INK JET RECORDING DEVICE

Abstract: The device includes a frame (1) provided with a recording head (3) comprising a plurality of nozzles (33) for the emission of droplets of ink, first driving means (9) for transporting the recording head in a first direction (7) and second driving means (57, 59) for transporting a record carrier (61) in a second direction (63) transverse to the first direction in such a manner that a surface of the record carrier faces the nozzles. The nozzles (33) are arranged in a linear array (55) extending parallel to the first direction (7) with equal spacings between adjacent nozzles. The first driving means (9) is conceived for imparting to the recording head (3) an oscillating motion having an amplitude (A) at least equal to the centre to centre distance (p) of adjacent nozzles (33). The device further comprises a control unit (71) conceived to activate the recording head (3) in such a manner that the emission of droplets of ink from each nozzle (33) is correlated with the momentary position of the recording head relative to the frame (1) during the oscillating motion of the recording head. As a result, a very compact device for depositing ink dots over the entire width of the record carrier (61) is obtained.

Description: Ink jet recording device. The invention relates to an ink jet recording device including a frame provided with a recording head comprising a plurality of nozzles for the emission of droplets of ink, first driving means for transporting the recording head in a first direction and second driving means for transporting a record carrier in a second direction transverse to the first direction in such a manner that a surface of the record carrier faces the nozzles. An ink jet recording device of this kind is known from US-A-4 864 328. In the known device the recording head is transported over the whole width of the record carrier (e.g. a sheet of paper) to make it possible to deposit ink droplets on the entire surface of the record carrier. Consequently, enough space must be available in the device to make this motion of the recording head possible. Moreover, the first driving means is rather bulky and it consumes rather much energy. It is an object of the invention to provide an ink jet recording device of the kind set forth in which the first driving means can be smaller than in the known device without sacrificing the possibility to deposit ink droplets on the whole surface of the record carrier. To achieve this object, the ink jet recording device in accordance with the invention is characterized in that the nozzles are arranged in a linear array extending parallel to the first direction with equal spacings between adjacent nozzles, the first driving means being conceived for imparting to the recording head an oscillating motion having an amplitude at least equal to the centre to centre distance of adjacent nozzles, the device further comprising a control unit conceived to activate the recording head in such a manner that the emission of droplets of ink from each nozzle is correlated with the momentary position of the recording head relative to the frame during the oscillating motion of the recording head. The length of the array of nozzles can be chosen freely. It can, for example, be made equal to the width of a sheet of paper (A4). For the centre to centre distance (pitch) of the nozzles a convenient value can be chosen that makes it easy to manufacture the recording head. A pitch in the order of magnitude of 1mm has been found to be very satisfactory. The oscillating motion of the recording head makes it possible to deposit ink droplets over the whole width of the record carrier. Because the amplitude of this motion is rather small, the size and energy consumption of the first driving means can be made relatively small. A preferred embodiment of the device in accordance with the invention is characterized in that position detection means are provided for locating the momentary position of the recording head relative to the frame. The momentary position of the recording head could be deduced from the time that has elapsed after the recording head has passed a predetermined position (e.g. its rest position or one of the extreme positions of the oscillating motion), but the use of the position detection means enables a more accurate determination of the momentary position of the recording head. A further preferred embodiment of the device in accordance with the invention is characterized in that resilient mounting means are provided for mounting the recording head on the frame so that the recording head and the mounting means form a system that can perform an oscillating motion in the first direction with a given natural frequency, the oscillating motion imparted to the recording head by the first driving means having a frequency equal to the said natural frequency. In this embodiment the energy consumption is further reduced because the oscillating system formed by the recording head and the mounting means is operated in resonance. It is possible to choose the amplitude of the oscillating motion of the recording head to be substantially equal to the pitch of the nozzles in the array. However, in that case ink droplets have to be emitted during the whole period of the oscillation. As is well known, the speed of the recording head is much lower near the extreme positions of the motion than when the recording head is near its rest position. Consequently, the control unit has to time the emission of droplets in accordance with the momentary velocity of the recording head. This is possible but it requires a relatively complicated control unit. In addition, the record carrier must be transported at a relatively low speed in order to enable the deposition of droplets over its entire width. Therefore, a further preferred embodiment of the device in accordance with the invention is characterized in that the amplitude of the oscillating motion of the recording head is at least twice the centre to centre distance of adjacent nozzles, the control unit being conceived to activate the recording head in such a manner that no ink is emitted during the periods in which the recording head is at a position that is at a greater distance from its rest position than half the centre to centre distance of adjacent nozzles. In this embodiment, the emission of droplets takes place only when the recording head is in a position that is about halfway the extreme positions. The velocity of the recording head is then more or less constant so that the timing of the emission of droplets is relatively simple. Moreover, the record carrier can be kept stationary during the periods in which droplets are emitted. It can be transported during the periods in which no droplets are emitted. The recording head is heavily shaken as a result of the oscillation motion. As a result, pressure waves can be formed in the ink that is supplied to the recording head by an ink supply system including an ink container. To prevent such pressure waves, a preferred embodiment is characterized in that the ink supply system further comprises damping means for suppressing pressure waves in the ink. The damping means may comprise, for example, a resilient wall in at least a part of the ink supply system or an aperture in a wall of the ink supply system, said aperture connecting the ink supply system to the atmosphere. A very simple embodiment of the device in accordance with the invention is characterized in that the first driving means comprise a permanent magnet fixed to the recording head and a coil of electrically conductive wire fixed to the frame. These and other aspects of the invention will be apparent from the embodiments described hereinafter. Figure 1 is a schematic view of an embodiment of an ink jet recording device in accordance with the invention, Figure 2 is a cross-section of an embodiment of an ink jet recording head for the device shown in Figure 1 , Figure 3 is a section according to the line III-III of a part of the recording head shown in Figure 2, Figure 4 is a diagram illustrating the operation of the ink jet recording device in accordance with the invention, and Figures 5 and 6 schematically show some details of parts of the device shown in Figure 1. Figure 1 is a schematic representation of an ink jet recording device showing at least those parts that are necessary to provide sufficient background for understanding the invention. The device comprises a frame 1 on which a recording head 3 is mounted by means of resilient mounting means 5, in this embodiment leaf springs. The mounting means 5 permit movement of the recording head 3 relative to the frame 1 in a first direction indicated by the double arrow 7. Other resilient mounting means, such as helical springs, could also be used. The recording head 3 and the resilient mounting means 5 form a system that can perform an oscillating motion in the first direction 7 with a given natural frequency that depends on the mass of the recording head and the properties of the resilient mounting means. A first driving means 9 is attached to the frame 1 for transporting the recording head 3 in the first direction 7. The first driving means 9 may, for example, be a linear motor. The first driving means 9 imparts to the recording head 3 an oscillating motion in the first direction 7, the frequency of this motion being equal to the natural frequency of the system formed by the recording head and the resilient mounting means 5. The recording head 3 comprises a plurality of nozzles for the emission of droplets of ink, the nozzles being arranged in a linear array extending parallel to the first direction 7. Various types of recording heads can be used in a recording device according to the invention. Examples of ink jet recording heads comprising a linear array of nozzles are described e.g. in US-A-4 599 628, EP-A-0 516 188 and the copending patent application No. ... (PHN 15.079). A cross-section of a recording head described in the latter publication is shown by way of example in Figure 2. This recording head comprises a piezoelectric actuator element 1 1 in the form of an actuator plate that consists of a plurality of layers of a ceramic piezoelectric material alternated with electrode layers. As shown schematically in Figure 2, the first, third, fifth, etc. electrode layers 13 are connected to a first terminal 15 and the second, fourth, sixth, etc. electrode layers 17 are connected to a second terminal 19. The odd-numbered electrode layers 13 are interrupted in a first zone 21 so as to isolate them from the second terminal 19 and the even-numbered electrode layers 17 are interrupted in a second zone 23 so as to isolate them from the first terminal 15. Between the first and second zones 21 , 23 an active part of the actuator plate 1 1 is situated. The terminals 15, 19 receive voltage pulses from a control unit as will be discussed later. When a voltage is applied between the terminals 15 and 19, the dimension of at least the active part of the actuator plate 11 in the vertical direction in Figure 2 is varied. In other words, the actuator plate 11 changes its thickness upon application of a voltage. The direction in which the dimension of an actuator plate is changed upon application of a voltage is called its active direction. The active part of the actuator plate 11 is provided with a recess 25 that forms a pressure chamber. A nozzle plate 27 has a first face 29 that is connected to the first face 31 of the actuator plate 1 1 so as to form a first wall of the pressure chamber 25. The nozzle plate 27 comprises a plurality of nozzles 33, one of which is visible in Figure 2. This nozzle 33 extends between the pressure chamber 25 and the space surrounding the recording head 3. A base plate 35 has a first face 37 that is connected to the second face 39 of the actuator plate 1 1 so as to form a second wall of the pressure chamber 25. An ink reservoir 41 communicates with the pressure chamber 25 via an ink supply channel 43. The ink reservoir 41 and the ink supply channel 43 are formed as recesses in the second face 39 of the actuator plate 1 1. As can be seen more clearly in Figure 3, the ink reservoir 41 is a relatively wide duct interconnecting the ink supply channels 43. In order to prevent a pressure wave in one of the pressure chambers 25 from causing a rise of pressure in one or more of the other pressure chambers, each ink supply channel 43 comprises a restricted portion 43a that serves as a choke. The first face 37 of the base plate 35 covers the ink reservoir 41 and the ink supply channel 43. The base plate 35 comprises one or more filling channels 45 formed as through-holes (shown in dotted lines in Figure 2) to connect the ink reservoir 41 to an ink supply system comprising an ink container (not shown in Figures 2 and 3). The construction of the recording head 3 from the actuator plate 11 , the nozzle plate 27 and the base plate 35 is very simple. The parts are aligned by means of an alignment pin 47 that extends through alignment holes 49 provided in all three components of the recording head. After the recording head 3 has been completed, the ink reservoir 41 , the ink supply channels 43 and the pressure chambers 25 are filled with a suitable ink. When a voltage of a predetermined polarity is applied between the terminals 15 and 19, the thickness of the actuator plate 1 1 increases so that the volume of the pressure chamber 25 grows. As a result ink flows from the ink reservoir 41 through the ink supply channel 43 to the pressure chamber 25. When the voltage between the terminals 15 and 19 is reduced to zero or when its polarity is reversed, the pressure chamber suddenly contracts so that a droplet of ink is expelled through the nozzle 33. The very small cross-section of the restricted portion 43a of the ink supply channel prevents the flow of ink from the pressure chamber 25 back to the ink reservoir 41 as a result of the contraction of the pressure chamber. This serves to reduce cross-talk between the nozzles 33 of a recording head 3 via the ink reservoir 41. Cross-talk via the actuator plate 1 1 is reduced by the provision of slits 51 between the pressure chambers 25, said slits extending through the active part of the actuator plate beyond the first zones 21. Due to these slits the actuator plate 11 is split into a plurality of fingers 53, each finger comprising one of the pressure chambers 25. The electrode layers 13, 17 in adjacent fingers 53 are electrically isolated by the slit 51 between these fingers and the fingers themselves are mechanically substantially isolated by the slits. As shown clearly in Figure 3, the nozzles 33 extend in a linear array 55. The length of the array 55 can be chosen freely. Usually it is preferred to choose this length equal to the width of a sheet of paper. For paper having the size A4 this amounts to an array length of about 216 mm. If the centre to centre distance of adjacent nozzles 33 (pitch) is chosen to be 1 mm, the recording head would then comprise 216 nozzles. Of course, Figure 3 shows only a small part of the recording head 3, which part comprises only four nozzles 33. The ink jet recording device shown in Figure 1 further comprises second driving means including a pair of pinch rollers 57 driven by a motor 59. The pinch rollers 57 cooperate to transport a record carrier 61 , for example a sheet of paper, in a second direction indicated by an arrow 63. As can be seen in Figure 1 , the second direction 63 is transverse to the first direction 7. The record carrier 61 is transported in such a manner that a surface of the record carrier (the upper surface in Figure 1) faces the nozzles 33 of the recording head 3. If no record carrier 61 is present, the nozzles 33 face a waiting and servicing station 65 that serves to keep the recording head in a proper condition when the device is not in use. An ink container 67 is fixed to the frame 1 for supplying ink to the ink reservoir 41 via a pipe system that comprises a flexible tube 69. The device further comprises a control unit 71 that is conceived to activate the recording head 3 by applying voltage pulses as discussed above in such a manner that the emission of droplets of ink from each nozzle 33 is correlated with the momentary position of the recording head relative to the frame 1 during the oscillating motion of the recording head. The control unit 71 may comprise a suitably programmed microprocessor (not shown). An example of the relation between the motion of the recording head 3 and the emission of droplets of ink will now be explained with reference to Figure 4. Figure 4 is a diagram with three curves showing the position x of the recording head 3, its velocity v and its acceleration a as a function of time t. The recording head 3 itself with some nozzles 33 is shown at the top of the Figure. The centre to centre distance (pitch) of the nozzles 33 is indicated as p. As has been explained above, the recording head 3 performs an oscillating motion with a frequency f that is preferably equal to the natural frequency of the system comprising the recording head and the resilient mounting means 5. Consequently, the momentary position x(t) of the recording head relative to its rest position x 0 is given by x(t) = x 0 + A sin 2τrft where A is the amplitude of the oscillation. The amplitude A is preferably chosen to be equal to at least twice the pitch p of the nozzles 33. In the example shown in Figure 4, the amplitude is equal to approximately 2.5 times p. Because the motion of the recording head 3 is sinusoidal, there are two intervals during each period T of the motion in which x varies approximately linearly as a function of time. The first one of these intervals is indicated as Δt j and the second one as Δt 2 . During these intervals, the velocity v(t) is at a maximum and the acceleration a(t) is close to zero. Because the displacement of the recording head 3 is, for all practical purposes, a linear function of time t during the first and second intervals Δt | and Δt 2 , these intervals are very suitable for emitting droplets of ink. The position on the record carrier 61 that is reached by a droplet emitted during one of these intervals is an approximately linear function of time. Of course, the intervals have to be chosen long enough to permit the recording head 3 to travel over a distance x equal to the pitch p during the interval in order to permit the deposition of droplets over the whole width of the record carrier. Consequently, the linearity of x as a function of t can be approached more closely by increasing the amplitude A. It has been found that satisfactory results are obtained if A >2p. Between the first and second intervals Δt j and Δt 2 there is a third interval Δt 3 during which x is a non-linear function of t. This third interval is less suitable for the emission of ink droplets, but it can be used advantageously for transporting the record carrier 61. Therefore, the control unit 71 is preferably programmed so that it activates the second driving means 57, 59 during the third interval Δt 3 and the recording head 3 during the first and second intervals Δt j and Δt 2 , respectively. Consequently, no ink droplets are emitted during the third interval when the recording head 3 is at a position that is at a greater distance from its rest position x 0 than half the pitch p of the nozzles 3. During the first and second intervals ink droplets are emitted from selected nozzles 33 at moments when the recording head 3 is at such a position that one of these nozzles is opposite a predetermined position on the record carrier 61. In order to determine the moment when a droplet of ink must be emitted from one of the nozzles, the control unit can deduce the position of the recording head from the phase of the oscillating motion of the recording head as imparted by the first driving means 9. A more accurate measurement of the position of the recording head 3 can be performed if position detection means are provided for locating its momentary position relative to the frame 1. Figure 5 schematically shows a view of an embodiment of the recording head 3 in which such position detection means are provided. The position detection means comprise a first portion 73 attached to the frame 1 and a second portion 75 attached to the recording head 3. The first portion 73 is electrically connected to the control unit 71. There are many different types of position detection devices that are suitable to be used in the ink jet recording device of Figure 1 , such as optical, capacitive or magnetic devices. A few examples can be found e.g. in US-A-5 136 192 and US-A-5 279 044. The position detection device needs to sense the position of the recording head only within a relatively small distance range, approximately equal to the pitch p. The first driving means 9 (see Figure 1) may be implemented very simply by providing a system similar to a well-known loudspeaker system as shown schematically in Figure 5. This system may comprise a permanent magnet 77 attached to the recording head 3 and a coil 79 of electrically conductive wire attached to the frame 1. The coil 79 is electrically connected to the control unit 71. When it is energized, a magnetic field is generated that cooperates with the field of the permanent magnet 77 to exert a force in the first direction 7 on the recording head 3. Such a driving system is very compact. As has been explained above, the ink jet recording device shown in Figure 1 comprises an ink supply system including the ink container 67 and the flexible tube 69. The flexible tube 69 is a part of a pipe system that connects the container 67 to the recording head 3. By way of example, an embodiment of the ink supply system is shown in Figure 6. The flexible tube 69 connects the ink container 67 to a main pipe 81 that may be rigid at least in part. The main pipe 81 is connected to the ink filling channels 45 of the recording head 3 by means of relatively short connecting pipes 83. The main pipe 81 and the connecting pipes 83 are rigidly attached to the recording head 3 and they participate in the oscillating motion of the recording head. Due to this motion, pressure waves could be formed in the ink. In order to diminish the effect of such pressure waves, the ink supply system comprises damping means. Two examples of such damping means are shown schematically in Figure 6. In a first embodiment, the damping means are formed by resilient walls in the ink supply system, for example resilient portions 85 in the wall of the main pipe 81. In a second embodiment, the damping means are formed by one or more apertures in a wall of the ink supply system, for example one or more additional connecting pipes 87 that connect the ink supply system to the atmosphere, either directly of via an open vessel 89. In every case, when a pressure wave tends to build up in the ink supply system, this immediately results in an increase in the volume of the ink supply system so that the pressure is decreased again. It is possible to use both described types of damping means together or to use only one of these types. Other types of damping means may be used as well. Because the ink container 67 is fixed to the frame 1 , there is no need for a damping element such as a sponge inside the ink container. As has been explained above, it is one of the advantages of the ink jet recording device in accordance with the invention that the first driving means 9 may be very compact and uses very little energy. A further advantage is that the print resolution can be varied by choosing the number of droplets to be emitted per oscillation cycle. The resolution in the first direction 7 (the x-direction in Figure 4) is defined by p/n where n is the number of droplets emitted during the first interval Δt ] or the second interval Δt 2 . The resolution in the second direction 63 is determined by the distance over which the record carrier 61 is transported during the third interval Δt 3 . For a high resolution, this distance is preferably chosen to be very small, e.g. in the order of magnitude of the diameter of a dot of ink on the record carrier 61. As a result, slight inaccuracies in the transport of the record carrier 61 cause only an insignificant mismatch between successive lines that are printed on the record carrier. This compares favourably with some conventional recording devices where a recording head comprises an array of nozzles that extends parallel to the direction of transport of the record carrier. In these devices the record carrier is transported in steps that correspond to the length of the array of nozzles, resulting in an increased mismatch between successively printed lines. Recording devices in accordance with the invention having a high printing speed can easily be provided by designing devices with an increased number of nozzles. Another method to increase the printing speed is to increase the frequency of oscillation of the recording head 3. This can be done by modifying the mass of the recording head or, preferably, by modifying the specifications of the resilient mounting means 5. Of course, if operation in resonance is preferred, the frequency of the first drive means 9 should be adapted to the new natural frequency of the system comprising the recording head and the resilient mounting means.

Claims: CLAIMS: 1. An ink jet recording device including a frame (1) provided with a recording head (3) comprising a plurality of nozzles (33) for the emission of droplets of ink, first driving means (9) for transporting the recording head in a first direction (7) and second driving means (57,59) for transporting a record carrier (61) in a second direction (63) transverse to the first direction in such a manner that a surface of the record carrier faces the nozzles, characterized in that the nozzles (33) are arranged in a linear array (55) extending parallel to the first direction (7) with equal spacings between adjacent nozzles, the first driving means (9) being conceived for imparting to the recording head (3) an oscillating motion having an amplitude (A) at least equal to the centre to centre distance (p) of adjacent nozzles, the device further comprising a control unit (71) conceived to activate the recording head in such a manner that the emission of droplets of ink from each nozzle is correlated with the momentary position of the recording head relative to the frame (1) during the oscillating motion of the recording head. 2. An ink jet recording device as claimed in Claim 1 , characterized in that position detection means (77,79) are provided for locating the momentary position of the recording head (3) relative to the frame (1). 3. An ink jet recording device as claimed in Claim 1 or 2, characterized in that resilient mounting means (5) are provided for mounting the recording head (3) on the frame (1) so that the recording head and the mounting means form a system that can perform an oscillating motion in the first direction (7) with a given natural frequency, the oscillating motion imparted to the recording head by the first driving means (9) having a frequency equal to the said natural frequency. 4. An ink jet recording device as claimed in any one of the preceding Claims, characterized in that the amplitude (A) of the oscillating motion of the recording head (3) is at least twice the centre to centre distance (p) of adjacent nozzles (33), the control unit (71) being conceived to activate the recording head in such a manner that no ink is emitted during the periods (Δt 3 ) in which the recording head is at a position that is at a greater distance from its rest position (x 0 ) than half the centre to centre distance of adjacent nozzles. 5. An ink jet recording device as claimed in any one of the preceding Claims, including an ink supply system for supplying ink to the recording head (3), the ink supply system comprising an ink container (67) that is fixed to the frame (1), the ink supply system comprising a tube (69) that is flexible over at least a part of its length, said tube connecting the ink container to the recording head, characterized in that the ink supply system further comprises damping means (81 ,87) for suppressing pressure waves in the ink. 6. An ink jet recording device as claimed in Claim 5, characterized in that the damping means comprise a resilient wall (85) in at least a part of the ink supply system. 7. An ink jet recording device as claimed in Claim 5, characterized in that the damping means comprise an aperture (87) in a wall of the ink supply system, said aperture connecting the ink supply system to the atmosphere. 8. An ink jet recording device as claimed in any one of the preceding Claims, characterized in that the first driving means (9) comprise a permanent magnet (77) fixed to the recording head (3) and a coil (79) of electrically conductive wire fixed to the frame (1).

Inventors: PANKERT JOSEPH ROBERT RENE, ROULAUX JOHANNES RICHARD, PANKERT, JOSEPH, ROBERT, RENE, ROULAUX, JOHANNES, RICHARD
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS PATENTVERWALTUNG, PHILIPS ELECTRONICS N.V., PHILIPS PATENTVERWALTUNG GMBH
Application year: 1996
Main classification: B41J2
IPC codes: B41J2