Patent ID: 7669476

Claim:
A system for sensing paper on a paper roll, comprising: an ultrasonic transducer positioned to be in contact with an outer surface of the paper roll; a transmitter circuit in communication with the ultrasonic transducer, the transmitter circuit providing a transmit signal to the ultrasonic transducer, the ultrasonic transducer generating an ultrasonic signal in response to the transmit signal, the ultrasonic signal for transmission into the paper roll; a receiver circuit in communication with the ultrasonic transducer, the receiver circuit providing a received signal indicative of an ultrasonic return echo from the paper roll, the ultrasonic return echo resulting from the ultrasonic signal interacting with the paper roll; and a processor circuit in communication with the transmitter circuit and the receiver circuit, the processor circuit determining an amount of paper remaining on the paper roll in response to the transmit signal and the receive signal, the processor providing an output signal when the amount of paper remaining falls below a threshold; wherein the processor determines the length of paper remaining on the paper roll is determined by: L =( D o 2− D i 2 )π/4 T, where, L is a length of paper remaining on the paper roll, D o is an outer diameter of the paper roll, D i is an inner diameter of the paper roll, wherein the inner diameter of the paper roll 10 is equivalent to an outside diameter of the carrier, and T is the thickness of one layer of the paper on the paper roll.