Patent Description:
Printing systems for large format media generally comprise a print head support member, often in the form of a wide beam, extending over a medium support surface. A print head assembly mounted on a carriage reciprocally moves along said print head support member to print an image on the medium support surface. Such printing systems may be e.g. flatbed printers or large format web printers. The movement of the print head carriage and/or the displacement of the print head support member may result in vibrations in the print head support member, which negatively affect the print quality, as these result in an undesired displacement of the print head assembly with respect to the print medium. Such vibrational problems are also known to occur in printing system with a page-wide print head assembly, which is mounted stationary on its print head support, though therein the vibrations originate from different components of the system, such as the print medium transport system and/or the ink supply system. It is known to reduce such vibrations by adjusting the stiffness and/or mass of the beam. This however increases the complexity of the beam, resulting in greater costs. A conventional inkjet printing system is known from <CIT>.

It is an object of the invention to provide an alternative manner of improving the image quality in a printing method and/or printing system, specifically with regard to vibration issues.

In accordance with the present invention, a method according to claim <NUM> and a printing system according to claim <NUM> are provided. This method of inkjet printing an image on a print medium supported on a medium support surface comprises the steps of:.

More specific optional features of the invention are indicated in the dependent claims.

In an embodiment, the vibration is continually detected during use and the exerted force is continually adjusted by passing the detected vibration through a predetermined control loop stored on a controller. The frequency and/or amplitude of the vibration in the print head support member may change over time and the controller is configured to adjust control of the actuator accordingly. A control loop is stored on the controller, for example in the form of a algorithm or electronic circuit. This ensures that any changes in the vibration are taken into account.

In an embodiment, the control loop is configured to apply the exerted force at a frequency substantially anti-phase to the detected vibration. Applying an anti-phase signal is an effective method of dampening a vibration. The exerted force is applied substantially <NUM>° out-of-phase with the vibration in the print head support member, as determined by the vibration sensor. The control loop is further configured to determine a suitable force amplitude to dampen the vibration.

In an embodiment, the control loop is further configured to correct the exerted force to compensate for a delay time required by the controller to determine the exerted force from the detected vibration. A certain delay time passes between the sensing of the vibration and the actuation of the actuator. The control loop has been configured to correct for this delay time, for example by adjusting the phase of an actuator control signal by a factor corresponding to the delay time. Said delay time may vary and this adjustment may be performed dynamically, for example by means of a suitable filter in the control loop.

In an embodiment, the force exerted by the actuator is applied in a contactless manner, preferably via electromagnetic force transfer. A one-way transfer of vibrational energy from the print head support member to the vibration absorption body is preferred. This may be achieved by a contactless actuator. Electromagnetic forces work well in this regard and allow for such a one-way transfer.

The invention further relates to an inkjet printing system comprising:.

The vibration sensor is provided on the print head support member, which may for example be formed as a first beam, on which the print head assembly is mounted, either moveably or in a stationary manner. The vibration sensor senses the vibration in the print head support member and transmits the corresponding sensor data to the controller. Based upon said sensor data the controller determines a control signal for driving the actuator, such that vibrational energy is transferred from the print head support member to the vibrational absorption body. Preferably the vibrational absorption body and the actuator are configured such that the transfer of vibrational energy is substantially one-way, resulting in a dampening of the vibration in the print head support member.

In one embodiment, the first beam may be a carriage support beam along which the print head carriage is translatable in a scanning direction for swath-wise printing an image. In a second embodiment, the print head support member supports and/or holds a page-wide print head array. The page-wide print head is stationary during printing and its width defines the maximum width of the printable image. In an embodiment, the print head support member comprises a first beam, which extends perpendicularly to the direction wherein the print head assembly and the print medium move with respect to one another.

In an embodiment, the vibration absorption body comprises a second beam and/or plate parallel to the first beam. The vibrational absorption body is preferably not directly mounted on the first beam to prevent an undesired return of vibrational energy to the first beam. A second beam may be mounted on opposite sides of the medium support surface without interfering in the print operation. The dimensions of the second beam may be selected to form a mass-damper system tuned to the main vibrational modes of the first beam (e.g. its eigenfrequencies). It is also common that a second beam is provided for supporting the cable connection between the controller and the print head carriage, such that this second beam may be provided with the dual function of cable support and vibration absorption.

In an embodiment, the controller is provided with a control loop to continually determine a suitable control signal for the actuator based in the detected vibration. The control loop may be formed by an electronic circuit and/or computer program. In this manner, any changes determined by the vibration sensor are constantly corrected, resulting in an efficient dampening system.

In an embodiment, the control loop is configured apply the force exerted by the actuator substantially in anti-phase with the detected vibration. Preferably, the control loop is further configured to correct a timing of the force for a delay time between the detection of the vibration and the application the control signal. This ensure that the applied force is anti-phase with the current vibration and efficiently dampens the vibration.

In an embodiment, the actuator is configured for electromagnetic transfer of forces between the print head support member, which preferably in the form of the first beam, and the vibration absorption body. An electromagnetic actuator may be contactless and prevents an undesired return of vibrational energy to the print head support member. The actuator may comprise an electromagnetic field generator which can move a magnetic element without direction contact. By mounting the generator on the first beam and the magnetic element on the vibrational body (or vice versa), the return of vibrational energy to the print head support member is prevented or reduced. Preferably, the actuator comprises a Lorentz force motor, as commonly known in the state of the art.

In an embodiment, the vibration sensor and/or the actuator are positioned halfway along a length of the print head support member, which is preferably the first beam. It was found that the dampening works best when sensed and applied halfway along the length of the first beam. It will appreciated that additional sensors and actuator may be applied.

In an embodiment, the vibration sensor and/or actuator are positioned on an opposite side of the print head support member, specifically of the first beam, with respect to the print head assembly. The actuator (and preferably the vibration absorption body) are an opposite side of the print head support member than the print head assembly or carriage. This allows the carriage to move unhindered.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only.

<FIG> is a so-called 'flatbed' printing system <NUM> as known in the prior art. The printing system <NUM> comprises a number of workstations 8B, 8C, which may be personal computers or other devices for preparing image data for prints to be printed. These workstations have access to a network N for transferring print jobs comprising the image data to a print controller 8A that is configured to receive the print jobs for prints and derive pass images. The print controller 8A may be part of the printing system <NUM> connected to a control unit of the printing system <NUM> via a connection <NUM>. The printing system <NUM> further comprises a print head <NUM> attached to an armature <NUM> for applying colorants, for example cyan (C), magenta (M), yellow (Y), black (K) and white (W) colorant, or varnish to pieces <NUM>, 9A of flat print media placed on a flatbed surface <NUM> in order to obtain a printed image. The armature <NUM> may be a first beam above the flat bed surface <NUM> as shown in <FIG> or a robot arm (not shown) moving in a plurality of directions over the flat bed surface <NUM>. The flatbed surface <NUM> is the surface of the flatbed which is at least partially printable by the print head <NUM>. The pieces of media may be so small that they are completely placed on the flatbed surface <NUM>, but a piece of media which is larger than the flatbed surface, in which case an image which is going to cover the whole piece of media must be printed into a plurality of parts of the image, is not excluded. A first piece 9A has already been printed upon, while the other pieces <NUM>, <NUM> are not provided with any recording material yet. The print head <NUM> reciprocally scans the flatbed surface <NUM> in the second direction X along the first beam <NUM> perpendicular to a first direction Y of the first beam <NUM> over the flatbed surface <NUM> along guiding parts <NUM>. During printing of an image on the piece <NUM>, 9A of media the piece <NUM>, <NUM>, 9A of media is not moved on the flatbed surface <NUM>. Movement of the first beam <NUM> in the first direction Y may results in vibrations in said beam <NUM>, which affect the positioning of ink droplets by the print head carriage <NUM>. Such vibrations may also be caused by the reciprocating movement of the print head carriage <NUM> along the first beam <NUM>.

<FIG> illustrates a roll printing system <NUM>, wherein the print media <NUM>, <NUM> are supplied as webs from respective rolls R1, R2 in a supply device <NUM> to an output tray <NUM> in cut form or to a take-up roll (not shown for re-winding. The rollers R1, R2 are driven to move the print media <NUM>, <NUM> along the first beam (inside the housing). The first beam in this case is stationary, while the print head carriage translates along the first beam. Generally the images are printed stepwise, wherein the print medium <NUM>, <NUM> is moved a step in between printing swaths. Like in <FIG>, the image quality may be negatively affected by vibrations in first beam. The roll printing system <NUM> may be controlled via a local user interface <NUM> or by means of a controller <NUM> which receives data from a network N via a connection <NUM>.

<FIG> illustrates a printing system <NUM> according to the present invention. The printing system <NUM> is embodied as a flatbed printing system similar to <FIG>, but the present invention may also be applied to a roll printer <NUM> as in <FIG>. The printing system <NUM> comprises a medium support surface <NUM> in the form of a table. The medium support surface <NUM> is provided with a plurality of suction holes in fluid connection to a suction force for applying an underpressure to a print medium on the medium support surface <NUM> for holding it flat and in position. A print head support member <NUM> in the form of a first beam extends in the second direction Y (commonly referred to as the scanning direction Y) over the width of the medium support surface <NUM>. The first beam <NUM> is moveably supported on a guide system <NUM> in the form of guide rails. A drive <NUM> is provided to move the first beam <NUM> along the guide system <NUM> in the first direction Y, which is perpendicular to the scanning direction Y. Any suitable drive and guide may be applied, such as a rack and pinion drive, linear drive motor, etc. The print head assembly <NUM> is provided as a print head carriage, which moves reciprocally along the first beam <NUM> in the scanning direction Y. The print head carriage <NUM> holds a plurality of print heads for jetting ink droplets onto the print medium for forming a swath of an image during a pass of the print head carriage <NUM> along the first beam <NUM>. The print head carriage is controlled via a controller by means of a connection <NUM>, which in <FIG> is illustrated as a cable connection. The cable connection <NUM> is configured to fold and/or roll-up with the movement of the print head carriage <NUM>. The cable connection <NUM> is supported on a second beam <NUM>, which also acts as a vibration absorption body. It will be appreciated that the connection between the controller and the print head carriage <NUM> may also be wireless.

The second beam <NUM> extends parallel and adjacent to the first beam <NUM>. An actuator <NUM> is positioned between the first and second beams <NUM>, <NUM> for transferring forces between them. The first beam <NUM> is provided with a vibration sensor <NUM> for detecting vibrations in the first beam <NUM>. Any suitable vibration sensor may be applied, such as an accelerometer. The vibration sensor <NUM> transmits sensor data to the controller, which is configured for processing said data into a control signal for the actuator <NUM>. By means of a predetermined control loop, the controller converts the sensor data into a control signal by which the actuator <NUM> at least partially transfers the vibration from the first beam <NUM> to the second beam <NUM>. The determination of the control signal will be discussed in detail with regard to <FIG> and <FIG>, but the actuator <NUM> substantially applies an anti-phase force to the first beam <NUM>. The applied force is substantially in anti-phase with the current vibration in the first beam <NUM>. In consequence vibrational energy is efficiently diverted from the first beam <NUM> to the second beam <NUM>. The second beam <NUM> herein is one example of a vibration absorption body. Preferably, such a vibration absorption body is suitably configured to absorb and hold the vibrational energy from the first beam <NUM>, without it returning to the first beam <NUM>. The vibration absorption body may for example be configured as a tuned mass-damper system tuned to e.g. an eigenfrequency of the first beam <NUM>. The vibration sensor <NUM> and the actuator <NUM> are preferably applied in the middle of the first beam <NUM> in the scanning direction Y, though multiple sensors and actuator may be also be applied. The vibration sensor <NUM> and the actuator <NUM> are on the other side of the first beam <NUM> than the print head carriage <NUM>.

To prevent vibrational energy from returning from the second beam <NUM> to the first beam <NUM>, the actuator is preferably configured as a contactless, electromagnetic actuator <NUM>, as shown in <FIG>. The actuator <NUM>, which may be a Lorentz force motor, comprises two mounts <NUM>, <NUM>. Each mount <NUM>, <NUM> is attached to its respective beam <NUM>, <NUM>. One of the mounts <NUM>, <NUM> is provided with a current driven electromagnetic field generator, for example a coil, while the other mount <NUM>, <NUM> comprises a magnetic element <NUM>, for example a bar magnet. By forcing a current through the coil, a force is applied to the magnetic element <NUM> and transmitted via the mount <NUM>, <NUM> to the respective beam <NUM>, <NUM> without direct contact. This prevent vibrations from the second beam <NUM> from returning to the first beam <NUM>.

<FIG> illustrates a print system <NUM>' with a page-wide print head assembly <NUM>'. The page-wide print head assembly <NUM>' extends as an array over substantially the full width of the medium support surface <NUM>'. The medium support surface <NUM>' is provided with through-holes to apply an underpressure to the print medium. The medium support surface <NUM>' may for example be a stationary plate over which the print medium passes by means of a transport system (not shown) or be part of an endless belt, which adheres and transports the print medium. It will be appreciated that multiple print media may be provided besides one another on the medium support surface <NUM>'. The vibration absorption body <NUM>' in <FIG> is formed as a pretensioned plate. The plate <NUM>' is mounted on the same bases <NUM> as the print head support member <NUM>', which is in the shape of a first beam <NUM>'. The materials, dimensions, and pretension in the plate <NUM>' may be attuned to certain harmonics of the first beam <NUM>'. The vibration sensor <NUM>' is mounted on the first beam <NUM>' and is used to control the actuator <NUM>'. Since vibrational energy is directed into the plate <NUM>' the first beam <NUM>' is prevented from excessively vibrating.

<FIG> illustrates the steps of the method of printing an image on a print medium. The print medium is supported on a medium support surface, such as the one's described in <FIG>. The steps are performed during the execution of a print job, wherein the print head carriage <NUM> moves along the first beam <NUM> to print consecutive swaths of an image on a print medium. In a first step i, a vibration in a first beam <NUM> extending over the medium support surface <NUM> is detected by means of a vibration sensor <NUM>. The vibration sensor <NUM> generates sensor data which is transmitted to the controller. In step ii the controller determines based on the sensor data a control signal for driving the actuator. This is illustrated in <FIG>, which shows a chart wherein the horizontal axis is the time axis and the vertical axis illustrates the relative amplitude or intensity. The top graph in <FIG> represents the sensor data SD as measured by the vibration sensor <NUM>. This reflects the current vibration in the first beam <NUM>. This vibration is continuously measured (which may include constant or intermittent measurements), though only a short portion of the sensor data SD is shown in <FIG>. To dampen this vibration, the controller in step iii applies a control loop or algorithm configured to generate a substantially anti-phase control signal CS. However, a certain delay time DT is required or taken for transmitting the sensor data SD to the controller and processing it. Merely inverting the phase of the sensor data SD would thus result in graph B, which shows a signal which would be anti-phase with the sensor data SD were it not for the shift caused by the delay time DT. To compensate for this the controller is configured to correct for this delay time DT, thereby arriving at the control signal CS, shown in the bottom graph. The delay time DT may be compensated in several ways, e.g. by a predetermined phase shift and/or by applying a low-pass filter in the control algorithm or loop. This results in a control signal CS which is substantially anti-phase with the sensor data SD, and thus with the actual vibration in the first beam <NUM>. In consequence, in step iv vibrational energy is efficiently transferred to the vibration absorption body. As described above this transfer is designed to be one-way only to prevent vibrational energy from returning to the first beam <NUM>.

It will also be appreciated that in this document the terms "comprise", "comprising", "include", "including", "contain", "containing", "have", "having", and any variations thereof, are intended to be understood in an inclusive (i.e. non-exclusive) sense, such that the process, method, device, apparatus or system described herein is not limited to those features or parts or elements or steps recited but may include other elements, features, parts or steps not expressly listed or inherent to such process, method, article, or apparatus. Furthermore, the terms "a" and "an" used herein are intended to be understood as meaning one or more unless explicitly stated otherwise. Moreover, the terms "first", "second", "third", etc. are used merely as labels, and are not intended to impose numerical requirements on or to establish a certain ranking of importance of their objects.

Claim 1:
A method of inkjet printing an image on a print medium supported on a medium support surface (<NUM>, <NUM>'), the method comprising the steps of :
- moving the print medium and a print head assembly (<NUM>, <NUM>') with respect to one another, resulting in a vibration in a print head support member (<NUM>, <NUM>'), which print head support member (<NUM>, <NUM>') extends over the medium support surface (<NUM>, <NUM>') and which print head support member (<NUM>, <NUM>') supports the print head assembly (<NUM>, <NUM>');
- detecting the vibration in the print head support member (<NUM>, <NUM>') by means of a vibration sensor (<NUM>, <NUM>'),
characterised in that the method further comprises the steps of:
- controlling an actuator (<NUM>, <NUM>') based on the sensed vibration to exert a force on the print head support member (<NUM>, <NUM>') to dampen the vibration in the print head support member (<NUM>, <NUM>') by at least partially transferring the vibration in the print head support member (<NUM>, <NUM>') to a vibration absorption body (<NUM>, <NUM>'); and
- inkjet printing an image on the print medium by means of the print head assembly (<NUM>, <NUM>').