Abstract: The edge of a web of material is inserted in a gap in a detector head between an ultrasonic blockage transmitter and an ultrasonic blockage receiver such that the magnitude of the pulse of sound from the transmitter that is received by the receiver is related to the portion of the web that blocks the beam of sound and thereby the position of the edge of the web. A compensation transmitter and compensation receiver are mounted proximate to the blockage transmitter and blockage receiver and transmit similar sound signals across the gap but are unoccluded by the web. The output of the compensation receiver is detected and used to normalize the values of the signal received from the blockage receiver to compensate for the effect of transient changes in ambient conditions.

Abstract: The edge of a web of material is inserted into a gap in a detector head between an ultrasonic blockage transmitter and an ultrasonic blockage receiver such that the magnitude of the pulse of sound from the transmitter that is received by the receiver is related to the portion of the web that blocks the beam of sound and thereby the position of the edge of the web. The ultrasonic frequency of each pulse is preferably very high to provide a narrow beam. The second half of the electronic drive pulse to the transmitter is preferably 180.degree. out of phase with the first half of the pulse to reduce excessive ringing of the transmitter. A compensation transmitter and compensation receiver, which are mounted proximate to the blockage transmitter and the blockage receiver, transmit similar sound signals across the gap but are unoccluded by the web.