WO1989011973A1 - Hammering printer head - Google Patents

Abstract

A fast hammering printer head including an electromagnet (25 to 37) and a hammering element (45 to 48, 50) fixed to a leafspring (38 to 44, 49) and constituted by a plate (46) made of magnetically attractable material from the centre of which extends a stem (47) which ends with a printing head (50). The plate and the stem protrude from opposite sides of the leafspring, a nut (48) being engaged on this stem so as to press the plate against the leafspring to which it is adjacent. The invention also concerns a method to print on moving documents, e.g. envelops of letters, using such a hammering printer head. The method consists in attracting the plate by a first force generated by the electromagnet and thereby bending the leafspring, in releasing this plate by suppressing the first force so as to allow the displacement of the hammering element by the bent leafspring until the impact on the document, and in generating a second force by the electromagnet prior to the occurrence of this impact.

Description

HAMMERING PRINTER HEAD The present invention relates to a hammering printer head including an electromagnetic device, spring means, and a hammering element fixed to said spring means and having a hammering head and a magnetically attractable element which protrude from opposite sides of said spring means, said magnetically attractable element including a plate-shaped part which is located outside an electromagnet forming part of said electromagnetic device. Such a hammering printer head is already known from the Belgian patent No 820413. Therein, the electromagnetic device includes a first and a second pot-shaped electromagnets which are coaxially mounted. The magnetically attractable element is constituted by a plunger of which one end is fixed to the spring means which are a leafspring and of which the other end terminates in said plate-shaped part. The plunger is mounted inside the second electromagnet and the plate-shaped part is disposed between the two electromagnets near their respective pole pieces and perpendicular to the common longitudinal axis thereof. Both the first and the second electromagnets are controlled by an electric control circuit. At rest, the plate-shaped part is located in the middle of the airgap between the two electromagnets and the leafspring is not bent. Before a printing operation, the electric control circuit only activates the first electromagnet as a consequence of which the latter produces an attraction force by which the plate-shaped part and thus also the plunger are displaced in a backward direction towards this first electromagnet, thereby bending the .leafspring . At the start of a printing operation on a document, the control circuit controls the two electromagnets in such a way that the first one is deactivated whilst the second one produces an attraction force on the plate-shaped part in order to move the electromagnetically attractable element in a forward direction towards the document. This force, together with that produced by the released leaf-spring, communicates a considerable speed to the hammering head. The displacement of the hammering head is such that the leafspring is reversely bent when impacting the document so^' that the only forward force then applied on the hammering head is produced by the second electromagnet. After the impact of the hammering head on the document, the electromagnets are again brought in their initial condition so that the plate—shaped part is then again attracted in the backward direction by the first electromagnet. During a first phase of this displacement of the head, the force produced by the first electromagnet is increased by the force of the reversely bent leafspring.

A drawback of the above known hammering printer head is that the magnetically attractable element is relatively heavy and that the electromagnetic device includes two electromagnets and an associated control circuit to control the reciprocating movement of this element. Furthermore, the displacement of the hammering head is mainly controlled by these electromagnets and because the generation and/or suppression of an electromagnetic force by such electromagnets is not instantaneous, this negatively affects the displacement speed of the hammering element. This is especially true when the force exerted by the second electromagnet has to be suppressed in order to bring the hammering head back in its initial position, i.e. after the impact.

An ob ect of the present invention is to provide a hammering head of the above known type but which does not present this drawback, i.e. which is of a simple structure, and wherein the displacement speed of the hammering element is increased .

According to the invention, this object is achieved due to the fact that said electromagnetic device is solely constituted by said electromagnet whilst said magnetically attractable element solely comprises said plate-shaped part which is mounted adjacent to and in mechanical contact with said spring means.

Because the magnetically attractable element only comprises the plate-shaped part it may be made relatively light as a consequence of which it is sufficient to use only a single electromagnet to control the reciprocating movement of the hammering element.

It should be noted that a hammering printer head including a single electromagnet is already known, e.g. from the US patent 4,708,501. However, therein the hammering element has a shaft portion which is slidably mounted into the electromagnet and which carries the plate-shaped part also located in this electromagnet and constituted by a permanent magnet. Moreover, the hammering element is not fixed to but only engaged in the spring means which are a coil spring. These spring means do not make any mechanical contact with the plate-shaped part. Before a printing operation the electromagnet is not activated so that no electromagnetic force is applied to the hammering element. However, the permanent magnet of this hammering element then attracts the pole pieces of the electromagnet so that the spring means are bent owing to this permanent magnet. At the beginning of a printing operation, an electromagnetic force repulsing the permanent magnet is generated by the electromagnet and is thus added to the force exerted by the bent spring means to overcome the attraction force of the permanent magnet and to move the hammering element in a forward direction, i.e. towards the document to be printed. During this forward displacement the force exerted by the spring means and the repulsive force exerted by the electromagnet are succesively suppressed so that the hammering element then only keeps moving owing to its inertia. After impact on the document, the hammering element is brought back in its initial position only by the force of the permanent magnet attracting the pole pieces of the electromagnet.

The present invention also relates to a method to print on documents using a hammering printer head such as the one described above. The present method is particularly characterized in that it comprises the successive steps of attracting said magnetically attractable element of said hammering element by means of a first electromagnetic attraction force generated by said electromagnet and thereby bending said spring means, of releasing said magnetically attractable element from said electromagnet by suppressing said first electromagnetic attraction force so as to allow the displacement of said hammering element by said bent spring means until the impact on said document to be printed, and of generating a second electromagnetic attraction force by said electromagnet prior to the occurrence of said impact.

In this way, the operation of the control circuit which controls the electromagnet remains relatively simple. The advantages of this method with respect to the one disclosed in the above mentioned US patent are the following. At the beginning of the printing operation, the force exerted by the electromagnet to move the hammering element towards the document has not to overcome the force of the permanent magnet. Moreover, at the end of the printing operation, the electromagnetic force exerted on the hammering element when it reaches again its initial position may be stronger than the force which is only exerted by the permanent magnet. This force is however necessary to retain the hammering element in its initial position in order to prevent bouncing thereof. These could otherwize cause unwanted second prints on the document.

The above mentioned and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to the following description of an embodiment taken in conjunction with the accompanying drawings wherein :

Fig. 1 is a longitudinal central cross-section of a hammering printer head according to the invention; and

Fig. 2 shows an electric current signal controlling an electromagnet 2 forming part of the hammering printer head of Fig . 1.

This hammering printer head forms part of a printer system able to print series of codes, e.g. bar codes, on moving documents such as letter envelopes. The hammering printer head includes a housing 1 wherein an electromagnet 2 and a hammering mechanism 3 are fixedly mounted.

The housing 1 is constituted by two parallel top 4 and bottom 5 plates and by two side plates 6 and 7. The left hand side plate 6 has an inner recess 8, whilst the right hand side plate 7 has an inner recess 9 which faces the recess 8 and is provided with an opening 10 made therein. The side plates 6 and 7 are provided with respective screw holes 11, 12 and 13, 1 and the top 4 and bottom 5 plates have corresponding threaded screw holes 15, 16 and 17, 18, the plates 4, 5, 6 and 7 being united by screws (not shown) fixed in these holes. The opening 10 in the right hand side plate 7 is covered by an external cover plate 19 which is fixed thereon by means of screws (not shown) engaged in cor esponding screw holes 20, 21 and 22, 23 in the plates 19 and 7 respectively. The cover plate 19 is provided with a recess 24 oriented towards the opening 10 and provided with a hole 25._. The plates 4, 5, 6 and 7 are made of brass whilst the cover plate 19 is of stainless steel.

The electromagnet 2 mounted in the housing 1 is constituted by a U-shaped yoke 25 and by two series connected coils 26 and 27 wound on distinct legs of the yoke 25 and terminating by electric wires 28, 29 and 30, 31 which are connected to terminals of an electric control circuit (not shown). The bottom of the U-shaped yoke 25 is engaged with a tight fit in the recess 8 of the left hand side plate 6. Rectangular pole pieces 32 and 33 each having a chamfered edge 34, 35 are fixed on the ends of the respective legs of the U-shaped yoke 25 in such a way that their respective small ends 36 and 37 face each other and delimit an airgap. These ends 36 and 37 constitute the poles of the electromagnet 2.

The hammering mechanism 3 includes a rectangular leafspring 38 whose ends 39 and 40 are secured in respective grooves 41 and 42 of distinct rubber blocks 43 and 44. This assembly 38, 43 and 44 is engaged with a thight fit in the opening 10 of the right hand side plate 7, the rubber blocks 42, 44 being mounted between the pole pieces 32 and 33 and the cover plate 19. The hammering mechanism also includes a hammering element 45 which is fixed to the leafspring 38 and comprises a hammering head and a magnetically attractable element 46 protruding from opposide sides of the leafspring 38. The magnetically attractable element 46 comprises a square plate-shaped part forming one end of a stem 47 which, together with an aluminium nut 48, constitute the hammering head. To this end, the stem 47 is engaged in a central hole 49 of the leafspring 38 and is fixed thereon by means of the nut 48. The leafspring 38 is thus fixedly mounted between the nut 48 and the plate-shaped part 46 which faces the airgap between the poles 36 and 37 of the electromagnet. The other end of the stem 47 of the hammering head constitutes the printing head 50. The latter has the shape of a screwdriver end and is therefore able to print a bar code. The stem 47 is engaged in the hole 25 of the cover plate 19 such that its printing head 50 sligthly protrudes from this plate 19 when the leafspring 38 is at rest. In this rest position, the square plate 46 is not in contact with the poles 36 and 37. The moveable mass of the hammering mechanism 3 is small because its constituent parts are relatively small and light.

Although only the printer head is shown in Fig. 1, it may be used in known manner for instance to print bar codes on mail envelopes moved along this head by an edgewise feeding mechanism. An ink ribbon is then disposed between the hammering head and the documents. These are pressed against the cover plate 19 and more particularly near the hole 25 thereof, i.e. near the printing head 50, by means of a pressure roller ensuring a substantially constant pressure of the document against the cover plate 19 independently from the thickness of this document. The profile of the external surface of the cover plate 19 is adapted to the kind of documents to be processed, e.g. letter envelopes of variable thicknesses.

The operation of the hammering printer head is described hereinafter by making reference to the Figs. 1 and 2. The Fig. 2 shows the current I, in Amperes (A), flowing through the series connected coils 26 and 27 in function of the time t.

At rest, indicated by an instant tl on Fig. 2, the leafspring 38 is not bent and, as already mentioned, the plate 46 is at a certain distance from the poles 36 and 37. When documents have to be printed, the electric control circuit supplies a current, e.g. of 2.2 Amperes, to the series connected coils 26 and 27. As a consequence, an electromagnetic field is induced in the yoke 25 and in the pole pieces 32 and 33 and an electromagnetic attraction force is created in the airgap between at the poles 36 and 37. By this force the plate 46 is attracted towards these poles 36 and 37 as a result of which the leafspring 38 is bent in a backward direction. When the left hand side of the plate 46 makes contact with the poles 36 and 37, the printer head is in a so-called stand-by position. It is to be noted that owing to their elasticity the rubber blocks 43 and 44 allow the displacement of the leafspring 38 without any sliding of pieces along each other. For this reason, the rubber blocks 43 and 44 operate not only as hinges for the leafspring 38 but also as vibration damping means.

When, at an instant t2 the approach of a document to be printed is detected, for instance by means of a photo-transistor (not shown), a timer forming part of the above control circuit is activated. This timer fixes the delays between the instant t2 and following instants t3, t4 and t5 defined below.

Because of the presence of a moving document to be printed detected at the instant t2, the hammering element 45 has to be released from the poles 36 and 37 at the instant t3 and moved in the forward direction to hit the passing document at a suitable place. More in detail, at the instant t3 the control circuit starts a first print cycle by rapidly reducing the current flowing through the coils 26 and 27 and thereby also reducing the electromagnetic attraction force exerted by the poles 36 and 37 on the plate 46. This happens for instance by rapidly reversing the voltage accross the two series connected coils. As a consequence, the leafpring 38 is liberated so that the plate 46 in disengaged from the poles 36 and 37 and the printing head 50 moves quickly in the forward direction towards the document to be printed.

From the instant t4 of this forward displacement the control circuit supplies to the coils 26 and 27 a current rising from 0 Ampere at the instant t4 to 4.5 Amperes at the instant t5 in an exponential way. During the time interval elapsing from the instant t4 to an instant t6 of the impact of the printing head 50 on the document, the effect of the electromagnetic attraction force thus generated on the hammering element 45, and more particularly on the plate 46 thereof, is negligible but becomes important afterwards. It is to be noted that the instant t6 occurs before the instant t5 at which the current reaches its maximum value of 4.5 Amperes.

At the instant t6, i.e. when the printing head 50 hits the document, the leafspring 38 is almost in its rest position whilst the electromagnetic attraction force exerted by the poles 36 and 37 is then strong enough to attract the plate 46.

At the instant t5, i.e. when the current reaches its maximum value of 4.5 Amperes, this current is exponentially brought back to its stand-by value of 2.2 Amperes which is reached at an instant t7 corresponding to the end of the first print cycle which has a duration of about 2 milliseconds. The plate 46 comes again into contact with the poles 36 and 37 at an instant tδ occurring after the instant t7. Although at this instant tδ the leafspring 38 is bent, the electromagnetic attraction force exerted by the poles 36 and 37 on the plate 46 overcomes the force exerted thereon by the bent leafspring 38 and keeps the hammering element 45 in contact with these poles 36 and 37. As a result, the element 45 is prevented from bouncing on the electromagnet thus avoiding double printing of bar code on the document by an unwanted second displacement of the printing head 50 towards the document. The delays between the instants t2, t3, t4 and t5 are defined by the required position of the impact of the printing head 50 on the document taking into account the displacement speed of this document, the distance covered by the hammering element 45 between the poles 36 and 37 and the document to be printed, and the speed at which this hammering element 45 covers this distance. The relative occurrences of these instants t2, t3, t4 and t5 are adjustable by the control circuit.

From the instant t7 onwards, a second print cycle may start. This second print cycle, shown in dashed lines on Fig. 2 is almost identical to the one previously described and ends at an instant t9. It only distinguishes from the first print cycle by the fact that, at this instant t7, the plate 46 is not yet in contact with the poles 36 and 37. In practice, this situation does not affect the parameters of the second impact of the printing head 50 on the document nor an instant tlO at which this second impact occurs with respect to the instant t7.

While the principles of the invention have been described above in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

Claims

1. Hammering printer head including an electromagnetic device (2; 25 to 37), spring means (38 to 44, 49), and a hammering element (45 to 48, 50) fixed to said spring means and having a hammering head (47, 50) and a magnetically attractable element (46) which protrude from opposite sides of said spring means, said magnetically attractable element including a plate-shaped part (46) which is located outside an electromagnet (25 to 37) forming part of said electromagnetic device, characterized in that said electromagnetic device (2, 25 to 37) is solely constituted by said elec romagnet (25 to 37) whilst said magnetically attractable element (46) solely comprises said plate-shaped part (46) which is mounted adjacent to and in mechanically contact with said spring means (3δ to 44, 49).

2. Hammering printer head according to claim 1, characterized in that said spring means (38 to 44, 49) are constituted by a leafspring (3δ to 40, 49) whose ends (39; 40) are secured (41, 42) to vibration damping means (43, 41, 44, 42).

3. Hammering printer head according to claim 2, characterized in that said vibration damping means (43, 41; 44, 42) are constituted by blocks (43; 44) of elastic material mounted with a tight fit in a cavity (10) of said printer head and each provided with a groove (41; 42) wherein a respective one of said ends (39; 40) of said leafspring (38 to 40, 49) is secured. 4. Hammering printer head according to claim 1, characterized in that said hammering head (47, 50) is constituted by a stem (47) extending perpendicularly in one direction from the center of said plate-shaped part (46) and mechanically connected thereto via a first of its ends, the other end of said stem constituting a printing head (50) .

5. Hammering printer head according to claim 4, characterized in that said printing head (50) has the shape of a screwdriver end.

6. Hammering printer head according to the claims 2 and 4, characterized in that said stem (47) is engaged in a hole (49) provided in said leafspring (38 to 40, 49).

7. Hammering printer head according to claim 6, characterized in that said stem (47) is at least partially threaded and in that a nut (48) is screwed thereon on one side of said leafspring (38 to 40, 49) in order to press said plate-shaped part (46) against the opposite side of said leafspring to securely fix said hammering element (45 to 4δ, 50) on said leafspring. δ. Hammering printer head according to claim 6, characterized in that said hole (49) is provided in the centre of said leafspring (3δ to 40, 49).

9. Hammering printer head according to claim 1, characterized in that said electromagnet (25 to 37) includes at least one coil (26; 27) and a U-shaped yoke (25) around which said coil is wound and of which the terminations are provided with respective pole pieces (34; 35) whose ends (36 37) face each other and delimit an airgap, said pole pieces ends constituting the poles (36; 37) of said electromagnetic means and both simultaneously face said magnetically attractable element (46) so as to be able to exert an electromagnetic attraction force thereon.

10. Hammering printer head according to claim 9, characterized in that said poles (36; 37) are located along the external surface of the volume occupied by said electromagnet (25 to 37) so that said magnetically attractable element (46) remains outside of said volume.

11. Hammering printer head according to claim 9, characterized in that, when said spring means (3δ to 44,

49) is in its rest position, said magnetically attractable element (46) is not in contact with said poles (36; 37) of said electromagnet (25 to 37).

12. Hammering printer head according to the claims 3 and 9, characterized in that said cavity (10) is constituted by a hole (10) in the housing (1; 4 to 25) of said hammering printer head and which is delimited on one side by said pole pieces (34; 35) of said electromagnet (25 to 37) and on the other side by a removable cover plate (19) provided with a hole (25) through which said hammering head (47, 50) is able to extend.

13. Method to print on documents using a hammering printer head as claimed in any of the claims 1 to 12, comprising the successive steps of attracting said magnetically attractable element (46) of said hammering element (45 to 4δ, 50) by means of a first electromagnetic attraction force generated by said electromagnet (25 to 37) and of thereby bending said spring means (38 to 44, 49), of releasing said magnetically attractable element from said electromagnet by suppressing said first electromagnetic attraction force so as to allow the displacement of said hammering element by said bent spring means until the impact on said document to be printed, and of generating a second electromagnetic attraction force by said electromagnet prior to the occurrence of said impact.

14. Method according to claim 13, characterized in that said second electromagnetic attraction force increases towards an amplitude relatively higher than that of said first electromagnetic attraction force. 15. Method according to claim 13, characterized in that at the occurrence of said impact said spring means (38 to 44, 49) are substantially in their rest position.

16. Method according to the claims 9 and 13, characterized in that said magnetically attractable element (46) is in contact with said poles (36; 37) of said electromagnet (25 to 37) prior to the suppression of said first electromagnetic attraction force.

17. Method according to the claims 14 and 16, characterized in that it includes a further step of reducing the amplitude of said second electromagnetic attraction force to the amplitude of said first electromagnetic attraction force after said impact but prior to the moment at which said magnetically attractable element (46) is again in contact with said poles (36; 37) of said electromagnet (25 to 37).