STATIC DISSIPATION IN A PRINTER

Abstract: A print head (22) is secured to a carriage (54) and is movable along shaft means (56) in printing operation. The carriage (54) includes bearings (61,62) engaging the shaft means (56) which are secured to the frame of the printer (12). The carriage (54) is made of conductive material to enable electrical coupling of the print head (22) through the carriage (54) and the bearings (61,62) and along the shaft means (56) to the ground potential of the printer (12).

Description: The present invention relates to a printer and more particularly, but not exclusively, to a dot matrix printer. One of the requirements in a dot matrix printer is to provide means for electrically coupling certain parts of the printer to a ground potential. Ground potential is defined as the chassis of an appropriately-connected integrated or a remote control unit providing electrical power to the dot matrix printer. In printing operations, the printer must be designed to provide static discharge control in order to dissipate the build-up of static charge resulting from the printing impact or electrostatic charge from human contact. In this respect, it is essential that the print head be connected by conductive means to a grounding part of the overall system. The bonding of the print head effectively provides for electrostatic discharge of any charges that may accumulate on the print head. The print head is accessible to the printer operator and therefore needs to be effectively bonded to the grounding system. Tinsel, grounding straps and the like have been used to directly connect the print head to ground potential. A dot matrix printer is described in United States Patent No. 4,452,542, issued to H. Akazawa on June 5, 1984, which discloses a print head carriage with an electrically conductive board mounted on the carriage, the board being electrically coupled via bearing members of the carriage and by shaft means to the frame of the printer for grounding the print head. One of the disadvantages of using a conductive board or the like is that the number of components for the printer is increased and the manufacturing and assembling steps are complicated so that a reduction in cost cannot be attained. Another dot matrix printer is described in the IBM-Technical Disclosure Bulletin, Vol.28, No 7, Dec. 1985, pages 2799-2800, "Static Electrical Charge Grounding in a Printer with Plastic Parts", which discloses a printer including a print head mounted on a carriage having bearing means for supporting said carriage on shaft means secured to the frame of said printer wherein said carriage is made of an electrically conductive moulded plastics material comprising carbon material wherein said print head is electrically coupled through said bearing means and through said shaft means to the ground potential of said printer. An object of the present invention is to provide a printer whereby the assembling operation can be facilitated and the manufacturing cost can be reduced. Accordingly, the present invention provides a printer including a print head mounted on a carriage having bearing means for supporting said carriage on shaft means secured to the frame of said printer, wherein said carriage is made of an electrically conductive moulded plastics material comprising polycarbonate with polytetrafluoroethylene and carbon material wherein said print head is electrically coupled through said bearing means and through said shaft means to the ground potential of said printer. The use of polycarbonate with polytetrafluoroethylene and carbon was found to be the most suitable material from which to mould the carriage. The use of this particular material not only allows the carriage to be moulded as an electrically conductive part of unitary construction but also enables the carriage to be formed in a complex shape with high tolerances thereby enabling the design of a carriage which accommodates the many functions which the carriage is required to perform. An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a perspective view of a printer incorporating an embodiment of the present invention; Fig. 2 is a plan view of the printer showing the relative positions of the pertinent parts; Fig. 3 is an enlarged perspective view of pertinent parts of the printer; Fig. 4 is a side elevational view of a portion of the printer and showing the arrangement of the print head, the print head carriage, and the supporting shafts; and Fig. 5 is a block diagram of the various elements included in the charge dissipative path. Prior to describing the structure in detail, it should be noted that the printer of the present invention is a multi-function type that can accommodate a receipt, a journal and a slip or form which form consists of one or more parts. The printer can be set in one of five different modes of operation which include printing a journal only, printing a receipt only, printing a receipt and a journal, printing a slip or form only, or printing a slip or form and a journal. The journal and the receipt can accommodate 42 columns of printing and the slip or form can accommodate 46 columns of printing. The printer is connected to a controlling device which contains a power supply whose negative secondary voltage terminals are connected to ground potential, i.e. chassis. Referring now to the drawing, Fig. 1 shows a perspective view of a printer 12 incorporating an embodiment of the present invention and having a front portion 14, a right side 16, a left side 18, and a rear portion 20. A wire matrix print head 22 is moved in a side-to-side manner by suitable motor drive means (Fig. 3) located at the right front corner of the printer 12. A journal station or module 24 is provided at the right side of the printer and includes a supply roll 26 of journal paper that is guided past the journal print station platen 28 and is rewound on a take-up roller 30 by a step-type drive motor (not shown). A receipt station or module 32 is provided at the left side of the printer 12 and includes a supply roll 34 of receipt paper that is guided past the receipt print station platen 36 and is driven by a step-type drive motor (not shown). The journal station 24 and the receipt station 32 are separated by a preferred number (19) of character spaces. A ribbon cassette (not shown) of the operator-changeable type is positioned to the rear of the print head 22 (toward the viewer of the illustration in Fig. 1) and the ribbon is driven in one direction from right to left in a path between the front portion of the print head 22 and the record media (journal, receipt or slip). A slot 40 is provided at the left front side of the printer 12 for insertion of a slip 38 which can be inserted from the front of the printer 12 or from the side thereof in a path in front of the receipt paper at the receipt station 32. A heat sink 42 is provided for the print head 22 to dissipate heat therefrom. Figs. 2 and 3 illustrate the drive mechanism wherein an endless toothed belt 46, driven by the print head carriage motor drive means in the form of a stepper motor 48, is trained around a first or motor pulley 50. The motor pulley 50 is secured to and extends upwardly from the stepper motor 48. The endless toothed belt 46 is trained around a second pulley 52 at the left side of the printer 12. A print head carriage 54 is secured by means of a curved or arcuate connector 55 (Fig. 4) to the timing belt 46 to move the carriage 54 back and forth across the printer in bi-directional printing operations. The carriage 54 supports the print head 22 in precise position for printing on the journal, the receipt, or the slip, as the case may be. The carriage 54 is supported by and rides on guide rails or shafts 56 and 58 (Fig. 2) by means of bearings 60, 61 and 62 (Figs. 3 and 4). Bearing 60 is a front bearing (looking from the front of the printer 12 in Fig. 1) and bearings 61 and 62 are rearward of bearing 60. The carriage 54 is molded from conductive plastic for satisfactory EDS (electrostatic discharge) performance and for static charge control. The carriage 54 includes a printed circuit board 64 as a part thereof which is connected to a ribbon-style cable 66. The cable 66 is connected to power and control devices in the form of additional printed circuit boards (not shown) coupled to printer control means or a printer controller providing a specific control program for operating the printer 12. The solenoids (not shown) for operating the wires of the print head 22 are connected to the printed circuit board 64 and to the cable 66. A timing strip 68 of elongated structure and made of plastics material is molded as an integral part of the frame portion 71 of the printer 12. This construction provides that the timing strip 68 is secured and fixed in one position on the printer 12 and is not subject to movement or to adjustment. The timing strip 68 includes a lower solid portion 70 and a plurality of slots 72 facing upwardly and extending substantially along the length of the timing strip 68. An optical sensor 74 of the light emitting diode and phototransistor type is secured to the underside of the carriage 54 and straddles the timing strip 68. Output signals from the optical sensor 74 are transmitted therefrom to the ribbon cable 66 and to the printer controller. Fig. 4 shows a side elevational view of the print head carriage 54 supporting the print head 22. The front of the printer 12 is shown at the left in Fig. 4. The print head 22 is shown as being rigidly secured to the carriage 54, however, the print head is removable or easily replaced. The carriage 54 includes the bearing means in the form of the pair of rear bearings 61 and 62 (Figs. 3 and 4) that are supported by and move along the main guide rail or supporting shaft 56. The front bearing 60 is a spherical bearing and provides a journal and support for the guide rail or supporting shaft 58 that is parallel to the shaft 56. The guide rail or supporting shaft 56 is supported in suitable manner from side frames of the printer 12 (Fig. 2). The shaft 58 is electrically floating and is likewise supported from side frames of the printer 12. The print head carriage 54 is a molded part which is made of a conductive material and is of unitary construction and of a complex shape to accommodate the many functions that the carriage is required to perform. The carriage includes a nose or front portion 76 of generally plate-like shape to support the nose portion 78 of the print head 22. The nose portion 76 of the carriage 54 includes a pair of spaced hubs 80 and 82 for receiving and securing the print head 22. A pair of projections 84 and 86 at the front of the nose portion 76 of the carriage 54 serve to guide the print head 22 when installing the print head and to maintain the nose portion 76 in position on the carriage. An upright plate portion 88 of the carriage 54 supports the nose portion 76 and connects a pair of bridging portions 90 and 92 that are integral with the rear bearings 61 and 62. The bridging portions 90 and 92 are integral with and connect with a generally horizontal plate portion 94 of the carriage 54, the plate portion providing an opening therein between the bridging portions 90 and 92 for receiving a lower portion of the print head 22. The carriage 54 also includes a pair of downwardly extending arms 96 and 98 (Figs. 3 and 4) which support the printed circuit board 64 (Fig. 3) that provides electrical power to the print head 22. The printed circuit board 64 includes a receptacle or like device 100 (Fig. 4) secured to the top of the board for receiving a printed circuit card 102 of the print head 22. The receptacle 100 is permanently affixed to the end of the board 64. The printed circuit card 102 is sandwiched between the nose portion 78 and the coil portion 104 of the print head 22. The carriage 54 has a plurality of lugs or guides 106 integral with the plate portion 94 and spaced thereacross for the purpose of routing the flat, flexible cable 66 (Figs. 2, 3 and 4). The cable 66 is also permanently affixed to the printed circuit board 64, as seen in Fig. 4, and is trained through a passageway formed by a clamp or like element 108 and a guide or like element 110, both integrally formed or molded as a part of the carriage 54. The cable 66 is trained in a suitable path across the top of the plate portion 94 of the carriage 54 to a power source or like connection. In accordance with the present invention, the carriage 54 is made of a conductive material that eliminates the need for a separate bonding strap or like element for direct connection to and grounding of the print head 22. The carriage 54 preferably is made of molded plastic polycarbonate with 15% teflon polytetrafluoroethylene and 30% carbon fiber material, as manufactured by RTP Company, Winona, Minnesota and designated as 385 TFE 15EM. The shaft bearing 60 preferably is made of nylon 6/6 with 15% polytetrafluoroethylene and 30% carbon fiber conductive material, as manufactured by LNP Engineering Plastics, Malvern, Pennsylvania and designated RCL 4036FR. The shafts 56 and 58 are steel for conducting any charges to ground potential. As mentioned earlier, the shaft 58 is electrically floating so that the carriage 54 and the spherical bearing 60, being made of conductive plastic material, provide a static charge dissipative path through the carriage and through the shaft 56 to the printer grounding system. The bearings 61 and 62 include self-oiling bronze inserts or bushings, as 112 and 114 in Figs. 2 and 4. The discharge path for any charges that are generated by reason of the impact printing of the print head 22 is from the print head, through the carriage 54, through the bronze inserts 112, 114 (Figs. 3 and 4) in bearings 61 and 62 and through the shaft 56 to ground potential. Additionally, the path of discharge or grounding includes the motor 48, a mounting plate 59 for printed circuit boards, an input/output connector shell 63, and an interconnect cable shield 65. Fig. 5 is a block diagram of the various elements and illustrating the path of discharge of any charges built up from print wire impact or by human contact. The print head 22 is carried by and secured to the carriage 54 with the shaft 58 being connected in the grounding path to the carriage through the spherical bearing 60. The carriage 54 is bonded to the shaft 56 through the bronze bushings 112, 114 of shaft bearings 61 and 62. The motor 48 is bonded to a strap 57 in contact with the shaft 56, in turn, connected with a baseplate 59 carrying one or more printed circuit boards. The baseplate 59 is bonded to the I/O connector shell 63, in turn, connected to the cable shield 65 and to the chassis of an appropriate printer controlling device which includes the power source for the printer 12. Other modules 69, such as a receipt/slip module, are bonded by means of a strap 67 to the shaft 56 (Fig. 2). It is thus seen that herein shown and described is a carriage on which is installed a dot matrix print head. The printed circuit card is received in a receptacle that is affixed to a printed circuit board. A flat, flexible cable is affixed to the printed circuit board. The carriage is made of a conductive material that provides a static charge dissipative path from the print head to the bronze inserts of the bearings that journal the carriage supporting shaft and then to ground potential.

Claims: A printer including a print head (22) mounted on a carriage (54) having bearing means (61, 62, 112, 114) for supporting said carriage (54) on shaft means (56) secured to the frame of said printer (12), wherein said carriage (54) is made of an electrically conductive moulded plastics material comprising polycarbonate with polytetrafluoroethylene and carbon material wherein said print head (22) is electrically coupled through said bearing means (61, 62, 112, 114) and through said shaft means (56) to the ground potential of said printer (12). A printer as claimed in claim 1 characterized in that the polytetrafluoroethylene is about 15% and the carbon is about 30% of the total material of said carriage (54). A printer as claimed in claim 1 or claim 2, characterized in that the carbon is in the form of carbon fiber.

Inventors: BRULL, ROBERT A., DEL SIGNORE II, JAMES R., WALKER, ALAN H.
Applicants: NCR INTERNATIONAL INC.
Application year: 1989
Main classification: B41J29
IPC codes: B41J29, B41J19