Printing head

Abstract: A piezoelectric printing head includes an annular base frame (29) and a plurality of piezoelectric actuator assemblies (28) mounted on the base frame. Each actuator assembly includes a piezoelectric element (30) having one end rigidly secured to the base frame and another free end, an armature (31) having a base end and a tip end to which a printing wire (37) is rigidly connected, and a magnifying mechanism (32). When the piezoelectric element is electrically energized, a displacement of the piezoelectric element is magnified to be transmitted to the printing wire. Between the base frame and the base end of the armature there is provided a vibration damping buffer member (42).

Description: The present invention relates to printing heads employing piezoelectric actuation elements. Electromagnetic-type printing heads are generally unsuitable for use in high speed impact dot-matrix printers. Therefore, printing heads using piezoelectric actuators have been considered. However, the amount of displacement caused directly by the expansion and contraction of a piezoelectric element is generally very small, of the order of only several microns, and so needs to be magnified by a magnifying mechanism in order that printing operations can be carried out. A conventional piezoelectric printing head, for example that shown in US 4,874,978, includes a common base frame, a piezoelectric element, an armature, and a magnifying mechanism. A plurality of piezoelectric elements are circumferentially provided in predetermined positions, and an armature and a magnifying mechanism are provided for each piezoelectric element. The piezoelectric element has an end block which is secured to the base frame by means of adhesive. The armature has a beam, and a printing wire is secured to a tip of the beam. The magnifying mechanism includes a support spring and a movable spring that are arranged in parallel with one another. Upper end portions of the springs are secured to the armature. A lower end portion of the support spring is secured to the base frame, and a lower end portion of the movable spring is secured to the piezoelectric element. The piezoelectric printing head is mounted on a carrier, and moved on a platen together with the carrier. Printing is conducted when a predetermined piezoelectric element is driven at a predetermined time in the process of moving. Application of a voltage to a piezoelectric element causes the upper end of the piezoelectric element to be displaced upward. The magnifying mechanism is activated and the armature is rotated, so that the wire is forced to conduct a printing motion. When the printing has been completed, the supply of voltage impressed upon the piezoelectric element is removed and the armature returns to its initial position together with the piezoelectric element. In such a printing head, when the movable spring is pushed up by displacement of the piezoelectric element, an adhesive agent which connects the piezoelectric element with the base frame may be deformed by a reaction force. Accordingly, a magnification ratio or characteristic frequency of the magnifying mechanism may be lowered, thereby causing a magnifying loss which can prevent high speed printing being carried out satisfactorily. According to the present invention, there is provided a printing head comprising a plurality of actuators mounted on a frame, each actuator comprising a piezoelectric actuation element secured at a first end region thereof to the frame and at a second end region thereof to an end region of an armature carrying a printing element, wherein activation of the said actuation element causes the said end region of the armature to be displaced through a first distance in a direction from the said first end region of the actuation element to the said second end region thereof, thereby causing the printing element to be displaced through a second distance in that direction, which second distance is larger than the said first distance, characterised in that the said first end region of the actuation element is secured to the frame by means of a screw-threaded member, one end of which is attached to the said first end region by means of adhesive material interposed between the screw-threaded member and the said first end region, the screw-threaded member being adjusted to apply a pre-stressing force to the said actuation element in the direction from the said first end region to the said second end region. Preferably, the said one end of the screw-threaded member is engaged in a recess formed in a block secured to the said first end region of the actuation element, the said adhesive material being located, in the said recess, between the said one end and the said block. Alternatively, a protruding part of a block secured to the said first end region of the actuation element may be engaged in a recess formed in the said one end of the screw-threaded member the said adhesive material being located, in the said recess, between the said one end and the said block. Anti-adhesion means may be provided, on the said frame over at least a region thereof that is opposed to the said first end region of the actuation element, for preventing the said adhesive material from becoming attached to the said frame. Such anti-adhesion means may comprise a film or sheet of polytetrafluoroethylene or silicon. The pre-stressing force applied in an embodiment of the present invention can serve to reduce the aforementioned problems caused by deformation of adhesive between the piezoelectric element and the frame. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, wherein: Fig. 1 is a perspective view of a printer in which a piezoelectric printing head embodying the present invention can be used; Fig. 2 is a perspective view of a piezoelectric printing head embodying the present invention; Fig. 3 is a sectioned side view of the general structure of the piezoelectric printing head of Fig. 2; Fig. 4 is a side view of a piezoelectric actuator assembly embodying the present invention; Fig. 5 is an enlarged side view of parts of the assembly of Fig. 4; Fig. 6 is a side view of parts of a second piezoelectric actuator assembly embodying the present invention; and Fig. 7 is a side view of parts of a third piezoelectric actuator assembly embodying the present invention. Fig. 1 is a perspective view of a printer in which a piezoelectric printing head is used. The printer includes a piezoelectric printing head 21, a platen 22, and a guide 23. The piezoelectric printing head 21 is mounted on a carrier which is movable along the platen 22 and guided by the guide 23. The head 21 is moved together with the carrier so as to conduct printing. Fig. 2 is a perspective view of the piezoelectric printing head 21 of Fig. 1. The printing head 21 has a nose 24, to guide printing wires for conducting the printing operation, a heat sink 25, a printed circuit board 26, and a connector 27. Fig. 3 is a side view of the general structure of the piezoelectric printing head 21. A piezoelectric actuator assembly 128 is provided in the heat sink 25 of the piezoelectric printing head 21. Fig. 4 is a side view of a piezoelectric actuator assembly 128 embodying the present invention and including a base frame 129, a piezoelectric element 130, and an armature 131 of a magnifying mechanism 132. An end block 133 is secured to a lower end portion of the piezoelectric element 130 and is connected with the frame 129 (the specific connection structure will be described later). A plurality of piezoelectric elements 130 are circumferentially disposed at predetermined intervals around the printing head, and a magnifying mechanism 132 with an armature 131 is provided for each piezoelectric element 130. The armature 131 comprises a beam 135, a tip portion of which carries a printing wire 134, the beam 135 being integrally secured at its other end to a base portion 136. The magnifying mechanism 132 includes a support spring 137 and a movable spring 138 that are arranged to be approximately parallel to one another. The two ends of the support spring 137 are connected with the base portion 136 and the frame 129 respectively, and the two ends of the movable spring 138 are connected respectively with the base portion 136 and a block 139 (different from the frame 129). In such a piezoelectric actuator assembly 128, the end block 133 is secured to the frame 129 as shown in Fig. 5. The securing system is constituted in the following manner: a screw-threaded member (an adjustment screw) 140, the tip of which has a protruding portion 141, is screwed into the frame 129 so as to be opposed to the end block 133. A recess 142 is provided in the end block 133, and the protruding portion 141 of the screw 140 is engaged in the recess 142. The adjustment screw 140 is fastened to the end block 133 by means of an adhesive agent 143 located in the recess 142, between the protruding portion 141 and the block 133. The screw 140 is adjusted so that the piezoelectric element 130 is subjected thereby to a pre-stressing force directed upwardly along the element condition. In a printing operation, a voltage is applied to the piezoelectric element 130 through a connector 144. As a result, the upper end of the piezoelectric element 130 is displaced upward, so that the movable spring 138 is pushed up. The armature 131 is thereby caused to rotate in the direction of an arrow shown in Fig. 4 by the action of the magnifying mechanism 132. A magnified displacement is thus transmitted to the wire 134. As a result, the wire 134 is moved in the direction of arrow C by a predetermined amount. In this manner, the printing is carried out. After the printing operation has been completed, the piezoelectric element 130 and the armature 131 return to their respective initial dispositions. Since, in the embodiment of the present invention described above, the end block 133 is connected with the frame 129 in such a manner that a pre-stressing force, in the direction from the end block 133 to the armature 131, is applied to the piezoelectric element 130, compressive deformation of the previously applied adhesive agent, which caused magnification losses in prior printing heads, is less likely to occur. This facilitates high speed printing operations. The adhesive agent 143 strengthens the engagement between the end block 133 and the protruding portion 141, as compared with piezoelectric printing heads not using such adhesive. A terminal 144a of a connector 144 is connected with the common connector 27 (Fig. 3) through the printed board 26 (Fig. 3). The terminal 144a is connected with each electrode of the piezoelectric element 130 through a lead wire 145. In this way, electrical power is supplied to the piezoelectric element 130 through the connector 27. Fig. 6 is a side view of parts of a second embodiment of the present invention, in which anti-adhesion means 152 made of polytetrafluoroethylene or silicon are added to the piezoelectric actuator assembly 128 shown in Figs. 4 and 5. A region of the frame base 129 that is opposed to the lower end of the element 130 is coated with the anti-adhesion means 152 so as to prevent adhesion of the adhesive material 143 onto the surface of the frame base 129. In the embodiment, if the adhesive material 143 spills downward from the gap between the protruding portion 141 of the screw threaded member 140 and the recess 142, it does not adhere to the base 129 since it is prevented from doing so by this anti-adhesion means 152. Fig. 7 is a side view of parts of a third embodiment of the present invention, in which a sheet-like anti-adhesion member 162 made of polytetrafluoroethylene or silicon is added to the piezoelectric actuator assembly 128 shown in Figs. 4 and 5. The member 162 is located on the frame base 129 at a location below the lower end of the element 130, as shown in the drawing, so that surplus adhesive agent is prevented from adhering to the frame base 129.

Claims: A printing head comprising a plurality of actuators (128) mounted on a frame (129), each actuator comprising a piezoelectric actuation element (130) secured at a first end region thereof to the frame and at a second end region thereof to an end region of an armature (131) carrying a printing element (134), wherein activation of the said actuation element causes the said end region of the armature to be displaced through a first distance in a direction from the said first end region of the actuation element (130) to the said second end region thereof, thereby causing the printing element (134) to be displaced through a second distance in that direction, which second distance is larger than the said first distance, characterised in that the said first end region of the actuation element (130) is secured to the frame (129) by means of a screw-threaded member (140), one end of which is attached to the said first end region by means of adhesive material (143) interposed between the screw-threaded member (140) and the said first end region, the screw-threaded member (140) being adjusted to apply a pre-stressing force to the said actuation element (130) in the direction from the said first end region to the said second end region. A printing head as claimed in claim 1, wherein the said one end of the screw-threaded member (140) is engaged in a recess formed in a block (133) secured to the said first end region of the actuation element, the said adhesive material being located, in the said recess, between the said one end and the said block. A printing head as claimed in claim 1, wherein a protruding part of a block (133) secured to the said first end region of the actuation element (130) is engaged in a recess formed in the said one end of the screw-threaded member (140), the said adhesive material being located, in the said recess, between the said one end and the said block. A printing head as claimed in claim 1, 2 or 3, wherein anti-adhesion means (152, 162) are provided, on the said frame (129) over at least a region thereof that is opposed to the said first end region of the actuation element (130), for preventing the said adhesive material (143) from becoming attached to the said frame (129). A printing head as claimed in claim 4, wherein the said anti-adhesion means comprise a film or sheet (152, 162) of polytetrafluoroethylene or silicon.

Inventors: KIMURA DAISUKE, OSHIME YUJI, YAMAKI SYUICHI, KIMURA, DAISUKE, OSHIME, YUJI, YAMAKI, SYUICHI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1993
Main classification: B41J2
IPC codes: B41J2