Impact dot printing head

Abstract: An impact dot printing head comprising an arm (1) to which is connected the distal end of a printing wire (2); movement effecting means (4,5) for causing or permitting the arm (1) to move from an initial position and in an operative direction (P) so that the printing wire (2) moves to effect printing; and positioning means (6,7) for positioning the arm in the initial position characterised in that return movement of the arm (1) back into the initial position does not cause it to be struck against a limit member.

Description: The present invention relates to an impact dot printing head. Previous impact dot printing heads have been of the two types shown in Figures 9(a) and 9(b) respectively (see also, for example, JP-A-58 217 374 and JP-A-62 204 958. In the head shown in Figure 9(a), a resilient arm B for driving a printing wire A is attracted by the magnetic force from a permanent magnet D provided in a core C. The resilient arm B is urged by the permanent magnet D in a direction opposite to that of the platen and is urged by its own resilience towards the platen. In printing, current flows in a solenoid E so as to overcome the magnetic force of the permanent magnet D, whereby the printing wire A is moved towards the platen by the resilient restoring force of the arm B. In the head shown in Figure 9(b), current is supplied during printing to a solenoid F, thereby attracting an arm H towards a solenoid core G and so moving a printing wire J in the direction of the platen. Since the initial position of the printing wire A is decided by the contact between the arm B and core C in the Figure 9(a) construction, and by the contact between the arm H and a static member, such as the inside surface of a case K, in the Figure 9(b) construction, impact sound is generated when the members B, C or H, K are brought into contact so that there is therefore considerable noise during printing. In order to solve this problem, an impact cushioning member has been provided at the contact position where the movable member B, H and the static member C, K contact each other, but such an impact cushioning member tends to be easily deformed, so that the original location of the initial position is gradually changed, thereby causing the printing quality to deteriorate. Moreover, since the use of an impact cushioning member does not fundamentally solve the problem, the noise still remains. According, therefore, to the present invention, there is provided an impact dot printing head comprising an arm to which is connected the distal end of a printing wire; movement effecting means for causing or permitting the arm to move from an initial position and in an operative direction so that the printing wire moves to effect printing; and positioning means for positioning the arm in the initial position characterised in that return movement of the arm back into the initial position does not cause it to be struck against a limit member. Preferably, the initial position is one about which the arm may be freely vibrated. Thus the arm may be dynamically balanced in the initial position. The arm, when in the initial position, may be subjected to magnetic and resilient forces which act in opposition to each other. Thus the positioning means may comprise a permanent magnet, while the movement effecting means may comprise a solenoid. Both the solenoid and the permanent magnet may act on the arm in the same direction, the arm also being acted on by a return spring. Alternatively, the arm may be urged by its own resilience in the operative direction and may be held in the initial position by the permanent magnet, the solenoid, when energised, cancelling the effect of the permanent magnet. In this case, the arm may comprise at least one plate spring secured to a static member, e.g. the core of the solenoid. In one embodiment of the present invention, the arm carries a permanent magnet which is disposed between poles of a magnetic yoke, the poles of the magnetic yoke having the same polarity as the adjacent poles of the permanent magnet so that the latter is urged thereby to the said initial position. In this case, the movement effecting means may comprise means for cancelling the magnetic force exerted by one of the poles of the magnetic yoke. Moreover, the arm may be provided with an armature which is adapted to be attracted to a solenoid when the latter is energised to cause the arm to move in the operative direction. In the case of the present invention, since the return movement of the arm does not cause it to be struck against a limit member, it may be arranged that substantially the only sound produced by a head according to the present invention arises from the printing wires striking paper supported on a platen. Consequently, there is considerable noise reduction and, since the energy losses are smaller, there is reduced power consumption. The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- Figure 1 is a sectional view of a first embodiment of an impact dot printing head according to the present invention; Figure 2 shows a waveform for the operation of the printing head of Figure 1; Figure 3 is a front view of a second embodiment of an impact dot printing head according to the present invention in a stand-by condition; Figures 4 and 5 are graphs illustrating the reduction of noise produced by an impact printing head according to the present invention; Figure 6 is a sectional view of a third embodiment of an impact dot printing head according to the present invention; Figure 7 is a sectional view of a fourth embodiment of an impact dot printing head according to the present invention; Figure 8 is a sectional view of a fifth embodiment of an impact dot printing head according to the present invention; and Figures 9(a) and 9(b) show examples of previous dot impact printing heads. Figure 1 shows a first embodiment of an impact dot printing head according to the present invention. The head comprises an arm 1 which is fixed to the distal end of a printing wire 2. The end of the arm 1 remote from the printing wire 2 is rotatably mounted on a pivot 3. The arm 1 is arranged to be attracted by a core 5 carrying a solenoid 4 when the latter is energised, so that the arm 1 is moved in an operative direction P towards a platen (not shown) so as to move the printing wire 2 to effect printing. In the neighbourhood of the core 5 there is provided a magnet 6. The magnet 6 urges the arm 1 in the direction of the platen so that, on the return movement of the arm 1 at the end of printing, the arm 1 is stopped at a position for starting printing, that is, an initial position. The end of the arm 1 adjacent the printing wire 2 is also acted on by a return spring 7. The return spring 7 generates a restoring force for bringing the arm 1 back to the initial position when the arm 1 is displaced from the initial position either in the direction of the arrow P or in the reverse direction. In this embodiment, when a printing signal is inputted, current is supplied to the solenoid 4. Then, the arm 1 is attracted to move in the direction of the platen (arrow P) and in opposition to the force of the return spring 7. The proximal end of the printing wire 2 remote from the arm 1 therefore strikes against the platen through an ink ribbon and a sheet of paper, thereby forming dots. In addition, the distal end of the printing wire 2 adjacent the arm 1 is stopped once and starts to move under the restoring force of the return spring 7 to the initial position. In the course of bringing the arm 1 back to the initial position, since the arm 1 is affected by the attraction exerted by the magnet 6 so as to move in the direction of the platen, the speed is rapidly decreased, and the arm is moved again to the initial position (the illustrated position) after a little overshooting in the direction opposite to that of the platen. Then the arm stops at the initial position, i.e. at the position where the restoring force of the return spring 7 is balanced by the force of attraction of the magnet 6. Since the amount of overshooting of the arm 1 is very small in the course of getting the arm back to the initial position, the arm 1 is located at the initial position without being bumped against the inside surface of the case K. Since a vibration system is formed at the initial position by the weight of the arm 1, the flexibility of the return spring 7 and the force of attraction exerted by the magnet 6 in the course of getting the arm 1 back to the initial position, the arm 1 is freely vibrated around a static point. In such conditions of vibration, when a printing signal for a dot to be printed is inputted, the arm 1 can be immediately displaced towards the platen. Then the suction of the solenoid 4 acts on the arm 1 so that the suction is added to the kinetic energy existing during the free vibration. Therefore, the speed of movement of the printing wire 2 is improved and, additionally, the printing wire 2 can be moved by less energy in comparison with an energization from a static condition. Moreover, in this embodiment, the magnetic force necessary for setting the initial position is obtained by an additional magnet 6, but the same effect can clearly be obtained by bringing the current to be supplied to the solenoid 4 down to a value at which the initial position can be formed in cooperation with the return spring 7, as shown in Figure 2. Figure 3 shows a second embodiment of the present invention. In Figure 3 there is shown in impact dot printing head comprising an arm 20 which is fixed at one end to a static member, in this case a core 21, the arm 20 comprising one or more plate spring members. The distal end of a printing wire 22 is fixed to the end of the arm 20 remote from the core 21. In addition, a magnetic member 24 is fixed to the arm 20 in a position opposite to a magnetic pole 21a of the core 21. The core 21 is provided with a permanent magnet 25 and a solenoid 26. The permanent magnet 25 produces the necessary force of attraction for urging the arm 20 away from the printing direction P while the solenoid 26, when energised, cancels this magnetic force. The distance between the surface of the magnetic pole 21a and the arm 20 is arranged to be such that the members 24, 21a do not contact each other at the initial position, that is, when the arm 20 is urged as shown in Figure 3. In the Figure 3 embodiment, when printing signals are inputted at a time when the arm 20 is urged by the magnet 25 towards the magnetic pole 21a at the initial position, as shown in Figure 3, then the solenoid 26 is energised so that the attraction exerted by the magnet 25 is cancelled. As a result, the power stored in the arm 20 is made free, and the latter causes the printing wire 22 to strike the platen. After the arm 20 has struck the platen, the current flow to the solenoid 26 is cut off and the arm 20 is brought back to the initial position by the force of attraction produced by the permanent magnet 25. In the process, the arm 20 overshoots from the initial position. However, since the distance between the surface of the magnetic pole 21a and the magnetic member 24 is kept sufficient, the members 21a, 24 do not contact each other. The suction of the magnet 25 and the resilience of arm 20 is balanced, and then the arm 20 stops at the initial position shown in Figure 3. In the course of the movement of the arm 20 brought back to the initial position, the arm 20 is vibrated freely. As a result, only a little electrical energy is required to energise the solenoid 26 so as to cancel the force of attraction exerted by the permanent magnet 25, so that when the solenoid 26 is so energised, the arm 20 begins moving towards the platen. Moreover, in the Figure 3 embodiment, the distance between the surface of the magnetic pole 21a and the magnetic member 24 is kept sufficient to prevent both the members from being brought into contact with each other while the arm 20 is being brought back to the initial position. However, if the magnetic power of the permanent magnet 25 is relatively reduced by energising the solenoid 26 while the arm 20 is being brought back to the initial position, the amount of overshooting of the arm 20 can be reduced. A printer may have four printing heads (not shown), each of which is of the kind illustrated in Figure 1, these printing heads being given the reference letters L, M, N, O in Figure 5. Each of the printing heads L, M, N, O has a different impact power, so that, with respect to such impact power L > M > N > O. The distance between the statically stable point and the casing K is referred to as b. The distance between the statically stable point and the impact point is referred to as a (see Figure 4). The ratio of the distances a, b was taken as a parameter and the ratio of printing noise/impact power was examined when the ratio of noise/impact power on contact between the arm 1 and the case K is 1. The result is shown in Figure 5. In the case of the head O having the smallest impact power, the noise is relatively reduced when the ratio b/a comes to about 5%; namely, the arm 1 will not contact the case K. When the ratio b/a comes to about 35%, the arm 1 does not contact the case K at all, and the only noise is caused by the printing wire 2 striking the paper. In the case of the head L having the greatest impact power, the noise is going to be reduced when b/a comes to about 30%. When the ratio b/a comes to about 70%, the noise is only that of the printing wire 2 striking the paper. In view of the above facts, there is hardly any effect for practical purposes when the ratio b/a is not more than 5% (zone I). The noise is going to be reduced to some extent when the ratio b/a comes to about 5 to 30% (II). When the ratio b/a comes to about 30 to 70% (zone III), the noise is going to be rapidly reduced. When the ratio b/a is not less than 70% (zone IV), the noise is limited to a substantially constant value regardless of the kind of head involved. Figure 6 is a sectional view of a third embodiment of an impact dot head according to the present invention. The lower half of Figure 6 shows the non-printing condition, and the upper half of Figure 6 shows the printing condition. A lever 31 is rotatably mounted on a pivot 32 by a bearing (not shown). The distal end of a printing wire 33 is fixed to one end of the lever 31 and a permanent magnet 34 is fixed to the other end of the lever 31. The permanent magnet 34 is magnetized so that it has S and N poles in the horizontal direction, as shown in Figure 6. A permanent magnet 35, which is carried by a magnetically permeable yoke 35a is magnetized so that the ends 36 and 37 of the yoke 35a have the same poles as that of the permanent magnet 34 which is mounted adjacent the ends 36 and 37 of the yoke 35a. Therefore, the magnetic forces exerted by the ends 36, 37 act on the permanent magnet 34 in opposite directions so that the permanent magnet 34 is urged to an initial position where the magnetic forces are balanced. In printing, the force exerted by the magnetic pole of the end 36 of the yoke 35a is cancelled by energising a coil 38. At this time, the force exerted by the end 37 of the yoke 35a on the permanent magnet 34 makes the lever 31 rotate around the pivot 32, thereby causing the printing wire 33 to strike the paper. In returning the printing wire 33 to the initial position, the force exerted by the end 37 of the yoke 35a on the permanent magnet 34 is cancelled by energising a coil 39. At this time, the force exerted by the end 36 of the yoke 35a on the permanent magnet 34 makes the lever 31 rotate around pivot 32, thereby returning the printing wire 33 to the initial condition. In addition, the energisation of the coil 39 is stopped, thereby stopping the printing wire 33 at a predetermined position at which printing does not occur. In this embodiment, as shown in Figure 6, the right hand sides of the permanent magnet 34 and yoke 35a have S poles and the left hand sides have N poles. However, the positions of the S and N poles are not limited to this example since the magnetic forces can be generated provided that the poles which are opposite to each other are the same poles. Therefore, the positions of the S and N poles as shown in Figure 6 can be reversed, and the same effects can obviously be obtained even if the coils 38 or 39 are wound or energised in the reverse direction. Figure 7 is a sectional view of a fourth embodiment of an impact dot head according to the present invention. The lower half of Figure 7 shows the non-printing condition, and the upper half of Figure 7 shows the printing condition. In this embodiment, an armature 40, a magnetic yoke 41 and a solenoid coil 42 are added to the structure shown in Figure 6, the armature 40 being carried by a lever 31a. Coils 48, 49 are employed which correspond to the coils 38, 39 of Figure 6. By energising the coil 42 to effect printing, attraction between the yoke 41 and the armature 40 is produced, thereby generating the rotary power required to rotate the lever 31a around the pivot 32. The other features of the Figure 7 embodiment are the same as that of Figure 6. Therefore, the kinds of magnetic poles and of coils to be energised can all be the same as in Figure 6. Moreover, since the power for urging the lever 31a is greater in the Figure 7 construction, higher speed driving can be obtained than in the construction of Figure 6. Figure 8 is a sectional view of a fifth embodiment of an impact dot head according to the present invention. The lower half of Figure 8 shows the non-printing condition and the upper half of Figure 8 shows the printing condition. This embodiment is the same as that of Figure 7, except that the coils 48 and 49 are not employed. By energising a coil 52 (corresponding to the coil 42) to effect printing, attraction between a yoke 51 (corresponding to the yoke 41) and an armature 50 (corresponding to the armature 40) is produced, thereby making a lever 51a, (corresponding to the lever 31a) rotate around pivot 52a counter-clockwise. Then a printing wire 53 strikes the paper, thereby printing. In returning the printing wire 53 to the initial position, the printing wire 53 is returned in accordance with the difference between the force acting between permanent magnet 54 and end 56 of yoke 55a and the force acting between the permanent magnet 54 and the end 57 of the yoke 55a. Moreover, as regards the polarity of the permanent magnet 54, this may be any of the possibilities discussed with respect to the Figure 6 construction, while the cost is lower than that of the Figure 7 construction.

Claims: An impact dot printing head comprising an arm (1) to which is connected the distal end of a printing wire (2); movement effecting means (4,5) for causing or permitting the arm (1) to move from an initial position and in an operative direction (P) so that the printing wire (2) moves to effect printing; and positioning means (6,7) for positioning the arm in the initial position characterised in that return movement of the arm (1) back into the initial position does not cause it to be struck against a limit member. A head as claimed in claim 1 characterised in that the initial position is one about which the arm (1) may be freely vibrated. A head as claimed in claim 2 characterised in that the arm (1), when in the initial position, is subjected to magnetic and resilient forces which act in opposition to each other. A head as claimed in claim 3 characterised in that the positioning means comprises a permanent magnet (6). A head as claimed in claim 4 characterised in that the movement effecting means (4,5) comprises a solenoid (4). A head as claimed in claim 5 characterised in that both the solenoid (4) and the permanent magnet (6) act on the arm (1) in the same direction, the arm (1) also being acted on by a return spring (7). A head as claimed in claim 5 characterised in that the arm (20) is urged by its own resilience in the operative direction (P) and is held in the initial position by the permanent magnet (25), the solenoid (26), when energised, cancelling the effect of the permanent magnet (25). A head as claimed in claim 7 characterised in that the arm (20) comprises at least one plate spring secured to a static member (21). A head as claimed in claim 8 in which the static member is a core (21) of the solenoid (26). A head as claimed in claim 1 or 2 characterised in that the arm (31) carries a permanent magnet (34) which is disposed between poles (36,37) of a magnetic yoke (35a), the poles (36,37) of the magnetic yoke (35a) having the same polarity as the adjacent poles of the permanent magnet (34) so that the latter is urged thereby to the said initial position. A head as claimed in claim 10 characterised in that the movement effecting means comprises means (38,39) for cancelling the magnetic force exerted by one of the poles (36,37) of the magnetic yoke (35a). A head as claimed in claim 10 or 11 characterised in that the arm (31a) is provided with an armature (40) which is adapted to be attracted to a solenoid (42) when the latter is energised to cause the arm (31a) to move in the operative direction.

Inventors: KOSHIISHI OSAMU C O SEIKO EPSO, MITSUISHI AKIO C O SEIKO EPSON, TAKEUCHI TAKASHI C O SEIKO EPS, KOSHIISHI, OSAMU C/O SEIKO EPSON CORPORATION, MITSUISHI, AKIO C/O SEIKO EPSON CORPORATION, TAKEUCHI, TAKASHI C/O SEIKO EPSON CORPORATION
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1989
Main classification: B41J2
IPC codes: B41J2