Machine Vision Option Optimizes Sheet Metal Inspection - Superior to the human eye
In collaboration with STEMMER IMAGING, Erichsen has developed an imaging module for routine sheet metal inspections according to ISO 16630 in order to further automate the inspection process, to eliminate the 'human factor' and to achieve a dramatic improvement in documentation. These measures are the foundation for Industry 4.0 concepts and cloud applications.
The first ever test method for sheet metal was the cupping test, which was patented in 1913 by the Norwegian A.M. Erichsen. This standard test method was used all over the world and formed the foundation for the global success of today's Erichsen GmbH and their universal sheet metal testing devices. Their recent devices also include test devices for examination of bore expansion test specimens (KWI test or ISO 16630). This standard test is used for quality assurance in sheet metal with a thickness of up to 6 mm. Test specimens are usually taken from the beginning of the coils to be processed. Erichsen sheet metal testing devices are used in rolling mills, the automotive industry and in research laboratories all over the world.
Good Reasons for Sheet Metal Testing
For decades, cars have been developed with a focus on passenger safety in such a way that as much kinetic energy as possible will be absorbed by deformation of car body parts outside the passenger compartment in case of a crash. Profiles such as bumpers, lateral supports, door sills, A, B and C columns, transversal supports or roof arches, etc. are shaped from sheet metal that should be as thin as possible in order to decrease the weight and they must have the calculated rigidity that results from the material characteristics of the sheets. Further examples include deep-drawn parts such as soda cans, cartridges, sleeves, troughs, nozzles, etc. that must retain a uniform wall thickness and must not be subject to crack formation during the manufacturing process. The raw metal sheet is checked prior to production in order to prevent rejects. Relevant data for this cannot be obtained using simple tensile tests for determining the yield strength.
Specimen Preparation and Test Procedure
The so-called hole expansion procedure according to ISO 16630 is an increasingly important test in the area of sheet metal testing. The specimens have to be manufactured before the actual test can be conducted. They are made using the same machine that is used as a test device. In our example it is the universal sheet metal test machine model 142-Basic from Erichsen GmbH & Co. KG located in Hemer, North Rhine Westphalia, Germany. A hole with a diameter of 10 mm is punched into the sheet metal specimen. After inserting the sheet metal specimen (one test requires 3 specimens), a conical steel spike with an electro-hydraulic drive is forced into the centre of the previously punched hole from below in the direction it was punched and expands it until cracks start to form on the inner edge of the punched hole that was created with the steel spike.
An Application for Imaging
The formation of cracks is conventionally monitored by an experienced operator who sees the specimen from above, stops the machine manually as soon as cracks start to form and who subsequently measures the diameter with a calliper gauge. This works well as otherwise it would not be an ISO standard that is recognised around the globe.
However, there is a better option - using machine vision: no involvement of personnel, no dependency on mood and physical condition, uniform visibility. The test procedure with its precisely defined parameters is optimised with a reproducible mode of observation using machine vision, which leads to increased reliability of results, increased accuracy and reproducibility, to narrower tolerances in the evaluation and a testing procedure that is fully documented in images and measured values available in digital format.
The Electronic Eye
It was Ludger Wahlers, general manager of Erichsen and an experienced user of vision systems, who came up with the idea of integrating machine vision in their sheet metal testing devices during a customer meeting. An initial approach to a solution was outlined quickly. The concrete implementation, however, was a bigger challenge. "Having a good idea is one thing", as Wahlers describes his situation back then, "the implementation using conventional components is an entirely different story. You need a supplier with whom you can discuss different variants in detail. I was looking to procure camera, lenses, illumination, and evaluation software, i.e. the entire imaging toolkit from a single source."
Mr. Wahlers found all of this at STEMMER IMAGING: An experienced sales engineer, Florian Mayr, with Europe's largest vision technology supplier was able to quickly select components from the company's very extensive product portfolio that were technically suitable and within the planned budget. The component selection was executed in the course of a two-step feasibility study conducted by STEMMER IMAGING and Erichsen.
The final installation can be seen in image (left): The metallic surface of the specimen to be observed is illuminated with a diffuse ring light with low reflection properties from CCS. The test mandrel moving vertically upwards by up to 40 mm during the test procedure was a challenge during the design phase. The ring-shaped hole edge in the sheet metal must be observed in this entire relatively wide range without any distortion in order to get images that can be used for precise diameter measurement. These circumstances made it clear that an optimal solution was only possible using a telecentric lens, in this case supplied by Opto Engineering.
The camera takes over the work that was previously done by the human operator. A monochrome area scan camera model with a GigE Vision interface from the Manta series by Allied Vision was selected. The capture speed of this camera of 30 images per second is slightly superior to the human eye while the resolution of approx. 30 µm/Pixel is significantly greater than the human eye and hence ensures reliable detection of cracks from 150 µm. The selected camera has the additional advantage of two integrated and freely programmable I/Os. These are used to control the PLC in the test device: As soon as the image processing software detects the beginning of a crack formation in the hole edge at a typical draw speed of 10 mm/min, a signal to stop the machine is sent immediately.
Subsequently, the inner hole diameter is determined using circle fitting. Crack detection and measurement are executed using various image processing tools of the Sherlock software from Teledyne Dalsa, which is a proven development environment for inspection tasks in various industries. The monitor image of the Erichsen test device is based on the graphic Sherlock interface and is an in-house product of Erichsen. It can easily be adapted to customer specifications.
The entire installation for observation is latched to a swivel arm so that the test chamber is easily accessible from above and yet all tests can be observed from a reproducible position. The additional cost for the imaging module is currently approx. 15 % of the cost of the sheet metal test device.
"The collaboration with STEMMER IMAGING in this project really was exceptional. Even though we approached the solution for this project from different angles, communication was clear and goal-oriented at all times. The support during the design phase as well as during implementation and programming the application was extremely helpful and crucial for the successful project implementation. I am sure that we will successfully realize further ideas on this basis", Mr. Wahlers says with great satisfaction.
Erichsen and STEMMER IMAGING have jointly developed an approach to imaging in routine sheet metal testing according to ISO 16630 in order to improve the repetitive accuracy of the test procedure and the reproducibility of the test results. "This significantly improves the reproducibility of a test conforming to standard", says Wahlers, "the results are more reliable and a higher documentation quality level is achieved at the same time" , the Managing Director of Erichsen explains.
"We want to make acting according to the standard easier for users. Thanks to digital records of the test procedures in the form of measurement data and image series, users can evaluate the test data and use them for improving their own product quality. I am certain that this data will open up entirely new possibilities for sheet metal testing with regard to Industry 4.0 concepts and cloud applications."