Method and apparatus for compensating for printer top-of-form and image stretch errors

In a printer having a platen roller driven to transport a print media and a print head disposed in proximity to said platen roller permitting transport of said print media therebetween, a method for controlling operation of the printer to compensate for errors associated with step size variations comprises:

• providing a series of clock pulses at a fixed rate;
• transporting said print media in step increments at a rate coinciding with each of a first number of said clock pulses, wherein said first number of said clock pulses defines a time interval between successive ones of said step increments;
• activating said print head to print lines of information onto said print media at a rate coinciding with each of a second number of said clock pulses, wherein said second number of said clock pulses defines a time interval between printing of successive ones of said lines of information;
• and
• updating said first and second numbers without interrupting printing operations in a manner that alters the ratio of step increments taken to lines of information printed so as to minimize image stretch errors.

An apparatus for a label printer for providing optimum registration between labels and printed information, said apparatus comprising:

• a clock adapted to provide a series of clock pulses at a fixed rate;
• a platen roller driven by at least one step motor to transport said labels in step increments in response to first interrupt signals corresponding to a first number of said clock pulses, said first number of said clock pulses defining timing between successive ones of said step increments;
• a print head disposed in proximity to said platen roller permitting transport of said labels therebetween, said print head being activated to print a line of information onto said labels in response second interrupt signals corresponding to a second number of said clock pulses, said second number of said clock pulses defining timing between successive activations of said print head;
• a sensor disposed a fixed distance from said print head, permitting transport of said labels thereunder, said sensor capable of sensing a leading edge and a trailing edge of each label that passes thereunder;
• and
• a processor and program memory storing instructions for controlling said processor, said instructions including:
• measuring an actual length of each label in terms of the number of steps taken by the step motor between the leading and trailing edge of each label;
• updating said first and second numbers in accordance with said actual length to maintain an optimum ratio of steps taken to lines of information printed to minimize image stretch errors;
• and
• calculating a number of steps of the step motor needed to move each label said fixed distance from said sensor to said print head in accordance with said actual length.

An apparatus for a label printer for correcting errors associated with step size variations, said apparatus comprising:

• means for providing a series of clock pulses at a fixed rate;
• means for transporting said labels in step increments at a rate coinciding with each of a first number of said clock pulses, said first number of said clock pulses defining a time interval between successive ones of said step increments;
• means for printing lines of information onto said labels at a rate coinciding with each of a second number of said clock pulses, said second number of said clock pulses defining a time interval between printing of successive ones of said lines of information;
• means for detecting a current step size;
• and
• means for maintaining an optimum step/line ratio in response to variations of said current step size to minimize image stretch errors by selection of said first and second numbers of said clock pulses.