Abstract:
A non-destructive testing and inspection (NDT/NDI) system and method operable to conduct an ultrasonic scanning test on a test object that synchronizes and merges the apertures of two or more NDT sub-instruments in frequency and phase. Disclosed are a method of using a Phased Lock Loop (PLL) as a synchronizing clock/trigger generator, and also a method of using a General Positioning Clock (GPS) and a pulse per second (PPS) output. Both methods combine ultrasonic scanning data acquisition from two or more NDT sub-instruments, and transform the sub-instruments into one bigger NDT instrument.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit and priority of U.S. Provisional patent application Ser. No. 62/029,051 filed Jul. 25, 2014 entitled A METHOD AND SYSTEM OF CONSOLIDATING MULTIPLE PHASED ARRAY INSTRUMENTS, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to non-destructive testing and inspection (NDT/NDI) technology instruments, and more particularly to a system and method of consolidating a group of phased array NDT instruments, each with different measuring apertures, any one or more of which may be selected by the user. 
     BACKGROUND OF THE INVENTION 
     A typical problem in phased array ultrasound (PAUT) NDT inspection utilizing multiple instruments is combining the data from multiple inspection instruments and obtaining phase-synchronous and frequency-synchronous data. Typically, the inspection instruments are neither synchronized in frequency nor in phase, making it hard to match the data from one instrument with another since they are not on the same time-base. 
     As the acquisition time gets longer, the problem is exacerbated. Assuming that one sub-instrument is the true time, the other sub-instruments acquire data slightly faster or slower. Thus, even if the data is synchronized using one particular NDT inspection feature, another inspection feature acquired at a later time is likely to be unsynchronized. 
     Inspection events with NDT instruments are often time critical. Having to adjust the phase and frequency of inspection features from multiple instruments can distract the effort of an inspection operator. Patent WO2010/017445A3 disclosed an apparatus that aims to increase the aperture of a medical ultrasound system using add-on ultrasound systems. However, clock synchronization is not performed. Instead, synchronization is performed by comparing the timing of received pulses from the different add-on instruments. This method is not good enough for industrial purposes because in industrial NDT data acquisition the acquisition time can be much longer, and time slip is more likely to occur. If one particular feature is time synchronized among multiple instruments, a feature acquired at a later time is likely to be unsynchronized because of the time slip. 
     A technical paper published on the Cornell University web site (“Multi-Channel Data Acquisition with Absolute Time Synchronization” by P. Wlodarczyk, S. Pustelny, D. Budker, and M. Lipiński) at http://arxiv.org/ftp/arxiv/papers/1311/1311.5849.pdf discloses a method to synchronize multiple channels of data on remote acquisition cards with absolute time synchronization. However, the application described in the paper is different from the one described in the present disclosure, and the sampling rate is about a thousand times slower. In contrast, the present invention deals with the most acute problems of consolidating data from multiple unsynchronized instruments. 
     Considering the background information above, a solution that provides a plurality of NDT instruments to service providers, while minimizing or eliminating manual adjustment of multiple phases and frequencies, would be of great utility and economic value. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present disclosure to overcome the problems associated with the background art by introducing a system and method of consolidating the apertures of multiple NDT instruments by synchronizing or compensating the sub-instruments both in frequency and in phase, wherein the aperture merging between the sub-instruments is seamless at the instrument supervisor level. Several embodiments are herein proposed to achieve this goal. 
     It is further an object of the present disclosure to overcome the problems associated with the background art by introducing a method and system that contains NDT instruments with different probes and apertures, any one of which may be selected by the user as the basis for the sole phase and frequency. 
     It is further an object of the present disclosure to maintain the integrity of the factory or service center calibration when switching between the aperture of one NDT sub-instrument to another. 
     It is further an object of the present disclosure to minimize the number of phase and frequency adjustments by combining them in such a way as to be shared by at least two or more NDT sub-instruments. 
     It is yet a further object of the present disclosure to provide a convenient manual or semi-automatic means of phase and frequency adjustment to allow the other NDT sub-instruments to configure themselves to work properly with the selected NDT sub-instrument&#39;s phase and frequency. 
     These and other objects of the present disclosure can be realized with a PAUT NDT instrument comprising multiple sub-instruments, wherein the NDT instrument includes: hardware and software compatible with the NDT sensor technology; a means to preserve calibration integrity when one NDT sub-instrument&#39;s phase and frequency is switched to follow another; a user interface suitable for each NDT sub-instrument&#39;s aperture; a sensor connection means allowing sharing among different NDT instrument apertures; and a means to automatically detect the aperture type and to configure the instrument to operate in a suitable manner. 
     Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE OF THE DRAWINGS 
         FIG. 1  is a schematic view of the consolidated PAUT system with a supervisor instrument and its sub-instrument components according to the present disclosure. 
         FIG. 2  is a schematic view of the PAUT instrument of the present disclosure, including the supervisor instrument and its sub-instrument software/firmware modules, showing how they relate to an NDT inspection. 
         FIG. 3  is a flow chart view illustrating set-up and test of the instrument, supervisor and sub-instruments, using a clock generator and a Phase-Locked Loop (PLL). 
         FIG. 4  is a schematic view of the supervisor instrument and sub-instrument interconnections of the present disclosure, using a clock generator and a PLL. 
         FIG. 4 a    is a schematic view showing details of the daisy-chain sub-instrument interconnections. 
         FIG. 5  is a flow chart view of the steps of a factory PLL calibration. 
         FIG. 6  is a grouped depiction of synchronizing clocks, triggers, and PLLs for each sub-instrument, using a clock generator and a PLL. 
         FIG. 7  is a schematic view of the NDT-instrument supervisor instrument and sub-instrument interconnections of the present disclosure, using a clock derived from a wireless connection with the Global Positioning System (GPS) and a pulse per second (PPS) output. 
         FIG. 8 a    shows sample points taken from two different sub-instrument components, and 
         FIG. 8 b    shows the cross-correlation result after correction of the out-of-phase data in  FIG. 8   a.    
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention is an apparatus for combining together and synchronizing or compensating the apertures of multiple PAUT NDT instruments so that the multiple instruments behave as one combined instrument. The invention is capable of other embodiments and may be practiced in various ways. Thus, it should be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description, or illustrated in the drawings. The embodiments described herein and the claims described hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
     Referring to  FIG. 1 , the consolidated PAUT system of the present disclosure comprises preferably an NDT-instrument supervisor  10 , sub-instruments  30   a,    30   b,    30   c,  and  30   d,  a clock managing unit  20  for each of the sub-instruments ( 20   a  for sub-instrument  30   a,    20   b  for sub-instrument  30   b,    20   c  for sub-instrument  30   c,    20   d  for sub-instrument  30   d,  each of them sending respective clock/trigger signals S 31   a,  S 31   b,  S 31   c,  and S 31   d ). Sub-instruments  30   a,    30   b,    30   c , and  30   d  are connected by data links S 30   a,  S 30   b,  S 30   c,  and S 30   d.  The specific components of this system are described in greater detail in  FIGS. 4 and 7 . Four (4) sub-instruments are shown along with NDT-instrument supervisor  10 , but this design can be applied to more than 4 sub-instruments. 
     Clock managing unit  20  has clock/trigger signals S 31   a,  S 31   b,  S 31   c,  and S 31   d,  which must be shared by all 4 sub-instruments  30   a,    30   b,    30   c,  and  30   d.  Clock managing unit  20  distributes both a clock and a trigger signal. The clock signal is distributed to all the sub-instruments to ensure that each sub-instrument acquires the ultrasound data at the same rate, thus making the 4 sub-instruments synchronous in frequency, but not necessarily in phase. The purpose of the trigger signal is to synchronize the start of the data acquisition for all the sub-instruments. 
     In the exemplary solution, clock managing unit  20  is an external stand-alone device, but the synchronizing clock and trigger generator can be internal to one of the sub-instruments and distributed to other sub-instruments, or it could be distributed from NDT-instrument supervisor  10 . The variations of this configuration are all within the scope and teaching of the present disclosure. Another key part is the configuration of data links S 30   a,  S 30   b,  S 30   c,  and S 30   d  between NDT-instrument supervisor  10  and sub-instruments  30   a,    30   b,    30   c,  and  30   d , respectively. The function of the NDT-instrument supervisor is further explained in  FIG. 2 . 
     Continuing with  FIG. 1 , NDT-instrument supervisor  10  has two (2) main functions. The first function of NDT-instrument supervisor  10  is to configure the aperture of each sub-instrument ( 30   a,    30   b,    30   c,  or  30   d ) according to the sub-instrument type, the type of material to be inspected, the geometry of the part to be inspected, and also according to the relative placement of the sub-instruments&#39; probes. The second function of the NDT-instrument supervisor  10  is to combine the data sent by all the sub-instruments, effectively merging their respective apertures into one aperture bigger than that of one sub-instrument in a seamless way. 
     Referring now to  FIG. 2 , an example of an NDT inspection setup using three (3) sub-instruments  30   a,    30   b,  and  30   c,  and NDT-instrument supervisor  10  is shown. As in  FIG. 1 , configuration and data links S 30   a,  S 30   b,  and S 30   c  between NDT-instrument supervisor  10  and sub-instruments  30   a,    30   b,  and  30   c  are shown. Clock managing unit  20  is omitted for clarity. One of the key components of the invention is the placement of NDT probes  40   a,    40   b,  and  40   c  connected to their respective sub-instruments  30   a,    30   b,  and  30   c,  along with the scan areas of their respective apertures  50   a,    50   b,  and  50   c,  on an inspected steel billet  60 . The area covered by one sub-instrument&#39;s aperture does not overlap with the area covered by another sub-instrument&#39;s aperture. Therefore, when the apertures are merged by NDT-instrument supervisor  10 , the combined aperture covers a much greater area than that of one sub-instrument. 
     First, NDT-instrument supervisor  10  configures each sub-instrument ( 30   a,    30   b,  and  30   c ) according to the sub-instrument&#39;s type, the type of material to be inspected, the geometry of the part to be inspected, and the relative placement of the sub-instruments&#39; probes. Configuring a sub-instrument includes, but is not limited to, configuring the sub-instrument&#39;s pulser voltage, pulse width, pulser delays, the receiver&#39;s acquisition duration, acquisition delays, analog filters, and digital filters. 
     Second, NDT-instrument supervisor  10  combines the data sent by all sub-instruments ( 30   a,    30   b,  and  30   c ), effectively merging their respective apertures into one bigger aperture. This bigger aperture allows the user to inspect more volume and area of the inspected part in less time. Effectively, all the sub-instruments work as a combined NDT instrument. 
     Reference is now made to  FIG. 3 , which is a flow chart view of the process of setting up the NDT sub-instruments, and performing an NDT test according to the present disclosure, using clock managing unit  20  which can be designed in a number of ways. One of the novel aspects of the present invention is to use a Phase-Locked Loop (PLL) to generate both sampling clocks and synchronizing clocks. Therefore the PLL functions as a clock/generator module shown later in  FIG. 4  as  210   a  and  210   b.    
     The PLL is a widely used electronic component and a novelty of the present disclosure includes configuring the PLL for the purpose of synchronizing phase for the present invention. 
     In step  301 , NDT-instrument supervisor  10  configures each sub-instrument and each of their apertures. In step  302 , the sub-instruments are connected to NDT-instrument supervisor  10 . In step  303 , clock managing unit  20  is connected from sub-instrument  30   a  to sub-instrument  30   b,  from sub-instrument  30   b  to sub-instrument  30   c,  and from sub-instrument  30   c  to sub-instrument  30   d.  The PLL of each sub-instrument is configured to a synchronizing delay in step  304 , and the clocks of the PLLs of each sub-instrument are matched in phase and frequency, which is described in greater detail in  FIG. 6 . In step  305 , once the sub-instruments are matched in phase and frequency, NDT-instrument supervisor  10  merges the respective apertures of the sub-instruments into one consolidated PAUT system shown in  FIG. 1 . This consolidated system then displays scan data in step  306  via NDT-instrument supervisor  10 . 
     Referring to  FIG. 4 , one of the novel aspects of the present disclosure is the interconnections between NDT-instrument supervisor  10  and the sub-instruments, using a clock generator and/or the PLL as described in association with  FIG. 3 . In  FIG. 4  the consolidated PAUT system is shown in a more detailed view than in  FIG. 1 . As can be seen, sub-instrument  30   a  is further comprised of a clock/generator module  210   a  and a trigger/generator module  220   a , a data acquisition module  310   a,  and a configuration module  320   a.  Similarly sub-instrument  30   b  is further comprised of a clock/generator module  210   b  and a trigger/generator module  220   b,  a data acquisition module  310   b,  and a configuration module  320   b.    
     It can also be noted in  FIG. 4  that clock managing unit  20  in  FIG. 1  is further presented as clock/generator module  210   a  and a trigger/generator module  220   a  in sub-instrument  30   a,  and clock/generator module  210   b  and a trigger/generator module  220   b  in sub-instrument  30   b , respectively. For simplicity and clarity, the clock/generator modules and trigger/generator modules of sub-instruments  30   c  and  30   d  are not shown in  FIG. 4 . 
     Clock/generator module  210   a,  trigger/generator module  220   a,  clock/generator module  210   b,  and trigger/generator module  220   b  are daisy chained. Sub-instrument  30   a  in the daisy-chain firstly generates a synchronizing clock, 20 MHz in this exemplary case, from an oscillator, which is fed to the clock/gen module  210   a,  which is preferably a PLL as previously described. 
     Continuing with  FIG. 4 , NDT-instrument supervisor  10  is further comprised of a remote instruments configurator  103 , a data combiner  110 , a scan display module  120 , and optionally a delay adjustment module  230 . 
     The functions of clock/generator module  210   a  are preferably fulfilled by a PLL, which uses a 20 MHz clock, as an exemplary case, from the oscillator (external, not shown) to generate all the sampling clock signals S 21  for sub-instruments  30   a,    30   b,    30   c,  and  30   d,  as well as a 20 MHz synchronizing clock signal  521   a  for daisy chained clock/generator module  210   b  and trigger/generator module  220   b  that are within sub-instrument  30   b,  the next remote sub-instrument. Sub-instrument  30   a  in the chain also generates trigger signal S 31   a  at a rate dictated by NDT-instrument supervisor  10 . Delay adjustment module  230  stores calibration data compensating the delay between  20   a  and  20   b,  and retrieves the calibration data to adjust the 20 MHz clock of sub-instrument  30   a,  as described in greater detail in  FIG. 5 . 
     Trigger signal S 31   a  from sub-instrument  30   a  is used to start the data acquisition of sub-instrument  30   b.  This trigger signal is fed to sub-instrument  30   b,  the first sub-instrument in the chain. The process is then repeated for sub-instrument  30   c,  and extended to sub-instrument  30   d . The process can be extended to more sub-instruments, and is limited only by the phase matching error buildup, the data rate limitations, and processing power limitations of NDT-instrument supervisor  10 . A 20 MHz clock is the exemplary clock for the disclosure, but this process can be used with a different frequency. 
     Referring to  FIG. 4 a   , preferably each sub-instrument is configured to have an oscillator on board, namely  222   a,    222   b,    222   c  and  222   d,  respectively. These oscillators serve to provide the basis of the internal clocks of each sub-instrument. The specific operating clock used in a particular inspection operation may be chosen to be either the original internal clock, or the adjusted clock based on a synchronizing clock. It should be noted that one consolidated oscillator can also be shared by the sub-instruments. 
     The clock from a local oscillator is fed to the PLL only if there is no input from a synchronizing clock. As shown in  FIG. 4 a   , only the first sub-instrument uses the oscillator  222   a  for its PLL input, while the other sub-instruments use the synchronizing clock signals from the previous sub-instrument. 
     Referring now to  FIG. 5 , a flow chart view of the steps of a factory PLL configuration/calibration is shown, using a PLL that functions as clock managing unit  20 . This flow chart is an example of a factory calibration of NDT sub-instruments done by an operator to determine a PLL configuration such that in the consolidated system all synchronizing clocks of each sub-instrument are in phase. The PLL of each sub-instrument uses clock managing unit  20  to generate synchronizing clocks  20   a,    20   b,    20   c,  and  20   d.  Three types of clocks are involved in the functions of the PLL: synchronizing clocks generating signals  21   a,    21   b,    21   c  respectively which are external to sub-instruments 2, 3 and 4; the internal clocks of each sub-instrument which are adjusted by the respective up-stream synchronizing clock and are the clocks in operation when a sub-instrument becomes the operating clock; and lastly the output clocks of sub-instruments  30   a,    30   b,    30   c,  and  30   d  (which are the internal clocks of the sub-instruments, adjusted for cable transmission delays). 20 MHz is the exemplary clock frequency for this disclosure, but this process can be used with a different frequency. 
     In step  501 , the output clock of sub-instrument  30   a  is connected to sub-instrument  30   b . Sub-instrument  30   a  does not have an input synchronization clock, because it is the first sub-instrument in the chain. For sub-instruments  30   b,    30   c,  and  30   d,  the clock used as the PLL input is the input synchronization clock, while for sub-instrument  30   a  the clock used is its internal 20 MHz clock. Since sub-instrument  30   b  uses the output synchronization clock of sub-instrument  30   a,  the synchronizing clock of  30   b  has the same frequency as that of  30   a,  but is likely out of phase because of cable and transmission delays. 
     In order to determine the phase of the various signals, it is necessary to sample the signals at a sampling clock frequency which may be any convenient multiple of the synchronization clock frequency. In the present embodiment, a 100 MHz sampling clock frequency is used, but this process can be used with a different frequency. 
     Continuing with  FIG. 5 , in step  502  the phase delay between the synchronizing clocks of sub-instruments  30   a  and  30   b  is measured, and stored in delay adjustment module  230  in NDT-instrument supervisor  10 . In step  503 , it is determined if the cumulative delay exceeds a certain threshold (for example 500 picoseconds). If yes, then in step  504  a delay adjustment for the output clock signal is performed. In the context of  FIG. 4 , if the phase delay is greater than a determined threshold, then delay adjustment module  230  retrieves the stored phase delay data and adjusts the output clock of sub-instrument  30   a  in step  504  (providing a synchronizing clock signal S 21   a ) until the phase difference is within the threshold. 
     Still in step  504 , the delay on the output synchronization clock is adjusted on sub-instrument  30   a,  until its internal 20 MHz clock signal and the internal 20 MHz clock signal of sub-instrument  30   b  have a delay less than the determined tolerance (500 picoseconds in the present example). Once performed, the internal clocks of sub-instrument  30   a  and sub-instrument  30   b  are matched both in frequency and in phase. 
     In step  505  the process is repeated between sub-instrument  30   b  and sub-instrument  30   c , between sub-instrument  30   c  and sub-instrument  30   d,  and so on for any number of sub-instruments. Once performed, the internal clocks of all the sub-instruments are matched in both frequency and phase. 
     Reference is now made to  FIG. 6 , which explains the phase relationship between synchronizing clock signals S 21   a  and S 21   b,  trigger signals S 31   a  and S 31   b,  and the associated functions of the PLL.  FIG. 6  also shows, for each sub-instrument, a sampling clock signal, a trigger signal from another sub-instrument with a time delay, a PLL clock in phase with another sub-instrument, and a synchronizing clock signal with or without a transmission delay for sub-instruments  30   a,    30   b,  and  30   c.  For simplicity and clarity, the signals for sub-instrument  30   d  have been omitted. 
     In  FIG. 6 , the sampling clocks of all the sub-instruments have the same frequency and the same phase, because the PLL of each sub-instrument is adjusted for the buffer and the cable transmission delays during the calibration process. This is essential for the synchronization. The sampling clock of each sub-instrument is phase-locked to its respective 20 MHz internal clock, but ticking at a higher rate, e.g., 100 MHz. 
     As shown in  FIG. 6 , three groups of waveforms are associated with sub-instruments  30   a ,  30   b  and  30   c  respectively. The groups of waveforms in the subsequent description can be referred to as “top group,” “middle group,” and “bottom group” for description purposes. Waveforms from sub-instrument  30   d  are omitted for the sake of simplicity. It should also be noted that the sequence of the sub-instruments and their associated waveforms are arbitrarily presented here, and any alternative arrangement is within the scope of the present disclosure. 
     As seen in the top group of waveforms, sub-instrument  30   a  has data link S 30   a  from NDT-instrument supervisor  10 , and a sampling clock that is in phase with sub-instrument  30   b  and sub-instrument  30   c.  It also has trigger signal S 31   a  sent to sub-instrument  30   b,  and synchronizing clock signal S 21   a  that is also sent to sub-instrument  30   b.  Since it is the first instrument in the chain, it does not receive an input synchronization clock as do the other sub-instruments. Its internal clock is the operating clock. Synchronizing clock signal S 21   a  from sub-instrument  30   a  is generated from clock/generator module  210   a,  and trigger signal S 31   a  is generated by trigger/generator module  220   a,  and both signals are sent to sub-instrument  30   b.    
     As seen in the middle group of waveforms, sub-instrument  30   b  has a sampling clock that is in phase with sub-instrument  30   a  and sub-instrument  30   c,  as well as a PLL that is configured to be in phase with sub-instrument  30   a.  Synchronizing clock signal S 21   a  from sub-instrument  30   a  with a transmission delay is received by sub-instrument  30   b.  This delay is adjusted by the PLL of clock/generator module  210   b  (more specifically, by step  504 ), and presented as waveform “PLL operating clock (in phase with  30   a )”, which is the operating clock for  30   b . Synchronizing clock signal S 21   b  is generated from clock/generator module  210   b.  Trigger signal S 31   b  is generated by trigger/generator module  220   b,  which is distributed to sub-instrument  30   c . As can be seen, trigger signal S 31   b  and synchronizing clock signal S 21   b  are sent to sub-instrument  30   c.    
     Continuing with  FIG. 6 , specifically the bottom group of the waveforms, sub-instrument  30   c  has a sampling clock that is in phase with sub-instrument  30   b  and sub-instrument  30   a,  as well as a PLL that is configured to be in phase with sub-instrument  30   b  and sub-instrument  30   a . Synchronizing clock signal S 21   b  from sub-instrument  30   b  is received with a transmission delay by sub-instrument  30   c.  This delay is adjusted by the PLL of the clock/generator module of sub-instrument  30   c  (more specifically, by step  504 ; the clock/generator module of sub-instrument  30   c  is not shown in  FIG. 4  for simplicity purposes), and presented as waveform “PLL operating clock (in phase with  30   a  and  30   b )”, which is the operating clock of  30   c.  Trigger signal S 31   b  is also sent from sub-instrument  30   b  with a transmission delay. 
     Referring to  FIG. 7 , another of the novel aspects of the present invention is a method to merge apertures by using a Global Positioning System (GPS) clock and a pulse per second (PPS) output. This method is embodied by having GPS clock/PPS modules  20   a,    20   b,    20   c  and  20   d  act as clock managing units  20  for sub-instruments  30   a,    30   b,    30   c,  and  30   d.  In the exemplary embodiment, GPS clock/PPS module  20   a  uses the commonly available GPS signal to generate a 10 MHz-clock and 1 pulse every second for trigger/generator module  220   a.  The 10 MHz clock is used for a maximum acquisition duration of 10 milliseconds in the present embodiment, although different maximum acquisition duration times are also within the scope of the invention. For a maximum acquisition duration of 10 milliseconds, the phase and timing of the clock/generators can be considered to be the same for each remote sub-instrument, since a time slip between 2 sub-instruments can be considered negligible on such a time scale. 
     However in this embodiment the sub-instruments are not phase-locked, since there is significant jitter between the synchronizing pulses per second of the different GPS modules. To work around this problem, a phase detection process is disclosed, which is comprised of NDT-instrument supervisor  10 , data acquisition module  310   a  of sub-instrument  30   a,  data acquisition module  310   b  of sub-instrument  30   b,  and Phase Detection Algorithm blocks  140   a,    140   b,    140   c , and  140   d  to synchronize the data acquisition start time of each sub-instrument. In  FIG. 7 , data acquisition modules for sub-instruments  30   c  and  30   d  are not shown for the sake of simplicity. 
     Continuing with  FIG. 7 , each sub-instrument starts the data acquisition at the same time delay after receiving its respective pulse per second input, and, in the present embodiment, data acquisition continues for 10 milliseconds. Each sub-instrument&#39;s 10 MHz clock can be considered to be locked in frequency; however significant jitter exists between each of the pulse per second inputs, and the sub-instruments are not yet phase-synchronous. 
     Once the acquisition is completed by the data acquisition modules, the data is sent to NDT-instrument supervisor  10 . The NDT-instrument supervisor synchronizes the data by comparing the first indications found on the Phase Detection Algorithm blocks  140   a,    140   b ,  140   c,  and  140   d  of each sub-instrument  30   a,    30   b,    30   c,  and  30   d.    
     Reference is now made to  FIGS. 8 a  and 8 b   , which illustrate use of a cross-correlation mathematical function which enables measurement of the relative time delay between the different sub-instruments using algorithms for Phase Detection Algorithm blocks  140   a,    140   b ,  140   c,  and  140   d.    FIG. 8 a    shows data from sub-instruments  30   a  and  30   b,  where the data is not in phase. NDT-instrument supervisor  10  assumes that sub-instrument  30   a  is in the true time, although sub-instrument  30   b,    30   c,  or  30   d  can also be used for the true time. The indication from sub-instrument  30   b  occurs later in time compared to sub-instrument  30   a.  NDT-instrument supervisor  10  then applies a cross-correlation function to the data from the two sub-instruments. 
     To correctly perform the cross-correlation, NDT-instrument supervisor  10  takes sample points before and after the indication for both sub-instrument  30   a  and sub-instrument  30   b,  and then performs the cross-correlation between the data from the sub-instruments. Assuming that N sample points are taken from each of sub-instruments  30   a  and  30   b,  then the length of the cross-correlation result is 2*N−1, which is depicted in  FIG. 8 a    in the bottom right corner. If the data were in phase, the maximum of the cross-correlation would be attained at the sample “N”, exactly the central sample of the Phase Detection Algorithm block of 2*N−1. 
     However, if the data from sub-instrument  30   a  and sub-instrument  30   b  are not in phase, the maximum is attained either before the sample N, or after the sample N. Therefore the position that is the maximum value needs to be determined. Simply taking the maximum value would yield a delay that is an integer multiple of the sampling period. However, instead of simply taking the maximum value of the cross-correlation output, a parabolic interpolation can be applied around the maximum sample in order to determine with better precision the actual delay between the 2 sub-instruments. The phase of the 2 sub-instruments can therefore be matched with precision in the nanosecond range. 
       FIG. 8 b    shows the result after application of the cross-correlation function to data from sub-instruments  30   a  and  30   b,  and correction of the phase difference. Referring to  FIG. 8 b   , once the phase difference between sub-instrument  30   a  and sub-instrument  30   b  has been determined, it is easy to adjust the data between the 2 sub-instruments so that they are in phase. Delays that are greater than a sampling period are compensated by shifting the data from the sub-instrument that generates the “early” data. 
     Delays that are smaller than a sampling period can be compensated by using an all-pass delay filter, though other techniques exist. Such filters can be implemented in various ways, either in Infinite Impulse Response (IIR) topology or Fixed Impulse Response (FIR) topology. For example, if the indication is seen 25 nanoseconds earlier by sub-instrument  30   a  than sub-instrument  30   b,  and the sampling period of the instrument is 10 nanoseconds, a delay of 2 samples is applied on the data from sub-instrument  30   a,  and then a delay filter of 5 nanoseconds is applied. The total is a delay of 25 nanoseconds, making the data from sub-instrument  30   a  and sub-instrument  30   b  synchronous in phase. The process can be easily extended to 4 sub-instruments and more. 
     Returning to  FIG. 7 , the data in this design from sub-instruments  30   a,    30   b,    30   c,  and  30   d  is synchronized both in frequency and in phase. NDT-instrument supervisor  10  then time shifts the data from the data acquisition modules of the sub-instruments to make the data from Phase Detection Algorithm blocks  140   a,    140   b,    140   c,  and  140   d  start at the same time. Once this is done for the current data acquisition, remote instruments configurator  103  synchronizes the configuration modules of the sub-instruments to one another by data combiner  110 , thereby creating a greater aperture from the sub-instruments for scan display module  120 .