Speed and rotation in motion

The speed of a moving material is a central physical quantity in process control: drive synchronisation, cutting to length, metre counting and line regulation all depend on it. In this Inspection section we examine the quantity “speed” in its various forms — surface, linear, multidimensional — and the methods for measuring it with non-contact optical speed meters and encoders.

Unlike the Technologies (theory) and Applications (practical cases) sections, here we concentrate on the measured quantity: how it is defined, in which units it is expressed and which instruments determine it accurately.

Speed as a process quantity

Speed expresses the distance covered in unit time and, in the case of materials in continuous production, it is typically measured in metres per second (m/s) or metres per minute (m/min). Knowing the real speed of the material is indispensable for many reasons: it makes it possible to derive the length by integration, to synchronise rollers and drives and to keep the process stable. Measuring it accurately, however, is not trivial, above all when the material is hot, shiny or moving at high speed.

Optical speed meters solve the problem by measuring speed without contact: the VLM500 using the spatial filter principle (up to 50 m/s, uncertainty < 0.025 %), the VLM60 using the correlation method (down to 0 m/s, bidirectional too).

The different forms of speed

The speed of a material can be described in different ways depending on the application. Surface and linear speed concerns the running of the material along the line; material speed and length links speed to the length produced; multidimensional speed takes into account movement in several directions (x and y) with direction recognition. Each of these forms is examined in a dedicated page.

Why measure speed optically

• Non-contact measurement: no wear and no slippage
• Accuracy down to < 0.025 % and a wide dynamic range
• Measurement down to 0 m/s and direction recognition (VLM60)
• Valid on hot, shiny, thin or delicate materials
• Encoder, analog and fieldbus outputs for integration

From the quantity to the solution

For the instruments, the optical speed meters and encoders page brings together the ASTech range; for real cases, Applications and solutions; for the theory, Optics and LASER and Doppler and optical correlation. The closely related quantity “length” is dealt with in Continuous length.

Rotation speed and linear speed

Two families of speed are encountered in industrial processes: rotation speed, expressed in revolutions per minute, and linear or surface speed, expressed in metres per minute. The two are linked by the diameter of the rotating member, but they are not interchangeable: measuring the revolutions of a roller is not the same as knowing the real speed of the material, because there can be slippage between roller and material. For this reason, when the actual speed of the product is what matters, direct optical measurement on the surface is the most reliable choice.

Why the real speed matters

Knowing the real speed of the material, and not the one estimated from a roller, is decisive for process quality. The calculated length, the synchronisation between drives and the stability of the settings all depend on it. An error in speed propagates to all these functions. Non-contact optical measurement supplies the actual speed of the product, eliminating the systematic errors typical of indirect measurements and improving line control as a whole.

Dynamics and response time

Besides accuracy, responsiveness also counts in speed measurement. A short update time makes it possible to follow accelerations, decelerations and rapid transients while keeping control stable. Optical speed meters update the measurement in fractions of a millisecond and offer configurable filters to balance responsiveness against signal stability. They are therefore suited both to slow lines with frequent stops and to fast lines running continuously.

Speed and product quality

An accurate speed measurement does not only improve the calculated length: it has a direct effect on product quality. In printing lines, for example, a stable speed favours correct registration; in drawing lines it helps to prevent excessive tension and breakage; in hot processes it makes it possible to calculate elongation. In all these cases, having the real speed value of the material is the prerequisite for effective process control and for a compliant product.

Robustness in an industrial environment

The speed measurement must remain reliable in real conditions: dust, vibration, heat and surface variations. With IP 65 protection, white LED illumination and, where necessary, protective housings, optical speed meters offer high availability even in continuous operation. The absence of moving parts reduces maintenance to a minimum, keeping accuracy constant over time.

One value shared with several systems

The speed value can be distributed simultaneously to several recipients: fast control loops through the encoder output, supervisory systems through the fieldbus and display devices through the analog output. In this way a single instrument feeds the entire control chain, simplifying the architecture of the line and reducing the number of sensors required.

Frequently asked questions