August 1998 OKI Technical Review Vol. 64

Transcription

1 August 1998 OKI Technical Review Vol. 64 Special Issue on Printers: UDC Hirokazu ANDO*, Tatsuya KOYAMA**, Tatsuhiko SHIMOMURA*** Abstract The speed of a print head can be increased by decreasing the weight of the armature-wire structure, increasing the magnetic force of a permanent magnet, and improving the degaussing efficiency of the electromagnetic coil. For this, we have greatly improved the structure of the armature lever and structure of the core, and succeeded in developing an armature-wire structure which responds to 4.3 KHz. We also developed an abrasion resistant wire and low abrasion ink ribbon, and implemented high reliability for the high-speed operation of a print head. 1. Introduction While non-impact printers are at their height of popularity, there remains a strong demand for impact printers, where most applications are slip processing and the output of large capacity computer data. As the performance of computers improves and software advances, high-speed is one never changing target of impact printers as well. This paper presents basic concepts to implement high-speed for print heads, which determine the printing speed of impact printers, and discusses the stabilization of reliability, which thus far has not been clarified. 2. Requirement for implementing high-speed Figure 1 shows the basic structure of a print head and a series of printing operations based on this structure. To implement high-speed operation, the challenge is how to decrease the time required for reciprocating time TC (= t 0 +t 1 +t 2 +t 3 ) of an impact vibration system, where each factor must be examined. To maintain this operation stably for a long period of time, the abrasion of each part, especially abrasion at the tip of wires, must be examined. 3. High-speed technology Print wire Core Coil Armature Plate spring Permanent magnet Bypass magnetic path The high-speed technology of a print head is summarized into the following three points: 1. Optimizing structure of impact vibration system of armature-wire structure. This is implemented by developing an impact center type printing mechanism 2). 2. Decreasing weight of armature-wire structure and increasing the magnetic force of permanent magnets. 3. Improving degaussing efficiency of permanent magnets by an electromagnetic coil. Print head board Impact force waveform Current waveform Speed waveform t0 t1 t2 t3 Wire displacement waveform Figure 1: Structure and operation of wire dot head * General Manager, Second Research Laboratory, Oki Data Corporation ** Manager, Second Research Laboratory, Oki Data Corporation *** Second Research Laboratory, Oki Data Corporation 3.1 Designing armature-wire structure Figure 2 shows the result of calculating the relationship between the equivalent mass of an armature-wire structure and printing speed. For Kanji printing, equivalent mass must be reduced to 45 mg for 120 cps (characters per second) and to 21 mg for 160 cps, which is less than half 120 cps. Decreasing the mass of the armature for 120 cps, however, causes an insufficient magnetic force in the permanent magnet. Figure 3 shows the relationship between the equivalent mass of an armature and magnetic force. As the armature becomes smaller and lighter, magnetic force drops, which will drop printing speed. To increasing printing speed, the flexibility of the spring must be increased, which requires greater magnetic force. Magnetic force F, which is required here, is 45

2 given by F = CB 2 S where C : constant determined by magnetic circuit and other factors B : magnetic flux density of attraction area (T) S : cross-section of attraction area (m 2 ) For an armature for 160 cps, we improved the structure by 1. increasing the cross-section of the attraction area of the armature 2. using magnetic material with a high saturation magnetic flux density. In order to increase the cross-section of the attraction area without increasing the mass of the armature, we made inside the area hollow. At the same time we stopped using thin plastic film which has been used between the core and armature as shock eliminating material, so that the characteristics of high magnetic material could be fully utilized. As a result, magnetic force was improved by 20% or more with the same equivalent mass. The lever section where wire is bonded must have a structure and material which supports light weight and can endure impact. Generally armature and lever are integrated into one unit. In this examination, we adopted maraging steel, which is a high strength material, to insure high-speed as much as possible. We also decreased mass as much as possible, and decided on a shape which has the least stress concentration using a stress calculation program based on a finite element method. For wires, we adopted powder highspeed steel wire and minimized the length to decrease mass and to increase rigidity. Figure 4 shows the small mass structure of the armature. Compared with a 120 cps print head, weight was decreased 40%. 3.2 Decreasing heating at driving As magnetic force is increased, the speed of releasing magnetic force by an electromagnetic coil must also be increased. For this, a high ampere-turn must be generated at the electromagnetic coil in a short time. Table 1 shows an example of the calculation result of the drive system. As Table 1 shows, the response time was decreased in a 160 cps Printing speed [characters / second] Magnetic force [gf] Equivalent mass of armature-wire structure Figure 2: Relationship between equivalent mass and printing speed 160 cps system 120 cps system Equivalent mass of armature [mg] Figure 3: Relationship between equivalent mass and magnetic force Plate spring 120cps Leaf spring cps Armature Wire Armature Lever Wire Unit: mm Figure 4: Comparison of armature-wire structures 46

3 August 1998 OKI Technical Review Vol cps drive system 160 cps drive system Number of turns of coil (turns) Coil resistance (Ω) Inductance (mh) Drive voltage (V) Response speed (msec) (when equivalent mass m = 30 mg) Required energy (mj) Table 1: Comparison of drive systems drive system, but this increased the required energy considerably, and throughput was obviously dropped by heating. To decrease heating, we laminated the core to reduce the generation of eddy current. Figure 5 shows the result of the experiment on the laminated core with the same external dimensions. Three or more layers of lamination does not drop the peak current value very much, however manufacturing becomes complicated, and this approach is not very effective in reducing heating. The practical lamination is 2 or 3 layers, which also has a simple structure. For a 160 cps drive system we adopted a 2 layer core, and decreased required energy 15%. 4. High-speed and reliability technology As printing speeds increase, insuring reliability is becoming very difficult. The mechanical stress applied to a print wire particularly increases at a higher rate than the increase rate of the operation speed, therefore materials, including ink ribbon, are the first consideration. 4.1 High strength and light weight abrasive resistant wire The requirement for print wire to increase printing speed is to use metal that has a low specific gravity, and has high strength and high abrasive resistance. We examined improving powder high-speed steel, which does not separate carbon very much, in choosing material that is suitable for wire of print heads. Generally high-speed steel contains large amounts of tungsten, molybdenum and vanadium, so that abrasive resistance is improved by the carbide of these elements. Since tungsten and molybdenum function in the same way, the value tungsten equivalent is used here. Tungsten equivalent = Tg + 2 Mng Tg = weight of tungsten (%) Mng = weight of molybdenum We first examined abrasive resistance and the fatigue strength while changing the percentage content of vanadium and keeping the tungsten equivalent constant. Figure 6 shows the relationship between abrasion loss and the percentage content of vanadium, and Figure 7 shows the relationship between the number of cycles until fatigue failure occurs and the percentage content of vanadium. Abrasive resistance radically drops when Current peak value [A] Abrasion loss [mm] Number of cycles [times] Drive voltage: 60 V Number of turns of coil: 205 turns Current carrying time: 80 ms Number of layers of laminated core Figure 5: Relationship between number of layers of laminated core and current peak value of coil Number of times of printing: 8 times Percentage content of vanadium [% on a weight basis] Figure 6: Relationship between percentage content of vanadium and abrasion loss Stress 120kg/mm 2 130kg/mm 2 140kg/mm Percentage content of vanadium Figure 7: Relationship between percentage content of vanadium and number of cycles 47

4 Abrasion loss [mm] 200 Number of times of printing: 8 times Tungsten equivalent Figure 8: Relationship between tungsten equivalent and abrasion loss Abrasion at the tip of wire increase wire stroke, and interrupts high-speed operation. To prevent this problem, ink which does not wear on the tip very much is necessary. As a result we examined color materials that can be substituted for carbon black. By examining soft color materials from various aspects that include tone, bleed and light resistance, and by repeated trial and error, we found that the following ink components are most suitable. mineral oil: 25 (on a weight basis) vegetable oil: 25 oil color: aniline organic pigment: 15 sorbitan fatty acid ester: 25 Figure shows the result of an abrasion test that we conducted using ink ribbons coated with this ink. Compared with carbon black-based ink ribbon, the abrasion loss improved times. Number of cycles [times] Tungsten equivalent Stress 120kg/mm 2 Figure 9: Relationship between tungsten equivalent and number of cycles 120kg/mm 2 120kg/mm 2 the percentage content of vanadium is 4% or less, but fatigue strength drops if the percentage is too high. We then examined the tungsten equivalent in the same way, keeping the percentage content of vanadium 4%. Figure 8 shows the relationship between the tungsten equivalent and abrasion loss of tip of wire, and Figure 9 shows the relationship between the tungsten equivalent and fatigue failure. Abrasive resistance radically drops when the tungsten equivalent is 14% or less. As these results show, a high-speed steel with a percentage content of vanadium of 4% or more and a tungsten equivalent of 14% or more is best. Based on this finding, we established optimum manufacturing conditions for powder high-speed steel wire. 4.2 Abrasive resistant ink Carbon black used for many ink ribbons mechanically wears on the tip of wire just like using polishing powder. 5. Conclusion We examined decreasing the mass of armature-wire, which is a basic factor to increase the speed of a print head, and clarified the requirement for each component. We also studied reliability to execute high-speed printing stably from the aspect of abrasion of wire. As these series of experiments and examinations show, it is clear that increasing speed at the component configuration level of a print head is reaching the limit. In the future, high-speed will be implemented by increasing the number of wires, which has already been adopted by some printers now on the market. As the number of wires increase, factors that make operation Abrasion loss [mm] Abrasive resistant ink Conventional carbon black ink Number of times of printing [x 8 ] Figure : Abrasion loss of abrasive resistant high-speed wire by newly developed abrasive resistant ink 48

5 August 1998 OKI Technical Review Vol. 64 unstable, such as magnetic interference, will increase. However, there are many possible countermeasures, including handling these factors instantly under various printing conditions, or recognizing a printing pattern in advance and performing controls that are suitable for that printing pattern. While demands for impact printers are definitely decreasing, the application fields are becoming more specific. The primary features of impact printers are simultaneous copying and good operability, including handling of printing media. Although non-impact printers may be better in terms of print quality, increasing speed while focusing on the above features is the development challenge of impact printers. 6. References 1. Ohmori, Ando: Wire dot head technology, Oki Kenkyu Kaihatsu, 126, 52, 2, 21 ~ Watanabe, Ando, Ohmori: A study of mechanism of wire dotprinter head, IECA, E 65, 7, (1982): 397~

6 50