WIRE DOT IMPACT PRINTER

Abstract: A device for controlling the gap between a wire dot head and a printing medium in a wire dot impact printer which effects the printing by impinging the printing wire upon the printing medium. The device comprises means (15) for changing the gap, means (10) for detecting the displacement, and control means (2) for changing the gap into a proper value. The amount of displacement is detected when the wire is moved, and the gap is properly adjusted between the wire dot head and the medium based on the amount of displacement. There is provided the wire dot impact printer which adjusts the gap within short periods of time between the wire dot head and the medium, and which effects the printing of high quality at high speeds.

Description: The present invention relates to a wire dot impact printer having a wire-dot printing head provided with a printing wire which strikes onto a printing medium, especially to a system for controlling an interval between the wire-dot printing head and the printing medium. There is conventionally a wire-dot printing head as illustrated in Fig. 1. Fig. 1 is a view showing a general arrangement of a printing mechanism of a prior art printer. In the same figure, designated at 101 is a wire-dot printing head having a printing wire (not shown), 102 is a carriage for supporting the wire-dot printing head 101, 103, 104 are guide shafts for movably supporting the carriage 102 in the direction of the arrow A and 105 is a platen for feeding the printing paper. The carriage 102 moves in the direction of the arrow A upon reception of a power from a spacing motor (not shown) for thereby moving the wire-dot printing head 101 in the direction of width of the printing paper while the platen 105 rotates upon reception of a power from a line feed motor (not shown) for thereby feeding the printing paper in the direction crossing the width direction. A printing operation can be carried out in the manner that the printing wire strikes onto the printing paper at the position to be printed via for example an ink ribbon while the wire-dot printing head 101 is moved in the width direction of the printing paper at the predetermined speed. When the wire-dot printing head 101 completed one line printing after reaching an end position of the printing paper in the width direction, the platen 105 rotates to feed the printing paper in the longitudinal direction thereof for the one line length while the wire-dot printing head 101 returnes to an original position so that the printing wire starts to strike onto the next line of the printing paper. In the printer having the printing wire for striking onto the printing paper to effect the printing operation in such a manner, a force to be applied to the printing paper affects the printing quality. The force to be applied to the printing paper varies according to an interval (head gap g) between the wire-dot printing head 101 and the printing paper. There is provided in the prior art printer a manual lever (not shown) so that the head gap g can be varied depending on the thickness of the printing paper. Frequently occured an erroneous operation of the manual lever by an operator which causes an inferior printing operation to lose the printing paper or to idle the time for printing. There is proposed an apparatus for automatically adjusting the head gap g without resorting to the manual operation. Fig. 2 is a view showing a general arrangement of the printer having such automatic adjusting mechanism, Fig. 3 is a side elevational view enlarging a main portion of Fig. 2, and Fig. 4 is a view explaining the main portion of Fig. 3. In the same figures, the constituents which are the same as those of Fig. 1 are denoted with the same symbols and described. In Fig. 2, the guide shaft 103 is attached to side frames 107, 108 via eccentric bushes 108, 109 which are rotatably supported by the side frames. As a means for rotating the eccentric bushes 108, 109, there is a mechanism as shown in Figs. 2 and 4 in which the rotary drive force of a pulse motor 110 is transmitted to a gear 112 engaged with a gear 111 via the gear 111 provided with a rotary shaft of the pulse motor 110 to thereby rotate the eccentric bushes 108 together with the gear 112. On the other hand, the carriage 102 has a holding portion 102a formed as a U-shape in cross section as shown in Fig. 3 and is movable in the direction of the arrow B to vary the head gap g. In the actual adjustment operation, the eccentric bushes 108, 109 are rotated until the wire-dot printing head 101 is brought into contact with the platen 105 to allow the pulse motor 110 to be in step out state, then the pulse motor 110 is rotated for the pulse number corresponding to the head gap g. However, inasmuch as the wire-dot printing head 101 of the prior art printer is brought into contact with the platen 105 every time the head gap g is adjusted so that there is generated such problems that firstly adjustment of the head gap g takes much time, secondly, the printing operation takes time since the adjustment of the head gap g is carried out every time the printing paper is changed or repeated every time the printing paper is changed in case of the single paper is inserted into the head gap g. The head gap g is normally adjusted at one end portion of the platen 105 in view of the restricted adjustment time. Hence, no attention was paid to a variation of the head gap g caused by an error of the diameter of the platen 105, a reflection of the guide shaft 103 or the platen 105 or the eccentric phase difference of the eccentric bushes 108, 109. There are generated other problems in that the head gap g can not be correctly set since a step out position of the pulse motor 11 is varied due to a variation of the press force against the platen 105 caused by the step out torque of the pulse motor 110, a variation of the load torque of the other transmission mechanism, a deformation of the platen 105 made of an elastic material, or a flexibility of the supporting shaft of the platen 105, etc. at the time when the head gap g is adjusted on the basis of the step out position of the pulse motor. Futhermore, it was impossible to have a function to set the head gap g in accordance with a partial variation of the thickness of the printing medium relative to the printing medium having a different thickness such as an envelope, a bank book or the printing medium having a perforation. Document D1 = US-A-4 173 927 describes a known hammer impact printer comprising a hammer print head arranged to be spaced in a predetermined interval relative to a printing medium at least one printing hammer provided in the hammer print head each having a "tip" end capable of selectively striking onto the printing medium further comprising an interval adjusting means for adjusting the interval between the hammer print head and the printing medium a flying time detector means for detecting the flying time of the print hammers when they are operated a control means for controlling the adjustment of the interval between the hammer print head and the printing medium to an appropriate value by supplying a control signal issued on the basis of the result of the detection of the flying time by the flying time detector means to the interval adjusting means. Accordingly, it is an object of the present invention to provide a wire-dot impact printer capable of solving the problems of the prior art printer, and capable of carrying out an apropriate adjustment of the head gap to effect printing operation with high quality. This object is solved by the wire-dot impact printer according to claim 1. Accordingly, the inventive wire-dot impact printer comprises an interval adjusting means for adjusting the interval between the wire-dot printing head and the printing medium, a displacement detector means for detecting the displacement of the printing wire provided at the wire-dot printing head, and a controller for controlling to vary the interval between the wire-dot printing head and the printing medium at an appropriate value on the reception of a signal issued at the result of the detection by the displacement detector means comprising means for detecting changes in capacitance between an armature connected to the print wire and an electrode adjacent to the armature. The wire-dot impact printer having the arrangement set forth above can adjust the head gap g in short period of time with accuracy and can print with high speed and high quality. Fig. 1 is a plan view of a general arrangement of a prior art; Fig. 2 is a plan view of a general arrangement of another prior art; Fig. 3 is a side elevation enlarging a main portion of Fig. 2; Fig. 4 is a view of assistance in explaining a gear portion of Fig. 3; Fig. 5 is a block diagram of a wire-dot impact printer according to an embodiment of the present invention; Fig. 6 is a plan view of a general arrangement of a printing mechanism of the embodiment of the present invention; Fig. 7 is a side elevation of Fig. 6; Fig. 8 is a longitudinal cross sectional view of the wire-dot impact printer of the embodiment of the present invention; Fig. 9 is a plan view of a printed circuit board; Fig. 10 is a perspective view of a main portion of the printed circuit board of Fig. 9; Fig. 11 is a circuit diagram of an electrostatic capacitor sensor circuit; Fig 12 is a view of assistance in explaining a principle of Fig. 11; Fig. 13 is a waveform of operation of Fig. 12; Fig. 14 is a graph showing a variation of the output of the A/D converter relative to a displacement of a printing wire; Figs. 15(a), (b) are an input waveform and an output waveform of respectively of the electrostatic capacitor sensor circuit; and Fig. 16 is a flow chart showing an operation of the wire-dot impact printer according to the embodiment of the present invention. Fig. 5 is a block diagram showing an arrangement of the wire-dot impact printer according to the present invention. In the same figure, designated at 1 is a centro I/F for receiving a printing data, 2 is a control circuit for controlling the whole constituents of the present printer, 3 is a head driver, 3b is a head coil, 4 is a wire-dot printing head, 5 is a motor driver, 6 is a spacing motor, 7 is a motor driver, 8 is a line feed motor, 9 is an operation switch, 10a is a sensor electrode, 10b is an electrostatic capacitor sensor (hereafter referred to as a sensor circuit), and the electrode 10a and the sensor circuit 10b constituting a displacement detector means 10. Designated at 11 is a sample and hold circuit, 12 is an A/D converter, 13 is a motor driver, 14 is a pulse motor, 15 is an interval adjusting means as a drive motor for adjusting the head gap. The arrangement of the present invention is different from that of the prior art in respect that the present invention has the displacement detector means 10 and the interval adjusting means 15 and the controller circuit 2 receives a head gap data detected by the displacement detector means 10 and issues an instruction of adjusting operation of the head gap to the interval adjusting means 15. This different arrangement will be described in detail hereinafter. The control circuit 2 comprises an input interface LSI 2a, an output interface LSI 2b, a CPU 2c, RAM 2d for storing the printing data and executing the printing data, and a ROM 2e for storing a control program or a printing font (data for displaying the shape of the character by dot). Next, the interval adjusting means 15 will be described hereinafter. Fig. 6 is a plan view of a printing mechanism according to an embodiment of the present invention. In the same figure, designated at 4 is the wire-dot printing head, 22 is a carriage for supporting the wire-dot printing head, 23, 24 are guide shafts for moving the carriage 22 in the direction of A, 25 is a platen for feeding the printing paper, and 26, 27 are side frames for supporting the guide shafts 23, 24. The carriage 22 receives a power from the spacing motor (denoted at 6 in Fig. 1) and moves in the direction of A to move the wire-dot printing head A in the width direction of the printing paper. The platen 25 receives a power from the line feed motor (denoted at 8 in Fig. 5) to rotate for thereby feeding the printing paper in the longitudinal direction crossing the width direction of the printing paper. At the time of printing operation, the printing wire strikes onto the printing paper at the predetermined position thereof via an ink ribbon while the wire-dot printing head is moved in a predetermined speed in the width direction of the printing paper. At the time when the one line printing is comple ted after the wire-dot printing head 4 reaches the end position of one line in the width direction of the printing paper, the platen 25 rotates to feed the printing paper for one line length while the wire-dot printing head 4 returns to its original position, then the printing wire carries out the next printing operation. An arrangement as shown in Fig. 7 is adopted as the interval adjusting means 15 according to the embodiment of the present invention. Although the carriage 22 moves along the two guide shafts 23, 24, the carriage 22 according to the embodiment of the present invention are not held directly by the two guide shafts 23, 24 but has at the rear portion thereof a height adjusting mechanism provided at the guide shaft 24. That is, fixed at the rear portion of the carriage 22 is a pulse motor 14 having a rotary shaft 14a which is directly connected with a screw gear 14b and protruded under the carriage 22. The carriage 22 has a guide pin 22a protruded from a lower surface thereof at the rear potion thereof. The guide pin 22a vertically slidably held by a guide hole 28a of a slider 28 which is movably supported by the guide shaft 24 so that the guide pin 22a can be vertically moved. The slider 28 has a gear (not shown) which is engaged with the screw gear 14b. Accordingly, the carriage 22 is supported by the guide shaft 24 via the slider 28, the screw gear 14b, the rotary shaft 14a and the pulse motor 14. As a result, when the pulse motor is rotated, the rear portion of the carriage 22 is vertically moved in the direction of the arrow C (in the direction of the guide pin 22a to be guided by the guide hole 28a) so that the carriage 22a is rotated about the guide shaft 23. Accompanied by this movement, the head gap g between the tip end 4a of the wire-dot printing head 4 and the platen 25 can be varied. The head gap g can be varied by a means other than that of the present invention such as the means to move the platen 25, etc. The displacement detector means 10 for detecting the displacement of printing wire will be described next. Fig. 8 is a cross sectional view of the aforementioned wire-dot printing head 4. In the same figure, designated at 30 is a plurality of print wires provided at the wire-dot printing head 4 (only two print wires are illustrated in the same figure), 31 is a guide frame having a guide groove 31a for guiding the printing wires, 30, 32 are armatures 32 each made of a magnetic material, 33 are plate springs 33 for supporting the armatures 32. On the other hand, designated at 34 is a base, 35 is an electromagnet having a core 35a and a head coil 3b wound around a periphery of the core 35a, 36 is a printed circuit board having a print wiring thereon and connector terminals for supplying a power source to the electromagnet 35, 37 is a permanent magnet, 39 is a spacer, 40 is a yoke, 41 is a printed circuit board, and 42 is a clamp. The clamp 42 presses and holds the base 34, the permanent magnet 37, the rack 38, the spacer 39, the plate springs 33, the yoke 40, the printed circuit board 41, the guide frame 31 in the manner that these members are laid one over another in turn and integrated. The armature 32 is supported at the side of a free end 33a of the plate Spring 33 while a base end 30a of one of the printing wires 30 is fixedly mounted on a distal end 32a of the armature 32. A distal end 30b of the guide frame 31 is guided by the frame groove 31a of the guide frame 31 so as to strike a predetemined position of the printingpaper (not shown). Fig. 9 is a plan view of the printed circuit board 41, and Fig. 10 is a perspective view of a main portion of the printed circuit board 41. In the same figure, the printed circuit board 41 has sensor electrodes 10a which are composed of copper foil patterns, positioned opposite the armatures 32 and connected to the connector terminals of the printed circuit 41. The printed circuit board 41 is coated by the insulating film for keeping insulation from the yoke 40 for thereby generating the electrostatic capacitance in the interval between the sensor electrodes 10a and the armatures 32. The larger the interval between the sensor electodes 10a and the armatures 32, the smallerthe value of the capacitance while the smaller the interval,the greater the capacitance value. With the arrangement of the wire-dot impact printer having the wire-dot printing head 4, at the time when the head coil 3a is deenergized, the armature 32 is attracted to the side of the base plate 34 (downward direction in the figure) by the attraction force of the permanent magnet 37 against the resilience force of the plate spring 33. When the head coil 31 is energized, a magnet flux of the permanent magnet 37 is cancel led by the magnet flux of the electromagnet 35 to release the armature 32 from the attraction force of the permanent magnet 37 to move the armature 32 toward the side of the guide frame 31 (upward direction in the same figure) by the resilience force of the plate spring 33. Hereupon, the yoke 40 constitutes a part of the mag netic circuit prepared by the electromagnet 35 and functions to insulate the mutual interference of the sensor electrodes 10a. Fig. 11 is a circuit diagram of the sensor circuit 10b, Fig. 12 is a view of explaining a principle of Fig. 11, and Fig. 13 is a waveform of operation of Fig. 12. In Fig. 12, designated at 50 is a digital IC (MSM74HCU04 made of Oki Electric Industry Co., Ltd.), 50a, 50b are MOSFET of internal equivalent circuits (field effector transistor). Designated at 51 is an oscillator, 52 is a resistor, 53 is an integrator, and 54 is an ac amplifier. With the circuit arrangement set forth above, the sensor electrode 10a is connected to an output terminal of the digital IC 50 while a square shaped signal S OSC shown in Fig. 12 from the oscillator is applied to the input terminal of the digital IC 50 for thereby permitting a current I C to flow at the output terminal of the digital IC 50. The current I C is a charging/discharging current to be supplied to the sensor electrode 10a so that the FETs 50a, 50b are alternately turned on or off on the reception of the signal S OSC . The discharging current I S flows to ground via the FET 50b and the resistor 52. A value of the integration of the discharging current I S for one periodic cycle corresponds to quantity Q of an electric charge to be substantially charged in the sensor electrode 10a. Assuming that an eleotrocapacitance of the sensor electrode 8a is C X , an oscillation frequency of the oscillator 51 is f, a resistance value of the resistor 52 is R S , an amplification factor of the amplifier 54 is a times, the means value of the current I S will be f Q = f · C X ·V DD while the output voltage of the amplier will be V Q = C X · R S · a · f · V DD whereby the desired voltage V Q proportional to the electrocapacitance C X is produced. However, actually the amplifier 54 is composed of an ac amplifier so that the offset (dc) such as the distribution capacitance etc. existing other than the sensor electrode 10a is cut off and only the displacement of the printing wire 30 is produced. Concretely, the output waveform of the sensor electrode 10a is illustrated in Fig. 15(a) while the output waveform of the sensor electrode 10b is illustrated in Fig. 15(b). Here, an interval between the plus peak and the minus peak in Fig. 15(b) corresponds to the head gap g. To know the value of the head gap g, the plus peak value and the minus peak value of the output of the sensor circuit 10b are held by the sample and hold circuit 11 and the difference of the voltage between the plus peak value and the minus peak value is converted into a digital value by the A/D converter 12 having a difference input. The head gap g thus subjected to the digital conversion is applied to the CPU 2c via the interface LSI 2b. Accordingly, the relationship between the displacement of the printing wire 30 and the output voltage V Q of the sensor circuit 10b is illustrated in a graph of Fig. 14 since the electrostatic capacitance of the sensor electrode 10a is approximately inverse proportional to the distance between the sensor electrode 10a and the armature 32. An operation process of the embodiment of the present invention having the arrangement set forth above will be described with reference to a flow chart of Fig. 16. First, after the power supply of the printer is supplied an initial operation is carried out as Step 1. The initial operation is an initial setting of the head gap g which is carried out in the same manner as that of the prior art, namely, once the wire-dot printing head strikes onto the platen 25 to step out the pulse motor 14 and thereafter the pulse motor is reversely rotated for the predetermined numbers of pulses to obtain a desired head gap g. In the succeeding Step 2, the CPU 2c judges whether the printing data is supplied or not, if supplied, the process goes to Step 3 where the CPU 2c supplies a control signal via the LSI 2b to the head driver 3a, the motor drivers 5, 7 to actuate the printing wire 30 of the wire-dot printing head 4, the spacing motor 6 and the line feed motor 8, etc. for effecting the printing operation. At Step 4, the head gap g is detected on the basis of the displacement of the printing wires 30 at the printing operation and judged to be appropriate or not. If the head gap is judged to be appropriate, the process is returned to Step 2, if judged to be inappropriate, the process goes to Step 5. At step 5, the control signal is supplied from the CPU 2c to the motor driver 13 via the interface LSI 2b to actuate the pulse motor 14 for adjusting the head gap g and thereafter the process is returned to Step 2. Actually, if an appropriate value of the head gap g is 0.45 mm, the value within a predetermined value (for example, within 0.45 to 0.48 mm) is judged to be the appropriate head gap g. If there occurs a case where the correction value of the head gap g is too large to follow within every time of one dot printing operation, the correction operations are carried out extending several printing operations. As mentioned above, the embodiment of the present invention is provided with a means for detecting the displacement of the printing wires 30. Inasmuch as the head gap g is adjusted on the basis of the data of the head gap g obtained from the displacement of the printing wires 30, it is not necessary to determine the initial position by striking the printing head onto the platen as made in the prior art as illustrated in Fig. 3 at the time of adjustment of the head gap g. Accordingly, the adjustment of the head gap g can be effected within short period of time to achieve the high speed printing. Accompanied by the reduction of time involved in the adjustment of the head gap g, it is possible to correct the head gap g every time of one-dot printing for thereby keeping the head gap to be at all times optimum and carrying out the clear and high quality printing. As mentioned above, the wire-dot impact printer according to the present invention can omit such an extra operation that an initial position is determined by striking onto the platen so that the time involved in adjustment of the head is reduced to thereby realized the high speed printing. Accompanied by reduction of the time involved in adjusting the head gap, it makes possible to finely correct the head gap, for example, every one dot printing for thereby keeping the head gap at all times at an optimum state. Accordingly, even if the printing medium having different thickness such as an envelope, a bankbook, etc. the head gap can be kept at the optimum state to thereby assure a high speed and high quality printing. As set forth above in detail, the wire-dot impact printer according to the present invention has an industrial applicability capable of adjusting the head gap in a short time and of printing with high speed and high quality.

Claims: A wire-dot impact printer comprising: a wire-dot printing head (4) adapted to be spaced at a predetermined interval (g) relative to a printing medium, said wire-dot printing head (4) including a plurality of print wires (30) each having a tip end (30b) capable of striking against the printing medium during a printing operation; interval adjusting means (15) for moving said wire-dot printing head (4) relative to the printing medium and thereby for adjusting the interval therebetween; displacement detector means (10a, 10b) for detecting displacement of said print wires (30) during the printing operation; and control means (2), operatively associated with said displacement detector means (10a, 10b) and with said interval adjusting means (15), for supplying to said interval adjusting means (15) a control signal to adjust the interval between said wire-dot printing head (4) and the printing medium to the predetermined interval as a function of result of the displacement of said print wires (30) detected by said displacement detector means (10a, 10b), wherein said displacement detector means (10a, 10b) comprises an armature (32) connected to said print wire (30) and movable therewith during the printing operation, an electrode (10a) adjacent said armature (32), and means (41, 10b) for detecting changes in capacitance between said armature (32) and said electrode (10a). A wire-dot impact printer as claimed in claim 1, wherein said electrode (10a) is formed on a printed circuit board (41). A wire-dot impact printer as claimed in claim 2, wherein said printed circuit board (41) has therethrough an opening, and said electrode (10a) extends radially adjacent said opening. A wire-dot impact printer as claimed in claim 3, wherein said electrode (10a) faces said armature (32). A wire-dot impact printer as claimed in one of the claims 1 to 4, wherein said interval adjusting means (15) comprises a carriage (22) supporting said wire-dot printing head (4), a motor (14) fixed to said carriage (22) and having a rotatable drive shaft (14a), a gear connected to said drive shaft (14a), a fixedly positioned support shaft (24), and a slider (28) movable along said support shaft (24) and having means meshing with said gear (14b), such that operation of said motor rotates said gear (14b) and moves said gear and thereby said motor (14), said carriage and said wire-dot printing head relative to said slider (28) and said support shaft (24). A wire-dot impact printer as claimed in claim 5, wherein said gear (14b) comprises a worm screw gear. A wire-dot impact printer as claimed in claim 5, wherein said motor (14) is attached to adjacent a first edge of said carriage, and said wire-dot printing head (4) is mounted on said carriage (22) adjacent an opposite second edge thereof. A wire-dot impact printer as claimed in claim 7, wherein said carriage (22) is mounted adjacent said second edge thereof on another support shaft (23) for pivotal movement thereabout.

Inventors: ISHIMIZU, HIDEAKI, KIKUCHI, HIROSHI, KOMORI, CHIHIRO, TANUMA, JIRO
Applicants: OKI ELECTRIC INDUSTRY CO. LTD.
Application year: 1988
Main classification: B41J25
IPC codes: B41J25