DOT MATRIX PRINTER

Abstract: A dot matrix printer in which, for the purpose of supervising the quality of the printing of dot matrix characters, an optical sensor (32) is provided for each dot producing element (11) of a print head (5). Each optical sensor (32) includes two glass fibers (34, 36) to carry light from a light-emitting diode to the surface of a record medium and to carry the reflected light to a photodiode. Each printed dot is detected and the sensor output is compared with a delayed or stored print signal which is applied to the drivers for the dot producing printing elements (11). The print head (5) may either be a wire matrix or an ink jet print head.

Description: DOT MATRIX PRINTER Technical Field The present invention relates to a dot matrix printer. Background Art In the field of impact printing, one of the most common types of printers has been the wire printer for printing characters in a dot matrix manner. As is well-known, the impact operation depends upon the move- ment of the wires which are typically moved by means of an electromechanical system which is believed to enable a precise control of the impact members. One manner of wire matrix printing utilizes a single wire carried in side-to-side motion and driven into impact with paper or like record media upon ener¬ gization of an electromagnetic solenoid. Another manner of wire matrix printing utilizes a print head having a plurality of wires disposed in alignment for printing characters as the print head is moved in side-to-side motion and as the wires are driven into impact with the paper upon energization of associated electromag¬ netic solenoids. Endeavors have been made to increase the print quality of wire matrix printers and accordingly arrangements have been proposed for detecting printing errors and for indicating when such errors have been made. Thus, for instance, the German Specification No. 2,617,902 discloses an error indicating device for a wire printer which includes force sensing means in the plane of the platen in face-to-face relationship with the print head and responsive to impact of the print wires and providing signals for interrupting the print¬ ing operation. The force sensors which are located at extreme left and right positions in the platen determine whether all the print wires strike with the proper force. However, with this arrangement it is impossible to detect whether the ink ribbon provides sufficient ink during the printing operation, whether the actual dot positions correspond with the desired positions and whether, in course of printing a line of characters, a print wire becomes detached or falls out. Disclosure of the Invention It is an object of the present invention to provide a dot matrix printer which is capable of detecting the omission, displacement or poor definition of a printed dot. Thus, according to the invention, there is provided a dot matrix printer having a print head mov¬ able along a line of printing relative to a record medium, said print head including at least one printing element, characterized in that each printing element has asso¬ ciated therewith an optical sensing element arranged to move with the print head and to sense each dot printed by the printing element on the record medium. In a preferred embodiment of the invention, a multi-wired print head has a multi-row of optical sensing elements arranged in parallel and as close as possible to the row of print wires in the print head and carried on the same carriage as the print head. Each sensing element may consist of at least two glass fibers, one for guiding the light from a light source to the paper or record medium and the other for guiding the reflected light to a photodiode. In a different arrangement, each sensing element may consist of four glass fibers for achieving a higher resolution. Brief Description of the Drawing An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which: Fig. 1 .s a diagrammatic view of a print head with an associated sensing element; and Fig. 2 is a view of a dot matrix character with the optical fibers associated with each dot thereof, Best Mode of Carrying Out the Invention Fig. 1 shows a wire matrix print head 5 with wire drive elements or solenoids (not shown but formed in a circular arrangement within a housing 10) for driving print wires 11 to effect printing in dot matrix manner on paper 12 or like record media. The paper 12 is caused to be transported or driven in a path between a platen 14 and the nose portion 16 of the print head 5. Wires or leads 18 are connected to a terminal block 20 and to the individual solenoids for driving the wires 11 for effecting printing upon pulsing of the solenoids. A pair of pressure rollers 22 and 24 are provided on the sides of the nose portion 16 to maintain the paper 12 in precise position on the platen 14 during side to side movement of the print head 5 in printing operation. A read head 26 is supported from the print head 5 and moves therewith to immediately sense the printing of the dots which make up the various charac¬ ters. In Fig. 1 the print head is seen to be moving from right to left across the ^ platen 14 and is printing the numeral "1" in repeated manner and the read head 26 is detecting the printed numerals immediately after printing thereof. Fig. 2 shows a composite view of a numeral "l" in dot matrix form along with an array 28 of optical sensing elements which are contained in a nose portion 30 of the read head 26. The array 28 is in the form of a window of the read head 26 wherein light passes to and from the paper 12. Each of the printed characters is defined within a 9 x 5 matrix pattern wherein the nine wires 11 are in alignment at the face of the nose por- tion 16 of the print head 5 and are individually ac¬ tuated by respective solenoids for the printing opera¬ tion. OMP The optical sensing element array 28 consists of an 11 x 4 arrangement for covering the nine dot high character and for an additional sensing element above and below the normal print area. A light-emitting fiber 34 and a light-receiving fiber 36 are repeated in the direction of travel of the print head to make up a sensing element or device 32 for each dot or each row of dots and comprises four glass fibers in adjacent manner. While one light-emitting fiber 34 for guiding light from a light source, such as a light-emitting diode, and one light-receiving fiber 36 for guiding reflected light from the paper 12 to a photodiode are sufficiient for sensing or detecting a dot, the two additional glass fibers 34 and 36 enable the achieving of a higher reso- lutioπ of supervision of the print quality. The optical sensing element 32 consisting of the four fibers is associated with each print wire 11 in a manner that each dot printed by a print wire on the paper 12 is detected individually by the associated sensing element at a predetermined time period after the printing operation. The time period depends on the distance between the end of the print wire 11 and the associated sensing element 32 and also on the speed of the relative movement between ^ the carrier for the print head 5 and the respective sensing elements 32 and the paper 12. The output signal of each sensing element 32 is compared in a comparator with the respective print signal which may be stored or may be delayed by the certain time period, all in a manner well known in the art. An error signal may also be produced in the con¬ trol system in well known manner to provide operator or machine readable indication in those cases where the comparison of these output signals provides a negative result. While a nine dot high column is shown in Fig. 2, the invention is also applicable for the well known 7 x 5 matrix character pattern. The additional sensing MPI elements above and below the numeral "l" are provided for reading documents having characters which are not in precise alignment. It is thus seen that the arrangement of the invention supervises continuously the printing of characters and the quality of the printed characters immediately after the printing operation and has the capability of detecting whether the inking medium is providing sufficient ink for printing the dots, whether the actual dot positions correspond with the desired positions and whether the print wires are operating in proper manner. The wire matrix printer just described enables the accomplishment of the objects and advantages mentioned above; however, it should be clear that the detecting scheme can be equally well applied to an ink jet matrix printer for supervising the quality of the printed characters. MPI

Claims: CLAIMS : 1. A dot matrix printer having a print head (5) movable along a line of printing relative to a record medium (12), said print head (5) including at least one printing element (11), characterized in that each printing element (11) has associated therewith an optical sensing element (32) arranged to move with the print head (5) and to sense each dot printed by the printing element (11) on the record medium (12). 2. A printer according to claim 1, charac¬ terized in that said print head (5) has a plurality of printing elements (11) arranged in a row extending perpendicularly to the line of printing and * that the optical sensing elements (32) are arranged in a respec¬ tive row (28) in alignment with the row of printing elements (11) . 3. A printer according to either claim 1 or 2, characterized in that each optical sensing element includes guide means (34, 36) for guiding light from a light emitting means to said record medium (12) and for guiding reflected light to a light receiving means for sensing the dots produced by the respective printing element. 4. A printer according to claim 3, charac¬ terized in that said light emitting means is a light emitting diode and in that said light receiving means is a photodiode. 5. A printer according to claim 3, character¬ ized in that each guide means includes a first optical fiber (34) for transmitting light to said record medium (12) and a second optical fiber (36) adjacent said first optical fiber (34) for receiving light from said record medium (12). 6. A printer according to claim 3, charac¬ terized in that each guide means includes in repeated sequence a first optical fiber (34) for guiding light to said record medium (12) and a second optical fiber (36) adjacent said first optical fiber (34) for receiving light from said record medium (12). 7. A printer according to either claim 1 or 2, characterized by a comparator for comparing the signal generated by an optical sensing element (32) upon sensing of a dot with the energization signal for the corresponding printing element (11), the signal generated by the optical sensing element (32) being delayed in dependence on the distance between said printing element (11) and the associated optical sensing element (32) and on the speed of the print head relative to the record medium (12), said comparator being arranged to generate an error signal if the com¬ parison provides a negative result. 8. A printer according to claim 1, charac¬ terized in that said print head (5) is a wire matrix print head. 9. A printer according to claim 1, charac¬ terized in that said print head (5) is an ink jet print head. OMPI

Inventors: BIEDERMAN H
Applicants: NCR CO, NCR CORP
Application year: 1981
Main classification: B41J3
IPC codes: B41J2, B41J7, B41J29, G06K5