds the difference signal based on the determination result of the polarity determined by the polarity determining section  23   b , that is, if the polarity is positive, the adding section  23   c  adds the difference signal without any inversion (that is, by +one time), and if the polarity is negative, the adding section  23   c  inverts the difference signal (that is, by −one time) and adds the inverted difference signal. Accordingly, the addition may be performed for each of the plurality of difference signals by unifying the polarity which may be variable depending on the sizes of the amplitudes of two reception signals. A white noise included in the added difference signal is removed by an averaging effect. 
     In a Step S 5 , the measurement mode of the identifying section  23   d  is selected by the mode selecting section  23   e . The mode selecting section  23   e  determines whether or not the amplitude of the added difference signal or the individual difference signal is equal to or more than a predetermined reference value, and if the amplitude is equal to or more than the reference value, the mode selecting section  23   e  selects the first measurement mode and moves on to a Step S 6 , and if the amplitude is less than the reference value, the mode selecting section  23   e  selects the second measurement mode and moves on to a Step S 7 . 
     Note that in the Step S 5 , the mode selecting section  23   e  may select the measurement mode of the identifying section  23   d  by using the added reception signal or the individual reception signal. 
     In the Step S 6 , the reception timing of each of the forward signal and the backward signal is identified by the identifying section  23   d  based on the target signal component included in the difference signal added by the adding section  23   c . For example, the identifying section  23   d  identifies the reception timing according to the timing at which the amplitude of the target signal component exceeds the predetermined threshold. After the identification, the measurement apparatus  100  advances the process to a Step S 9 . 
     In the Step S 7 , the reception signal stored in the memory  22  is added by the adding section  23   c  for each of the forward signal and the backward signal. Here, the adding section  23   c  adds the reception signal without operating the polarity. The white noise included in the added reception signal is removed by the averaging effect. 
     In a Step S 8 , the reception timing of each of the forward signal and the backward signal is identified by the identifying section  23   d  based on the reception signal added by the adding section  23   c . For example, the identifying section  23   d  identifies the reception timing according to the timing at which the amplitude of the reception signal exceeds the predetermined threshold. After the identification, the measurement apparatus  100  advances the process to the Step S 9 . 
     In the Step S 9 , the flow velocity of the medium  98  is calculated by the flow velocity calculating section  23   f  based on the identified reception timing of each of the forward signal and the backward signal. The flow velocity calculating section  23   f  calculates a difference (represented as ΔT) of the reception timing of each of the forward signal and the backward signal, and, for example, uses the difference AT of the reception timing and a separation distance between the sensors  11  and  12  related to the flow direction of the medium  98  to calculate the flow velocity V. 
     Note that the flow velocity calculating section  23   f  may multiply the calculated flow velocity by the cross-sectional area of the pipe  99  to calculate the flow rate of the medium  98 . 
     The measurement apparatus  100  of the present embodiment includes a measurement unit  10  to propagate an ultrasonic wave in the medium  98  flowing inside the pipe  99  by using the sensors  11  and  12  which are provided in the pipe  99  and to receive the ultrasonic wave, a difference calculating section  23   a  to calculate a difference signal obtained by taking a difference of waveforms between reception signals of at least two received ultrasonic waves, and an identifying section  23   d  to identify a reception timing of transmission of an ultrasonic wave based on a target signal component included in the difference signal. The interference noise is offset by the difference of waveforms between two reception signals, and only the target signal component corresponding to the amplitude difference of the component of the signal wave only becomes to be included in the difference signal. Accordingly, based on the target signal component included in the difference signal, the reception timing of transmission of the ultrasonic wave can be identified precisely. 
     Note that in the measurement apparatus  100  of the present embodiment, the ultrasonic wave is used as the measurement wave to be propagated in the medium; however, the measurement wave is not limited to the ultrasonic wave. Any measurement wave which can be propagated in the medium  98  flowing inside the pipe  99  from the outside of the pipe  99 , for example, a sound wave, maybe used. In this case, if the interference noise derived from the measurement wave transmitted to the pipe is steadily superimposed on the signal component derived from the measurement wave propagated in the medium, the reception timing can be identified based on the signal component only by eliminating the interference noise from the reception signal. 
     Also, the measurement apparatus  100  of the present embodiment is set to be an apparatus to use the trigger method, that is, the measurement apparatus  100  measures a propagation time period when an ultrasonic wave is propagated in a medium in a flow direction and a propagation time period when the ultrasonic wave is propagated in an opposite direction to the flow direction to obtain a flow velocity from a difference between results of these measurements; however, the measurement apparatus  100  is not limited to the above and may be set as an apparatus to measure the propagation time period of the ultrasonic wave in the medium and identify the medium based on the measurement result or an apparatus to measure a propagation distance arose from a level and the like of the medium. 
       FIG. 6  shows one example of a hardware configuration of a computer  1900  according to the present embodiment. The computer  1900  according to the present embodiment includes a CPU peripheral section having a CPU  2000 , a RAM  2020 , a graphic controller  2075  and a display apparatus  2080  which are interconnected via a host controller  2082 , an input/output section having a communication interface  2030 , a hard disk drive  2040 , and a CD-ROM drive  2060  which are connected to the host controller  2082  via an input/output controller  2084 ; and a legacy input/output section having a ROM  2010 , a flexible disk drive  2050 , and an input/output chip  2070  which are connected to the input/output controller  2084 . 
     The host controller  2082  connects the RAM  2020  to the CPU  2000  accessing to the RAM  2020  at a high transfer rate and the graphic controller  2075 . The CPU  2000  operates based on a program stored in the ROM  2010  and the RAM  2020  to perform controlling on each section. The graphic controller  2075  acquires image data generated on a frame buffer provided within the RAM  2020  by the CPU  2000  and the like, and displays the image data on the display apparatus  2080 . Instead of this, the graphic controller  2075  may include the frame buffer therein, which stores the image data generated by the CPU  2000  and the like. 
     The input/output controller  2084  connects the host controller  2082  to the communication interface  2030  which is a relatively-high-speed input/output apparatus, the hard disk drive  2040 , and the CD-ROM drive  2060 . The communication interface  2030  communicates with other apparatuses via a network. The hard disk drive  2040  stores a program and data used by the CPU  2000  which is within the computer  1900 . The CD-ROM drive  2060  reads a program or data from the CD-ROM  2095  and provides the read program or data to the hard disk drive  2040  via the RAM  2020 . 
     Also, the input/output controller  2084  is connected to the ROM  2010 , the flexible disk drive  2050 , and a relatively-low-speed input/output apparatus of the input/output chip  2070 . The ROM  2010  stores a boot program executed when the computer  1900  runs and/or a program and the like depending on a hardware of the computer  1900 . The flexible disk drive  2050  reads a program or data from the flexible disk  2090 , and provides the read a program or data to the hard disk drive  2040  via the RAM  2020 . The input/output chip  2070  connects the flexible disk drive  2050  to the input/output controller  2084 , and also connects various input/output apparatuses to the input/output controller  2084  via, for example, a parallel port, a serial port, a keyboard port, a mouse port, and the like. 
     The program provided to the hard disk drive  2040  via the RAM  2020  is stored in a recording media, such as the flexible disk  2090 , the CD-ROM  2095 , an IC card, or the like, and is provided by a user. The program is read out from the recording media, installed in the hard disk drive  2040  within the computer  1900  via the RAM  2020 , and executed on the CPU  2000 . 
     The program installed in the computer  1900  and causing the computer  1900  to serve as the measurement apparatus  100  includes a difference calculating program, a polarity determining program, an adding program, an identifying program, a mode selecting module, and a flow velocity calculating program. These programs or modules work on the CPU  2000  and the like to cause the computer  1900  to serve respectively as the difference calculating section  23   a , the polarity determining section  23   b , the adding section  23   c , the identifying section  23   d , the mode selecting section  23   e , and the flow velocity calculating section  23   f.    
     By reading therein information processing described in these programs, the computer  1900  serves as the difference calculating section  23   a , the polarity determining section  23   b , the adding section  23   c , the identifying section  23   d , the mode selecting section  23   e , and the flow velocity calculating section  23   f  which are specific means realized by cooperation among software and various hardware resources described above. Then, according to these specific means, the specific measurement apparatus  100  corresponding to a purpose of usage is constructed by achieving operations or processing on information corresponding to a purpose of usage of the computer  1900  in the present embodiment. 
     As one example, when a communication between the computer  1900  and an external apparatus and the like is to be performed, the CPU  2000  executes a communication program loaded on the RAM  2020  and instructs the communication interface  2030  to perform the communication processing based on the processing contents described in the communication program. The communication interface  2030  is controlled by the CPU  2000 , reads out transmission data stored in a transmission buffer region and the like provided on a storage apparatus such as the RAM  2020 , the hard disk drive  2040 , the flexible disk  2090 , the CD-ROM  2095 , or the like, and transmits the transmission data to the network, or writes the reception data received from the network into the reception buffer region and the like provided on the storage apparatus. In this way, the communication interface  2030  may transfer the transmission/reception data between the storage apparatuses by a DMA (direct memory access) method, or, instead of this, the CPU  2000  may transfer the transmission/reception data by reading out data from a storage apparatus or the communication interface  2030 , which is a transfer source, and writing the data into the communication interface  2030  or the storage apparatus, which is a transfer destination. 
     Also, the CPU  2000  reads all or necessary parts from among files, database, or the like stored in external storage apparatuses such as the hard disk drive  2040 , the CD-ROM drive  2060  (the CD-ROM  2095 ), the flexible disk drive  2050  (flexible disk  2090 ), and the like into the RAM  2020  by the DMA transfer and the like, and performs various processing on the data which is on the RAM  2020 . Then, the CPU  2000  writes back the data on which the processing has be done to the external storage apparatuses by the DMA transfer and the like. In such a processing, since the RAM  2020  may be regarded as temporarily holding the contents of the external storage apparatuses, the RAM  2020 , the external storage apparatuses, and the like in the present embodiment are collectively called a memory, a storage section, a storage apparatus, or the like. Various information, such as various programs, data, tables, databases and the like in the present embodiment, are stored on such a storage apparatus and become the target of the information processing. Note that the CPU  2000  may hold a part of the RAM  2020  in a cache memory, and may also perform writing on the cache memory. Because in such an embodiment also, the cache memory plays a role of a part of the functions of the RAM  2020 , the cache memory is also regarded as being included in the RAM  2020 , a memory and/or a storage apparatus in the present embodiment, unless otherwise they are distinguished from each other. 
     Also, the CPU  2000  performs, on data read out from the RAM  2020 , various processing including various operations, information processing, conditional judgments, information searches/replacements or the like described in the present embodiment that are specified according to an instruction sequence of a program, and writes the data back into the RAM  2020 . For example, in a case where the conditional judgment is to be performed, the CPU  2000  compares various variables shown in the present embodiment with other variables or constants to determine whether or not a condition, such as “larger”, “smaller”, “equal to or more than”, “equal to or less than”, “equal to”, and the like, is satisfied, and if the condition is satisfied (or not satisfied), branches to a different instruction sequence or calls up a subroutine. 
     Also, the CPU  2000  may search information stored in the files, database, or the like within the storage apparatus. For example, in a case where a plurality of entries in which attribute values of second attributes are respectively associated with attribute values of first attributes are stored in the storage apparatus, the CPU  2000  may search an entry, from among the plurality of entries stored in the storage apparatus, which meets the condition specified by the attribute value of the first attribute, and may obtain the attribute value of the second attribute associated with the first attribute which satisfies the prescribed condition by reading out the attribute value of the second attribute stored in the entry. 
     The operations, procedures, steps, and stages of each process performed by an apparatus, system, program, and method shown in the claims, embodiments, or diagrams can be performed in any order as long as the order is not indicated by “prior to,” “before,” or the like and as long as the output from a previous process is not used in a later process. Even if the process flow is described using phrases such as “first” or “next” in the claims, embodiments, or diagrams, it does not necessarily mean that the process must be performed in this order. 
     The program or module shown above may be stored in an external recording media. As the recording media, other than the flexible disk  2090  and the CD-ROM  2095 , an optical recording media such as DVD or CD, magneto-optical recording media such as MO, a tape media, a semiconductor memory such as an IC card, and the like may be used. Also, a storage apparatus such as a hard disk, a RAM, or the like provided to a server system connected to a dedicated communication network or the Internet may be used as the recording media, and a program may be provided to the computer  1900  via the network. 
     While the embodiments of the present invention have been described, the technical scope of the invention is not limited to the above described embodiments. It is apparent to persons skilled in the art that various alterations and improvements can be added to the above-described embodiments. It is also apparent from the scope of the claims that the embodiments added with such alterations or improvements can be included in the technical scope of the invention. 
     The operations, procedures, steps, and stages of each process performed by an apparatus, system, program, and method shown in the claims, embodiments, or diagrams can be performed in any order as long as the order is not indicated by “prior to,” “before,” or the like and as long as the output from a previous process is not used in a later process. Even if the process flow is described using phrases such as “first” or “next” in the claims, embodiments, or diagrams, it does not necessarily mean that the process must be performed in this order. 
     As apparent from the above-described description, according to the embodiments of the present invention, the measurement apparatus, the measurement method, and the program can be achieved.