Abstract:
A system and method for carrying out non-destructive testing and inspection of test objects to assess their structural integrity uses a calibration module configured to provide V-Path time of flight (TOF) correction data over a plurality of object thickness points, obtained from an object or objects having known thicknesses using the same physical probe as is used for the inspection measurements. When a probe launches acoustical waves into a test object and an instrument and a control system compute a time of flight value of the acoustical waves launched by the probe, the pre-obtained V-Path TOF correction data is used to correct the measured time of flight computed by the instrument.

Description:
FIELD OF THE INVENTION 
     The present invention relates to non-destructive testing and inspection systems (NDT/NDI) and more particularly to a method to compensate for acoustic V-Path time of flight errors and thereby optimize inspection measurement accuracy. 
     BACKGROUND OF THE INVENTION 
     The measurement data from NDT/NDI devices used for the routine monitoring of structural integrity must be of sufficient accuracy to allow a valid assessment to be made of the condition of the structure under test. Examples of such structures are pipes and vessels widely used in the petrochemical and other industries. Examples of measurement data are pipe wall thickness and other geometric conditions, including, but not limited to, the presence of irregular surfaces (e.g. corrosion, oxide, etc.) and flaws (e.g. porosity, cracks, etc.). 
     The decision to perform or not perform maintenance on a structure is made based on the assessment of the measurement data. Therefore, the measurement accuracy will have a direct impact on the decision. The consequence of inaccurate measurement data that underestimates an unfavorable condition of a structure can result in failures occurring before maintenance is performed. Conversely, inaccurate measurement data that overestimates an unfavorable condition of a structure can result in performing expensive and unnecessary maintenance. 
     One of the most common NDT/NDI devices used for assessing structural integrity is a corrosion gage, such as the instant assignee&#39;s 37 DLP product. Products of this type typically employ a ‘dual-element’ probe or probe system that contains one element for acoustic transmission and another for acoustic reception, preferably packaged in an integral housing. The two elements are set at a fixed angle, thereby setting a fixed focal depth and ‘V-Path’ within the object being tested. Although this element positioning provides advantages for measuring corrosion wear, measurement errors, known as ‘V-Path errors’, can be introduced when measuring thicknesses at depths other than that of the focal depth. 
     The specific challenge herein dealt with is to provide a method that will ameliorate the measurement errors resulting from V-Path echo, which is the energy path traveled by the acoustic wave after the energy is transmitted into the target material and reflected from the back-wall of the material and into the receive element of the transducer. Particularly, V-Path errors occur when thickness measurements are being made on a material thinner than the focal thickness of the transducer. 
     Existing efforts have been made to eliminate or reduce such errors as described above. Thus, embodiments employing pre-defined data for the V-Path, or time distortion, correction in the calculation of a thickness measurement are well known by those skilled, and are therefore not described in detail herein. 
     One conventional solution for V-Path error compensation employs pre-defined static data tables to compensate for the time distortion; however, this solution has the drawback of not accounting for actual material sound velocity, transducer wear and manufacturing variances in transducer population. 
     Materials under inspection have their own individual velocities denoted as V, where V=material velocity. U.S. Pat. No. 3,554,013 teaches a hardware error correction circuit for ultrasonic thickness gauges. It is not a software method and presents the drawbacks of thermal and other electronic drift and material costs. 
     U.S. Pat. No. 4,570,486 teaches V-Path calibration for UT thickness measurement using hardware error correction circuits for ultrasonic thickness gauges. It is not a software method and presents the drawbacks of thermal and other electronic drift and material costs. 
     Current V-Path methods using a pre-determined data table, called “V-Path Table,” use empirical methods of deriving data to generate the Table. The predetermined Table is generated by using TOF measurement methods on a batch of typical transducers of one model. It is then used for hundreds of the transducers of the same model for many years. The existing V-Path Table is herein referred to as the “Empirical V-Path Table”. 
     Using an Empirical V-Path Table to compensate all the transducers of one model is less accurate because of variations of transducer factors such as acoustical focal depth and saturation of the acoustic barrier. The factors causing such variations include manufacturing tolerance changes in different batches of transducers, changes in material characteristics, and changes caused by wear-and-tear. 
     Accordingly, a solution that overcomes the drawbacks described above and results in advantages highly valued by potentially affected industrial and public infrastructure concerns, needs to:
         a. Improve measurement accuracy;   b. Extend the longevity of transducers along with their measurement accuracy; and   c. Improve measurement accuracy of generic transducers for which the pre-defined V-Path data is unknown.       

     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a system and method for obtaining more accurate V-Path correction data. 
     It is a further object of the invention to provide a system and method that is able to extend the longevity of proprietary transducers and their measurement accuracy. 
     Yet another object of the invention is to provide a system and method to improve measurement accuracy of generic transducers for which a pre-defined V-Path correction data is unknown. 
     The foregoing and other objects of the invention are realized with a thickness measuring system for measuring the thicknesses of test objects. The system includes a calibration module which is configured to provide V-Path time of flight (TOF) correction data over a plurality of object thickness points, obtained from one or more objects having known thicknesses. A probe configured to launch acoustical waves into a test object and to receive returning waves is employed, to produce an electrical output representative of the returning waves. An instrument, including control and computation hardware and software, is coupled to the probe and is configured to compute a time of flight value of the acoustical waves launched from the probe. A correction module associated with the instrument and configured to receive the V-Path TOF correction data from the calibration module is used to correct the time of flight computed by the instrument, based on the V-Path TOF correction data provided by the calibration module. 
     In accordance to various embodiments of the system and method of the present disclosure, the probe is preferably a dual element probe. Further, the V-Path TOF correction data can be provided in the form of a plurality of discreet correction values and those values can be used to compute correction values, needed to correct the TOF in real-time as the measurement is being made. Alternatively, linear equations or higher order polynomials can be fitted to the V-Path TOF correction data and these equations can be used to compute the needed TOF correction information in real-time as the measurement is being made. 
     Other features and advantages of the present invention will become apparent from the following description of the invention that refers to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a Dual Element Ultrasonic Transducer depicting the Effective Angular Sound Energy Path for Thick Material targets after the Transducer has been electrically excited by the UT apparatus. 
         FIG. 2  is a diagram of a Dual Element Ultrasonic Transducer depicting the Effective Angular Energy Sound Path for Thin Material targets after the Transducer has been electrically excited by the UT apparatus. 
         FIG. 3  is a diagram showing the Measured Time Intervals that comprises a Time Of Flight measurement for a Dual Element Ultrasonic Transducer. 
         FIG. 4  is a diagram showing the functional modules used for deriving and employing ultrasonic V-Path correction data according to the present invention. 
         FIG. 5  is a module or component embodiment showing the module and steps required for acquiring input calibration data from the operator. 
         FIG. 6  is a module or component of the embodiment showing the module and steps required for deriving and creating the User Created V-Path correction values. 
         FIG. 7  is a module or component of the embodiment showing the module and steps required for the application of the V-Path correction data during the measurement calculation phase. 
         FIG. 8  is a chart showing the Actual Thickness vs. the Measured Thickness by an UT apparatus, and the associated calibration points of the embodiment. 
         FIG. 9  is a chart showing a plot of time correction factors at different measured wave flight times. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to assist the understanding of presently disclosed V-Path error calibration method, the following description is given in association with background  FIGS. 1-3 . 
     It should be noted that ‘sensor’, ‘probe’ and ‘transducer’ are herein used in the present disclosure interchangeably. The term ‘real-time measurement’ is used in the present disclosure to mean the immediate measurement result provided to the user or external device by measurement device  111  ( FIGS. 1 and 2 ) using one or more probe excitation/response cycles. The measurement result may be provided to the user by means of display  111   b , an integral audio device (not shown), and/or an external device by means of input/output port  111   a . The measurement result may be comprised of, but not limited to, values representing thickness, relative thickness and/or an alarm indication. 
     Referring to  FIG. 1 , the presently disclosed V-Path error compensation method as disclosed in used in conjunction with a dual-element transducer ultrasonic inspection system. The inspection system comprises a transducer  102 ; a measurement device  111  wherein the algorithm of the present disclosure is executed; and a target test object  107 . 
     The invention is a system and method employing a software program that may be used for producing, and employing time distortion correction data, henceforth referred to as a V-Path Table for probes, that may be employed by ultrasonic thickness measuring apparatus. Ultrasonic thickness measuring apparatus will henceforth be referred to as measurement device  111 . It should noted that although the preferred embodiment of the present disclosure describes an exemplary ultrasonic measuring apparatus, the teachings of the present disclosure may be applied at acoustic frequencies below the ultrasonic range (typically &lt;20 kHz). 
     V-Path correction data may be used in the measurement calculation when the need to compensate for time distortion introduced by Angular Sound Energy Paths is required. Refer to the Effective Angular Sound Energy Paths  108  in  FIG. 1  and  FIG. 2 . Note that the effective Angular Sound Energy Path distortion is greater in thin material targets  207   FIG. 2 , than that of thick material targets  107  of  FIG. 1 . Thus, as shown by curve  802  of  FIG. 8 , time distortion effects increase as the thickness of the target material decreases when V-Path correction is not present. Hence, the application of V-Path correction factor, t v , (of Eq. 1 below) may be essential for accurate thickness measurement in thin material targets. 
     Referring again to  FIG. 1 , the Transmit Element side of the Transducer  101  generates an ultrasonic energy wave after being excited by electrical signals at its Input Connector  105  by measurement device  111 . The Ultrasonic Energy Wave will hereby be referred to as Wave. 
     Referring to  FIG. 3 , the Wave  108  generated by the Transmit Element  101 , travels through the Transmit Delay Material  100  and into the coupled target material, such as  107  or  207 , through front-surface  110 . The Wave is then reflected from the coupled target back-surface  109  and back through front-surface  110  and Receive Delay Material  106  into the Receive Element  104 , where it is converted back to an electrical signal. 
     As can be appreciated by those skilled in the art, the measurement device  111  is capable of precisely measuring the Time Interval (TI) comprised the Time Of Flight (TOF) of each of the elements depicted in  FIG. 3 , i.e., T 1 , T 2 , T 3 , and T 4 . It should be noted that the TI measurements associated with T 1  and T 4  are typically made when transducer  102  is decoupled from the target material and element  101  and  104  are each operated in pulse-echo mode to measure the TI to and from their respective transducer contact surfaces  101   a  and  104   a . Accordingly, the TI values T 2  and T 3  associated with the target material may be measured because the T 1  and T 4  are TI components of the total TOF are accounted for. 
     Therefore, the ‘thickness’ calculation H for a target material, whether it be a calibration block or test object, may be calculated by Eq. 1 as shown below,
 
 H =[(TI+ t   v ) V]/ 2  [Eq. 1]
 
     where, 
     TOF=T 1 +T 2 +T 3 +T 4   
     t x =T 1 +T 4   
     TI=TOF−t x    
     t v =V-Path correction factor 
     V=sound velocity in test blocks  107  and  207 . 
     It should be noted that the above equations are also used to generate the Empirical V-Path Tables typically provided by transducer manufacturers. An Empirical V-Path Table is usually provided for a specific transducer model number. 
     The preferred embodiment of the present disclosure is a system and method employing a software program that may be used for producing, and employing time distortion correction data in real-time, henceforth referred to as ‘V-Path Table’ for Transducers that may be employed by ultrasonic thickness measuring apparatus. A key aspect of the present invention includes deriving an ‘User Created V-Path Table’ and employing such for dual-element transducer calibrations and test object measurement. As further described below, it should be noted that in comparison with the existing Empirical V-Path Table, ‘User Created V-Path Table’ is derived using TOF data measured for a specific physical transducer. The following method/software program can be used to generate the ‘User Created V-Path Table’ on any specific transducer and at any point during the service life of the transducer. 
     As shown in  FIG. 4 , the presently disclosed method in combination with a software program performs process  400  that is comprised of modules (or steps) including Setup and Calibration Data Acquisition  500 , User Created V-Path Table Creation  600 , and Creation and Output of Compensated Value H  700 . 
     Referring now to  FIGS. 4 ,  1  and  2 , in step  500 , transducer  102  is coupled to a calibration block such as  107  or  207 , to acquire the required data elements for the creation of the V-Path table in step  600 . Next, in step  700  the V-Path Table is employed during real-time measurement acquisition to correct for time distortion, thereby resulting in a compensated thickness measurement value H. It should be noted that process  400  is performed within inspection device  111  when connected to transducer  106 . 
     It should be noted that the combined ‘steps’ above are also called modules. The present disclosure is focused on a combination of a software program and a method. The terms ‘Step’ and ‘module’ are interchangeably used, wherein ‘step’ is used in the context of the method and ‘module’ is used in the context of the system and associated software program. 
     Turning now to  FIG. 5 , which provides a more detailed description of the module  500 , note that the operator is required to perform a calibration on a range of blocks of different thicknesses. The range or number of thickness points is determined by the operator. After the Setup and Calibration Data Acquisition process within module  500  is completed, the resulting data is provided to the module  600  of  FIG. 6  to derive the V-Path Table. The data in the V-Path Table is then used in module  700  of  FIG. 7  wherein it may be utilized in the calculation of compensated measurement H to yield an accurate thickness as shown in Eq. 1. 
     It should be noted that the term “actual” as used in the present disclosure denotes the precise metrics of the target material, and the term ‘measured’ denotes the metrics of the target material acquired by the measurement device  111 . In step  502 , the operator couples the transducer  102  to the calibration block and enters the actual thickness of the block in step  503 . The actual TI is then calculated in step  504  using Eq. 2 shown below.
 
ActualTI i =[ActualThickness i   /V]* 2  [Eq. 2]
 
     where, 
     V=Material Velocity of the block 
     The measurement device  111  then acquires the TI in step  505  ( FIG. 5 ), where the measured time is calculated using Eq. 3 shown below.
 
MeasuredTI i   =T 2 +T 3  [Eq. 3] (see FIG. 3)
 
     The measured thickness is calculated in step  506  by Eq. 4 shown below.
 
MeasuredThickness i =[MeasuredTI i   *V]/ 2  [Eq. 4]
 
     where 
     V=Material Velocity of the block 
     The data obtained for ActualTI i , ActualThickness i , MeasuredTI i  and MeasuredThickness i  are then stored into CalData[i] in step  507 , as multiple-element array or array of data structures. The procedure repeats steps  502  through  508  until the desired range of calibration thicknesses have been entered. The module in  FIG. 5  is completed in step  509 . 
     Reference is now made to  FIG. 6 , wherein detailed steps of module  600  are elaborated. The CalData[i] data structure is preferably sorted from smallest ActualTI i  value to largest ActualTl k  value in step  602 , where k is the number of calibration points entered in step  510  of  FIG. 5 . Once sorted, a time correction factor is calculated for each calibration point by Eq. 5 shown below.
 
 t   c   [i ]=ActualTI[ i ]−MeasuredTI[ i]   [Eq. 5]
 
     In practice, t c [i] and MeasuredTI[i] are stored in step  604  into VPathData[i], which is a multi-element array or array of data structures. The process of calculating t c [i] and storing it along with MeasuredTI[i] in steps  603  through  606  continues until i=k, where k is the number of calibration points entered in step  510 . 
     Module  700  is now described with reference to  FIG. 7 . The “User Created V-Path Table” was created as described above within the software program modules  500 , and  600 , and is then used in module  700  to compensate for time distortion in the thickness measurement of the measurement device  111  and thereby produce V-Path compensated measurement value H. 
     For module  700 , the operator couples the Transducer to the material under test in step  702  and the measurement device  111  acquires a time measurement LiveTI in step  703  using Eq. 6 shown below.
 
LiveTI= T 2 +T 3  [Eq. 6](see FIG. 3)
 
     LiveTI is then utilized to determine an index n into VPathData[ ] in step  704  for selecting the appropriate data in the table for that thickness and then using the data for deriving the live (real-time) V-Path correction factor, t v  in step  705  using Eq. 7 shown below.
 
 t   v   =ΔT 1+(LiveTI− T 1)*[(Δ T 2−Δ T 1)/( T 2 −T 1)]  [Eq. 7]
 
     and referring to  FIG. 8 , 
     T 1 =VPathData[n].MeasuredTI, the MeasuredTI value in VPathData[n]. 
     ΔT 1 =VPathData[n].t c , the t c  value in VPathData[n]. 
     T 2 =VPathData[n+1].MeasuredTI, the MeasuredTI value in VPathData[n+1]. 
     ΔT 2 =VPathData[n+1].t c  the t c  value in the VPathData[n+1]. 
     Finally, V-Path compensated measurement value H of the target object is calculated in step  706  by using Eq. 8 shown below.
 
 H =[(LiveTI+ t   v )* V]/ 2  [Eq. 8]
 
       FIG. 8  shows time correction factors designated as ΔT 1 , ΔT 2 , etc., at specific calibration points, plotted against the thickness of the test object. It also shows time correction factors for live measurements and identifies them as LiveTI. These “Δ” designated time correction factors can be plotted as shown in  FIG. 9 , as specific (x,y) coordinate points where, in  FIG. 9 , the ordinate (y-axis) indicates the time correction factors, at each of the calibration points. The x-axis designates the measured TOF (Time of Flight), at the calibration points. 
     In the preceding embodiment, time correction factors for the live measurement points are obtained via interpolation calculations, as previously described. See, for example, equation 7. Those skilled in the art appreciate that the plot ( FIG. 9 ) of the actual time correction factors, relative to the measured times at the calibration points, produces a non-linear function, which is not easily fitted to a rigid mathematically expressed formula. 
     However, in accordance with the presently described alternative embodiment, the software system of the present invention aggregates these discrete data points, for example, between points A and B; B and C; C and D; and D and E, and produces a linear transfer function (formula) for each such section. This allows calculation of the Δ time correction factors, for live measurements, using a specific equation for each section. 
     Thus, for the Section A-B, the transfer function, i.e. the formula, for calculating the correction factor for the TOF obtained on a test object may be expressed as: Δy A-B =f(Δx); according to a general form of the equation, which is y A-B =−A(x)+B, with A and B being constants which are unique to each of the sections A-B; B-C; etc., in  FIG. 9 . The variable “x” is the live measured TOF obtained during a test. By adding the thus obtained Δy to the live TOF measurement, one obtains a TOF for thickness values falling on and between the discrete time correction data points, which readily allows calculating the thickness parameter based on the acoustical wave speed through the test object. 
     The approach of this embodiment does not require accessing the V-Path data tables and calculating interpolated corrections during live measurements on test objects. Instead, it allows the use of direct conversions, using the above formulas. 
     In creating the segmentized linear transfer functions shown in  FIG. 9 , it should be noted that the software of the present invention uses actual measurement data to determine the starting and ending points for each section of the curve, using well known mean deviation methodologies to enable the fitting of a linear equation over the selected data ranges. 
     In operation, a TOF measurement is taken. Then, it is determined to which linear segment the measurement TOF value belongs. Lastly, the appropriate equation is used to calculate the TOF correction. As noted above, the calculation of the thickness is then readily obtained. 
     In an alternate embodiment, V-Path correction data may be determined for a specific physical transducer by some other means than the measurement device  111  and provided to the measurement device  111  to conduct V-Path correction during real-time measurements. One of the other means may be a measurement device  111  other than the one that will be used to conduct the real-time measurements. 
     In another alternate embodiment, as shown in  FIGS. 1 ,  2  and  3 , ID  112  may provide a means of physical probe identification (i.e. a ‘probe identifier’), such as a serial number, coupled to measurement device  111  to be used to recall from its memory the V-Path correction data table associated with the probe identifier. ID  112  may be a non-volatile (NV) digital memory device or a component that maintains a substantially constant value over time—such as a resistor. ID  112  is preferably packaged in an integral manner with probe  102  in order to ensure that ID  112  remains with the probe. For example, ID  112  may be packaged with the probe, the probe cable assembly, or any other device attached to probe  102  on a permanent or semi-permanent basis. 
     If ID  112  is a NV digital memory device of adequate capacity, the V-Path correction data table may be stored with the physical probe it applies to (i.e. ‘V-Path stored in probe’), thereby allowing the probe to be used with any measurement device  111  without the need for the measurement device  111  to store a database of V-Path correction data tables associated with probe identifiers. 
     The primary advantage provided by the ‘probe identifier’ and ‘V-Path stored in probe’ embodiments is improved inspection process efficiency by eliminating the need to perform the V-Path correction data table calibration process before starting an inspection measurement session. 
     Although these embodiments are described in relation to a V-Path correction data table associated with a specific physical probe, V-Path correction data may also be created by empirical means, such as derivation from a sample population of probes. Analytical means may be used as well, such as a mathematical model of a distinct probe type. 
     It should be noted with respect to these embodiments that the V-Path correction data table stored in the NV digital memory device may be updated by the user from time to time to account for changes in physical probe properties, thereby maintaining optimal accuracy of the V-Path correction data. 
     Other arrangements of embodiments of the invention include software programs to perform the method embodiment steps and operations summarized above and disclosed in detail below. More particularly, a computer program is one embodiment that has a computer-readable medium including computer program logic encoded thereon that when encoded and executed in a computerized device provides associated operations providing V-Path error calibration as explained herein. The computer program logic, when executed on at least one processor with a computing system, causes the processor to perform the operations (e.g., the methods and algorithms) indicated herein as embodiments of the invention. Such arrangements of the invention are typically provided as software, code and/or other data structures arranged or encoded on a computer readable medium such as but not limited to an optical medium (e.g., CD-ROM, DVD-ROM, etc.), floppy or hard disk, a so-called “flash” (i.e., solid state) memory medium, or other physical medium, such as but not limited to firmware or microcode in one or more of ROM or RAM or PROM chips, or as an Application Specific Integrated Circuit (ASIC) or as downloadable software images in one or more modules, shared libraries, etc. The software or firmware or other such configurations can be installed onto a computerized device to cause one or more processors in the computerized device to perform the techniques explained herein as embodiments of the invention. Software processes that operate in a collection of computerized devices, such as in a group of data communications devices or other entities may also provide the system of the invention. The system of the invention may be distributed between many software processes on several data communications devices, or all processes may run on a small set of dedicated computers, or on one computer alone. 
     It is to be understood that embodiments of the invention may be embodied strictly as a software program, as software and hardware, or as hardware and/or circuitry alone. The features disclosed and explained herein may be employed in computerized devices and software systems for such devices such as those manufactured by Olympus NDT Inc. of Waltham, Mass. 
     Although the present invention has been described in relation to particular exemplary embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention not be limited by the specific disclosure.