Editor's Note: This is the first in a four-part series of articles on web guiding. A system that responds quickly to web alignment issues will ensure that waste is minimized. This article will show the true cost of web-guiding accuracy and provide a benchmark with which converters can judge their own guiding systems and processes.

As a supplier of automatic web guides, one of the most common questions we encounter is, "How accurate is your web guide?" If you ever get a quick answer such as "typically +/- X mils," you should be suspicious. Why? As it is posed, the question does not include enough information to be properly answered.

A web guide's accuracy is a function of three things: the web, the guide and the guide installation. Asking about accuracy based solely on the web-guide design is like asking how quickly a car will stop solely on car and tire design. Without knowing how fast the car is going and what type of road it's on, the question cannot be answered.

The web's behavior, how far it is off center and how fast it moves laterally are critical to determining the accuracy of the web position at the web sensor.

It's easy to understand that a web guide has an offset limit. A guide can't correct a web that is outside its actuation range. How the web's lateral velocity affects web guide accuracy is a little more complicated. Any control system must also be faster than the parameter it's chasing. To keep the following error low, a web guide must be able to shift faster than the web's motion.

The web guiding control loop starts when the sensor detects the web error and outputs a signal to the controller. The controller then outputs the correction signal to the actuator/guide. The actuator/ guide provides the correction velocity to the web.

The most responsive web-guide systems will have high gain, allowing the fastest correction velocity in response to the web position error. The overall system gain is a function of the individual components in the control system.

The sensor gain (K1) is the proportional output signal in current or voltage per inch of offset. The controller's gain (K2) is adjusted to tune the closed-loop control. The actuator gain (K3) is the actuation rate in in./second in response to input voltage. The overall open-loop system gain will be: KS(System) = K1 × K2 × K3.

The accuracy (or following error at the sensor) can be calculated by dividing the error velocity by the system gain. For example, if the system gain is 20 inverse seconds (20/seconds) and the lateral error rate is 0.5 in./second, the accuracy will be 0.025 in. If the system gain is increased to 40 inverse seconds, the accuracy will be 0.0125 in. Figure 1 (below left) illustrates accuracy versus error velocity for a high-gain system with no backlash.

Gain is one element of a highly accurate web guide. Other considerations include system backlash, sensor deadband, actuator thrust, and system bandwidth.

"How accurate is your web guide?" As you can see, the answer lies in knowing a multitude of guide and web factors. You must know the web's input error magnitude, lateral velocity, and frequency. You should also know that your guide has high gain, low backlash, small sensor deadband, and that actuator thrust is sufficient.

When thinking about buying a web guide and the accuracy needed, consider both the web's behavior and the guide's performance. If more accuracy allows the converting of a more competitive product or reduces waste, then buying a web guide with higher gain, better response time, and less backlash will be a good investment.