Looking like the pimply knee of a giant, a 14-ton valve body rests on heavy wooden blocks at Siemens’ steam turbine plant in Mülheim an der Ruhr, Germany. The valve body is covered with white dots on black markings. Photogrammetry enables the huge component’s three-dimensional shape to be measured without having to use a yardstick or a protractor. It’s use in Mülheim marks the first time that this application is being used to measure turbines at Siemens.
From Big Data to Smart Data
A Photographic Technique that Measures Parts
Siemens is using photogrammetry to quickly and precisely measure turbine parts in order to accelerate their processing and ensure they meet specifications.
Photogrammetry is by no means new — it has been used to measure buildings since the mid-19th century. For the past two decades, it has also been used to measure the surface of the Earth from the air. The method enables users to calculate the measurements of three-dimensional objects on the basis of photographs taken from a variety of angles. To do so, associated software searches the photos for matching details, which it then uses to reconstruct a subject’s three-dimensional shape on the basis of the camera’s known imaging properties.
In order to measure the three-dimensional shape of a turbine part, minute holes are drilled into its surface – a process that takes about two hours. The tiny holes serve a second purpose: they function as reference points for CNC-based milling steps designed to eliminate any discrepancies detected by the photogrammetric processing.
During the photogrammetry itself, a part is photographed using a single-lens reflex camera equipped with a distortion-free lens. Because each picture has to show at least four markers, around 200 images are needed for a valve body.
It previously took up to 24 hours to measure such a component manually with the help of a yardstick, a compass, and a special surface plate. “Measuring large components was a major bottleneck in our production process,” says mechanical engineer Sven Illberger, an expert in 3D measurement technology at the Mülheim plant.
Comparing Physical Parts to CAD Models
After a workpiece has been photographed, the images are processed by software that turns the marker holes into a point cloud, from which the component’s three-dimensional shape is computed.
The information is then processed further with the help of Siemens know-how. In this step, software compares the actual photogrammetric measurements with the planned dimensions from the workpiece’s CAD data. The difference generally amounts to only a few millimeters. This excess metal must be milled off.
The Mülheim plant’s quality assurance team has been using photogrammetry for about a year now. The method is particularly well suited for parts that are larger than one meter and that do not require machining tolerances of less than half a millimeter. There are many such products at Siemens — from locomotives to wind turbine rotor blades. But the engineers in Mülheim first want to use photogrammetry to measure additional turbine parts. According to Illberger, they are discussing this possibility with Siemens’ gas turbine production facility in Berlin, as well as its compressor production facility in Leipzig. Photogrammetry is also being used for final inspections at supplier companies, where the method is helping to increase accuracy and accelerate the processing of turbine parts.
Among other things, photogrammetry will be introduced at a Siemens plant in Cilegon, Indonesia that supplies many of the steel components for steam turbines. The earlier in the manufacturing process photogrammetry is used, the more time and money can be saved during processing, says Illberger. “We’ve taken a big step forward in the digitization of large components,” he adds.