Chains perform important tasks in many areas of industry. They are used in lifting gear for bearing heavy loads, as timing chains in combustion engines, and for many other applications. Depending on the application, these components have to meet high standards of quality and reliability-as a broken link in the chain hoist of a crane, for example, could have grave consequences. Therefore, it is important that a consistently high level of quality and precision is maintained in the manufacturing of chain links. Quality control in this continuous production process, however, encounters tight technical limits. Until now, it could only be carried out manually by using tactile methods. As a result, the tester had to fall back on mechanical tools such as gauges or callipers. It is clear: this procedure is linked with high expense in terms of time and personnel, and only really allows for random sampling. But 100 % coverage is what we want, which means testing every single chain link individually-that's the only way to ensure the high levels of quality required.
Inline procedure for contactless testing of chain links
An image processing system, developed by the Technical University of Ilmenau together with Vision & Control GmbH (based in Suhl) as an interdisciplinary research project, can do just that: using a contactless inline procedure, it allows for complete test coverage in the production cycle. Without having to stop production for even a fraction of a second, the system measures the chain links optically and determines with the help of a special algorithm whether the tested objects fulfil the criteria.
The development project was able to profit from Vision & Control's comprehensive expertise in the field of industrial image processing. The aim of the project was to develop an inline test device capable of real-time contactless testing of chain links with 100 % test coverage. Industrial image processing offers a comprehensive tool kit of various methods for resolving problems. Important fundamental decisions for agreeing on the design of the measurement system concerned the illumination technique used, the choice between transmitted light and reflected light, as well as the nature of the optical path to be used. Even the choice of the type of camera is a fundamental decision; there's a selection of matrix and line scan cameras to choose from.
The developers decided on a telecentric optical path. This approach enables objects to be recorded without perspective distortion. Additionally, an axial shift in the position of the object-i.e. its distance from the lens-does not result in any changes to the image scale. That means that the image always appears to be the same size, regardless (within certain limits) of how far away from the lens it is. In practice, this means a high positional tolerance for the component being tested. Variable distances between the component and the lens cannot distort the results. Regarding the illumination of the object, the developers settled on the transmitted light with the flashing light source, which is advantageous for measuring. Matrix cameras were preferred to line scan cameras due to the direct two-dimensional recording of the chain link being tested.
Image processing system as a modular system
Regarding the camera, lenses, and lighting, the developers settled on using the vicosys® 4400 image processing system from Vision & Control. The system, which is configurable in modular design, fulfils the requirements for various reasons: a wide range of components are available for the development of various types of solutions. In addition, all elements are built for industrial purposes and tested in manufacturing applications. The availability of support services for supporting the development work was also important.
Finally, the system offers a good price/performance ratio in the eyes of the project participants. The modular vicosys® 4400 system, which can be used anywhere, offers the option of connecting up to 16 cameras. Accordingly, it offers an appropriate number of Gigabit Ethernet ports with power supplied via the Ethernet port (Power Over Ethernet). The use of at least two cameras is necessary for planned measuring to set both to a 90-degree angle to measure the chain. A third camera would be advantageous for measuring multiple chain links, but we were restricted to the use of two cameras for this project for financial reasons. With a resolution of 2 megapixels, the black and white Manta cameras used offer the necessary attention to detail to measure chains of various sizes. With a frame rate of 30 frames per second (fps), the camera is also able to complete running production processes relatively quickly and sufficiently accurately.
Optical measurements + algorithms = actual measurements
A significant point of view for measuring tasks in the context of automated production processes is to keep the number of interventions by measuring technology as low as possible. Lifting every link from the chain for the purpose of measuring has, therefore, been withdrawn as an approach. So, a process needed to be developed for measuring the chain while it was constantly moving. Direct optical measurement is not possible in this case, because parts of the chain links are covered. As a solution for this problem, the development team devised an indirect measurement process that uses appropriate algorithms for determining actual measurements of the chain links. By using specific, model-based algorithms, actual measurements are calculated from the measurements recorded visually. It should be noted that the diameter of the wire used was not used as a constant value. Instead, this had to be corrected in various ways depending on the state of its bend, and its location in the chain link. The recording of the chain position on two planes is advantageous because it broadens the possibility for correcting systematic errors.
Standard deviation <0.015 mm
The measuring system that emerged from this exercise can record and measure up to 5 chain links per second. It can interpret results according to the various parameters of the chain links, including partition, internal width, or diameter. After an optimisation of the process, the development team succeeded in bringing the standard deviation down to less than 0.015 mm: a requirement for an industrial measurement system. Alongside quality control, possible fields of application are adjusting chain link bending machines and welding machines as well as guaranteeing product safety.
published in: inspect issue 2_2018