- June 01, 1996, Mykytiuk, Andrew
There are four proven web-guiding systems available to the converter. The common denominator in each is that the web edge is monitored by a sensor that sends a signal, proportional to the amount of deviation detected by the sensor, to an actuator that positions the guide structure laterally, moving the web to the correct position in the sensor's field of detection.
The first type is a true pneumatic system that uses air for both sensing and web positioning. An air-flow sensor sends a low-pressure signal to the servo controller, which converts the signal to a higher actuator pressure. An all-air system uses no electricity, is tied into the plant's air supply, uses very low air pressures, and requires virtually no maintenance. It's not powerful, in terms of moving heavy loads, but, according to Doug Fife of Coast Controls, when fitted with an 8-in.-diameter actuator, it can move as much as 2,000 lb.
The second type is an air pressure hydraulic system, or pneumohydraulic. These use air pressure for sensing but, unlike the all-air system, use hydraulics for actuation and web positioning. Hydraulic web-positioning systems are strong enough to move heavy loads, such as shiftable unwind and rewind stands. They are rugged systems which, if regularly maintained, can last for decades.
Type three is called electrohydraulic and uses photo-electric sensing, instead of an air signal. The signal from the sensor is routed through an electrically operated servo valve, and the positioning of the web-guide structure is performed by hydraulics. According to Tom Herold, WTI, electromechanical web-guiding systems can accommodate 95% of all converting applications.
The final type, electromechanical, is the most sophisticated and versatile system, but it's also the most expensive. These systems typically use one of three types of sensors: photocell, ultrasonic, or infrared. There is no universal sensor; each has advantages and disadvantages, but because there is a choice, converters can select the best sensor for specific environments or converting applications. The signals are sent to an amplifier and then to an electrically operated actuator.
"With webs traveling in excess of 1,500 fpm, it's important that the web-handling device be sensitive and work quickly," says Ray Buisker, president of Accuweb. Electromechanical systems are undergoing a quantum technology shift from analog to digital.
The Future is Digital
"Digital systems with SIC or line array cameras are the hot ticket as far as high tech goes," says Matt Tielkemeier of Dri Tec. "A digital system can do all kinds of things: It can download data into a host computer for statistical process control; it can take a digital guiding system with a servo motor and do slaving and get position feedback, something that was never practical with analog systems; and, in case of trouble, you can modem in to your service people. You can't do that with analog or pneumohydraulic."
Each of the four systems accomplishes the same ends by different means. And although it is true that the high-end systems can do many things, you must ask yourself if you really need all of those features.
"Pneumatic systems will last a long time; just keep the filters clean and don't blow high pressure air up into the sensors," says Tielkemeier. "Year after year, they will outrun electronic systems, and they are less expensive to repair."
The key when choosing a web-guiding system is to consider your own application.
"The first thing I do is take a look at the application and ask the user or OEM what our end purpose is here and then try and tailor the system to fit that need," says Allen Rausch, application manager for BST Pro Mark.
The communication between the manufacturer and converter is more important than how many bells and whistles are on a system.