Process-capable length and speed sensors - Reliable under the toughest of operating conditions (Company news)
Reliable measurement of product speed and length is essential for production processes in many areas. And the measuring system requirements are as diverse as their applications. For example, in the steel industry, ambient temperatures of 200 °C as well as dust and oil vapors are quite common. In the food industry, on the other hand, things are cleaner, but the most stringent hygiene regulations must nevertheless be observed. Under these ambient conditions, the measurement system must meet the highest standards of accuracy and reproducibility.
Figure: Polytec's Laser Surface Velocimeters have been especially designed for use in harsh environments, work without wear and tear and are maintenance-free in 24-hour continuous operation. (© Polytec)
Non-contact optical measurement systems are the ideal solution for high-precision, non-invasive length and speed measurement in the most different areas of application. Polytec's Laser Surface Velocimeters, especially designed for use in harsh environments, are a good example of this (Figure 1). Their non-contact working principle does neither show wear nor tear leading to a maintenance-free, continuous operation. Their reliable measurement data can easily be integrated into process control systems. The basic function is simple: Using the differential laser Doppler principle, they evaluate the laser light scattered back from a moving object (see technology box). The LSV measures during standstill and reliably detects the direction of movement. This is why non-contact, optical measurement technology is more and more the first choice in a many applications, from the production of food and to paper and steel production.
Increasing profitability in continuous casting In the continuous casting of steel, it is important to keep material losses to a minimum to ensure economical production. Measurement systems for cut-to-length control are therefore obligatory. But often the inherent inaccuracy of conventional contacting methods – caused, for example, by slippage or wear – means that a considerable amount must be added to the minimum length guaranteed to the customer; so manufacturers give away material for free with every cut. The exact knowledge of the speed and length of the strand is the key to optimize the process and the costs. Here, due to their laser precision and reproducibility, non-contact LSVs offer the most practical solution, improving quality and increasing yield. This pays off quickly for the user. If the extra length of each cut is reduced by only a few millimeters, the investment can be paid off in less than one year. In addition, the mounting and start-up of this optical measurement system is easy. It is mounted at a height of 1.5 m above the strand (Figure 2) and is ready for use immediately after entering the application-related parameters.
Another advantage is the environmental protection: In spite of the hot environment, there is no need for the user to protect the measurement system with additional cooling measures. The protective cooling housing, made of aluminum, with cast-in cooling water pipes made of stainless steel, has such an effective cooling performance that additional enclosing is unnecessary. This saves further costs and effort.
Mass flow control in rolling mills
Rolling mills have similar requirements. The processing industry demands highest accuracy and quality of today's steel and aluminum products (Figure 3). In order to continuously reduce thickness tolerances of rolled strips, modern rolling mills are controlled according to the mass flow principle. The mass flow relationship (Figure 4) allows the roll gap to be calculated for the required strip thickness. It requires measuring speed and thickness before the roll stand and measuring speed again after the roll stand. This way the roll gap can be adjusted fast and very precisely.
The strip thickness is usually determined by x-ray or optical measuring instruments; Laser Surface Velocimeters are perfect for reliably measuring speed here, because the sensors are easy to integrate and – in contrast to traditional contacting speed measurement techniques (e.g. measuring wheels) – LSV work without slippage and wear, avoiding measurement errors. In addition, the different available stand-off distances offer flexibility (300 to 3000 mm), simplifying the mounting as well as retrospective installations.
Uninterrupted paper production
In paper manufacturing (Figure 5), flying splicing is the usual procedure used to join paper rolls during ongoing operation. The start of the new roll is bonded to the end of the used roll. The high operating speed of roll decoilers can make speed matching of the rolls quite a challenge. Speed differences, however, impair the splicing process; the web can tear due to tension fluctuations, leading to loss of production and costs of several thousand Euros. Such problems can be prevented by using Laser Surface Velocimeters.
LSVs measure the true surface speed of the web on the decoiler and send an accurate and reliable measurement signal to the control system. The decoiling speeds can then be synchronized to within +/- 1.5 m/min of each other, corresponding to an error of 0.1% for absolute values of 1500 m/min. In practice, this leads to a significant reduction in tension fluctuations, preventing the expensive downtimes caused by differing roll speeds.
Non-destructive material testing
Various non-destructive measurement procedures are combined in the quality control of seamless steel tubes. Most important are longitudinal and transverse defect testing as well as wall thickness and lamination measurement. For exact localization of defect, the actual test procedure is combined with Polytec velocimeters. Using the laser Doppler principle, they can determine the precise movement of the tube and the current tube position in the test system based on the backscattered laser light. The use of several LSVs is common in ultrasonic testing equipment. Typically, the tubes rotate around their longitudinal axis at 2 m/s. Two LSVs (Figure 6) simultaneously record both the longitudinal movement of the tube in feed direction and the rotary motion perpendicular to it. This provides unambiguous position data during the entire test. With the LSV position data, defect can be located precisely and marked in color for further post-processing.
Cake in top form
Potential applications for the versatile Laser Surface Velocimeters can also be found in many other fields. In the food industry, for example, they measure the speed of dough (Figure 7): An extruder feeds the dough onto the conveyor belt. At this point in time, the dough has a roughly rectangular shape, but is still very irregular. The speed here is approximately 1.5 m/min. A subsequent rolling process smooths the dough and reduces its thickness. The outer parts of the dough strip are then folded inwards; and the dough is rolled out again. Just before the oven, the dough is approx. 30 mm thick and moving at a speed of approximately 2.5 m/min. To achieve the optimum dough thickness and shape, the mass flow must be controlled for the rolling processes. A certain degree of elongation, resulting from the speed difference between dough and belt, is desired. Too much, however, causes deformation and tearing of the dough. Precise speed measurement is therefore obligatory. The speed of the dough must also be measured to monitor the baking time in the oven and ensure optimal baking results. Measuring wheels would touch and damage the dough and are therefore unsuitable for these tasks. Deposits could also change the diameter of the wheel and cause inaccuracies. Here, Polytec LSV are the first choice again.