Wire-dot print head

Abstract: In a wire-dot print head having an armature (10) with a tip to which a print wire (11) is secured, a core (16) provided in confrontation with the armature, plate spring (7) which is supported in a canti lever fashion and to which the armature is fixed, a permanent magnet assembly (4, 18) generating a magnetic flux to cause the armature to be attracted toward the core overcoming the resilient force of the plate spring, and a coil (13) wound on the core to generate a magnetic flux upon energization to cancel the magnetic flux from the permanent magnet assembly thereby to release the armature, the permanent magnet assembly comprises a first permanent magnet (4) of samarium-cobalt type, and a second permanent magnet (18) of neodyminum-iron-boron type or lanthanoid-­ iron-boron type.

Description: The present invention relates to a wire-dot print head for a printer, which utilises print wires driven by means of armatures. Serial printers employing a wire-dot print head can be used to print on a variety of print media, such as multi-ply print papers, and they are used extensively. In wire-dot print heads, print wires are driven selectively by the interaction of a permanent magnet and electromagents. Reference is directed to our prior EP-A-0 191 549. Figure 6 hereinafter, shows a cross-sectional view of the above-described prior art wire-dot print head. Broadly stated, the prior head consist of a wire-dot print head including a print wire, an armature for moving the print wire, a spring for moving the armature, permanent magnet means for attracting the armature against the force of the spring, and an electromagnet for producing on the armature a force in opposition to the force of the permanent magnet means to release the armature therefrom so as to be moved by the spring. Considering now the figure in more detail, provided between the guide frame 1 and the cap 2 are a base plate 3, a permanent magnet 4, an upright support 5, a spacer 6, a plate spring 7 and a yoke 8 which are stacked successively with each other and clamped by a clamp 9. Provided on the flexible part of the plate spring 7 is an armature 10. Fixed to the tip of the armature 10 is a base part of a print wire 11, whose tip is guided by a guide 1a to project toward the platen. A core 12 is provided in the centre of the base plate 3. 14 is a circuit board for energising the coil 13. 15 is a space sheet for positioning the board 14. 16 is a temperature-detecting thermistor. 17 is a filler having a high thermal conductivity and covering the coil 13 and the thermistor 16. With the above structure, a magnetic circuit is formed whereby the magnetic flux from the permanent magent 4 is passed through the upright support 5, the spacer 6, the yoke 8, the armature 10, the core 12 and the base 3 and returns to the permanent magnet 4. Because of this magnetic circuit, the armature 10 is attracted to the core 12 into a biased state to store distortion energy in the plate spring. In this biased state, if the coil 13 is energised to generate a magnetic flux in opposition to the magnetic circuit, the force for attracting the armature 10 is reduced. For this reason, the distortion energy stored in the plate spring 7 is released and the plate spring 7 is restored, so that the print wire 11 fixed to the tip of the armature 10 projects from the guide 1a and presses the ink ribbon and the print medium against the platen. In this way, characters and graphic patterns are printed. As a result of energising the coil 13 during printing, heat is generated and transmitted to the thermistor 16 through the filler 17 made of epoxy resin or the like which has a high heat conductivity, and the temperature within the wire-dot print head it can be supervised so that the coil 13 is controlled below its maxiumum operating temperature. For the permanent magnet 4, materials of samarium-cobalt type having a high energy product (BH product) and low temperature coefficient of magnetic flux density are frequently employed. With the above-described prior art wire-dot print head, instead of the permanent magnet of the samarium-cobalt type containing rare, samarium and cobalt as main constituents, permanent magnets of neodyminum type are used to increase the printing speed and to lower the price of the printer. In such a case, the temperature coefficient of the residual magnetic flux density of the permanent magnet is four to five times greater than that of the permanent magnet of the samarium-cobalt type, and the attracting force generated by the permanent magnet 4 varies due to the heat generated by the coil 13 within the wire-dot print head. Moreover, in the worst case, the plate spring 7 cannot be attracted. Figure 7 shows the relationship between the attraction stroke, and the spring force and the attracting force in the prior art wire-dot print head. The attracting force curves both at a high temperature and at a low temperature are shown. The attracting force F0 at the fully attracted point of the armature 10 decreases with the temperature rise, and keeps decreasing to F1 for the highest operating temperature of the wire-dot print head. At this temperature, the attracting force may become smaller than the spring force and a failure to attract the plate spring may occur. Where the design printing speed is increased, the weight of the wire-dot pring head is reduced, heat generatred from the coil 13 during printing is increased, and the head radiation capacity is reduced. The temperature rises more quickly and reaches the maximum operating temperature in a shorter time. Because heat control must be performed to suppress the temperature rise, printing needs to be suspended or one-way printing is performed or some other action to reduce the duty ratio must be taken. As a result, the printing speed (throughput) is lowered. Another problem relates to magentic interferences between adjacent cores. Figure 8 is a developed view of the core for explaining the magnetic interference in the prior art wire-dot print head. In the Figure, 12a, 12b and 12c are cores provided in juxtaposition. 13 is a coil. 7 is a plate spring having a print wire. When the coil 13 is not energised, the plate spring 7 is attracted by the magnetic flux, shown by solid line, generated by the permanent magnet. When a drive current is made to flow through the coils 13 on the core 12a and the core 12c, a magnetic flux, shown by dotted line, is generated and part of this passes through the core 12b. The direction of this leakage flux is identical to the direction of the magnetic flux for attracting the plate spring 7, so the attracting force is increased. Because of the effect of the magnetic flux, when a plurality of print wires 11 are driven simultaneously, the printing force is lowered. To compensate for this, control is made whereby the drive time for which the drive current is made to flow through the coils 13 are varied with the number of the print wires simultaneously driven, and the drive time is lengthened with the number of the driven print wires. In this case, heat generated from the coils 13 is increased, so the print duty cycle is further lowered. The present invention aims at solving the problems of the prior art wire-dot print head described above by providing a wire-dot print head maintaining the maximum energy product of the permanent magnet while reducing the temperature variation, and increasing the printing duty cycle and enabling printing speed to be increased, and to permit a reduction in the size of the print head. For this purpose, the wire-dot print head of the present invention uses, as the permanent magnet forming the magnetic circuit, a combination of a first permanent magnet of samarium-cobalt type, and a second permanent magnet of neodyminum-iron-boron type of lanthanoid-iron-boron type. According to the present invention, since a first permanent magnet of samarium-cobalt type, and a second permanent magnet of neodyminum-iron-boron type or lanthanoid-iron-boron type, are used for the permanent magnet forming the magnetic circuit as described above, the maximum energy product is increased, while the rate of variation of the residual magnetic density is reduced, and the printing speed of the wire-dot print head can be increased, and the size thereof can be reduced. Moreover, the print head according to the invention may be included in a printer that includes means for detecting the temperature of the wire-dot print head, and means for shortening the coil energisation time (drive time) linearly or step-wise in accordance with increase in the detected temperature. This is for the purpose of compensating for the decrease in the attracting force due to increase in said temperature. The drive time can be shortened to such an extent that, at a temperature above a threshold temperature, the printing force against the print medium is not lowered (by compensating the decrease in the attracting force), and the heating of the coil is suppressed and the printing duty is prevented from being lowered. In order that the invention may be more fully understood, reference will be made to the accompanying drawings wherein: Figure 1 is a cross sectional view of a wire-dot print head of an embodiment of the present invention; Figure 2 is a cross sectional view of a wire-dot print head of another embodiment of the invention; Figure 3 is a diagram showing another example of diposition of the permanent magnet; Figure 4 is a diagram showing the relationship between the attracting stroke, and the spring force and the attracting force; Figure 5 is a diagram showing a circuit for controlling the coil energisation time; Figure 6 is a cross sectional view of a prior art wire-dot print head; Figure 7 is a diagram showing the relationship between the attracting stroke, and the spring force and the attracting force for the prior art head of Figure 6; and Figure 8 is a developed view of cores of the head of Figure 6, for explaining the magnetic interference. Referring to Figure 1 front (upper as seen in Figure 1) ends of print wires 11 project output guide holes 1c in a guide frame 1 and directed toward a platen PL over which a printing paper PP is passed. An ink ribbon IR is interposed between the tips of the print wires 11 and the printing paper PP on the platen PL. Rear (lower as seen in Figure 1) ends of the wires 11 are fixed to tips (inner ends) of respective armatures 10 supported by inwardly projecting parts 7a of a plate spring 7. The projecting parts 7a are supported in a canti-lever fashion and are flexible. The rear (lower as seen in Fig. 1) surfaces of the armatures 10 are in confrontation with front (upper as seen in Fig. 1) ends of cores 12 on which coils 13 are wound to form electromagnets 23 for the respective wires 11. The rear (lower as seen in Fig. 1) ends of the cores 12 are fixed to a central part of a disk-shaped base plate 3 which is formed of a magnetically permeable material. A space sheet 15 is provided for positioning a printed circuit board 14. A thermistor 16 is provided for detecting the temperature. A filler 17 having a high thermal conductivity is provided to cover the coils 13 and the thermistor 16. A first annular permanent magnet 4, a second annular permanent magnet 18, an annular upright support 5, an annular spacer 6, an an annular part 7b of of the plate spring 7, and an annular part 8b of an armature yoke 8 as well as an annular peripheral part 1b of the guide frame 1 form a cylindrical wall of the print head. The first and the second permanent magnets 4 and 18 in combination form a permanent magnet assembly 24. A magnetic circuit is formed by the first and second permanent magnets 4 and 18, the upright support 5, the spacer 6, the yoke 8, the armature 10, the core 12 and the base 3. A magnetic flux passing through the magnetic circuit attracts the armature 10 to the corresponding core 12, and the inwardly projecting part 7a of the plate spring 7 is brought into a biased state and distortion energy is stored in the plate spring 7. The coils 13 of the electromagnets 23 are electrically connected by electric conductors on the printed circuit board 14 to a drive circuit, not shown, for controlled selective energization in accordance with data for printing. The printed circuit board 14 is covered with a rear cap 2 provided at the rear (lower side as seen in Fig. 1). The members forming the cylindrical wall of the print head drive part are clamped by a clamp member 9. When the electromagnets 23 are not energized, the armatures 10 are attracted toward the cores 12 of the electromagnets 23 because of the magnetic flux from the permanent magnet assembly 24. The projecting parts 7a of the plate spring 7 are thereby resiliently deformed. When the electromagnets 23 are energized the magnetic flux due to the electromagnets 23 and the magnetic flux due to the permanent magnet assembly 24 cancel each other, and the attracting force acting on the armature 10 is reduced, being overcome by the resilient force of the plate spring 7. The distortion energy is thereby released, and the plate spring 7 returns to the free original state. As a result, the print wires 11 project out of the guide holes 1c, and their tips are pressed against the ink ribbon IR and the print paper PP on the platen PL. Printing of characters and graphic patterns is thereby achieved. Heat generated because the coil 13 is energized is conducted by the filler 17 of epoxy resin or the like having a high thermal conductivity to the thermistor 16 and the temperature within the wire-dot print head is supervised and the coil 13 is controlled to be below its maximum operating temperature. The permanent magnet 4 is formed of a material consisting of samarium-cobalt (Sm-Co) type which has a high energy product (BH product) and a low temperature coefficient of the magnetic flux. The second permanent magnet 18 consists of neodyminum-iron-boron type or lanthanoid-iron-boron type. The materials for the first and the second permanent magnets may be interchanged. In the embodiment of Fig. 1, the first and the second permanent magnets 4 and 18 are stacked being adjacent to each other. But the first permanent magnet 4 and the second permanent magnet 18 may be separated by other members, such as the upright support 5, as shown in Fig. 2. In the above description, the first permanent magnet 4 of samarium-cobalt type, and the second permanent magnet 18 or 19 of neodyminum-iron-boron type or lanthanoid-iron-boron type are provided in series. However, as shown in Fig. 3, they may be provided in parallel, but still the similar results are obtained. In the example illustrated in Fig. 3, the first and the second permanent magnets 4 and 18 are both annular, with the second permanent magnet 18 hating a larger diameter. More specifically, the inner periphery of the second annular permanent magnet 18 is adjacent to the outer periphery of the first permanent magnet 4. The inner periphery of the first permanent magnet 4 and the outer periphery of the second permanent magnet 18 respectively form inner and outer surfaces of the cylindrical wall. When only the permanent magnet of neodyminum-iron-boron type or lanthanoid-iron-boron type having a higher maximum energy product than the permanent magnet of samarium-cobalt type is used, the temperature variation rate of the residual magnetic flux density is -0.11 %/°C to 0.15 %/°C, larger than that of permanent magnet of samarium-cobalt type, which is 0.03 %/°C. When the first permanent magnet 4 of samarium-cobalt type, and the second permanent magnet of neodyminum-iron-boron type are used in combination, compared with the case where only the first permanent magnet 4 is used, the maximum energy product is increased without increasing the volume. Moreover, compared with the case where only neodyminum-iron-boron type or lanthanoid-iron-boron type is used, the temperature variation rate of the residual magnetic flux density can be reduced (e.g., -0.06 %/°C). Fig. 4 shows a relationship between the attraction stroke, and the spring force and the attracting force. As illustrated, the degree of reduction of the attracting force curve for a high temperature is reduced, and it does not become lower than the curve of the spring force of the plate spring 7, and failure in attraction does not occur. As a result, it is possible to attract a plate spring having a high spring force constant, and the size of the permanent magnet can be reduced. The attracting force F at the fully attracted point of the armature 10 is F₂, and it is decreased with increase of the temperature, and it is F₃ at the maximum operating temperature of the wire-dot print head. The holding force F₃ - F₄ of the plate spring 7 at a high temperature is lower than the holding force F₂ - F₄ at room temperature, and is smaller than the holding force F₁ - F₄ at high temperature where only samarium-cobalt type permanent magnet is used. That is, the armature is released more easily at high temperature than at room temperature. The drive time for which the coil 13 is energized above the temperature threshold level determined by the thermistor 16, either linearly or step-wise in accordance with increase of the detected temperature. The shortening may be so made as to compensate for the decrease in the attracting force due to increase in said temperature. Moreover, the shorting is made at or above the threshold temperature. As a result, heating of the coil 13 is suppressed and the reduction of the printing duty can be prevented. Fig. 5 is a diagram of a circuit for controlling the coil energization time. In the figure, a resistor 30 and a thermistor 16 are connected in series between a power supply (5V) and the ground to form a voltage divider providing a temperature signal indicative of the temperature of the print head. A comparator 31 receives at one input terminal the temperature signal and receives at another input terminal at a reference voltage defining the threshold level. The output of the comparator 31 is connected to a microprocessor 32, which controls a print head driver 33. When the temperature of the coil 16 exceeds the threshold level, the output of the comparator 31 is active, responsive to which the microcomputer operates, and a wire-dot print head driver 33 is controlled to vary the coil energization time. The present invention is not limited to the embodiments described above. Various modifications are possible without departing from the scope of the present invention. As has been described according to the invention, a First permanent magnet of samarium-cobalt type, and a second permanent magnet of neodyminum-iron-boron type or lanthanoid-iron-boron type are used as the permanent magnet forming the magnetic circuit. The maximum energy product is increased, while the rate of variation of the residual magnetic density is reduced, and the speed of the wire-dot print head can be increased, and the size thereof can be reduced. Moreover, since there are provided a means for detecting the temperature of the wire-dot print head, and a means for shortening the coil energization time linearly or step-wise in accordance with increase in the temperature determined by the detecting means, the drive time can be shortened to such an extent that, when the temperature determined by the thermistor is above the threshold temperature, the printing force against the print medium is not lowered. The heating of the coil is therefore suppressed and the printing duty is prevented from being lowered. Furthermore, when the drive time of the coil is lengthened for preventing the magnetic interference, the peaks of the drive currents are reduced, so the printing duty (throughput) is not lowered.

Claims: A wire-dot print head including a print wire (11), an armature (10) for moving the print wire, a spring (7) for moving the armature, permanent magnet means for attracting the armature against the force of the spring, and an electromagnet (12, 13) for producing on the armature a force in opposition to the force of the permanent magnet means to release the armature therefrom so as to be moved by the spring, characterised in that the permanent magnet means comprises a first permanent magnet of samarium-colbalt type (4) and a second permanent magnet (18) of neodyminum-iron-boron type or lanthanoid-iron-boron type. A wire-dot print head according to claim 1 wherein the electromagnet includes a core (12) provided in confrontation with said armature, and a coil (13) wound on the core to generate a magnetic flux upon energisation to cancel the magnetic flux from the permanent magnet means (4, 18) thereby to release the armature. A wire-dot print head according to claim 2 wherein the spring (7) comprises a plate spring which is supported in canti-lever fashion and to which said armature is fixed. A wire-dot print head according to any preceding claim, wherein said first permanent magnet (4) and said second permanent magnet (18) are provided in series with each other along a magnetic circuit for passing said magnetic flux. A wire-dot print head according to claim 1, 2 or 3, wherein said first permanent magnet (4) and said second permanent magnet (18) are provided in parallel with each other along a magnetic circuit for passing said magnetic flux. A wire-dot print head according to claim 3, wherein a plurality of print wires (11), a plurality of corresponding armatures (10), connected by an annular yoke and a plurality of corresponding cores (12) are provided; said plate spring (7) has a plurality of inwardly projecting parts (7a) for the respective armatures; ends of the print wires (1) are fixed to the respective armatures (10) supported by the respective inwardly projecting parts (7a) of the plate spring; the armatures (10) are in confrontation with front ends of the respective cores (12) on which the respective coils (13) are wound to form respective electromagnets for the respective print wires; rear ends of the cores (12) are fixed to a disk-shaped base plate (3) which is formed of a magnetically permeable material; said first permanent magnet (4) and said second permanent magnet (18), are annular; said plate spring (7) has an annular part; said guide frame 1 for the print wires, which has an annular peripheral part, said first permanent magnet, said second permanent magnet, said annular part of the plate spring, and said annular part of said armature yoke, form a cylindrical wall of the print head. A wire-dot print head according to claim 6, wherein said cylindrical wall forms part of a magnetic circuit through which a magnetic flux for attracting the armature toward the corresponding core passes. A wire-dot print head according to claim 6 or 7, wherein said first annular permanent magnet (4) and said second annular permanent magnet (18) are stacked in series with each other in said magnetic circuit. A wire-dot print head as set forth in claim 5, one of said first and second annular magnets (4, 18) are arranged one within the other, so that they are in parallel in said magnetic circuit. A wire-dot print head according to any one of claims 6 to 9, wherein the coils of the electromagnets are electrically connected by electric conductors on a printed circuit board (14) to a drive circuit for controlled selective energisation in accordance with data for printing. A printer comprising: a wire-dot print head as set forth in any preceding claim; means for detecting the temperature of the wire-dot print head; and means for shortening the coil energisation time linearly or step-wise in accordance with an increase in said temperature determined by said detecting means. A printer according to claim 11, wherein said shortening means is operative to shorten the coil energisation time to compensate for the decrease in the attracting force due to increase in said temperature.

Inventors: ANDOU HIROKAZU OKI ELECTRIC IN, KISHIMOTO MITSURU OKI ELECTRIC, OOISHI NOBORU OKI ELECTRIC IND, UMEZAWA YOICHI OKI ELECTRIC IN, ANDOU, HIROKAZU, OKI ELECTRIC INDUSTRY CO., LTD., KISHIMOTO, MITSURU, OKI ELECTRIC IND. CO., LTD., OOISHI, NOBORU, OKI ELECTRIC INDUSTRY CO., LTD., UMEZAWA, YOICHI, OKI ELECTRIC INDUSTRY CO., LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY CO., LTD.
Application year: 1990
Main classification: B41J2
IPC codes: B41J2, B41J29, B66F7