Non-contact speed and length measurement

Continuous length measurement concerns materials produced without interruption: strip, wire, cable, tube, profiles, panels and fabrics. In these cases the length is not measured with a fixed rule, but is derived from the speed of the material, integrated over time. It is one of the most important quantities in process control, because it determines cut-to-length, meter counting and the invoicing of the product. Optical speed meters and encoders measure it without contact, with high accuracy.

This page belongs to the Dimension and geometry section of the Inspection area and examines the physical quantity “length” in the case of moving materials.

How continuous length is measured

The principle is simple: if we know the instantaneous speed of the material and integrate it over time, we obtain the distance traveled. The quality of the result therefore depends on the quality of the speed measurement. A measurement affected by slip, such as that of contacting counter wheels, accumulates error meter after meter; a non-contact optical measurement, by contrast, keeps the accuracy constant. As a result, over long lengths the difference between the two methods becomes very significant.

The VLM500 measures speed up to 50 m/s with an uncertainty below 0.025 % and can totalize lengths up to millions of meters without drift. The VLM60, a compact optical encoder, measures down to 0 m/s and is the direct replacement for measuring wheels.

Accuracy, resolution and repeatability

Three parameters matter in length measurement. Accuracy indicates how close the measured value is to the real one; resolution is the smallest detectable increment; repeatability is the ability to give the same result in successive measurements. Optical measurement excels in all three aspects: the absence of contact eliminates slip, the encoder output offers fine resolution, and stability over time ensures repeatability. For this reason it is also suited to applications with contractual value, where the declared length must match the real one.

Cut-to-length and meter counting

Continuous length measurement feeds two key functions directly. Cut-to-length requires knowing the running length precisely in order to command the shear at the exact instant; meter counting documents the production and the length of the rolls or of the pieces. In both cases a length error translates into scrap, into material given away or into disputes with the customer. On a rewinder, for example, the length determines the size of the finished roll.

Advantages of non-contact length measurement

• No accumulation of error due to slip
• Totalizing up to millions of meters without drift
• Accuracy up to < 0.025 % (VLM500)
• Measurement down to 0 m/s (VLM60), valid during stops and restarts
• Encoder output compatible with existing counters and PLCs

Where it is applied

Continuous length measurement cuts across sectors: from the steel industry to converting, from wood to cable production. For practical cases we refer to Applications and solutions; for the theory of the optical method to the Optics and LASER page. The quantity “speed”, closely linked to length, is examined in Speed and rotation in motion.

Slip error and its impact

The slip of a measuring wheel generates an error that grows with the length and varies with the speed and with the condition of the surface. Over a few tens of meters the effect may seem negligible, but over thousands of meters it becomes significant and unpredictable. Optical measurement, having no contact, eliminates this source of error at its root: every meter measured is independent of the previous ones, so the uncertainty does not accumulate. As a result the total length remains reliable even in the longest production runs.

Signal integration and totalizing

The length is obtained by totalizing the speed over time. Optical instruments perform this integration internally and make available both the speed value and the totalized length. Totalizing can reach millions of meters without drift, a typical requirement of continuous production lines. In addition, the ability to reset and manage the count through trigger inputs makes it easier to control cut-to-length and to manage batches.

Reliability and maintenance

Since there are no contacting parts, optical length measurement requires neither the periodic replacement of coverings nor frequent recalibration. This translates into less line downtime, reduced maintenance costs and consistent data quality over time. RODER technical support assists the customer in selecting the model, in defining the working distance and in commissioning, so as to obtain maximum accuracy in the specific application.

Length and process digitalization

Reliable and documented length data is today a prerequisite of industrial digitalization: it feeds MES systems, batch traceability and line performance analysis. Because the optical signal is stable and free of drift, integration with these systems is direct and requires no compensation. In this way length measurement becomes not only an inspection tool, but also a source of data for the continuous optimization of the process.

Selecting the model according to the application

The choice between VLM500 and VLM60 depends on speed, required accuracy and type of surface. The VLM500 is suited to high speeds and maximum precision; the VLM60 to the direct replacement of measuring wheels and to measurement down to 0 m/s. RODER technical support assists the customer in defining the working distance, the accessories and the commissioning, so as to ensure maximum performance in the specific application.

Frequently asked questions

Related products

The RODER instruments for this application. Go to the relevant catalogue pages for models, technical data and documentation.