Self-aligning thermal print head
BACKGROUND OF THE INVENTION 
     The present invention relates to thermal printers and more particularly to an improved thermal print head which maintains a uniform pressure against the print medium. 
     A printing wafer which contains a series of in-line dot thermal print elements as used in the present invention is described in U.S. Pat. Nos. 3,774,899 and 3,808,669. This wafer contains a plurality of elements each of which is heated when energized by an electrical signal. The heated element causes the formation of a dot upon a thermal sensitive paper which is in contact with the heating element. The intensity and clarity of the dot produced depends upon the proximity of the heating element to the paper and the tension exerted on the element against the thermal sensitive paper. The paper is supported by either a round or flat platen. 
     In previous high speed thermal printers the print element wafers are aligned on a printer frame in a given position in relation to the platen and are then rigidly fixed in place. This structure, however, is subject to a number of problems which can cause the intensity of the dot to vary and thereby adversely affect the clarity of the characters printed on the thermal sensitive paper. The letter quality will be degraded when either the proximity of the element to the paper or the tension of the element against the paper changes from some optimum setting. These conditions are present when the surface of the platen is irregular, the platen has a tapered shape or the sizes of the printer components vary due to thermal expansion or contraction. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a thermographic printing unit includes a frame, a platen and thermal print head. The thermal print head includes a dot-generating thermal element and a member rigidly holding the thermal element. The member is supported by the frame and movable relative to the platen. A spring biases the member in relation to the platen. 
     The print head includes a printing element that floats against the printing medium with a constant pressure thereby providing automatic and continuous alignment of the thermal element with the printing medium and eliminates the problems of spatial variations. This maintains the intensity and clarity of the printed characters, makes possible increased printer speed and provides a simple method for adjusting the thermal element wafer pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the following drawings: 
     FIG. 1 is a plan view of the self-aligning thermal print head of the present invention; 
     FIG. 2 is a section view of the present invention taken along line 2--2 of FIG. 1; and 
     FIG. 3 is a plan view of a thermal line printer utilizing the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A thermal print head embodying the present invention is shown in FIGS. 1 and 2 and includes a printer head module 10 having an upper member 10a, a lower member 10b, and a module heater strip 10c. A thermal print element wafer 11 is clamped between the module members 10a and 10b and includes a set of parallel dot-printing elements 11a. The module heater strip 10c provides heat to the module 10 and to the thermal print element wafer 11 to maintain these units at a fixed predetermined temperature. 
     Each of the print elements 11a is connected to an individual conductor included within the electrical cable 12 thereby enabling the activation of any individual element or combination thereof. 
     Thermal sensitive paper 13 is carried by a platen 18 in a conventional manner past the thermal element wafer 11 whereby the print element wafer 11 can be activated by an electrical signal to form a dot upon the thermal sensitive paper. The print elements 11a have very low thermal time constants to enable the formation of a dot in a very short time period. The dots must be printed in a proper sequence in relation to the moving thermal sensitive paper to form a matrix representing a particular character. The most common layout is a 5×7 matrix wherein, for example, the number 1 would be formed by a single print element 11a generating seven vertically stacked dots on the thermal sensitive paper 13. 
     The module 10 is supported by a frame 16 and the horizontal movement of the module 10 is limited by a module retainer 14. Compression springs 20 are positioned between the fixed module retainer 14 and the movable module 10 so as to force the thermal print element wafer 11 into contact with the thermal sensitive paper 13. The magnitude of the force applied by the compression springs 20 is adjusted by tension adjustment screws 22. 
     The quality of the dot produced by a print element 11a and therefore the quality of the resulting character is a function of the intensity of the dot burn on the thermal sensitive paper. The quality of the burn is in turn determined by the spaced relationship of the print element 11a to the thermal sensitive paper 13. Under the optimum conditions, the module 10 forces the print element 11a against the thermal sensitive paper with a constant force at all times. But when the thermal wafer element 11 is fixed in place on the frame, as in the prior art, the spaced relationship of the wafer to the paper can vary due to irregularities in the surface of the platen 18 and to the thermal expansion and contraction of the various elements of the printer. 
     The compression springs 20 bias the module 10 to compensate for the above condition by allowing the module 10 to move horizontally on the frame 16 wherein the print elements 11a are in effect floating on the thermal sensitive paper 13 supported by the platen 18. Therefore, when surface irregularities appear on the platen 18 as it rotates, the module 10 slides forward or backward on the frame 16 to accommodate the irregularities. Variations in the size of structural components due to thermal expansion and contraction are similarly accommodated. 
     Since the print elements 11a maintain a uniform pressure over a period of time against the paper, wear on the elements is reduced and the lifetime of the print head is extended. This uniformity in addition makes possible operation of the printer at a much higher speed. 
     FIG. 3 shows the arrangement of a number of modules 10 to form a full page width thermal line printer 24. The compression springs 20 force the modules 10 against the paper 13. The modules 10 are supported by the frame 16 and are limited in their movement by the module retainer 14. In a conventional system, each wafer 11 has 80 dot print elements 11a to accommodate the printing of a 16 character field using a 5×7 matrix. With a total of five modules, the printer 24 can print out a standard 80 character line. The modules 10 are independently tensioned against the paper 13 and the platen 18 thereby accommodating any nonuniformity in the shape of the platen 18 and any changes in component sizes due to thermal expansion or contraction. 
     Although only one embodiment of the invention has been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the scope of the invention.