Thermal print head unit

Abstract

 The invention provides a thermal printer with a removable print head unit (14) having a multitude of very small resistance heating elements (34-1...,34-n) for printing small dots or pixels (e.g., 300 pixels per inch) of an image. Minor variations in the printing characteristics of the multitude of individual head elements are electronically compensated for to achieve uniform print densities for that head unit (14). Print compensation data in the form of correction numbers respective to each resistance element of the print head are permanently stored in a low cost device, such as an electronically erasable programmable read only memory (EEPROM), which is furnished as an integral part of the removable head unit (14). Each such head unit is uniquely calibrated by its permanently stored correction numbers to achieve uniform printing action so that for all intents and purposes all such head units perform the same in any printer. Any head unit may be operated in a given thermal printer without further adjustment to the printer. When the printer is powered-up, the correction numbers unique to the head unit (14) then installed in the printer are automatically loaded into the high speed memory and compensation circuits of the printer. This arrangement greatly simplifies the replacement of the head unit of a printer in the field.

Description

Field of the Invention

[0001] This invention relates to a thermal printer, and more particularly to a removable thermal print head unit with integral compensation to correct for non-uniformity in the multiple resistance heating elements of the head unit by means of a low cost, easily programmable permanent memory device.

Background of the Invention

[0002] One type of thermal printer employs a thermal head assembly comprising a multitude of closely spaced resistance heating elements which are separately energized to print very small dots or pixels from a thermally reactive dye element onto a print medium or page, in accordance with input data. The data is in the form of binary words (e.g., 8-bit words) which define each of the positions and densities of the respective pixels being printed, and the characters or image so defined. For visual sharpness and fidelity to an original image, the printed pixels should be very small and have a nearly uniform tone scale over a wide range of densities. A thermal head assembly is typically a linear, one-dimensional array of thick (or thin) film resistance heating elements individually formed on a ceramic substrate which is then mounted on a heat sink, such as a metal bar. At present, print heads with 300 miniature heating elements per lateral inch (2400 elements for an 8 inch wide page) are commonly used. These elements are small enough to give a good degree of visual sharpness. However, minor variations in the printing action of these individual print elements give rise to visually objectionable non-uniformity (for a supposed equal density) in the pixels across a line on a page being printed. This non-uniformity manifests itself in visible lines, streaks and bands in a direction parallel to page motion past the thermal head.

[0003] There are minute random differences in the physical, electrical and thermal characteristics of the ultra-small heating elements in a thermal head assembly such as described above. As a result, it is very difficult, if not impossible, even with state-of-the-art manufacturing techniques to produce a print head assembly with a sufficiently high degree of printing uniformity from element to element to avoid residual visual distortion and blemishes in a printed page. In view of this, circuits to compensate electronically for print head non-uniformity in a thermal printer have been developed and are currently in use. In U.S. Patent No. 4,827,279, which is assigned to an assignee in common with the present patent application, there are described a method and apparatus for correcting non-uniformity in the printing action amongst the heating elements of a thermal print head. In the compensating system disclosed in this patent, correction numbers for each heating element of the head assembly are derived from microdensitometer measurements of a line of print; the correction numbers are then computed according to a specified formula. These correction numbers, which are unique to a particular head assembly are then entered in a high speed electronic memory circuit of the overall thermal printer apparatus. The numbers so stored are used along with input print data during subsequent normal operation of the printer to achieve a desired quality of printing. But since the correction numbers for a given head assembly are unique to that particular head assembly, changing or replacement of the head assembly of a printer (such as disclosed in the above patent) causes difficulty. On the one hand, where it is desirable to replace a head assembly in the field, correction for a new head must be separately entered into the electronic memory and correction circuits of the printer. This is difficult to do in the absence of specialized electronic instruments or skilled technicians. But sending the entire printer to a repair center for installation of a new head and for re-calibration costs substantial time and money. The present invention provides a simple and cost effective solution to this problem. The invention provides for permanent and unique calibration of each print head assembly unit so that any calibrated head unit may readily be used with any printer. There is no need for further calibration of a particular head unit when it is installed in a given printer. The invention also simplifies and makes more efficient the head unit manufacturing and testing operation in the first place, as well as the final assembly of a complete printer.

Summary of the Invention

[0004] In accordance with the present invention there is provided a physically separable print head unit for a thermal printer comprising a multi-element resistance print head, a mounting base, and an electronically erasable programmable read only memory (EEPROM) device. These members are put together as an integral but separable unit of an overall printer. In the course of assembly the head unit is tested and the individual heating elements of the head are energized to determine the respective printing characteristic of each element. Actual print data thus acquired are fed into a computer which in turn calculates the compensation data (correction numbers) corresponding to each heating element so that each and every element prints with a uniform density when energized by the same data word calling for such density. These respective correction numbers, as derived by the computer, are thereupon stored permanently in the EEPROM device associated with that particular print head and forming a permanent part of the print head unit. Thus each print head unit is made identical, for all intents and purposes, to all other such head units being produced. The EEPROM device incorporated into each print head unit can be a relatively slow, hence inexpensive, device. The correction numbers permanently stored in the EEPROM device are outputted to the high-speed correction circuitry of an associated thermal printer when it is powered-up. This process which may take a number of seconds. Thus the correction numbers in the EEPROM device can be serially fed at very slow speed into the memory portion of the high speed printing correction circuitry of the printer. By virtue of this arrangement each print head unit may be accurately, permanently, and inexpensively calibrated to achieve substantially uniform print-out when used in any given printer. This in turn offers considerable savings and efficiency in the initial manufacturing cost and in the subsequent field maintenance of thermal printers utilizing the present invention.

[0005] The cost of the EEPROM device used in accordance with the present invention is relatively low because such a device is not directly in the high-speed data path of the thermal printer and hence need not have high speed capability. The correction numbers from the EEPROM unit can be serially outputted at very slow speed, this being done when power to the printer is first turned on (the various other elements and circuits of the printer take some time to reach equilibrium). The EEPROM device itself is in the form of a simple 8-pin dual-in-line (DIP) package which in turn occupies only a minimum of circuit board space. Furthermore, since the individual heating elements of the print head, in spite of unavoidable variations from element to element, are nonetheless closely similar in their printing characteristics, the correction numbers stored in the EEPROM device need only a limited range of numbers to achieve nearly exactly uniform compensation. Thus where a data word to be printed as a pixel by the thermal printer may have a range of 0 to 255 (8-bits), the range of compensation for the individual heating elements of the print head may require only numbers with a range of 0 to 63 (6-bits). For a 2400 element print head (300 pixels per inch x 8 inch wide page) a single, low cost EEPROM with 2048 "byte" positions (8-bit) has more than enough single-bit positions to store the necessary 6-bit correction numbers (6x2400 or 14,400) when read out serially by appropriate circuitry.

[0006] A better understanding of the invention, together with its important advantages will best be gained from a study of the following description given in conjunction with the accompanying drawings and claims.

Brief Description of the Drawings

[0007] 

FIG. 1 is a schematic view (partly in phantom) of a thermal printer embodying the present invention;

FIG. 2 is a schematic view, not to scale, of a print head unit of the printer in FIG. 1 provided according to a preferred embodiment of the invention and shown here removed from the printer. FIG. 2 also shows a separate block diagram of circuit elements of the printer of FIG. 1 and further shows an externally provided computer for deriving correction numbers for permanent storage in the print head unit; and

FIG. 3 is a chart illustrating actual variation in density of the pixels printed by a thermal head in accordance with the numbered print elements of the head across a page.

Detailed Description

[0008] Referring now to FIG. 1 there is shown in schematic form, partly in phantom, a thermal printer 10. The printer 10 comprises a drum 12 adapted to support a thermal print medium (not shown) and to move the medium past a print head unit, indicated at 14. The drum 12 is mounted within a housing and frame 16 (shown in dashed outline) for rotation as indicated by a curved arrow 18. The print head unit 14 is mounted on the frame 16 by means (not shown) in fixed relation closely adjacent to the drum 12. The print head unit 14, which will be described in more detail in connection with FIG. 2, is shown here in FIG. 1 electrically connected by a cable-connector 20 to a circuit module 22. Mounted within the module 22 (but not shown here) are various data and control circuits (to be described shortly) of printer 10. The print head unit 14 is easily separated physically and electrically from the other elements of the printer 10.

[0009] Referring now to FIG. 2, the print head unit 14 is schematically shown here as being physically and electrically separated from the remainder of the printer 10. The print head unit 14 is shown upside down here for the sake of illustration. An end 20a of the electrical cable-connector 20 (shown here in dashed outline) is a permanent part of the print head unit 14. Another end 20b of the cable-connector 20 is a permanent part of the circuit module 22. The print head unit 14 comprises a mounting board (base) 30 to which a multi-element thermal print head 32 is affixed. The print head 32 has a large number of thermal heating elements 34-1 through 34-n (not shown to scale) which are adapted to print across a page of a print medium a like number of individual closely spaced printed pixels shown schematically as 36-1 through 36-n (not to scale). The construction and operation of the print head 32 and its respective resistive elements 34-1 through 34-n may be as described in the above-mentioned U.S. Patent No. 4,827,279. Mounted closely adjacent to the head 32 on the base board 30 is a memory device 38 which advantageously is an electronically erasable programmable read only memory (EEPROM) device. This memory device 38, installed as an integral part of the print head unit 14, permanently accompanies a particular print head 32 (identified by date and serial number) and provides electrical print compensation data unique to that print head unit 14. If desired, there may also be mounted on the base 30 of the unit 14, head driver circuitry 40 for electrically powering the individual heating elements 34-1 through 34-n of the head 32 in accordance with print data sent from the circuit module 22.

[0010] Shown to the right of print head unit 14, though not part of it, is a separate computer 44 which is employed as follows. Actual head print data derived from density measurements of the printed pixels 36-1 through 36-n are applied to the computer 44 as indicated by the dashed line 46. Based on these actual data, the computer calculates respective print compensating data corresponding to each of the resistance elements 34-1 through 34-n of the head 32. These compensating data, in the form of binary correction numbers are then, as indicated by the dashed line 48, loaded into and permanently stored in the EEPROM device 38 which is an integral part of the head unit 14. The calculation of the correction numbers may be as described in the above mentioned U.S. Patent No. 4,827,279. These correction numbers are respective to each of the resistance elements 34-1 through 34-n of the print head 32. This set of correction numbers (unique to a given head unit 14), are subsequently entered automatically into and used in the high speed memory and correction circuits of a printer 10. These correction numbers and the respective circuits substantially eliminate non-uniformity in the printing action of the individual resistance heating elements 34-1 through 34-n irrespective of which one of many print head units 14 is used with a given printer 10. There is no need for subsequent adjustment of the printer when a different print head unit 14 is installed.

[0011] The print head unit 14 contains the end 20a of the cable- connector 20 shown in FIG. 1. When the two ends 20a and 20b are connected by cable-connector 20 (shown here in FIG. 2 in dashed outline) and the head unit 14 mounted for operation in the printer 10 (see FIG. 1), electrical connections are made to the circuit module 22. The circuit module 22 need not be removed from the printer 10 and is shown in FIG. 2 for the purpose of illustration. The circuit module 22 contains (as shown in FIG. 2 in block diagram form) a correction circuit and data memory (CCDM) unit 50, a data processor 52, an image data unit 54, and a power supply unit 56. The set of correction numbers from the EEPROM device 38 (when the print head unit 14 is installed in the printer 10), are applied via a buss 57 running from the EEPROM device 38, through the cable-connector 20 (shown in FIG. 2 in dashed outline), to the CCDM unit 50. The loading of the correction numbers into the CCDM unit 50 is initiated when the power to the printer 10 is turned on. This is schematically illustrated by an on-off switch 58 connecting the power supply 56 to an input terminal 60 connected to an external power source (not shown). Another input power terminal 62 is connected to the external power source and directly to the power supply 56. A lead 64 running from the power supply 56, through the cable-connector 20, to the EEPROM device 38 supplies an energizing signal to the device 38 and enables it to load its permanently stored correction numbers into the CCDM unit 50. Circuits in the CCDM unit 50 permit the respective correction numbers from the EEPROM device 38 to be sent serially via the buss 57 at low speed and loaded into appropriate memory positions in the high speed CCDM unit 50. Image data from the image data unit 54 are fed into the data processor unit 52 which in turn sends data to be printed to the CCDM unit 50. The printer 10 then operates with electronic print compensation (i.e., the print head 32 prints with a high degree of print pixel uniformity) in the way described in the above mentioned U.S. Patent No. 4,827,279.

[0012] Referring now to FIG. 3, there is shown a graph of print pixel density corresponding to the numbered head elements 34-1 through 34-n. The line 70 is a plot of the actual densities of the print pixels 36-1 through 36-n as printed by the respective head elements 343-1 through 34-n for the same energizations of each of the head elements. It will be seen that the line 70 runs slightly above and below a dashed line 72, which indicates a desired uniform density level. However, actual densities plotted along the line 70 deviate only slightly from the uniform density level line 72 and these deviations are much less than the total density range indicated along the ordinate axis. Thus a correction number for each head element to compensate for the deviation of its printed pixel from the desired density level indicated by the line 72 is much smaller than a data number for the full range of densities. If, by way of example, the full range of densities indicated from "0" to maximum in FIG. 3 is represented by an 8-bit number (data word), then a correction number may be represented by a 6-bit number for the small magnitudes of the deviations between the values along the line 70 and the level, uniform line 72. As mentioned previously there may be as many as 2400 head elements 34-1 through 34-n across an 8 inch wide page. Thus the requirement for a correction number for each head element of only 6-bits permits the use of a single EEPROM device 38 with 2048 "byte" positions to compensate for all of the 2400 head elements (2400x6 = 14,400. This is less than 2048x8 = 16,384). The correction numbers respective to each head element 34-1 through 34-n as stored in the EEPROM device 38 and applied to the CCDM unit 50 of the printer 10 result in the print pixels 36-1 through 36-n having a desired uniform density lying substantially along the dashed line 72.

[0013] It is to be understood that the embodiment of the printing apparatus described herein is illustrative of the general principles of the invention. Modifications may readily be devised by those skilled in the art without departing from the spirit and scope of the invention. For example, the invention is not limited to the use of a particular EEPROM device or to just one device. Moreover data numbers other than 8-bit words and correction numbers other than 6-bit words may be used. Still further, the print head 32 is not limited to a particular number of heating elements per inch or to a particular page width.

Claims

1. A thermal printer comprising:
   a frame;
   supporting means mounted on the frame for supporting and moving a medium to be printed on past a printing location;
   circuit module means for driving the printer and being mounted on the frame, the circuit module means comprising power means, image and data processor means, and memory and print data correction means for controlling the printing action of multiple heating elements of a thermal print head;
   a print head unit comprising a base, a thermal head mounted on the base and having a multitude of small resistance heating elements for thermally printing individual pixels on the print medium, and an electronic memory device mounted on the base and permanently storing multi-bit correction numbers corresponding respectively to the multiple heating elements of the head, each multi-bit correction number providing print compensating data for its respective resistance element so that all such elements print with uniform density when energized in accordance with uniform image data;
   mounting means for mounting the print head unit on the frame in close proximity to the supporting means at the printing location for printing an image on the print medium, the mounting means permitting the easy removal of the print head unit from the frame and printer; and
   means for providing a data buss to output the correction numbers permanently stored in the memory device to the memory and print data correction means, and for effecting the automatic transfer of correction numbers from the memory device to the memory and print data correction means when the thermal printer is powered-up such that the printer prints uniformly with any given head unit and need not be further adjusted after the head unit is installed.

2. The printer of claim 1 wherein the memory device of the print head unit is an electronically erasable programmable read only memory (EEPROM) device, the correction numbers for the respective heating elements being stored in sequential addresses in the EEPROM device.

3. The printer of claim 2 wherein the means for providing a data buss output the correction numbers stored in the EEPROM device serially at slow speed to the memory and print data correction means when the printer is powered-up.

4. The printer of claim 3 wherein each correction number stored in the EEPROM device is a digital data word having a maximum weighted value substantially less than the maximum weighted value of an image data word.

5. In a thermal printer having a frame, means for moving a thermal print medium past a printing location, and circuit module means including electronic correction and data memory means for driving the multiple heating elements of a thermal print head, a thermal print head unit comprising:
   a print head having a large number of substantially identical resistance heating elements closely positioned along its length for printing on the thermal medium;
   a base on which the print head is mounted and which is adapted to be removably positioned on the frame at the printing location with the head and its resistance elements in printing relation to the thermal medium;
   memory means integral with the base and print head for permanently storing print compensating data in the form of correction numbers respective to each of the resistance heating elements of the head, the correction numbers enabling the printer to print with substantially uniform density in spite of minor variations in the printing characteristics of the heating elements of the print head; and
   circuit means for connecting the circuit module means with the memory means of the print head unit, the circuit means downloading the correction numbers stored in the EEPROM device to the electronic correction and data memory means when the print head unit is installed in the printer and the printer is powered-up, such that any print head unit can be installed in a given printer and the printer will print with the same desired high degree of uniformity.

6. The thermal print head unit of claim 5 wherein the memory means is an electronically erasable programmable read only memory (EEPROM), and the correction numbers are stored as 6-bit words in the EEPROM.

7. The thermal print head unit of claim 5 wherein the memory means is loaded with the correction numbers by a separately provided computer, the correction numbers being downloaded serially at low speed by the circuit means to the electronic correction and data memory means when the printer is turned on.

8. In a thermal printer having a frame, means for moving a thermal print medium past a printing location, and circuit module means including electronic correction and data memory means for driving the multiple heating elements of a thermal print head, a thermal print head unit comprising:
   a print head having a large number of substantially identical resistance heating elements closely positioned along its length for printing on the thermal medium;
   a base on which the print head is mounted and which is adapted to be removably positioned on the frame at the printing location with the head and its resistance elements in printing relation to the thermal medium;
   an electronically erasable programmable read only memory (EEPROM) device integral with the base and print head for permanently storing print compensating data in the form of correction numbers respective to each of the resistance heating elements of the head, the correction numbers enabling the printer to print with substantially uniform density in spite of minor variations in the printing characteristics of the heating elements of the print head; and
   circuit means for connecting the circuit module means with the EEPROM device means of the print head unit, the circuit means downloading the correction numbers stored in the EEPROM device to the electronic correction and data memory means when the print head unit is installed in the printer and the printer is powered-up, such that any print head unit can be installed in a given printer and the printer will print with the same desired high degree of uniformity.

9. The thermal print head unit of claim 8 wherein the heating elements are positioned at 300 per inch, the EEPROM device has 2048 byte positions, and the correction numbers for each of the heating elements of the head are stored as respective 6-bit data words.

Drawing