Web guides have evolved dramatically since their first appearance in the 1930s. The earliest units used pneumatic edge detectors and guiding devices powered by hydraulics to keep the web “in the process.” Electronic edge detectors and electronic motors gradually displaced pneumatics and hydraulics delivering greater accuracy and reliability, eliminating leaky hydraulics. Still, the earliest electronic systems were far from perfect; they were sensitive to dust and temperature swings.
In 1988, web guide technology took a big leap forward when AccuWeb Inc. patented a new edge detector technology — the compensated ultrasonic edge detector.
Most web guides use optical or ultrasonic through-beam edge detectors to determine the lateral position of the web. These detectors have a transmitter and a receiver located inches apart and oriented to direct their energy from one to the other. The detector is mounted so the beam intercepts the edge of the web. As the web moves laterally, it blocks the beam to a greater or lesser extent and this, in turn, varies the amount of energy arriving at the receiver. The received energy level is roughly proportional to web position, and is used to determine which way to move the guide.
During operation, the web guide system monitors the received energy level and tries to maintain it as a target level by moving the web laterally. Moving the web into the beam causes the received energy level to drop, moving it out causes it to rise. The target level energy is typically preset to one half of the level measured when the beam is totally unblocked. This target level will cause the guide to move the web until it covers about half of the beam, aligning the edge of the web with the center of the beam.
Early electronic edge detectors could be influenced by a variety of environmental and process factors that degrade accuracy, including temperature, humidity, air turbulence, vapors and gases, dust, ink, coating overspray, web flutter, curl and web path changes.
Dust or other contaminants accumulating on the optical detector’s lenses will absorb some of the bean’s energy and prevent it from reaching the receiver, resulting in incorrect redirection of the web. In a similar manner, the early ultrasonic edge detectors were affected by changes in ambient air temperatures given cool air conducts ultrasonic energy (sound) better than warm air. As ambient temperatures rose or fell, ultrasonic energy levels to the receiver would vary, resulting in the web being moved in or out of the beam.
In addition, web flutter still caused reflected ultrasonic energy. Direct energy by itself provides an excellent indication of web position, but when mixed with reflected energy bouncing off the web and nearby surfaces, position readings become erratic. Subtle changes in the flatness, position or orientation of the web can greatly affect the amount of reflected energy arriving at the receiver, giving it a random, unpredictable quality.
Compensating for Changes
The solution to the ultrasonic detector’s inherent issues came with another new technology patented again by AccuWeb — the fully compensated ultrasonic edge detector.
Now two ultrasonic beams would be used — a beam that senses the edge of the web and a beam that monitors ambient conditions. The two beams are located less than an inch apart, but oriented so the web passes only through the sensing beam.
Because the web never blocks the reference beam, it provides a direct, real-time indication of how environmental and other changes are affecting the performance of the detector. This information is combined with sensing beam measurements to produce the truest indicator of the web’s position.
By transmitting very short ultrasonic pulses every few milliseconds, the edge detectors can also filter out unwanted reflected energy. After each pulse is transmitted, the receiver will typically see a train of pulses arrive. These are actually the same pulse as it makes several trips back and forth across the gap from transmitter to receiver, followed by several smaller pulses that took a longer path. The detector looks for the first (and largest) pulse and discards the rest, making the detector insensitive to curl, flutter and other vertical motion of the web.
Ultrasonic Increases Sensing Area
The next evolution in compensated ultrasonic edge detectors utilized an array of two overlapping sensor beams that form one large sensing area. Any number of beams can be combined in one detector to provide a sensing area of any required size.
As the web moves, pairs of beams are sequentially activated to chase the edge of the web. The ultrasonic array detectors are ideal for applications handling a variety of web widths or requiring large adjustments of web positions.
While fully compensated ultrasonic array edge detectors are ideal for most applications, infrared detectors may be better suited to extremely lightweight nonwovens or mesh materials. These materials are so porous that they don’t fully block ultrasonic energy beam.
Compensated ultrasonic and infrared array edge detectors provide accurate, stable and reliable operation in a wide range of demanding converting applications, including printing, coating, slitting and rewinding. Ultrasonic and infrared edge detection technologies continue to team up to handle web guiding challenges.
These advances and the continued enhancement of web guiding control systems and programs makes web guiding almost invisible. It requires little initial set-up, no routine maintenance or adjustment, and often no operator intervention during changeover.