In a web aligning apparatus for multi-level web material such as carpeting, tufted fabrics and the like having a backing web and pile on the backing web spaced inwardly from the edge of the backing web, it is frequently desirable to maintain the edge of the pile in a preselected relation to an apparatus which operates on the web, independent of the lateral position of the edge of the backing web. This necessitates sensing the line of juncture between the backing web and the pile and this poses some problem if the backing web is such that it is not possible to pass either an air stream or a light beam through the backing web to sense the edge of the pile.
The U.S. Pat. No. 3,244,418 to Henderson discloses an electrical pile edge sensing apparatus which utilizes a plurality of contact fingers, some of which normally rest on the pile and others of which normally rest on the backing adjacent the pile, and which contact fingers operate switches to control positioning of the pile edge in accordance with the relative positions of the contact fingers. Such pile edge sensing apparatus, however, can only effect a step-type control and the pile edge can shift laterally a significant distance before one of the contact fingers either rides onto or off of the pile to change the electrical signal.
U.S. Pat. No. 3,935,979 owned by the assignee of the present application discloses a web aligning apparatus for use with multi-level web material in which the sensing apparatus included a gap-type pneumatic sensor, a pile engaging member mounted for movement along a first path crosswise of the course of travel of the web at the sensing location and generally parallel to the plane of the web to sense lateral deviations of the course of travel of the pile edge and also along a second path generally perpendicular to the plane of the web at the sensing location to sense when the pile engaging member rides onto the pile, and a vane connected to the pile engaging member for movement therewith in the gap to vary the signal produced by the sensor in response to movement of the web engaging member along either the first or the second paths. Thus, in this prior apparatus, the same sensor sensed both motion of the pile engaging member in a direction paralleling the plane of the web and laterally of the pile edge and motion of the pile engaging member perpendicular to the plane of the web, and adjustment of the device to enable a single sensor to properly sense both motions of the pile engaging member, was somewhat critical. Moreover, while the prior pile edge sensing apparatus worked well in applications where the web was traveling generally horizontally with the pile upright at the sensing location, it was difficult to adapt it for use in applications where the web was not traveling generally horizontally, for example where the web was traveling vertically or where the pile face of the web was inverted at the sensing location. Further, since the pile engaging member was supported for movement along two relatively perpendicular paths that were both perpendicular to the path of travel of the web, the pile engaging member could not readily absorb impact forces acting in a direction paralleling the course of travel of the web at the sensing location, such as would occur when a seam or other discontinuity in the web struck the web engaging member.