Patent Publication Number: US-RE41342-E

Title: Coating thickness gauge

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to coating thickness gauges and more particularly to a novel method and apparatus for measuring and recording coating thickness data and associated descriptive data through a graphical user interface. 
     2. Description of the Related Art 
     The art of measuring the thickness of a coating on a substrate has produced a wide variety of coating thickness gauges for measuring a variety of materials. In general, coating thickness gauges include a probe which produces an electronical signal responsive to a measured physical quantity representative of a coating thickness. For example, when measuring the thickness of an electrically nonconductive coating on a conductive substrate, the probe can include an inductor which registers a change in impedance based on its proximity to the conductive substrate. The impedance change of the inductor is reflected by a change in frequency in an LC oscillator which can be mathematically related to the thickness of the coating. 
     Conventional coating thickness gauges have also provided the capability of transforming the electronic signal representative of coating thickness into digital data and of storing a number of data points for later downloading and analysis. Typically, the coating thickness measurements are later sequentially correlated to a written description of the article being measured. Such a procedure, however, requires the user to manually keep track of which data points correspond to which locations on the object being measured, and are thus time consuming and susceptible to recording errors. 
     Thus, although coating thickness gauges have been developed to provide very accurate digital readings, the industry has not yet produced a coating thickness gauge with a user interface which facilitates recording and analysis of data, despite the ongoing advances in computer technology. Prior to the present invention, there was a need in the art, therefore, for a method and apparatus for measuring and recording coating thickness data which is easy to use and which ensures accuracy and reliability in the recording of measurements. 
     OBJECTS AND SUMMARY 
     It is an object of the invention to provide a novel coating thickness gauge which allows a user to record thickness measurement data along with descriptive data through a user interface on a computer screen. 
     It is a further object of the invention to improve the accuracy of coating thickness measurement data by providing an apparatus which allows a user to alternate between recording a coating thickness measurement data point and recording descriptive textual or graphical data relating to the data point. 
     It is a further object of the invention to provide a modularized coating thickness apparatus which includes a probe which produces an electrical signal representative of a measured coating thickness and a PCMCIA card which receives the electrical signal and converts the electrical signal into a digital data signal in a standard PCMCIA output format. The coating thickness apparatus preferably includes a portable computing unit or Personal Digital Assistant (PDA) with a port for receiving the PCMCIA card and a screen for providing a graphical user interface. 
     An exemplary method according to the present invention includes the steps of obtaining a plurality of coating thickness values with a probe electrically connected to an electronic memory, recording in the electronic memory the plurality of coating thickness values, and recording in the electronic memory a plurality of descriptive data units, each descriptive data unit being associated with one of the coating thickness values and defined, for example, with reference to an electronic pictorial representation of the coated article. The steps of recording the coating thickness values and of recording the descriptive data units may be performed alternately. 
     Exemplary embodiments of the invention provide the on-site user with the power of a personal computer together with an easy-to-use interface that does not require a keyboard. Among other advantages, the gauge improves the accuracy and reliability of coating thickness measurements, provides the flexibility of plugging in any probe (e.g., magnetic, eddy current, ultrasonic, etc.) to an PCMCIA-compatible device, and allows the user to perform data analysis on-site. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the present invention will be more readily understood upon reading the following detailed description in conjunction with the drawings in which: 
         FIG. 1  is a perspective view of a coating thickness gauge according to an exemplary embodiment of the invention; 
         FIG. 2  is a schematic diagram of an exemplary PCMCIA card/probe unit; 
         FIG. 3a  is an enlarged view of a portion of a first exemplary probe assembly; 
         FIG. 3b  is a diagram of a second exemplary probe assembly; 
         FIG. 4  is a schematic diagram of a portable computing unit; 
         FIG. 5  is a diagram of an exemplary control display on the portable computing unit; and 
         FIG. 6  is a diagram of an exemplary output display on the portable computing unit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a perspective view of a coating thickness gauge according to an exemplary embodiment of the invention. The portable gauge  10  comprises a probe  20  connected by a cable  30  to an interface unit  40  such as a Personal Computer Memory Card International Association (PCMCIA) card. The PCMCIA card  40  is adapted to communicate with a portable computing unit  50  via a port  60 . The portable computing unit  50  is small enough to be held comfortably in the palm of one&#39;s hand. However, it preferably includes a relatively large screen display  70  to provide a graphical interface to the user. The screen  70  is preferably, though not necessarily, a touch-sensitive screen which can be activated, for example, with an index finger or with any suitable pointed writing instrument  80 . The portable computing unit  50  can be of the type generally known as a Personal Digital Assistant. The Apple NEWTON®, which provides a graphical user interface without a keyboard, is a preferred example of such a portable computing unit  50 . 
     The PCMCIA card  40  can be adapted to support a wide variety of peripheral devices, and due to its versatility, allows virtually any type of probe  20  to be incorporated into the thickness gauge  10 . For the purpose of illustration, two exemplary embodiments will now be described briefly in which a known type of probe  20  is implemented to measure the thickness of a coating on substrate. However, those skilled in the art will recognized that the PCMCIA card  40  can be adapted to support many other types of probes  20  in conjunction with the portable computing unit  50 . 
     According to one embodiment, as shown in FIG.  2  and as further described in commonly owned U.S. Pat. No. 5,293,132, entitled “Coating Thickness Measurement Gauge”, which is hereby incorporated herein by reference, the probe  20  of the coating thickness gauge  10  can be the inductor  75  of an LC oscillator  85  of suitable, known type. The LC oscillator  85  allows for the measurement of the thickness of an electrically nonconductive coating on an electrically conductive substrate. The inductor  75  can be a simple air-core solenoid-type coil. The phrase “air-core” is meant to refer to a coil having a core made of nonmagnetic, nonmetallic material. In practice, the wire is wound around a nonmagnetic, nonmetallic rod. During measurement, a probe structure housing the probe is placed in contact with the surface of the coating such that the separation of the coil  75  and the electrically conductive substrate is a function of the geometry of the probe structure and the coating thickness. 
     The impedance of the coil  75  varies with its proximity to the electrically conductive substrate resulting in a corresponding variation in the oscillation frequency of the LC oscillator  85 . This frequency is determined by a counter  90  which is used in conjunction with a microprocessor  100 . For instance, a timing loop may be programmed into the microprocessor  100  such that it resets the counter  90  at the beginning of the timing loop and measures the period of time elapsed until a predetermined number of oscillations has occurred as indicated by an overflow signal. The number of measured oscillations should be large enough to achieve the desired accuracy. 
     The relationship of the change in frequency of the oscillator  85  to the coating thickness is dependent on the particulars of the geometry of the probe assembly  20 , shown in expanded detail in FIG.  3 a. The most significant parameters affecting the relationship of the change in frequency to the coating thickness are the diameter r of the coil  75 , the number of turns of the coil  75 , the height I of the coil  75 , the gauge of the wire as it affects the dimension b, and the material of the wound wire. Furthermore, the relationship is different depending on the material composition of the substrate. For a nonmagnetic substrate such as aluminum, the relationship may be approximated by the fourth-order polynomial:
 
Y=A 0 +A 1 F+A 2 F 2 +A 3 F 3 +A 4 F 4  
 
where the coefficients A 0-4  are determined by the geometry of the probe  20  and the electrical characteristics of the substrate.
 
     For a six-turn single layer wound coil using 26-gauge copper wire, the coefficients A 0-4  may be empirically determined and represented as follows for nonmagnetic aluminum substrates, with F representing the frequency change in KHz and Y representing the thickness in microns:
 
Y=10090.44−(26.965)F+(3.0195×10 −2 )F 2 −(1.60−374×10 −5 )F 3 +(+3.25473×10 −9 )F 4  
 
     A complete set of coefficients A 0-4  can be stored in a ROM portion  110  associated with the microprocessor unit  100  during production of the thickness 10 gauge for any desired substrate material. For example, an additional set of coefficients B 0-4  can be stored for use with magnetic substrates. Thus, upon selection by the user of one of the substrate materials stored in memory, the coefficients associated with the selected substrate material can be recalled from the ROM  110  and employed along with the measured frequency change in the appropriate equation shown above for determining coating thickness. 
     According to a second exemplary embodiment, a second gauge probe can be used in conjunction with the present invention to determine automatically, with a single probe, the substrate characteristics, and to effect a measurement of the coating thickness on that substrate. Such a probe is described for example in commonly owned U.S. Pat. No. 5,343,146, entitled “Combination Coating Thickness Gauge Using a Magnetic Flux Density Sensor and an Eddy Current Search Coil”, which is hereby incorporated herein by reference. The probe tests for a ferrous substrate, measuring the temperature-compensated magnetic flux density at a pole of a permanent magnet using a Hall effect magnetic sensor and a thermistor.  FIG. 3b  shows a probe  25  which includes a permanent magnet  35 , a Hall effect magnetic sensor  45 , and a thermistor  55 . The magnetic flux density and temperature measurements are converted into a temperature-compensated magnetic flux density value that is proportional to the coating thickness on the ferrous substrate. If no ferrous substrate is detected, the coating thickness gauge automatically switches over to test for a conductive nonferrous substrate, measuring the effects of eddy currents generated in the conductive nonferrous substrate by the coating thickness gauge magnetic fields using an eddy current search coil  65 , as shown in FIG.  3 b. The eddy current measurements are converted into an eddy current frequency value that is proportional to the coating thickness on the conductive nonferrous substrate. 
     Various other types of known probes may also be incorporated into the present invention, for example probes which measure coating thicknesses on ferrous substrates with a magnetic induction technique using two coils and a ferrous core. As discussed with regard to the first embodiment, the PCMCIA card  40  can be adapted to include hardware elements such as a counter or a ROM chip to support a desired coating thickness gauge probe. The gauge electronics  120  in  FIG. 2  are thus intended to generally represent a capacity of the PCMCIA card  40  to include hardware elements to support any type of gauge probe. For example, as will be readily appreciated by those skilled in the art, the PCMCIA card  40  can be modified by one skilled in the art to include hardware to support probes which measure thicknesses of nonmagnetic coatings on ferrous substrates, non-conductive coatings on nonferrous substrates, combination probes which measure both, or probes which ultrasonically measure coating thicknesses on nonmetals. 
     In addition to the hardware support elements  120  included in the PCMCIA card  40  for a particular application. The PCMCIA card  40  also includes the microprocessor  100  and a PCMCIA interface  130  which creates a standardized communication path from the microprocessor  100  to the portable computing unit  50 . Included in the PCMCIA interface  130  is a Universal Asynchronous Receiver Transmitter (UART)  140 , an I/O device which sends and receives information in bit-serial fashion. The microprocessor  100 , in conjunction with the supporting hardware  120 , converts the signal from the probe  20  into a digital representation of a coating thickness which is transmitted through the UART  140  to the portable computing unit  50  in a standardized PCMCIA format. For brevity, the details of this process are omitted, as those skilled in the art are capable of adapting a particular signal to the PCMCIA format. 
     The physical attributes and internal operation of the PCMCIA card  40  are defined in detail by the Personal Computer Memory Card International Association, which updates the PCMCIA specifications periodically. The PCMCIA standard includes detailed specifications regarding the physical attributes of the card such as dimensions and mechanical tolerances, card interface information such as signal definitions for the connecting pins  125  of the PCMCIA card, and data organization on the card. Because the PCMCIA card is a standard interface, the present invention provides a versatile coating thickness gauge which can be used in a wide variety of hardware environments. 
     The portable computing unit  50  receives the PCMCIA card  40  via a port  60  to communicate with the probe  20 . The portable computing unit  50  includes, among other elements, a microprocessor  150  for controlling the operations of the coating thickness gauge  10 . See FIG.  4 . The portable computing unit  50  can be programmed, for example, to automatically recognize the type of probe which is connected to the portable computing unit  50 . The microprocessor  150  is associated with a memory  160  which can store computer programs which control the operation of the gauge  10 . The microprocessor  150  exchanges data with the memory  160  and with the user via the screen  70  which is large enough to provide a graphical interface for the user. The versatility provided by the memory  160 , the microprocessor  150 , the large screen  70 , and the standard PCMCIA interface thus provide the coating thickness gauge  10  of the present invention with many important advantages. Exemplary embodiments of the invention, for example, provide the user with the ability to perform complete data analysis or statistical process control on-site, the flexibility of using any probe with any PCMCIA-compatible portable computing unit  50 , and the capability of providing a sophisticated user interface which allows the user to easily annotate coating thickness measurements with descriptive textual and graphical data. 
     According to one exemplary method of the invention, a user of the gauge  10  alternates between recording a thickness measurement reading with the probe  20  and entering descriptive data via the screen  70 . The descriptive data can be entered in a number of ways. For example, a virtual typewriter keyboard can be graphically simulated on the screen  70  for entry of descriptive comments relating to a particular thickness measurement using an index finger or a pointed writing instrument  80 . Alternatively, the portable computing unit  50  can be adapted to convert a handwritten image, created by handwriting on the screen with the writing instrument  80 , into textual data. The process of converting a handwritten image of “electronic ink” or typed letters into digital textual data, which has been incorporated into the Apple NEWTON®, greatly facilitates the entry of descriptive data associated with a particular coating thickness measurement. The ability to label all or selective individual data points with descriptive text also enhances the reliability of the measured coating thickness data by ensuring that data points are properly labeled and by allowing the user to immediately record any abnormalities as measurements are taken. 
     According to a further exemplary method, a two- or three-dimensional image of the object to be measured can be created on the screen  70  by the user as a reference for input coating thickness data points. According to this method, a user first recalls or sketches a diagram of the object to be measured on the screen  70  of the portable computing unit  50  using the writing instrument  80 . This process can be facilitated with a program, included in the Apple NEWTON, which transforms user-created images into various geometrical forms such as rectangles and circles. The drawing is then stored in the memory  160  as a reference for the measured thickness values. As coating thickness values are obtained with the probe  20 , the user identifies, with reference to the screen drawing, the locations on the object at which the coating thickness values were obtained. In addition, the user can input for any coating thickness value, a textual description relating to the measured data point.  FIG. 1  is an example which depicts a drawing of a coated pipe  170  which a user would measure to obtain coating thickness values at various locations. After taking a measurement of the actual pipe with the probe  20 , the user simply indicates the location of the data point with reference to the pictorial representation on the screen  70  using the writing instrument. The screen thus serves as a graphical interface to record the location of data points  180 , as shown in FIG.  1 . 
     The large touch-sensitive screen  70  of the portable computing unit  50  can be further adapted to facilitate operation of the coating thickness gauge  10  with a number of virtual buttons. As shown in  FIG. 5 , the screen  70  can include several virtual buttons  190  which, for example, allow the user to enter a memory mode to begin storing thickness measurements, enter high and low tolerance limits, command the gauge to compute and display statistics on the data thus obtained, enter parameters specifying a particular process used in applying a coating, specify units for the coating thickness readings, or any other desired function. The process control button can be used for, among other functions, labeling any batch with a particular process used in coating. This feature facilitates data analysis by allowing the user to analyze a group of batches associated with the same coating process. Calibration buttons  200  are provided to calibrate the gauge when a reading differs from a known thickness. 
     At the top of the screen  70 , a display section  210  may be provided which displays thickness readings with units, an indicator of whether a ferrous or nonferrous material was measured, a textual description of a particular batch, and a label for a particular process used in coating. The screen  70  shown in  FIG. 5  is of course intended to provide an example illustrating the versatility of one embodiment of an exemplary coating thickness gauge. It will be readily appreciated by those skilled in the art, however, that many modifications in the screen interface can be affected without departing from the scope of the invention. 
     The screen  70  can also be adapted to provide graphical output, which advantageously allows the on-site user to use statistical process control in analyzing coating thickness measurements.  FIG. 6  shows an exemplary output screen which includes graphs  220  and  230  of x-bar and range for a set of batches, a histogram  240 , and a list of desired statistics  250  for the stored readings. The x-bar graph  220  shows on the screen  70  a computed average thickness value for each batch. The range graph  230  shows a computed difference in thickness between the greatest and least measured thickness in a particular batch. These graphs thus allow the user to easily monitor any anomalies or trends in the coating process. Moreover, according to an exemplary embodiment of the invention, the user can access any annotations or other descriptive data associated with a batch or thickness measurement simply by touching the displayed batch number, data point, or other indicia on the screen  70  with the writing instrument  80 . This capability allows the user to determine, for example, whether anomalies illustrated in the output graphs are associated with any anomalies described in annotations recorded during measurement. 
     The histogram  240  provides an additional visual indicator of the consistency of recorded coating thickness measurements. This list of statistics  250  can include, among other parameters, a standard deviation calculated from measurements of selected batches, a maximum and a minimum reading, upper and lower set limits (USL, LSL) set by the user, and upper and lower control limits (UCL, LCL) which represent the average thickness plus or minus three standard deviations. Like the screen of  FIG. 5 , the output screen in  FIG. 6  is, of course, intended to show one embodiment which may be modified, for example, to accommodate other statistical process control operations without departing from the scope of the invention. 
     The present coating thickness gauge according to exemplary embodiments of the invention thus provides many important advantages in obtaining coating thickness measurement data. By combining a portable computing unit such as a Personal Digital Assistant with a coating thickness gauge probe via a PCMCIA interface, the invention greatly enhances the computing options available for obtaining and processing coating thickness measurements on-site. Thus, the user may perform data analysis, enter descriptive comments, control the gauge with icons, and generally harness the power of a large display, resident software, and regular upgrades of the portable computing unit. Moreover, these advantages are provided in a coating thickness gauge which is substantially less expensive to manufacture than commercially available gauges. 
     The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims.