Using Laser Line Sensors to Measure Precise Thickness

Strip thickness is an important parameter in the production of aluminum strips as it is significant for further processing. In addition to strip thickness, strip width also plays a major role, especially when the material is slit into individual rings.

It costs money and time if there is production deviations. Highly accurate production monitoring is absolutely essential in the production process, and it can be reliably ensured with the use of Micro-Epsilon’s laser line sensors.

Keen competition exists in the aluminum production and processing market, which is a hard-fought one. In order to be competitive, manufacturers face a lot of challenges. The increasingly strict demands on the production process include optimization of the use of raw materials, and compliance with the various standards with various limits.

Measuring instruments are the only rational and economical solution to comply with regulations, conditions, standards and parameters, and to ensure precision measurements. In the production process, measuring instruments function as reliable, consistent, and accurate control units.

Deviations from the specified dimensions during hot-rolling and cold-rolling processes usually occur at the beginning of the production chain. Fluctuations and deviations from nominal values for thickness and/or width result in non-acceptable material costs as well as quality deteriorations, which increases the difficulties in downstream processing of production goods, finally resulting in complaints and heavy financial losses.

Comparison of Measurement Methods

Three different principles dominate the sector of metal thickness measurement. The first are contact methods, where two measurement heads are preferably used, one above and one below the object.

Such devices often wear out more rapidly and create problems during production because of the contact during measurements. In addition, only approximate information can be achieved on thickness variation, since measurements are only made at individual points.

Radiometric methods work with isotope radiation or an X-ray source, which however is damped by the sheet itself. A transmitter is used to emit and recieve radiation, and the difference between the emitted and received radiation is used to determine the mean thickness.

Improvements in the method’s reliability depend on the alloy and the condition of the material. Additionally, there are also costs for radiation protection and regular safety testing, which entails regular expenses.

Optical methods based on laser triangulation are advantageous when compared with other methods, since measurements are made without contact, and hence, without wear. Additionally, an exact geometrical measurement in relation to the strip surface can be carried out, irrespective of the condition of the material.

Micro-Epsilon’s latest thickness measuring units use laser line triangulation sensors (profile sensors) that provide further advantages. Large vertical movements often take place during the processing of cold strip, as seen in longitudinal slitting machines due to the forces exerted by the blades on the strip.

Such constraints are overcome by laser point sensors. The higher information density produced by a profile sensor highlights its benefits. The laser spot is extended to a line in the profile sensor, and the measurement is obtained from a “best-fit line” through the cloud of points produced by the sensor.

As the variation of the said line is computed from the interplay of several partial resolutions, the distance-to-resolution relation is significantly better than that obtained with the point sensor. Consequently, more measurement values are available over a larger area, which when averaged provide better precision.

The resolution of the line sensor at a larger measurement distance is better than that of the point sensor due to the best-fit line feature. Due to these measures, a working gap of 190 mm with a measuring range of 40 mm and a precision of ±5 µm are achieved in line-scanners, compared to about ±25 µm achieved in the same range with point sensors.

Solution: C- and O-Frame Systems

A constant sensor distance is critical for differential thickness measurement with distance sensors. Usually, two different design types are used, which are referred to as C-frames or O-frames due to their shapes.

The sensors are fixed on an upper and a lower arm in the C-frame, and the frame moves as a unit to reach the measurement position. By increasing material width, the oscillation susceptibility of the upper boom increases, which  makes C-frames best suited for applications involving narrow strips.

When calibrating the C-frame during coil exchange, a master-component automatically moves into the measuring gap, and balances the system for new measurements.

The advantage of the C-frame is that during threading-in or in hazardous situations caused by the so-called ski effects (the strip curves upward on one side) or alligator effects (the strip curves downward and upward) at the start of the strip, the C-frame can be completely removed from the line. However, this requires space, which in a service center is often not available.

C-frames are particularly suitable for narrower strips. They can be moved completely clear of the production line, but for that they need more space than an O-frame.

This is where an O-frame is a better option due to its compact structure. As the constant measuring gap is a decisive criterion for the precision of such a unit, the O-frame has major advantages. This version is designed based on a stable frame, which is integrated in the production line.

Strip widths of up to 4,000 mm can be inspected for thickness, width, and profile, due to the rigid frame. An auto-calibration unit is also a part of this version. The sensor system continually passes across the metal strip during measurement, and collects profile data over the entire width of the strip material.

Measurements made using an O-frame are extremely precise and, thanks to their compact structure, can also be made when space is limited.

Constant Measuring Gap

A constant measuring gap in combination with the O-frame, is a fundamental prerequisite for precise results. It is possible to achieve effective monitoring of the measuring gap with an additional displacement-measuring sensor technology or by iterative calibration at process-uncritical times.

The geometry of the machine frame, and hence, the measuring gap is affected by changes in temperature. However, as a rule, enough time is available to take the necessary action without compromising the manufacturing process, because the temperature-related changes occur slowly.

Micro-Epsilon’s patented concept of the “compensation frame” provides help in this context. An extra temperature-invariant frame is integrated into the system for this purpose, running parallel to the upper and lower booms, and the support of each measurement sensor is extended with the so-called compensation sensors.

The distance of the support from the compensation frame is determined by these sensors, and the variation of the measuring gap is completely converted to the distances of the compensation sensors from the compensation frame. The change can be eliminated, and the measuring gap is kept constant at an uncritical level.

Thickness Measurement with Integrated Width Measurement in Longitudinal Slitting Shears

In addition to thickness measurement, the edges of the material can also be accurately measured due to the high lateral resolution of the linear sensors. It is possible to determine the transverse profile for every individual ring in longitudinal slitting shears.

However, this can be difficult for methods with a large measurement spot, as the lateral resolution of the method is often insufficient for this measurement when the slit strips are narrow. The yield obtained from a coil can be increased for rings that are cut from strips and are very close to the minimum tolerance limits, using a thickness measurement unit based on profile scanners.

A ring produced can still be within tolerance while those adjacent to it can no longer be sold, or not as part of the order concerned. A ring with acceptable dimensions is no longer brought into circulation when there is only one thickness profile, whose lateral resolution is not precise.

Precise width measurement is also enabled by the possibility of edge detection. In addition to the sensor system for measuring thickness, a third laser line sensor is integrated in the system and can be independently positioned. Therefore, synchronous detection of the two edges of a strip is made possible, and clustering does not affect the measurement result.

The width sensor is positioned at the next cutting gap when the thickness sensor system is continuously moved across the material’s width as a whole. The width of the strips is measured when both sensors detect the edges of the strip.

Thickness Measurement of Laminate Sheets

Thickness measurement with integrated width measurement in a single system

Thickness measurement of laminate sheets is another application for which the laser line sensors are predestined. However, methods involving penetrating radiation are not suitable for this.

For instance, the radiation methods only measure the material fraction in the measuring gap and not the product’s dimensional accuracy when the laminates are made with outer sheets and have a web structure on the inside. Laser line scanners determine the geometrical dimensions of such sheets and recognize undulations, which provide information on the problems that occur during processing.

The laser spot is extended to a laser line, so that more measurement values over a larger area are averaged (best-fit line), giving substantially greater precision

Thickness Measurement of Corrugated and Dimpled Sheets

Measurement on structured surfaces is where the laser profile sensors show their strengths. The basic thickness of the material and the overall thickness of the finished product are of interest in the manufacture of corrugated or dimpled sheets.

This task cannot be performed by methods with a large measurement spot, such as the use of X-ray or isotope radiation, or by the ones with a very small measurement spot such as point-shaped laser triangulation, or even contact methods.

Laser line sensors feature a maximum line width of up to 64 mm, depending on the measuring range. The sensor can be positioned so that the dimple peaks and the basic thickness of the sheet can be determined reliably by the laser line. This is because the geometry of the rolls used to roll the embossed or dimpled profile into the sheet is known.

This information has been sourced, reviewed and adapted from materials provided by Micro-Epsilon.

For more information on this source, please visit Micro-Epsilon.