1. Field of the Invention
This invention relates to an edge sensing apparatus, and more particularly, to an apparatus for sensing the edge of a glass ribbon as it advances on a molten metal bath through a forming chamber.
2. Discussion of the Technical Problems and Presently Available Edge Sensing Apparatus
In the manufacture of flat glass, batch materials fed into the upstream end of the melter or furnace melt into raw molten glass as they advance downstream through the melter and thereafter the raw molten glass is refined. The refined molten glass exits at the downstream end of the furnace between a tweel and a glass supporting member e.g., a lip or threshold onto a pool or bath of molten metal. The tweel is mounted for movement toward and away from the glass supporting member to meter or control the flow of molten glass onto the pool of molten metal. The refined molten glass as it advances downstream on the molten metal pool is controllably cooled to form a glass ribbon which is lifted from the pool by lift out rolls and moved through an annealing lehr.
When supported on the molten metal pool, the molten glass naturally stabilizes at a thickness of about 0.271 inch (0.69 millimeter). This thickness is called equilibrium thickness and in the absence of a controlled application of forces to the advancing molten glass during forming, the subsequently formed glass ribbon has this thickness. For example, a glass ribbon less than equilibrium thickness can be obtained by pulling the molten glass or heated glass ribbon by a force downstream of the delivery channel or near the lehr end (often called lehr force) which stretches or attenuates the glass ribbon and causes it to move faster than when a glass ribbon of equilibrium thickness is produced. The glass ribbon not only becomes thinner, it also becomes narrower unless the edges of the ribbon are restrained, e.g., by edge roll machines or attenuating apparatus such as the type taught in U.S. Pat. Nos. 3,709,673 and 3,998,616.
In general, the attenuating apparatus includes an attenuating wheel that engages the upper surface of the body of hot glass at an angle. The angle subtended by the axis of the attenuating wheel and the glass path defines an angle of attenuation. A pair of opposed attenuating apparatus have their respective attenuating wheels applying longitudinal and transverse forces at the ribbon edge to maintain the ribbon within a desired width. During the attenuation of the ribbon, it is important to engage the ribbon a sufficient distance from the ribbon edge to have positive traction while minimizing the distance between the wheel and adjacent ribbon edge to reduce ribbon edge loss. The ribbon portion between the edge and point of attenuating wheel engagement is usually referred to as nip width. Techniques employed for maintaining an acceptable nip width include mounting a ribbon edge detector or nip sensor on an extending arm of the attenuating apparatus adjacent the wheel.
U.S. Pat. No. 3,500,548 teaches that the edge of a glass ribbon is detected by suspending a plurality of spaced electrodes at each opposite side of the ribbon. The electrodes conduct current when in contact with the molten metal bath. When the ribbon width decreases, one of the spaced electrodes not supported on the glass contacts the molten metal bath. The resultant electric current of each one of opposed edge sensors is monitored to determine the ribbon width. A limitation of the above technique is that the electrodes are spaced from one another and therefore the actual ribbon width is difficult to determine when the ribbon edge is between spaced electrodes. In U.S. Pat. No. 3,482,954 the ribbon edge is sensed by projecting an annular gas stream toward the ribbon edge and sensing the back pressure. A limitation of this technique is that air is directed onto the molten metal pool and may result in turbulence that may detrimentally effect the optical quality of the subsequently formed glass.
In U.S. Pat. No. 3,805,072 a glass ribbon edge detector includes scanning telescopes each having a pair of cells responsive to infrared energy. The object ends of the telescopes are mounted above the ribbon edge in a home position as determined by sensed infrared energy. When the ribbon shifts in a direction generally transverse to its direction of motion, the output of the cells varies and the telescopes move in response thereto until the telescopes are each in the home position. U.S. Pat. No. 3,998,616 teaches the use of a plurality of photodetectors which are mounted over the edge of the ribbon and the molten metal bath. Changes in output of the photodetectors indicate the position of the glass ribbon within the chamber. In U.S. Pat. No. 4,008,062 the radiation receiving end of a radiation pyrometer is inserted transversely of and above a glass ribbon supported on a molten metal bath. The radiation receiving edge reciprocally moves until a rapid temperature increase is sensed by the pyrometer to indicate the edge of the ribbon. In U.S. Pat. No. 3,977,858 two optical detectors move in a direction perpendicular to that of a glass ribbon advancing on a roller conveyor. The detector at each side of the ribbon stops upon detecting its respective ribbon edge. The distance between the detectors which corresponds to the width of the ribbon is indicated by an electrical signal. A limitation of the above type ribbon edge detectors is that the sensing elements are usually mounted in a water cooled housing to prevent thermal damage to the sensing elements in the housing. Mounting a water cooled housing in a heated glass forming chamber necessitates specially designed equipment. Further, condensates may form on water cooled surfaces within the forming chamber and excess condensates may drop onto the ribbon surface causing surface defects. Still further, initial insertion of the water cooled members may upset the thermal equilibrium in the forming chamber.
In view of the above, it would be advantageous to provide a ribbon edge detector that does not have the limitations of the presently available edge detectors.