GB2046670A - Printer - Google Patents

Abstract

A printer has an electromagnetic portion consisting of an iron core (4) and a coil (3) thereon, and a printing portion consisting of a hammer (7) and a permanent magnet (31) attached thereto. The permanent magnet is attracted to the core to retain the hammer in the rest position when the coil is not energised. A cam (33) controls the position of the hammer and in particular when it begins to move during a printing operation. A spring (32) moves the hammer during the printing operation when the printing portion is released from the core by energisation of the coil. <IMAGE>

Description

SPECIFICATION Printer This invention relates to printers.

According to the present invention there is provided a printer comprising: an electromagnetic portion consisting of a ferromagnetic core and a coil thereon; a printing portion consisting of a movable printing member and a permanent magnet attached thereto, the permanent magnet being attracted to the core to maintain the printing member in a rest position when the coil is not energised; a cam for controlling the position of the printing member; and resilient means for moving the printing member during a printing operation when the printing portion is released from the core upon energisation of the coil.

In the preferred embodiment the printing member is a hammer.

The core may have buffer means to prevent direct contact between the core and the permanent magnet.

The printer may include means for translating the electromagnetic portion and the printing portion laterally relative to a printing medium which is to be printed.

Preferably said cam is arranged to determine when the printing member initially begins to move during the printing operation.

Preferably said cam is arranged to determine the time taken for the printing member to strike a desired print character during a printing operation.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 illustrates one conventional printer; Figure 2 illustrates another conventional printer; Figure 3 illustrates the alignment of printing produced by the conventional printers of Figures 1 and 2; Figure 4 illustrates the operation of the conventional printer of Figure 1, Figure 5 shows graphically the relationship between voltage applied to an electromagnet and movement of a hammer of the conventional printer of Figure 1, Figure 6 shows graphically the relationship between forces acting on a hammer of the conventional printer of Figure 2; Figure 7 illustrates schematically one embodiment of a printer according to the present invention; and Figure 8 illustrates the operation of the printer of Figure 7.

Referring first to Figure 1, there is illustrated a conventional printer in which a hammer is moved by an attractive force generated by an electromagnet. The conventional printer of Figure 1 has a character ring 1 carrying, on its periphery, a plurality of print characters 2, which may be letters, symbols, etc., a coil 3, and iron core 4, a yoke 5 and an attraction plate 6. The coil, the iron core, the yoke and the attraction plate constitute the electromagnet. A hammer 7 is connected at one end to the attraction plate 6. When the coil 3 is energised, the attraction plate 6 is attracted to the iron core 4 against the action of a spring 8 which fixed between the hammer 7 and the yoke 5, and the hammer 7 strikes against the given one of the print characters 2 to print a desired letter, symbol etc. on the printing paper 9.After this printing operation has taken place and the coil 3 de-energised, the hammer is returned to the rest position by the spring 8.

Referring now to Figure 2, there is shown another conventional printer in which a hammer is maintained in a rest position by a permanent magnet, the hammer being released to perform a printing operation by the energisation of a coil. In Figures 1 and 2 like parts have been designated by the same reference numerals. The conventional printer of Figure 2 has yokes 12 which are attached to opposite sides of a permanent magnet 11. A ferromagnetic member 13 which is attached to one end of the hammer 7 is thus attracted to the yokes 12 against the action of a spring 14. The permanent magnet 11 thus maintains the hammer 7 in a rest position until the coil 3 is energised.

When the coil 3 is energised the hammer 7 is caused by the springs 14 to strike a given one of the print characters and a desired letter, symbol etc. is printed on the printing paper 9. After this printing operation has taken place and the coil 3 de-energised the hammer 7 is returned to the rest position against the action of the spring 14 by the permanent magnet 11.

Such conventional printers have the following disadvantages: 1. Printing alignment of printed letters, symbols etc. on one line is irregular as shown in Figure 3.

2. In the case of the conventional printer of Figure 2, when the hammer 7 is released to perform a printing operation it moves relatively slowly to strike the desired print character because it is still attracted by the permanent magnet -- this is referred to as "the reattracting phenomen." The reason why the conventional printer of Figure 1 produces poor alignment of printing will be explained with reference to Figure 4. A timing signal 21 comprises pulses corresponding to each print character. When a desired print character 2 comes just under the hammer 7, a driving pulse 22 is applied to the coil 3 in synchronism with the timing signal 21 and this causes the hammer 7 to move as indicated by waveform 23.When the driving pulse 22 is applied to the coil 3, the iron core 4 becomes magnetised, the attraction plate 6 is attracted to it against the action of the spring 8, but the hammer 7 does not move for a period t measured from the leading edge of the driving pulse 22, and then the hammer 7 moves rapidly and strikes the desired print character 2 at the end of a period T,. After impact with the desired print character, the hammer 7 rebounds towards the rest position even though the coil 3 is still energised. The rebounding force, the spring force of the spring 8, and the attraction between the iron core 4 and the attraction plate 6 interact with one another as the hammer 7 rebounds and the hammer 7 contacts a supporting guide 10.The point when the driving pulse 22 is terminated is theoretically when the hammer 7 has rebounded to its rest position so that it is maintained in that position by the spring 8 until the coil 3 is energised by the next driving pulse.

Period T, varies within the limits indicated by broken lines and chain dotted lines due to (1) residual magnetism of, for example, the iron core 4, (2) friction between the attraction plate 6 and the yoke 3 and ageing thereof, (3) change of magneto-motive force produced by the coil 3 when energised, with temperature, (4) the change of magnetomotive force with voltage, etc. In the case shown by chain dotted lines, since the period T, of the hammer 7 is increased the desired print character 2 will have passed the proper printing position and the resulting printed letter, symbol etc. will be shifted upwards as shown by letter A in Figure 3, and the upper part of the letter, symbol etc. will not be printed. On the other hand in the case shown by broken lines, the upper part of the printed letter is shifted downwards as shown by letter B in Figure 3.

The effects of variation of voltage E are illustrated in Figures 5 and 6. Figure 5 shows the effect on the conventional printer shown in Figure 1 and Figure 6 shows the effect on the conventional printer shown in Figure 2. In Figure 5, the change of voltage E appears as a change of velocity vofthe hammer. In Figure 6, an attracting force Fc of the electromagnetic increases when the coil 3 is energised: The hammer 7 starts to move after a period t2 when the force Fc exceeds a spring force Fs. The period t2 thus varies with change of the force Fc which itself varies with the voltage within the limits indicated by broken lines and chain dotted lines.

The conventional printer shown in Figure 2 suffers from the reattracting phenomen already mentioned because the yoke 12 round which the coil 1 3 is wound is a N-pole in the rest position of the hammer but is magnetised to a S-pole by a energising the coil 3. The ferromagnetic member 1 3 is reattracted due to this S-pole, so that the hammer does not move. This reattracting phenomenon due to the magnetised S-pole occurs when the force Fc exceeds the spring force Fs.

This reattraction can be prevented by strengthening the spring 14, but this is disadvantageous in itself, since it is necessary to strengthen the magnetic force of the permanent magnet 11 in the rest position and the magnetomotive force produced by the coil 3 when energised.

In addition to these two disadvantages, in view of the effects of change of voltage, variation of spring force, variation of elements making up each printing column, etc., mass-production of conventional printers such as shown in Figures 1 and 2 is difficult.

Referring now to Figure 7, there is illustrated one embodiment of a printer according to the present invention to overcome the disadvantages of the conventional printers of Figures 1 and 2.

Like parts in Figures 1 and 6 have again been designated by the same reference numerals. The coil 3 is attracted to the iron core 4 and together constitute an electromagnet portion. A permanent magnet 31 is fixed to an end of the hammer 7 to constitute a hammer portion. A cam 33 engages with a projection 34 on the hammer 7, to return the hammer portion to the rest position where it engages the iron core 4 and to control the position where the hammer portion begins to perform a printing operation. A spring 32, one end of which is attached to the projection 34 of the hammer 7, and the other end of which is attached to fixed structure (not shown) urges the hammer to move during the printing operation.

Figure 8 illustrates the operation of the printer of Figure 7. A driving pulse 42 is applied to the coil 3 in response to a pulse of a timing signal 41 corresponding to a desired print character 2. The permanent magnet 31 and the iron core 4 constitute a magnetic circuit. The iron core 4 has a polarity opposite to that of the permanent magnet 31 when the coil 3 is energised. Thus the permanent magnet 31 and the iron core 4 repulse each other and the hammer portion begins to move as shown by waveform 43 after a period t3 when the cam 33 is released as shown by waveform 44 which illustrates the operation of the cam 33. The hammer portion strikes against the desired print character 2 after a period T3.The hammer portion rebounds after impact with the desired print character and is returned by the cam 33 against the action of the spring 32 to the rest position and is maintained in the rest position by the magnetic circuit including the permanent magnet 31 until the next printing operation. At this time, the stability of the period t3 largely contributes to the stability of the period T3.

Therefore, the alignment of letters, symbols, etc.

printed on the printing paper 9 is greatly improved compared to the conventional printers.

Since the hammer portion is mechanically returned to the rest position, the return time is reduced and so the printing speed may be increased and the return operation is stabilised.

Since the force of the permanent magnet 31 need only resist the force of the spring 32 at the moment when the magnetic circuit is closed, it is possible to make the permanent magnet small.

To absorb sound when the hammer is returned to the rest position, the permanent magnet 31 may be made of rubber, or dampers 35 may be provided between the permanent magnet and the iron core 4. Moreover, the dampers 35 decrea'se harmful influences due to attachment of iron particles to the permanent magnet in the rest position.

The printer illustrated in Figure 7 has the following advantages: (1) the period T3 is stabilised, (2) the reattracting phenomenon experienced with the conventional printer of Figure 2 does not occur, (3) printing speed is increased, (4) the period T3 is controlled by the came, and (5) since the electromagnetic portion is.

operated when the magnetic circuit is closed the power consumption of the coil is reduced.

Whilst the present invention has been described in relation to a printer with a hammer, it is also applicable to other types of printer e.g. a dot printer with a needle for forming letters made up of dots. A printer according to the present invention may have a plurality of columns, the number of coils 3 being proportional to the number of columns. The present invention may also be applied to a serial printer in which the hammer portion and the electromagnetic portion are laterally translated relative to the printing paper 9.

Claims (8)

1. A printer comprising: an electromagnetic portion consisting of a ferromagnetic core and a coil thereon; a printing portion consisting of a movable printing member and a permanent magnet attached thereto, the permanent magnet being attracted to the core to maintain the printing member in a rest position when the coil is not energised; a cam for controlling the position of the printing member; and resilient means for moving the printing member during a printing operation when the printing portion is released from the core upon energisation of the coil.

2. A printer as claimed in claim 1 in which the printing member is a hammer.

3. A printer as claimed in claim 1 or 2 in which the core has buffer means to prevent direct contact between the core and the permanent magnet.

4. A printer as claimed in any preceding claim including means for translating the electromagnetic portion of the printing portion laterally relative to a printing medium which is to be printed.

5. A printer as claimed in any preceding claim in which said cam is arranged to determine when the printing member initially begins to move during the printing operation.

6. A printer as claimed in any preceding claim in which said cam is arranged to determine the time taken for the printing member to strike a desired print character during a printing operation.

7. A printer substantially as herein described with reference to and as shown in Figures 7 and 8 of the accompanying drawings.

8. A printer to print letters, symbols, etc.

comprising: an electromagnet portion including an iron core and a coil attached to said iron core, said electromagnet portion being excited by energizing said coil; a flying member having a flying object and a permanent magnet means attached to said flying object, said permanent magnet means adapting to make said flying object attracted to said iron core at the time of non-printing; a cam controlling a position of said flying member; and a spring means forcing said flying member to fly from non-printing position to printing position.