Webs do not always track down through a machine in a consistent location. On a roll-to-roll operation, for example, any offsets in the unwinding roll will cause the web to get started through a machine in a variable CD (cross direction) position. However, even if a web starts out in the position, it may take slightly different paths through the machine depending on several factors.
First, a variably baggy or cambered web will cause the web to track toward the variably tighter side. Second, variabilities in roller traction or drag can cause a steering of the web. Third, changes in tension will cause changes in how straight the web travels downstream. At zero tension, edge position control is essentially lost. However, there are other edge movement factors which include nip roller draw variations, aerodynamics and so on.
Web Guiding Systems are used to bring the edge or center of a web to a specific CD position. The guide location may be at an unwind to get the web started down through a machine in a consistent position, at an intermediate location, or at the winder to improve roll edge quality. The accuracy demands for the guide may vary from merely keeping the web on the rollers, to minimizing trim loss, to registering several print colors to within a couple of mils. Guides may be either active or passive.
Active guides systems are composed of a sensor, an actuator, and a controller. The sensor can be any detector which can reliably pick up the edges of a web. The most common are pneumatic (nonporous webs), photoelectric (opaque webs), or paddies (thick webs). The web must be flat (free of curl) and stable (free of flutter) through the edge sensor. For this and other reasons, the sensor is often placed near a roller. If two sensors are used, the web could be guided to the front edge, back edge, or center.
The output of the sensor goes to a controller which outputs a movement command to the actuator. If the gain of the controller is too low, the response of the guide will be sluggish and slow to correct. If the gain of the controller is too high, the guide will be hot but overshoot, or may even be unstable. The actuator which moves the guide mechanism may be a stepper motor and ball screw for smaller assemblies or a hydraulic cylinder for larger assemblies. The actuator and framework must be stiff for responsive operation.
In Part 2 of the Web Guiding Series we discuss Web Steering Guides.
Written for Faustel by David R. Roisum, Ph.D.