Patent Application
20150015631
The present invention relates to the field of ink jet printing and, more particularly, to controlling operating parameters in a continuous ink jet printing system.
In continuous ink jet printing systems, it is necessary to control operating parameters, for example, vacuum and pressure levels, to specific target levels. These target levels change as the system is stepped through various operating states, for example, operating states associated with preparing the printhead for printing, shutting down the printhead, cleaning the printhead, or flushing the system. Accordingly, there is an ongoing need to improve control of the dynamic operating parameters included in the various operating states of ink jet printing systems.
According to one aspect of the present invention, a method of controlling a selected operating parameter in a device that operates in multiple operating states which include differing natural response characteristics is provided. At least one of the natural response characteristics in at least one of the multiple operating states is variable over time. The selected operating parameter of the device is controlled by changes in a drive parameter of the device with a servo controller that includes control parameters which are responsive to at least one of the natural response characteristics. The method includes providing sets of control parameters to the servo controller with the sets corresponding to the multiple operating states of the device. One of the sets of control parameters corresponding to a given operating state of the device is selected to be used by the servo controller to initiate control of the selected operating parameter in the given operating state. A response of the selected operating parameter in the given operating state of the device is measured while the drive parameter of the device is being controlled by the servo controller using the selected set of control parameters. The selected set of control parameters used by the servo controller is modified based on the response. The selected operating parameter of the device in the given operating state continues to be controlled with the servo controller using the modified set of control parameters.
In the detailed description of the example embodiments of the invention presented below, reference is made to the accompanying drawings, in which:
Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.”Additionally, directional terms such as “on”, “over”, “top”, “bottom”, “left”, “right” are used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration only and is in no way limiting.
The present description will be directed in particular to elements forming part of, or cooperating more directly with, an apparatus in accordance with the present invention. It is to be understood that elements not specifically shown, labeled, or described can take various forms well known to those skilled in the art. In the following description and drawings, identical reference numerals have been used, where possible, to designate identical elements. It is to be understood that elements and components can be referred to in singular or plural form, as appropriate, without limiting the scope of the invention.
The example embodiments of the present invention are illustrated schematically and not to scale for the sake of clarity. One of ordinary skill in the art will be able to readily determine the specific size and interconnections of the elements of the example embodiments of the present invention.
As described herein, the example embodiments of the present invention provide a printhead or printhead components typically used in inkjet printing systems. However, many other applications are emerging which use inkjet printheads to emit liquids (other than inks) that need to be finely metered and deposited with high spatial precision. Such liquids include inks, both water based and solvent based, that include one or more dyes or pigments. These liquids also include various substrate coatings and treatments, various medicinal materials, and functional materials useful for forming, for example, various circuitry components or structural components. As such, as described herein, the terms “liquid” and “ink” refer to any material that is ejected by the printhead or printhead components described below.
Inkjet printing is commonly used for printing on paper. However, there are numerous other materials in which inkjet is appropriate. For example, vinyl sheets, plastic sheets, textiles, paperboard, and corrugated cardboard can comprise the print media. Additionally, although the term inkjet is often used to describe the printing process, the term jetting is also appropriate wherever ink or other liquids is applied in a consistent, metered fashion, particularly if the desired result is a thin layer or coating.
In continuous ink jet printing systems, ink is supplied under pressure by a fluid system 100 to the printhead 112, as shown in
If vacuum is too high, uncharged drops, which should end up being printed, are instead sucked into the catcher. Large amounts of foam can also be generated in the ink reservoir. If vacuum is too low, the system cannot remove ink quickly enough from the catcher face, and ink drips down onto the print media, degrading the quality of the printed product. Similarly, if the ink pressure is too high or too low, drop formation and drop deflection can be seriously affected, again resulting in degradation of the final product.
To maintain the proper vacuum and ink pressure levels, ink jet printers typically incorporate a controller system. The controller system includes sensors to determine the values of the parameters to be controlled, for example, pressure and vacuum; a controller including control electronics, and the ability to adjust a drive parameter(s), for example, the voltage supplied to a pump. The parameter to be controlled to a target value by the control system by adjusting a drive parameter is called a controlled parameter. One common form of control electronics includes a Proportional-Integrate-Differentiate (PID) control system. PID controllers are used as they can provide reduced fluctuation of the control parameters, fast settling times, and lack of oscillation.
When such PID controllers are used, the stability of the controller, the response rate, and the amount of overshoot depend on the control parameters such as multiplier values, used for each stage of the controller. Note as used herein, control parameters are distinct from the controlled parameter and the target value of the controlled parameter. The control parameters serve to tune the control system to account for natural system response of the controlled parameter to changes in the drive parameter, which is also called a driven parameter. The control parameters used in the PID controller are ideally selected based on the natural response rates of the system to be controlled. Using control parameter values that are sufficiently to one side of the ideal values can produce instability, that is, the value of the controlled parameter can go into oscillation. Control parameters values that are sufficiently to the other side of the ideal values can result in slow settling times and larger than ideal fluctuations of the controlled parameter. If the system gain and response rates change for different conditions, the control parameters used by the PID controller may no longer be optimal. If the changes are small, there may be a minor effect on the controller settling rate or overshoot. If the values change more significantly, the controller may become unstable, and could oscillate, or the settling rates or overshoot may become unacceptable.
Other control systems such as deadbeat control systems also have set of control parameters, whose values affect the stability of the controlled parameter as well as the rate at which the controlled parameter approaches or settles at a target value. The invention is appropriate for use with any such control systems that have sets of one or more control parameters that govern the response characteristic of the control system.
Referring back to
Under the control of the system controller, individual ink pumps 110 withdraw ink from the ink tank 104 and supply the ink to the individual print heads 112. Each pump 110 is typically driven by a variable speed brushless VDC motor which allows the pressure or flow rate of liquid supplied to each print head by varying the voltage applied to the pump. The pressure of the liquid supplied to the printhead 112 is measured by pressure sensor 114. While in print mode, the outlet valve 116 is closed enabling the liquid pressure in the droplet generator to rise to a level sufficient for liquid to continuously jet from the nozzles of the droplet generator. During startup and shutdown sequences of the printhead, it is desirable to flush fluid through the droplet generator of the printhead 112 to flush debris from the droplet generator. This crossflush function is enabled by opening the outlet valve 116 while liquid is pumped to the printhead and vacuum is maintained on the ink tank facilitate the return of the liquid from the printhead to the ink tank 104.
In a typical ink jet printer, the vacuum level provided by the vacuum pump 132 is controlled by adjusting the pump speed or, for control purposes, pump voltage. Alternatively, the vacuum level can be controlled by adjusting one or more air bleeds 144 through which air is bred into the vacuum system, as is described in U.S. Pat. No. 5,394,177. As the controller steps through the various sequences of operating states, the servo controller 30 portion of the system controller 20 measures the vacuum level by means of vacuum sensor 120 and adjusts the vacuum level to bring it to the target level associated with each operating state.
A concentration control sensor 124 monitors the ink concentration. Ink is circulated through the concentration sensor from the ink tank by a small separate fluid pump 126. In this way, the flow through the sensor is independent of the flow to either of the print heads. The concentration control system is configured such that when the fluid system 100 fills with fresh ink, ink passes through a valve 128 at the inlet of the concentration sensor and passes through the sensor. In this way, the sensor can be calibrated against fresh ink. The fluid system control electronics monitors the output of this sensor and the output of the ink tank level sensors as it controls the addition of ink to the ink tank from the ink supply 138 via ink refill valve 128 or of replenishment fluid to the ink tank 104 from replenishment supply 20 via replenishment valve 129.
Checking the concentration of makeup ink with the concentration sensor as the ink is added to the fluid system can also provide a failsafe test to prevent the wrong type or color of ink from being added to the fluid system.
A positive air pump 130 supplies clean air into the fluid lines. The positive air pump in fluid system 100 provides clean air through air valves 108 to each droplet generator during shutdown to help remove ink from the print heads of both print heads. The function of this air pump is described in more detail in U.S. Pat. No. 6,273,013.
As mentioned, the printing system has various operation sequences made up of a series or sequence of operating states. These sequences facilitate the startup, shutdown and cleaning of the printhead and fluid system and potentially over functions. Each of the operating states has associated with them the open/closed configuration for each of the valves as well as target values for the controlled parameters such as supplied liquid flow and system vacuum, and also the duration of the operating state.
The natural response of the controlled parameters to changes in a drive parameter in such a fluid system depends on the fluid system configuration as defined by the configuration parameters. To understand this, the natural response of the vacuum level in the reservoir to changes in the vacuum pump drive level is considered. The vacuum in the reservoir depends not only on the voltage applied to the vacuum pump, but also the amount of air allowed to enter the vacuum system through various routes, such as the catcher and catch pan lines via catcher valve 134 and catch pan valve 136, respectively, and other possible air bleeds 144. As more air bleed routes are closed through the closing of the appropriate valve, a small change in vacuum pump voltage will make a bigger change in the vacuum level. That is, the gain of the vacuum system response increases as air bleed valves are closed. As the amount of damping of the vacuum response is affected by the flexibility of the various fluid lines, opening or closing the various air bleed valves, which effectively can alter the amount of fluid lines present that can affect damping. In a similar manner, the natural response of the ink pressure depends on whether the outlet valve 116 from the printhead is open or closed.
The effect of these changes in the natural system response produced by the different valve or bleed conditions is illustrated in
In addition to being sensitive to the status of various valves which differ from one fluid system state to another, the natural system response can also be sensitive to the component makeup of the fluid system. For example, different fluid pumps 110 may have different drive voltage to output flow characteristics, which can affect the natural system response. Similarly, the flow impedance of the ink filters 118 can also affect the natural system response. The flow impedance of the ink filters 118 can vary with the particle loading of the filter; as more and more particles are captured by the filter the flow impedance increases. The flow impedance of the filters and other components also depends on the viscosity of the ink. As a result of such component related differences, the natural system response can vary from printing system to printing system, and also over time for a given printing system.
To provide the desired control of the controlled parameters (i.e. pressure and vacuum level), the invention uses a servo control system that automatically determines the natural system response of the system for the different fluid system states and then it determines appropriate control parameters to use with the individual fluid system states based on the determined natural system response for the individual fluid system states. In one embodiment, the determination of the natural system response by the servo control system involves the servo control system 30 imposing a step 200 of a defined step size amount 202 in the value of the drive parameter 204, such as pump voltage, while the fluid system is in a defined fluid system state, as shown in
The change in the controlled parameter value is measured using the appropriate sensor, such as vacuum sensor 120. The servo control can analyze the measured response of the controlled parameter to the imposed step of the drive parameter to determine the natural system response in the defined fluid system state. Based on the natural system response, determined in this manner, the servo control system can identify control parameter values to be used when the fluid system is in this defined fluid system state. The identified fluid system parameters for the defined fluid system state are stored in memory for future use by the servo controller when the fluid system is in this defined fluid system state.
In a similar manner, the servo controller can determine the control parameters to use for each of the fluid system states in the set of fluid system states. The identified control parameters for each of the fluid system states can be stored in memory as elements in a table linking the identified control parameters with the corresponding fluid system states. It is common for the sequences of fluid system states to have multiple states which have the same set of configuration parameters, for example a start up sequence might have a series of states alternately open and close the same valve, with no other changes in configuration. As one would expect the multiple states which have the same set of configuration parameters to all have the same response characteristics, some embodiments link the sets of control parameters so that control parameters do not need to be determined for each instance of a state that shares a set of configuration parameters with other states.
The set of control parameters determined for the given fluid system state may then replace the previous sets of control parameter values in the state tables, so that the newly determined set of values of the control parameters are subsequently used in place of the previous sets of values of the control parameters.
In one example embodiment, the newly determined set of control parameter values are compared with the previous set of control parameter values to determine whether the difference is statistically significant. In some embodiments, the analysis to determine whether the currently determined set of control parameter values are statistically different from the previous sets of control parameter values can involve comparisons of the current set of values to a sequence of previous sets of values of the control parameters, using the tools of statistical process control such as the analysis of Shewhart charts, to determine whether the newly determined control parameter values are indicative of some drift or of an outlier in the control parameters. Depending on the results of such an analysis, the value of the control parameters stored in the state table may be left unchanged from the previous control parameter values, it may be replaced by the newly determined values, or the stored value may be replaced by values derived from a combination of previous values and the newly determined value such as through a moving average calculation or through a regression type analysis. In some embodiments, the individual control parameter values that make up the set of control parameter values are individually compared to the corresponding individual control parameter value in one or more previous sets of control parameter values. In other embodiments, a more complex comparison is made between the newly determined set of control parameters and one or more previously determined sets of control parameters that analyzes the set of control parameters as a combined entity to one or more previously determined sets of control parameters. In some embodiments, the operator may be able to prompt the controller to replace the stores sets of control parameters with the newly determined control parameters, following a maintenance operation in which some portion of the fluid system has been replaced or altered such as a pump filter or fluid line.
The results of comparing the set of newly determined control parameter values to one or more sets of previously determined control parameters are used as a diagnostic tool of the fluid system. Through the use of statistical process control tools, determinations can be made to identify system drifts or some other indication that set of control parameters is outside the normal bounds for such control parameters. By analysis of changes in sets of control parameter values for different ones of the fluid system states, the controller may be able to determine not only that something has changed but it may be able to pinpoint the cause for the changes. For example, based on the results of such diagnostics, the controller may be able to determine when operator intervention is needed. The intervention can include but is not limited to replacement of one of the fluid system filters, valves or pumps when the diagnostics indicate an impending failure. The controller may then alert the operator concerning the corrective action to take.
In another example embodiment, the measurements of the response characteristics, which are used to determine the set of control parameter values for a given state, are compared to previous values of those response characteristic measurements as a diagnostic tool of the fluid system rather than comparisons of the determined control parameters. By analysis of changes in response characteristic measurements for different ones of the fluid system states, the controller may be able to determine not only that something has changed but it may be able to pinpoint the cause for the changes. Using such diagnostics, the controller, for example, can determine when various filters should be replaced, or whether a particular valve or pump is showing signs of impending failure. The controller may then alert the operator concerning the corrective action to take.
Referring to
The control parameters are normally determined or confirmed using the simple step in the driven parameter value described above. However, if the diagnostic analysis of the response characteristic measurements or of the determined control parameters indicates a significant change has occurred in these values, a follow up set of diagnostic tests may be initiated. One such follow up diagnostic test can include the use of a complex drive parameter function and the analysis of the response of the controlled parameter to this complex drive parameter function in one or more of the fluid systems states. The use of the more complex drive parameter drive function, for example, the one shown in
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.
