Apparatus and method for ultrasonic control of web

Abstract

A turning guide for a running web and which guide has a curved guide surface with air slots therein and pressurized air supplied from a blower is delivered through a plenum to the guide to form a cushion of pressurized air which floatingly supports the web. An adjustable damper operated by a servomotor regulates air flow and controls the clearance spacing between the curved guide surface and the web. Control apparatus for the servomotor comprises an electronic control unit, including a memory and a central processing unit (CPU), to which are connected an ultrasonic signal generator, an ultrasonic signal receiver, temperature sensing devices located with the ultrasonic signal sensors, a sensor responsive to blower operation, and a sensor responsive to web presence. In operation, after a desired web clearance set-point is entered into the memory and blower operation and web presence are established, an ultrasonic signal from the signal generator is reflected off of the web to the signal receiver. The time interval between initiation and reception of the ultrasonic signal, compensated for ambient temperature, is compared to the stored web clearance set-point value and, if necessary, a control signal from the electronic control unit operates the damper servomotor to change the pressure of the air cushion and thus move the web relative to the running guide surface to establish and maintain the selected web clearance spacing.