Apparatus for driving printing head of wire-dot impact printer

Abstract: An apparatus for driving a printing head of a wire-dot printer, the apparatus including a plurality of electroexpansive elements (2) for driving respective printing wire (4) cooperatively constituting a wire-dot matrix. The electroexpansive element (2) is expanded and shrunk by an electrical charge/discharge, to move the printing wire (4). The times for the charge T1 and discharge T2 are: in the case of continuous dots, T1 = A, T2 = B' for the first dot, T1 = A', T2 = B' for later dots, and T1 = A', T2 = B for the last dot, and in the case of a single dot, T1 = A, T2 = B, provided that A > A', and B > B'.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a wire-dot printer, and more particularly, to an apparatus for driving a printing head of such a wire-dot printer, which apparatus includes actuating devices for driving dot-impact wires or rods, comprising electroexpansive elements which are expanded and shrunk by an electrical charge and discharge, respectively. 2. Description of the Related Art Recently, high-speed wire-dot printing heads have become more widely used, and accordingly, to drive the dot-impact wires of such a high-speed printing head, an actuating means comprising electroexpansive elements has been developed and used instead of the usual electromagnetic type driving elements. For example, U.S. Patent No. 4,435,666 and page 92 of a publication "NIKKEI (Japan Economic) MECHANICAL" issued on March 12, 1984, suggest that a printing head including such electroexpansive elements can be used. This electroexpansive element is made by the following steps of preparing a plurality of green sheets made of piezo-electric ceramics, forming a metal paste film on one of the surfaces of each of the green sheets, to form an inner electrode, and laminating and sintering the plurality of green sheets. To make a printing head using such an actuating device, the provision of a means for effectively enlarging a very small displacement of such an electroexpansive element is essential. Further, very sophisticated drive means is required to meet the requirements for high speed wire-dot printing. SUMMARY OF THE INVENTION An object of the present invention is to provide an apparatus for driving a printing head of a wire-dot printer, this apparatus including electroexpansive elements for driving dot-impact wires or rods, and capable of effectively enlarging a very small displacement of the electroexpansive elements to drive the dot-impact wires or rods, by appropriately setting the timing for the charging and discharging of the electroexpansive elements. Another object of the present invention is to provide an apparatus for driving the printing head of a wire-dot printer, which apparatus provides a stable operation of the printing wires to thereby improve the printing quality. According to the present invention, there is provided a an apparatus for driving a printing head of a wire-dot printer, this apparatus including a plurality of electroexpansive elements for driving respective dot-impact printing wires, which cooperatively constitute a wire-dot matrix, wherein each of the electroexpansive elements is expanded and shrunk by an electrical charge thereof and electrical discharge therefrom, respectively, to move an impact printing wire connected to the electroexpansive element to thereby conduct a printing operation: This apparatus is characterized in that it comprises a means for detecting an existence of a dot, in each printing cycle, to determine whether continuous dots appear throughout continuous printing cycles or only a single dot appears in the printing cycle, and a means for setting the times for an electrical charge to and discharge from the electroexpansive elements, as follows; in the case of the continuous dots, T1 = A , T2 = B' for the first dot, T1 = A' , T2 = B' for the second or later dot or dots, and T1 = A' , T2 = B for the last dot, and in the case of the single dot, T1 = A , T2 = B , provided that T1 is a time for an electrical charge, T2 is a time for an electrical discharge, A > A', and B > B'. In this invention, if the dot appearance is continuous, the discharge is completed before the electroexpansive element is fully discharged, and therefore, the shrinkage of the element does not return it to its initial position, whereby an overshoot of the impact printing wire is prevented. In the next charge time, as the charge time is reduced in response to the incomplete discharge during the previous cycle, the mechanical parts of the printing head are not subjected to an excess load and thus the amplitude of the printing wire can be reduced. Also, at the appearance of the last dot of the continuous dots, the discharge time is not shortened, and thus a stable and reliable operation of the printing wire is ensured. In another aspect of the present invention, there is provided an apparatus for driving a printing head of a wire-dot printer having, which apparatus includes a plurality of electroexpansive elements for driving dot-impact printing wires, respectively, which cooperatively constitute a wire-dot matrix, and each of these electroexpansive elements is expanded and shrunk by an electrical charge thereto and discharge therefrom, respectively, to move an impact printing wire connected to the electroexpansive element, in such a manner that a motion of the electroexpansive element is enlarged by an enlarging means and transmitted to the impact printing wire to conduct a printing operation: This apparatus is characterized in that it comprises a means for controlling the times for an electrical charge to and discharge from the electroexpansive elements in such a manner that the electrical charge is continued once it is started at (a) so that the impact printing wire performs an impact operation, and after the impact operation and immediately before (c'), when the impact printing wire reaches a maxims retracted position (c), the electric discharge from said electroexpansive element is started. In this aspect, since the shrinkage of the electroexpansive element is started immediately before the printing wire reaches the most retracted position (c), a force for moving the printing wire forward due to the remaining energy and an opposite force for moving it in the opposite direction due to the shrinkage of the electroexpansive element are mutually balanced, and thus the remaining energy is considerably reduced. Therefore, a kinetic energy of the printing wire, per se, is almost extinguished, and therefore, the printing wire can be quickly returned to its initial position. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a partial perspective view of a printing head, particularly of an actuator for driving dot-impact wires or rods of a printer; Figure 2 is a schematic view of the printing head and a block diagram illustrating a drive apparatus for actuating dot-impact wires according to the present invention; Figure 3 is a diagram illustrating an operation of an electroexpansive element; Figure 4 illustrates operations of a printing head driven according to the prior art and its improvements thereof; Figure 5 illustrates operations of a printing head driven according to the present invention; Figure 6 is a schematic view of a printing head according to a second embodiment of the present invention; Figure 7 illustrates an operation of an electroexpansive element when electrically charged; and Figure 8 illustrates an operation of the second embodiment of a printing head of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Figs. 1 and 2, which illustrate a printing head of a dot-impact printer according to the present invention, the printing head, generally indicated by reference numeral 10, comprises a substantially cylindrical housing 20 and a plurality of actuators 30 arranged radially in the cylindrical housing 20. Each of the actuators 30 comprises a base frame 1, an electroexpansive element 2, a movable member (or armature) 3, an impact printing wire or rod 4, and hinge members 5 and 6. The frame 1 is substantially L-shaped and has a base 11a and a side wall 11b extending upward and substantially perpendicular to the base 11a. The electroexpansive element 2, such as a piezo-electric device, has a base portion 2a which is rigidly mounted on the frame base 11a, and therefore, the top free end of the electroexpansive element 2 is displaced upward when an electric power is charged to the element 2. The armature 3 is connected at one end to a top of the side wall 11b, by the hinge 5, and to the top of the electroexpansive element 2 by the hinge 6, which is positioned relatively near to the hinge 5. Therefore, a displacement of the electroexpansive element 2 is enlarged by the armature 3 and transmitted to the impact printing wire 4 fixed to the top end of the armature 3. A plurality (for example, 24 x 24 dots) of such printing wires 6 driven by the respective actuators 30 constitute a wire-dot matrix, as well known in the prior art. As shown in Fig. 3, in one printing cycle, an electric power is charged to the electroexpansive element 2 for a predetermined time T1. In this case, the upper portion of the electroexpansive element 2 is displaced upward, and therefore, the armature 3 is turned in the counterclockwise direction (in Fig. 2) about the hinge 5. Accordingly, the displacement of the electroexpansive element 2 is enlarged by the armature 3 and transmitted to the impact printing wire 6, which is moved in the upward direction as shown by an arrow P to conduct a dot-printing. In Fig. 3, after the predetermined time T1, the electric power is discharged from the electroexpansive element 2 for a predetermined time T2, and thus the armature 3 is shrunk, and accordingly returned in the clockwise direction (in Fig. 2) to its original position. Figure 4 illustrates, the operation of the printing heads in the prior art and improvements thereto, in which the common abscissa of (1) to (5) indicate the time (t). In Fig. 4, (1) shows the print pattern with reference to continuous or discontinuous dots, wherein a solid circle indicates an existence of a print dot and a dotted circle indicates a nonexistence of a print dot; (2) refers to the charging and discharging, i.e., the voltage applied to the electroexpansive element, in the prior art; (3) refers to the displacement of the printing wire in the prior art; (4) refers to the charging and discharging, i.e., the voltage applied to the electroexpansive element, in an improved printing head disclosed in Japanese patent Application No. 63-282369, filed on November 10, 1988, by the assignee of this application; and (5) refers to the displacement of the wire of this improved printing head. As understood from (2) and (3), in the printing head of the prior art, the electrical charging and discharging of the electroexpansive element is conducted in the same manner as in Fig. 3, regardless of the existence of continuous or discontinuous dots. In the case of a discontinuous or single dot, the top of the armature 3 is slightly higher than the initial position when the wire 4 is returned, i.e., a slight overshoot of the armature 3 occurs. In the next cycle, however, a dot does not exist, and therefore, the electrical charging or discharging is not conducted, and thus the overshoot of the armature 3 no longer has any affect. In the case of the continuous dots, however, immediately after the top of the armature 3 overshoots when the wire 4 is returned, the electrical charging and discharging are conducted for the next cycle, and therefore, the displacement of the wire 4 by the second charge becomes larger than that due to the previous charge, and thus the overshoot of the armature 3, i.e., the amplitude of the wire 4, becomes larger and larger. Finally, the accumulated overshoot becomes significantly large at the last dot of the continuous dots, and therefore, at the next cycle in which there is no dot, although an electric charge is not applied to the electroexpansive element, the wire 4 may be moved in the printing direction due to the energy accumulated of the armature 3 by the excess stress imposed by the overshooting, and accordingly, a ghost dot may appear at a point C, which reduces the print quality. According to the improvements shown by (4) and (5), in the case of the discontinuous (single) dot, the charge time A and the discharge time B are the same as in (2), but in the case of the continuous dots, the discharge is completed sooner, i.e., the discharge time B' is a shorter than the time B. The operation (the amplitude) of the wire 4 thus is improved, compared to the above-mentioned case (3). Nevertheless, when considering the discharge operation, in some printing cycles a full discharge is made, but in other cycles the discharge operation is completed before a full discharge is obtained. Accordingly, the initial conditions when operating the wire become uneven, and therefore, a reliable operation of the wire cannot be expected, particularly in a last half D of the cycle of continuous dots. According to the present invention, as shown in Fig. 2, the printing head controller comprises a data input line buffer 10, a head actuator (actuating time set), a drive circuit 12, a print pattern detector 13, and a dot detector (actuating time set). Figure 5, illustrates the operation of the printing head of this invention. In Fig. 5, the common abscissa from (1) to (3) indicates the time (t). Also, in Fig. 5, (1) shows the same dot pattern as in Fig. 4, with reference to the continuous or discontinuous dots, i.e., the solid circle indicates the existence of a print dot and the dotted circle indicates the nonexistence of a print dot; (2) refers to the charging and discharging, i.e., the voltage applied to the electroexpansive element, in this invention; and (3) refers to the displacement of the printing wire. In this invention, in the case of the discontinuous (or single) dot, the charge time T1 and the discharge time T2 are set in the same manner as in the prior art, i.e., T1 = A , and T2 = B , but in the case of the continuous dots, the operation is as follows. At the first dot the charge time T1 is still the same as A, but the discharge is completed sooner, i.e., the discharge time T2 = B' (B > B'). At the second dot and thereafter, except for the last dot, the charge time T1 and the discharge T2 are both completed sooner, i.e., the charge time T1 = A' (A > A') and the discharge time T2 = B' (B > B'). At the last dot, only the charge is completed sooner and the discharge time is not shortened, i.e., the charge time T1 = A' (A > A') and the discharge time T2 = B . As mentioned above, with the control according to this invention, if the dot is continuous, the discharge is completed before the electroexpansive element is fully discharged, and therefore, the shrinkage thereof does not reach the initial position, thereof and thus the overshooting the printing wire is prevented. At the next charging, since the charge time is shortened due to the incomplete discharge in the previous cycle, the mechanical parts of this printing head are not subjected to an excess load and the operation wave (amplitude) of the wire can be lowered. Also, at the last dot of the continuous dots, the discharge time is not shortened, and therefore, the electroexpansive element can be fully discharged, and thus a stable and reliable operation of the printing member, i.e., the printing wire, is obtained. The dot pattern as shown in Fig. 5 (1) with regard to the existence of continuous or discontinuous dots can be detected by the print pattern detection area 13 in Fig. 2 before the printing head is actuated and discriminated by the dot detector (actuating time set), and set the charge time T1 and discharge time T2 of either A or A' and B or B', respectively, set accordingly. Referring to another embodiment shown in Figs. 6, 7, and 8, an actuator of a printing head according to this embodiment comprises a base frame 21, an electroexpansive element 22, a leaf spring 23, a movable member (or armature) 24, and an impact printing wire or rod 25. The electroexpansive element 22, such as a piezo-electric device, has a base end which is rigidly mounted on the frame base 21 and a top free end thereof connected to the leaf spring 23 near a fulcrum point thereof, which is rigidly supported at the base frame 21, to function as a cantilever. The leaf spring 23 is rigidly connected at the free end thereof to the armature 23 having a free end thereof connected to the printing wire 25. Therefore, in one printing cycle, a displacement of the electroexpansive element 22 is enlarged by the leaf spring 23 and the armature 24, and transmitted to the impact printing wire 25, in the same manner as the previous embodiment. When an electric power is charged to the electroexpansive element 22, the voltage applied thereto is abruptly increased, since the charge is started and reaches a maximum or saturated voltage Eo after a predetermined time To, as shown in Fig. 3 or 7. Nevertheless, and electric discharge is not started immediately after the time To, at which the maximum voltage Eo is obtained, but the electric discharge is continued to maintain the maximum voltage Eo until a predetermined time period T1 at which an electric charge is stopped and an electric discharge is started. This is because, although the expansion or shrinking stroke of the electroexpansive element 22 is substantially proportional to the voltage applied thereto, and occurs at substantially the same time sequence, a remaining energy due to the deformation of the leaf spring 23 is accumulated on the enlarging mechanism including the leaf spring 23 and the armature 24, and therefore, the timing of the electric charging and discharging must be altered. Thus, after the voltage applied to the electroexpansive element 22 reaches the maximum value and is saturated, and when the printing wire 25 reaches the maximum forward stroke at an impact point thereof, an electric discharge is started. According to the electric charge and discharge timing as mentioned above, however, when the printing wire 25 reaches the maximum forward stroke thereof at an impact point, an electric discharge is started. Therefore, a force for returning the wire due to a vibration energy remaining in the printing wire itself and a force for returning the wire due to the shrinkage of the electroexpansive element at the discharge timing thereof are accumulated, and therefore, the wire is returned with a relatively large energy to overshoot the initial position thereof, an overshoot occurs. Thus the amplitude of the wire 25 becomes larger and the accumulated overshoot causes an unstable or unreliable operation of the wire, to thereby reduce the printing quality. As shown in Fig. 7, when an electric power is charged to the electroexpansive element 22, the voltage applied thereto is abruptly increased, since the charge is started, and reaches a maximum or saturated voltage Eo. This maximum voltage Eo is maintained for a predetermined time. The electroexpansive element 22 is expanded according to the voltage applied thereto and the movement thereof is enlarged and transmitted via the leaf spring 23 and the armature 24 to move the printing wire 25 upward from the initial position (a) thereof. After the voltage applied to the electroexpansive element 22 reaches the maximum value Eo and the electroexpansive element 22 is almost fully expanded, the printing wire 25 still continues to move upward due to a kinetic energy accumulated on the leaf spring 23 and the armature 24, and comes over a central line (d) to reach a maximum forward stroke point (b) at which an impact or printing operation is conducted. At this maximum forward stroke point (b), however, the leaf spring 23 and the armature 24 still hold the vibration energy by which the printing wire 25 is to be moved in the opposite direction, i.e., downward. Also, the printing wire 25 moves upward again from a most retracted point (c), and thus the vibration thereof about the central line (d) is continued and the amplitude thereof is reduced to finally stop on the central line (d). According to this embodiment, after the printing wire 5 reaches the maximum forward stroke point (b) and an impact operation is conducted, and immediately before the printing wire 25 reaches the most retracted point (c), i.e., at a point (c') as shown in Fig. 8, electric discharge from the electroexpansive element 22 is started. Thus, since the shrinkage of the electroexpansive element 22 is started immediately before the printing wire 25 reaches the most retracted point (c), a force for moving the printing wire 25 forward (upward) due to the remaining energy and an opposite force for moving it downward due to the shrinkage of the electroexpansive element 22 are mutually balanced, so that the remaining energy is considerably reduced. Therefore, as shown in Fig. 8, the kinetic energy of the printing wire 25 perse is almost extinguished, and therefore, the printing wire 25 can be quickly returned to the initial point (a) thereof. In the above-mentioned embodiment, the leaf spring 23, supported as a cantilever on the frame 21, and the armature 24 cooperatively constitute an enlarging means of which the remaining energy is accumulated, but this invention is not limited to such an enlarging means, and is also applicable to a wire-dot printer having another type of enlarging mechanism in which a timelag between a motion of the electroexpansive element 22, which expands and shrinks according to an electrical charge and discharge, and a movement of the printing wire 25, which reciprocally operates to conduct an impact printing, occurs.

Claims: An apparatus for driving a printing head of a wire-dot printer, said apparatus including a plurality of electroexpansive elements (2, 22) for driving respective dot-impact printing wires (25), which cooperatively constitute a wire-dot matrix, each of said electroexpansive elements (2, 22) being expanded and shrunk by an electrical charge thereof and discharge therefrom, respectively, to move an impact printing wire (25) connected to the electroexpansive element (2, 22) to thereby conduct a printing operation; characterized in that said apparatus comprising means (14) for detecting an existence of dot, in each printing cycle, to determine whether continuous dots appear throughout continuous printing cycles or only a single dot appears in a printing cycle, and means (14) for setting times for an electrical charge of and discharge from said electroexpansive elements, as follows; in a case of continuous dots, T1 = A , T2 = B' for a first dot, T1 = A' , T2 = B' for a second or later dot or dots, and T1 = A' , T2 = B for a last dot, and in a case of a single dot, T1 = A , T2 = B , provided that T1 is a time for an electrical charge, T2 is a time for an electrical discharge A > A', and B > B'. A driving apparatus as claimed in claim 1, wherein said apparatus further comprises a frame (1) and a movable member (3, 23) having one end defining a first pivotal point (5) pivotably connected to the frame (1) and an other end connected to said impact printing wire (4), and said electroexpansive element (2, 22) has a base end rigidly connected to the frame (1) and a free end pivotably connected to said movable member (3, 23) at a second pivotal point (6) between said first pivotal point and said other end thereof, so that a motion of said electroexpansive element (2, 22) is enlarged and transmitted to said impact printing wire (4). A driving apparatus as claimed in claim 2, wherein a distance from said first point (5) to said second point (6) is smaller than a distance from said second point (6) to said other end of said movable member (3) to which said impact printing wire is connected. A driving apparatus as claimed in claim 2, wherein said frame (1) is substantially L-shaped, having a base (11a) and a side wall (11b) extending substantially perpendicular to said base (11a), said electroexpansive element (2) is rigidly mounted on said base, and movable member is pivotably connected to a top of said side wall at said first pivotal point. An apparatus for driving a printing head of a wire-dot printer, said apparatus including a plurality of electroexpansive elements (22) for driving dot-impact printing wires (25), respectively, which cooperatively constitute a wire-dot matrix, each of said electroexpansive elements (22) being expanded and shrunk by an electrical charge thereof and discharge therefrom, respectively, to move an impact printing wire (25) connected to the electroexpansive element (22) in such a manner that a motion of said electroexpansive element is enlarged by an enlarging means (23, 24) and transmitted to said impact printing wire to thereby conduct a printing operation; chracterized in that said apparatus comprises a means for controlling times of an electrical charge to and discharge from said electroexpansive element (22), in such a manner that the electrical charge is continued once started at an initial position (a), so that said impact printing wire (25) performs an impact operation, and after said impact operation and immediately before (c') said impact printing wire (25) reaches a maximum position (c), said electric discharge from said electroexpansive element (22) is started, provided that said maximum retracted position (c) is defined as a point at which the impact printing wire would be most retracted, if the electrical charge to the electroexpansive element is continued. A driving apparatus as claimed in claim 5, wherein said apparatus further comprises a frame (21) and a leaf spring (23) having one end supported to said frame (21) and the other end connected via an armature (24) to said impact printing wire (25) in such a manner that said leaf spring and said armature cooperatively constitute said enlarging means. A driving apparatus as claimed in claim 6, wherein said leaf spring (23) constitutes a cantilever. A driving apparatus as claimed in claim 6, wherein said frame is (21) substantially L-shaped, having a base and a side wall extending substantially perpendicular to said base, said electroexpansive element (22) is rigidly mounted on said base, and said leaf spring (23) is rigidly supported to said side wall of said frame (21). An apparatus for driving a printing head of a wire-dot printer, said apparatus including a plurality of electroexpansive elements (2) for driving respective dot-impact printing wires (4), which cooperatively constitute a wire-dot matrix, each of said electroexpansive elements being expanded and shrunk by an electrical charge thereof and discharge therefrom, respectively, to move an impact printing wire connected to the electroexpansive element to thereby conduct a printing operation; characterized in that said apparatus comprising means (14) for detecting an existence of dot, in each printing cycle, to determine whether continuous dots appear throughout continuous printing cycles or only a single dot appears in a printing cycle, and means (14) for setting times for an electrical charge of and discharge from said electroexpansive elements, as follows; in a case of continuous dots, T1 = A , T2 = B' for a first dot, and in a case of a single dot, T1 = A , T2 = B , provided that T1 is a time for an electrical charge, T2 is a time for an electrical discharge, and B > B'.

Inventors: HOSOKAWA KOUJI, YANO AKIO SEIJYO-YAYOI CORP, HOSOKAWA, KOUJI, YANO, AKIO, SEIJYO-YAYOI CORP.
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: B41J2
IPC codes: B41J2