In many web lines, it is not enough to know the thickness alone. If the web also wanders sideways, or if the actual web width needs to be kept within tight tolerances, real-time position and width control becomes a direct quality issue. This is where the combination of contact-based thickness measurement and simultaneous width or side position measurement becomes particularly valuable.
Vollmer's contact measurement sensors work with two diamond-tipped sensors that simultaneously touch the upper and lower sides of the strip. The sum of the distances gives the actual, absolute strip thickness. For applications where strip width and side position must also be checked, an additional sensor pair is used to measure the side position in parallel.
Why is thickness alone not enough?
The thickness may be stable, but you will still encounter quality issues if the tape is not positioned correctly.
Common consequences when the belt wanders sideways:
- Edge defects, grade problems, or edge bending that occur when the strip does not pass centrally through subsequent steps.
- Unstable trimming or wear, where knives, scissors, or laser trimmers end up in the wrong position relative to the edge of the belt.
- Problems with coating, lamination, or surface treatment, where even coverage and the correct edge zone require stable tape positioning.
- Flushing and rolling up, resulting in telescoping and poorer roll quality.
When measuring thickness without simultaneously monitoring the side position, you risk misinterpreting the stability of the process. You may see “good” thickness but miss that the belt is drifting, which in practice moves the process window.
What does simultaneous width, or side position measurement, mean?
In Vollmer's contact measurement setup, an additional contact pair can read the side position of the strip parallel to its thickness. This means that you get two types of information at the same time:
- Absolute thickness from the upper and lower pairs of cards.
- Lateral position, and in practice also width control, from the extra pair of cards that follow the edge position of the band.
The point is not just to “log breadth,” but to create stable, rapid feedback on where the band actually is in the transverse direction at any given moment, following the edge position of the band.
When does it make the biggest difference?
Here are some typical situations where simultaneous width and side position control have a clear effect.
1) For grinding, trimming, and edge-critical steps
If the belt's side position varies, you will get variation in the edge zones, especially when tolerances are tight. Real-time data on side position allows you to see causes more quickly and make corrections before it becomes scrap.
2) When you want to reduce changeover time and achieve more stable operation
Many lines lose time at the start of runs when the belt “settles” and the control system chases. With lateral position measurement in parallel with thickness, the operator and control system get a better picture of the situation immediately, which often results in fewer manual interventions and faster stabilization.
3) At high bandwidth and dynamic processes
The higher the speed, the more costly minor instabilities become. This makes it particularly valuable to obtain thickness and side position data simultaneously and continuously, rather than troubleshooting after the fact.
4) When you need to secure “real bandwidth”
If the end product is sold based on width and edge quality, not just thickness, width control becomes part of your quality chain. Vollmer also describes how contact measurement can be combined with tactile width measurement, especially for narrow strips.
Why an extra pair of contacts may be worth the investment
Adding side position measurement in parallel can provide three clear benefits:
- Fewer quality deviations related to edges
You detect belt wandering immediately and can take action before defects spread throughout the roll. - Better process understanding and easier troubleshooting
Many “mysterious” problems turn out to be related to side position. When you see the position at the same time as the thickness, the causal relationships become clearer. - More stable control and better basis for automation
Real-time lateral position is a data point that often makes control more robust, especially when you want to reduce operator dependency.
Practical checklist: when should you consider simultaneous width and side position measurement?
You have a strong case if at least two of the following apply:
- You drive on narrow roads or have critical edge zones.
- You have grinding, trimming, coating, or other edge-sensitive processes in your line.
- You see recurring edge errors with no apparent cause.
- You have problems with belt drift during start-up, acceleration, or load changes.
- You want to shorten changeovers and achieve more stable operation in real time.
Conclusion and next steps
Contact-based thickness measurement provides absolute thickness with two sensors on the top and bottom sides. When you add an extra pair of sensors for side position, you also gain control over width and lateral position in real time. For many slitting lines, it is precisely this combination that takes quality and stability from “good enough” to consistently predictable.