Abstract:
A system for inspecting a component includes a fixture for holding the component, a light source disposed on one side of the component, and an optical detector disposed on the other side of the component. The detector images a portion of the outer edges of the component. A translation stage is operative to move the light source and detector in unison along the length of the component. The component is rotated through a predetermined angle about its axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/510,402 filed Aug. 25, 2006, which is presently pending. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to component inspection and, in particular, to a system and method for inspecting the profile of threaded and cylindrical components. 
       BACKGROUND OF THE INVENTION 
       [0003]    There is a need in many industries for the measurement of thread characteristics on screws, bolts and other fasteners and components. Apparatus for this purpose may be broadly categorized into contact and non-contact approaches. Contact-type gages typically employ feelers which contact the threaded surface to be measured at set points and mechanically measure the tolerances. One disadvantage of such devices is that the contact feelers can become worn with usage or become out of adjustment. Also, because the gage only takes a reading when the feeler is contacting the component, 100% coverage of the threaded surface during the inspection is not possible. 
         [0004]    Numerous non-contact thread measurement techniques have been developed, including systems that employ lasers and video cameras. U.S. Pat. No. 4,315,688 shows an apparatus for inspecting threaded objects, such as bolts, moving on a conveyor line past the inspection station. A light-sensitive detector picks up reflected light and produces an analog output which is used to determine the quality of the threads. The light-sensitive area of the detector is sufficiently small to resolve the individual threads of the threaded object, to determine whether the correct number of threads are present. 
         [0005]    U.S. Pat. No. 4,598,998 discloses a screw surface flaw inspection method and an apparatus therefore. The system projects light onto the surface of a screw, the light being scanned axially of the screw. A detector picks up the reflected light from the projected surface and sends an output signal to a signal processing circuit to thereby detect the flaw on a basis of time base variation of the intensity of reflected light. The method and apparatus enables the inspection of minute flaws rapidly and exactly. 
         [0006]    The apparatus of U.S. Pat. No. 4,644,394 has a light source for illuminating the threaded surface to be inspected, a mirror system for directing the light from the light source, and a video camera for receiving the directed light, for forming an optical image of the illuminated threaded surface, and for converting the optical image into electrical video signals. An encoding means converts the video signals from the camera into digital information representative of special information in the optical images viewed by the video camera. Processing means receive and interpret the digitized information provided by the encoding means for analyzing the thread characteristics and for detecting defects in the threaded surface being inspected. 
         [0007]    An apparatus for measuring the profile of portions of an article located within a predefined plane is disclosed in U.S. Pat. No. 4,906,098. Each portion is scanned, such as by an optical micrometer providing a beam of radiant energy, to determine its dimension. The distance between each portion and a vertical reference is also scanned to determine its dimension. The article is rotated about an axis intersecting the predefined plane within the scan of the beam and is axially moved along an axis parallel to the intersecting axis within the scan of the beam so that the dimension of each portion and its distance from the vertical reference can be determined. The apparatus may be used in combination with a cavity identification system to control manufacturing employing multiple molds. 
         [0008]    U.S. Pat. No. 5,521,707 uses laser triangulation to quickly build a precise profile of a thread form. The sensor is mounted on a precision mechanical system that moves the sensor to scan the thread form, producing a set of digitized images of a thread form that are digitally stored. The digitized images are analyzed to derive quantitative information about thread characteristics such as pitch, lead, root radius, flank angle, surface roughness, helix variation, and pitch diameter. Thread characteristics may be stored and later retrieved in order to provide traceability and verification of the part. 
         [0009]    U.S. Pat. No. 5,608,530 utilizes a laser for producing a beam of radiation which is then refined in cross section through use of plano-cylindrical lenses. The refined beam of radiation falls incident on a part to be measured. The unobstructed portions of the beam are then redirected by a pair of reflective surfaces producing non-parallel radiating beams; each beam comprises of the unobstructed portion of radiation which has passed radially opposed halves of the part. The magnitude of radiation present in each non-parallel radiating beam is then measured. The magnitude of radiation measured is proportional to a dimensional measurement. However, the assumption must be made that the part is placed perfectly in the center relative to lens; if not, diameter measurement may be problematic. In addition, since the resolution limit is a strong function of laser wavelength, performance may suffer if component layout is not properly aligned. For example, if the light from the laser varies, this will result in a change at the detectors which, in turn, could be misinterpreted as a diameter irregularity. 
         [0010]    Thus, despite the advances of these and other approaches, the need remains for simple yet effective inspection apparatus and method. 
       SUMMARY OF THE INVENTION 
       [0011]    This invention resides a system and method for inspecting a component having a length, a width, and an axis. According to an apparatus aspect, the system includes a fixture for holding the component, a light source disposed on one side of the component, and an optical detector disposed on the other side of the component. In the preferred embodiment, the detector has a field of view wider than the width of the component, thereby enabling the detector to image a portion of the outer edges of the component. A translation stage is operative to move the light source and detector in unison along the length of the component and a processor, in communication with the detector and the translation stage, is operative to: 
         [0012]    a) receive electrical signals representative of the outer profile imaged by the detector, 
         [0013]    b) move the translation stage incrementally along the length of the component, and 
         [0014]    c) record the outer profile imaged by the detector at each increment and form a composite profile of the component. 
         [0015]    The light source and optical detector and generally linear and spaced apart from one another in a generally parallel relationship. The optical detector may be a line scanned or may form part of a two-dimensional image detector. The light source preferably has a width co-extensive with the width of the optical detector. 
         [0016]    In the preferred embodiment, the processor is further operative to record the composite profile of the component at one or more angular orientations by rotating the component through a predetermined angle about its axis. Using the composite profile of the component at one or more angular orientations, the processor may generate a three-dimensional model the component by assembling the composite profiles of the component taken at a plurality of angular orientations. 
         [0017]    The preferred apparatus includes a display device, and the processor is further operative to generate a display of the profile of the component. The fixture holds the component in a generally vertical orientation, and the light source and optical detector are horizontally disposed. In a comprehensive embodiment, the processor is further operative to generate a virtual object that interacts with the composite profile and calculate parameters associated with the component as a function of the interaction. For example, if the composite profile includes a thread pattern, the processor may perform a 3-wire analysis or other investigations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a simplified drawing used to illustrate the way in which this invention operates; 
           [0019]      FIG. 2A  shows how, if a light source is a point source, or has a horizontal dimension less than that of the detector elements used in the image sensor, the features of the component and the diameter may be misinterpreted; 
           [0020]      FIG. 2B  illustrates how a lens may be used to collimate a divergent beam; 
           [0021]      FIG. 2C  shows how a telecentric lens arrangement may be used between an image sensor and a component under inspection to provide a consistent profile of the part regardless of the light source structure; 
           [0022]      FIG. 2D  shows a light source obtained via a line-generating laser; 
           [0023]      FIG. 3A  depicts a component moving down a V-shaped track having a separation allowing a light source and detector to be positioned to scan the part as it moves along the track; 
           [0024]      FIG. 3B  is an end-on view of the profile  303  of the component  302  seen as the part moves; 
           [0025]      FIG. 4  shows how it is possible with the invention to perform virtual a 3-wire thread analysis; and 
           [0026]      FIG. 5  is a drawing of a chuck components according to the invention used to hold components for rotational purposes. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]      FIG. 1  is a drawing which illustrates the preferred embodiment of the invention. A part  102  to be inspected is placed in a fixture (discussed later) allowing the component to be rotated about axis  104 . Behind the component  102  is a bright, horizontally oriented light source  106  and, in front of the component  102  is a horizontally oriented light detector  108 . It must be kept in mind that this is a highly simplified drawing, such that details of the light source, detector and other features are not illustrated for the purposes of clarity. Nor are any of the enclosures that would be used for most of the components. 
         [0028]    The light source  106  and detector  108  are physically coupled to one another, such that they move as a unit up and down along the axis of the part  104 . This is accomplished by connecting the light source and detector to a column  110 , including a translation mechanism and linear encoder for precise movement. In the preferred embodiment, the light source and detector move in fixed increments of 1 micron, though this is adjustable. 
         [0029]    The light source  106  is of a high intensity, creating a bright field and an image on the detector  108  which is extremely high in contrast. With such an arrangement, that the light-sensing elements of detector  108  essentially see white where the light is able to pass by the component  102 , and dark or black where the component blocks the light reaching detector. The elements of the detector  108  are coupled to a processor (not shown) which receives the profile information for each up/down increment of the light source and detector as it moves. The various readings are compiled, providing for an accurate profile of the component at a particular orientation. Once the length of the component is scanned as desired or necessary, the component is rotated about axis  104 , and the process is repeated. The angle through which the component is rotated is preferably user controllable, though a total rotation of 180.degree. as opposed to 360.degree. is sufficient, since the detector need not look at the “backside” of the component. 
         [0030]    According to the invention, the detector  108  is comprised of a linear set of sensor elements, each element being positioned at a known, fixed distance relative to the overall set. As these dimensions are known, the profile and other characteristics of the component  102 , including diameter and other features, may be precisely measured and stored through appropriate calibration. Although the detector  108  may be implemented utilizing a line-scan camera, these are currently quite expensive. Consequently, according to the invention, a line-scan camera is simulated using a two-dimensional image sensor, which is very reasonably priced due to its numerous applications in digital cameras and other equipment. 
         [0031]    Since the detector elements of most two-dimensional image sensors are grouped in a plurality of lines, a number of rows of pixels are used during each up/down increment. For example, if 16 lines are coupled to one another in the image sensor, the stage  110  is directed under computer control to move up or down at a distance calibrated to the 16 lines of the sensor. Again, since the distance between rows of pixels on the image sensor is known in advance, it is a straightforward calibration procedure to increment the light source  106  and detector  108  by any given number of rows per increment. Note, further, that the invention is not limited in terms of the length of the part, so long as it fits into the machine and the translator  110  is able to move the light source  106  and detector  108  along the length of the part. 
         [0032]    Given the versatility of the system and method, the scanning process may utilize multiple, distinct scanning processes. For example, in addition to the linear scan just described (set angle, variable position along the part axis), a rotational scan may be used with a set linear position and a variable angle. The rotational scan can, for example, be used very efficiently to detect the flat-to-flat or peak-to-peak distance on a hex screw head. This can be used for any local analysis requiring significantly more data than the ones given by the somewhat limited number of planes available through the linear scan. Checking for roundness for instance would require at least 4 planes which the user may not necessarily want for the rest of the inspection. Use of the rotational scan allows for the acquisition of more data to perform a roundness analysis. 
         [0033]    These different scan modalities may be used separately, together, and/or repeated as desired. For example, a complete scan process (linear+rotational) may be repeated several times, (i.e., “observations”), which may be important for six-sigma type quality control process. 
         [0034]    Since it is important for the pixel elements of the detector  108  to receive an accurate representation of the right and left profiles of the component  102 , the structure of the light source is important to the invention. As shown in  FIG. 2A , if light source  106  is a point source, or has the horizontal dimension less than that of the detector elements used in the image sensor  108 , the features of the component and the diameter will be misinterpreted as larger if the component  102  is moved from axially position  104  toward the light source to a position  102 ′. Various options are available according to the invention to correct this deficiency. As shown in  FIG. 2B , if the light source is divergent, a lens  200  may be used to collimate the beam. Alternatively, as shown in  FIG. 2C , a telecentric lens arrangement  202  may be used between the image sensor  108  and the component  102 , such that the image sensor of “C” is a consistent profile of the part regardless of the light source structure. 
         [0035]    In the preferred embodiment, however, the arrangement of  FIG. 2D  is used, wherein the width of the light source  204  is physically matched to the detector elements  108 . This is done with a light source formed from a linear row of surface-mount high-brightness-emitting diodes, with an overall length on the order of 2 inches. The width of the rows of pixels on the image sensor are also on the order of 2 inches, such that, in essence, a plane of light emerges from the light source, past the component, and on to the image sensor. 
         [0036]    A light source in the form of a line of LEDs ( FIG. 2D ) has advantages and disadvantages. LEDs are relatively inexpensive, compact and easy to implement, but the light is not collimated. This mean that the outer edges LEDs can create reflections on the part if it shiny or if it has a succession of flat surfaces such as a hex screw head. This problem may be resolved by implementing a line-generated laser lighting scheme (collimated in nature), but is bulkier and more expensive. A prudent approach is to offer the LED lighting with the current “low-res” 2D camera and standard (non-telecentric) optics for the entry level operation, and collimated lighting, telecentric optical mode, and “hi-res” linescan for a higher-performance operation. 
         [0037]    An advantage of the invention is that the component need not be perfectly centered in the inspection plane in order to measure features, such as threads, diameter, and so forth. Since the light source and detector are laterally fixed as they move up or down, the profile of the part may be registered to one end of the detector, measurements being normalized regardless of minor variances. This is an advantage over some prior art systems, which must be placed exactly in the center of a beam in order for multiple detectors to accurately measure component features. The current invention not only measures the width of the part in the area of inspection but also its location in the field of view with respect to one end of the detector. Thus, the invention may measure the features on components that would be a challenge to existing systems, including screws of the type called “trilobes.” These are self-tapping screws that are not, at all times, perfectly cylindrical. According to the instant invention, however, since measurements are referenced to one end of the detector, accurate details may be determined following a sufficient number of angular displacements. For instance the trilobular feature is only detected by the oscillation of its edges versus the angular displacement, while displaying a constant width. The invention can also utilize exotic parts, having diameters that change dramatically from end to another. Components that are bent and which may include flat surfaces may also be examined. A distinct advantage of the physical measurement process described here is that it directly measures a length (or diameter) but also its horizontal location in the inspection plane. This is particularly important in the case of the trilobular parts since at any given angle the diameters are equal. The only tell-tell sign of its trilobular feature is its horizontal “wobble,” which this invention takes into account. 
         [0038]    The invention is also applicable to both 100% inspection and sorting operations. Reference is made to  FIGS. 3A and 3B  which show, respectively, a component  302  moving down a V-shaped track  304  having a separation  306 , allowing a light source and detector  310 ,  312 , to be positioned to scan the part as it moves along the track.  FIG. 3B  is an end-on view of the profile  303  of the component  302  seen as the part moves. The velocity of the component can be measured optically. Although the gap in the track may cause the part to move or vibrate, a continuous track which is light-permeable may be used according to the invention. 
         [0039]    With the invention it is also possible to perform a virtual 3-wire thread measurement. Traditionally this is done by mounting a component in a fixture the, using 3 feelers coupled to micrometers, thread pitch and other parameters may be determined when the feelers are nested in the thread grooves. As shown in  FIG. 4 , this test may be simulated with the invention using computer-generated ‘wires’ of an appropriate diameter. The component is indicated at  402 , and three virtual wires  404 ,  406  and  408  are shown in within threads, in the way that an existing contact-type measurement is made. 
         [0040]    Also according to the invention if sufficient profiles are taken along an adequate number of angular rotations, interpolation or other forms of “morphing” algorithms may be used to estimate component geometry in between the actually measured profiles, thereby allowing a three-dimensional model of the part to be created. With this data, the use of a computer aided design program may be used to view a component from different angles, rotate, flip, magnify and perform other operations on the part seen on a computer screen. Furthermore, the 3D reconstruction of the part can be used to reverse engineer parts. Also the inspected part 3D model can be compared to the 3D design model and geometric and dimensional differences can be highlighted using a color scale proportional to the dimensional difference. 
         [0041]    Referring now to  FIG. 5 , there are shown preferred chucks  502 ,  504  according to the invention used to hold components for rotational purposes. Assuming the part is a bolt with a hex head, lower chuck  502 , removably attached to shaft  505 , includes a hex-shaped well into which the head of the bolt fits. Similarly, upper chuck  504  is adapted to receive the other end of the part. The component-contacting ends may be spring-loaded, allowing a part to be easily and quickly snapped into position. Even if a user requires custom adapters for particular part geometries, these are easily machined and relatively inexpensive.