Abstract:
Disclosed is a method and an NDT/NDI calibration process that automatically detects erroneous TOF readings by providing a predetermined time acceptance window. During the calibration process, TOF readings acquired by a UT device are validated to determine whether the TOF reading for the thin test block falls within the range of the predetermined time acceptance window. If the TOF reading for the thin block (T 2 ) falls out of the predetermined time acceptance window, the operator is alerted of an error and to repeat the TOF test for the thin block.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/317,446, filed Mar. 25, 2010, entitled “Automatic Calibration Error Detection for Ultrasonic Inspection Devices”. The complete contents of the priority application are hereby incorporated by their reference for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to non-destructive testing and inspection devices (NDT/NDI) and more particularly to a method that automatically detects calibration data errors for ultrasonic inspection systems, such as thickness gauge devices. 
     BACKGROUND OF THE INVENTION 
     Ultrasonic apparatus calibration procedures fall into three categories, namely Transducer Zero Compensation, Material Velocity Calibration and Zero Offset Calibration. Presently, “Two Point Calibrations”, which is widely known in the art, utilizes the combination of “Zero Offset Calibration” and “Material Velocity Calibration.” 
     “Two Point Calibration” for thickness measurement instruments typically involves the process of adjusting an ultrasonic inspection device (UT device) so that it takes measurement on known-thickness test objects for a particular material, using a particular transducer at a particular temperature. In most cases, Material Velocity and Zero Offset Calibration may be combined using a thick and a thin calibration block of the same material, which is referred to as “Two Point Calibration”. 
     Material Velocity Calibration is typically performed using a thick test block of known thickness that is fabricated out of the same material to be measured, by measuring the time of flight of the ultrasonic signal that travels from the front surface to the back surface of the test material. This calibration needs to be completed for each batch of test objects. 
     Zero Offset Calibration is typically performed using a thin test block of known thickness made of the same material to be measured, by measuring the time of flight of the ultrasonic signal that travels from the front surface to the back surface of the test material. This calibration only needs to be performed once for each new transducer and material combination. 
     An existing conventional Two Point Calibration process for a given transducer typically involves the following steps:
         1) Select a calibration block comprising a few sub-blocks with different but known thicknesses. Select two sub-blocks, with the thinner one called “thin block” and the thicker one called “thick block”. The thicknesses of both thin block and thick block are known. The pertinent parameters of the transducer selected to be calibrated are either recalled from the UT device&#39;s memory or provided by the operator.   2) Determine T 1 , the measured time of flight (TOF) of the thick block, by using the UT device and the selected transducer. T 1  is the TOF measured for the ultrasonic signal to travel from the front surface to the back surface and back to the front surface of the thick block.   3) Provide H 1 , the known thickness of the thick block, to the UT device.   4) Determine T 2 , the measured time of flight (TOF) of the thin block, by using the UT device and the selected transducer. T 2  is the TOF measured for the ultrasonic signal to travel from the front surface to the back surface and back to the front surface of the thin block;   5) Provide  112 , the known thickness of the thin block, to the UT device.   6) Lastly, the UT device performs the Two Point Calibration calculations and stores the results.       

     However, it is quite frequent that the UT device acquires an erroneous T 2  for the thin block an account of factors such as incorrect gain or signal noise. The calibration would be therefore erroneous when an operator mistakenly accepts the erroneous T 2 . This has been problematic particularly for inexperienced operators who might unknowingly perform erroneous calibrations, which subsequently produce erroneous inspections. For experienced operators, erroneous readings slow down the calibration process, since the operator needs to stop and verify the calibration manually, which decreases productivity. Moreover, if there is not a waveform display on the UT device to view the signal during the calibration session, the operator has no means to determine if T 2  is correct. 
     The accuracy of non-destructive testing (NDT) is well known to be critical for many industries. 
     Existing efforts are exemplified in U.S. Pat. No. 3,554,013 to Jerry Berg which deploys hardware circuitry to minimize the problems caused by erroneous calibration due to wrong signal detection. However, the hardware solution is comparatively not cost effective and adaptable and suffers from instability with thermal drift. 
     Thus, given the existing problems and tried efforts, there is a critical need to automatically remove erroneous calibration signals, especially for ‘thin block’ or “Zero Offset Calibration” to improve the inspection certainty, accuracy and to increase productivity. 
     SUMMARY OF THE INVENTION 
     The disclosure herein solves the problems related to the calibration of ultrasonic inspection devices used in NDT/NDI devices, where existing “Two Point Calibration” procedures typically encounter the aforementioned erroneous TOF readings, particularly for thinner blocks. 
     Note that the terms “probe”, “transducer”, and “sensor” used herein may be used interchangeably. 
     Time of Flight measurement is herein referred to as TOF, which is the time of flight measurement of the ultrasonic signal travelling from the front surface to the back surface and back to the front surface of either the thin or thick block. 
     The ultrasonic depth measuring apparatus is herein referred as the UT device. 
     Accordingly, it is a general object of the present disclosure to provide a method and an associated software procedure that may be employed to automatically determine if there is an error in detection of calibration signals during a “Two Point Calibration” process. 
     It is further an object of the present disclosure to carry out a Two Point Calibration with automatic erroneous signal detection according to the present invention. The process involves taking readings of TOF, T 1  and T 2  of a thick block and a thin block with known thickness H 1  and H 2 , respectively, with the TOF reading for the thin block T 2  verified for error before proceeding to calibrating the UT device. 
     It is further an object of the present disclosure to define a time acceptance window for the TOF reading for the thin block during the Two-Point Calibration process. 
     It is further an object of the present disclosure to validate whether the TOF reading for the thin block (T 2 ) falls within the range of the predetermined time acceptance window. If the TOF reading for the thin block (T 2 ) falls out of the predetermined time acceptance window, the operator is alerted for an error and to repeat the TOF of the thin block. 
     It also can be understood that the presently disclosed method for automatic calibration error detection provides the advantanges of improving calibration and therefore measurement/inspection confidence, accuracy and avoids erroneous readings. 
     It also can be understood that the presently disclosed method for automatic calibration error detection provides the advantanges of improving the calibrating productivity by eliminating time wasted in guessing whether the readings are valid, particularly if the UT device does not have a waveform display to verify (T 2 ). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a  and  1   b  depict the ultrasonic waveforms reflected from the backwall boundary of the thick and thin block respectively.  FIGS. 1   a  and  1   b  are collectively used to describe the principle used by the auto calibration error detection according to the present invention. 
         FIG. 2  is a flow chart of a procedure for Two Point Calibration with automatic error detection according to the present invention. 
         FIG. 3  is a flow chart showing the detailed procedure identifying erroneous readings outside of a predetermined window according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As described in the Background of the Invention, during a typical Two Point Calibration procedure, a transducer selected for calibration is engaged with a thick and then a thin block with known thickness (H 1  and H 2  respectively). The transducer is triggered and ultrasonic echo signals are captured by the UT device. Typical waveforms are plotted in  FIGS. 1   a  and  1   b.    
     Referring to  FIGS. 1   a  and  1   b , the ultrasonic waveforms reflected from the thick and the thin blocks are shown respectively. For both  FIGS. 1   a  and  1   b , the X-axis depicts the time required for the ultrasonic signals to travel to and be reflected from the backwall boundary of the testing objects, namely the thick block and the thin block. The Y-axis is the ultrasonic echo signal amplitude detected by the UT device. 
     In  FIG. 1   a , A represents the excitation pulse of the ultrasonic signal. B represents the first echo signal from the bottom boundary of the thick block which is detected by the UT device. T 1  is the TOF measurement of the first echo. 
       FIG. 1   b  shows the waveform of the detected echo signal when the calibration procedure is performed on the thin block. H is the excitation pulse of the ultrasonic signal. I is the first echo signal reflected from the back surface of the thin block which is detected by the UT device. Subsequently, J is the second echo signal and K is the third echo signal. T 2  is the Time Of Flight measurement of the first echo I. 
     As can be noted in  FIG. 1   a , the first echo signal reflected from the back surface of the thick block is easily distinguishable, and there are no other major echos detected to confuse the echo reflected from the back surface. 
     However, for the case of the thin block, as shown in  FIG. 1   b , in addition to the first echo I reflected from the back surface of the thin block, there are other echoes (J and K) that could be easily confused with echo I. In existing practice, it often occurs that the UT Device misreads the TOF for echoes J or K, for the TOF of echo I. 
     In accordance with one novel aspect of the present invention, a predetermined time acceptance window is provided, where a correct reading of the TOF for the first echo is expected to fall. As shown in  FIG. 1   b , the time acceptance window, herein referred to as W is given based on a calculated thin block thickness T using the three known values of T 1 , H 1  and H 2 . The details for such calculation are given later in the description associated with  FIG. 3 . 
     Also shown in  FIG. 1   b , dT is a predetermined tolerated window size. W is the time acceptance window. 
     If the echo TOF reading falls out of W, the UT device automatically aborts the reading and alerts the operator to repeat the TOF for the thin block. 
     Reference is now made to  FIG. 2 , which is a flow chart depicting the procedure of two point calibration with the auto error detection according the present invention. 
     The calibration procedure is started at step  201 . At step  202 , the transducer selected for calibration is engaged to the thick block of the calibration. At step  203 , TOF for the thick block, T 1 , is measured by the UT device. The actual known thickness of the thick block H 1  is then provided to the UT device at step  204 . The acquired ultrasonic waveform and T 1  are shown in  FIG. 1   a.    
     Continuing with  FIG. 2 , at step  205 , the transducer is engaged with the thin calibration block. At step  206 , TOF for the thin block T 2  is measured by the UT device. At step  207 , the actual known thickness of the thin block H 2  is then provided. The ultrasonic waveform and T 2  are shown in  FIG. 1   b.    
     It should be noted that steps  201  through  207 , together as steps  200 , constitute the procedure of how an existing conventional Two Point Calibration is carried out. 
     Continuing with  FIG. 2 , after obtaining the TOF reading of the thin block, an aspect of the invention herein includes the check step  300  for the UT device to automatically verify if T 2  falls within the predetermined valid range of W as shown in  FIG. 1   b . If T 2  falls within W, the UT device applies the calibration result at step  208 . If T 2  falls outside of range W, the UT device issues a warning to the operator, alerting the operator to the need to adjust the gain of the UT device or verify other factors and retake the T 2  measurement for the thin block by going back to step  206 . The check step  300  is further elaborated in the following  FIG. 3 . 
     Referring now to  FIG. 3 , also referring back to  FIG. 1   b , the detailed process of the automatic T 2  signal error detection is described. During this automatic T 2  error detection process, the acceptance window W as defined in  FIG. 1   b  and the validity of the TOF for the thin block T 2  using the acceptance window W is determined. 
     At step  301 , the signal error detection procedure is started. At step  302 , calibration material velocity V is calculated using H 1  and T 1  according to V=2·H 1 /T 1 . H 1  and T 1  are obtained in steps  202  and  203  in  FIG. 2 . 
     At step  303 , the thin block TOF value T is calculated using H 2  and V according to the equation T=H 2 /V. Then the TOF measurement detection window W is set using the predetermined dT at step  304  according to W=[T−dT, T+dT]. The value of dT is preferably given in a range of 30% -80% of value T. 
     At step  305 , and as shown in  FIG. 1   b , the measured TOF of the thin block, T 2 , is verified using the acceptance window W. 
     If T 2  is within this window W, the error detection procedure  300  is ended at step  307 . At this point a valid T 2  is provided and is used for the calibration procedure for the material velocity and zero offset in step  208 . 
     If T 2  does not fall within this acceptance window W, a warning message is generated at step  306 , which prompts the operator to repeat the calibration reading for T 2  as shown in  FIG. 2 . 
     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. For example, the scope of the present disclosure may be applied to a wide range of probes such as, but not limited to acoustic single element, multi-element, and array probes.