Ultrasonic edge sensor for the detection of a web edge

An ultrasonic edge sensor for detecting an edge position of a traveling web, said edge sensor comprising:

• a support;
• at least two ultrasonic sensor units on said support, each of said ultrasonic sensor units being comprised of a transmitter emitting short ultrasonic wave pulses and a receiver responsive to said ultrasonic wave pulses spaced apart across a gap through which an edge of said traveling web can pass, all of said ultrasonic sensor units producing measurement fields across said gap which can be masked by said webs, said receivers having respective receiver outputs dependent upon a degree of masking of the respective measurement field by said web;
• switching means connected to said ultrasonic sensor units and responsive to a degree of penetration of said edge into said gap for switching on the transmitter and the receiver of only a selected one of said ultrasonic sensor units depending upon the degree of penetration of said edge into said gap in a width direction of said web, and for switching over between said sensor units;
• at least one signal converter for forming a digital receiver signal (W) from a respective receiver output of a switched on transmitter and receiver;
• a computer connected to said at least one signal converter for producing a sensor output signal (S) representing an edge position of the edge from the digital receiver signal (W) for use in controlling a position of said web, said measurement fields each extending in said width direction between imaginary boundary edge positions at which the receiver signal (W) is a maximum at one side of the respective field and at which the receiver signal (W) is a minimum at the opposite side of the measurement field, said measurement fields being arrayed in said direction without a gap between them;
• a forward/backward counter having a number of counter states equal to the number of said sensor units, said counter being connected to said switching means to shift from counter state to counter state and switch over from sensor unit to sensor unit successively as said counter states change and as said edge of the web changes position in said gap;
• a comparator circuit connected to said signal converter for comparing a receiver signal (W) for an actual position of said edge with the maximum (W.sub.max) and the minimum (W.sub.min) of the respective sensor unit and stepping said counter forward and back selectively upon said receiver signal (W) exceeding said maximum (W.sub.max) and said receiver signal (W) falling below said minimum (W.sub.min), thereby turning on the transmitter and receiver of a respective one of the sensor units having an adjacent measurement field;
• and
• a calibration data memory storing for receiver signals (W) of the respective switched-on transmitter and receiver and for respective sensor output signals (S) calibration values (W.sub.kn, S.sub.kn) associated with respective calibration edge positions (Z.sub.kn) distributed over the measurement fields of all of said ultrasonic sensor units, the computer being provided with means for calculating an actual edge position (Z) from the sensor output signal (S) for a receiver signal (W) of the switched on sensor unit from the stored calibration values (W.sub.kn, S.sub.kn) thereof.