Patent Publication Number: US-2012044292-A1

Title: Vacuum Control For Print Head of A Printing System

Description:
BACKGROUND 
     This disclosure relates to printing systems, and in particular, to ink jet printing systems. 
     Ink jet printing systems include a print head having small orifices through which ink is ejected in a controlled manner to form an image on an adjacent substrate. To counteract the effect of capillary action in the small orifices which would otherwise cause ink to seep out of the print head when not in use but, at the same time, prevent air from being drawn into the print head through the orifices, the ink in the print head must be maintained at a selected negative pressure which is dependent upon the orifice size and the ink characteristics. In ink jet printing systems having a remote ink supply connected to the print head through a supply line, however, the pressure of the ink in the print head can be affected by the relative vertical positions of the print head and the remote ink supply. Moreover, some ink jet printing systems are designed to operate with multiple available orientations of the print head, which can also affect the pressure of the ink in the print head. 
     SUMMARY 
     This disclosure describes systems and techniques to set and maintain desired pressure values in print heads of ink jet printing systems. According to an aspect of the disclosed subject matter, a controller to maintain negative pressure in a print head of a printing system includes: control loop feedback logic to receive a set point and an output of a vacuum sensor associated with the print head; a regulator coupled with an output of the feedback logic; and a driver coupled with the regulator and configured to output a drive signal to a pump, which is associated with the print head, responsive to an output of the regulator. The regulator can include a voltage regulator, and the control loop feedback logic can include proportional-integral-derivative (PID) circuitry. The driver can include: a programmable logic device (PLD) to generate a pulse width modulated (PWM) signal; and integrated circuitry coupled with the PLD to modulate the PWM signal, responsive to a motor drive voltage output of the voltage regulator, to generate the drive signal. 
     The PID circuitry can include a closed loop circuit including exactly six operational amplifiers. The PLD can be configured to generate a sixty hertz square wave pulse. In addition, the controller can include a processor programmed to establish the set point by performing operations including: ramping, at initialization, the set point to a value selected to maintain a negative pressure at a desired level; maintaining, at run time, the set point at a constant value; replacing, during a purge cycle, the negative pressure with a positive pressure using a set point change; and ramping, after the purge cycle, the set point back to the value selected to maintain the negative pressure at the desired level. 
     According to another aspect of the disclosed subject matter, a vacuum control assembly for a printing system includes: a body having one or more associated accumulators; a pump coupled with the body; and one or more flexible tubes coupled with the one or more accumulators associated with the body and configured to restrict air flow within the vacuum control assembly. The one or more flexible tubes can include Polyvinyl Chloride (PVC) microbore tubing manufactured for medical and laboratory environments. The one or more flexible tubes can include multiple, fixed diameter tubes having tube lengths selected to produce target amounts of air flow restriction. In addition, the body can include a machined plate structure that forms the one or more associated accumulators therein. 
     According to another aspect of the disclosed subject matter, a hot melt ink jet printing system includes: a jetting assembly having at least one ink reservoir; a vacuum sensor associated with the at least one ink reservoir of the jetting assembly; a vacuum control assembly coupled with the at least one ink reservoir of the jetting assembly, the vacuum control assembly including a pump; and a controller coupled with the vacuum control assembly to maintain a negative pressure in the at least one ink reservoir of the jetting assembly; wherein the controller includes: control loop feedback logic to receive a set point and an output of the vacuum sensor, a regulator coupled with an output of the feedback logic, and a driver coupled with the regulator and configured to output a drive signal to the pump responsive to an output of the regulator. 
     The regulator can include a voltage regulator, and the control loop feedback logic can include proportional-integral-derivative (PID) circuitry. The driver can include: a programmable logic device (PLD) to generate a pulse width modulated (PWM) signal; and integrated circuitry coupled with the PLD to modulate the PWM signal, responsive to a motor drive voltage output of the voltage regulator, to generate the drive signal. The PID circuitry can include a closed loop circuit including exactly six operational amplifiers, the PLD can be configured to generate a sixty hertz square wave pulse, and the controller can include means for establishing the set point by performing a ramping process. 
     The vacuum control assembly can include: a body coupled with the pump and having one or more associated accumulators; and one or more flexible tubes coupled with the one or more accumulators associated with the body and configured to restrict air flow within the vacuum control assembly. The one or more flexible tubes can include Polyvinyl Chloride (PVC) microbore tubing manufactured for medical and laboratory environments. The one or more flexible tubes can include multiple, fixed diameter tubes having tube lengths selected to produce target amounts of air flow restriction. Moreover, the body can include a machined plate structure that forms the one or more associated accumulators therein. 
     The systems and techniques described herein may provide several advantages. A low vacuum negative pressure can be maintained in the print head at a lower cost than solutions employed currently, while improving yield and performance in the field. The accuracy of the vacuum pressure can be improved, making it less susceptible to changes when filling the print head reservoir. Using proportional-integral-derivative (PID) logic implemented using analog componentry can provide 100% linearity in the control signal for the vacuum pressure, which can result in very high accuracy of control. Moreover, variations from a defined set point can be minimized, which can reduce air ingestion issues, since overshooting the set point can cause air ingestion in the print head. Minimizing air ingestion can also minimize jet instability. 
     Another attribute of the voltage control scheme described herein is that a greater vacuum range can be achieved, allowing for improved operation in high altitude applications by pulling lower vacuums than previously done. Using the PID circuit design described herein also allows for more accurate detection of high vacuum line irregularities and consequently reduced fluctuation in low vacuum levels. The low vacuum level typically fluctuates by only 5-7% in implementations of the present invention, allowing the system to differentiate between a low vacuum leak and high vacuum leak. With previous system, low vacuum could fluctuate as much as 30% during an extreme high vacuum leak, which could result in inaccurate faults being generated by software. Moreover, using the tubing in the described implementations can facilitate adaptability to future design changes, such as changes in the pump, jetting assembly, assembly requirements, etc. 
     Details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages may be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an example of an ink jet printer system in association with a product packaging line. 
         FIG. 1B  shows an example of a print head used in the system of  FIG. 1A . 
         FIG. 1C  shows a rear view of the print head shown in  FIG. 1B  positioned vertically for horizontal ejection of ink with the orifice array oriented in a horizontal line. 
         FIG. 1D  shows a rear view of the print head shown in  FIG. 1B  positioned in a sidewise orientation for horizontal ejection of ink with the orifice array oriented in a vertical line. 
         FIG. 1E  shows a side view of the print head shown in  FIG. 1B  positioned horizontally for downward ejection of ink from the orifices. 
         FIG. 2  shows another example of an ink jet printer system. 
         FIG. 3A  shows an example of a controller that maintains negative pressure in a print head. 
         FIG. 3B  shows an example of a PID circuit that can be used in the controller of  FIG. 3A . 
         FIG. 4A  shows an example of a processor implementation of defined set point, ramp, and digital to analog converter elements from the controller of  FIG. 3A . 
         FIG. 4B  shows the output from the microprocessor of  FIG. 4A  in accordance with the defined set point. 
         FIG. 4C  shows an example of a ramping process to establish the set point. 
         FIG. 5  is a schematic diagram showing an example of a vacuum purge control assembly for a printing system. 
         FIG. 6A  is a top view showing an example of an implementation of the vacuum purge control assembly of  FIG. 5 . 
         FIG. 6B  is a bottom view showing the example of the vacuum purge control assembly from  FIG. 6A . 
         FIG. 6C  is an exploded bottom view showing the example of the vacuum purge control assembly from  FIG. 6A . 
         FIG. 6D  is an exploded top view showing the example of the vacuum purge control assembly from  FIG. 6A . 
     
    
    
     Like reference characters in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1A  shows an example of an ink jet printer system in association with a product packaging line. In this example, a main control unit  10  includes a remote ink supply reservoir  12  connected through an ink supply conduit  14  in a cable  15  to an ink jet print head  16  and a pressure control unit  18  connected to the ink jet print head  16  through three air conduits  19 ,  84  and  86 , also carried by the cable  15 . In addition, the main control unit  10  includes a temperature control unit  22  for controlling the temperature of hot melt ink in various portions of the ink jet system. 
     To facilitate positioning of the print head  16  adjacent to different types of objects to which printing is to be applied, the print head  16  can be movably supported on a vertically disposed column  24  so as to be locked by a clamp  26  at any desired vertical position on the column. In addition, the print head  16  can be supported for pivotal motion in any vertical plane by a clampable universal joint  28  so that the print head can be oriented to permit a linear array of ink jet orifices  30  therein (as shown in  FIG. 1B ) to project ink horizontally, either in a horizontal line or in a vertical line, or downwardly. 
     In the arrangement shown in  FIG. 1A , the print head  16  is disposed in a horizontal orientation (as shown in solid lines) to cause the print head orifices  30  (shown in  FIG. 1B ) to project a train of ink drops  31  downwardly onto the top surfaces  32  of a series of containers  34  which are conveyed in the horizontal direction by a conveyor  36 , thus permitting appropriate information to be printed on the top surface of each of the containers. If desired, the print head  16  can be lowered on the column  24  and the universal joint  28  can be arranged to clamp the head  16  in a sidewise orientation with the array of orifices  30  extending vertically and facing the near sides  37  of the containers  34 , as viewed in the drawing, so as to cause information to be printed on the sides of each of the containers as they are conveyed past the print head by the conveyor  36 . 
     In still another print head position, the printing can be arranged to print a series of labels  38  conveyed on a tape  40  in a vertical direction from one reel  42  to another reel  44  by adjusting the universal joint  28  to clamp the print head in a vertical orientation (as shown in dotted outline in  FIG. 1A ) so that the array of orifices  30  extends horizontally and faces the labels  38  as they are conveyed in the vertical direction. 
     The ink supply reservoir  12  in the main control unit  10  which has a sealing cover  46 ; is arranged to receive a block  48  of solid hot melt ink and has a thermostatically controlled heater  50  connected by a line  52  to the temperature control unit  22 . The temperature control unit  22  is arranged to control the heater  50  so as to heat the block of hot melt ink  48  sufficiently to melt it and to maintain the ink in the supply reservoir  12  at a temperature just above its melting point so that it is sufficiently liquid that it can be transferred by a pump  53  through the supply conduit  14  to the print head  16  as required. At the same time, the ink temperature in the supply reservoir  12  is kept low enough so that no appreciable degradation will take place even though the ink is maintained continuously at that temperature for several days or weeks. Similarly, the ink supply conduit  14  contains a thermostatically controlled heater  54  connected through a line  56  to the temperature control unit  22  so that the ink in the supply line is also maintained continuously in liquid condition, but at a temperature low enough that no appreciable degradation occurs. 
     As shown in  FIGS. 1B-1E , the print head  16  includes two ink reservoirs  58  and  60  containing ink at different levels, a passage  62  leading from the high level reservoir  58  to a deaerator  64  and another passage  66  leading from the low level reservoir to the deaerator  64 . The passages  62  and  66  pass downwardly as viewed in  FIGS. 1B and 1C  in the deaerator  64  adjacent to a membrane  68 , which separates those passages from a vacuum chamber  70  connected to the vacuum line  19  from the pressure control unit  18 . That line and the chamber  70  can be maintained at a pressure level of about 25 inchesHg to extract dissolved air from the ink passing through the passages  64  and  66  adjacent to the membrane  68 . After passing through the deaerator  64 , the ink passages  62  and  66  extend downwardly to supply alternately adjacent orifices  30  respectively in the array, ink from the low level reservoir being supplied through a passage  72  shown in  FIG. 1B  which extends downwardly adjacent to an orifice plate  74  to supply alternate odd-numbered orifices in the array, and ink from the high level reservoir being supplied downwardly to the bottom of the orifice plate  74  and upwardly adjacent to the orifice plate to the alternate even-numbered orifices  30  through a passage  73  shown in dotted line in  FIG. 1B . 
     Each of the orifices  30  in the print head  16  has an associated transducer  76  arranged to respond to electrical signals to eject ink drops through the corresponding orifice in the usual manner, as described, for example, in the Fischbeck et al. U.S. Pat. No. 4,584,590. An appropriate arrangement of the ink passages  72  and  73 , transducers  76 , orifices  30  and supply passages  62  and  66  is described in detail in the Hoisington et al. U.S. Pat. No. 4,835,554. 
     In order to maintain the ink in the orifice passages  72  and  73  at the temperature required for jetting through the orifices  30 , a heater  78  is mounted in the print head adjacent to the passages  72  and  73  and is connected through a line  79  in the cable  15  to the temperature control unit  22 . In addition, a further heater  80  is mounted adjacent to the reservoirs  58  and  60  and is connected to the control unit  22  by a line  81 . The control unit is arranged to maintain the temperature of the ink in the reservoirs  58  and  60  at a temperature sufficiently below the jetting temperature to avoid degradation, but close enough to the jetting temperature to permit the orifice passage heater  78  to heat the ink quickly to the jetting temperature as the ink is supplied through the passages  72  and  73  to the orifices  30 . 
     As an example, for a hot melt ink which has a melting point of about 90° C. and tends to degrade when maintained for substantial periods of time at temperatures above 130° C., the temperature control unit  22  can be arranged to maintain the temperature of the ink in the remote ink supply reservoir  12  and in the ink supply conduit  14  at a temperature of about 100° C. and to control the heater  80  to maintain the ink in the reservoirs  58  and  60  at a temperature of about 125° C., but to control the heater  78  so as to maintain the ink in the passages  72  and  73  leading to the orifices  30  at a jetting temperature of 137° C. Since only a small quantity of ink is maintained in the passages  72  and  73  and, during operation, the ink passes through those passages relatively rapidly, no significant degradation of ink can occur during operation of the ink jet system. 
     When the ink jet system is not in use, but is being maintained ready for use as, for example, during the course of a working day in which the system is used only periodically, the temperature control unit  22  reduces the temperature of the ink in the passages  72  and  73  to a lower level, such as the 125° C. temperature of the ink in the reservoirs  58  and  60 . Moreover, if the capacity of the reservoirs  58  and  60  is small enough to permit rapid heating of the ink in those reservoirs to the normal 125° C. operating temperature, the temperature control unit  22  can be arranged to maintain the ink in those reservoirs as well as in the orifice passageway  68  at an even lower temperature such as 120° C. when the system is in the stand-by condition. 
     Since the solidification of molten hot melt ink normally causes the ink to contract in volume, air can be drawn into the passages  72  and  73  when the printing system is turned off and the ink in the system solidifies, leading to start-up problems. In order to avoid such problems, the temperature control unit  22  is arranged to cause the ink in the reservoirs  58  and  60  and the deaerator  64  to be maintained in the molten condition until the ink in the passages  72  and  73  has solidified when the printing system is turned off, thereby preventing air from being drawn into those passages as the reservoir ink solidifies. In addition, the negative pressure normally applied to the reservoirs as described hereinafter can be terminated while the ink in the passages  72  and  73  is cooling to reduce the tendency of air to be drawn into the orifices  30 . 
     In order to maintain the pressure of the ink in the orifices  30  at the desired negative pressure level during operation regardless of the elevation or orientation of the print head  16  with respect to the remote ink supply reservoir  12 , the ink supply conduit  14  leading from the remote ink supply reservoir  12  to the print head can include a check valve  82  which is spring-biased toward the closed position with sufficient force to require an ink pressure of for example, at least 5 psi to open the valve and permit ink to pass from the line  14  into the low level reservoir  60 . Since the check valve  82  is closed except when ink is being supplied to the reservoir  60 , the relative elevation of the print head  16  with respect to the ink supply reservoir  12  will have no effect on the pressure of the ink in the reservoirs  58  and  60  and in the passages  72  and  73  leading to the orifices  30 . 
     To maintain the pressure in the orifices  30  at the desired negative level during normal operation, the print head pressure control unit  18  in the main control unit  10  is connected through two conduits  84  and  86  to the reservoirs  58  and  60 , respectively, so that a negative air pressure of approximately 2.8 inches of water is normally maintained in those reservoirs. With the orifice array extending in the horizontal direction slightly less than one inch below the reservoirs, as shown in  FIG. 1B , this pressure level produces a negative air pressure of about two inches at the orifices  30  which is sufficient to prevent ink from seeping out of the orifices as a result of capillary action, but is not low enough to cause air to be drawn into the passages  72  and  73  through the orifices  30 , which would interfere with the operation of the system. Further details regarding setting and maintaining the negative air pressure in the print head  16  are described below in connection with  FIGS. 3-4C . 
     As also described in the Hoisington et al. U.S. Pat. No. 4,835,554, each of the ink passages  72  and  73  is connected through a return flow path (not shown) to the ink passages  62  and  66  leading to the other of the two reservoirs  58  and  60 . With this arrangement, when the printer is not operating, ink is caused by the difference in the levels in the reservoirs to flow continuously at a low rate from the high level reservoir  58  to the low level reservoir  60  through the deaerator  64  in order to maintain the ink at the orifices  30  in a deaerated condition. As a result, the difference in the ink levels in the reservoirs is gradually reduced thereby reducing the pressure which causes the ink to flow through the deaerator and the associated passages leading to the orifices  30 . In order to restore the difference in the ink level in the reservoirs  58  and  60 , the pressure control unit  18  periodically applies a higher negative pressure of about 3.2 inches of water through the line  84  to the ink in the reservoir  58  thereby drawing ink through a check valve  87  from the low level reservoir  60  to the high level reservoir  58  until the difference in the ink levels in the reservoirs balances the applied pressure difference. 
     In addition, when the ink jet system is started up after being cold, for example after having been turned off overnight, it may be necessary to purge air bubbles and debris from the orifice passages  72  and  73  in order to assure proper operation of the system This is accomplished by applying a positive pressure of about 2 psi through both of the lines  84  and  86 , thereby forcing ink from both reservoirs through the orifice passages  68  and out of the orifices  30  to remove any air bubbles and debris which may be trapped in those passages. 
       FIG. 1D  shows the print head  16  oriented in a position in which the array of orifices  30  extends in the vertical direction, such as to print information on the sides of the containers  34  as described above with reference to  FIG. 1A . In this case, because of the different elevations of the reservoirs  58  and  60 , the ink pressure will normally be less at the orifices supplied by the low level reservoir  60  than at the orifices supplied by the high level reservoir  58 , which could cause air to be drawn into the ink passages  72  receiving ink from the low level reservoir or produce seepage of ink at the orifices connected to the high level reservoir  58 . In order to avoid this potential problem, the pressure control unit  18  is arranged to reduce the negative pressure applied to the high level reservoir while maintaining the desired negative pressure at the low level reservoir. For example, a negative pressure of about 1.1 inches of water can be applied through the line  86  to the low level reservoir  60  while the usual negative pressure of about 2.8 inches of water is applied through the line  84  to the high level reservoir  58 , providing a difference of about 1.7 inches of water between the negative pressures applied to the reservoirs to compensate for the difference in the height of the reservoirs (as shown in  FIG. 1D ) when the array is oriented in the vertical direction. 
       FIG. 1E  shows the print head when positioned to project ink downwardly from the orifices  30 , for example, to the top surfaces of the containers shown in  FIG. 1A . In this case, the two reservoirs are at the same elevation and the elevational difference between the reservoirs and the orifices is approximately the same as that of  FIGS. 1B and 1C . Consequently, the same negative pressure of about 2.8 inches of water is applied to both reservoirs. Further details regarding the exemplary pressure control unit  18 , and its interconnections with the print head  16 , are described in the Brooks et al. U.S. Pat. No. 5,489,925. Nonetheless, it will be recognized that other implementations of the present invention need not include the details of the example system described above in connection with  FIGS. 1A-1E . 
     In other implementations, the pressure control unit can have it elements separated from each other and integrated with other portions of the larger system. For example, the pressure control can be implemented using a vacuum purge control assembly and separate control electronics (e.g., on one or more circuit boards). The vacuum purge control assembly and separate control electronics can be combined together in a single unit, such as the print head itself or the main control unit  10  shown in  FIG. 1A , which can also include a user interface device, a power supply, as well as other components. Alternatively, the vacuum purge control assembly and separate control electronics can be placed in separate units. 
       FIG. 2  shows another example  200  of an ink jet printer system. In this case, two print heads  210  are movably supported on a vertically disposed column  230  so as to be locked at any desired vertical position on the column  230 . Each of the print heads  210  includes its own ink reservoir, vacuum purge control assembly, and jetting array. In addition, separate control electronics are included in a control unit  220  that is also movably supported on the vertically disposed column  230  so as to be locked at any desired vertical position on the column  230 , and the control unit  220  is electrically coupled with the print heads  210  to control purging and negative pressure setting and maintenance for the ink reservoirs and jetting arrays. The control unit  220  can also include a user interface device and power supply. Thus, as will be understood, the systems and techniques described in this application can be employed with many different printing system configurations, including different numbers of ink reservoirs at different locations. 
       FIG. 3A  shows an example of a controller  300  that maintains negative pressure in a print head. The controller  300  can receive a defined set point  305 , and can include a ramp  310  and digital to analog converter (DAC)  315 . The set point  305  can be defined by a user of the system, by print head orientation, or by a combination of these. Moreover, the set point  305 , ramp  310 , and DAC  315  can be implemented using a processor, as is described further below in connection with  FIGS. 4A-4C . 
     The controller  300  includes control loop feedback logic  320 , which can be proportional-integral-derivative (PID) logic (as shown), PI logic, PD logic, P logic, I logic, or D logic. For example, the PID logic can be a closed loop circuit  380 , as shown in  FIG. 3B ; having six operational amplifiers. Note that if PI logic or PD logic is used instead, the number of operational amplifiers can be reduced. Other forms of control loop feedback logic  320  are also possible. For example, rather than implementing this control logic entirely in analog componentry, various implementations can employ integrated circuitry (IC), a processor, firmware, or some combination thereof. 
     The controller  300  includes a voltage regulator  325 . The voltage regulator  325  can be a switching voltage regulator, a linear regulator, an amplifier controlled regulator, or other regulators that have an adjustable feature. The feedback logic  320  provides feedback bias to the voltage regulator  325  to control the output of the voltage regulator  325  (motor drive voltage), which is then provided to a driver configured to output a drive signal to a pump, which is associated with the print head. Thus, the amplitude of the motor drive voltage is controlled at the voltage regulator  325  to generate an appropriate drive signal for the pump and its associated vacuum chamber  350  and vacuum sensor  355 . 
     The driver can be implemented using a direct current (DC) motor drive IC  330  and a square wave generator  335 . The square wave generator  335  can be implemented using a programmable logic device (PLD) that generates a 60 Hz pulse width modulated (PWM) signal. Note that different frequencies other than 60 Hz may be needed for implementations using different types of pumps. In addition, rather than the motor drive IC  330 , field-effect transistors (such as MOSFETs or JFETs) on an electronics board can be used, provided they have the current and voltage capabilities needed for a given implementation. 
     The drive signal can be a 50% duty cycle 60 Hz square wave, with the voltage being adjusted based on a variable but algorithmically determined vacuum set point. Voltage control can provide smoother operation (quicker to the set point and more uniform vacuum control) which can provide more consistent and uniform meniscus properties and may provide longer pump life. The voltage control circuit can also provide a smoother pump output throughout its range. This is due in part to keeping the drive frequency and phase constant. This in turn, gives the oscillations of the pump diaphragm the ability to stay in a tightly matched synchronous pattern. This is in contrast with previous PWM circuits, where the frequency and voltage were constant, and the phase was altered to adjust pump output. At various points across its drive range, the existing PWM circuit/algorithm can cause the diaphragm to become out of synch and unstable. This is most often noticeable at the upper ends of the output scale. In addition, in order for the low vacuum circuit to operate with the types of pumps typically used in industrial printing systems, a method of restricting air flow to the pump can be provided, as described further below in connection with  FIGS. 5-6D . 
     A PID circuit can be used to establish a set point via a processor based on the appropriate vacuum level for the print head jetting orientation, which can be selected from a user interface or automatically determined The low vacuum sensor output can be used as the feedback loop and adjusting the feedback loop, based on the set point, can be accomplished by biasing the switching regulator&#39;s feedback loop, which adjusts the output voltage amplitude. The output amplitude determines the force at which the pump is driven to generate the low vacuum level. Properly tuned, the PID circuit can provide a feedback bias that will result in a steady output voltage amplitude to drive the low vacuum pump. 
     The use of a switching regulator allows for a large output voltage amplitude swing throughout the range of low vacuum settings and adjustments. The switching regulator can be supplied with 24V DC and still manage an output voltage amplitude as low as 1.225V DC without the worry of heat and power dissipation that might result from using a linear voltage regulator. By using the output of the PID circuit to directly control the output amplitude of the voltage regulator, any need for AD conversion by a processor and non-linear response can be eliminated, and the low vacuum control can be made a linear function. Low vacuum adjustments are almost instantaneous, responding to each jetting cycle, ink purges and high vacuum changes. 
       FIG. 4A  shows an example of a processor implementation of defined set point, ramp, and DAC elements from the controller of  FIG. 3A . These elements are implemented using a microprocessor  400 , where code provided in firmware can define the set point based on print head orientation.  FIG. 4B  shows the output from the microprocessor of  FIG. 4A  in accordance with the defined set point in a chart  405 . The set point (e.g., in mm water) is converted to digital value using a calculation determined using the voltage regulator circuit and vacuum sensor transfer functions. Once low vacuum control is enabled, or there is a change in the set point, the algorithm ramps to the new setpoint. 
       FIG. 4C  shows an example of a ramping process to establish the set point. At initialization, the set point is ramped  450  to a value selected to maintain a negative pressure at a desired level. At run time, the set point is maintained  460  at a constant value (e.g., the selected value). During a purge cycle (which can be initiated either manually or automatically), the negative pressure is replaced  470  with a positive pressure using a set point change. This positive pressure can be employed to purge the system or evacuate debris from the faceplate of the print head. After the purge cycle, the set point is ramped  480  back to the value selected to maintain the negative pressure at the desired level. Note that multiple purges can be performed in sequence to push ink out the front end of the orifices in the jetting assembly (with the operator wiping the print head with a lint free wipe) to completely clean the jetting assembly and recover any jets that were not previously printing properly. This design can allow a greater purging pressure by applying a specific fixed maximum voltage to the pump for the purging operation. This voltage can be designed to ensure that the jets are being cleared during the purge cycle. 
     Furthermore, pump drive adjustment can be performed by a PID circuit with machine program code (software or firmware based) providing the vacuum set point via a machine user interface. The set point for the PID circuit can be the sole program code intervention in the voltage controlled circuit. This PID circuit can allow full linear adjustment, based on the feedback loop, as compared to previous approaches in which the control was accomplished by a software method using a PWM circuit. Comparatively, the PID approach can provide for a faster response time and settle time of the pump output after a purge function or ink fill cycle. With the software controlled PWM circuit, response time was often slow as the software would “search” for the proper vacuum level to settle on in an iterative and time consuming approach. Vacuum levels can still be monitored by a processor in implementations of the present invention, but adjustments need not be made by the program code based on the monitored readings. 
       FIG. 5  is a schematic diagram showing an example of a vacuum purge control assembly for a printing system. In this example, a pump P 1  has an air vent  540  connected through a restriction R 3 . The pump P 1  represents the low vacuum (LO-VAC) pump that provides an appropriate negative pressure to the print head. The LOW-VAC pump P 1  is connected through an accumulator A 2 , a restriction R 2 , an accumulator A 1 , and a first filter  520  to a two-position valve  510  (e.g., a solenoid valve). In addition, an air intake  545  is connected through a second filter  530  and through a restriction R 1  to the accumulator A 1 . Further, the second filter  530  is coupled with a second (PURGE) pump P 2  to the two-position valve  510 . 
     The first and second filters  520 ,  530  can each be  10  micron filters. The restrictions R 1 , R 2 , R 3  can be flexible tubing, connected with accumulators A 1 , A 2  as described further below. The restrictions R 1 , R 2 , R 3  provide continuous passages of constant reduced cross-sectional area providing flow resistance proportional to their length and diameter, and can be constructed so as to avoid clogging. The restriction R 2  is placed between the accumulators A 1 , A 2  and typically set at a value designed to dampen pump oscillation to the print head. The restrictions R 1  and R 3  are typically set at values designed to get the LO-VAC pump P 1  to run in its natural sweet spot over a variation of different settings for a given operation. 
     The LO-VAC pump P 1  and the accumulators and restrictions are arranged so that a continuous flow of air is drawn through the filter  520  to provide substantially constant negative pressures (as specified) to the print head (e.g., via tubing to a reservoir or volume of ink, as described above, to which the low vacuum is applied). The positive pressure side of the pump P 1  is connected to a line that opens to the atmosphere through the restriction R 3  arranged to provide a constant positive air pressure (as specified) at the pump output line  540 . A pressure sensor  550  is coupled with the vacuum purge control assembly for use in setting and maintaining the correct pressure level. 
     When it is necessary to purge the system to remove debris or air bubbles from the orifice passageways of the jetting assembly, the valve  510  is moved to a position connecting the positive pressure line from the PURGE pump P 2  to the print head. After purging of any contaminants and air bubbles (which may have accumulated in the print head components) is completed, the valve  510  is restored to the position shown in  FIG. 5 , causing a negative pressure to be applied once again. Note that in various implementations, more than one valve, more than one output line, or both can be used to provide negative pressure and positive pressure (as needed) to various portions of the print head. For example, different negative pressures can be provided to the print head based on orientation of the print head, as described in U.S. Pat. No. 5,489,925 to Brooks et al. In addition, a line can be run from the pressure side of pump P 1  through the valve  510  for use during a purge cycle, thus eliminating the need for the second pump P 2 . 
       FIG. 6A  is a top view showing an example of an implementation of the vacuum purge control assembly of  FIG. 5 . This view shows detailed implementations of the LO-VAC pump and the PURGE pump (which can be identical pumps from the same manufacture, or different pumps), as well as a tube  610  to the print head, and tube  615  to the pressure sensor (e.g., on a circuit board holding the separate control electronics).  FIG. 6B  is a bottom view showing the example of the vacuum purge control assembly from  FIG. 6A , including detailed implementations of accumulators A 1 , A 2  and the restrictions R 1 , R 2 , R 3 . The restrictions R 1 , R 2 , R 3  can be implemented using microbore tubing, as shown, which acts as a restrictor in the pneumatic circuit of the low vacuum control assembly. Restrictors are used to limit/govern the flow of the pump and to dampen resonance of diaphragm oscillation. Accumulator chambers A 1 , A 2  add to total system volume, which lessens the impact of system variations. 
       FIG. 6C  is an exploded bottom view showing the same example of the vacuum purge control assembly from  FIG. 6B . As shown, the accumulators A 1 , A 2  and the restrictions R 1 , R 2 , R 3  are contained within a vacuum purge control (VPC) body  630  and sealed by a VPC seal  635  (e.g., a plate structure) that attaches thereto. The VPC body  630  and the VPC seal  635  can each be made of plastic or aluminum. The accumulators A 1 , A 2  are machined out areas of the VPC body  630 , and tubing  640  can be used to connect the restrictions R 1 , R 2 , R 3  with the accumulators A 1 , A 2 . Note that other implementations can use different shapes for the VPC body and its seal (e.g., a block, a cylinder, etc.), and rather than being formed within the VPC body, the accumulators A 1 , A 2  can also be formed in separate structures (e.g., a block, a cylinder, etc.) external to the VPC body. 
     The tubing  640  can be implemented using six silicone tubes, each having an inner diameter of one sixteenth of an inch and an outer diameter of three sixteenths of an inch, and each being 25 mm in length. The restrictions R 1 , R 2 , R 3  can be implemented using three flexible tubes  650   a,    650   b,    650   c.  The flexible tubes  650  can be different lengths to control the restrictive values needed in a given application of the VPC assembly. For example, each of the tubes  650   a,    650   b  can be 205 mm in length, whereas the tube  650   c  can be 610 mm in length. These flexible tubes  650  can be microbore tubing, such as Polyvinyl Chloride (PVC) tubing manufactured for medical and laboratory environments, where the tubing has a defined diameter (e.g., 0.040 inches in diameter) that is maintained within tight tolerance. For example, the PVC tubes can be cut from TYGON® PVC microbore tubing, available from Saint-Gobain Performance Plastics Corporation of Aurora, Ohio. This microbore tubing is available in a durometer that holds its shape well without kinking or collapsing. It can be coiled tightly to provide a compact package within the design. It is available in a range of sizes. It has a tightly controlled inner diameter, which will protect the design from process variations. 
     The combination of tubing bore times length will produce certain restrictive values. Various combinations of bore size(s) and length(s) work in the design. As it is often difficult to visually discern diameter differences, a single bore size of tube can be used to ease burden on manufacture. The bore size can be selected to produce the required range of desired outputs with a manageable length of tubing. The close tolerance of the tubing bore utilized opens up the required length tolerance to acceptable manufacturing limits. 
     Note that given the defined diameter of the tubing, this structure for the assembly allows one to cut the tubing to a desired length to obtain the desired restrictive result within the tolerances of the given application. In addition, this structure provides flexibility for future modifications. For example, if a later generation of the print head requires a different negative pressure with respect to the reservoir and jetting assembly (e.g., because of a changed reservoir design), the length of the tubing  650  can be readily changed by determining the new lengths, cutting new tubing to the new lengths, and replacing the old tubing with the new tubing in a simple pull-out-old and plug-in-new process. 
     The microbore tubing in this design acts as a replacement for the machined restrictors and orifice restrictors in the pneumatic circuit of previous low vacuum control assemblies. The use of microbore tubing can provide a more manufacturable solution and can provide a more robust solution when compared to the previous orifice restrictors, which are often prone to contamination failure. As will be apparent, the flexible tubing implementation of the restrictors can reduce contamination failure and also allows for replacement of the restrictors to be readily performed. In particular, use of microbore tubing, as described, can reduce system costs (due in part to the commercial availability of the tubing), improve resistance to particle contamination, improve chemical compatibility with ink, improve resistance to heat, an improve the ease of assembly. 
       FIG. 6D  is an exploded top view showing the example of the vacuum purge control assembly from  FIG. 6A . Tubes  620  and  622  are used to connect the LO-VAC pump and the PURGE pump, respectively, into the VPC body  630 . The tube  615  can be coupled to a barb tee, one eighth inch fitting  665 , which can in turn be coupled with a solenoid valve  660  through a twenty mm tube  624 . Each of the tubes  610 ,  615 ,  620 ,  622 ,  624  can be implemented using silicone tubing having an inner diameter of three thirty seconds of an inch and an outer diameter of seven thirty seconds of an inch. In addition, the VPC assembly can include two inline filters  670 , a connector screw terminal  680  (to provide connection for control of the LO-VAC pump, the PURGE pump, and the solenoid), and appropriate pan screws and cable ties to hold the components of the VPC assembly together. 
     Although the invention has been described herein with reference to specific embodiments, many modifications and variations therein will readily occur to those skilled in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention.