Patent Publication Number: US-9415600-B2

Title: System for detecting inoperative inkjets in three-dimensional object printing using a digital camera and strobe light

Description:
TECHNICAL FIELD 
     The device disclosed in this document relates to printers that produce three-dimensional objects and, more particularly, to the accurate detection of inoperative inkjets in such printers. 
     BACKGROUND 
     Digital three-dimensional manufacturing, also known as digital additive manufacturing, is a process of making a three-dimensional solid object from a digital model of virtually any shape. Three-dimensional printing is an additive process in which one or more printheads eject successive layers of material on a substrate in different shapes. Three-dimensional printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling. 
     The production of a three-dimensional object with these printers can require hours or, with some objects, even days. One issue that arises in the production of three-dimensional objects with a three-dimensional printer is consistent functionality of the inkjets in the printheads that eject the drops of material that form the objects. During printing of an object, one or more inkjets can deteriorate by ejecting the material at an angle, rather than normal, to the printhead, ejecting drops that are smaller than an inkjet should eject, or by failing to eject any drop at all. An inkjet suffering from any of these operational deficiencies is known as an inoperative inkjet. If the operational status of one or more inkjets deteriorates during object printing, the quality of the printed object cannot be assessed until the printing operation is completed. Consequently, print jobs requiring many hours or multiple days can produce objects that do not conform to specifications due to inoperative inkjets in the printheads. Once such objects are detected, the printed objects are scrapped, restorative procedures are applied to the printheads to restore inkjet functionality, and the print job is repeated. An apparatus that enables detection of inoperative inkjets while printing would enable restorative procedures to be applied during object printing so a properly formed object can be produced. In this manner, product yield for the printer is improved and its printing is more efficient. The apparatus should be able to detect inoperative inkjets that eject a multitude of printing materials, such as clear, colored, translucent, phosphorescent, and waxy materials. 
     SUMMARY 
     An apparatus that enables inoperative inkjet detection in three-dimensional printers includes an optical sensor having a focal plane at a predetermined distance from the optical sensor, the optical sensor being configured to generate image data of the focal plane, an illumination source positioned to illuminate the focal plane of the optical sensor, and a controller operatively connected to the optical sensor, the controller being configured to operate a printhead positioned to eject drops from inkjets in the printhead into the focal plane of the optical sensor, to activate the illumination source as the printhead ejects drops into the focal plane of the optical sensor, and to receive image data of the focal plane from the optical sensor. 
     A printer that incorporates the apparatus for detecting inoperative inkjets includes a printhead configured for movement in a plane in two perpendicular directions in the plane, an optical sensor having a focal plane at a predetermined distance from the optical sensor, the optical sensor is positioned to enable the focal plane to be perpendicular to a face of the printhead and the plane in which the printhead is configured for movement, an illumination source positioned to illuminate the focal plane of the optical sensor, and a controller operatively connected to the printhead, the illumination source and the optical sensor, the controller being configured to operate the printhead to eject drops from inkjets in the printhead, to activate the illumination source as the printhead ejects drops through the focal plane of the optical sensor, and to receive image data of the drops passing through the focal plane of the optical sensor generated by the optical sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of an apparatus or printer that detects inoperative inkjets during three-dimensional printing are explained in the following description, taken in connection with the accompanying drawings. 
         FIG. 1  is a perspective view of a three-dimensional object printer. 
         FIG. 2  is front view of a three-dimensional object printer having a housing that depicts a space within the housing for a module that enables inoperative inkjets in the printhead to be detected during a printing operation. 
         FIG. 3A  is a perspective view of a module for detecting inoperative inkjets that fits in the space  112  shown in  FIG. 2  and  FIG. 3B  depicts a position of the focal plane of the camera in a space beneath a printhead. 
         FIG. 4  is a flow diagram of a method for operating the module of  FIG. 3 . 
         FIG. 5  is an illustration of material drops ejected by a printhead in the field of view of the camera shown in  FIG. 3 . 
         FIG. 6  is a perspective view of a printhead face that illustrates the X and Y directions of movement for imaging the ejections from inkjets in the printhead. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the device disclosed herein as well as the details for the device, reference is made to the drawings. In the drawings, like reference numerals designate like elements. 
       FIG. 1  shows a configuration of components in a printer  100 , which produces a three-dimensional object or part  10 . As used in this document, the term “three-dimensional printer” refers to any device that ejects material with reference to image data of an object to form a three-dimensional object. The printer  100  includes a support material reservoir  14 , a build material reservoir  18 , a pair of inkjet printheads  22 ,  26 , a build substrate  30 , a planar support member  34 , a columnar support member  38 , an actuator  42 , and a controller  46 . Conduit  50  connects printhead  22  to support material reservoir  14  and conduit  54  connects printhead  26  to build material reservoir  18 . Both inkjet printheads are operated by the controller  46  with reference to three-dimensional image data in a memory operatively connected to the controller to eject the support and build materials supplied to each respective printhead. The build material forms the structure of the part  10  being produced, while the support structure  58  formed by the support material enables the build material to maintain its shape while the material solidifies as the part is being constructed. After the part is finished, the support structure  58  is removed by washing, blowing, or melting. 
     The controller  46  is also operatively connected to at least one and possibly more actuators  42  to control movement of the planar support member  34 , the columnar support member  38 , and the printheads  22 ,  26  relative to one another. That is, one or more actuators can be operatively connected to structure supporting the printheads to move the printheads in a process direction and a cross-process direction with reference to the surface of the planar support member. Alternatively, one or more actuators can be operatively connected to the planar support member  34  to move the surface on which the part is being produced in the process and cross-process directions in the plane of the planar support member  34 . As used herein, the term “process direction” refers to movement along one axis in the surface of the planar support member  34  and “cross-process direction” refers to movement along an axis in the planar support member surface that is orthogonal to the process direction axis in that surface. These directions are denoted with the letters “P” and “C-P” in  FIG. 1 . The printheads  22 ,  26  and the columnar support member  38  also move in a direction that is orthogonal to the planar support member  34 . This direction is called the vertical direction in this document, is parallel to the columnar support member  38 , and is denoted with the letter “V” in  FIG. 1 . Movement in the vertical direction is achieved with one or more actuators operatively connected to the columnar member  38 , by one or more actuators operatively connected to the printheads  22 ,  26 , or by one or more actuators operatively connected to both the columnar support member  38  and the printheads  22 ,  26 . These actuators in these various configurations are operatively connected to the controller  46 , which operates the actuators to move the columnar member  38 , the printheads  22 ,  26 , or both in the vertical direction. 
     A three-dimensional object printer having a housing is shown in  FIG. 2 . That printer  60  has a housing  64 . Within the housing  64  are six compartments that are generally cubic in shape. The housing  64  is shown in  FIG. 2  without the doors that close to conceal the compartments. Compartment  72  includes a planar support  78  on a movable platform  82 . Movable platform  82  is configured with one or more actuators and guide members (not shown) to enable the movable platform  82  to move up and down in a vertical direction. The planar support  78  is the surface on which a three-dimensional object is formed. In some embodiments, the printhead  86  has a length that is approximately equal to the length of the planar support  78  in the direction from the back wall of compartment  72  to the opening at the front of the compartment. In these embodiments, printhead  86  is mounted on support member  92  in the space between sidewalls  96  and  100  of housing  64  for linear reciprocating movement only. In other embodiments, the printhead  86  has a length that is less than the length of the planar support  78  in the direction from the back wall of compartment  72  to the opening at the front of the compartment. In these embodiments, printhead  86  is mounted on support member  92  in the space between sidewalls  96  and  100  of housing  64  for reciprocating movement in two orthogonal directions in a plane above compartment  72 . In these various embodiments, one or more actuators  104  are operatively connected to the printhead  86 . Controller  108  operates the actuators  104  to move the printhead  86  either linearly back and forth on support member  92  or to move the printhead in two orthogonal directions within a plane. By selectively operating the inkjets in the printhead  86 , vertically moving the support platform  82 , and horizontally moving the printhead  86  on the member  92 , a three-dimensional object can be formed on the planar support  78 . 
     The area  112  outlined in dashes in  FIG. 2  identifies the placement of a module that uses a digital camera and light source to detect inoperative inkjets in the printer  60 . As noted above, if an inkjet fails during printing of an object by either completely or partially failing to eject material or by errantly ejecting material in a skewed direction, the object being produced is malformed. Currently, this malformation cannot be detected until production of the object is finished. By using area  112  for optically imaging the material ejected from inkjets in the printhead  86 , printer  60  can be configured to detect inoperative inkjets during object production as described more fully below. Some components within the module  300  can move in the horizontal direction H, depth direction D, and vertical direction V as shown in the figure. 
     One embodiment of a module that detects inoperative inkjets during object printing is shown in the block diagram of  FIG. 3A . The module  300  is configured to fit within area  112  of printer  60 . The module  300  includes a high speed digital camera  304 , a strobe light  308 , a waste receptacle  312 , and a controller  320 . The controller is operatively connected to the camera  304 , the strobe light  308 , and the controller  108  that moves the printhead  86 . The strobe light is tuned to produce illumination for a period of time that material drops are present in the field of view of the camera once the light is activated. The camera  304  has a focal plane  306  at a predetermined distance from the magnification lens  310  of the camera  304  as shown in  FIG. 3B . The field of view of the camera also has a predetermined height H and width W. As explained below, the printhead  86  is maneuvered by the controller  108  to align a plane normal to the face of the printhead  86  with the focal plane  306  of the camera  304  at a distance from the printhead face that enables drops ejected from the inkjets in a row of the printhead  86  to pass through the focal plane  306  of the camera  304 . Image data of the drops passing through the field of view of the camera  304  are captured and analyzed to identify inoperative inkjets ( FIG. 5 , for example). 
     To detect inoperative inkjets during printing operations, the module  300  is operated with reference to the method shown in  FIG. 4 . The method of  FIG. 4  is implemented with controllers configured to perform the method. As used in this document, configuring a controller means storing programmed instructions in a memory operatively connected to the controller so when the controller executes the programmed instructions the controller generates signals to manipulate data and operate electronic components to perform the method. 
     At predetermined times in the printing operation, the controller  108  ( FIG. 2 ) operates an actuator  104  to move the printhead  86  into the module  300  located in the area  112  (block  404 ). In response to the controller  320  detecting the printhead in the module  300 , controller  320  generates a signal to the controller  108  to operate some of the inkjets in the printhead to eject material (block  412 ). The controller  320  then operates the strobe light to illuminate the area beneath the printhead  86  and the camera is activated to generate image data of the illuminated area (block  414 ). The controller  320  analyzes the image data received from the camera to identify any inoperative inkjets (block  416 ). For example, the size of the drops in the image data can be measured and compared to an empirically determined drop size range to determine whether the drop mass/volume of the drops is within an acceptable range. Also, the time of travel for the drops across the field of view can be measured and compared to an empirically determined velocity range to determine whether an inkjet is firing correctly. Image data of material drops ejected from a group of inkjets in a staggered manner are shown in  FIG. 5 . Controller  320  checks to see if more inkjets are to be tested (block  418 ) and, if inkjets remain to be tested, generates electrical signals indicating an amount of movement for the printhead in an X or Y direction (block  422 ). The Y direction is movement along a row of inkjets and X direction is movement from one row in a printhead to another row in the printhead. This pattern of movement is shown in  FIG. 6 . In response to the controller  108  sending electrical signals to controller  320  that the printhead  86  has been moved (block  426 ), controller  320  generates the signals for controller  108  to operate the printhead (block  412 ), and then controller  320  activates the strobe light and the camera to capture image data of the material ejection (block  414 ). The process continues until all of the inkjets are tested (block  418 ). A list of the inoperative inkjets can be generated for the operator (block  430 ) so appropriate action can be taken. 
     One advantage of the module described is the ejection of the material drops into the waste receptacle  312 . This configuration does not require substrates for the printing of a test pattern since the drops are imaged while they are in flight. The waste receptacle can be removed and either replaced or cleaned and then reinstalled from time to time to prevent the receptacle from overflowing. 
     In one embodiment, only a predetermined number of inkjets in a single row are operated. This predetermined number corresponds to the number of inkjets that can been seen in the field of view of the camera  304 . The printhead can then be moved in the Y direction by a distance that corresponds to the width of the camera&#39;s field of vision. In this manner, all of the inkjets in a row of inkjets can be successively imaged as they eject material. Any inkjet that does not produce a drop of the material in the field of view is identified as being inoperative. After a row of inkjets have been operated and imaged, the printhead can be moved in the X direction to transition to a new row and the inkjets in this row successively imaged until all the inkjets in that row have been imaged as they eject material. This process is repeated until all of the rows of inkjets have been tested. Alternatively, a corresponding section of each row can be imaged successively by moving the printhead in the X direction and then moving the printhead in the Y direction by a distance corresponding to the width of the field of vision of the camera before successively imaging a portion in each row. This type of pattern can be repeated until all of the inkjets have been tested. Alternatively, other combinations of X and Y direction movement can be used to test all of the inkjets in a printhead. 
     It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems, applications or methods. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art that are also intended to be encompassed by the following claims.