A METHOD AND DEVICE FOR PRINTING QUALITY CONTROL

Abstract: A method and device for checking the printing quality of matrix or liquid jet printers (2). A character (3) is printed in the paper margin and is optically sensed in connection with line shift. The signal pattern from the symbol is compared with a predetermined pattern corresponding to a correct reproduction of the character. An alarm signal is sent if there is a difference between the signal patterns.

Description: A method and device for printing quality control. TECHNICAL FIELD The invention relates to a method and device for chec ning the printing quality of preferably matrix or liquid jet printers with the aid of photo-sensitive sensing means. BACKGROUND ART In printing with a matrix printer, the readability is dependent, inter alia, on the condition of the printing ribbon, and that all the printing elements of the printing head are functioning. Particularly obvious problems with control of these functions are present where printers are used without their being given continuous supervision. Examples of such printers are computer controlled printing apparatus operating at certain times during the day, and cash dispensers which print out dtails of transactions carried out. Different methods exist for checking the printing quality of matrix printers. For example, in the PCT application US 81/01207 there is described a device for detecting every dot that is printed by means of optical sensors following the printing head. The result of the detection is compared in a computer with a signal pattern corresponding to the printed character. A fault signal is produced on deviation from the signal pattern. The device is also intended for use with ink jet writers. Since the two most important factors affecting printing quality of a matrix printer are the condition of the printing ribbon and the function of the printing elements, the device according to the mentioned patent is an unnecessarily sophisticated and expensive solution. DISCLOSURE OF INVENTION With the present invention there is obtained a simple and effective check on the printing quality of a matrix printer or an ink jet printer. This is done by each printing element simultaneously printing a dot pattern within an area of the paper where no normal printing takes place. In connection with line shift a photo-sensitive device reads the printed pattern. The signals thus generated are processed and compared with a predetermined signal pattern corresponding to correct printing quality. Difference between these patterns activates an alarm circuit. The characterizing features of the invention are disclosed in the accompanying claims. BRIEF DESCRIPTION OF DRAWINGS The invention will now be described in detail in with reference to the accom¬ panying drawing where Figure 1 schematically illustrates the parts of a matrix printer which are of interest in connection with the invention, and Figure 2 illustrates the included electrical circuits. BEST MODE FOR CARRYING OUT THE INVENTION Figure 1 schematically illustrates a writing paper 1 with characters which are printed with the aid of a printing head 2 of the matrix printer type. The printing head has also printed a dot pattern 3 in the left hand margin of the paper for ech line of print. The pattern is shaped as a row of dots at right angle to the direction of movement of the printing head and has been produced by all the writing elements of the printing head. An optical sensing means 4 is disposed such that the dot pattern 3 passes under the sending means 4 during line shifting. Figure 2 illustrates in more detail a device in accordance with the present invention. The optical sensing means 4 includes a light emitting diode 7 illuminating the writing paper under a row of photo diodes 8 provided with a shift register, e.g. of the type EG & G Reticon RL128G. On a line shift the dot row 3 passes in the direction of the arrow P past the photo diode row 8, where the diodes at a spacing of 25 um are arranged at right angle to the direction of movement of the dot row 3. By means of the shift register, each diode is read out in turn with a speed determined by a control means 12 included in a computer 6. In this case the clock frequency of the control means has been selected to 50 KHz and the sampling frequency to 300 Hz, which gives a scanning time of 2,56 ms and about 12 scans per dot of the dot pattern 3. The signal from each photo diode is transferred to a comparator means 5 where it is amplified in a DC amplifier F and compared with the voltage from a reference voltage source R in a differential amplifier D. To signals with higher amplitude than the reference voltage are assigned a "one" is allotted in the differential amplifier, while to signals with lower amplitude is allotted a "zero". The digitalized signals are read by the computer 6, where a counter 9 adds all the "ones" according to given conditions. One condition, for example, may be that at least four consecutive "ones" are required for the counter to add them. After three scans of all diodes without the above-mentioned condition being filled, the counter is read, subsequent to which it is set to zero. A condition for a dot to be registered at the passage of the dot pattern 3 past the sensing means is that the counter 9 adds 40 "ones", for example. The number of registered dots is compared in a register 10 with the number of printing elements in the printing head 2. If there is a difference between these numbers a signal circuit 11 is actuated to give an alarm signal. OMPI to>_. wi p o →&)

Claims: CLAIMS 1 A method of checking the typing quality of writers, preferably matrix or ink jet printers, by means of a photo sensitive circuit, characterized in that the printer prints a predetermined character in an area predetermined area in relation to a line of print, in that the character is read by means of the photo sensitive circuit in connection with line shifting, in that the signal pattern from the photo-sensitive circuit is compared with a signal pattern corresponding to correct reproduction of the printed character, and that an error signal is generated if there is adifference between the two signal patterns. 2 Apparatus for checking the printing quality of preferably matrix or liquid jet writers, by means of a photo-sensitive sensing means (4) according to claim 1, characterized in that the photo-sensitive sensing means of the device is disposed such that in connection with line shift, a predetermined symbol printed by the printer in an area determined in relation to a printed line passes under the sensing means, electrical signals thus being generated, the amplitude of which are dependent on the colour contrast of the character against the printing agent, and circuits for comparison between the signals and a pre¬ determined signal pattern, a signal being produced in response to difference between the pattern generated by the sensing means and the predetermined signal pattern. 3 Device as claimed in claim 2, characterized in that the printed character forms a row of dots produced by all the printing elements simultaneously and running in the direction of paper feeding. 4 Device as claimed in claim 2, characterized in that for alternate lines the printed character originates from printing elements with odd numbers and for the other rows from printing elements with even numbers. 5 Device as claimed in claim 2, characterized in that the signal produced in response to difference between the two signal patterns controls an alarm circuit.

Inventors: FALK PER IVAR ROLAND, FALK, PER, IVAR, ROLAND
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON
Application year: 1984
Main classification: B41J3
IPC codes: B41J29