EP0020975A1 - Driving circuit for the supply of a current to a coil and its use in a printing device - Google Patents

Abstract

The print hammer control circuit is controlled by a supply voltage having inherent voltage variations. The control current is applied to the winding (30) of the printing hammer and the current level in the winding detected using the resistor (32). After the current level in the winding has reached a predetermined maximum level, a chronology circuit (41) is actuated to control the duration of application of the maximum current. Variations in supply voltage over time and strike force are significantly reduced because the timing is based on when the predetermined control current level is achieved and no longer includes rise time . Variations in inductance can also be compensated for by adjusting the timing circuit. Can be used in any type of hammer printer.

Description

Technical area

The present invention relates to solenoid control systems in general, and more particularly to systems which include a solenoid control system in which the duration and force of the output of the solenoid regardless of voltage variations must be precisely controlled. The present invention relates more particularly to the precise control of an impact printer operating at high speed and controlled by a solenoid, enabling quality printing to be obtained.

State of the Prior Art

Printers using the principle of the daisy-shaped print wheel and the high speed hammer are well known in the prior art and commercially available.

Precise control of the printer is necessary for good quality printing. Several techniques have been used to analyze and thus control the striking force, the flight time and the duration of application of the force of a striking hammer. These data are used according to the area to be printed, the number of forms to be printed, etc.

We know in the prior art several devices designed to try to solve the problem posed by the hammer control.

For example, U.S. Patent No. 3,712,212 discloses an impact printer in which the striking force varies depending on the printing surface of the character to be printed. In this patent, a rotary printing drum or wheel or an endless printing belt is used in combination with one or more printing hammers for printing on a printing medium. In a conventional manner, the electromagnetic field produced by the solenoid, triggers the flight of the hammer against the document to be printed. While in this device, the problem of striking the hammer is treated as a function of the printing surface of the characters, no attempt is made to vary the striking force of the hammer, in order to compensate for the variations in tension of food. On the contrary, the impulse applied to the winding of the hammer solenoid is effective from its application without taking into account possible variations in its rise time, caused by fluctuations in the supply voltage or variations in inductance.

US-A-No. 3,866,533 discloses another system for varying the striking force applied by a striking hammer. In this system, the width of the pulse applied to the hammer solenoid is varied according to the thickness of the forms on which printing is performed. This patent also presents a technique for regulating the input voltage to reduce variations in the hammer strike force to a minimum. However, this patent does not deal with the chronology of the printing pulse from the moment when a predetermined current level is reached in the winding of the solenoid, in order to eliminate the problems posed by the voltage variation.

US-A-No. 4,030,591 also discloses a hammer hammer control circuit. In this system, the hammer is synchronized according to the speed printing. In this case, the triggering of the hammer is still based on the moment of reception of a pulse. The timing of the pulse is based on the printing speed. No attempt is made to command the hammer after receipt of the conditioning pulse. The duration of the pulse is also counted from the start of the excitation of the hammer winding and not from the moment when the current in the winding reaches a predetermined level. Thus, variations in the rise time can severely affect the system presented in this patent.

Statement of the invention

The device of the present invention consists of a solenoid control circuit excited by a power supply with inherent voltage variations. The excitation current is applied to the winding and the current level in the winding is detected. After the current level in the winding has reached a predetermined maximum level, a timing circuit is initiated to control the duration of the application of the maximum current. Variations in the supply voltage have little effect on the net electromagnetic field produced by the winding since the entire chronology is based on the instant when the level of the predetermined control current is reached, unlike the chronology which includes the rise time of the control current signal. The effects of inductance variations in solenoid type systems can also be offset by the use of the appropriate chronology.

In the preferred embodiment of the present invention, a solenoid control circuit is used in a daisy-shaped print wheel printer to precisely control the flight time, the strike force and the duration of its application by a hammer mounted on winding.

Other characteristics and advantages of the present invention will emerge more clearly from the following description, made with reference to the drawings appended to this text, which represent a preferred embodiment of the latter.

• La figure 1 représente une imprimante dans laquelle le dispositif de la présente invention peut être utilisé.
• La figure 2 est une représentation graphique de l'effet des variations de tension sur le temps de montée du courant dans un système à marteau et solénoïde.
• La figure 3 est une représentation graphique des variations du temps de montée provoquées par des variations d'alimentation et d'inductance.
• La figure 4 est une représentation schématique du circuit de commande de solénoïde de la présente invention, et
• La figure 5 est une représentation schématique des signaux associés au circuits de commande de la figure 4.

Brief Description of Drawings

• Figure 1 shows a printer in which the device of the present invention can be used.
• Figure 2 is a graphical representation of the effect of voltage variations on the current rise time in a hammer and solenoid system.
• FIG. 3 is a graphic representation of the variations in the rise time caused by variations in supply and inductance.
• FIG. 4 is a schematic representation of the solenoid control circuit of the present invention, and
• FIG. 5 is a schematic representation of the signals associated with the control circuits of FIG. 4.

Description of an embodiment of the invention

Figure 1 shows the main mechanical components of a printing system. They are shown schematically since they are well known in the prior art and the present invention relates to the control system of the hammer hammer control circuit. It is obvious that the system of the present invention could be used for other applications.

In Figure 1, a laterally sliding carriage 1 is mounted on a guide rod la and a worm 7 and carries a wheel or a rotary printing disc 2 driven by a stepping motor 3. The carriage 1 is driven by the worm 7, even driven by a stepping motor 8. The motor 8 could also drive a belt, which, in turn, would drive the carriage 1.

The printing wheel 2 is formed of a disc comprising a certain number of movable elements with character, for example in the form of flexible radial tongues 9A, 9B, 9C, etc., each bearing a printing character. The impression of a desired character is caused by the actuation of a striking hammer 10 controlled by a solenoid 11, these two elements being mounted on the carriage 1. When the appropriate character-carrying tongue arrives in the vicinity of the position the solenoid 11 applies the hammer 10 against the selected character carrier tab bringing the latter into contact with the paper 12 or any other printing medium. A transmitting wheel 13 fixed to the printing wheel 2 and rotating with the latter, operates in association with a detector FB2 to generate a current of transmitter indexing pulses controlling the operation of the printer. The emitting wheel 13 comprises a series of teeth each corresponding to one of the tongues 9A, 9B, 9C, etc. A reference pulse is generated at each revolution of the printing wheel by a single tooth on another emitter ( not shown). The printer commands can thus determine at any time the angular position of the print wheel 2 by counting the pulses received since the last reference pulse. A notched transmitter 15 is mounted on the shaft of the motor 8 and, in association with a detector FB1, delivers pulses which indicate the position of the carriage 1.

Stepper motors 3 and 8 are energized by conventional control circuits 21 and 22. US-A-No. 3,636,429 presents examples of this type of control circuit which can be used. A hammer solenoid 11 is actuated by a hammer control circuit 23 which constitutes the object of the present invention.

Figure 2 taken from US-A-No. 3,866,533 shows the variation in pulse width caused by variations in the supply voltage. The curves shown in Figure 2 are for 1, 3 or 6 forms. Thus, with regard to the signal F1, the figure shows that a variation of the supply voltage from 22 to 23 volts causes a variation of the width of the pulse of approximately 100 microseconds. As indicated previously, although the systems of the prior art try to vary the duration of the pulse applied to the solenoid controlling the hammer according to the different prints, the number of forms to be printed, etc. , once the input pulse has been applied to the solenoid, its chronology is established from the start of its application to the winding of the solenoid without taking into account the effect brought to the duration of said pulse by the variations of the supply voltage while the pulse is rising.

FIG. 3 is a graphic representation of the problem associated with variations in supply voltage during the rise of the pulse, appearing when said pulse is synchronized from the moment of application of the voltage to the winding. Figure 3 shows that a relatively high voltage applied to the winding will obviously cause a relatively rapid rise which, in turn, will cause a time of application of the maximum current relatively long compared to the time T2 measured from the moment where the nominal current in the winding reaches a predetermined level. FIG. 3 further shows that the application of a relatively low voltage to the winding causes an application time to the winding of the relatively low maximum current compared to the duration of T2. The graph in Figure 3 simply shows that variations in voltage during the signal rise can cause significant variations in the time of application of the maximum current to the winding. These rise times result in an indefinite application time of the maximum winding current, which poses very difficult problems to solve with regard to the control of the hammer. According to the present invention, the chronology of the application of the signal to the winding is established from the moment when the signal reaches its maximum selected current unlike prior art devices in which it is established from 'moment when the current is initially applied to the winding. It has been shown that extremely precise control of the electromagnetic field produced can be ensured by establishing the chronology from the application of the maximum current to the winding instead of establishing it from the initial application of said current.

FIG. 4 represents an example of a circuit making it possible to establish the chronology of a signal applied to a winding for a preselected time from the moment when a preselected maximum current is reached. FIG. 4 shows that the AND gate 25 receives a signal A by the line 24. The signal A is simply the control signal coming from the system, indicating that the hammer must be triggered. Gate A receives its other conditioning signal D via line 42. Signal D will be described later. The output of the AND gate 25 is applied by the line 26 to the AND gate 27 which receives its other conditioning input C from the chronology unit 41. The AND gate 27 delivers an output by the line 28 to the transistor switch 29 Switch 29 is simply a conventional transistor current switch. The switch 29 connects the winding 30 to ground. The switch 29 is also connected by line 31 through the resistor 32 to ground to complete the winding current circuit. Resistor 32 is a detection resistance crossed by a detected current and applied by lines 33 and 34 to a comparator 35. Comparator 35 also receives an input by line 36 to from a current reference unit 37. The current flowing in the detection resistor 32 is compared to the predetermined current delivered by the unit 37. When the current flowing in the resistor 32 is equal to the reference current unit 37, this equality is noted and an output signal B is generated. This output signal is applied to the chronology unit 41 by the line 39 and to the oscillator 40 by the line 38. The output of the oscillator 40 is applied to the line 42.

The operation of the circuit of FIG. 4 will now be described with reference to the signals of FIG. 5. The priming signal coming from the control logic of the printer or any other system, is applied by line 24 to the AND gate 25. This signal is the signal A in FIG. 5. At this instant, the signal D coming from the oscillator 40 is at the high level and therefore the AND gate 25 applies a positive logic level via line 26 to the gate. AND 27. The other input of the AND gate 27 receives the signal C from the chronology unit 41. The signal C coming from the chronology unit 41 is again at this instant at a positive logic level which causes the application of a positive logic level by line 28 to the transistor switch 29. The transistor switch 29 is a conventional transistor switch and the application of a positive potential by line 28 causes the transistor to conduct for application of current through winding 30 of the positive potential at mass e. Thus, the current begins to flow in the winding 30 which is the control winding of the solenoid. When the current flows in the winding 30 towards ground, it crosses the resistor 32 which is, as indicated above, a detection resistor. The current flowing in the detection resistor 32 is applied to the comparator 35 where it is compared to the reference current applied to the line 36 by the reference current unit 37. When the current flowing in the resistor 32 and therefore in the winding 30, reaches the level of the reference current, the comparator 35 delivers the signal B which is applied by line 38 to the oscillator 40 and by line 39 to the chronology unit 41. At this instant, the chronology unit 41 begins a count based on the selected instant. In the figure, the chronology unit 41 is a single shot for the sake of simplicity and the instant selected will be that required for the application concerned. In a similar way, the signal B applied by the line 38 will cause the start of oscillation of the oscillator 40. The obvious function of the oscillator 40 is to deliver conditioning pulses to the system to prevent any overshooting of the current. Thus, it provides by line 42 the conditioning and the unconditioning of the AND gate 25 and therefore of the transistor 29 to supply the sawtooth winding current signal represented in FIG. 5. Finally, after the unit of chronology 41 has finished its countdown based on the preselected value, its output C falls to a negative level which causes the application by the AND gate 27 of a low logic level by the line 28 to decondition the transistor switch 29 and cut the current in winding 30.

Representative values of certain components and signals shown in Figures 4 and 5 are shown below:

In summary, the invention consists of a hammer control circuit controlled by a power supply having inherent voltage variations. The control signal is applied to the hammer winding and the current level in the winding is detected. After the current level in the winding reaches a predetermined level, the timing circuit is energized to establish the length of the signal. Variations in the supply voltage do not affect the impact hammer since the entire chronology is based on the instant when the predetermined level of the control current is reached, unlike the chronology which includes the signal rise time control. Similarly, variations in inductance can be compensated for by varying the duration of the current pulse.

Although the essential characteristics of the invention applied to a preferred embodiment of the invention have been described in the foregoing and represented in the drawings, it is obvious that a person skilled in the art can provide all of them. modifications of form or detail which he judges useful, without departing from the scope of said invention.

Claims (8)

1. A circuit for controlling the application of a current of a preselected level to a winding for a preselected time of the type comprising a current source selectively connected to said winding and means connecting said current source to said winding, said circuit control being characterized in that it comprises: means for interrupting said current to said winding a predetermined time after said current in said winding has reached a predetermined level. 2. Control circuit according to claim 1 characterized in that said interruption means comprises a current detector for detecting said predetermined level. 3. Control circuit according to claim 2 characterized in that said interrupting means further comprises a reference current source and a comparator, said comparator being connected both to said current detector and to said reference current source . 4. Control circuit according to claim 2 or 3 characterized in that it comprises a chronology unit connected to said predetermined level detector to cut said current to said winding a predetermined time after said current has reached said predetermined level. 5. Printing device for printing at a plurality of printing positions arranged along a printing line on a printing medium, of the type comprising a carriage moving in front of said printing positions, printing means for print one of several print characters on said printing medium in response to the application of a striking force and a striking means for applying said striking force to said printing means, said printing device being characterized in that it comprises: actuating means responsive to a predetermined level of an electrical signal for actuating said striking means so as to apply said striking force to said printing means. 6. Device according to claim 5 characterized in that said predetermined level of said electrical signal is a current level. 7. Device according to claim 5 or 6 characterized in that said actuating means continues the actuation of said striking means for a time practically equal to the duration of the maintenance of said electrical signal at said predetermined level. 8. Device according to claim 7 characterized in that said current level is maintained for a predetermined time after said predetermined level has been reached.

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