Printing wire driving apparatus

Abstract: In a printing wire driving apparatus wherein a plurality of printing wires mounted on a carriage of a printer are driven in accordance with printing data, there are provided with a voltage regulator (10) including a transistor chopper (10a) for periodically chopping input voltage, and a smoothing circuit (10b) for smoothing the output voltage of the transistor chopper, a plurality of printing wire driving circuits connected in parallel across output lines of the smoothing circuit (106), each of the printing wire driving circuits including a printing wire operating coil (18), a driving circuit (19) connected in series with the operating coil and a transistor (10) which are connected in series across the output lines. There is also provided a system controller (30) for controlling the operation of the transistor chopper (10a), and for controlling a printing signal generator (31) which generates electric signals adapted to control the printing wire driving circuits.

Description: This invention relates to a driving apparatus which is not expensive but capable of efficiently controlling the driving of printing wires of a wire dot printer which depicts characters, figures, patterns, etc. by dot printing. One example of a prior art printing apparatus will be described with reference to Fig. 1. Fig. 1 is a simplified drawing showing an embodiment disclosed in Japanese Laid Open Patent Specification No. 161,549 of 1987 invented by the applicant. As shown, a voltage V₁ is applied across a capacitor 2 from a power supply, not shown, through input terminal 1. To one terminal of the capacitor 2 is connected a voltage regulator 3 including a control terminal 3a for controlling generation of its output voltage V p . In parallel with the voltage regulator 3 is connected a series circuit including a diode 4, a coil 5 adapted to drive a printing wire, not shown, and an FET transistor 6 for controlling current supplied to coil 5 from the voltage regulator 3. To the gate electrode 6a is applied the printing data. A diode 7 is connected to a junction of the coil 5 and transistor 6. The purpose of the diode 7 is to feed back to the capacitor 2 the current generated by electromagnetic energy stored in the coil 5. In Fig. 1, i₁ designates the current flowing through the coil 5 from the voltage regulator 3 when the transistor 6 is ON, while i₂ designates the current flowing through the diode 4, coil 5 and transistor 6 generated by the electromagnetic energy stored in the coil 5 while the voltage regulator 3 is OFF. i₃ designates the current generated by the electromagnetic energy stored in the coil 5 while the voltage regulator 3 is OFF and the transistor 6 is also OFF. In this prior art apparatus, since the voltage regulator 3 is usually of the dropper system, where a voltage difference V₁-V p is large, the power consumption of the regulator 3 itself increases, thereby decreasing the overall efficiency. In other words, in the case of the dropper system, the voltage regulator 3 consumes a large power expressed by (V₁-V p )xi(current). US-A-4637742 discloses a wire drive circuit which has the features of the preamble of claim 1. EP-A-0306347 discloses a chopper arrangement for a wire printer drive. An object of this invention is to provide an improved printing wire driving apparatus capable of decreasing power consumption of a printer system as a whole by designing the voltage regulator as a chopper type. Another object of this invention is to provide an inexpensive printing wire driving apparatus by mounting constituting elements of the printer on a carriage together with a printing head. A further object of this invention is to provide a novel printing wire driving apparatus in which input voltage to the voltage regulator can be set to high voltage whereby electromagnetic energy stored in driving circuits can be rapidly fed back to a capacitor at the input of the voltage regulator for obtaining a high energy efficiency and a high response speed. According to this invention there is provided printing wire driving apparatus wherein a plurality of printing wires mounted on a carriage of a printer are driven in accordance with printing data, the driving apparatus comprising: a voltage regulator inputted with a d.c. voltage for outputting a d.c. voltage lower than the inputted d.c. voltage; a plurality of printing wire driving coils respectively energized by the output voltage of the voltage regulator; a plurality of switch means provided between the voltage regulator and driving coils for opening and closing electrical connection therebetween; switch actuating means for actuating the plurality of switch means in response to the printing data; and characterised in that the driving apparatus further comprises unidirectional elements for feeding back electromagnetic energy stored in the driving coils to an input side of the voltage regulator, and in that the voltage regulator includes chopper means for intermittently chopping the input d.c. voltage to generate the output d.c. voltage and wherein the voltage regulator further comprises a smoothing coil and a capacitor for smoothing the chopped output voltage of the voltage regulator and wherein circuit constants of the smoothing coil and capacitor are determined such that the energy stored therein would be smaller than the electromagnetic energy stored in the driving coils. In the accompanying drawings: Fig. 1 is a connection diagram showing a prior art printing wire driving apparatus; Fig. 2 is a connection diagram showing one embodiment of the printing wire driving apparatus according to this invention; Fig. 3 is a graph showing waveforms at various parts of a voltage regulator utilized in this invention; Fig. 4 is a graph showing the relation between current waveforms and the timing of driving the printing wire; and Fig. 5 is a perspective view showing an application of the printing wire driving apparatus of this invention. A preferred embodiment of this invention will now be described with reference to Fig. 2 in which the same reference numerals and letters designate the same elements as in Fig. 1. In the embodiment shown in Fig. 2, across input terminals 1 are parallelly connected a capacitor 2 and voltage regulator 10 of the chopper type. The voltage regulator 10 is constituted by a controller 10a and smoothing circuit 10b. The controller 10a is constituted by an FET transistor 11, a transformer 13 supplying an interrupted high frequency control signal to the transistor 11 through a bias resistor 12 and a control circuit 14 which applies a control signal to the transformer 13 upon receival of an output voltage V p fed back through a wire 14b. A direct coupling may be used without using the transformer 13, but electric power consumed for controlling increases. The smoothing circuit 10b is constituted by a diode 15, a reactor 17 and a capacitor 16. Connected to the voltage regulator 10 are series circuits of coils 18-1 ... 18-n adapted to drive printing wires not shown, and coil driving circuits 19-1 ... 19-n, so as to be applied with the output voltage of the voltage regulator 10. To n printing wires are connected n coils 18-1 ... 18-n and n coil driving circuits 19-1 ... 19-n. Each of coil driving circuits 19-1 ... 19-n is constituted by a transistor 20 and a diode 21. The cathode electrodes of n diodes 21 are connected to the capacitor 2 via a wire 22. The control circuit 14 is connected to a system controller 30 via a wire 14a. A printing signal generator 31 is connected between the system controller 30 and respective gate electrodes or n transistors 20. The printing signal generator 31 is made up of a shift register 31a, a latch circuit 31b and an enabling circuit 31c. The circuit shown in Fig. 2 is driven by voltage V DD of about 5 volts. When a voltage V₁ is applied from a power supply, not shown, across the capacitor 2 and constant voltage source 10 through the input terminals 1, the transistor 11 generates an output voltage of interrupted waveform of from several tens KHz to several MHz at its output line A. This output voltage is smoothed by the smoothing circuit 10b to produce an output voltage V p . This output voltage V p is fed back to the control circuit 14 via the wire 14b so that circuit 14 applies a control signal to the transformer 13. When no output voltage appears at the output line A, current generated by the electromagnetic energy stored in the reactor 17 would be smoothed out by the serially connected coils 18-1 ... 18-n, coil driving circuits 19-1 ... 19-n, and diode 15. Or tooth shaped output voltage V p would be smoothed out by so-called chopper action. When the transistor 11 is ON, its voltage drop is less than 0.5 V meaning a small power loss. However, in the case of the dropper system, the power loss due to the difference between the voltages V₁ and V p becomes large. Usually ranges of V p = 20-35 V and V₁ = 40-60 V are selected. The graphs shown in Fig. 3 show the operation of the voltage regulator 10 in which curve (a) represents an ON-OFF signal applied to the control circuit 14 from the system controller 30 via the wire 14a, curve (b) shows the interrupted control signal applied to the transistor 11, and curve (c) the waveform of the output voltage V p . Rise time t r and fall time t f are required to be several microns, respectively. Because the rise time and fall time should be sufficiently smaller than about 200 µs of the driving period of the printing wires. When a printing data is supplied from outside to the system controller 30, it sends out an ON-OFF signal to the voltage regulator 10 via the wire 14a. The system controller 30 also sends out a printing data signal, shift clock, latch pulse and enabling signal, to the printing signal generator 31 over conductor 30a. Although not shown, the system controller 30 generates other control signals for controlling other mechanisms of the printer. When applied with various signals described above, the shift register 31a of the printing signal generator 31 sequentially shifts the print data signal supplied from the system controller 30 and stores the signal. The latch circuit 31b latches the stored printing data, and enabling circuit 31c supplies stored printing data to the coil driving circuits 19-1 ... 19-n over wires 31d-1 ... 31d-n. The method of driving the printing wires will be described with reference to the graphs shown in Fig. 4 in which curve (a) shows the output waveform of the voltage regulator 10, curve (b) the ON time of the transistor 20, curve (c) the current flowing through the coils 18-1 ... 18-n The curve (c) shows that the printing dire is driven for forming dots on a recording paper before current i₂ terminates The letter i₁ designates the current flowing to the transistor 20 from the voltage regulator 10. Current i₂ is caused to flow by the electromagnetic energy stored in the driving coils 18, and flows through a loop formed by the transistor 20, diode 15 and reactor 17. Current i₃ is caused to flow by the electromagnetic energy stored in the coals 18 and corresponds to the current at the end of current i₂ (that is at the time of turning OFF the transistor 20). The current i₃ flows through a loop formed by the diode 21, capacitor 2, diode 15 and reactor 17 so as to feed back energy to the capacitor 2, thereby improving the driving efficiency of the printing wire. We now derive out equations representing the currents i₁, i₂ and i₃. In the following equations, R and L respectively represent the resistance and inductance of coils 18 and voltage drops of the diodes 15 and 21 and transistor 20 are neglected. i₁ = V p /R·(1-EXP(-t/t₀)) i₂ = i₂₀·EXP(-t/t₀) i₃ = (i₃₀+V₁/R)·EXP(-t/t₀)-V₁/R where t=time, i₂₀ and i₃₀ show final values of the currents i₁ and i₂, respectively, t₀ = L/R and EXP is an abbreviation of exponential. With this method of driving the printing wire a unique utilization of the electromagnetic energy stored in the coil 18 can be realized. With this driving method, it is possible to decrease the power supplied to the voltage regulator 10 by about 30 to 40 % when compared with another driving method. It is advantageous to quickly extinguish the current i₃ for the purpose of improving the frequency response of the printing wire. From the foregoing equation, the time t₃ at which the current i₃ becomes zero can be shown as follows: t₃ = -t₀·log[V₁/(Ri₃₀+V₁)] This equation shows that t₃ can be made smaller as V₁ icreases. Since V p is constant, even if V₁ is increased, power consumption does not change in any appreciable amount because the voltage regulator 10 is of the chopper type. In other words, this invention has a feature of obtaining a high speed response characteristic by decreasing the power consumption. The other methods of driving shown by curves (d), (e) and (f) in Fig. 4 will be described. Curve (d) shows the output voltage V p of the voltage regulator 10, while curve (e) the ON time of the transistor 20. The latter half of the curve (e) is chopped under the control of the printing signal generator 31. As a consequence, the current flowing through the coil 18 is shown by curve (f). In this case too, it is possible to efficiently drive the printing wire. It should be understand that the number of turns of the coil 18 can be decreased to decrease the inductance thereof for decreasing the current rise time. The printing wire driving apparatus of this invention is suitable to use for a serial printer. Preferably, the number n of the printing wires is about 8 to 48. The response frequency is 1-4 KHz, the peak value of the current i₁ is about 0.5-2 A, and the driving energy of the printing wires is usually 3-6 mJ in one cycle. One example of a printer utilizing the printing wire driving apparatus of this invention is shown in Fig. 5. In this printer, a printing head 42 incorporated with n printing wires and n coils 18-1 ... 18-n shown in Fig. 2 is fixed on a carriage 43. A printing data signal, a shift clock pulse, a latch pulse and an enabling signal are supplied to the printing head 42a from printer control board 47 through a cable 46. Upon receival of these signals, the printing head 42 is moved to the left or right on a belt 44 driven by pulleys 45a and 45b for printing characters or dots on a recording paper 41 wrapped abut a platen 40. An ink ribbon, and an electric motor for driving the pulleys are not shown in Fig. 5. The number of conductors contained in the cable 46 is usually larger than (n+1) representing the sum of n of the number of the coils 18-1 ... 18-n shown in Fig. 2 and one conductor common to the coils. Where the number of the coils is large, the cable 46 is divided into 2 to 4 subcables. In such case, not only the cost increases, but also the load of the driving motor increases. For this reason, the driving circuit 19-1 ... 19-n and printing signal generator 31 are commonly mounted on the carriage 43 for decreasing the numbers of cables and conductors contained therein. With this construction, the number of conductors of the cable can be reduced to a sum of conductors 30a, 22, a common line to the coils 18-1 ... 18-n, V DD and GND (grounded line). Since the number of wires included in the conductor 30a is 4 for transmitting printing data signal, a shift clock pulse, a latch pulse, and an enabling signal so that the total number of conductors is 9 at the minimum. Where the number n coils 18-1 ... 18-n is equal to 48, usually the minimum number of conductors are 49, but with the improved construction just described, the necessary number of conductors can be reduced to only 9. The result of analysis of the energy consumed by the driving apparatus of this invention is as follows: the copper loss of the coils 18-1 ... 18-n amounts to 60-80 %, the iron loss of the electromagnetic circuit of the coils 18-1 ... 18-n amounts to 5-15 %, the energy converted into a mechanical energy including a dot forming energy amounts to 5-10 %, and the energy consumed by such circuit elements as transistors and the like amounts to about 5 %. The iron loss of the electromagnetic circuit can readily be reduced by fabricating the magnetic circuit with laminated iron sheets or ferrite, which also improves the driving efficiency of the printing wire. In working out the invention into practice, where the number of printing wires is 48 and when the peak value of i₁ is 1 ampere, the total peak current reaches 48 A. In such case, a stable switching operation of the voltage regulator 10, shown in Fig. 2 can not always be obtained. In this case, by dividing the number of coils into four groups each consisting of 12 coils, by providing four voltage regulators 10 and by driving these regulators in a time division fashion, the peak current of each voltage regulator 10 can be reduced to 12 A (48 A/4). Control of the peak current contributes to the decrease of the load of a power supply, not shown, on the input side of the voltage regulator 10. In addition, it is also possible to decrease the line drop due to resistance and inductance of the wirings, which enables to supply stable driving power to the printing wires. Examples of the concrete values of the constituting elements of this invention and setting conditions of the elements will now be described. Assume that V₁ = 60 V, V p = 20 V, the inductance L₁ of the reactor 17 is 10⁻⁴H, the capacitance C of the capacitor 16 is 10⁻⁷F, the equivalent inductance L₂ of the coils 18-1 ... 18-n is 2.4 x 10⁻³H and that the equivalent resistance R is 12 ohms. Furthermore, let us assume that voltage build up equations are V R1 and V R2 when current is passed through the coils 18-1 ... 18-n only once and 12 times respectively. V R1 = 60 - 2.4 exp(α₁t) - 57.6 {cos (ω₁t) + 9.3 x 10⁻⁴ sin(ω₁t)} exp(β₁t) where α₁ = - 4.8 x 10³, β₁ = -1.0 x 10², ω₁ = 3.23 x 10⁵ V R2 = 60 - 20 exp(α₂t) - 40 { cos (ω₂t) + 6.45 x 10⁻³ sin(ω₂t)} exp(β₂t) where α₂ = -3.33 x 10³, β₂ = -8.33 x 10², ω₂ = 3.87 x 10⁵ These equations show that the rise time in which the voltage V p of the voltage regulator 10 reaches 20 v is not largely influenced by the number of the coils 18-1 ... 18-n but the voltage V p builds up in only several µs. Assuming that build down voltages at the time of interruption are V F1 and V F2 at the end of an interval in which the output voltage V p is maintained at a constant value of 20 V, and that the initial value i₀ = 1 A per one of the coils 18-1 ... 18-n, we obtain V F1 = - 2.88 exp(α₁t) + 22.88 { cos(ω₁t) - 1.6 x 10⁻³ sin(ω₁t) } exp(β₁t) where α₁ = -4.8 x 10³, β₁ = -1.0 x 10², ω₁ = 3.23 x 10⁵ V F2 = -24 exp(α₂t) + 44 {cos(ω₂t) -2.5 x 10⁻³ sin(ω₂t)} exp(β₂t) where α₂ = -3.33 x 10³, β₂ = -8.33 x 10², ω₂ = 3.87 x 10⁵ V F1 and V F2 are almost the same as each other and not influenced by the number of the coils 18-1 ... 18-n. These equations show that the build up and build down characteristics of the voltage regulator 10 do not depend upon the number of energizations of the coils 18-1 ... 18-n so that there is no difference between energies for printing dots, which ensures high quality printing. In the above described examples, the values of the constituting elements were determined under the conditions that L₁ = 10⁻⁴H < 2.4 x 10⁻³/12 = 2 x 10⁻⁴ , that 1/2CV p ² = 1/2 x 10⁻⁷ x 10² < 1/2 L₂i₀² = 1/2 x 2.4 x 10⁻³ x 1² . In other words, various conditions are determined such that the energy stored in the smoothing circuit 10b shown in Fig. 2 is sufficiently smaller than the energy 3 mJ-5 mJ necessary for energizing the coils 18-1 ... 18-n, when J means joule. The chopping frequency f is determined as follows. The electric power supplied from the voltage regulator 10 is expressed by P s = 1/2L₁i²f , provided that this electric power is larger than the power V p i₀n consumed by the coils 18-1 ... 18-n. Assuming that n=12, i₀ = 1 A, V p = 20 V, L₁ = 10⁻⁴H and i = ni₀ , as determined by two foregoing equations, f should be larger than a value expressed by an equation f > V p .i₀ n/½L₁i² = 34 KHz . Furthermore, f should satisfy the build up and build down characteristics of the voltage regulator 10. In various equations described above, there are products of the attenuation term of EXPONENTIAL and a SINUSOIDAL oscillation term. Consequently, based on the constant ω₁, when using aforementioned values of the constitutional elements, it is advantageous to make the chopping frequency f to be above 1 MHz, which ensures a predetermined build up characteristic of the constant voltage source as well as the flat characteristic in the normal state. In addition, by making sufficiently high the chopping frequency, it is possible to decrease the value of the reactor 17 and capacitor 16 of the smoothing circuit 10b so that the volume of the printing wire driving apparatus can be reduced and its cost of manufacturing can also be reduced. Thus, this invention is characterised by the setting of the values of the constitutional elements. As above described, according to this invention, since an ON-OFF type chopper system is used for driving printing wires with a high efficiency driving circuit and the constant voltage source which supplies an operating power to the printer, the operating efficiency of the apparatus can be improved. Moreover, as it is possible to set the input voltage regulator at a high value, the energy stored in the coils can be more rapidly fed back, thus obtaining a high energy efficiency and a high speed response characteristic. The invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims: Printing wire driving apparatus wherein a plurality of printing wires mounted on a carriage of a printer are driven in accordance with printing data, the driving apparatus comprising: a voltage regulator (10) inputted with a d.c. voltage for outputting a d.c. voltage lower than the inputted d.c. voltage; a plurality of printing wire driving coils (18-1, ..., 18-n) respectively energized by the output voltage of the voltage regulator (10); a plurality of switch means (19-1, ..., 19-n) provided between the voltage regulator (10) and driving coils (18-1, ..., 18-n) for opening and closing electrical connection therebetween; switch actuating means (31) for actuating the plurality of switch means (19-1, ..., 19-n) in response to the printing data; and characterised in that the driving apparatus further comprises unidirectional elements (21) for feeding back electromagnetic energy stored in the driving coils (18-1, ..., 18-n) to an input side of the voltage regulator (10), and in that the voltage regulator (10) includes chopper means (10a) for intermittently chopping the input d.c. voltage to generate the output d.c. voltage and wherein the voltage regulator (10) further comprises a smoothing coil (17) and a capacitor (16) for smoothing the chopped output voltage of the voltage regulator (10) and wherein circuit constants of the smoothing coil (17) and capacitor (16) are determined such that the energy stored therein would be smaller than the electromagnetic energy stored in the driving coils (18-1, ..., 18-n). The printing wire driving apparatus according to claim 1 further comprising means (30) for driving the voltage regulator at a predetermined timing. The printing wire driving apparatus according to claim 1 or claim 2 wherein the switch actuating means (31) includes means for effecting chopping operation of respective switch means at a portion of an interval in which the electrical connection is closed by respective switch means. The printing wire driving apparatus according to any preceding claim wherein the driving coils (18-1, ..., 18-n), switch means (19-1, ..., 19-n), switch actuating means (31) and said unidirectional elements (21) are mounted on the carriage. The printing wire driving apparatus according to any preceding claim wherein the chopper means (10a) of the voltage regulator (10) comprises a transistor (11).

Inventors: FUTAGAWA YOSHIKIYO C O SEIKO E, MITSUISHI AKIO C O SEIKO EPSON, FUTAGAWA, YOSHIKIYO C/O SEIKO EPSON CORP, MITSUISHI, AKIO C/O SEIKO EPSON CORP
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1991
Main classification: B41J2
IPC codes: B41J2