Sensing amount of medium and medium roll malfunction in a printer

Abstract

An optical detector senses the teeth on a wheel rotating with an unwinding roll of medium in a printer and provides a signal which is used to count the sensed teeth to measure the rotation of the roll. When a predetermined proportion of a rotation is completed, the measured length of medium movement along a path through the printer during the predetermined proportion of a rotation is used to sense the condition of the medium. The conditions which may be sensed include amount of medium, medium thickness and medium malfunction. The circumference may, for example, be calculated and compared with a low medium circumference for the preselected or calculated thickness of the medium to detect a low medium condition. Periodically, the present medium position may be compared with its position when the last tooth was detected to sense a medium malfunction, either a medium out condition or a medium break. The teeth are triangular and shaped to provide a slowly varying signal from the optical detector which is converted to a purely digital signal by a high hysteresis Schmitt Trigger device. The processor which receives the digital signal also receives a signal indicating the length of medium movement.