Title of Invention

CIRCUIT FOR DRIVING DOT IMPACT HEAD

Abstract

A circuit adapted to drive head pins of a dot impact head is disclosed.Each of head coils has a first terminal and a second terminal, and is adapted to 5 actuate One of the head pins. A first control element is operable to control current flowing through the first terminal of each of the head coils, and has: a first terminal electrically connected to the first terminal of each of the head coils; and a second terminal, adapted to be electrically connected to a power supply circuit. Each of second control elements is operable to control current flowing through the second terminal of each of the head coils, and has; a first terminal electrically connected to the second terminal of an associated one of the head coils; and a second terminal being grounded. A first circuit is operable to return to the power supply circuit, a back electromotive force avalanche energy generated when at least one of the head coils is deactivated. The first circuit includes: a first end electrically connected to the first terminal of the first control element and to the first terminal of each of the head coils; and a second end being grounded. A second circuit is operable to return the back electromotive force avalanche energy to the power supply circuit, and includes:a first end electrically connected to the first terminal of each of the second control element and to the second terminal of each of the head coils; a second end adapted to electrically connected to the power supply circuit; and a third circuit configured to be activated when the first control element is deactivated and to be deactivated when the first control element is activated.

Full Text

CIRCUIT FOR DRIVING DOT IMPACT HEAD BACKGROUND OF THE INVENTION The present invention relates to a circuit for driving a dot impact head (hereinafter, referred to as a head driving circuit) including an induced current cutoff circuit. In a printer incorporating a dot impact head, a current flows through a head coil, and an iron core disposed in the center of the head coil is operated by a magnetic attractive force generated in the head coil, so that printing is performed when a printing medium is struck with a head pin coupled to the iron core. Fig. 1 shows an example of a conventional head driving circuit 10. Specifically, this is a circuit configuration in which current allowed to flow through head coils 12a, 12b, 12c is controlled by a power supply side control element 11 and ground side control elements 13a, 13b, 13a The power supply side control element 11 is controlled by a control signal supplied by a control port H so as to be normally in an ON state while printing is executed. By controlling the ground side control elements 13a, 13b, 13c with control signals supplied by control ports La, Lb, Lc, current flowing to the head coils 12a, 12b, 12c, which operate the head pins, is controlled, thereby executing the printing. However, when the current flowing to the head coils is cutoff in order to stop the operation of the head pin, owing to a transient phenomenon, there is generated a so-called back electromotive force avalanche energy. Since this back electromotive force avalanche energy is large, the following problems have been raised. The back electromotive force avalanche energy generated in the head coils is applied to the ground side control elements and is radiated there as heat This causes a power loss which thermally influences the elements of the circuit. Also, the ground side control elements must be elements that can stand the thermal influence and, therefore, the elements themselves are expensive. Further, because the ground side control elements must be elements that are not easily influenced by heat, it is necessary to use large-size elements (for example, large-size transistors) as the ground side control elements. As a result, the area of a circuit board must be increased. Fig. 2 shows another example of a head driving circuit 20 capable of solving the above problem. The head driving circuit 20 comprises: an FET (p-channel type) 21; head coils 22a, 22b, 22c; FETs (n-channel type) 23a, 23b, 23c; a diode 24; diodes 25a, 25b, 25c; and a power supply circuit 26. The FET 21 is controlled by a control port H, while the FETs 23 are controlled by control ports La, Lb, Lc respectively. The source terminal of the FET 21 is connected to the power supply circuit 26, the gate terminal thereof is connected to the control port H, and the drain terminal thereof is connected to the higher-voltage input terminals of the head coils 22a, 22b, 22c and to the cathode of the diode 24. The source terminals of the FETs 23a, 23b, 23c are respectively grounded, the gate terminals thereof are connected to the control ports La, Lb, Lc respectively, and the drain terminals thereof are respectively connected to the lower-voltage output terminals of the head coils 22a, 22b, 22c and to the anodes of the diodes 25a, 25b, 25c. The cathodes of the diodes 25a, 25b, 25c are respectively connected to the power supply circuit 26. The anode of the diode 24 is grounded. The FET 21 controls current which flows to the head coils 22a, 22b, 22c. A voltage applied to the gate terminal of the FET 21 is controlled through the control port H to control a voltage between the gate terminal and the source terminal. When the FET 21 is turned ON, the higher-voltage input terminals of the head coils 22a, 22b, 22c are electrically connected to the power supply circuit 26. When current is allowed to flow into any one of the head coils 22a, 22b, 22c, a magnetic attractive force is generated to move an associated iron core and a head pin to perform printing. The FETs 23a, 23b, 23c respectively control the current that flows into the head coils 22a, 22b, 22c. A voltage applied to the gate terminal of each of the FETs 23a, 23b, 23c is controlled through each of the control ports La, Lb, Lc to control a voltage between the gate terminal and the source terminal thereof. When the FETs 23a, 23b, 23c are turned ON, current is fed from the lower-voltage output terminals of the head coils 22a, 22b, 22c to the ground. The FETs 23a, 23b, 23c are connected so as to correspond to the head coils 22a, 22b, 22c respectively and, by individually turning ON the FETs 23a, 23b, 23c, the current flowing to any one of the respective head coils 22a, 22b, 22c can be controlled. The power supply circuit 26 supplies power voltage (24V) to the head driving circuit 20 and charges back electromotive force avalanche energy generated in the head coils 22a, 22b, 22c and fed back to the power supply circuit 26. When charging the back electromotive force avalanche energy, a capacitor (not shown) provided in the power supply circuit 26 is used. With the above configuration, there are formed a first circuit capable of grounding the higher-voltage input terminals of the head coils 22a, 22b, 22c, and a second circuit capable of electrically connecting the lower-voltage output terminals of the head coils 22a, 22b, 22c to the power supply circuit 26. Thus, the back electromotive force avalanche energy generated in the head coils 22a, 22b, 22c can be fed back to the power supply circuit 26 through the second circuit. This is effective in saving power. Also, because the back electromotive force avalanche energy is not applied to the FETs 23a, 23b, 23c, the circuit elements are not subjected to a thermal influence. Further, since the need for selection of elements that are not easily influenced by heat is eliminated, the use of the expensive elements is not necessary. In addition, because the large-size control elements are not necessary, it is not necessary to increase the size of the circuit substrate. However, if a so-called half-dot printing, in which printing operations using adjacent head coils (e.g., 22a and 22b) are sequentially performed by the above head driving circuit 20, is executed, the FET 21 must be kept ON to activate the head coil 22b while deactivating the head coil 22a. In this situation, a current path, indicated by the dashed lines in Fig. 2, is formed in the second circuit. The current path includes the head coil 22a, the diode 25a and the FET 21 for maintaining an induction field generated in the head coil 22a. When such a current path is formed, an induced current is allowed to flow to the head coil 22a, which provides resistance to the returning operation of the head pin actuated by the head coil 22a, thereby deteriorating the printing quality. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a head driving circuit capable of preventing the generation of the induction field in the head coil to thereby eliminate the resistance to the returning operation of the head pin in order to maintain the printing quality. In order to achieve the above objects, according to the invention, there is provided a circuit, adapted to drive head pins of a dot impact head, comprising: a plurality of head coils, each of which has a first terminal and a second terminal and is adapted to actuate one of the head pins, a first control element, operable to control current flowing through the first terminal of each of the head coils, and having: a first terminal, electrically connected to the first terminal of each of the head coils; and a second terminal, adapted to be electrically connected to a power supply circuit; a plurality of second control elements, each of which is operable to control current flowing through the second terminal of an associated one of the head coils, and has: a first terminal, electrically connected to the second terminal of the associated one of the head coils; and a second terminal, being grounded; a first circuit, operable to return, to the power supply circuit, a back electromotive force avalanche energy generated when at least one of the head coils is deactivated, the first circuit including: a first end, electrically connected to the first terminal of the first control element and to the first terminal of each of the head coils; and a second end, being grounded; and a second circuit, operable to return the back electromotive force avalanche energy to the power supply circuit, and including: a first end, electrically connected to the first terminal of each of the second control element and to the second terminal of each of the head coils; a second end, adapted to be electrically connected to the power supply circuit; and a third circuit, configured to be activated when the first control element is deactivated and to be deactivated when the first control element is activated. With the above configuration, the second circuit is deactivated when current flows through any one of the head coits, and is activated when current flowing to the head coils is cutoff. Accordingly, the back electromotive force avalanche energy generated after one printing operation is executed (the first control element is deactivated) can be returned to the power supply circuit by way of the third circuit When the first control element is again activated in order to perform a subsequent printing operation, the third circuit is deactivated so that the induced current is prevented from being generated in the head coils. As a result, since no induced magnetic field is generated in the head coils, the head pins can be returned to their original positions without any resistance. Each of the first control element and the second control element may be a switching element having three terminals such as a transistor, and more specifically a field effect transistor. When the first control element and one of the second control elements are respectively turned ON, current is allowed to flow through an associated one of the head coils, a magnetic attractive force is generated in the head coll. Upon the generation of the magnetic attractive force, an associated one of the head pins is moved to thereby execute printing. After the printing, the second control element is turned OFF to thereby stop the current flowing to the one head coil, so that the one head pin is returned to its initial position. The first circuit may include a diode. A cathode of the diode may serve as the first end, and an anode of the diode may serve as the second end. The second circuit may include a plurality of diodes. An anode of each of the diodes may serve as the first end. A cathode of each of the diodes may be electrically connected to the third circuit. With the above configurations, the direction from the ground to the power supply circuit by way of the head coils becomes a forward direction so that the back electromotive force avalanche energy generated in the head coils can be returned to the power supply circuit. On the other hand, since the direction from power supply circuit to the ground by way of the head coils becomes a backward direction, no current flows to the head coils from the power supply circuit by way of the second circuit and no current flows to the ground from the head coil side by way of the first circuit. The third circuit may include: a field effect transistor; and a capacitor, one end of which is electrically connected to a gate terminal of the field effect transistor. BRIEF DESCRIPTION OF THE DRAWINGS The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein: Fig. 1 is a circuit diagram showing a first example of a conventional head driving circuit; Fig. 2 is a circuit diagram showing a second example of a conventional head driving circuit; Fig. 3 is a circuit diagram showing a head driving circuit according to a first embodiment of the invention; Figs. 4A to 4D are time charts for explaining operations of the head driving circuit of Fig. 3; and Fig. 5 is a circuit diagram showing a head driving circuit according to a second embodiment of the invention. DETAILED DESCRIPTION OF THE EMBODIMENTS Embodiments of the invention will be described below in detail with reference to the accompanying drawings. As shown in Fig. 3, a head driving circuit 30 according to a first embodiment of the invention comprises: an FET (p-channel type) 31; head coils 32a, 32b, 32c; FETs (n-channel type) 33a, 33b, 33c; a diode 34; diodes 35a, 35b, 35c; a power supply circuit 36; and an induced current cutoff circuit 37. The FET 31 is controlled by a control port H, while the FETs 33 are controlled by control ports La, Lb, Lc, respectively. In the present embodiment, for convenience of explanation, there three head coils 32a, 32b, 32c are used as driven members; however, it goes without saying that the number of head coils can be set arbitrarily according to the number of dots required of the dot impact head. The source terminal of the FET 31 is connected to the power supply circuit 36, the gate terminal thereof is connected to the control port H, and the drain terminal thereof is connected to the higher-voltage input terminals of the head coils 32a, 32b, 32c and to the cathode of the diode 34. The source terminals of the FETs 33a, 33b, 33c are respectively grounded, the gate terminals thereof are connected to the control ports La, Lb, Lc respectively, and the drain terminals of the FETs 33a, 33b, 33c are respectively connected to the lower-voltage output terminals of the head coils 32a, 32b, 32c and to the anodes of the diodes 35a, 35b, 35c. The cathodes of the diodes 35a, 35b, 35c are respectively connected to the induced current cutoff circuit 37. The anode of the diode 34 is grounded. The FET 31 controls current which flows to the head coils 32a, 32b, 32c. A voltage applied to the gate terminal of the FET 31 is controlled through the control port H to control a voltage between the gate terminal and the source terminal. When the FET 31 is turned ON, the higher-voltage input terminals of the head coils 32a, 32b, 32c are electrically connected to the power supply circuit 36. When current is allowed to flow into any one of the head coils 32a, 32b, 32c, a magnetic attractive force is generated to move an associated iron core and a head pin to perform printing. The FETs 33a, 33b, 33c respectively control the current that flows into the head coils 32a, 32b, 32c. A voltage applied to the gate terminal of each of the FETs 33a, 33b, 33c is controlled through each of the control ports La, Lb, Lc to control a voltage between the gate terminal and the source terminal thereof. When the FETs 33a, 33b, 33c are turned ON, current is fed from the lower-voltage output terminals of the head coils 32a, 32b, 32c to the ground. The FETs 33a, 33b, 33c are connected so as to correspond to the head coils 32a, 32b, 32c respectively; and, by individually turning ON the FETs 33a, 33b, 33c, the current flowing to any one of the respective head coils 32a, 32b, 32c can be controlled. The power supply circuit 36 supplies power voltage (24V) to the head driving circuit 30 and charges back electromotive force avalanche energy generated in the head coils 32a, 32b, 32c and fed back to the power supply circuit 36. When charging the back electromotive force avalanche energy, a capacitor (not shown) provided in the power supply circuit 36 is used. The induced current cutoff circuit 37 comprises an FET (n-channel type) 371, a capacitor 372, and a resistor 373. The FET 371 is turned ON when the voltage of the gate terminal thereof is higher than the voltage of the source terminal thereof. The capacitor 372 is connected so as to control the ON time duration of the FET 371. That is, the flow of the current in the FET 371 is allowed until the capacitor is charged completely. The flow of current causes a voltage drop in the resistor 373, thereby turning the FET 371 ON. After completion of the charging of the capacitor 372, the FET 372 is turned OFF and the current is cutoff. Now, description will be given below of an operation to be executed when the half-dot printing is executed using the above head driving circuit 30. For example, a case will be considered where printing with the head coil 32a and printing with the head coil 32b are sequentially executed. Fig. 4A shows current (la) flowing to the head coil 32a. Fig. 4B shows voltage (V) applied to the FET 33a. Fig. 4C shows the displacement amount (Xa) of the head pin. Fig. 4D shows current (Ib) flowing to the head coil 32b. When the FET 31 and FET 33a are turned ON, current starts to flow to the head coil 32a (Point a). In response to this, there is generated a magnetic field in the coil head 32a, the iron core is moved by a magnetic attractive force due to such magnetic field, and the head pin is moved because it is linked with the iron core. When the value of the current flowing to the head coil 32a reaches the peak value (Point b), the displacement of the head pin becomes greatest and the head pin is struck on a printing medium to thereby execute the printing. After execution of the printing, the FET 33a is turned OFF and the supply of the current to the head coils 32a is cutoff. However, since a current to maintain the magnetic field generated in the head coil 32a flows to the head coil 32a, current la does not drop suddenly but attenuates gently (Point c). Also, as a result of this, there is generated back electromotive force avalanche energy in the head coil 32a; however, the back electromotive force avalanche energy is fed back from the head coil 32a to the power supply circuit 36 by way of the diode 34a and induced current cutoff circuit 37. The back electromotive force avalanche energy generated in the head coil 32a passes through the diode 35a and applies a voltage to the resistor 373 while charging the capacitor 372. Owing to this, the FET 371 is turned ON and the back electromotive force avalanche energy is fed back to the power supply circuit 36. Until the capacitor 372 is charged completely, the FET 371 is kept in the ON state; however, after completion of the charging, the FET 371 is turned OFF, thereby disabling the supply of the current to the power supply circuit 36. Thus, the induced current generated in the head coil 32a is cutoff and, even when the FET 31 is turned ON, the half dot printing (printing with the head coil 32b) can be executed without any problem. In the present embodiment, when the FET 31 is turned ON, the induced current cutoff circuit 37 is turned OFF; whereas when the FET 31 is tuned OFF, the induced current cutoff circuit 37 is turned ON. When the induced current cutoff circuit 37 is turned OFF, the path of the induced current generated in the head coil 32 is cutoff, so that the induced current is prevented from flowing to the head coil 32a. Therefore, the head pin is able to return to its original position without resistance (Point d), which makes it possible to execute the half dot printing with high printing quality. Next, a second embodiment will be described with reference to Fig. 5. Similar components to those in the first embodiment will be designated by the same reference numerals or characters, and repetitive explanations for those will be omitted. In this embodiment, a head driving circuit 40 comprises; head coils 32a, 32b, 32c; an FET (p-channel type) 41; FETs (n-channel type) 33a, 33b, 33c; a diode 34; diodes 35a, 35b, 35c; a transistor 47; a capacitor 48; an FET (p-channel type) 50; a transistor 51; and resistors 49, 52, 53, 54, 55. The source terminal of the FET 41 is connected to the power supply circuit 36, the gate terminal thereof is connected to the emitter terminal of the transistor 51, the resistor 54 and the resistor 55, and the drain terminal thereof is connected to the higher-voltage input terminals of the head coils 32a, 32b, 32c and the cathode of the diode 34. The lower-voltage output terminals of the head coils 32a, 32b, 32c are connected to the drain terminals of the FETs 33a, 33b, 33c and the anodes of the diodes 35a, 35b, 35c, respectively. The gate terminals of the FETs 33a, 33b, 33c are connected to control ports La, Lb, Lc respectively, while the source terminals thereof are grounded respectively. Also, the anode of the diode 34 is grounded. The base terminal of the transistor 47 is connected to a control port H, the emitter terminal thereof is connected to the ground, and the collector terminal thereof is connected to the capacitor 48 and resistor 49. The gate terminal of the FET 50 is connected to the intermediate portion of the drive circuit existing between the capacitor 48 and resistor 49, the source terminal thereof is connected to the cathodes of the diodes 35a, 35b, 35c and resistor 49, and the drain terminal thereof is connected to the power supply circuit 36. The base terminal of the transistor 51 is connected to the intermediate portion of the drive circuit existing between the resistors 52 and 53, while the emitter terminal thereof is connected to the power supply circuit 36. Next, description will be given below of the operation of the head driving circuit 40. in order to cause current to flow to the head coil 32a to thereby move a head pin and execute printing, the transistor 47 is turned OFF by turning OFF the control port H. in this state, no current is allowed to flow to the base terminal of the transistor 51, and the transistor 51 also remains OFF. On the other hand, since a power supply voltage is applied to the resistor 54, a voltage applied to the gate terminal of the FET 41 is lower than a voltage applied to the source terminal thereof, the FET 41 is turned ON, and thus current is supplied from the power supply circuit 36 to the head coil 32a. Also, by turning ON the control port La at the time when the control port H is turned OFF, a current is supplied from the head coil 32a to the ground. As a result, a current is allowed to flow through the head coil 32af and thus the head pin is struck on a printing medium, thereby executing the printing. After the printing, first back electromotive force avalanche energy generated in the head coil 32a must be fed back to the power supply circuit 36, and then the flow of the induced current to the head coil 32a must be prevented. Therefore, the following operation must be performed. By turning on the control port H at the completion of the printing, the transistor 47 is turned ON, and thus current flows to the base terminal of the transistor 51 as well to thereby turn ON the transistor 51. As a result of this, a voltage applied to the gate terminal of the FET 41 is equal in potential to a voltage applied to the source terminal thereof, so that the FET 41 is turned OFF. Also, by turning ON the control port H, the transistor 47 is turned ON and thus current is supplied from the resistor 49 to the ground side. As a result of this, there occurs a voltage drop between the source and gate terminals of the FET 50, thereby turning ON the FET 50. During the time when the FET 50 is kept in the ON state, the back electromotive force avalanche energy is fed back the power supply circuit 36. The time duration to turn ON the FET 50 is controlled by the electrostatic capacity of the capacitor 48. When the charging of the capacitor 48 is completed, the FET 50 is turned OFF and thus the supply of the current from the head coil 32a to the power supply circuit 36 is cutoff. As a result of this, the induced current Is cutoff. As described above, in the present embodiment as well, when the induced current cutoff circuit is activated, while an induction field is being generated, the path of the induced current generated in the head coil is cutoff by the induced current cutoff circuit, the induced current is prevented from flowing to the head coil. Therefore, the head pin can return to its original position without resistance, thereby being able to execute the half dot printing. By the way, in the present embodiment, in order to cutoff the induced current at a given time, a circuit including two or more elements is incorporated into the second circuit; however, alternatively, a switching element may be incorporated into the second circuit, and the ON/OFF of the switching element may be controlled using a port signal. The invention is not limited at all to the above-mentioned embodiments of the invention and the description thereof. According to the invention, other various changes and modifications are also possible without departing from the scope of the appended patent claims and within the range where the persons skilled in the art can understand easily.

Documents:

1234-che-2006 drawings.pdf

1234-che-2006 form-2.pdf

1234-che-2006 correspondence po.pdf

1234-che-2006 power of attorney.pdf

1234-CHE-2006 CORRESPONDENCE OTHERS.pdf

1234-che-2006-abstract.pdf

1234-che-2006-claims.pdf

1234-che-2006-correspondence-others.pdf

1234-che-2006-description-complete.pdf

1234-che-2006-form 1.pdf

1234-che-2006-form 18.pdf

1234-che-2006-form 3.pdf

1234-che-2006-form 5.pdf