Abstract:
A particle inspection device includes a feeder configured to drop a particle through an image area, a reflector configured to provide a reflected view of the particle in the image area, and an image capturing device configured to capture an image of the particle in the image area such that the image includes at least a direct view of the particle and the reflected view of the particle. In addition, a method for inspecting a particle includes dropping the particle through an image area, providing a reflected view of the particle in the image area using a reflector, and capturing an image of the particle in the image area using an image capturing device so that the image includes a direct view of the particle and the reflected view of the particle.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to inspection systems and particularly to a method and device for three-dimensional inspecting of particles. 
   It is often desirable to inspect particles that are produced or created during various industrial processes. Inspection may be useful for determining properties of the particles, including, for example, size, shape, purity, surface roughness, color, and uniformity. The particles may be inspected for a variety of reasons, for example, as part of a quality control process, for sorting, or for identifying particular qualities of the particles including defects. 
   Several devices and methods are known for inspecting and analyzing particles. For example, many such methods and devices employ laser diffraction, spectroscopy, and various forms of visual image analysis. 
   One known image analysis technique of particle inspection captures a two-dimensional image of particles being inspected as they fall from a feeder through an image area. The captured image is analyzed using software running on a microprocessor to determine certain properties of the particles, such as size and shape. For non-spherical particles, for example, rock fragments and particles produced in mining and aggregate industries, analysis of a two-dimensional image can lead to an incorrect determination of the true size or shape of the particle. 
   One known inspection system uses three-dimensional image analysis to inspect the shape of coarse aggregates. That known system relies on the analysis of two separate images taken at right angles from two separate cameras of aggregate particles moving on a conveyor belt. The use of separate cameras and separate images has several disadvantages including additional cost of the inspection device as well problems in calibrating the two separate images. In addition, obtaining high image quality of particles as they are being transported on a conveyor belt can be problematic and can diminish the accuracy and precision of the particle observations and/or measurements. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a method and device for three-dimensional inspection of particles in an efficient and cost effective manner. A further object of the present invention is to provide a method and device that presents particles for inspection in an advantageous way for capturing high quality images of the particles. 
   The present invention provides a particle inspection device that includes a feeder configured to drop a particle through an image area, a reflector configured to provide a reflected view of the particle in the image area, and an image capturing device configured to capture an image of the particle in the image area such that the image includes at least a direct view of the particle and the reflected view of the particle. In this manner, the present invention provides a device in which a single image capturing device can obtain an image of the particle in free-fall that includes three-dimensional information about the particle being inspected. Particularly when non-spherical particles are being inspected, information about a third dimension of a particle may be especially advantageous to the particle inspection. 
   The reflector may be advantageously disposed in a field of view of the image capturing device such that a vertical axis of the reflector is perpendicular to a sighting axis of the image capturing device. When the reflector is disposed vertically with respect to the sighting axis of the image capturing device and in the field of view, the vertical position of the particle in the image will be the same in both the direct and reflected views. Thus the direct and reflected views of the particle are easily correlated with one another. If the angle of the vertical axis is not perfectly perpendicular to the sighting axis of the image capturing device, the analysis software may include a calibrating routine in order to correlate the direct and reflected views to the particle. Preferably, the reflector is also disposed so that the reflected view is a reflected side view of the particle. Though a reflected view of the particle from nearly any angle will provide additional information about the particle, a side view provides a full view of the dimension perpendicular to the front face of the particle. 
   The device preferably also includes a first light source disposed opposite the image capturing device that is configured to provide a backlighting for the direct view, and may further include a second light source disposed opposite the reflector and configured to provide a further backlighting for the reflected view. The first and second light sources may include illuminated panels, which may be LED panels. The backlighting provides an improved image of the profile of the direct and/or reflected views of the particle, which is advantageous, for example when inspecting for particle size and/or shape. If surface characteristics are desired to be inspected, front lighting may be provided in place of and/or in addition to the backlighting of the direct and reflected views. 
   The feeder preferably includes a tray surface angled downward toward a first end of the feeder disposed above the image area and the particle inspection device preferably also includes a vibration device configured to jog the particle toward the first end of the feeder. The first end of the feeder may advantageously include a downwardly curved edge portion. A first section of the curved edge portion is preferably tangential to the tray surface, and a second section of the curved edge portion is preferably tangential to a drop angle of the particle. The curved portion of the end of the feeder is preferably shaped so as to encourage a translation of the particle and to discourage a rotation of the particle, so that the particle slides off the end of the tray with minimal rotational movement as it falls. If the end of the tray ends abruptly, with no curved transition surface, the particles, particularly oblong-shaped particles, will tend to tumble as they fall through the image area. If the particle is tumbling during its free-fall through the image area, the orientation of the particle with respect to the image capturing device is not well-controlled, and is unlikely to include a principal face of the particle. Particularly when inspecting particles having elongated shapes, it is desirable to have at least one view that shows a principle face of the particle. As the particle vibrates along the tray surface, it will tend to settle in a position such that its principal face is facing downwards against the tray surface. When the particle reaches the curved edge portion, it will tend to slide along the curved edge portion with the principal face facing the surface of the curved edge portion. Thus, as the particle slides down the curved edge surface, the principle face is slowly being rotated so as to be facing the image capturing device as it falls from the end of the curved edge portion and through the image area. 
   The particle inspection device preferably also includes an image processing device in operative connection with the image capturing device, wherein the image processing device is configured to determine a property of the particle. The property may includes a size property, a shape property, a color property, and a surface roughness property, or any combination of these. The image processing device is preferably configured to make calculations to derive further properties of the device, including, for example, volume and weight of the particle and statistical analyses based on the distribution of properties among large number of inspected particles. 
   The particle inspection device is preferably configured to inspect many particles, and the feeder is preferably configured to drop a second plurality of particles through the image area. The image capturing device is preferably configured to provide an image that includes a direct view of the particle and of the second plurality of particles and a reflected view of the particle and of the second plurality of particles. The flow rate of the particle being jogged through the feeder may be adjusted (for example by adjusting the vibrations of the vibration device and/or the angle of the feeder), so that more or less particles are captured in the image area by the image capturing device. 
   The device may also include a second image capturing device configured to capture a second image of a second particle in a second image area and wherein the second image includes a direct view of the second particle and a reflected view of the second particle. The use of more than one image capturing device may provide calibration advantages and may increase the rate at which images can be captured and processed and therefore the rate at which a large number particles can be inspected. 
   The image capturing device is preferably disposed such that a sighting axis of the image capturing device is substantially perpendicular to a drop angle of the particle. In most cases, the motion of the particle as it falls from the end of the tray surface will include a horizontal component and therefore the drop angle will not be vertical, at least not at the upper part of its free fall. Therefore, the image capturing device is preferably disposed so that the sighting axis is at an angle from horizontal. 
   The present invention also provides a method for inspecting a particle. The method includes the steps of dropping the particle through an image area, providing a reflected view of the particle in the image area using a reflector, and capturing an image of the particle in the image area using an image capturing device such that the image includes a direct view of the particle and the reflected view of the particle. 
   The dropping is preferably performed using a feeder having a downwardly curved edge portion. Also, the dropping is preferably performed so that a principle face of the particle is oriented so as to be facing the image capturing device. Backlighting is also preferably provided to the direct and/or reflected views, when size and shape characteristics of the particle are desired. The backlighting is preferably provided using one or more panels, such as LED panels. The method may further include analyzing the direct view and the reflected view of the image so as to determine a property of the particle, the analyzing preferably being performed using a microprocessor. The method preferably also includes dropping a second plurality of particles through the image area so that the image includes a direct view of the particle and the second plurality of particles and a reflected view of the particle and the second plurality of particles. 
   The method may be performed on a particle having a major diameter between 50 microns and 6000 microns, and/or on a particle has a major diameter between 0.1 inches and 3.0 inches, and/or on a particle having a major diameter greater than 1 inch. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be discussed in the following with reference to the drawings, in which: 
       FIG. 1  shows a perspective view of a particle inspection device according to the present invention; 
       FIG. 2  shows a perspective view of the imaging assembly of the particle inspection device shown in  FIG. 1 ; 
       FIG. 3  shows a schematic view of a portion of the imaging assembly shown in  FIG. 2 ; 
       FIG. 4  shows a perspective view of the feeder and vibration device of the particle inspection device shown in  FIG. 1 ; 
       FIG. 5  shows a schematic view of an image captured from the particle inspection device shown in  FIG. 1 ; and 
       FIG. 6  show a perspective view of an exemplary embodiment of a gate mechanism. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a perspective view of one embodiment of a particle inspection device  10 , which includes housing  20 . Inside the housing, a feeder  11  is suspended from the housing using mounting cables  27 . A vibration device  12  is rigidly connected to the feeder  11  and also suspended from the housing  10  using mounting cables  27 . A particle inlet opening  25  enables particles to be placed into the feeder  11 . In a laboratory setting, a user of the device may place a sample of particles to be inspected through the particle inlet opening  25 . Alternatively, the device could be used in-line so that the particles flow through the opening from a previous process operation. 
   The feeder includes a tray surface that is preferably slightly inclined downward from the end proximate the particle inlet opening  25 . When particles are in the feeder  11  and the vibration device  12  is switched on, such as by switching on switch  17 , the feeder is vibrated by vibration device  12 , which jostles the particles so that they may migrate toward the downward end of the feeder  11 , which is adjacent the vibration device  12  in  FIG. 1 . When the particles reach the downward end of the feeder, the particles fall into catch tray  26 . The particles can be removed from the housing through opening  28  in the rear of the housing by the device user. Alternatively, if the inspection device were to be used in-line with a larger production or inspection process, the particles could fall into a chute or otherwise flow to a subsequent process operation. An imaging assembly  30  is mounted to supports  23  and  24 , which each include a plurality of holes, using bolts passing through slots  38  and  39  respectively, of imaging assembly  30 . In this way, the imaging assembly  30  is mounted in a manner such that its position and angle can be adjusted to provide optimal viewing and imaging conditions. 
   Imaging assembly  30  is shown in more detail in  FIG. 2 . Two image capturing devices, for example CCD cameras  15  and  16 , are mounted at one end of imaging assembly  30 . At an opposite end, an illumination panel  33  is mounted opposite camera  15  and illumination panel  35  is mounted opposite camera  16 . Image area  31  includes the area in front of illumination panel  33  through which particles fall from the feeder  11  to the catch tray  26 . A second image area  32  includes the area in front of illumination panel  35  through which particles fall from the feeder  11  to the catch tray  26 . Because the particles fall between a camera and illumination pair ( 15  and  33 , or  16  and  35 , respectively), the illumination panels  33  and  35 , when illuminated, provide backlighting for a direct view of the particles from cameras  15  and  16 , respectively. LED panels may be used as the illumination panels. 
   Although the embodiment shown includes a pair image capturing devices  15 ,  16  and a pair image areas  31 ,  32 , this is not necessary for the functioning of the invention. An imaging assembly including a single image capturing device  15  and single image area  31  would work as well. The use of two cameras merely increases the rate at which particles can be inspected as two images can be captured of different particles and simultaneously processed. 
   In addition, imaging assembly  30  includes reflector  13 , such as a mirror, which is positioned within a field of view of the first image capturing device  15  such that it provides a reflected side view of particles falling through the image area  31  to image capturing device  15 . Illuminated panel  34 , which is oriented 90 degrees with respect to illumination panel  33 , provides backlighting to the reflected side view taken from camera  15 . Similarly, reflector  14  is positioned within the field of view of second camera  16  such that it provides a reflected side view of particles falling through the second image area  32  to second camera  16 . Illuminated panel  36 , which is oriented 90 degrees with respect to illumination panel  35  (and back-to-back with respect to illumination panel  34 ) provides backlighting to the reflected side view taken from camera  16 . 
     FIG. 3  shows a schematic view of the components of the imaging assembly  30 . Particles  45  and  46  are shown falling within first image area  31 . Particles  47  and  48  are shown falling within second image area  32 . First camera  15  defines sighting axis  49  and a field of view between boundary lines  41  and  42 . The direct view  43  of the image area  31  taken from camera  15  is shown schematically by arrow  43  and the reflected side view taken from camera  15  is shown schematically by arrow  44 . 
   The feeder  11  and vibration device  12  are shown in more detail in  FIG. 4 . Feeder  11  includes two mounting elements  51 . Feeder  11  is rigidly attached to vibration device  12 , which also includes two mounting elements  52 . Mounting elements  51  and  52  each including a loop connected to a spring. Mounting cables  27  are connect to the loops of mounting elements  51  and  52  in order to suspend the feeder  11  and the vibration device  12  from the housing. The springs in mounting elements  51  and  52  provide damping action in order to smooth out the vibrations to feeder  11  and to allow a smoother migration of the particles from one end of the feeder to the other. Feeder  11  includes tray surface  53  at its bottom. Feeder is preferably disposed within housing  20  in such a manner that tray surface  53  slopes downward slightly toward the open end of the feed tray (disposed underneath vibration device  12  in  FIG. 4 ). The slight downward slope coupled with the vibrations induces a migration of the particles from one end of the feeder to the other. 
   At its open end, tray surface  53  includes downwardly curved portion  55 . Curved portion  55  provides a smooth transition to the particles as they fall off the edge of tray surface  53  and helps to orient the particles so that a principle surface of the particle is directed toward the camera during free-fall through the image area. Through the vibration of the feeder  1 , the particles, which may include rock fragments or other particles having oblong shapes, will tend to settle into a position with their principle face (i.e. the face having the largest substantially flat surface area) downward. If the tray surface were to include an abrupt edge without a downwardly curved edge portion, the oblong-shaped particles would tend to tumble off the edge of the feeder and rotate end-over-end as they fell through the image area. In effect, the edge would act to flip the trailing edge upward as the leading edge of the particle began to fall. With the curved edge portion  55 , the particles will tend to slide down the edge portion with their principle faces adjacent to the surface of the curved edge portion  55 . Thus, as the particles slide down the curved edge portion, they become oriented such that their principal faces are facing toward image capturing device and in a direction perpendicular to the direction of movement of the particle as it begins to fall from the feeder. The end of the curvature of edge portion  55  is preferably tangential with the initial angle of fall  57  of the particle from feeder  11 . In addition, imaging assembly  30  is preferably mounted within housing  20  so that the sighting axis of the camera is perpendicular to the direction of fall of the particles. In this way, the particles will tend to fall with only minimal rotational movement, if any. During the fall, the principal faces of the particles will be oriented substantially toward the camera. In this way, the direct view of the particle from the camera will show the principle face of the particle, which is useful, especially for size and shape determinations of oblong-shaped particles. 
   The direction of fall of the particle, will typically not be directly vertical, at least not during the upper portion of its fall. Rather, as it leaves the end of curved portion  55 , the particle will be sliding along in the direction of the angle  57  of the curved portion. The direction of fall will become more vertical later in the fall as gravity accelerates the particle downward . Therefore, as shown in  FIG. 1 , in order to capture an image of the particles perpendicular to their direction of fall through the image area, the imaging assembly  30  is typically mounted in housing  20  at an angle from direct horizontal. 
   Tray surface  53  of feeder  11  also includes screened recess  54  at an intermediate portion between the two ends, which may be provided in order to remove particularly fine particles (“fines”) from a particle sample being inspected. In some instances, the volume of fines that are mixed with the larger particles can create a “dirt curtain” through the image area, or otherwise interfere with optimal imaging of the larger particles. Depending on the type of particles being inspected, and the type of analysis being performed, the gage of the screened recess may be adjusted or a feeder without a screened recess may be used. 
   A gating mechanism may be optionally used in the feeder with or without a screened recess to separate out particles according to size and/or to control the rate of migration of the particles through the feeder. One example of a gating mechanism, shown in  FIG. 6 , includes a low-profile raised portion  61  of tray surface  53  of the feeder. Raised portion  61  may be a strip of material connected, for example by welding, to tray surface  53 . Raised portion  61  is sized so as to extend above the rest of surface  53  enough to divert fine particles toward the edges of tray surface  53  while enabling larger particles to vibrate over raised portion  61  without being significantly diverted. By diverting the fines to the edges of the tray surface  53 , interference with the imaging of the larger particles is reduced or eliminated. The optimal height of raised portion  61  for diverting fine particles will depend on, among other factors, the size of particles being imaged and the size of fine particles to be diverted. 
   For applications in which the fines are an important component of the measurement, the fines can be extracted from the main flow, for example by using the screened recess  54 , and sent down a chute so as to pass through a supplemental image area. A supplemental image capturing device may capture images of the fines and send them to the processor for inclusion in the total analysis of the sample. Flow of fines through a supplemental image area may be viewed with backlighting and/or using a reflector as are the particles through the first and second image areas  31  and  32 . 
   An example of an image  100  of particles  45  and  46  (as shown in  FIG. 3 ) is shown in schematic form in  FIG. 5 . The left half of the image  100  shows a direct view  101  of image area  31  and the right half of the image shows a reflected view  102  of image area  31 .  45   d  represents a direct view of particle  45  and  45   r  represents a reflected side view of particle  45 . Likewise,  46   d  represents a direct view and  46   r  represents a reflected side view of particle  46 . As can be seen from two views of the image, particle  45  has a rather flat shape with considerably less thickness than particle  46 . The image  100 , shows an example of the importance of the additional information shown in the reflected side view, especially in determining size or volume of the particles. For example, if only the direct view of the particles were available, the particle  46  may be judged to be only slightly larger than particle  45 . When both views are available, it becomes clear that the volume of particle  46  is substantially greater than the volume of particle  45 . 
   In the preceding specification, the invention has been described with reference to a specific exemplary embodiment thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative rather than a restrictive sense.