Editor's Note: This is the third in a four-part series of articles on web guiding. Part 1 covered understanding web-guide accuracy and how to achieve it. Part 2 explained how to properly size web-guide actuators.

The sensor is the eye of the automatic web guide. As you stand and watch a moving web, sensing the position of the material doesn't seem like a difficult task, but think about the different challenges that a web sensor may face. Could you watch the web in a harsh environment? Could you watch both edges at once? Could you watch the web in that small, hard-to-get-to place? If you were the right web guide sensor, you could.

Finding the right sensor is a combination of knowing what you need and what a web guide sensor can do.

Web Material Properties: What is your web? To the sensor, it's not the material, but the material properties that are important. Does your web reflect or block light? Does your web block ultrasonic energy in the air? Does the density of your web restrict airflow through it?

Edge, Center or Line Guiding: Do you want to detect your web's edge, center or a feature on your web's surface?

Edge or Line Quality: What is the quality of your edges or the surface feature you want to detect? For edges, are they sharp or ragged? For surface features, are they visually distinct? Continuous or intermittent? Do you have step changes in the edge position at splices, especially if you have width changes?

Physical Constraints: What physical constraints do you have? Does the sensor have to fit into a tight location? Does the web edge flutter? How much does the web plane change, especially considering an offset-pivot or steering-guide exit span twisting?

Environment: What is the environment at the detection point? Is the web in a hot, hazardous, low-pressure or dusty environment?

Range and Resolution: What is the range of lateral position you need to detect? What is your edge position variation from lateral motion and width variation? Do you want your web guide to automatically adjust for width changes or have your operators manually reposition the sensor? How fine of resolution do you need?

Did you count the number of question marks in this discussion? Taking the time to answer as many of these questions as possible will help in selecting which of the many standard or customized web-sensor designs will be best for your application.

Choosing the right sensor is more than just selecting between pneumatic, optical or ultrasonic models. Finding the right sensor for your application should consider the following half-dozen options:

1. Consider the linearity of the sensor's proportional feedback.
2. Decide if a center-guiding option would reduce downtime and waste in your process.
3. Understand the benefits of analog vs. digital sensors.
4. Review a sensor's sensitivity to ambient signals, time, temperature, contamination and web motion.
5. Determine your geometric constraints for both lateral and out-of-plane motion.
6. Survey less conventional or customized sensor options

For examples of different sensor configurations for web-guiding systems, see Figure 1.

Choosing a web guide sensor is more than just "pneumatic, optical, or ultrasonic" or "edge vs. center." For any new product or process challenge, an experienced web guide supplier will help you answer these questions and show which sensor provides the best solution. No matter the application, there is a sensor waiting to be your eyes in the web-guiding process.