Patent Application
20200086638
The present invention relates to drive circuits for driving a plurality of actuators of printheads, to printhead circuits having such drive circuits and to printhead assemblies having such printhead circuits and to corresponding methods.
It is known to provide printhead circuits for printers such as inkjet printers. For example, the inkjet industry has been working on how to drive piezoelectric printhead actuators for more than fifty years. Multiple drive methods have been produced and there are multiple different types in use today, some are briefly discussed now.
Hot Switch: This is the class of driving methods in which the generation of drive waveforms for the actuators takes place within the print head itself. Typically, the electronics in the print head are implemented in an integrated circuit (ASIC). In this approach, all of the power dissipation associated with generating the waveforms and connecting them to the actuators (a total of 0.5 CV2 per driven actuator) occurs in the print head. This was the original drive method, before cold switch became popular.
Cold Switch: This describes an alternative structure using a Common Drive Waveform (CDW). in which the electronics that generate the CDW is located outside of the print head. The electronics within the print head (typically an ASIC) is then only required to provide multiplexer functionality to connect this externally generated CDW to the appropriate actuator nozzles. A key advantage of this approach is that a significant proportion, in some cases perhaps around 80%, of the 0.5 CV2 energy dissipation occurs in the external waveform generation electronics and, consequently, the dissipation in the print head and the ASIC is reduced. This makes it much easier to maintain the print head at or around a suitable operating temperature.
However, for printed image quality reasons, it is highly desirable to provide a mechanism for trimming the drop velocity or drop volume on a per actuator nozzle basis. This requires that the drive circuits are capable of generating an individually-tailored waveform to each actuator nozzle. In a hot switching environment, in which the waveforms are generated in the print head itself (typically in an ASIC) this is straightforward to achieve. In a cold switching environment, however, where a common drive waveform (CDW) is generated outside of the print head, the modification of the waveform on a per actuator nozzle basis is more difficult to achieve.
us 2005200639 shows a printer with drive circuitry for actuators using a common drive waveform applied to one side of the actuators and with switches for coupling the other side of the actuators to a common return path. The switches are controlled to switch on sloping edges of pulses of the common drive waveform to adjust a height of the pulses, for an array of actuators. Adjustments can be made for each printed line so that blocks can be varied around an average weighting.
U.S. Pat. No. 8,303,067 shows a stepped common drive waveform with multiple different pulses having multiple levels, switching is carried out to select which of the different pulses to use to generate different sizes of droplet. There is adjustment of ejection speed by widening or narrowing intervals between successive droplets.
us 2009/0278877 shows common drive waveforms A and B with multiple levels, with adjustment of hi, a hold time when the chamber is at maximum volume before contraction and ejection.
us 2011/0128317 shows a common drive waveform and adjustment of timing of gating during a ramp so as to change a height of the ramp.
US20120262512 shows a common drive waveform and shows changing a height of part of a pulse by controlling a timing of a switch to couple the common drive waveform to an actuator, to compensate for variations between different actuators.
Embodiments of the invention can provide improved apparatus or methods or computer programs. According to a first aspect of the invention, there is provided a drive circuit for driving at least one of a plurality of actuators of a printhead from a common drive waveform, and having a switching circuit for coupling the common drive waveform to provide a drive pulse to a selected at least one of the actuators, and a timing circuit coupled to receive a trimming signal and having a control output coupled to control the switching circuit so as to form the drive pulse from at least part of a pulse in the common drive waveform, and so as to trim the drive pulse by controlling according to the trimming signal a duration of a step at an intermediate level in the drive pulse.
Any additional features can be added to any of the aspects, or disclaimed, and some such additional features are described and some set out in dependent claims. One such additional feature is the timing circuit being arranged to control the duration of the step by causing the switching circuit to couple the common drive voltage to the selected at least one of the actuators to provide a transition in the drive pulse, to decouple for a period to provide a flat portion of the step, and to recouple the common drive waveform to the selected at least one of the actuators to provide another transition of the same drive pulse.
Another such additional feature is the switching circuit also having a circuit to selectively couple the selected at least one of the actuators to a reference voltage, and the timing circuit being arranged to control the duration by causing the switching circuit to couple the common drive voltage to the selected at least one of the actuators to provide a transition in the drive pulse, and to couple the reference voltage to the selected at least one of the actuators for a period of the same drive pulse to provide a flat portion of the step.
Another such additional feature is the timing circuit being configured to control the duration of the step independently of control of a height of the step. Another such additional feature is the timing circuit being arranged to change a state of the switching circuit during a flat portion of the common drive waveform. Another such additional feature is the drive circuit being arranged so that, where the common drive waveform comprises a multilevel pulse having a portion at the intermediate level before a portion at another level, the timing circuit is arranged to cause decoupling from the common drive waveform to occur during the portion at the intermediate level and to cause a recoupling to occur during the portion at the another level, to control the duration of the step. Another such additional feature is the drive circuit being arranged so that where the common drive waveform comprises a multilevel pulse having a portion at another level before a portion at the intermediate level, the timing circuit is arranged to cause decoupling from the common drive waveform to occur during the portion at the another level and to cause a recoupling to occur during the portion at the intermediate level to control the duration of the step.
Another such additional feature is the switching circuit being arranged to cause a transition in the step of the drive pulse where it does not follow the common drive waveform, to have a different slew rate to that of a transition in the common drive waveform. Another such additional feature is the switching circuit having at least two separately controllable switching paths having different series resistances, and the timing circuit being arranged to control the switching paths to provide a higher series resistance during the transition. Another such additional feature is the timing circuit being arranged to receive a reference timing signal, and to receive the trimming signal as a digital value corresponding to a time interval between the reference timing signal and a desired timing of the step, and having a digital circuit for using the digital value and the reference timing signal to generate the control output.
Another such additional feature is the drive circuit being arranged such that when the common drive waveform has no step at the intermediate level, the timing circuit is arranged to change the switching circuit as the common drive waveform passes through the intermediate level. Another such additional feature is the switching circuit having a holding circuit for maintaining a level in the drive pulse without isolating it from the common drive waveform.
Another aspect provides a printhead assembly having at least one drive circuit for driving at least one of a plurality of actuators of a printhead from a common drive waveform, and a common drive waveform circuit for generating the common drive waveform with a pulse having a flat portion. The drive circuit has a switching circuit for coupling the common drive waveform to provide a drive pulse to a selected at least one of the actuators, and a timing circuit coupled to receive a trimming signal and having a control output coupled to control the switching circuit so as to form the drive pulse from at least part of the pulse in the common drive waveform, and so as to trim the drive pulse by controlling according to the trimming signal a duration of a step in the drive pulse, by changing a state of the switching circuit during the flat portion in the common drive waveform. Another such additional feature is the common drive waveform circuit having a level adjustment circuit for adjusting the intermediate level. The printhead assembly can have a drive circuit with any of the additional features set out above.
Another aspect provides a printer having a printhead assembly having any of the drive circuits set out above.
Another aspect provides a method of operating a printhead having a plurality of actuators, having the steps of: using a switching circuit for coupling a common drive waveform having a pulse, to a selected at least one of the actuators to provide a drive pulse, generating a trimming signal and controlling the switching circuit to form the drive pulse from at least part of a pulse in the common drive waveform. The drive pulse is trimmed by controlling according to the trimming signal a duration of a step at an intermediate level in the drive pulse.
Numerous other variations and modifications can be made without departing from the claims of the present invention. Therefore, it should be clearly understood that the form of the present embodiments of the invention is illustrative only and is not intended to limit the scope of the present invention.
How the present invention may be put into effect will now be described by way of example with reference to the appended drawings, in which:
The present invention will be described with respect to particular embodiments and with reference to drawings but note that the invention is not limited to features described, but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes.
Where the term “comprising” is used in the present description and claims, it does not exclude other elements or steps and should not be interpreted as being restricted to the means listed thereafter. Where an indefinite or definite article is used when referring to a singular noun e.g. “a” or “an”, “the”, this includes a plural of that noun unless something else is specifically stated.
References to programs or software can encompass any type of programs in any language executable directly or indirectly on any computer.
References to circuits or circuitry or logic or processor or computer, unless otherwise indicated are intended to encompass any kind of processing hardware which can be implemented in any kind of logic or analog circuitry, integrated to any degree, and not limited to general purpose processors, digital signal processors, ASICs, FPGAs (Field Programmable Gate Arrays), discrete components or logic and so on, and are intended to encompass implementations using multiple processors which may be integrated together, or co-located or distributed at different locations for example.
References to nozzles are intended to encompass any kind of nozzle for ejecting any kind of fluid from a fluid reservoir for printing 2D images or 3D objects for example, onto any kind of media, the nozzles having actuators for causing the ejection in response to an applied electrical voltage or current.
References to actuators are intended to encompass any kind of actuator for such nozzles, including but not limited to piezoelectric actuators, provided they have a predominantly capacitive characteristic, so that the voltage across it does not change significantly when it is decoupled from the CDW during the step in the pulse.
References to groups or banks of the actuators or nozzles are intended to encompass linear arrays of neighbouring nozzles, or 2-dimensional rectangles or other patterns of neighbouring nozzles, or any pattern or arrangement, regular or irregular or random, of neighbouring or non-neighbouring nozzles.
References to a step in a pulse are intended to encompass any kind of notch or protrusion in the typically trapezoidal shaped pulse including and not limited to those having one or more flat portions each next to a sloping portion, sloping up or down, and the flat portion may be flat or have a small gradient less than a gradient of the sloping portion.
References to level are intended as encompassing a portion of a pulse, such as a step, or shelf or a flat or sloping part with a shallower gradient than edges of the pulse.
References to decouple are intended to encompass switching to isolate from a drive circuit, or if not isolating, then applying holding circuit to hold the voltage against being changed by the drive circuit, such as applying a relatively large capacitor or a voltage supply circuit to hold the voltage temporarily without isolating.
An actuator's motion creates the pressure and flow that pushes fluid through the nozzle. The performance of each nozzle is characterized mostly by the drop speed, drop weight, appearance of satellites, and drop shape. Variability in actuator motion can cause errors and artefacts in the image quality during printing. Sources of the variability can be due to manufacturing variability or due to the operating environment; for example, the frequency at which an actuator is fired affects the drop speed. It is desirable to be able to control individual actuators to allow the printing system to compensate for these effects.
Effects to be compensated for can include for example:
Existing printhead circuits such as hot switch or cold switch drive ASICs for driving print actuators have limitations in terms of their cost and power dissipation for compensation of the above effects. So there is a question of how to provide electrical drive for actuators such as piezoelectric actuators at the lowest circuit area (to reduce the cost) and with the lowest power dissipation, which reduces thermal effects, while still meeting minimum drive requirements. Using hot switch methods that vary the pulse width of the drive pulse to each actuator or vary the voltage level at each pulse has a large thermal impact. All of the drive power plus baseline power is dissipated in the ASIC which is located within the printhead close to the actuators and there tend be larger areas for these designs, meaning added costs in the ASIC. In cold switch designs, on the other hand, the majority of the power dissipation occurs in the circuitry creating the CDW which is located outside the printhead and much easier to cool than an ASIC inside the printhead.
Jetting performance, specifically the volume and velocity of ejected drops, of the individual actuators/nozzles on for example a MEMS printhead can vary as a result of manufacturing tolerances. In addition, the drop ejection velocity or drop shape or volume can be influenced by the jetting of adjacent nozzles (crosstalk) and, in cases where high frequency jetting is required, is also influenced by the time elapsed since the actuator under consideration itself last ejected a drop of ink. Compensating for these variations and effects requires a mechanism in which the drop ejection velocity can be trimmed on a per-nozzle and in some cases per-ejection basis. If such a mechanism can be successfully implemented, image artefacts that would result from differences in droplet ejection velocity between nozzles can in principle be corrected.
One current method for providing a trimming mechanism is based on changing the amplitude of the trapezoidal waveform applied to individual actuator channels. The droplet velocity (and also droplet volume) is a function of the waveform amplitude and therefore, by changing this, the droplet velocity can be trimmed. But implementing this “voltage trimming” approach in a cold switching environment is difficult without using excessive silicon area and increasing the power consumption of the ASIC, thus losing the thermal advantages of the cold switching approach. The description below of embodiments of the present invention shows various ways to provide generation of individually tailored waveforms for the actuator nozzles in a cold-switching environment where the additional heat dissipated by the circuits that modify the CDW can be reduced or minimised.
Embodiments as described below provide for trimming to vary the resulting droplet by controlling a duration of the proposed step in the drive pulse. Various implementations are possible. Some are based on perturbing the slew rate of the leading or the trailing edge of the pulse in the Common Drive Waveform (CDW) so that the drive pulse does not follow that edge of the CDW, by isolating or forcing or holding the voltage, to provide the step in the drive pulse for the trimming function rather than producing the step by providing a ledge in the common drive waveform. Some of the proposed implementations involve a holding circuit to create the step, though some implementations of this use an on-ASIC capacitance per nozzle, which has disadvantages in terms of amount of silicon area used. Some implementations cause the switch to decouple the drive pulse from the CDW during a flat portion in the CDW. This enables the timing precision of the decoupling to be more relaxed than if the decoupling takes place during a sloping portion since the slope makes the level of the intermediate level very sensitive to the precise timing of the decoupling.
Trimming may be effected by varying the height of the step (shown in the Figures as the
To avoid a high peak current which can cause voltage disturbance on the ground resulting from parasitic resistance and inductance effects, a ‘high resistance’ switch can be used to reduce a slew rate. This can be implemented in various ways, for example by using a separate MOS transistor or by having a transistor with multiple separately controllable gate fingers having different resistances. The trimming signal can be loaded as a digital value to give dynamic trimming.
Some consequences of particular embodiments are as follows.
1. Simpler circuitry is possible having little silicon real estate overhead (e.g. in an ASIC implementation), especially for cases where the timing of decoupling is less critical, in simpler embodiments only the addition of 1 timer and 1 level shifter per channel in the ASIC is used.
2. The trimming range and resolution can be adjusted by control of the CDW (by changing the height of the voltage ledge in the CDW).
3. The trimming range and thermal dissipation trade-off can likewise be changed by changing the ledge voltage in the CDW.
4. Some implementations may have a fast slew rate of the hot-switch part of the drive pulse where it has a transition which does not follow the CDW. The fast slew rate can be reduced by increasing the resistance of the switch i.e. by using a separate smaller switch or by using a part of the right hand side portion of the switch e.g. one or two fingers of the right hand side transistor only. The lower slew rate can reduce the high peak currents and thus reduce ground or voltage rail spikes.
5. The trimming concept is a step based trimming, overall this class of driver is a hybrid hot/cold switch type. For instance, in one embodiment, on the trailing edge of the waveform (the second or rising edge) all the energy into the load is provided by a cold switch multiplexer, whereas on the first (i.e. falling or leading) edge, all of the driving energy is provided by a cold switch multiplexer up to the ledge voltage, but with a hot switch transistor for driving from the ledge voltage back to the waveform (now at zero) after a programmed delay. This is because on the falling edge and first part of the leading edge, the CDW generating circuit is controlling the maximum slew rate. The transition from the ledge voltage back to the waveform is controlled by the RC time constant formed by the pass gate switch ON resistance and the load capacitance, and is therefore a hot transition. The net result is still a driver that has lower thermal impact than a hot switch design.
The print signal input to the timing circuit is provided so that the switching circuit can cause the actuator to be decoupled for the duration of the cycle of the waveform so that no drive pulse is produced for a given pixel of an image if the print signal indicates that there is no dot to be printed for that pixel. There are many ways to generate the timing to control the duration, synchronised to an internal clock or to a level or slope of the CDW or to some timing reference for example.
To compensate for differences between actuating elements, and/or in some cases to compensate for parameters varying over time such as temperature, ageing or crosstalk from neighbouring pixels, a trimming signal is applied as necessary for each actuator to modify the CDW. The trimming signal can be generated for example from a look up table, or by a processor based on measurements of output or temperature for example, or from information such as manufacturing calibration results, or print image information for example, or a combination.
A more detailed explanation of the operation of the example of
1. Prior to the leading edge of the CDW, the switch is turned ON (if not ON already). The leading edge of the CDW is coupled via the switch to the actuator.
2. When the CDW voltage reaches
3. After the short period of e.g. ο.ï μs to 0.5 μs, the CDW continues down to
4. Meanwhile with the switch off, the actuator remains at
5. The actuation activity is then completed by the CDW slewing back to
Note that:
a). The duration of the step,
b). The ledge on the leading edge of the CDW is optional but is useful for two reasons:
(i) it defines the
(ii) the required accuracy of the switch turn OFF event is determined by the duration of the ledge in the CDW waveform—the requirement being that the switch turns OFF while the CDW voltage is at
c). The magnitude of the trimming effect is determined by both
d). There can be many variants. The step can take various shapes, for example the flat portion forming the ledge can be sloping to some degree and still achieve much of the benefit. There can be multiple sub-steps within the step; the step can be on the leading edge or the trailing edge of the pulse, or on both edges, or away from either edge. There can be a series of steps within the pulse. The polarity of the pulse can be reversed, the slew rates of the edges can be limited, and the flat portions can be formed by coupling to a voltage reference or to a holding circuit such as a capacitor.
This can enable the timing of the change in coupling to be more relaxed since the resulting level is not so sensitive to the timing compared to the case that the decoupling occurs while the CDW is in transition through the intermediate level for example. Relaxing the precision of timing can enable cost, complexity and thermal loading to be reduced, or precision of trimming to be increased.
The simple implementation of the passgate in
The operation is shown in
The end of the step in the drive pulse is caused by recoupling after a controlled duration
Note that this different slew rate should not affect the drop ejection as the ejection is typically only weakly dependent on the slew rate if the slew rate is above a threshold value. Note also that in the Figures, M2A and M2B are shown as separate MOS devices. In practice, these would likely be implemented as a single MOS device with multiple gate fingers, with one set of gate fingers driven by one timer, and the remaining gate fingers driven by the other timer. The number of gate fingers driven by each timer will determine the relative ON resistances of M2A and M2B.
This represents an example of the switching circuit being arranged to cause a transition in the step of the drive pulse where it does not follow the CDW, to have a different slew rate to that of a transition in the CDW. This can help reduce noise caused by excessive ground plane voltage movement due to higher current flowing during faster slew rates.
This represents an example of the drive circuit being arranged so that where the CDW comprises a multilevel pulse having a portion at another (lower in
This shows another example of the drive circuit being arranged so that where the CDW comprises a multilevel pulse having a portion at the intermediate level before a portion at another level, the timing circuit is arranged to cause decoupling from the CDW to occur during the portion at the intermediate level and to cause a recoupling to occur during the portion at the another level, to control the duration of the step in the actuator drive pulse. This is one way of enabling the timing of one of the changes in coupling to be relaxed, by making it occur in a flat portion such as where the portion at the intermediate level is part of a leading edge of the pulse, or a leading edge of a secondary peak in the pulse. Notably the timing of the decoupling need not affect the shape and thus need not be so precise. The timing of the recoupling directly affects the pulse shape and so the precision of its timing affects the precision of trimming.
The reference timing signal can be a global reference for all actuators, or specific to one of a number of banks of actuators, or specific to each of the actuators for example. It should have some defined relation to the timing of whatever part of the pulse in the CDW represents one end of (or some other given point along) the step, so that the duration of the step can be defined relative to this reference timing signal. There are various ways of achieving this, for example the reference timing signal could be derived directly from that given end or point along the step, or it could be derived indirectly, from some other timing signal which has itself been derived from that given end or point along the step. Or the reference timing signal could be derived indirectly in the sense of being derived from a common timing source down a different branch of a timing hierarchy or tree to a branch used to derive the pulse in the CDW for example. So the trimming signal could be for example a digital value of a number of clock pulses starting from a change of state of the print signal, or from a change of state of the control output where it decouples the drive pulse from the CDW for example.
Examples of alternative switching circuits are shown in
When the holding capacitor
These
The printhead arrangements described above can be used in various types of printer. Two notable types of printer are:
In both types of printer, the printheads can optionally be operating several different colours, plus perhaps primers and fixatives or other special treatments. Other types of printer can include 3D printers for printing fluids such as plastics or other materials in successive layers to create solid objects.
The printer also has a fluid supply system 420 coupled to the nozzles, and a media transport mechanism and control part 400, for locating the print medium 410 relative to the nozzles. This can include any mechanism for moving the nozzles, such as a movable printbar. Again this part can be coupled to the processor to pass synchronizing signals and for example position sensing information. A power supply 450 is also shown.
The common circuitry 170 in this case has a CDW circuit 174 for generating the CDW, typically with a power amplifier to handle the currents needed if there are many actuators to be driven. Optionally the CDW circuit is coupled to a level adjust circuit 178 for adjusting the intermediate level, either based on the trimming signal or a different global or per nozzle trim signal. There is a trim generator 176 for generating the trimming signals, which are fed to each drive circuit, optionally as digital values, updated as often as needed. There may be a static part and a dynamic part of the trimming signal for each drive circuit, representing time invariant and time varying differences between the actuators. The common circuitry also has a timing reference circuit 172, for generating a timing reference for use by the timing circuits of the drive circuits. In principle this may not be necessary if the timing could be obtained from the CDW by the timing circuit in each drive circuit, though in practice the higher currents and noise in the CDW may make it less useful for synchronising the timings of the switching.
This Figure shows an example of a printhead assembly having at least one drive circuit for driving at least one of a plurality of actuators of a print head from a common drive waveform, and a common drive waveform circuit for generating the common drive waveform with a pulse having a flat portion. The drive circuit has a switching circuit for coupling the common drive waveform to provide a drive pulse to a selected at least one of the actuators, and a timing circuit coupled to receive a trimming signal and having a control output coupled to control the switching circuit so as to form the drive pulse from at least part of the pulse in the common drive waveform, and so as to trim the drive pulse by controlling according to the trimming signal a duration of a step in the drive pulse, by changing a state of the switching circuit during the flat portion in the common drive waveform. This Figure also represents an example of the common drive waveform circuit having a level adjustment circuit for adjusting the intermediate level. This can enable adjustment of the range and resolution of the trimming.
Other embodiments and variations can be envisaged within the scope of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1504108.0 | Mar 2015 | GB | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15557088 | Sep 2017 | US |
| Child | 16687768 | US |
