Printer and pressure assembly therefor

Abstract

A printer with a high-speed advancing and printing head suitable for printing on thermally sensitive paper, in response to computer commands. Good contact between the printing head and the paper is provided by completion of a magnetic flux circuit formed in part by a flux-carrying means that extends parallel to the travel of the head. This magnetic circuit permits the use of a small, low-inertia printing assembly and thus facilitates high speed movement from one print position to the next. The use of the same magnetic flux circuit for braking of the printing head and carriage by sliding friction upon the paper prevents overshoot and oscillation when the head approaches a new print position.

Description

BACKGROUND OF THE INVENTION

This invention relates to a printer for high-speed printing. One problem associated with such printers for thermal printing is difficulty in obtaining good thermal contact between the fast advancing printing head and the thermally sensitive paper, thus causing indistinct printing. A second problem is that use of heavy mechanical systems for increasing contact pressure is incompatible with high-speed advance, since high mechanical inertia prevents rapid start and stop motion. A third problem is that printing head carriages which advance rapidly from one print position to another may overshoot and oscillate while approaching the new print position.

SUMMARY OF THE INVENTION

Objects of the invention are to provide an improved printer with regard to the various problems just noted and in particular to provide a thermal printer for high-speed printing on thermally sensitive paper suitable for receiving signals from a computer, translating those signals into commands and utilizing the commands to produce carriage movement, paper movement and printing.

According to the invention, pressure exerted by the printing head on the paper for ensuring good contact is produced by the completion of a magnetic circuit including a flux-carrying means extending parallel to the advancing path of the head. This permits the application of pressure by means of a small, low-inertia part or armature mounted on the head, which is compatible with rapid advancement of the head for high printing speeds. The use of such electromagnetic systems also permits simple and rapid control together with a minimum of moving parts, increasing durability. Another feature of the invention is a braking system employing a structure as above, to eliminate overshoot and oscillation when the head approaches a new position, thus to increase printing speed. Other features concern the particular structural relationship illustrated in the Figures which enable a compact and effective design to be achieved.

Other objects, features, and advantages of the invention will appear from the following description of a preferred thermal printer embodiment taken in conjunction with the drawings wherein:

FIG. 1 is a vertical cross-sectional view of the embodiment taken on lines 1--1 of FIG. 5;

FIG. 2 is a perspective view, partially broken away of the embodiment of FIG. 1;

FIG. 3 is a perspective view, partially broken away and on an enlarged scale of the elements defining the magnetic flux circuit of the invention;

FIG. 4 is an end view of the elements of FIG. 3 together with the carriage of the printer; and

FIG. 5 is a front side view of the printer.

DESCRIPTION OF THE PARTICULAR EMBODIMENT

Referring to FIG. 1, a platen assembly 10 and a carriage assembly 12 define a printing system for action upon a sheet 14 of thermosensitive paper fed upwardly between them. The sheet is fed from a supply roll 16, under a dancer assembly 18, through upper and lower paper guides 20 and 22 respectively, past a feed roll drive assembly 24 and then upwardly between the platen and carriage assemblies.

Referring to FIG. 2, a perspective view of the preferred embodiment, the platen assembly 10 is elongated, extending throughout the width of the printer, parallel to feed drive roll assembly 24 and shaft 26. Shaft 26 guides and supports the carriage assembly in its advances across the width of the sheet. More specifically, referring to FIGS. 3 and 4, the platen assembly comprises an elongated elastomeric bar 28 held in position by an iron core member 30. This member in vertical cross section as shown in FIGS. 3 and 4 is of U-shape and the elastomeric bar 28 is positioned at the open end of the U-shape, protruding slightly beyond the end faces 31 of the core member 30. Each leg 30a, 30b of the U, as well as bar 28, extends throughout the width of the printer, each leg serving as a core for a coil, 32a and 32b respectively. Each coil has elongated strands extending the length of the platen on each side of its respective core, turning about the ends thereof. Thus in vertical cross-section, the device forms a horseshoe magnet with coils wound and connected to augment each other in the production of flux within core 30 during energization of the two coils. The platen assembly 10 is joined by end mounts 34 to the housing side plates 36 of the printer. FIG. 5, a side view of the preferred embodiment, also illustrates many of these features.

Referring now to FIG. 4, the carriage assembly 12 is comprised of four sections: carriage body 38, carriage bearings 40, printing head assembly 42 and plate 44 forming a cable restraint and a photocell flag tab used in conjunction with a photocell (not shown) to indicate approach to the zero position upon carriage return. The carriage body 38 is mounted on shaft 26 via carriage bearings 40, thereby to permit the carriage assembly to advance freely along the shaft in an axial direction and to rotate freely upon the shaft, thereby allowing the printing head assembly to increase or decrease its pressure upon sheet 14 during operation of the printer. Printing head assembly 42 (FIGS. 3, 4) comprises a printing element 46 and a ceramic mounting block 48. Printing element 46 provides an assemblage of silicon transistors mounted to form a 5 .times. 7 matrix of thirty-five heatable dots or elements with drive electronics and temperature compensation built into the printing element. A suitable printing element is Model DC 1157012 Thermal Print Head--5 .times. 7 Dot Matrix manufactured by Displaytek Corp. The ceramic mounting block 48 attaches the printing element to carriage body 38 and carries electrical leads and contacts by means of which electrical power and signals can be transmitted to the printing element. The printing element 46 is mounted at the upper left edge of block 48 (relative to the operator, or right edge when viewed from the back as in FIG. 3) to permit the operator to read a character immediately after it has been printed.

A flexible harness 50 extends from plate 44 on carriage assembly 12 to printed circuit board 52, which provides the logic for actuating the various dots in the matrix, thus forming desired letters or symbols through known thermal printing techniques. The carriage assembly is advanced along shaft 26 by the advancer 54 in the form of a carriage timing belt 56 driven by carriage stepper motor 58. The various drives of the printer, i.e. the carriage stepper motor 58 and the paper advance drive 62, are controlled by power circuits on a second printed circuit board (not shown) mounted parallel to housing endplate 36.

Crucial to effective printing is the amount of pressure with which the printing head bears upon the sheet. The pressure is needed to make good thermal contact between each dot in the matrix and the paper despite watermarks and grain in the paper itself. According to the invention, the force of the head against the paper is provided by interaction of the platen magnet previously described with a force clip member 60. The printing head assembly 42 is mounted on carriage assembly 12 with two small screws. Force clip member 60 is comprised of an H-shaped piece of ferro magnetic material having legs 60a, 60b joined by bridge 60c (FIG. 5), mounted so that faces 61 of these legs oppose faces 31 of legs 30a, 30b of the U-shaped platen magnet, all of the faces being parallel to the paper, and normals thereof corresponding to the direction of application of the pressure of the head on the paper. Printing element 46, surrounded by legs 30a, 30b, opposes the projecting elastomeric bar 28. When the platen magnet is energized, the legs of the force clip will complete a path for magnetic flux which includes the U-shaped steel core 30 of the electromagnet and the air gaps between the faces 31 of the legs of the platen magnet 30a, 30b and the faces 61 of the legs of the force clip 60a, 60b. The air gaps will be of the order of 3 to 5 mils when the magnet is energized to the first current level and 4 to 7 mils when de-energized, depending on the thickness of the paper used, etc.

Paper is advanced through the printer by means of a paper stepper motor 62 and paper timing pulley and belt 64 and 66 respectively. This drives a roller shaft 68 against which the paper is pushed by pinch rollers 70. This action pushes the paper upwards past the head and platen assembly. The paper feed system 72 is comprised of two end supports which hold the paper roll in the printer frame. One end is a free turning pivot point. The other is comprised of a spring and washer arrangement 74. The frictional action between the washer and a slide plate prevents unrolling of the paper under light vibration.

In a typical use the printer is employed in a remote terminal such as might be carried by a salesman and connected to a computer by telephone line. The control logic of the printer receives instructions from the computer and initiates a series of letter or symbol commands. Each of these commands involves a cycle which includes a print operation followed by a carriage movement operation. In a typical cycle the following steps occur: At time zero the electromagnet 32 is energized to a first current level, increasing the force applied by the force clip and the U core which brings the printing element into good thermal contact with the paper, regardless of the particular position of the printing head along the width of the paper. In achieving this good thermal contact both the paper and the elastomeric bar are slightly compressed by the pressure, as a matter of design the pressure being determined by appropriate selection of the elastomeric bar and the current level. The printing elements, i.e., those dots needed for the printing of the selected letter or symbol, are turned on for a period of approximately 6 to 12 milliseconds, the electromagnet remaining energized at the first current level during this period to provide good thermal contact. The paper changes color due to its thermo-chemical properties and a letter or symbol is produced on the paper. At the end of this period a short time of the order of a millisecond is allowed to permit cooling of the print element and to permit collapse of the magnetic field, the magnet 32 having been de-energized to a second, lower current level. The carriage advance stepper motor is now energized to advance the carriage one character position along shaft 26, the head sliding on the paper, the lower level of energization ensuring that the head does not move away from the paper. As the head advances to its new position, braking to reduce overshoot and oscillation is provided by a momentary electrical pulse (e.g. to the first current level) through the electromagnet coils, providing increased frictional force between the printer head assembly and the paper, (and with momentary compression of the elastomeric bar by the head acting through the paper) as the head slides thereupon. The head comes to rest much more quickly than it would without the braking pulse. The total time for this cycle is approximately 16-20 milliseconds and the logic circuitry is now ready for the next instruction. The carriage is allowed to advance 81 print positions across the page. The 81st position initiates a carriage return as can also be done with an external command. By reverse action of the carriage stepper motor this causes the head to return to its left-most starting point and also initiates a paper advance command. The paper advance is forward through the machine. However, to print overscored and underscored characters, the paper may be rolled one-eighth of a line either reverse or forward by the stepper motor and control logic.

By the positioning of the electromagnet in the relation shown a very effective printer construction is achieved, e.g. it provides ready visibility of the print-out as it occurs, etc. However it is possible under certain circumstances to provide the elongated electromagnet on the same side of the sheet as the print head and armature, e.g. in an arrangement where an armature associated with the head (e.g. protruding to the front from mounting block 48) serves to push the head against the paper instead of the pull arrangement shown. This and other embodiments will occur to those skilled in the art and are within the following claims .

Claims

We claim:

1. In a thermal printer for printing successive lines of selected discrete characters upon a sheet, the printer comprising a thermal printing head having a multiplicity of energizable and de-energizable print points positioned over an area for defining the desired characters by selective energization of respective sets of said print points by a character control circuit, a carriage means for advancing said head stepwise across said sheet to a plurality of discrete character positions and for returning said head, a sheet transport mechanism for advancing said sheet lengthwise after the printing of each line, and a pressure applying system for causing said thermal printing head to thermally contact said sheet, the improvement wherein said pressure applying system is an electromagnetic system comprising a first flux carrying member attached to and secured to move with said head and carriage, said flux carrying member having at least a pair of spaced apart faces, and a second flux carrying member which is elongated, extends parallel to the path of advance of said head and presents a pair of elongated faces opposed and parallel to respective faces of the first said flux carrying member, at least one of said flux carrying members associated with electromagnet coil means, and said first and second flux carrying members together defining a magnetic circuit for flux produced by said coil means regardless of the position of advance of said head, and control circuitry for energizing said coil means while said printing head is stopped in a said discrete character position, said first flux carrying member being arranged to press said printing head to thermally contact said sheet in response to flux produced by energization of said coil means, whereby a selected set of said print points on said print head, energized by said character control circuit, are caused simultaneously to firmly press against said sheet in the absence of relative lateral movement to produce a character on said sheet.

2. The thermal printer of claim 1 wherein said printing head is positioned to slidably contact said sheet while said head advances.

3. The thermal printer of claim 1 wherein said control circuitry is adapted to selectively energize said coil means to a first level at the time of energizing said thermal head to press said printing head firmly against said sheet for printing a character and after said printing to de-energize said coil means to a second level, lower than the first, reducing pressure of said head upon said sheet to enable advancing movement of said head while said head remains in sliding contact with said sheet under the influence of said coil means.

4. The printer of claim 1 wherein said control circuitry is adapted during advancing movement of said head to energize said electromagnetic coil means to brake said carriage by sliding friction of said head upon said sheet as said head approaches a new discrete print position.

5. The thermal printer of claim 1 wherein said second flux carrying member is on the opposite side of the sheet from said first flux carrying member, said magnetic circuit being defined across the thickness of said sheet.

6. The thermal printer of claim 1 wherein said first flux carrying member comprises a ferromagnetic armature and said second flux carrying member is associated with said coil means.

7. The printer of claim 6 wherein said second flux carrying member comprises a pair of legs extending in parallel throughout the range of movement of said carriage, said legs connected to each other by a bridge member throughout their length and free portions of said legs defining a pair of pole faces disposed for interaction with said armature to complete said magnetic circuit.

8. The printer of claim 7 wherein said coil means comprises two coils, one wound about and throughout said extent of each of said legs.

9. The printer of claim 7 wherein the normals to said pole faces correspond to the direction of application and release of pressure of said head, said armature having corresponding pole faces, energization of said coil means being effective to draw respective pairs of said leg and armature pole faces together thereby to urge said head against said sheet.

10. The printer of claim 9 wherein said second flux carrying member is on the opposite side of the sheet from said first flux carrying member, said magnetic circuit being defined across the thickness of said sheet, the respective pole faces of said armature and said second flux carrying member being parallel to the plane of said sheet and opposed to each other across the thickness of said sheet.

11. The printer of claim 10 further including an elongated elastomeric mass extending parallel to said second flux carrying member, said mass defining a surface parallel to the surface of said sheet, said surface being opposed to said thermal printing head and projecting beyond said flux carrying member toward said sheet.

12. The printer of claim 7 wherein said armature comprises a plurality of leg members connected by a bridge means, each of said armature leg members positioned closely in opposition to a leg member of said second flux carrying member.

13. The printer of claim 4 further including an elongated elastomeric mass extending parallel to the path of said carriage and head, said mass defining a surface parallel to the surface of said sheet, and being opposed to said head, on the opposite side of said sheet therefrom, and said control circuitry adapted to energize said coil means during a braking period before said head comes to rest in a new discrete print position and to energize said coil means while said head is at rest and thermal printing elements of said head are energized, to press said head against said sheet and thereby compress said elastomeric mass.

14. The printer of claim 13 wherein said carriage is slidably mounted on a shaft to advance said printing head, said carriage being rotatable on said shaft, to allow said armature and head to freely respond to press said head against said sheet upon energization of said coil means.

15. For use in a printer in combination an electromagnet coil means and an elongated flux carrying member associated with said coil means and extending along one face of a sheet to be printed, said flux carrying member having at least a pair of spaced apart elongated faces, a print head adapted for printing characters upon said sheet, a carriage means to advance said head stepwise in the direction of extent of said elongated flux carrying member to a plurality of discrete character positions therealong, an armature of ferromagnetic material associated with said head and having faces opposed and parallel to faces of said elongated flux carrying member, said elongated flux carrying member and associated electromagnet coil means being adapted to interact with said armature to press said head against said sheet upon energization of said electromagnet coil means over the range of said positions of advance of said head and control circuitry adapted to momentarily energize said electromagnet coil means to a braking level during advancing movement of said head from one discrete character position to the next thereby to press said head against said sheet to produce sliding friction sufficient to brake said head and carriage means as it comes to rest at said next discrete character position.

16. The mechanism of claim 15 wherein said print head is disposed at the opposite side of said sheet from said electromagnet coil means, said sheet lying in said gap therebetween and means selectively to advance said sheet through said gap in a direction perpendicular to the direction of advance of said head.

17. The mechanism of claim 15 wherein said control circuitry is adapted to receive commands from a computer and in response thereto to initiate a printing cycle including energization of said print head and energization and de-energization of said electromagnet coil means.