Paper size detection using ultrasound

A detection system for detecting the size of a print medium, comprising:

• a hollow tube having a first end and a second end;
• an ultrasonic drive signal generator for generating pulses of electrical drive signals in an ultrasonic frequency range;
• ultrasonic transducer apparatus responsive to said electrical drive pulses for launching pulses of ultrasonic energy into the tube and responsive to ultrasonic echoes of the pulses to convert echo pulse energy into electrical detected signals;
• flag positioning apparatus for selectively positioning a blocking flag within the tube at a location spaced from the transducer indicative of a print medium size;
• a controller coupled to the ultrasonic transducer apparatus and responsive to the electrical detected signals to determine a print medium size in dependence on a time interval between launching an ultrasonic pulse into the tube and the arrival of the echo energy at the transducer apparatus.

An inkjet printer, comprising:

• a housing structure;
• an input tray for holding a supply of input print media in sheet form;
• an inkjet printing apparatus positioned at a print zone for ejecting droplets of ink onto a print medium;
• apparatus for advancing a sheet of print media from the stack to the print zone;
• and
• a detection system for detecting the size of a print medium in the stack, comprising:
• apparatus defining a elongated chamber adjacent the input tray, the chamber having a longitudinal extent;
• a drive signal generator for generating pulses of electrical drive signals;
• transducer apparatus responsive to said electrical drive pulses for launching pulses of energy into the elongated chamber and responsive to echoes of the pulses to convert echo pulse energy into electrical detected signals;
• flag positioning apparatus for selectively positioning a blocking flag within the chamber at a location spaced from the transducer indicative of a print medium size;
• a controller coupled to the transducer apparatus and responsive to the electrical detected signals to determine a print medium size in dependence thereon.

A method for detecting the size of a print medium, comprising the steps of:

• positioning an apparatus defining a elongated chamber having a longitudinal extent adjacent the print medium;
• positioning a blocking flag within the chamber at a location corresponding to a print medium size;
• launching one or more pulses of energy into the chamber;
• detecting the position of the blocking flag in the chamber by determining the time between the launching of the one or more pulses and the receipt of corresponding echoes of said one or more pulses at a transducer apparatus;
• and
• determining the print medium size in dependence on the detected position of said blocking flag.