Detecting apparatus

Abstract: An ultrasonic non-destructive inspecting apparatus is constructed to comprise a transmission signal generator, an ultrasonic probe, a correlator and an adder wherein a transmission singal based on a sequence is generated and transmitted to a specimen, its echo is correlation processed with an appropriate reference signal, and the results of the correlation processes are summed to judge a reflection matter within the specimen.

Description: The present invention relates to an apparatus for detecting a flaw or the like within a specimen and more particularly to an apparatus for performing a non-destructive inspection utilizing transmission waves such as ultrasonic waves, electric waves or micro-waves. Several aspects of prior art will be touched upon. As to the first aspect, an impulse system is often employed in a non-destructive inspecting apparatus utilizing ultrasonic waves. A construction of an example of prior art employing an impulse system is explained referring to Fig. 1. Fig. 1 illustrates a conventional ultrasonic non-destructive inspecting apparatus in a block diagram, shown for example on pages 114 - 140; 173 - 174, "Ultrasonic Detecting Method" compiled by Steel Production 19th Committee, Nihon Gakujutsu Shinkokai published from Nikkan Kogyo Shimbunsha, December 20, 1977. In Fig. 1, a conventional ultrasonic non-destructive inspecting apparatus is constructed by comprising a pulse-generating circuit 1, an ultrasonic probe 2 connected to the pulse-generating circuit 1, a receiving circuit 3 connected to the ultrasonic probe 2, and a display 4 such as CRT or the like connected to the receiving circuit. Incidentally, the ultrasonic probe 2 is contacted with a specimen S made of steel or the like. Next, operation of the conventional example noted above is explained referring to Figs. 2(a), (b) and (c). The pulse-generating circuit 1 generates pulses having short duration of vibration. The ultrasonic probe 2 is driven by these pulses to transmit ultrasonic waves of short pulse width into the specimen S. The probe receives echo reflected within the specimen S and transmit it to the receiving circuit 3. The receiving circuit 3 amplifies the reflection echo and transmits it to the display 4. The display 4 displays the amplified reflection echo. As shown in Fig. 2(b), the reflection echo appears at time t = t₀. The time t₀ is a required time for ultrasonic wave to make a round trip to a reflecting object within the specimen S and therefore it is possible to calculate a position of the reflecting object by measuring the time t₀. However, in the example explained above, there has been a problem in that positional detection of the reflecting object cannot be made with accurate preciseness when a level of the reflection echo is low and a level of noise overlapping the reflection echo is high, that is, an S/N ratio is not good. One countermeasure for solving the above problem would be enlargement of a pulse amplitude for driving the probe 2. However, such countermeasure as above cannot become a perfect solution because there is an upper limitation in enlarging pulse amplitude due to reasons regarding power limitation for elements constituting the pulse-generating circuit 1 and power limitation for the probe 2 in withstanding electric power. On the other hand, as is well known, an ultrasonic inspecting apparatus for detecting a flaw in a metallic material has been manually operated and an automatic ultrasonic inspecting apparatus has been widely introduced recently for the purpose of promoting reliability and performing an effective inspection. However, there has been several problems in automatically inspecting flaws and one of them is to secure a good S/N ratio. While several factors are considered which make an S/N ratio worse, they may be roughly classified into two cases, namely one being the case wherein a detecting signal level is low and the other being the case wherein a level of a signal not used for detection (i.e. noise level) is high. As to the case wherein the detecting signal level is low, it is considered in general that an input signal transmitted to a signal receiving amplifier is quite low. Such a fact that, if the level of the detecting signal is low, it would be burried within an amplifier noise inherently produced due to thermal ion motion in a receiving signal amplifier thereby degrading the S/N ratio has been publicly known as described on pages 177 - 181 of "Ultrasonic Testing Technology" published from Inc. Association - Nihon Nohritsu Kyokai, February 25, 1980. In order to improve an S/N ratio in the case of a low input signal level, it is generally proposed to make an amplitude of an electric signal driving a probe larger so that the signal level is made higher. However, there is an upper limitation with respect to an electric pulse voltage because of limitation on electric elements constituting a transmitting portion and an withstand voltage of a probe. Also, it is not permissible to make an electric voltage impressed on a signal transmitting portion higher for the purpose of prohibiting it from catching fire within explosive atmosphere due to possible electric discharge. As to the case wherein a signal level not used for detection (i.e. noise level) is high, it is noted that one of these problems is that the noise is caused from an outside source. It is well known as disclosed on pages 296 - 298 of "Ultrasonic Testing Technology" published from Inc. Association - Nihon Nohritsu Kyokai, February 25, 1980 that intermixing of the outside electric noise into an amplifier amplifying a detection signal or a signal processing system causes to degrade reliability of the inspection. Further, it is quite rare that all the noises coming from outside can be completely removed. As to the other case, there may be such problems as presence of interfering echo due to an internal construction of a specimen or reverberation echo interfering with a high speed inspection. Regarding the interfering echo due to the internal construction of a specimen, it is a phenomenon experienced in the case where an internal construction of such material as stainless steel or cast iron is relatively rough. The presence of interfering echo, referred to as "Grove echo", from the internal construction observed in such a specimen as above at the time of inspecting the specimen is well known as disclosed on pages 548 - 553 of "Ultrasonic Inspecting Process" published from Kikkan Kogyo Shimbunsha, June 1, 1984. It is difficult to find out a small flaw because situation of interfering echo fluctuates depending on the variation of manufacturing process of the specimen and/or the inspecting frequency to be used. As to interference of echo due to reverberation which causes a problem in a high speed inspection, it becomes critical when attenuation degree of an ultrasonic signal within a specimen is low and a repetition frequency for inspection is high in an automatic inspection. Such a problem as above is encountered in a case where a subsequent transmission pulse is fed into a specimen during a time when the previous ultrasonic signal is under transmission within the specimen without being sufficiently attenuated. This problem is understood as a phenomenon wherein the ultrasonic signal reflected at the time of previous transmission causes interference and it is generally coped with by lowering repetitive inspecting frequency which inherently accompanies a lower inspection speed or by increasing diffusion rate of an ultrasonic beam with the direction of the ultrasonic beam being slightly inclined. Now, a conventional method hereinbefore practiced for improving an S/N ratio is explained hereunder. It is generally considered as effective in order to improve an S/N ratio in an inspection signal to utilize a correlation process. At first, the correlation process is touched upon. Figs. 3(a) and (b) are graphs showing an operational principle of a Barker sequence as an example of finite binary sequence having a sharp correlation function often used as a synchronizing pattern in a pulse transmission, Figs. 4 are graphs explaining, similarly to Figs. 3, an operational principle of a complementary sequence as an example of finite binary sequence having a sharp correlation function, Fig. 5 shows a constitution of an ultrasonic inspecting apparatus performing a correlation process using a random code sequence, Fig. 6 is a drawing for explaining an operation of Fig. 5, and Fig. 7 is a graph indicating frequency characteristics of a transmitting signal, probe and specimen shown in Fig. 6. Figs. 3 show a Barker sequence as one of finite binary sequences having a sharp correlation function as detailedly disclosed on, for example, pages 488 - 490 of "Coding Theory" published from Shokodo, June 30, 1981. In this case, the "binary" means that it possesses two states of "+" and "-". A correlation function ρ aa (k), wherein k is variable relative to a binary sequence, is generally expressed by the following equation (1): where n is a length of the sequence. In this case, a finite sequence "a" is treated as an infinite length sequence having series of "O" at the both sides as expressed by the following equation (2): {a j } = O ··· Oa₀a₁ ····· a n-1 OO ··· O Since a j = 0 for the range of j ≦ -1 and j ≧ n, the equation (1) may be converted to the following equation (3): As seen from the equation (3), noise levels such as thermal noise having no correlation with the sequence may be reduced, and a correlation process is considered to be an effective means for improving an S/N ratio relative to thermal noise. This is also considered to be effective against random noise interfering with an inspecting apparatus such as electric noise derived from a motor or an welding equipment. Further, at the time of inspecting flaws, a certain sequence whose autocorrelation function ρ aa (k) has a sharp peak at k = 0 and becomes sufficiently small in other range (0 < k < n) is required. An absolute maximum value |ρ| max in the range other than the peak (i.e. referred to as a range sidelobes) for evaluating an autocorrelation degree of a binary sequence is expressed by the equation (4) as below: |ρ| max = max{|ρ aa (k)|} In this connection, a finite binary sequence satisfying |ρ| max = 1/n is specifically referred to as a Barker sequence for distinction. Fig. 3(a) shows a signal of binary sequence expressed by the following equation (5) wherein n = 7: {a j } = + + + - - + - Fig. 3(b) shows an autocorrelation function calculated based on the equation (3) wherein -n ≦ k < n. It is noted that the maximum value is recorded at k = 0 and, at the other portions, it is 1/n as the maximum (in this instance 1/7). Figs. 4 show a complementary sequence which is one of finite binary sequences having a sharp correlation function. From the equation (3), it is clear that there is no finite length binary sequence whose autocorrelation function becomes zero at all the points other than at the point k = 0. However, there may be a case wherein the sum of respective autocorrelation functions ρ aa (k) and ρ bb (k) of two binary sequences {a j } and {b j } each having a length of n, i.e. ρ(k) = ρ aa (k) + ρ bb (k) becomes zero at all the points other than at the point k = 0. These two sequences {a j } and {b j } are referred to as a complementary sequence. Fig. 4(a) shows an example of a complementary sequence wherein n = 4 and represents a signal of a binary sequence expressed by the equation (7): {a j } = + + + - {b j } = + - + + Fig. 4(b) shows autocorrelation functions of {a j } and {b j } in the range -n ≦ k < n and calculated based on the equation (3). Fig. 4(c) indicates the sum ρ(k) of the respective autocorrelation functions calculated based on the equation (6). As seen from this drawing, it is theoretically possible to make levels of range sidelobes zero. Fig. 5 shows an example detailedly discussed on pages 888 - 891 of "High-Speed Digital Golay Code Flaw Detection System, IEE 1981 Ultrasonic Symposium Proceeding", by B.B. Lee and E.S. Furgason (hereinafter referred to as Reference A) wherein 1 designates a probe, 2 a transmitter, 5 a code generating source, 10 an analog correlator, 11 a display, 12a and 12b bipolar converters, 13 a selector, 14 a digital delay line, 15 a system controller, 16 a water bath and 17 a target. Figs. 6 are drawings for explaining the operation of Fig. 5. Figs. 6(a), 6(b), 6(c) and 6(d) indicate a synchronizing signal, a code sequence, a selection signal and an output of the bipolar converter 12, respectively. With the synchronizing signal of Fig. 6(a) from the system controller 15, the code generating source 5 generates a binary code sequence such as shown in Fig. 6(b). The code sequence generated at the code generating source 5 is inputted into the bipolar converter 12a connected to the transmitter 2 through the selector 13 as well as into the digital delay line 14. The code sequence inputted into the bipolar converter 12a connected to the transmitter 2 is given an amplitude of +/- depending on its code at the bipolar converter and, thence, the output of Fig. 6(d) is inputted into the transmitter 2 and applied, as a transmission pulse with being amplified, to the probe 1. An ultrasonic signal radiated from the probe 1 is reflected at the target 17 within the water bath 16, fed back as a receiving signal to the probe 1 and, thence, directed to the analog correlator 10. The code sequence of Fig. 6(b) inputted into the digital delay line 14 is delayed by a time (t′) designated by the system controller 15, inputted into the other bipolar converter 12b as a reference code sequence [Fig. 4(f)] and directed, as a signal for a correlation processing with an amplitude of +/- being given at the bipolar converter 12b depending on the input code, to the analog correlator 10. Within the analog correlator 10, the left side operation of the equation (3) is performed with using a multiplier 10a and an integrator 10b wherein the delay time "t′" of the reference signal code [Fig. 4(f)] corresponds to "k" in the equation (3). Therefore, an inspected wave form after the correlation process can be displayed in the display by varying the delay time t′ at each cycle of repetitive transmission in the system controller 15. With employment of the correlation process as above, an S/N ratio has been improved. In case where the process of the complementary sequence explained regarding Fig. 4 is carried out with using the above apparatus, it is not possible to use the apparatus shown in Fig. 3 and addition of some memory means and some means for summing the operational results of correlation operation on two sequences would be required. Incidentally, in a conventional ultrasonic inspecting apparatus performing a correlation process with using finite length binary sequence, transmission signals are given at a level of +/- to a probe. Fig. 7 shows frequency characteristics of a probe, etc. used for a flaw inspecting apparatus and "a" of Fig. 7 indicates a frequency response of a probe 1, "b" a frequency characteristic of a transmission signal wave form used in a conventional apparatus and "c" a frequency responding characteristic of a specimen in which, for example, attenuation is relatively large. There has been a problem in that the transmission signal applied to the probe 1 possesses, due to its feature of a pulse signal, its energy in a relatively low frequency range and the energy in the low frequency range has not been effectively utilized by the probe 1 due to its frequency characteristics. In addition to the above, it is seen that a frequency characteristics of an ultrasonic beam radiated into a body of specimen depends on a frequency characteristics of a probe because a frequency range of a transmission signal is wide. This matter indicates that manufacturing tolerances of a probe affect a flaw detection frequency whereby there has been a problem in that results of flaw detection would not be constant if the probe is switched to the other. Further, Fig. 7 indicates that a certain part of a frequency range wherein a probe is effectively used is cut off depending on an attenuation characteristic of a specimen. Therefore, there has been another problem in that noise may remain at a certain level in a case where attenuation in a specimen is high and an S/N ratio likely becomes worse even an S/N ratio is improved by a conventional correlation process whereby enlarging of an amplitude in transmission signal has been required. Also, there has been a further problem in that a component of code sequence may remain even if a correlation process is carried out and it cannot be reduced because interfering echo is due to an internal construction of a specimen or due to the fact that it is derived from reverberation echo in a high speed inspection. There are further references in the names of the same authors as those of Reference A with respect to a conventional ultrasonic non-destructive inspecting apparatus as follows. "An Evaluation of Ultrasound NDE Correlation Flaw Detection Systems", IEEE Transactions on Sonics and Ultrasonics, Vol. SU-29, No. 6, November, 1982, pp 359 - 369 (Reference B); and "High-Speed Digital Golay Code Flaw Detection Systems", Ultrasonics, July, 1983, pp. 153 - 161 (Reference C). An operation of a conventional example shown in Fig. 5 is touched upon again from further aspect referring to Figs. 8 and 9. Fig. 8 shows a wave form of a transmission signal in a conventional ultrasonic non-destructive inspecting apparatus, for example shown in Reference B and Fig. 9 shows a wave form of a compressed pulse in a conventional apparatus shown in Reference B. In Fig. 8, abscissa is expressed by unit of bits and, thus, it may be regarded as a time if a unit time corresponding to a unit of bit is used. In Reference B, a unit time corresponding to a unit bit is represented by a symbol δ. Therefore, the pulse width of the transmission signal is 63 × δ. This transmission signal is one having a frequency band as a base band wherein an amplitude thereof is coded by a special sequence. As to the coding process, it will be discussed later but the sequence employed therein is explained. A finite length sequence having a length of 63 bits is used and it is produced by cutting one cycle of M-sequence (maximal length sequence) having a cycle length of 63 bits. As to M-sequence, it is detailedly explained, for example, on pages 474 - 499 of "Coding Theory" coauthored by Hiroshi Miyagawa, Yoshihiro Iwataru and Hideki Iwai, published from Shokodo, June 29, 1989 (Reference D) which reference was touched upon earlier. M-sequence is an infinite length cyclic sequence and its component constituting the sequence is a binary sequence comprising two elements. For the two elements, codes "+" and "-", or numerical values of "+1" and "-1" or "1" and "0" are assigned depending on the case. In the example shown in Fig. 8, a finite length sequence is produced by taking out one cycle from M-sequence having an infinite length and a cycle length of 63 bits. Next, explanation is given with respect to an amplitude coding process using this finite length sequence. An amplitude is modulated to ±1 with their relative values per each unit time δ according to appearing order of the sequence elements "±" by assigning an amplitude "+1" and an amplitude "-1" to an element "+1" and an element "-1" constituting the finite sequence, respectively. These signals are referred to as coded signals. In Fig. 9, abscissa is expressed, similar to Fig. 8 by a unit of bit and it will be regarded as a time if a unit time δ is assigned to a unit bit. This compressed pulse is an example wherein a transmission signal coded in its amplitude by a finite length sequence of 64 bits. This sequence is produced by adding 1 bit to the finite length sequence of 63 bits used for producing the transmission signal shown in Fig. 8. Therefore, the pulse width of this transmission signal is 64 δ. The pulse width of echo is almost equal to the above length. As shown in Fig. 9, the majority of the compressed pulse energy is concentrated to the central time range (several bits × δ) in the drawing. The center signal portion having a large amplitude is referred to as a main lobe of a compressed pulse. The pulse width of the main lobe is small. This means that the energy of echo is substantially concentrated to a point on the time axis. Portions of signals having small amplitudes at the opposite sides are referred to as range sidelobes of a compressed pulse. A transmission signal such as shown in Fig. 8 is produced from the signal source 5 and digital delay line 14 through the bipolar converter 12a and the transmitter 2. The ultrasonic probe 1 is driven by this signal. The ultrasonic wave radiated from the ultrasonic probe 1 is reflected at the target 17 and received by the probe 1. The echo received by the probe 1 is transmitted to the multiplier 10a of the analog correlator 10. The pulse width of the above echo is almost equivalent, in length, to that of the transmission signal. That is, energy of the echo is distributed almost uniformly over the time (approximately 63 × δ), that is the pulse width of the transmission signal. On the other hand, the same signal as the above transmission signal is transmitted to the multiplier 10a of the analog correlator 10. The analog correlator 10 performs a correlation operation between the echo and the transmission signal. With this correlation operation, the echo energy distributed approximately uniform over the time on the time axis equivalent in length to that of the transmission signal is compressed substantially to a single point on the time axis. The pulse obtained by the above operation is referred to a compressed pulse. The compressed pulse obtained from the analog correlator 10 is transmitted to the display 11 and displayed as a final result. The distance resolution of the conventional ultrasonic non-destructive inspecting apparatus explained above is dependent on a main lobe pulse width of a compressed pulse (simply referred to as a pulse width of a compressed pulse). While the pulse width of the transmission signal is wide, the pulse width of the compressed pulse is, as touched upon above, narrow. Therefore, in the conventional apparatus, the resolving power is obtained which is similar to that in the case of an ultrasonic non-destructive inspecting apparatus employing a pulse-echo method with the use of a transmission signal originally having a narrow pulse width. On the other hand, an S/N ratio becomes higher as a mean energy of a transmission signal becomes larger. The mean energy of a transmission signal becomes larger as a pulse width of the transmission signal becomes wider. Therefore, in a conventional non-destructive inspecting apparatus, a higher S/N ratio can be obtained compared to that obtained by a pulse-echo method using a transmission signal originally having a narrow pulse width. As explained above, a good resolution and a high S/N ratio can be obtained in a conventional ultrasonic non-destructive inspecting apparatus. However, in a conventional non-destructive inspecting apparatus, there is a problem in that, if levels of the range sidelobes are high, a position of a reflecting body (such as a flaw, etc.) may be misrecognized as in a position corresponding to a time when a range sidelobe of a compressed pulse appears. To avoid such a problem as above, it is necessary to lower levels of the range sidelobes of the compressed pulse. To such end, it is required to lower levels of range sidelobes in an autocorrelation function of a sequence used for coding a transmission signal. In other words, as a sequence to be used for coding a transmission signal, it is necessary to use a sequence in which a level of an autocorrelation function range sidelobes is low. Hereinafter, explanation is given referring to Fig. 10 with respect to range sidelobes of a compressed pulse and range sidelobes of an autocorrelation function of a sequence. As to definition regarding an autocorrelation function of a sequence there is a detailed description in Reference D. Fig. 10 shows a wave form indicating an autocorrelation function of a finite length sequence used for coding the transmission signal shown in Fig. 8. M-sequence is, as described on pages 479 - 483 of Reference D, the sequence wherein an autocorrelation function thereof possesses a sharp peak referred to as a main lobe and range sidelobes having low levels. However, as noted on page 489 of Reference D, if a finite length sequence is produced by picking up one cycle of a cyclic sequence having a low range sidelobe level, a range sidelobe level of an autocorrelation function of the finite length sequence produced as above is not necessarily low as shown in Fig. 10 even the range sidelobe level of an original cyclic sequence autocorrelation function is low. It is described on pages 479 - 480 of Reference D that a range sidelobe level of M-sequence autocorrelation function is 1/n wherein a cycle length is expressed by "n" and a peak value of a main lobe is given as "1" by normalization. Therefore, in the case where a cyclic length is 63 bits, a range sidelobe level is 1/63 = 0.0159. However, it is noted by referring to Fig. 10 that, in the autocorrelation function of finite length sequence used for coding the transmission signal of Fig. 8, a range sidelobe level is at least more than 0.1 and larger, by one order, than the level of M-sequence autocorrelation function range sidelobes. Thus, there is a problem in that, if a sequence produced as a finite length by cutting from M-sequence is used for coding a transmission signal, levels of a compressed pulse range sidelobes becomes high. The compressed pulse shown in Fig. 9 is an example that a range sidelobe level is high. Also, it is disclosed in Reference B that a trial was made with using a sequence cut off by 63 bits from M-sequence. In this case, it is disclosed that, while a shape (pattern) of range sidelobes was different from that of Fig. 9, the difference was not beyond 3 dB compared to the case of Fig. 9. In these examples, it is noted that, if a sequence having a low level in its autocorrelation function range sidelobes is not used as the sequence for coding a transmission signal, lowering a compressed pulse range sidelobe level is not possible. From the foregoing, it is clear that it is considered to be best if a sequence is available which has completely no range sidelobe in its autocorrelation function. However, as disclosed in Reference D, there is no finite length binary sequence with zero range sidelobe. However, as described in the same Reference D, there may be a case wherein, if respective autocorrelation functions of two finite length binary sequences each having the same length are summed, a range sidelobe will completely disappear after the summing operation. Two sequences constituting a pair having such a feature as above is referred to as a complementary sequence. As to the complementary sequence, it is disclosed in detail on pages 82 - 87 of "Complementary Series, IRE Transactions on Information Theory, Vol. IT-7, April, 1961 by M.J.E. Golay. This complementary sequence is referred to either as Golay Complementary Sequence or simply Golay Code. Now, an autocorrelation function of a complementary sequence is explained referring to Figs. 11(a), 11(b) and 12. Figs. 11(a) and 11(b) show wave forms of autocorrelation functions of 1st and 2nd sequences constituting, for example, a complementary sequence shown in Reference D. Fig. 12 shows a wave form indicating the result of the summing operation of autocorrelation functions of the 1st and 2nd sequences. As seen from Figs. 11(a) and 11(b), a high level is observed in the range sidelobes of both the autocorrelation functions. An autocorrelation function (hereinafter referred to as a composite autocorrelation function) obtained by a summing operation has, at the center as shown in Fig. 12, only a sharp peak corresponding to a main lobe and completely no range sidelobe. In References A and C, an ultrasonic non-destructive inspecting apparatus is disclosed. Two sequences constituting a complementary sequence are alternately and repeatedly used and this matter is explained referring to Figs. 13(a), 13(b) and 14. Figs. 13(a), 13(b) and 14 illustrate wave forms showing computer simulation results of 1st and 2nd compressed pulses and composite pulse disclosed in Reference C. Two sequences constituting a complementary sequence are, hereinafter, referred to as 1st and 2nd sequences. Also, two transmission signals produced in a manner similar to that in the case of Fig. 8 are referred to as 1st and 2nd transmission signals, respectively. Two echoes obtained when the ultrasonic probe is driven by the 1st and 2nd transmission signals are referred to as 1st and 2nd echoes, respectively. Further, two compressed pulses obtained by a correlation process similar to Fig. 9 with the use of 1st the 2nd transmission signals are referred to as 1st and 2nd compressed pulses, respectively. First and second transmission signals are alternately repeated with a certain cycle period. In the period when the ultrasonic probe 1 is driven by the 1st transmission signal, the 1st echo is obtained and this 1st echo is correlation processed within the same period with using the 1st transmission signal so that the 1st compressed pulse is obtained. Similarly, the 2nd echo is obtained in the period when the ultrasonic probe 1 is driven by the 2nd transmission signal and this 2nd echo is correlation processed within the same period with using the 2nd transmission signal so that the 2nd compressed pulse is obtained. Further, by making an integration time of the analog correlator 10 longer than the twice of the repetition period of transmission signals, an operation for summing the 1st and 2nd compressed pulses is carried out. The result of summing the 1st and 2nd compressed pulses is referred to as a composite compressed pulse which is displayed on the display 11. As shown in Figs. 13(a) and 13(b), the levels of range sidelobes are high in both the 1st and 2nd compressed pulses. However, as shown in Fig. 14, in the composite compressed pulse only a main lobe appears at the center and there is completely no range sidelobe. As explained above, the conventional ultrasonic non-destructive inspecting apparatus possesses a superior feature in that there is no range sidelobe. However, a complementary sequence having a feature that there is no range sidelobe in its composite auto-correlation function is not always existing for all of natural numbers "n" representing the length of the sequence. A complementary sequence is existing for limited lengths. For example, below 50 in respect of the length, it exists, as disclosed in Reference D, with respect to the cases wherein n = 2, 4, 8, 10, 16, 20, 26, 32 and 40. However, it is not clear in the cases wherein n = 34, 36 or 50. The US 4,167,879 discloses a method and an apparatus for examining a solid having a transmitting signal generating means for generating a transmission signal which consists of a pseudorandom code wave form modulating the sinusoidal wave form of the frequency synthesizer. The echo signal of such a transmission signal is received by a receiving element amplified and demodulated and then subjected to an analyzing process. Accordingly, there has been a problem in the conventional ultrasonic non-destructive inspecting apparatus in that levels of range sidelobes are high and, in the case where a complementary sequence is used which can make the level of range sidelobes zero, a sequence length "n" is available only for limited numbers. In the foregoing, discussion has been made rather at length with respect to prior art and several problems were touched upon. Those problems may be classified as follows. I Insufficient Accuracy in Detecting a Flaw and Insufficient Utilization of Transmission Energy; II Interferences; and III Limitation in Selecting Length of Complementary Sequence. Accordingly, it has been desired to have a detecting apparatus for performing a non-descructive inspection which is free from the drawbacks above and, thus, it is an object of the present invention to provide a non-destructive detecting apparatus utilizing waves such as ultrasonic waves, electric waves or micro-waves which can overcome the drawbacks of prior art. Further, it is an object of the present invention to provide a means for identifying kinds of flaws in the non-destructive detecting apparatus. The object is solved by the features of claim 1. In the detecting apparatus according to the present invention, a wave form having a frequency component f₀ is generated with respect to either positive code or a negative code of a binary code sequence having a sharp auto-correlation function and the phase thereof is altered by 180° in correspondence to the positive or negative code. With the above arrangement, almost all the energy of the transmission signal can be assigned to a frequency band passing through the probe so that the utilization of the transmission energy is improved. In some of the embodiments, the signal, which has already passed through the signal transmitting passages such as the probe and specimen, etc. having different frequency characteristics, is used as a reference signal at the time of performing a correlation operation of a received signal so that a particular wave form may be emphasized. Also, in some embodiments, a plurality of correlation operation portions are provided and the above reference signal added with frequency characteristics of several kinds of flaws are used as reference signals for the respective operation portions. The present invention will be further explained referring to the accompanying drawings following a brief explanation of drawings summarized below. Fig. 1 shows a conventional ultrasonic non-destructive inspecting apparatus in a block diagram; Figs. 2(a) - (c) explain an operation of the conventional ultrasonic non-destructive inspecting apparatus; Figs. 3(a) and (b) are graphs showing an operational principle of a Barker series; Figs. 4(a), (b) and (c) are graphs showing a complementary sequence for explaining a conventional example; Fig. 5 shows a conventional ultrasonic inspecting apparatus having a function of correlation operation; Figs. 6 are drawings for explaining the operation of Fig. 5; Fig. 7 are a drawing for explaining a problem of prior art; Fig. 8 shows a wave form of a transmission signal in a conventional ultrasonic non-destructive inspecting apparatus; Fig. 9 shows a wave form of a compressed pulse in a conventional ultrasonic non-destructive inspecting apparatus; Fig. 10 is a wave form of a finite length sequence autocorrelation function in a conventional ultrasonic non-destructive inspecting apparatus; Figs. 11(a) and (b) are wave forms of a complementary sequence autocorrelation function in a conventional ultrasonic non-destructive inspecting apparatus; Fig. 12 shows a summing result of autocorrelation functions of a complementary sequence in a conventional ultrasonic non-destructive inspecting apparatus; Figs. 13(a) and (b) show a compressed pulse in a conventional ultrasonic non-destructive inspecting apparatus; Fig. 14 shows a composite compressed pulse in a conventional ultrasonic non-destructive inspecting apparatus; Fig. 15 shows a first embodiment according to the present invention in a block diagram; Figs. 16(a) and (b) show a wave form of a transmission signal in the first embodiment of the present invention; Figs. 17(a) and (b) show a result of a correlation operation in the first embodiment of the present invention; Fig. 18 shows a summing result of correlation operations in the first embodiment of the present invention; Fig. 19 shows frequency characteristics of a transmission signal and an ultrasonic probe in the first embodiment of the present invention; Figs. 20(a) and (b) show a wave form of another transmission signal in the first embodiment of the present invention; Figs. 21(a) and (b) show a wave form of another reflection echo in the first embodiment of the present invention; Figs. 22(a) and (b) show a wave form of another correlation operation results in the first embodiment of the present invention; Fig. 23 shows a summing result of another correlation operation in the first embodiment of the present invention; Figs. 24(a) and (b) show a wave form indicating an operation of a conventional ultrasonic inspecting apparatus; Fig. 25 shows a second embodiment according to the present invention in a block diagram; Figs. 26(a) and (b) show a result of a correlation operation in the second embodiment of the present invention; Fig. 27 shows a wave form of a summing result of correlation operations in the second embodiment of the present invention; Fig. 28 is a block diagram showing an example of a correlator of the present invention; Fig. 29 is a block diagram showing another example of a correlation of the present invention: Fig. 30 shows a third embodiment according to the present invention; It is to be noted that the embodiments explained hereinafter with respect to ultrasonic waves are merely for the convenience and they are not limited to use ultrasonic waves but also applicable to use other transmission waves such as micro-waves or electric waves. A construction of a first embodiment of the present invention is explained with reference to Fig. 15. Fig. 15 is a block diagram showing the first embodiment and an ultrasonic probe 102 and a display 104 are the same as those of prior art. In Fig. 15, the first embodiment comprises the same components as those of prior art and others, namely, a transmission signal generator 101B, a correlator 105A coupled to the transmission signal generator 101B and the ultrasonic probe 102, and an adder 106 having a memory function and connected to the correlator 105A. Incidentally, the ultrasonic probe 102 is connected to the transmission signal generator 101B and the display 104 is connected to the adder 106. Next, an operation of the first embodiment is explained referring to Figs. 16, 17, 18 and 19. The transmission signal generator 101B generates transmission signals S a *(t) and S b *(t) and transmits them to the ultrasonic probe 102. As shown in Figs. 16(a) and (b), the transmission signal S a *(t) employs (+, +, +, -) as a 1st sequence A and the transmission signal S b *(t) employs (+, +, -, +) as a 2nd sequence B, the 1st and 2nd sequences A and B constituting a complementary sequence. For the positive sign "+", sin 2πf₀t (0 ≦ t ≦ T) is assigned and for the negative sign "-", -sin 2πf₀t (0 ≦ t ≦ T) is assigned and the above sinusoidal wave portions are arranged on the time axis in the appearing order of the signs, wherein t is a time variable, T a fixed time and f₀ a fixed frequency. The ultrasonic probe 102 is driven by the above transmission signal S a *, radiates ultrasonic waves towards a specimen S, receives a reflection echo G a *(t) reflected at some reflecting portion inside the specimen and transmits it to the correlator 105A. The same signal as the above transmission signal is also transmitted to the correlator 105A. The correlator 105A performs a correlation operation between the reflection echo G a *(t) and the transmission signal S a *(t). The result ρ a * of the correlation operation is expressed by the following equation: ρ a *(t) = ∫G a *(t′)S a *(t′ - t)dt′ [integrating range: -∞ ∼ ∞] Similarly, in the next time phase, the ultrasonic probe 102 is driven by the above transmission signal S b *(t) and the correlator 105A performs a correlation operation between the reflection echo G b *(t) and the transmission signal S b *(t) and its result ρ b *(t) is expressed by ρ b *(t) = ∫G b *(t′)S b *(t′)(t′ - t)dt′ [integrating range: -∞ ∼ ∞] The adder 106 sums the results of correlation operations of equations (101) and (102) and transmits it to the display 104 which displays ρ a *(t) + ρ b *(t). Incidentally, the adder 106 stores ρ a *(t) in itself until ρ b *(t) is transmitted. An operational principle of the first embodiment according to the present invention is explained hereunder. If the 1st sequence A and the 2nd sequence B are in a complementary relationship, the transmission signals S a *(t) and S b *(t) are also in a complementary relationship whereby range sidelobe levels become theoretically zero. Autocorrelation functions of the transmission signals S a *(t) and S b *(t) are expressed, in the following description, by ρ a *(τ) and ρ b *(τ), respectively. These ρ a *(τ) and ρ b *(τ) are defined as follows: ρ a *(τ) = ∫S a *(t)S a *(t - τ)dt [integrating range: -∞ ∼ ∞] ρ b *(τ) = ∫S b *(t)S b *(t - τ)dt [integrating range: -∞ ∼ ∞] As shown in Figs. 17(a) and (b), comparison between the autocorrelation functions ρ a *(τ) and ρ b *(τ) reveals that their amplitudes are the same in respect of their primary lobes as well as their range sidelobes while their phases are the same in respect of their primary lobes but are opposite in respect of their range sidelobes. Accordingly, sidelobe level become zero as shown in Fig. 18, when the autocorrelation functions ρ a *(τ) and ρ b *(τ) are summed. In other words, if the 1st sequence A and the 2nd sequence B are in a complementary relationship, the transmission signals S a *(t) and S b *(t) are in the same relationship. In Figs. 16(a) and (b), there is shown a case wherein an interval T′ that is an arranging interval of sin 2πf₀t and -sin 2πf₀t is larger than an interval T. However, the relationship explained above is also applicable to a case wherein an arrangement interval T′ equals to T. Also there is shown a case wherein a cycle number of sin 2πf₀t and -sin 2πf₀t is 3 (= f₀T) but the relationship explained above holds regardless of a cycle number. Now consideration is given to a case where ultrasonic waves are used in a non-destructive inspection. Let's express a round trip time of a ultrasonic wave between the ultrasonic probe 102 and a reflecting portion within the specimen S by t₀. When the probe 102 is driven by the signal S a *(t), the reflection echo is approximately expressed as S a *(t - t₀). If a correlation operation is carried out between the reflection echo S a *(t - t₀) and the transmission signal S a *(t), the right hand side of the following equation is obtained. ∫S a *(t - t₀)S a *(t -τ )dt = ρ a *(τ - t₀) [integrating range: -∞ ∼ ∞] Similarly, the reflection echo is approximately expressed as S b *(t - t₀) when, at the next time phase, the ultrasonic probe 102 is driven by the transmission signal S b *(t). If a correlation operation is carried out between the reflection echo S b *(t - t₀) and the transmission signal S b *(t), the right hand side of the following equation is obtained. ∫S b *(t - t₀)S b *(t - τ)dt = ρ b *(τ - t₀) [integrating range: -∞ ∼ ∞] When the results of the above correlation operations are summed, the following is obtained. ρ a *(τ - t₀) + ρ b *(τ - t₀). This result is the same as a case where the wave form shown in Fig. 18 is shifted by t₀ along the time axis. That is, it has a primary lobe at τ = t₀ and zero range sidelobe levels. Therefore, a position of the reflection body can be determined by detecting the time when the primary lobe appears. An advantageous effect obtained in the first embodiment according to the present invention is explained hereunder. If the frequency f₀ of the transmission signals S a *(t) and S b *(t) is set at near the center frequency of the ultrasonic probe 102, the frequency characteristics of the transmission signals S a *(t) and S b *(t) and the frequency response characteristic of the ultrasonic probe 102 become as shown in Fig. 19. The transmission signals S a *(t) and S b *(t) have substantially no energy in the low frequency region and, so most of the energy passes through the ultrasonic probe 102. Therefore, compared to prior art, efficiency for utilizing energy in the first embodiment is made higher. Next, in order to evaluate the improvements in the first embodiment with respect to range sidelobes and S/N ratios, the operation under the following conditions is explained referring to Figs. 20, 21, 22 and 23. An ultrasonic probe, so-called a broad band probe, having a center frequency of 5 MHz and a relative band width of 60% or more was used as the ultrasonic probe 102 and steel was employed as the specimen S. As a complementary sequence, the following sequences were employed, each having a length of 8. A = (+, +, +, -, +, +, -, +) B = (+, -, +, +, +, -, -, -) As the transmission signals S a **(t) and S b **(t) corresponding to these complementary sequences A and B, respectively, signals shown in Figs. 20(a) and (b) were employed wherein f₀ = 5 MHz, f₀T = 1.5. An interval T′ between the unit wave forms (the sinusoidal wave portions) was set to be equivalent to T. At first, the ultrasonic probe 102 was driven by the transmission signal S a **(t) and the reflection echo G a **(t) such as shown in Fig. 21(a) was measured. Similarly, the reflection echo G b **(t) such as shown in Fig. 21(b) was measured by using the transmission signal S b **(t). The reason why the wave forms of the reflection echoes G a **(t) and G b **(t) are different from those of the transmission signals S a **(t) and S b **(t) is due to a filtering function of the ultrasonic probe 102 with a finite band width. This filtering function operates doubly since the transmission signals S a **(t) and S b **(t) pass through the ultrasonic probe 102 twice, that is, at the time of transmitting and at the time of receiving. By performing the correlation operation between the transmission signal S a **(t) and the reflection echo G a **(t), the result shown in Fig. 22(a) was obtained. Similarly, by performing the correlation operation between the transmission signal S b **(t) and the reflection echo G b **(t), the result shown in Fig. 22(b) was obtained. Then, by summing the above two operation results, the result shown in Fig. 23 was obtained. The levels of range sidelobes are below -32 dB which is relatively low, and the result also shows that the S/N ratio is higher than 32 dB. Further, for the sake of comparison, an operation of the conventional apparatus of an impulse type shown in Fig. 1 is explained referring to Figs. 24(a) and (b). Incidentally, the ultrasonic probe 102 and the specimen S are the same as those in the first embodiment. By the signal shown in Fig. 24(a), the ultrasonic probe 102 was driven. This signal is the same with respect to its wave form and its peak amplitude as those of the unit portion of the transmission signals S a **(t) and S b **(t). The reflection echo shown in Fig. 24(b) was measured. On this echo, noises were seriously superimposed and an S/N ratio was only about 25 dB. Therefore, it is noted that, upon comparing the results shown in Figs. 23 and 24(b), the first embodiment according to the present invention achieves an advantageous effect in that an S/N ratio is improved by around 7 dB, and also achieves an advantageous effect in that the efficiency of utilizing transmission energy is made higher as shown in Fig. 19. A construction of the second embodiment according to the present invention is explained referring to Fig. 25 wherein a transmission signal generator 101B, an ultrasonic probe 102, a correlator 105A, an adder 106 and a display 104 are the same as those in the first embodiment. In the second embodiment shown in Fig. 25, it is constructed by the same components as those in the first embodiment and, as different components, a reference signal generator 107 an input side and an output side of which are connected to the transmission signal generator 101B and the correlator 105A, respectively. An operation of the second embodiment is explained referring to Figs. 26 and 27. Incidentally, the operating conditions are the same as those in the case of the first embodiment. At first, the transmission signal generator 101B generates transmission signals S a **(t) and S b **(t) to drive the ultrasonic probe 102. These transmission signals S a **(t) and S b **(t) are the same as those in the first embodiment. The ultrasonic probe 102 is driven by the transmission signals S a **(t) and S b **(t), transmits ultrasonic waves to the specimen S, receives their reflection echoes G a **(t) and G a **(t) and transmits them to the correlator 105A. On the other hand, the reference signal generator 107 generates reference signals h a (t) and h b (t) based on the transmission signals S a **(t) and S b **(t) respectively and transmits them to the correlator 105A. These reference signals h a (t) and h b (t) are signals obtained by passing the transmission signals S a **(t) and S b **(t) twice respectively through the ultrasonic probe 102 exhibiting filtering function at the time of both transmitting and receiving. That is, let us express the filtering characteristics (impulse response characteristics) of the ultrasonic probe 102 exhibited at the time of transmitting and receiving by U₁(t) and U₂(t), respectively. Then the reference signals are expressed by the following equations. h a (t) = ∫∫S a **(t′)U₁(t″-t′)U₂(t″-t)dt′dt″ [integrating range: -∞ ∼ ∞] h b (t) = ∫∫S b **(t′)U₁(t″-t′)U₂(t″-t)dt′dt″ [integrating range: -∞ ∼ ∞] Incidentally, the reference signals h a (t) and h b (t) actually used are those obtained beforehand by measuring the reflection echo from the bottom surface of the specimen when the ultrasonic probe 102 is driven by the transmission signals S a **(t) and S b **(t), respectively. The reason why such reference signals were chosen is that the reflection echo from the bottom surface of the specimen S is equivalent to the signal obtained by passing the transmission signals S a **(t) and S b **(t) through the ultrasonic probe 102 having the filtering function twice and that the reflection echo from the bottom surface can be measured with a high S/N ratio. The correlator 105A performs correlation operations between the reflection echoes G a **(t) and G b **(t) and the reference signals h a (t) and h b (t) with a variable τ being changed. The equations regarding the above correlation operations are given as follows: ∫G a **(t)h a (t - τ)dt [integrating range: -∞ ∼ ∞] ∫G b **(t)h b (t - τ)dt [integrating range: -∞ ∼ ∞] The actual integrating ranges in the equations (109) and (110) were limited to finite ranges. That is, ranges where the reference signals may be, without no serious harm, regarded as substantially zero were cutted off. In other words, if symbols T a and T b are assigned to time width, respectively wherein the reference signals h a (t) and h b (t) cannot be regarded as zero, the following integrating ranges were employed in the above equations (109) and (110), respectively. -T a /2 + τ ∼ T a /2 + τ, -T b /2 + τ ∼ T b /2 + τ. Figs. 26(a) and (b) show the results of the correlation operations according to the equations (109) and (110) with the use of the above integrating ranges. Finally, the adder 106 sums the results of the correlation operations according to the equations (109) and (110). That is, the result of the correlation operation according to the equation (109) is stored in an internal memory of the adder 106 and, in the next time phase, the result of the correlation operation according to the equation (110) and the stored result of the equation (109) are summed. Fig. 27 shows the result of the summing operation. Upon comparing this result with the result, shown in Fig. 23, of the correlation operations and the summing operation in the first embodiment wherein the levels of the range sidelobes are below -32 dB, it is noted that the levels of range sidelobes are below -48 dB in Fig. 27 and the level of range sidelobes is improved approximately by 16 dB in the second embodiment. Since the second embodiment according to the present invention is constructed as above, it is useful even under unfavorable condition where an S/N ratio is inferior and achieves an advantageous effect in that the levels of range sidelobes can be remarkably lowered. Incidentally, in the first and second embodiments, the signals were all digitized and correlation operations and summing operations were performed using a computer. As a means for performing a correlation operation, several kinds of means may be considered. Two practical means for performing a correlation operation are explained referring to Figs. 28 and 29. In Fig. 28, a correlator 105B comprises a tapped delay line 150 connected to the ultrasonic probe 102, plural number (Ka) of multipliers 151 connected to the tapped delay line 150 and an adder 152 connected to the multipliers 151. This correlator 105B performs a correlation operation with utilizing the feature of the equation (109) which may be modified as follows: That is the equation (109) can be modified as noted below. ∫G a **(t)h a (t - τ)dt [integrating range: -∞ ∼ ∞] = ∫G a **(t + τ)h a (t)dt [integrating range: -∞ ∼ ∞] ≒ ∫G a **(t + τ)h a (t)dt [integrating range: 0 ∼ T a ] ≒ ΣG a **(kΔt + ℓΔt)h a (kΔt) [k = 1 ∼ K a ] wherein k, ℓ : integer Δt : sampling interval K a : constant t = kΔt , τ = ℓΔt , and T a = K a Δt . In the correlator 105B, Δt is a delay time between the taps of the tapped delay line 150 and K a is a total number of taps. When the reflection echo G a **(t) is inputted to the tapped delay line 150, an output of a tap, for example, of the kth tap is multiplied with a pre-known weight h a (kΔt) by the multiplier 151. Thereafter, the adder 152 sums outputs from all the taps and the result thereof is equal to the above equation (111). Also, the equation (110) can be modified as follows: ∫G b **(t)h b (t - τ)dt [integrating range: -∞ ∼ ∞] ≒ ΣG b **(kΔt + ℓΔt)h b (kΔt) [k = 1 ∼ K b ] wherein T b = K b Δt . The respective correlation operations of the equations (111) and (112) may be performed at different timing phases. In this case, it is possible to use the same and single correlator 105B and simply replace the weights h a (kΔt) in the same construction by the weights h b (kΔt), respectively. Also the same result and effect can be expected with utilizing two systems comprising another tapped delay line, another multipliers and another adder in addition to the tapped delay line 150, multipliers 151 and the adder 152 for performing the two correlation operations independently. In Fig. 29, another correlator 105C is shown and it comprises a Fourier transformer 153 connected to the reference signal generator 107, a corrector 154 connected to the Fourier transformer 153, a Fourier transformer 155 connected to the ultrasonic probe 102, a corrector 156 connected to the Fourier transformer 155, a multiplier 157 connected to the correcting units 154 and 156 and an inverse-Fourier transformer 158 connected to the multiplier 157. This correlator 105C utilizes the fact that the results of the correlation operations expressed by the equations (109) and (110) are equivalent to results obtained by Fourier transforming the respective signals with following multiplication and further inverse-Fourier transforming. That is, the Fourier transformer 153 transforms the reference signals h a (t) and h b (t) from the reference signal generator 107 and the Fourier transformer 155 transforms the reflection echoes G a **(t) and G b **(t). The correcting units 154 and 156 can correct frequency response characteristics of the specimen S, such as frequency characteristics of ultrasonic attenuation, also can correct the frequency response characteristics of the ultrasonic probe 102. The multiplier 157 performs multiplication between the corrected reflection echo G a **(t) and the corrected reference signal h a (t) and similarly multiplication between the corrected reflection echo G b **(t) and the corrected reference signal h b (t). The inverse-Fourier transformer 158 performs inverse-Fourier transformation of the result of the above multiplication. This correlator 105C can, for the reason explained hereinafter, reduce the levels of range sidelobes to zero, i.e. an ideal state. That is, the correcting units 154 and 156 can also correct the frequency response characteristics, i.e. filtering characteristics, of the ultrasonic probe 102. For example, the reference signals h a (t) and h b (t) are, as shown in equations (107) and (108), dependent on the impulse responses U₁(t) and U₂(t) of the ultrasonic probe 102. Therefore, if the impulse responses U₁(t) and U₂(t) are corrected so that they approximate as possible as close to δ function in the time domain, the reference signals h a (t) and h b (t) become close to the transmission signals S a **(t) and S b **(t), respectively. On the other hand, reflection echoes G a **(t) and G b **(t) are also corrected regarding the specimen S and the ultrasonic probe 102. Accordingly, it is possible to approximate the level of range sidelobes to an ideal state, i.e. zero as shown in Fig. 18. While, in the correlator 105C explained above, the reference signals h a (t) and h b (t) were Fourier transformed by the Fourier transformer 153, the similar effect and advantage can also be expected by transmitting the computed results of the Fourier transforms of the reference signals h a (t) and h b (t) to the multiplier 157 at a proper time. In this case, the computed results are previously obtained by Fourier transformation and stored at appropriate memories. In the correlator 105C explained above, two correcting units 154 and 156 are employed; however, a single correcting unit may be connected to a rear stage of the multiplier 157 to achieve the anticipated same effect. In the above explanation regarding the correlators 105B and 105C, correlation operations between the reflection echoes and reference signals are explained; however, they may be utilized to perform correlation operations between the reflection echoes and the transmission signals. In the latter case, portions corresponding to the reference signals may be replaced by the transmission signals. The correlators 105B and 105C may be constructed by means of either software or hardware. It has been confirmed that the respective embodiments explained above can also achieve the same several effects and advantages as those explained under the following conditions. The test was practiced with a cycle number f₀T being changed in the range of 0.5 ∼ 3.0 by each increment of 0.5 and further with an interval T′ between sinusoidal wave portions for each cycle number f₀T being changed in a range of f₀T ∼ f₀T+2.0 by each increment of 0.5. The test was also practiced using metals exhibiting a high attenuation such as an austenite stainless steel and a titanium alloy, etc. as a specimen S. Further, in the second embodiment, actual measured data of the reflection echoes from the bottom surface of the specimen S were employed as the reference signals h a (t) and h b (t); however, the similar effect can be expected by using data obtained from the following calculation. That is, if material constants of the specimen S and components constituting the ultrasonic probe 102 are known, the impulse response characteristics of the ultrasonic probe 102 and the frequency response characteristics derived by Fourier transformation thereof can be obtained by calculations, for example, noted on pages 762 - 767 of "Non-Destructive Test", Vol. 30, No. 10, 1981, and also the frequency response characteristics of the specimen S can be calcualted. A third embodiment according to the present invention will be explained referring to Fig. 30. In Fig. 30, the third embodiment of the present invention comprises the same elements as those in the second embodiment (except for the probe), and others, namely a transmitting ultrasonic probe 102A connected to the transmission signal generator 101B and a receiving ultrasonic probe 102B connected to the correlator 105A. In the third embodiment, the impulse responses of the ultrasonic probes 102A and 102B are used as U₁(t) and U₂(t), respectively, in the equations (107) and (108). The third embodiment exhibits the similar effect and advantage as those in the second embodiment. It is, of course, possible, to apply the transmitting ultrasonic probe 102A and the receiving ultrasonic probe 102B to the first embodiment of the present invention. Also, the application of the present invention is not limited to the previous embodiments and a combination of a multiple complementary sequence comprising more than four sequences and a conventional complementary sequence may be employed. For instance, in the case where a conventional complementary sequence having a length of "4" and a multiple complementary sequence comprising four sequences each having a length of "5" employed in the sixth embodiment are used in combination, six sequences in total are used to repeatedly generate six transmission signals obtained either in amplitude-coding or phase-coding in a manner similar to that in the sixth and seventh embodiments, and the echoes are processed in a manner similar to those in the eighth, ninth and tenth embodiments. In this case, it provides an advantage in that an S/N ratio is further improved to become larger since N in the right side of the equation (302) becomes "6". Also, a length of the conventional complementary sequence and that of a multiple complementary sequence may not be necessarily the same and they may differ as explained above. Also, plurality of multiple complementary sequences and plurality of complementary sequences may be used in combination as in the foregoing for coding transmission signals. In this case, N in the right side of the equation (302) becomes large whereby an S/N ratio is made larger. In addition to the above, since many kinds of sequences are available, a freedom is made large in selecting a combination of a sequence for coding the "i"th transmission signal and a sequence for coding the (i+1)th transmission signal so that the cross-correlation function of the two sequences is made small. Therefore, interferences of reverberation echoes affecting on inspection are effectively reduced. In many of the embodiments explained above, the ultrasonic probe has been explained about a case where it is contacted with the specimen. However, the transmission of the ultrasonic waves between the probe and the specimen may be performed through an appropriate medium such as water. Further the present invention can be applied to a transmitter-receiver system of an independent element constituting an array type ultrasonic probe. In the foregoing, explanation has been given with respect to detection of a flaw or flaws in the specimen; however, the present invention is not limited to such usage as above and it is also applicable to other purposes such as diagnosis, and medical applications, etc. Also, the embodiments utilizing ultrasonic waves have been explained. However, the present invention is not limited to usage of ultrasonic waves but it is applicable to employ other waves such as electric waves or micro-waves. Summarizing the above, with the use of the present invention, the following effects and advantages are expected. a. Range sidelobes can be reduced to substantially zero in a composite compressed pulse; b. Required values for dynamic ranges of a receiving circuits can be reduced; c. Affection of noises on inspection can be reduced; d. Affection of reverberation echoes on inspection can be reduced; e. Improved S/N ratios are available; and f. efficiency for utilizing transmission energy can be improved. The present invention has been explained in detail with respect to the embodiments referring to drawings; however, it is not limited to those explained and it can be modified/changed by those skilled in the art within the scope and gist of the present invention as defined in the claims appended.

Claims: A detecting apparatus comprising: a first transmission signal generating means (101B) for generating a first transmission signal based on a first sequence; a second transmission signal generating means (101B) generating a second transmission signal based on a second sequence which has a complementary relationship to said first sequence; a first transmitting means (102, 102A) for transmitting waves to an object (S) by said first transmission signal; a second transmitting means (102, 102A) for transmitting waves to said object (S) by said second transmission signal; a first receiving means (102, 102B) for receiving a first echo corresponding to said first transmission signal to provide a first echo signal; a second receiving means (102, 102B) for receiving a second echo corresponding to said second transmission signal to provide a second echo signal; a first correlation operation means (105A) for performing a first correlation operation with respect to said first echo signal, and a second correlation operation means (105A) for performing a second correlation operation with respect to said second echo signal, characterized in that said first and second transmission signals are in a complementary relationship and comprise first and second substantially sinusoidal signals, and that an adder means (106) is provided for adding the results obtained by said first and second correlation operations. A detecting apparatus as claimed in Claim 1, wherein said first transmission signal is formed by allocating said first substantially sinusoidal signal to a first component (+) of said first sequence and allocating said second substantially sinusoidal signal to a second component (-) of said first sequence; and said second transmission signal is formed by allocating said first substantially sinusoidal signal to a first component (+) of said second sequence and allocating said second sustantially sinusoidal signal to a second component (-) of said second sequence. A detecting apparatus as claimed in Claim 2, wherein said second substantially sinusoidal signal is obtained by changing the phase of said first substantially sinusoidal signal by 180°. A detecting apparatus as claimed in Claims 1, 2 or 3, wherein said first and said second sequences are finite binary sequences. A detecting apparatus as claimed in any one of Claims 1 to 4, further comprising first and second reference signal generating means (101B, 107) for generating first and second reference signals, and wherein said first correlation operation is performed between said first echo signal and said first reference signal, and said second correlation operation is performed between said second echo signal and said second reference signal. A detecting apparatus as claimed in Claim 5, wherein said first and second reference signals are the same as said first and second transmission signals, respectively. A detecting apparatus as claimed in Claim 5, wherein said first reference signal is the same as said first echo signal previously obtained by transmitting said first transmission signal from said first transmission means (102, 102A) and receiving said first echo from the surface or bottom of a trial object at said first receiving means (102,102B), and said second reference signal is the same as said second echo signal previously obtained by transmitting said second transmission signal from said second transmission means (102,102A) and receiving said second echo from the surface or bottom of said trial object at said second receiving means (102,102B). A detecting apparatus as claimed in Claim 5, wherein said first reference signal is a signal obtained by computing on the basis of frequency response characteristics of a signal path from an output portion of said first transmission signal generating means (101B) through said object to an input portion of said first correlation operation means (105A) and said first transmission signal, and said second reference signal is a signal obtained by computing on the basis of frequency response characteristics of a signal path from an output portion of said second transmission signal generating means (101, 101B) through said object to an input portion of said second correlation operation means (105A) and said second transmission signal. A detecting apparatus as claimed in Claim 8, wherein each of said frequency characteristics of the signal paths includes frequency characteristics relating to reflection of a reflection body in said object. A detecting apparatus as claimed in one of Claims 1 to 9, wherein said first and second substantially sinusoidal signals are sinusoidal signals having a frequency f₀, and said frequency f₀ is determined by frequency characteristics of said first and second transmitting means (102, 102A), frequency characteristics of said object and frequency characteristics of said first and second receiving means (102, 102B) to maximize a signal/noise ratio. A detecting apparatus as claimed in Claim 1, wherein a single generator (101B) is commonly utilized as said first and second transmission signal generating means. A detecting apparatus as claimed in Claim 1, wherein a single transmitter (102, 102A) is commonly utilized as said first and second transmitting means. A detecting apparatus as claimed in Claim 1, wherein a single receiver (102, 102B) is commonly utilized as said first and second receiving means. A detecting apparatus as claimed in Claim 1, wherein a single correlator (105A) is commonly utilized as said first and second correlation operation means. A detecting apparatus as claimed in Claim 1, wherein said first and second transmitting means and said first and second receiving means are constituted by a single common transmitting/receiving means (102). A detecting apparatus as claimed in Claims 1 or 11, wherein said first and second transmission signals are alternately generated in timing. A detecting apparatus as claimed in Claim 16, wherein said first and second sequences are alternately generated in timing. A detecting apparatus as claimed in any one of Claims 1 to 17, wherein said first and second transmitting means are constituted by a ultrasonic probe to transmit ultrasonic waves in accordance with said first and second transmission signals, and said first and second receiving means are constituted by a ultrasonic probe.

Inventors: KOIKE MITSUHIRO C O KAMAKURA S, MISU KOICHIRO C O JYOHODENSHI, NAGATSUKA TSUTOMU C O JYOHODEN, WADAKA SHUSOU C O JYOHODENSHI, KOIKE, MITSUHIRO, C/O KAMAKURA SEISAKUSHO OF, MISU, KOICHIRO, C/O JYOHODENSHI KENKYUSHO OF, NAGATSUKA, TSUTOMU, C/O JYOHODENSHI KENKYUSHO OF, WADAKA, SHUSOU, C/O JYOHODENSHI KENKYUSHO OF
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: G01N29
IPC codes: G01N29