Defects Fast Recognized
Making Chatter Marks Measurable
- Metal strips must satisfy high quality requirements. Shadow formation due to chatter marks is not desired
- Picture of a metal strip. The light/dark transitions are called chatter marks
- Changes in the surface curvature of the measurement object produce clear distortions of the stripe pattern
- The “RC-Compact” system for smaller measurement objects up to the size of a DIN A4 page
- Result of a measurement with reflectControl. The light/dark transitions are the different surface curvatures
- Recording of the surface curvature. The noise has already been reduced using filters. An undulation of the surface can already be seen.
Process stability is critical in rolling mills. Maintenance and monitoring of the technology used is therefore a high priority. Where monitoring is not carried out, production shutdowns due to defective parts can result in significant costs. However, general wear of components cannot be avoided. Defective roller bearings, for example, are subject to wear. Knowing which roller is operating with a defective bearing can be very time-consuming to detect. A deflectometry principle used by sensor specialist Micro-Epsilon Messtechnik can now assist companies in monitoring roller bearings.
In order to cold roll metal strip with the desired thickness from slabs, various stands with different rollers are required. Due to the laws of physics, the rollers at the end of the rolling line turn faster than at the beginning, since the strip is thinner and longer towards the end. Therefore, each roller has a specific speed. If the bearing on a roller is defective, it begins to make "chatter" noises. This non-spherical shape of the roller is due to the defective bearing and leaves traces on the sheet, so-called chatter marks. These marks are hardly visible to the naked eye. A striped pattern across the sheet can only be recognized under a certain angle of light. These light/dark transitions indicate an undulation of the strip in the micrometer range, which are the result of in-line variations.
The mechanical quality of the metal strip is not significantly affected by these chatter marks; nevertheless, they do indicate an error in the process, which must be rectified for in order to achieve process stability. If these errors are not detected, process stability is endangered due to the defective roller bearing(s).
Multiple different rollers makes it difficult to define at exactly which point the chatter marks are produced, which means it is also difficult to find the defective roller. Defining exactly which roller is responsible for the effect requires a measurement of the distance between the chatter marks. If this distance is known, the speed of the strip can help to locate where the effect is occurring. If the roller speeds and the strip speed in the process are known, the error can be localized using this information.
The problem here is that the chatter marks are hardly visible to the naked eye and so cannot be measured easily. In addition, a high-precision measurement of the distance is required for an exact definition of the position.
This task can be accomplished using the deflectometry principle. A sine-shaped light/dark pattern alternating in its position is reproduced on an industrial design TFT display. Cameras record the image reflected from the surface of the measuring object and transfer the data to an industrial PC for evaluation. The recorded mirror images are further processed and evaluated in the computer in several CPU-intensive operations. Distortions of the reflected image, which indicate differences in the curvature of the surface, are used for the evaluation. Although the sheet shows undulations of only a few micrometers due to the chatter marks, the characteristic curvatures of the surface can be evaluated using this technique. The distance between each curvature peak represents the distance of the chatter marks. If the measurement system is calibrated to the target, the size of each pixel is known. Therefore, the distance between two chatter marks is determined with micrometer precision.
Turnkey Measurement Solution
The reflectControl system developed by Micro-Epsilon is very well suited to this task. A section of the metal strip is sufficient to measure the distance between the chatter marks. This is inserted in the RC-compact system. The system is designed for smaller measuring objects and for laboratory operation with fixed measurement systems. The target is measured without contact and the result is available within a matter of seconds.
Dipl.-Ing. Hannes Loferer, Surface Technology Product Manager at Micro-Epsilon explains: "Often, the brushed surface of the metal causes relatively high noise in the measurement data. Due to the two-dimensional measurement of the surface and the high point density of reflectControl, this noise can be reliably eliminated using special software algorithms. Therefore, the pure surface information remains."
This is completely in contrast to alternative sensors that measure the curvature values in one dimension.
With reflectControl, the ratio of the measurement area to the resolution is superior to all other methods. Micrometer-sized defects are reliably detected and automatically evaluated by the integrated software algorithms. For subsequent processes, the data can be further displayed in simple forms such as defect reports; evaluation by a person is no longer necessary.
In addition to the RC-compact solution, Micro-Epsilon also provides alternative systems. The RC-Robotic system, for example, is intended to be utilized for very large objects such as complete automobile body shells. Here, the optical part of the measurement system is located on the end effector of a robot arm. A measurement process covers an area of approximately 70 x 30 cm; the robot moves the measurement system to different positions on the object to be inspected. For example, four RC-Robotic systems can be used in one cycle to inspect a complete vehicle body shell on a production line. Placed on both sides, these systems perform the complete inspection within 60-80 seconds. Afterwards, the detected defects can be marked up using special marking robots.
The manufacturer also provides RC-custom versions for specific measurement applications. The optimum system arrangement for the measuring object is developed, produced specifically for the customer and integrated for this appropriate variant in every environment.
If the exact speed of the strip, as well as the distance between the chatter marks, have to be measured, Micro-Epsilon also provides a non-contact speed sensor. The ASCOspeed 5500 was developed for use in foil and strip systems, as well as tube and profile lines in order to replace incremental sensors that are subject to slippage.
Solutions from the Sensor Specialists
The systems and equipment division of Micro-Epsilon develops, plans and produces turnkey measurement systems for process monitoring and quality control applications for many different industry sectors. This means that new technologies and systems are constantly being developed and integrated. The software is also programmed specifically for each system by the company's own software engineers. The company has special expertise in constructing special machinery, particularly in the development of new technologies. Everything from mechanical design through to software and custom sensor solutions is developed and produced in-house. This enables shorter, faster communication channels, resulting in faster response times. This is a huge benefit when it comes to the construction of special equipment and systems, where unexpected situations can often occur. Solutions can be devised in the shortest possible time and the electrical and mechanical engineering can be integrated on site.
With the reflectControl surface inspection system, the user is able to subject his products to completely objective and repeatable assessment. Errors in production are quickly detected after an inspection and can be corrected immediately. All reflecting surfaces such as polished and painted metal, smooth and painted plastics and galvanized surfaces, can be accurately measured. For borderline objects, anon site feasibility study at the customer's premises is normally required.