GB1577884A

GB1577884A - Printing head drivers

Info

Publication number: GB1577884A
Application number: GB2023677A
Authority: GB
Original assignee: Sycor Inc
Current assignee: Sycor Inc
Priority date: 1977-05-13
Filing date: 1977-05-13
Publication date: 1980-10-29

Description

(54) IMPROVEMENTS IN AND RELATING TO PRINTING HEAD DRIVERS (71) We, SYCOR, INC., a Corporation organised under the laws of the State of Delaware, United States of America, of 100 Phoenix Avenue, Ann Arbor, Michigan, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to printing head drivers such as are used for impact matrix, or dot, printers. and to a method of impact printing.

In impact matrix printing apparatus, indi vldual needle-like printing elements are thrown longitudinally to impact endwise against a recording medium, thereby forming a dot on the medium. Each such dot it usually part of a dot pattern which forms a given alphanumeric character, the pattern being selected from a matrix of possible dot positions. Typically, individual electromagnetic actuators are used for each printing needle. or wire, as they are often referred to. The actuators have to move the wire with sufficient force to impact the medium, and then withdraw the wire so it is clear of the medium. While speed is a major consideration in such printing apparatus.

the impact force of the wire against the medium is also very important since the force must be sufficient to produce the desired image. Clearly, a force sufficient merely to produce an image on a single sheet of paper when the wire is separated from the paper by only an ink-carrying ribbon is probably insufficient to produce the required image on each sheet when the medium is a plurality of sheets of paper with intervening sheets of carbon paper.

One impact needle actuator which has been proposed in United States patent No.

3.592.311 has a printing wire associated with an electromagnet armature and a leaf spring. The wire is connected to the armature and the armature is connected to the leaf spring. An electromagnet holds the armature so that the leaf spring is in a flexed position. When the magnetic coil is deenergized, the armature and the wire are propelled forward by the leaf spring to a printing position. Energizing the magnetic coil then retracts the wire from the printing position to a retracted position and at the same time flexes the leaf spring once again, recocking the actuator. Much the same principle is used in the construction proposed in United States Patent No. 3,672,482.

However, instead of having an electromagnet which is de-energized to release the armature and the spring, there is a permanent magnet whose field is overcome by activating an electromagnet having a field which is the reverse of the permanent magnet field and approximately equal in magnitude. Typically, one can expect a force of about one and one-half pounds from such spring-actuated printing wires. A typical operating speed is about 40 characters per second.

A somewhat greater force, for example about three pounds, can be obtained by a system which uses an electromagnet to impel the wire from a retracted position into a printing position. A typical operating speed for such an arrangement is about 165 characters per second. The wire may be retrained in a retracted nonprinting position by a spring. The force created by the electromagnet overcomes the retaining force of the spring. For example, United States Patent No. 3.584.575 described a construction having a wire connected to an armature which in turn is connected to a coil spring that holds it in a retracted nonprinting position. The armature is within a magnetic coil and near a core piece. When the magnetic coil is energized, the coil and the core piece act to attract the armature and move it and the connected printing wire into a printing position.In the printing position. the spring is resiliently flexed from its normal position. De-energizing the magnetic coils allows the spring to return to its normal unextended position, which returns the print wire to a retracted, nonprinting position. Similarly, United States Patent No. 3,690,431 described a system where energization of the solenoid coil rapidly moves the printing wire in the impact printing direction, against the bias of a spring. In this system, the spring, instead of being a coil spring. is in the shape of a wagon wheel. The armature is connected to the hub of the wagon wheel, movement of the armature causing the spokes of the wheel to deflect elastically relative to the rim.

According to one aspect of the present invention, a printing head driver comprises: a printing element and a magnetically responsive armature connected to the printing element for moving such element in re spouse to magnetic force; permanent magnet means acting on the armature to bias the printing element towards a non-printing rest position; mechanical means for providing a force acting in a direction to move the printing element away from the rest position towards a printing position: and magnetic print means arranged so that upon actuation such means counteracts the permanent magnet bias means and additionally applies to the armature a force to impel the printing element towards the printing position in cooperation with the mechanical means. the effective combined force of the magnetic print means and the mechanical means exceeding the operative force of the permanent magnet bias means of an amount greater than the magnitude of the force provided by the mechanical means alone.

The mechanical means preferably comprises a spring, for example a disc-shaped washer having central portions operatively coupled to the printing element to move the latter and having peripheral portions fixed relative to at least part of the print head when the printing element is in a retracted rest position.

Thus. in a printing head driver constructed according to the invention. the armature and the connected printing wire are accelerated to a printing position by combined and jointly-acting forces obtained from mechanical means such as by releasing the energy stored in a deflected spring and energizing an electromagnet to create a forwardly-impelling magnetic field. The force imparted to a printing wire in a printing head driver constructed in accordance with the present invention can be as much as seven pounds. using the same order of component size and actuating power as that producing merely the one and one-half to three pounds provided by the previously proposed devices referred to above.Such a magnitude of force could not be obtained in those devices from a practical point of view because of the magnetic saturation and/or practical limits on the allowable size and mass of the actuating mechanism. Typically, to increase the printing force would require increasing the driving spring force in one case and the driving magnetic field force in another case. If the driving spring force is increased then the magnetic force which overcomes the spring force to return the printing wire to a retracted position must be increased. As discussed further below, this requires an increase in the size and mass of the spring and of the magnetic flux circuit which can adversely affect the speed of operation. Increasing the driving magnetic field force requires a corresponding increase in the capability of the magnetic circuit to handle the increased magnetic flux.The movable armature attached to the printing wire is part of the magnetic flux circuit and must be increased in flux-handling ability and therefore mass. Thus, the increase in spring and/or magnetic field forces does not produce a correspondingly great increase in printing force because of the greater masses to be moved. Further, the greater mass of the armature or spring results in a longer time being required for the printing wire to cycle from a retracted position to a printing position and back to a retracted position.

While printing force is important, printing speed is also a major consideration. Also, the increase in spring size and magnetic circuit size makes a compact arrangement of numerous wires, as is required in a printing head. a difficult objective to achieve.

The present invention using a combination of both mechanical forces such as spring forces and magnetic field forces to propel a printing wire increases printing force while maintaining printing speed and permitting a compact arrangement of the driving apparatus for printing wires.

The invention also includes, according to a second aspect, a method of impact printing including: biassing an armature and an associated printing element together towards a retracted. non-printing rest position by means of a permanent magnet while storing potential energy in mechanical means arranged to impel the armature upon release from such bias towards a printing position; releasing the armature bias and impelling the armature towards the printing position by use of a combination of the energy stored and the application of an electromagnetic field to the armature the force resulting from the combination being greater than that resulting from release of the restraint alone.

Another way of considering the invention is as a printing head driver comprising: a printing element; a magnetic circuit including a permanent magnet producing magnetic flux tending to move an armature mechanically connected to the printing element and hold the armature in a rest position; an electromagnetic coil located to counteract the magnetic flux in the magnetic circuit; a mechanical bias acting on the printing element in a direction away from the rest position and towards a printing position; and an electromagnetic coil located to exert a magnetic force on an armature mechanically connected to the printing element in the same direction as the mechanical bias.

The invention may be carried into practice in various ways but one printing head driver embodying the invention and a method of printing using the drive will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a central longitudinal crosssection of the printing head driver; Figure 2 is a graphical representation of the forces produced to act on the printing wire, with respect to time; Figure 3 is a perspective view of the printing head driver; and Figure 4 is a transverse cross-section along section line IV-IV of Figure 1 through a central portion of the printing head driver.

Referring to the drawings, a printing wire 10, which may be of tungsten, extends through and is connected to an elongated cylindrical armature 12 of magnetic material which is axially aligned in the central cylindrical opening of a casing 11, also of magnetic material, which houses a pair of electrical windings or coils 14 and 15 having connection posts or terminals 14a and 15a, respectively. extending outwardly of the casing 11.

The magnetic casing 11 is basically cylindrical, with a central generally cylindrical opening defined by the interior of plastics or other non-magnetizable spools 14b and 15b carrying the windings 14 and 15, the armature 12 being axially movable within such central openings. The print wire or needle 10 is axially aligned with the aramture 12 and disposed within a central cylindrical opening therein. extending axially beyond the casing 11 and the winding 15 in the direction of a printing medium (not shown).

An ink-carrying ribbon (not shown) is typically positioned between the printing medium and the end of the print wire 10.

For support and improved control, a cylindrical end guide 26 is positioned at the forward end of the housing 11. the guide 26 being threaded into the end of the housing to close it and cover the end of the magnetic winding 15. The guide 26 has a central guide passage 28, through which the print wire 10 extends rearwardly, towards (and through) the armature or plunger 12. The guide passage 28 is necked down near its end at portion 29, to help support the print wire and provide a seat for a jewel or other suitable bearing 30, mounted in the guide passage 28 at its extreme outer end. The end guide 26 has an integral rearwardlyextending portion 32 which fits inside the coil or winding 15 and forms a pole-piece therefore, such portion being of magnetic material.The front projecting portion 27 of the guide 26 may be externally threaded, as shown, to function as a mount for the solenoid, or for other purposes such as mounting a ribbon guide or mask (not shown).

A permanent magnet 16 is mounted at the end of the central cylindrical opening of the device, inside the end of the spool 14b on which the coil 14 is wound. A central pole piece 13 of magnetic material is also mounted in this cylindrical opening inside the spool 14b and adjacent the permanent magnet 16, the pole piece extending towards the armature 12 and abutting the latter when the armature is moved rearwardly into its retracted, non-printing position. The pole piece 13 is necked down at 23 and mounts a bearing 22 for the print wire to slide in, as in the case of the threaded pole piece 32.

An anti-residual washer-shaped member 17 of magnetic metal is disposed between a non-magnetic collar 20 and the near end of the winding spool 15b to provide a magnetic return path for flux conduction between the armature and the housing during each cycle of armature movement.

A disc-type spring 18 is disposed directly behind the armature 12 and held in fixed position by, and between, the collar 20 and another non-magnetic collar 19.

As can be seen from Figure 4, the spring 18 has a series of radial slots extending outwardly from its inside diameter to form spring tines or fingers which could alternatively be separate segments from one another. The armature 12 has an outwardlyprojecting circuit flange or skirt 21 which indexes against the front face of the spring 18 to flex the inner portion thereof (i.e., the slotted spring fingers) rearwardly relative to the outer portion. The forward face of the collar 19 is chamfered or bevelled to allow such rearward axial flexure of the spring 18.

The armature 12 is attached to the print wire or needle at the forward end of the armature, where it is necked-down to closely fit about the print wire as it passes through the armature. Thus, the print wire is secured to the armature but is axially slidable through the pole pieces. A typical distance of axial travel for the armature 12 is about ().070 inches. A typical radial gap between the armature 12 and the inside diameter of the spools or bobbins 14b and 15b on which the coils 14 and 15 are wound is about 0.01() inches.

Operation In a totally quiescent state, without energization of either of the coils 14 and 15, the permanent magnet 16 attracts the armature 12 with sufficient force to retract it into contact with the end of the pole piece 13, thereby spacing the end of the wire 10 from the printing medium. This is accompanied by flexure of the spring 18 to a coneshaped structure. The effect of energizing the coil 14 under these conditions is to create a magnetic field which opposes and overcomes the magnetic field of the permanent magnet 16. As a result, the armature 12 and the wire 10 are urged forwards away from the pole piece 13 and towards a printing position by the energy stored in the spring.

At the same time, the energizing coil 15 creates an additional impelling force which throws the armature forwards with in crcased force produced by the magnetic field of the winding 15 acting on the armature, in cooperation with the spring force.

De-energizing of the coils 14 and 15 eliminates the magnetic fields which oppose (and which overcome) the magnetic field of the permanent magnet 16. Accordingly, the field produced by the permanent magnet 16 will then withdraw the armature 12 and the wire 10 from the printing position to a retracted position. The armature 12 then travels along the cylindrical opening in the casing 11 towards the permanent magnet 16 until the armature abuts the pole piece 13.

In this retracted position, the spring 18 is again flexed. and again has stored energy due to its deflection.

The advantages of using both stored mechanical energy (spring force) and magnetic forces to drive a printing wire are shown in Figure 2. A graphical representation of resultant armature driving force with respect to time shows that spring-driven armatures have an instantaneous high force which rapidly decreascs with time while magnetically-dliven armatures have an initially low force which builds with time. Each has disadvantages, in that there is either a slow acceleration at first (magnetic drive) or a low ultimate force at the time the printing wire strikes the printing medium (spring drive). In contrast. combining both spring and magnetic drive produces a relatively constant force on the armature. This is advantageous for both good operating speed and striking force of the printing wire.

A further advantage of using the combination of spiing and magnetic forces to drive a printing wire is avoiding an undesirable increase in the physical size and mass of components used. As pointed out above, relying on only magnetic or spring forces for driving the printing wire produces a structure generally larger than is possible by combining magnetic and spring forces when considering comparable force-equivalent structures. Thus groups of printing wires can be conveniently clustered into a printing head and speed is not reduced because of increases in mass or size.

WHAT WE CLAIM IS: 1. A printing head driver comprising: a printing element and a magnetically responsive armature connected to the printing element for moving such element in response to magnetic force; permanent magnet means acting on the armature to bias the printing element towards a non-printing rest position; mechanical means for providing a force acting in a direction to move the printing element away from the rest position towards a printing position; and magnetic print means arranged so that upon actuation such means counteracts the permanent magnet bias means and additionally applied to the armature a force to impel the printing element towards the printing position in cooperation with the mechanical means, the effective combined force of the magnetic print means and the mechanical means exceeding the operative force of the permanent magnet bias means by an amount greater than the magnitude of the force provided by the mechanical means alone.

2. A printing head driver as claimed in Claim 1 in which the mechanical means comprises a spring.

3. A printing head driver as claimed in Claim 1 or 2 in which the mechanical means comprises a disc-shaped washer having central portions operatively coupled to the printing element to move the latter and having peripheral portions fixed relative to at least part of the print head when the printing element is in a retracted rest position.

4. A printing head driver as claimed in Claim 3 in which the disc-shaped washer is disposed to contact the armature, with the axis of the washer substantially coincident with the axis of the armature.

5. A printing head driver as claimed in Claim 1, 2. 3 or 4 which includes a magnetisable casing having central axial passage for axial movement of the armature therewithin. and in which the printing element is a wire connected to the armature, axially aligned with the direction of axial movement of the armature. and extending axially beyond the casing, the permanent magnet bias means comprises a permanent magnet secured relative to the casing for providing magnetic force acting axially on the arma

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. about ().070 inches. A typical radial gap between the armature 12 and the inside diameter of the spools or bobbins 14b and 15b on which the coils 14 and 15 are wound is about 0.01() inches. Operation In a totally quiescent state, without energization of either of the coils 14 and 15, the permanent magnet 16 attracts the armature 12 with sufficient force to retract it into contact with the end of the pole piece 13, thereby spacing the end of the wire 10 from the printing medium. This is accompanied by flexure of the spring 18 to a coneshaped structure. The effect of energizing the coil 14 under these conditions is to create a magnetic field which opposes and overcomes the magnetic field of the permanent magnet 16. As a result, the armature 12 and the wire 10 are urged forwards away from the pole piece 13 and towards a printing position by the energy stored in the spring. At the same time, the energizing coil 15 creates an additional impelling force which throws the armature forwards with in crcased force produced by the magnetic field of the winding 15 acting on the armature, in cooperation with the spring force. De-energizing of the coils 14 and 15 eliminates the magnetic fields which oppose (and which overcome) the magnetic field of the permanent magnet 16. Accordingly, the field produced by the permanent magnet 16 will then withdraw the armature 12 and the wire 10 from the printing position to a retracted position. The armature 12 then travels along the cylindrical opening in the casing 11 towards the permanent magnet 16 until the armature abuts the pole piece 13. In this retracted position, the spring 18 is again flexed. and again has stored energy due to its deflection. The advantages of using both stored mechanical energy (spring force) and magnetic forces to drive a printing wire are shown in Figure 2. A graphical representation of resultant armature driving force with respect to time shows that spring-driven armatures have an instantaneous high force which rapidly decreascs with time while magnetically-dliven armatures have an initially low force which builds with time. Each has disadvantages, in that there is either a slow acceleration at first (magnetic drive) or a low ultimate force at the time the printing wire strikes the printing medium (spring drive). In contrast. combining both spring and magnetic drive produces a relatively constant force on the armature. This is advantageous for both good operating speed and striking force of the printing wire. A further advantage of using the combination of spiing and magnetic forces to drive a printing wire is avoiding an undesirable increase in the physical size and mass of components used. As pointed out above, relying on only magnetic or spring forces for driving the printing wire produces a structure generally larger than is possible by combining magnetic and spring forces when considering comparable force-equivalent structures. Thus groups of printing wires can be conveniently clustered into a printing head and speed is not reduced because of increases in mass or size. WHAT WE CLAIM IS:

1. A printing head driver comprising: a printing element and a magnetically responsive armature connected to the printing element for moving such element in response to magnetic force; permanent magnet means acting on the armature to bias the printing element towards a non-printing rest position; mechanical means for providing a force acting in a direction to move the printing element away from the rest position towards a printing position; and magnetic print means arranged so that upon actuation such means counteracts the permanent magnet bias means and additionally applied to the armature a force to impel the printing element towards the printing position in cooperation with the mechanical means, the effective combined force of the magnetic print means and the mechanical means exceeding the operative force of the permanent magnet bias means by an amount greater than the magnitude of the force provided by the mechanical means alone.

ture, and the magnetic print means comprises an electromagnet winding carried in the casing and circumferentially surrounding the axial passage.

6. A printing head driver as claimed in Claim 5, which includes stop means within the axial passage and spaced from the armature and defining a gap establishing the allowable forward travel of the armature from a centered position along the axial length of the central passage.

7. A printing head driver as claimed in Claim 6 in which the said allowable forward travel is approximately 0.070 inch.

8. A printing head driver as claimed in Claim 1 or Claim 2 or Claim 3 in which the magnetic print means includes a pair of electromagnetic windings disposed generally adjacent one another along a common axis, the permanent magnet bias means includes a permanent magnet generally adjacent at least one of the windings, the permanent magnet being aligned with the printing element and having a field oriented for providing a force to so bias the printing element, and that of the electromagnetic windings which is generally adjacent the permanent magnet is arranged to produce on energisation a field opposite that of the permanent magnet.

9. A printing head driver substantially as described herein with reference to Figures 1, 3 and 4 of the accompanying drawings.

10. A method of impact printing including: biassing an armature and an associated printing element together towards a retracted, non-printing rest position by means of a permanent magnet while storing potential energy in mechanical means arranged to impel the armature upon release from such bias towards a printing position; releasing the armature bias and impelling the armature towards the printing position by use of a combination of the energy stored and the application of an electromagnetic field to the armature the force resulting from the combination being greater than that resulting from release of the restraint alone.

11. A method of impact printing substantially as described herein with reference to the accompanying drawings.