Abstract:
An apparatus detects inoperative inkjets during printing. The apparatus includes a light transmitting substrate onto which a test pattern of material is ejected. A light source directs light into the substrate and an optical sensor generates image data of a surface of the substrate. Light propagates through the substrate unless it reaches a position where the material is present on the surface. Thus, the material emits light so a contrast exists between the surface of the substrate and the material emitting light. By comparing the image data to the positions of the light emitting areas on the surface, inoperative inkjets are detected.

Description:
TECHNICAL FIELD 
     The device disclosed in this document relates to printers that produce three-dimensional objects and, more particularly, to accurate detection of inoperative inkjets in such printers. 
     BACKGROUND 
     Printing of documents on substrates, such as paper, are well-known. Newer forms of printing now include digital three-dimensional manufacturing, also known as digital additive manufacturing. This type of printing is a process of making a three-dimensional solid object of virtually any shape from a digital model. 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. Similar maladies in printheads are known in document printing with printheads. If the operational status of one or more inkjets deteriorates during three-dimensional 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. Even in document printing at high speeds on a moving web, unacceptable images may be produced over a long length of the web and this portion of the web may have to be scrapped. 
     Although systems have been developed in document printing systems to detect inoperative inkjets, the detection of inoperative inkjets in object printing systems is more problematic. Particularly problematic in both object printing and document printing systems are the use of the clear materials and inks. These materials and inks are difficult to detect by imaging systems because the contrast between the clear inks/materials on the substrates on which they are ejected is low. Consequently, the noise in the image data of the patterns on the substrate makes analysis of the test pattern difficult. An apparatus that enables detection of inoperative inkjets while printing with clear ink or clear materials would enable restorative procedures to be applied during object printing so a properly formed object or document could be produced. In this manner, product yield for the printer is improved and its printing is more efficient. 
     SUMMARY 
     A printer that detects inoperative inkjets includes a substrate having a surface and an edge along a perimeter of the substrate, a printhead configured to eject material onto the surface of the substrate, a light source positioned to direct light into the edge of the substrate, an optical sensor positioned to receive light emitted by the surface of the substrate, the optical sensor being configured to generate image data corresponding to the surface of the substrate, and a controller operatively connected to the printhead, the light source and the optical sensor, the controller being configured to operate the printhead to eject material onto the surface of the substrate with reference to a predetermined pattern, to activate the light source selectively, to receive image data generated by the optical sensor while the light source is directing light into the edge of the substrate, and to detect inoperative inkjets in the printhead with reference to the received image data and the predetermined pattern. 
    
    
     
       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. 3  is a perspective view of a module for detecting inoperative inkjets that fits in the space shown in  FIG. 2 . 
         FIG. 4  is a flow diagram of a method for operating the module of  FIG. 3 . 
     
    
    
     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 to control movement of the planar support member  34 , 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 either the planar support member  34  or the columnar support member  38  to move the surface on which the part is being produced in the process and cross-process directions. 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  are configured with an actuator to move in a direction that is orthogonal to the planar support member  34 . This direction is called the vertical direction in this document and is denoted with the letter “V” in  FIG. 1 . Movement in the vertical direction can be effected by one or more actuators operatively connected to the columnar support 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  and 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 light transmitting substrate 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 sensing material ejected onto a light transmitting substrate, 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 ejecting clear materials during object printing is shown in the block diagram of  FIG. 3 . The module  300  is configured to fit within area  112  of printer  60 . The module  300  includes an optical sensor  304 , a light transmitting substrate  308 , a substrate transport  312 , a light source  314 , one or more actuators  316 , a cleaning member  320 , a controller  324 , and a waste receptacle  328 . The optical sensor  304  is configured for bidirectional movement in both directions W and L by an actuator  316 . This configuration enables optical sensor  304  to move over the surface of endless belt  330  of transport  312 . The transport  312  includes endless belt  330 , which is entrained about rollers  332 . An actuator  316  drives the rollers  332  to rotate the belt bi-directionally to move the substrate  308  to a position where it can be printed and then to a position where it can be cleaned. The controller  324  is operatively connected to the actuators  316  to move the optical sensor  304 , to drive the rollers  332  to move the light transmitting substrate  308  with the belt  312 , and to sweep the light transmitting substrate  308  with the cleaning member  320 . In some embodiments, optical sensor  304  need only be configured to move bi-directionally in direction L provided the sensor is at least as wide as substrate  308 . 
     The light transmitting substrate  308  is a planar member made of a material that supports the build material and the support material ejected from the printhead  86  and that provides total internal reflection of light entering the substrate along an edge of the substrate. These materials enable light entering along an edge of the substrate to remain within the planar substrate unless some material on the surface of the substrate alters the total internal reflection property at the interface of the material and the planar surface. For printers that eject materials or ink having a refractive index in a range of about 1.3 to about 1.5, the light transmitting substrate typically has a refractive index in a range of about 1.4 to about 1.8. For example, the planar substrates could consist essentially of polycarbonate, acrylic, or glass. When the substrate is printed, the similar index of refraction between the material ejected onto the planar surface of the substrate and the substrate enables light propagating down the substrate to enter the material despite the shallow angle of incidence relative to the interface of the substrate and the material. The light inside the material has a steep angle of incidence to the internal surfaces of the material on the substrate, which enables the light to exit into the ambient air. Other portions of the light experience multiple internal reflections before eventually exiting the material. The escaping light provides a visual indication of the position of the material on the planar surface of the light transmitting substrate because the light escaping from the material piles contrasts well with the uncovered surface where the light does not escape. When the sensor  304  is passed over the substrate  308 , the sensor  304  generates electrical signals that form image data of the test pattern on the substrate  308 . 
     A method of operating a printer that produces three-dimensional objects is shown in  FIG. 4 . In the description of this method, statements that a process is performing some task or function refers to a controller or general purpose processor executing programmed instructions stored in a memory operatively connected to the controller or processor to manipulate data or to operate one or more components in the printer to perform the task or function. The controller  324  noted above can be such a controller or processor. Alternatively, the controller  324  can be implemented with more than one processor and associated circuitry and components, each of which is configured to form one or more tasks or functions described herein. 
     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  324  detecting the printhead in the module  300 , controller  324  operates the transport  312  to move light transmitting substrate  308  opposite the printhead  86  (block  408 ). Controller  324  then generates a signal to the controller  108  to operate the inkjets in the printhead to print a test pattern on the substrate  308  (block  412 ). In one embodiment, each inkjet in the printhead is repetitively operated to form a pile of material, also called a test dot, on a portion of the substrate  308  opposite the inkjet. After the test pattern is printed, controller  108  moves the printhead  86  out of the module  300  and generates a signal for controller  324 . In response to the signal from controller  108 , controller  324  operates an actuator  316  to move the optical sensor  304  to a position opposite the test pattern on the substrate  308  (block  416 ). This movement can be accomplished by moving the optical sensor to the side of the module where the substrate  308  was printed or the actuator driving rollers  332  can be operated to move the substrate to the side where the optical sensor is located. The controller  324  then activates the light source  314  to shine light into one edge of the substrate  308  (block  418 ). In the figure, the light source injects light that extends in direction L into the edge of substrate  308  on the right edge as shown in the figure, although the light can be directed into any of the other three edges. The light source  314  can be an array of light emitting diodes (LEDs), an array of laser diodes, a cold cathode fluorescent lamp, a filament, or the like. The arrays can be one dimensional, that is, linear, or two dimensional arrays. The light produced by the light source  314  can be infrared, ultraviolet, polychromatic, or monochromatic. The light source can be separate from the substrate or attached to the substrate to enable injection of the light into the substrate. The process continues by operating an actuator to move the optical sensor  304  in the direction L over the substrate  308  to generate electrical signals that are provided to the controller  324  as image data of the planar surface of the substrate  308  (block  420 ). The areas where build material and support material have been ejected emit light as explained above. The portions of the surface that internally reflect light and those portions that emit light should correspond to the test pattern used to eject the build and support material. The image data of the planar surface are analyzed with reference to expected positions for the build and support material used to form the test pattern to identify inoperative inkjets (block  424 ) and, if inoperative inkjets are identified, a signal indicative of the defective printhead is generated for the operator of the printer (block  428 ). The operator can then take appropriate action. If the substrate was imaged at the position where it was printed, the process continues by the controller  324  operating an actuator  316  to rotate the transport  312  in reverse and return the substrate to its starting position (block  432 ). Otherwise, the substrate is already in position for cleaning. Controller  324  operates an actuator  316  to engage the substrate  308  with the cleaning member  320  and then move the cleaning member  320  in direction L to remove material from the substrate (block  436 ). The removed material is collected in the waste receptacle  328 , which is shown in the figure as being positioned at the forward end of the endless belt  330 , although positions and directions of cleaning member movement can be used. The receptacle  328  can be removed from the printer from time to time and either replaced or emptied and then re-installed. If the substrate was imaged in the position where it was printed, the controller  324  operates actuator  316  to return the optical sensor  304  to the position over the substrate  308  (block  440 ). 
     The process of  FIG. 4  in some embodiments continues with an evaluation of the substrate cleaning. In these embodiments, the controller activates a light source after the cleaner has removed material from the surface of the substrate (block  444 ), and moves the optical sensor over the substrate to generate image data of the cleaned substrate (block  448 ). These image data are compared to a predetermined threshold to identify whether the light being emitted from the substrate exceeds the threshold since light is only emitted where material is present on the surface of the substrate (block  452 ). If the threshold is being exceeded, noise is present in the image data received from the optical sensor. In response to the detection of noise, another cleaning operation can be performed (blocks  436 - 440 ) and another evaluation of the cleaning occurs (blocks  444 - 452 ). If the substrate has been cleaned more than once in the present test (block  456 ), the image data of at least a portion of the substrate is stored in memory operatively connected to the controller (block  460 ). These noise data are subtracted from image data obtained in the next testing of the printhead to enable identification of the test pattern without the interference of the noise so the controller can detect inoperable inkjets. 
     The cleaning member  320  is mounted to a support member  348  that is operatively connected to an actuator  316 . As noted above, the controller  324  operates the actuator to move the support member  348  to swipe the substrate  308  with the cleaning member  320 . This action sweeps build and support material from the substrate  308  into the waste receptacle  328  to renew the surface of substrate for another test pattern printing. The cleaning member  320  can include a supply of cleaning solvent  340  that is configured to spread cleaning solvent onto the substrate before the cleaning member sweeps the substrate. The cleaning solvent chemically interacts with the build and support material to loosen the material before the cleaning member encounters it. Additionally or alternatively, a heater  344  can be operatively connected to the controller for selectively connecting the heater to a power supply. The heater is positioned with respect to the cleaning member  320  to heat the build and support material before the cleaning member sweeps the substrate  308 . 
     While the embodiments discussed above are within a printer that forms three-dimensional objects, a light transmitting substrate and the system that detects inoperative inkjets from the light emitted by such a substrate can also be used in two dimensional document printing systems, particularly those that use clear inks. Thus, as used in this document, the word “material” refers to substances that can be used to form three dimensional objects as well as inks used in document printing. In document printing systems, a light transmitting substrate can be positioned proximate a printing zone in the printer and, from time to time, the printhead is moved opposite the substrate for the ejection of ink onto the substrate. Light is then injected into the substrate and the substrate is imaged so the image data can be analyzed to identify inoperative inkjets. Likewise, printheads ejecting clear ink onto a moving web or an imaging member, such as a drum, can be moved opposite a light transmitting substrate for printing and detection of inoperative inkets. 
     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.