Patent Publication Number: US-2016221260-A1

Title: Automatic ejector head drop mass adjustment in a three-dimensional object printer

Description:
TECHNICAL FIELD 
     The device disclosed in this document relates to printers that produce three-dimensional objects and, more particularly, to the accurate production of objects with such printers. 
     BACKGROUND 
     Digital three-dimensional manufacturing, also known as digital additive manufacturing, 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 or ejector heads eject successive layers of material on a substrate in different shapes. The substrate is supported either on a platform that can be moved three dimensionally by operation of actuators operatively connected to the platform, or the printhead or printheads are operatively connected to one or more actuators for controlled movement of the printhead or printheads to produce the layers that form the object. 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 ejectors in the printheads that eject the material drops that form the objects. During printing of an object, one or more ejectors can eject material with a drop volume that is slightly different from the drop volume of the ejectors surrounding the ejector. These volumetric differences can accumulate during the printing of the multiple layers that form an object so the column of material formed by the ejector ejecting the smaller or larger drops can be shorter or taller, respectively, than the surrounding material columns formed by the other ejectors. These surface variations can be significant enough to require the scrapping of the object. Because the print jobs can require many hours or multiple days to produce objects, this scrapping of objects can be expensive and time consuming. A three-dimensional object printer capable of compensating for the volumetric variations in material drops ejected by ejectors in such printers would be advantageous. 
     SUMMARY 
     A three-dimensional object printer that detects volumetric drop variations in the ejectors during printing and adjusts the firing signal parameters used to operate the ejector heads to compensate for these variations has been developed. The three-dimensional object printer includes a track; a at least one ejector head disposed along the track, the at least one ejector head being configured to eject drops of a material onto a substrate; a platform configured to move along the track to convey the substrate to a position to enable the at least one ejector head to eject drops of the material onto the substrate; a scale configured to measure a weight of at least one of the platform and the substrate; and a controller operatively connected to the scale and the at least one ejector head, the controller being configured to: operate the scale to identify a first weight measurement of at least one of the platform and the substrate; operate the at least one ejector head with first firing signal parameters to eject material onto the at least one of the platform and the substrate; operate the scale to identify a second weight measurement of the least one of the platform and the substrate; calculate a first drop mass of at least one ejector in the at least one ejector head based on a difference between the first weight measurement and the second weight measurement; calculate a relationship between a drop mass of the at least one ejector and firing signal parameters for the at least one ejector based on the calculated first drop mass; and adjust the firing signal parameters for the at least one ejector based on the calculated relationship between the drop mass of the at least one ejector and the firing signal parameters for the at least one ejector to compensate for variations in drop volumes between the at least one ejector and other ejectors in the at least one ejector head. 
     A method has been developed for operating a three-dimensional object printer that detects volumetric drop variations in the ejectors during printing and adjusts the firing signal parameters used to operate the printheads in the printer to compensate for these variations. The method includes operating a scale to identify a first weight measurement of at least one of a platform and a substrate; operating an at least one ejector head with first firing signal parameters to eject material onto the at least one of the platform and the substrate; operating the scale to identify a second weight measurement of the least one of the platform and the substrate; calculating a first drop mass of at least one ejector in the at least one ejector head based on a difference between the first weight measurement and the second weight measurement; calculating a relationship between a drop mass of the at least one ejector and firing signal parameters for the at least one ejector based on the calculated first drop mass; and adjusting the firing signal parameters for the at least one ejector based the calculated relationship between the drop mass of the at least one ejector and the firing signal parameters for the at least one ejector to compensate for variations in drop volumes between the at least one ejector and other ejectors in the at least one ejector head. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of an apparatus and method that detects volumetric drop variations in the ejectors during printing and adjusts firing signal parameters to compensate for these variations are explained in the following description, taken in connection with the accompanying drawings. 
         FIG. 1  shows a printing system having an in-line scale and configured to automatically adjust firing signal parameters to compensate for variations in drop volume in ejectors during printing. 
         FIG. 2  shows a side view of a printing system having a scale that is not in-line. 
         FIG. 3  shows a calculated relationship between drop mass and a voltage parameter for two printheads. 
         FIG. 4  depicts method for compensating for variations in drop volume between ejectors. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the device and method disclosed herein as well as the details for the apparatus and method, reference is made to the drawings. In the drawings, like reference numerals designate like elements 
     As used herein, the terms “electrical firing signal,” “firing signal,” and “electrical signal” are used interchangeably to refer to an electrical energy waveform that triggers an actuator in an ejector to eject a material drop. Examples of actuators in ejectors include, but are not limited to, piezoelectric, and electrostatic actuators. A piezoelectric actuator includes a piezoelectric transducer that changes shape when the firing signal is applied to the transducer. The transducer is proximate to a pressure chamber that holds material, and the change in shape of the transducer urges some of the material in the pressure chamber through an outlet nozzle in the form of a material drop that is ejected from the ejector. In some embodiments, rather than being pressurized, the pressure chamber is designed to fill through capillary action and to hold the material by vacuum. In an electrostatic actuator, the material includes electrically charged particles. The electrical firing signal generates an electrostatic charge on an actuator with the same polarity as the electrostatic charge in the material to repel material from the actuator and eject a material drop from the ejector. 
     As used herein, the term “peak voltage level” refers to a maximum amplitude level of an electrical firing signal. As described in more detail below, some firing signals include a waveform with both positive and negative peak voltage levels. The positive peak voltage level and negative peak voltage level in a firing signal waveform may have the same amplitude or different amplitudes. In some ejector embodiments, the peak voltage level of the firing signal affects the mass and velocity of the material drop that is ejected from the ejector in response to the firing signal. For example, higher peak voltage levels for the firing signal increase the mass and velocity of the material drop that is ejected from the ejector, while lower peak voltage levels decrease the mass and velocity of the ejected material drop. Since the object receiving surface moves in a process direction relative to the ejector at a substantially constant rate and typically remains at a fixed distance from the ejector, changes in the velocity of the ejected material drops affect the relative locations of where the material drops land on the object receiving surface in the process direction. 
     As used herein, the term “peak voltage duration” refers to a time duration of the peak voltage level during a firing signal. The peak voltage duration can refer to the duration of both a positive peak voltage level and negative peak voltage level in a signal. Different electrical firing signal waveforms include positive peak voltage durations and negative peak voltage durations that are either equally long or of different durations. In one embodiment, an increase in the duration of the peak voltage level in the firing signal increases the ejection velocity of the material drop while a decrease in the duration of the peak voltage level decreases the ejection velocity of the material drop. These velocity changes reduce the variation in the material drop velocities ejected by the printhead. When the material drop velocity variation is reduced, the accuracy of the material drop placement is increased. 
     As used herein, the term “waveform component” refers to any parameter in the shape or magnitude of an electrical firing signal waveform that is adjusted to affect the velocity of a material drop that is ejected from an ejector in response to the generation of the waveform with the adjusted component parameter. The peak voltage level and the peak voltage duration are examples of waveform components in electrical firing signals. As described below, an ejector printer adjusts one or more waveform components including either or both of the peak voltage level and peak voltage duration to adjust the ejection velocities of material drops on a drop-by-drop basis during an imaging operation. Since different drop ejection patterns result in variations of the material drop velocity due to the characteristics of the ejector and printhead, the adjustments to the waveform components enable more accurate placement of material drop patterns on the object receiving surface during the imaging operation. In some embodiments, the waveform has smaller non-firing pulses that also affect mass and velocity of a drop by changing a resonance. 
     The term “firing signal parameter adjustment,” as used in this document, refers to a change in a waveform component, such as, one of a peak voltage level parameter, a peak voltage duration parameter, or a frequency parameter for the firing signal. The change can be a relative increase or decrease in a peak voltage level defined for a firing signal, a relative increase or decrease in the duration of the peak voltage for the firing signal, or a relative increase or decrease in the frequency of the firing signal. Additionally, a combination of changes of two or all three parameters can be made. The firing signal parameter adjustment normalizes the material drop volumes ejected by the ejectors in the printheads so that the effective material drop volume is approximately the same for all material drops ejected by the printheads. 
     A three-dimensional object printing system  100  is shown in  FIG. 1 . A platform  104 , called a cart, includes wheels  108  that ride upon a track  112  to enable the cart  104  to move along the track between printing stations, such as the printing station  116 . The printing station  116  includes four ejector heads  120  as shown in the figure, although fewer or more ejector heads can be used in a printing station. Once the cart  104  reaches the printing station  116 , the cart  104  transitions to enable wheels  108  to roll upon precision rails  124 . Precision rails  124  are cylindrical rail sections that are manufactured within tight tolerances to help ensure accurate placement and maneuvering of the cart  104  beneath the ejector heads  120 . Linear electrical motors are provided within housing  128  and are operatively connected to the wheels  108  of cart  104  to move the cart along the track rails  112  and to the wheels  108  to maneuver the cart  104  on the precision rails  124 . Once the cart  104  is beneath the printing station  116 , ejection of material occurs in synchronization with the motion of the cart. The electrical motors in housing  128  are also configured to move the cart in an X-Y plane that is parallel to the ejector heads  120  as layers of material are formed in the object. Additional motors move the printing station  116  vertically with respect to the cart  104  as layers of material accumulate to form an object. Alternatively, a mechanism can be provided to move the cart  104  vertically with respect to rails  124  as the object is formed on the top surface of the cart. Once the printing to be performed by a printing station is finished, the cart  104  is moved to another printing station for further part formation or for layer curing or other processing. 
     The printing system  100  further includes a controller  132  that is operably connected to the printing station  116  to operate the ejector heads  120 . The controller  132  is also operably connected to a scale which is configured to measure the weight of the cart  104  or the substrate. In one embodiment, the scale  136  is built as an in-line component of the track  112 , as show in  FIG. 1 . The scale  136  is configured to measure the weight of the cart  104  as it rests on the track  112 . In one embodiment, the scale  136  is configured directly beneath the printing station  116 . 
       FIG. 2  shows a printing system  200 , which is similar to the printing system  100 , wherein a scale  236  is separate from the track  112  and configured to receive the substrate from the cart  104  and to measure the weight of the substrate directly. In this embodiment, the scale  236  includes a robotic arm  204  that enables the scale  236  to receive the substrate from the cart  104 . The controller  132  controls the robotic arm  204  to grasp the substrate and to move it onto a platform  208  of the scale  236 . The controller  132  controls the scale  236  to measure the weight the substrate, and then controls the robotic arm  204  to return the substrate to the cart  104 . Other embodiments use other methods to move the substrate to and from the scale  236 . 
     As noted previously, one source of error in three-dimensional object printing arises from variations in the volumes of material drops from ejector to ejector. The printing system  100  is configured to detect material drop variations between ejectors and to compensate for these variations by adjusting firing signal parameters of the ejectors. The controller  132  is configured to operate the scale  136  to take weight measurements of the cart  104  before and after the printing of a test pattern to calculate drop mass values of drops ejected by the ejectors. The controller  132  operates the scale  136  to take a first weight measurement of the cart  104 , then operates the ejectors of the ejector heads  120  to eject a test pattern onto the cart  104  using a set of firing signal parameters, and then operates the scale  136  to take a second weight measurement. The controller  132  calculates a drop mass value for the ejector based on a difference between the first and second weight measurements. 
     The test pattern comprises a fixed number of material drops jetted onto the cart  104  using a specified pattern. In one embodiment, the test pattern comprises just one drop of material from one ejector. In this embodiment, the controller  132  determines the mass of the drop of material directly based on the difference between the first and second weight measurements. In another embodiment, the test pattern comprises several drops of material ejected from one ejector. In this embodiment, the controller  132  determines an average drop volume based on the difference between the first and second weight measurements and the number of drops ejected from the ejector. This test pattern provides increased accuracy for calibrating ejectors that produce minor variations in drop volumes from one drop to the next. In yet another embodiment, the test pattern comprises several drops ejected by several ejectors. In this embodiment, the controller  132  determines an average drop volume for the several ejectors based on the difference between the first and second weight measurements and the number of drops ejected from the ejectors. Test patterns including multiple ejectors can be used when the ejectors are expected to degrade similarly, such as ejectors of the same type or ejectors in the same ejector head. 
     In one embodiment, the controller  132  is configured to use many test patterns that are selectable by an operator of the printing system  100 . For example, certain test patterns can be used for a more thorough high-accuracy calibration, and other test patterns can be used for a less time consuming quick calibration. In other embodiments, the controller  132  uses different test patterns based on the particular firing signal parameter being calibrated, the type of ejectors being calibrated, or the type material in the ejectors being calibrated. In some embodiments, velocity and drop mass are variable to enable an operator to calibrate for a particular job. 
     In this way, the controller  132  collects one or more drop mass values using different firing signal parameters for the ejectors. After collecting drop mass values using various firing signal parameters, the controller  132  calculates an approximate relationship between one or more of the firing signal parameters and the material drop mass for an ejector. For example, the controller  132  may calculate the relationship between a peak voltage parameter of the firing signal for a particular ejector and the drop mass ejected by the particular ejector.  FIG. 3  shows a relationship between a change in voltage and a corresponding change in drop mass ejected by two printheads, X and Y. The controller  132  calculates three drop mass values  404  for printhead X, and three drop mass values  408  for printhead Y. Next, the controller  132  calculates the relationship  412  between a voltage offset from a nominal voltage for the printhead X and a change in drop mass for the printhead X. Similarly, the controller  132  calculates the relationship  316  between a voltage offset from a nominal or prior voltage for the printhead Y and a change in drop mass for the printhead Y. In other embodiments, the controller  132  is configured to determine relationships between drop mass and other firing signal parameters, such as a duration of a peak voltage parameter, a frequency parameter, and other waveform components. 
     In the example of  FIG. 3 , the controller  132  is configured to calculate the curve representing the relationship by linearly connecting each of points for the drop mass values. However, in other embodiments, the controller  132  is configured to calculate the curve using other curve-fitting methods such as linear and nonlinear regression analysis. Collecting several drop mass values is more time consuming, but enables increased accuracy in approximating the curve. However, in some embodiments, the controller  132  is configured to collect only one drop mass value for each ejector. In these embodiments, the controller  132  calculates the curve by assuming a general shape of the curve based on known characteristics of ejector degradation and performance. For example, in one embodiment, the controller  132  assumes that the curve is linear and has a particular slope that is characteristic of the type of ejector. In this way, the controller  132  uses the calculated drop mass value to determine how much the assumed curve has drifted up or down. In some embodiments, the controller  132  is configured to collect a number of drop mass values based on a selection by an operator of printing system  100 . In other embodiments, the controller  132  is configured to collect a number of drop mass values based on the particular firing signal parameter being calibrated, the type of ejectors being calibrated, or the type material in the ejectors being calibrated. 
     After the controller  132  approximates one or more curves representing relationships between drop mass and various firing signal parameters for one or more ejectors of the ejector heads  120 , the controller  132  is configured to adjust the nominal firing signal parameters for the ejectors so that each of the ejectors ejects drops of material having about the same volume or mass. To accomplish this, the controller  132  uses the calculated curves to interpolate values for the firing signal parameters of each ejector such that the ejectors eject drops of material having an ideal or target volume. In the example of  FIG. 3 , the controller  132  calculates that the voltage for printhead X should be decreased by 0.3 volts and that the voltage for printhead Y should be increased by 0.7 volts. In this way, the controller  132  calibrates each of the ejectors to compensate for volumetric drop variations in the ejectors during printing. 
     A method  400  for operating a printing system to compensate for volumetric drop variations in ejectors during printing is shown in  FIG. 4 . In the description of the method, statements that the method is performing some task or function refers to a controller or general purpose processor executing programmed instructions stored in non-transitory computer readable storage media 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  132  noted above can be such a controller or processor. Alternatively, the controller 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. 
     When the method  400  is performed, it begins by identifying a first weight measurement of a platform or substrate (block  404 ). The controller  132  operates the scale  136  to identify a first weight measurement of the cart  104 . Alternatively, in the case of a scale that is not built into the track  112 , the controller  132  operates the scale to receive the substrate from the cart and to identify a first weight measurement of the substrate. In one embodiment, the controller  132  is configured to tare the scale with the weight of the cart  104  or the substrate. Next, the method  400  ejects material onto the platform or the substrate with first firing signals (block  408 ). The controller  132  operates the ejectors of the ejector heads  120  to eject drops of material corresponding to a test pattern onto the cart  104  or the substrate. Next, the method  400  identifies a second weight measurement of the cart  104  or the substrate (block  412 ). The controller operates the scale  136  to identify a second weight measurement of the cart  104  or the substrate. Next, the method  400  calculates a first drop mass for at least one ejector (block  416 ). The controller  132  calculates a first drop mass for at least one ejector based on a difference between the identified first and second weight measurements. In the case of a test pattern having several drops ejected from the at least one ejector, the controller  132  calculates an average first drop mass based on the difference between the first and second weight measurements and the number of drops ejected by the at least one ejector. The portion of the method depicted in blocks  404 ,  408 ,  412 , and  416  are optionally repeated to collect several drop mass values using different firing signal parameters. 
     Next, the method  400  calculates a relationship between a drop mass and firing signal parameters for the at least one ejector (block  420 ). The controller  132  calculates a relationship between the drop mass and a firing signal parameter for at least one ejector based on the collected drop mass values, at least including the first drop mass. The controller  132  is configured to approximate a curve that fits the collected drop mass values using a curve fitting method. Next, the method  400  adjusts the firing signal parameters for the at least one ejector based on the relationship between the drop mass and the firing signal parameters (block  424 ). The controller uses the calculated relationship to interpolate a value for the firing signal parameter that is expected to cause the at least one ejector to eject a drop of material having an ideal or target volume. The controller  132  sets this value as the new nominal firing signal parameter for the at least one ejector, thereby compensating for variations in the drop volume of the at least one ejector compared with other ejectors of the printing station  116   
     In one embodiment, the controller  132  is configured to periodically perform the method  400  automatically at predetermined times or after a predetermined number of printing operations. In other embodiments, the controller  132  is configured to perform the method  400  at the command of an operator of the printing system  100 . In some embodiments, the cart  104  must be cleaned or otherwise prepared to accept a large volume of uncured material before performing the method  400 . Additionally, in some embodiments, the height of the ejector heads  120  of the printing station  116  must be appropriately adjusted before performing the method  400 . In one embodiment, the controller  132  is configured to automatically perform this preliminary setup. 
     The method  400  for calibrating ejectors to compensate for drop variations between ejectors can be augmented by performing small automatic firing signal adjustments between executions of the method  400 . For example, small adjustments to a peak voltage parameter can automatically be made based on an expected degradation curve, or “drift curve.” These small adjustments can help to ensure continued robust performance between calibrations. 
     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 therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.