Kim's research was part of her doctoral thesis. In it, she researched methods for charting and optimizing the precision of machines for dimensional measurement. Her promoters Dr. Wim Dewulf (Group T) and Prof. Jean-Pierre Kruth (K.U.Leuven) were the ones who, in the spring of 2010, advised her to submit a paper for the symposium in Japan. It was accepted immediately. In fact, Kim also got the chance to give a talk. “No fewer than 150 speakers were present,” Kim relates. “Coming from every continent. I was the only Belgian speaker at this world summit of measurement techniques and quality control.”
Calibration object
Kim presented a new 'calibration object' in Osaka. She called it a 'cactus step gage block,' a neologism of size. “I designed it specifically to improve dimensional measurements with computer tomography,” she explains. “You have to imagine it as a little aluminum block with an inside structure that truly looks like a cactus. With the CT scanner, we take black and white images of it from different 360° angles. These images are put together to form a 3D model, composed of voxels, the 3-D equivalent of pixels. Dimensional measurements can be taken of this model.”
“The image of the object is enlarged by moving it. The closer to the source of the X-ray, the larger the image becomes. This can be compared with a candle that throws a shadow on a wall. The closer your hand is to the flame, the larger the shadow becomes. The more the image is enlarged, the better the resolution of the 3D model.”
“Two problems arise when measurements are being taken. First, after taking the measurements, you have to rescale the object to its real size. That is, you have to know precisely how many times it has been magnified. Second, the edge of the object is made up of a whole range of grayscales between white and black and you have to determine which grayscale determines the border between the material and the background (surrounding light). The software here sometimes goes wrong, so a correction is necessary. Well, the method illustrated using this calibration object allows both rescaling to the correct size and correcting the grayscale.”
“To do the calibration (rescaling and correcting the edge), a number of measurements of the object are also taken with a coordinate measuring machine,” Kim continues. “It's true, this can only measure outer forms, but it is much more precise so that we have a reference to compare with our CT measurements. Based on this, we can correct the inside measurements so that these become more precise. It is precisely here where the difference lies between the conventional measurement techniques and computer tomography. This offers an indispensable addition to the conventional techniques. Alone, these measurements are less accurate and therefore corrections are needed.”
Real workpieces
The method that Kim has developed can also be used on industrial workpieces. In the TETRA project, in which not only Group T, Lessius University College and K.U.Leuven, but also a number of high-tech companies were involved, the method was tested on real cases. Kim uses LayerWise and Materialise as examples. “They make objects through a layered production method. Using this, complex workpieces can be made with internal forms, for example, internal cooling channels. Whether these channels have the correct measurements can only be checked using computer tomography. The method developed here can significantly improve the accuracy of these measurements.”
Further research
According to Kim, the demand for dimensional measurement techniques with computer tomography will rise rapidly over the next few years. And so she is happy that she and her colleagues at Group T, Lessius University College and K.U.Leuven can buy a very expensive machine through the Hercules program, a machine that will let them continue and expand their research. “In fact, we've submitted a project application to the Research Foundation Flanders – FWO and the Industrial Research Fund to pursue this research further. Here we will also look at the workpieces that are composed of different materials, something that makes it more complex. More than enough material for more exciting doctoral work...”
Yves Persoons
