WO2010089234A1

WO2010089234A1 - A print head and a method for measuring on the print head

Info

Publication number: WO2010089234A1
Application number: PCT/EP2010/050903
Authority: WO
Inventors: David D.L. Wijngaards
Original assignee: Oce-Technologies B.V.
Priority date: 2009-02-03
Filing date: 2010-01-27
Publication date: 2010-08-12

Abstract

Ink Jet print head (4a,4b,4c,4d) comprising a pressure chamber (201), the pressure chamber having a deformable wall (210), an actuator (202) operationally coupled to the pressure chamber for generating a pressure wave in a fluid present in the pressure chamber, a sensor for determining a pressure signal from the fluid in the pressure chamber, characterized in that the sensor comprises a strain gauge (415), the strain gauge separate from the actuator and being in operative coupling with the deformable wall, for detecting a deformation of the deformable wall, and a readout system (413) operationally connected to the strain gauge. The invention also comprises a method for determining a pressure signal from a fluid in a pressure chamber, the pressure chamber having a deformable wall, said pressure chamber being part of a print head, said method comprising the step of detecting a strain of the deformable wall for determining a deformation of the deformable wall.

Description

A print head and a method for measuring on the print head

The invention relates to an ink jet print head comprising a pressure chamber, the pressure chamber having a deformable wall, an actuator operationally coupled to the pressure chamber for generating a pressure wave in a fluid present in the pressure chamber, and a sensor for determining a pressure signal from the fluid in the pressure chamber.

Ink jet print heads with an actuator and a sensor are well known from the prior art. A pressure chamber of such a print head is operationally connected to an actuator. By actuating the actuator, a volume change is achieved in the pressure chamber. The actuation results in a pressure wave in the fluid in the pressure chamber and, provided it is strong enough, leads to a drop of fluid being ejected from the pressure chamber.

Once the pressure wave has become sufficiently damped, the associated actuator may be re-actuated to eject another fluid drop. After the actuation pressure in the pressure chamber may be determined as a function of the time lapsed since the actuation, to get information about the state of the fluid in the pressure chamber and about the state of the pressure chamber itself. For example, air bubbles in the pressure chamber, mechanical disturbances of the pressure chamber or pollution in the pressure chamber may be determined from the determined pressure signal. Pressure in the pressure chamber may be determined using the sensor. The pressure signal may be analyzed for obtaining the above-mentioned information about the state of the fluid and/or pressure chamber. Herein, inkjet is intended to refer to ejection of droplets of a fluid, not only including ink, but also including other fluids such as, for example, molten metals, chemicals, water, etc.

From patent application US 7527363 a print head is known in which a piezoelectric element comprises a drive piezoelectric body section for contributing to discharge liquid droplets and a determination piezoelectric body section for determining a pressure in a pressure chamber of the print head. When the drive piezoelectric body section is actuated a liquid droplet is discharged resulting in a pressure wave in the pressure chamber. The pressure wave effectuates the determination piezoelectric body section in such a manner that the determination piezoelectric body section generates a voltage corresponding to a distortion in the piezoelectric element. The height of the voltage is dependent on the degree of distortion of the piezoelectric element.

From patent application US 7387374 a print head is known in which a piezoelectric element comprises an electromechanical transducer for drive and a mechanoelectrical transducer for determination. When the electromechanical transducer is actuated a liquid droplet is discharged resulting in a pressure wave in the pressure chamber. The pressure wave effectuates the mechanoelectrical transducer in such a manner that an electrical charge is generated in the mechanoelectrical transducer.

From patent application EP 1057634 a liquid discharge head is known in which heat generating bodies are arranged along non-deformable walls of a bubble generating area. A sensor for measuring a pressure caused by an air bubble in the bubble generating area is disposed on a solid structure portion which is protruding from a wall of the bubble generating area. The sensor is therefore not integrated in the wall but greatly surrounded by the liquid.

From patent application US 7497543 a print head is known in which a piezoelectric element comprises a piezoelectric actuator on a wall of a pressure chamber and a pressure sensor on a wall of the pressure chamber so as to oppose the piezoelectric actuator. A pressure wave in the pressure chamber generates a distortion of the pressure sensor which generates a voltage, the height of which is dependent on the degree of distortion of the pressure sensor.

From patent application US 6565172 a piezo-resistive thermal detection apparatus for detecting a temperature of ink inside an inkjet print head.

From patent applications US 7232199 and US 5757392 a print head is known in which a piezoelectric element functions as an actuator as well as a sensor. A disadvantage is that while the piezoelectric element is actuating, it cannot sense, and while the piezoelectric element is sensing, it cannot actuate, because a switch mechanism controls the separation of the actuating and sensing function. This is particularly a problem when the frequency of jetting ink drops is increasing, e.g. in an ink jet print head based on micro-electromechanical systems, also known as MEMS. The objective of the present invention is to obviate the above problems and to integrate a sensor in an ink jet print head. According to the invention, this objective is achieved by an ink jet print head according to the preamble, characterized in that the sensor is separated from the actuator and comprises a strain gauge, the strain gauge being in operative coupling with the deformable wall, for detecting a deformation of the deformable wall, and a readout system (413) operationally connected to the strain gauge for measuring a resistance change of the strain gauge. A strain gauge is a piece of material of which an electric property changes, when a strain in the material changes. The sensor may register a resistance change of the strain gauge by means of the readout system, however, depending on the type of strain gauge and the type of readout system, a capacitance or an inductance may be registered instead of the resistance change of the strain gauge.

From patent application US 5861558 a pressure sensor is known which has elements for measuring strain on the surface of a diaphragm caused by a pressure of a fluid. The thickness of the diaphragm is selected for a pressure range to be measured for a minimum pressure of about 100 kPa. Such a pressure sensor is not suitable for measuring a pressure signal lower than 100 kPa, which is actually part of the range of the print head being invented according to this application. Further, the pressure sensor is too large for application and integration in an inkjet print head.

The strain gauge has been arranged on a deformable wall of the pressure chamber and the strain gauge outputs an output signal dependent on a strain of the deformable wall. In particular when the actuator is actuated causing a pressure wave in the fluid of the pressure chamber, the deformable wall is deformed by the pressure wave. Due to the deformation of the deformable wall the strain changes and the output signal of the strain gauge changes. Particularly, the resistance of the strain gauge changes and the readout system outputs a different output signal. The output signal may be used to determine the state of the pressure chamber, e.g. the presence of pollution or an air bubble or a mechanical defect in the pressure chamber. Such a determination may take place in a control system, which reads out the output signal. The sensitivity of the output signal may be optimized by placing the strain gauge in specific regions of the deformable wall, depending on the expected deformation pattern of the deformable wall.

Such an arrangement of a strain gauge may be particular suitable for a print head which has been manufactured with MEMS structures. MEMS allows easy integration of electrical and electronic components in the print head, such as diodes and resistors. The integration of a strain gauge in an according to MEMS technology produced print head is also particular convenient. A strain gauge is easier to be manufactured by means of MEMS structures, since a strain gauge does not need any additional electrodes in contrast with a sensing piezoelectric element.

In an embodiment according to the invention a print head with a sensor comprises a strain gauge and therefore may be easily arranged inside or on the surface of the deformable wall of the pressure chamber. The arrangement may be realized in such a way that the sensor is completely embedded in the deformable wall. An advantage of this embedding is that the pressure chamber wall still proportions equal to proportions of a wall in which no sensor is embedded. In another embodiment at least a part of the sensor is embedded in the deformable wall. In yet another embodiment the sensor may be on top of the surface of the deformable wall of the pressure chamber. In case that the surface of the deformable pressure chamber wall is covered with one or more layers of a different kind of material, the sensor may be on top of the most outside layer. At least part of the sensor may also be protruding from the deformable wall into the pressure chamber. This may be advantageous because the protruding part of the sensor may be able to accurately register the pressure wave without any disturbances since the protruding part is the first part of the deformable wall that is effected by the pressure wave in the duct.

In an embodiment according to the invention a print head comprises a strain gauge of any kind of thin-film material like metal or silicon. A strain gauge is sensible for any change in the stretchiness and a space dimension, for example length, of the strain gauge. Any deformation of the deformable wall to which the strain gauge is operatively coupled automatically leads to a change in the stretch and the dimension and corresponding resistance of the strain gauge.

In an embodiment according to the invention the strain gauge is embedded in the deformable wall of the pressure chamber, which wall is made of silicon material. This has the advantage that the wall is functioning as an electrical isolator for the strain gauge. Another advantage is that no additional layers need to be added to the printhead, so no additional mechanical load is applied on the deformable wall. In this way a magnitude of the deformation of the deformable wall is not influenced or decreased. Hence, the efficiency of the actuator is not influenced or decreased.

In an embodiment according to the invention the strain gauge is arranged on a deformable wall of the pressure chamber, which wall is made of another material than silicon material, such that the deformable wall is separated from the actuator by a thin dielectric film. The thin dielectric film has an electrically isolating effect between the deformable wall and the actuator.

The print head contains a fluid duct which may be filled with fluid and leads fluid via a smaller passage from the fluid duct to the pressure chamber to which an actuator is operatively coupled. The actuator may be actuated in order to generate a pressure wave in the pressure chamber in order to eject a fluid drop from the pressure chamber. This pressure wave may be determined in order to detect air bubbles or pollution in the pressure chamber.

However, air bubbles may also emerge in the fluid duct which is leading fluid via the smaller passage from the fluid duct to the pressure chamber. These air bubbles may not be detected and localized by the determining of the pressure wave in the pressure chamber. Therefore the following embodiments are provided. A pressure wave may be generated and determined in the fluid duct in order to yield information about the presence of air bubbles in the fluid duct or about the fluid level in the fluid duct. Several embodiments of such an embodiment may be constructed.

In one embodiment a PVDF (Polyvinylidene Fluoride) foil is used with piezoelectric properties. To give PVDF foil its piezoelectric properties, it is mechanically stretched to orient the molecular chains and then it is poled under tension. This foil may be used to generate and register a pressure wave in the fluid duct in order to detect the presence of an air bubble. The main difference between PVDF and other materials is the fact that PVDF is flexible and can be formed into a variety of shapes to suit special applications, e.g. PVDF can be used as a coating on the walls of an ink duct. The fact that PVDF can be bent and formed into complex shapes - it is a material that can be formed to some very complex shapes - allows some previously very costly transducers to be made inexpensively.

In another embodiment a surface acoustic wave (SAW) transducer is put on a compliant membrane which may generate and register a pressure wave in order to detect the presence of an air bubble. In a SAW transducer a conversion from electrical into mechanical energy takes place upon input of electrical energy in the transducer and a conversion of mechanical to electrical energy occurs upon input of mechanical energy. The determination of the electrical signal upon receipt of mechanical energy from a pressure wave makes it possible to use the SAW transducer as a sensor.

The invention also relates to a method for determining a pressure signal from a fluid in a pressure chamber. The pressure chamber has a deformable wall and is part of a print head. The method comprises a step of detecting a strain of the deformable wall for determining a deformation of the deformable wall.

The invention will now be further explained with reference to the following examples.

Fig. 1 is a schematic view of an inkjet printing apparatus.

Fig. 2a is a schematic top view for illustrating a print head comprising a pressure chamber and an actuator.

Fig. 2b-2c are cross sections of the print head according to Fig. 2a.

Fig. 3a is schematic cross-sectional view for illustrating a print head comprising a pressure chamber in which a sensor has been embedded in a deformable wall of the pressure chamber.

Fig. 3b is a schematic top view of the print head according to Fig. 3a.

Fig. 4a-4c are schematic views for illustrating possible arrangements of a strain gauge relative to the deformable wall. Fig. 5a is a schematic top view illustrating a fluid duct provided with SAW transducers.

Fig. 5b is a schematic cross-sectional view illustrating the same fluid duct as in Fig. 5a.

An inkjet printing apparatus is shown in Fig. 1. According to this embodiment, the inkjet printing apparatus comprises a roller 1 used to support a receiving medium 2, such as a sheet of paper or a transparency, and to move it along the carriage 3 in direction A. This carriage 3 comprises a carrier 5 on which four ink jet print heads 4a, 4b, 4c and 4d have been mounted. Each print head may contain its own color, in this case cyan (C), magenta (M), yellow (Y) and black (K) respectively but in an embodiment each print head may comprise a same substance to be applied onto the medium 2, for example. The roller 1 may rotate around its own axis as indicated by arrow A. In this manner, the receiving medium may be moved in a sub-scanning direction relative to the carrier 5, and therefore also relative to the print heads 4. The carriage 3 may be moved in reciprocation using suitable drive mechanisms (not shown) in a direction indicated by double arrow B, substantially parallel to roller 1. To this end, the carrier 5 is moved across guide rods 6, 7. This direction is generally referred to as the main scanning direction. In this manner, the receiving medium may be fully scanned by the print heads

4.

According to the embodiment as shown in this figure, each ink jet print head 4 may comprise a number of internal pressure chambers (not shown), each with its own exit opening (nozzle) 8. The nozzles 8 in this embodiment form one row per print head 4 perpendicular to the axis of roller 1 (i.e. the row extends in the sub-scanning direction). However, embodiments with a print head with nozzle oriented in the axial direction of the roller 1 are also included in the scope of this application. In a practical embodiment of an inkjet printer, the number of pressure chambers per print head may be greater and the nozzles may be arranged over two or more rows. Each pressure chamber comprises an actuator (not shown) that may generate a pressure wave in the pressure chamber so that an ink drop is ejected from the nozzle of the associated pressure chamber in the direction of the receiving medium. The actuators may be actuated image-wise via an associated driver circuit 9. In this manner, an image built up of ink drops may be formed on receiving medium 2.

A schematic view of an ink jet print head is shown in Fig. 2a-2c according to a first embodiment.

Fig. 2a is a top view of an ink jet print head. The print head comprises a pressure chamber 101 , which may be filled with a fluid such as ink, an actuator 102, an exit opening (nozzle) 103, a feed through 104 and a chamber wall 105. The pressure chamber 101 is enclosed by the chamber wall 105 and comprises a feed through 104 which ends up in the exit opening (nozzle) 103 for ejecting ink drops. The actuator 102 is operatively coupled to the chamber wall 105. The coupling of the actuator 102 may be arranged such that the actuator 102 is part of the chamber wall 105. The actuator may be a piezoelectric actuator, a thermal actuator or a thermoelectric actuator or any other kind of actuator. Hereinafter the actuator will be assumed to be a piezoelectric actuator. The pressure chamber 101 may be relatively small and elongated, for example about 50 micron in width and about 500 - about 2000 micron in length, or almost square, for example about 500 micron in width and about 500 micron in length. The chamber wall 105 may be made of Si or graphite being coated with TiC, for example.

A cross-section A-A' is shown in Fig. 2b.

According to the cross-section A-A' the actuator 202 comprises a first electrode 207, a second electrode 209 and a layer 208 of piezoelectric material. The layer 208 is embedded between the first electrode 207 and the second electrode 209. Also other arrangements of the electrodes with respect to the piezoelectric layer 208 are envisaged. A driver circuit 206 is connected via separate lines to the first electrode 207 and the second electrode 209. The actuator 202 is coupled to an optional dielectric layer 212. The optional dielectric layer 212 is coupled to a part of the chamber wall 105, being a deformable wall 210. The chamber wall 105 also comprises a non-deformable wall 21 1. The deformable wall 210 and the non-deformable wall 211 together form the chamber wall 105 of the pressure chamber 201. The actuator 202 may be situated on the deformable wall 210 as well on the non-deformable wall 21 1. In Fig. 2b-2c the actuator 202 is situated nearby the deformable wall 210. The deformable wall 210 is a wall of the pressure chamber 210 which deformation will be relatively large as a result of the actuation of the actuator 202. The other walls may in practice also deform, but they deform in such a manner that the deformation is small with respect to the deformation of the deformable wall 210 and preferably they do not deform. Therefore the other walls are hereinafter called non-deformable walls.

Another cross-section B-B' (Fig. 2a) is shown in Fig. 2c.

According to the cross-section B-B' the actuator 202 is actuated by the driver circuit 206. The driver circuit may establish a voltage difference between the first electrode 207 and the second electrode 209. Upon such an actuation the layer 208 of piezoelectric material is configured to bend in the direction of the pressure chamber 201. The layer 208 of piezoelectric material is actually able to bend in the direction of the pressure chamber 201 since the top part of the chamber wall 105 is a deformable wall 210. The bending of the deformable wall 210 changes the pressure on the fluid in the pressure chamber 201. If the pressure is large enough, an ink drop is ejected via the exit opening 203.

A schematic view of a second embodiment of a print head is shown in Fig. 3a-3b. Fig. 3a is a side view of the ink jet print head. A read out system 413 is connected to an electrical connection 414. The electrical connection 414 is coupled to a strain gauge 415. The strain gauge 415 is integrated in the deformable wall 410 in a meandering way. A meander of the strain gauge 415 may have a length of about 10 - about 200 micron. A resistance, a capacitance or an inductance of the strain gauge 415 depends on a strain of the deformable wall 410. Hereinafter the resistance is mentioned, but may be replaced by the capacitance or the inductance depending on the type of readout system coupled to the strain gauge. On top of the deformable wall 410 an actuator 402 is situated. When the actuator 402 is actuated by the driver circuit, the deformable wall is bent towards the pressure chamber. Since the strain gauge 415 is integrated in the deformable wall 410, it also bends. When the strain gauge 415 bends, the strain, particularly the resistance, of the strain gauge 415 changes and this change is detectable by the read out system 413, such as a Wheatstone bridge configuration, which generates a current.

The strain gauge may consist of metals or alloys like Constantan, Nichrome V, platinum alloys, lsoelastic or Karma-type alloy wires, foils, or semiconductor materials such as (doped) Si or (doped) polysilicon. Preferred alloys are copper-nickel alloys and nickel- chromium alloys, as well as typical metals used in the semiconductor manufacturing such as Al, AISi, AISiCu, TiW or TiN. Metals are preferred in case that the strain gauge is present between the deformable wall 210 and the layer 208 of piezoelectric material. An advantage of a metal strain gauge is higher design freedom and independency of the selected material of the deformable wall. A metal strain gauge may be attached to an electrically conducting surface, e.g. to a conducting oxide surface. When the strain gauge is included in the deformable wall 210 a strain gauge of doped Si or doped polysilicon is preferred, which has a much higher sensitivity than a metal strain gauge. Moreover, if the deformable wall consists of Si, the deformable wall may partially consist of doped Si, (e.g. locally doped Si) which part of doped Si may form the strain gauge. An advantage of a semiconductor strain gauge is that such a strain gauge has a high sensitivity. It is noted that a high temperature sensitivity of such a strain gauge is not relevant for an ink jet print head because temperature will be controlled accurately anyway to obtain no large viscosity variations in the ink in order to control a viscosity of the fluid to be ejected.

As soon as the printing process starts, the actuator 402 may be actuated by the driver circuit (not shown). The actuation generates a pressure wave in the fluid present in the pressure chamber. As a result of the pressure wave a fluid drop is ejected from the pressure chamber via the exit opening. Another result of the pressure wave is the deformation of the deformable wall 410. Since the strain gauge 415 is coupled to the deformable wall 410, the strain gauge 415 changes its resistance because of the deformation of the deformable wall 410. The change in resistance of the strain gauge 415 generates a signal which is provided to the read out system 413 via a suitable electrical connection 414.

The signal may preferably determined close in time to the actuation. Further it may be advantageous to situate the strain gauge 415 in the vicinity of the actuator 402. An advantage of situating the strain gauge 415 in the vicinity of the actuator 402 may be that the pressure wave caused by the actuation of the actuator 402 is sensed by the nearby strain gauge 415 at such a location that the pressure wave has a relatively large amplitude.

An advantage of the separation of the actuator 402 and the sensor, e.g. the strain gauge 415, is that the sensing by the strain gauge 415 may be real-time processing. The readout of the strain gauge 415 may occur in real-time. Since read out system 413 has been separated from the actuator 402, the strain gauge 415 may be read out even while the actuator 402 is being actuated.

The read out system 413 may take care of analyzing the signal in time before a next activation of the actuator.

A cross-section C-C (Fig. 3a) is shown in Fig. 3b. Fig. 3b shows two electrical connectors 514 coupled to the strain gauge 515. The shape of the strain gauge 515 is such that a large area, preferably the area with a maximum deformation of the deformable wall is covered in order to optimize the sensing of the pressure wave which is generated in the pressure chamber after actuation of the actuator by the driver circuit.

The read out system 413 may analyze the signal from the strain gauge 415. The analysis may have influence on a subsequent actuation signal from the driver circuit. This subsequent signal may have the same properties as the previous signal, for example if the fluid ejection from the exit opening has been successful. In case that the fluid ejection from the exit opening is not successful, the subsequent signal from the signal generator may be adapted to be different from the previous signal in order to even in this case manage successful further ejections. A desired or even required adaptation may be transmitted from the read out system 413 via a suitable connection (not shown) to the driver circuit. It is noted that a subsequent signal does not need to be for ejection of a droplet. For example, the subsequent actuation signal may be adapted for damping the residual pressure wave in the fluid of the pressure chamber.

Fig. 4a shows another embodiment of the print head. The strain gauge 615 is not integrated into the deformable wall 610, but is arranged on a surface of the deformable wall 610. A space between parts of the strain gauge 615 is filled by a filler or planarizing material 616 like adhesive. The strain gauge 615 is coupled to optional electrical connectors 614. The electrical connectors 614 are connected to a read out system 613. An actuator 617 is arranged on a surface of the strain gauge 615 and the filler material 616. Optionally a dielectric layer 619, for example a thin dielectric film, is present between the strain gauge 615 and the actuator 617.

Fig. 4b shows another embodiment similar to the embodiment shown in Fig. 4a. A difference is that the filler material 616 shown in Fig. 4a is omitted. The actuator is shaped in such a way that it is situated on a surface of a strain gauge 715 and in a space between parts of the strain gauge 715, which space is - in Fig. 4a - occupied by the filler material 616. The strain gauge 715 is coupled to optional electrical connectors 714, which are coupled to a read out system 713. A deformable wall 710 separates the pressure chamber from the strain gauge 715. Optionally a dielectric layer 719, for example a thin dielectric film, is present between the strain gauge 715 and the actuator.

Fig. 4c shows another embodiment similar to the embodiment shown in Fig. 4b. A difference is that a strain gauge 815 is arranged on a surface of the actuator. The actuator comprises a first electrode 807, piezoelectric material 808 and a second electrode 809. A read out system 813 is coupled to optional electrical connectors 814. The electrical connectors 814 are coupled to the strain gauge 815. Optionally a dielectric layer 819, for example a thin dielectric film, is present between the strain gauge 815 and the actuator.

In an embodiment in which the deformable wall is made of silicon Si, a strain gauge may be implanted, i.e. embedded in the deformable wall as shown in Fig. 3a. A strain gauge, like a p- or n-doped Si resistor strain gauge, has the advantage of being very sensitive. When the deformable wall is made of a non-silicon, e.g. an oxide or a nitride, it may be possible to deposit a thin-film metal strain gauge directly on the deformable wall, e.g. a thin-film strain gauge of NiCr, that is protected and isolated by a thin dielectric film, e.g. an isolating oxide or a nitride. Alternatively, instead of a metal strain gauge, a Polysilicon strain gauge may be deposited.

In one embodiment the strain gauge may be a polyvinylidene fluoride (PVDF) foil with piezoelectric properties. Such a PVDF foil has the before-mentioned advantageous properties.

In an embodiment, the sensitivity of the strain gauge may be optimized depending on the expected deformation pattern of the deformable wall. This may be achieved by placing the strain gauge in specific regions of the deformable wall, e.g. on the surface of the deformable wall, inside or outside the pressure chamber. More specifically, the strain gauge may be deposited at the surface of the deformable wall facing the pressure chamber or may be deposited at the surface of the deformable wall facing the actuator. Also the strain gauge may be deposited partially or wholly inside the deformable wall. Exemplary shapes of the strain gauge in combination with the deformable wall are illustrated in Fig. 3b and Fig. 4a-4c.

In Fig. 3b a rectangular piece of the deformable wall is shown (outer dashed lines in Fig. 3b), in which a strain gauge 515 (shown in partially in black lines and partially in dashed lines, partially overlapping the actuator) is arranged in such a way that the sensitivity is optimized depending on the expected deformation pattern of the deformable wall. Therefore, the strain gauge 515 is loopy draped or meandered from the edge of the rectangular piece of the deformable wall around the middle of the rectangular piece of the deformable wall. The skilled person may vary in the shape of the strain gauge as well as the position of the strain gauge on the deformable wall, for example a circular shape, an oval shape, a polygon shape or any other suitable shape.

The pressure chamber 201 as shown in Fig. 2b is connected to a fluid duct (not shown in Fig. 2b) from which the fluid is being advanced to the pressure chamber 201. Air bubbles or other pollution may be present in the pressure chamber or even in the fluid duct. Air bubbles or other pollution in the fluid duct may not be detected by analyzing the signal which is registered by the strain gauge present in the pressure chamber. In order to detect air bubbles or other pollution in the fluid duct, a pressure wave may be generated and determined in the fluid duct in order to yield information about the presence of air bubbles or other pollution in the fluid duct. It is noted that the signal may as well be analyzed for detecting other defects or obstructions preventing the print head from correct operation.

To generate and register such a pressure wave in one embodiment a resistive polyvinylidene fluoride (PVDF) foil may be used as coverage of a wall in the fluid duct. Such a PVDF foil has the before-mentioned advantageous properties.

To generate and register such a pressure wave in another embodiment a surface acoustic wave (SAW) transducer is put on a compliant membrane coupled to the fluid duct. Such a SAW transducer is able to generate and register a pressure wave in order to detect the presence of an air bubble. In a SAW transducer a conversion from electrical into mechanical energy may take takes place upon input of electrical energy in the transducer and a conversion of mechanical to electrical energy occurs upon input of mechanical energy. The determination of the electrical signal upon receipt of mechanical energy from a pressure wave makes it possible to use the SAW transducer as a sensor. A surface acoustic wave is also known as a Rayleigh wave.

In an embodiment, a voltage may be applied to the electrodes of the transducer in order to let the piezoelectric material alternately being compressed and expanded resulting in two pressure waves being generated which will propagate with acoustic velocity under the SAW transducer in both directions (see Fig. 5a and Fig. 5b). Absorbent material is placed at the edges to prevent unwanted reflections of the pressure waves. Fig. 5a shows a piezoelectric substrate with transducers and absorbers formed of absorbent material. Fig. 5b shows an electric field between the electrodes of the transducers.

By putting several transducers along a wall of the fluid duct a relation between a frequency and a period of the transducers may be derived. An attractive feature of surface acoustic wave transducers is that a frequency, which is a quotient of a propagation velocity of the acoustic wave and a wavelength of the acoustic wave, can be obtained by a photolithographic pattern formation process which can be very well controlled. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

1. Ink jet print head (4a, 4b, 4c, 4d) comprising a pressure chamber (201 ), the pressure chamber having a deformable wall (210), an actuator (202) operationally coupled to the pressure chamber for generating a pressure wave in a fluid present in the pressure chamber, a sensor for determining a pressure signal from the fluid in the pressure chamber, characterized in that the sensor is separate from the actuator and comprises a strain gauge (415), the strain gauge being in operative coupling with the deformable wall, for detecting a deformation of the deformable wall, and a readout system (413) operationally connected to the strain gauge.

2. Ink Jet print head according to claim 1 , wherein the strain gauge is a thin-film strain gauge.

3. Ink Jet print head according to any of the claims 1-2, wherein the actuator is one of a piezoelectric actuator, a thermal actuator and a thermoelectric actuator.

4. Ink Jet print head according to any of the claims 1-3, wherein the actuator is a piezoelectric actuator which is separated by a thin dielectric film (619) from the deformable wall.

5. Ink Jet print head according to any of the claims 1-4, wherein the deformable wall of the pressure chamber is made of silicon.

6. Ink Jet print head according to any of the claims 1-4, wherein the deformable wall of the pressure chamber is made of a non-silicon material, the strain gauge is a thin film metal strain gauge deposited directly on the deformable wall, the strain gauge being isolated by a thin dielectric film.

7. Ink Jet print head according to any of the claims 1-6, wherein at least a part of the sensor is at least partly embedded in the deformable wall.

8. Ink Jet print head according to any of the claims 1-7, wherein at least a part of the sensor is attached to a side of the deformable wall, which side is facing the pressure chamber.

9. Ink jet print head according to any of the claims 1-8, wherein at least a part of the sensor is protruding from the deformable wall into the pressure chamber.

10. Ink Jet print head according to any of the claims 1-8, wherein the actuator is situated between the strain gauge and the deformable wall.

1 1. Ink jet print head according to claim 1 , in which the strain gauge comprises doped Si and/or doped polysilicon.

12. Ink Jet print head according to claim 1 , in which the strain gauge consists of a metal, in particular the metal selected from the group comprising Pt and Al.

13. Ink Jet print head according to claim 1 , in which the strain gauge consists of an alloy, in particular the alloy being selected from the group comprising CuNi, NiCr, AISi,

AISiCu, TiW, TiN and NiFe.

14. A method for determining a pressure signal from a fluid in a pressure chamber, the pressure chamber having a deformable wall, said pressure chamber being part of a print head, said method comprising the step of detecting a strain of the deformable wall for determining a deformation of the deformable wall.