International Symposium on Computerized Tomography for
Industrial Applications and Image Processing in Radiology
March, 15 - 17, 1999 Berlin, Germany Proceedings BB 67-CD
published by DGZfP
|TABLE OF CONTENTS|
This paper reports experiences with a flat panel detector which replaces conventional X-ray imaging systems. An amorphous silicon array detector is used within a fully automated X-ray inspection unit designed for non destructive testing of aluminum components for the automotive industry. The new technology enables YXLON to considerably reduce the number of necessary views (and thus the cycle time) for complete coverage of the inspected part.
Non destructive testing using X-ray imaging techniques gains increasing importance in (but is not restricted to) the automotive industry where a great amount of production parts is required. Cost effectiveness, high quality standards at any time as well as reliable and reproducible inspection results are major reasons to use fully automated inspection units to meet the demands of today's market.
Fully automated YXLON units are designed as real time X-ray inspection systems for the batch production of aluminum castings in high production numbers. Handling of the specimen, image acquisition and evaluation (including decisions whether or not quality standards are met) are performed automatically by the units.
Image processing is a vital part in this automatic inspection procedure. For acquiring the X-ray images an amorphous silicon array detector is used. It is connected to a Pentium 1 PC via parallel interface, where both frame grabbing and controlling the whole unit (including image processing) are performed.
Advantages of using the flat panel detector over the conventional image intensifier systems are distortion-free images at full image size with a dynamic range of 16 bit. The size of the active area to detect and process incoming X-ray beams is larger than the input screen of conventional image intensifiers normally used. Because of the mechanical design and electronic features of the amorphous silicon array detector YXLON is able to build more compact customized X-ray inspection systems with considerably lower inspection cycle times per part.
|Fig 1: Flat panel detector mounted on C- arm|
The general concept of the device (a HEIMANN detector RID 512- 400) is a flat panel detector which serves as a complete electronic camera for X-ray images. Incoming X-ray beams are directly converted into electronic signals which then form digital images on a connected PC. Its physical dimensions are much smaller and thus lighter than a conventional image acquisition system which normally consists of a camera and an X-ray image intensifier. This enables YXLON to mechanically build more compact and flexible inspection units. During inspection the specimen is located at a fixed position and the imaging system moves to required angles (see Fig. 1) rather than vice versa (e.g. with systems using robots handling the specimen in the X-ray beam).
X-ray radiation is detected using scintillators to convert the incoming radiation into light. The active area for X-ray detection is 204.8 x 204.8 mm2 and the sensors are implemented in amorphous silicon technology. The pixel number of 512 x 512 yields a pitch of 400 µm. Image acquisition is similar to crystalline silicon photodiode arrays. All necessary electronics (driving and data readout circuits, timing control and digital data interface) is typically placed in a separate box apart from the detector outside the X-ray beam. The detector itself is highly resistant to X-rays (radiation energy range: 20- 200 keV) and can be installed directly in the beam.
The generated digital X-ray images have a size of 512 x 512 pixels (according to the number of sensors in the detector) with a dynamic range of 16 bit. This gray value range suits the need to evaluate both very dark and very bright regions in the same image, in order to reduce the total number of views for an inspection program. With amorphous silicon array detectors it is now possible to automatically evaluate views in which high absorbing areas (e.g. thick cross sections) are located next to regions with little material thickness (low absorbing areas). Where formerly two or more views were needed, each with different X-ray energies to cover a specific area of a specimen, now only one view is sufficient to achieve the same result. This reduction of the number of necessary views is possible due to the lack of blooming effects.
Fig 2: View at MU69 from input conveyer
Fig 3: Pick and place unit at output gate
The YXLON MU69 unit is designed as a real time X-ray inspection system for the batch production of aluminum castings in high production numbers. Depending on its set-up it is capable of inspecting several different parts. Feeding the unit with production parts can be done either manually by a human operator or automatically with a handling device (e.g. robot or manipulator), so that the MU69 can be integrated into the production line. At the loading station each part is placed onto a pallet and then conveyed into the X-ray protection cabinet (see Fig. 2).
Before entering the X-ray protection cabinet the pallet stays in a waiting position in front of the input gate for the preceding inspection cycle to be finished. Moving the inspected part out of the cabinet and moving the waiting pallet into the cabinet is done at the same time. During this waiting period a part recognition system determines the part type to select the appropriate ADR program. After inspection the specimen is automatically sorted according to the inspection result, separating rejected parts from accepted castings. For moving the inspected parts from the pallet at the output gate to predefined locations a shuttle combined with a pick and place unit is installed (see Fig. 3).
The PXV5000 software package running on a Pentium PC with Windows NT 2 is supplied with the YXLON MU69. It is not only a program to process X-ray images, but also to control the complete unit within a single graphical user interface. Automatic Defect Recognition (ADR) is done each time an image is acquired. The images taken represent the views of the inspected part. The sequence of views is predefined in an ADR inspection program which controls the movements of the manipulator as well as the X-ray settings. For every part type which is to be inspected an ADR program has to be established. After ADR set-up is completed the unit is capable of running in different modes (Automatic, Semi-Automatic or Manual).
The actual image processing part of the PXV5000 system is freely configurable, depending on the needs at hand. Up to eight different image processing algorithms can be applied to every single image. Every algorithm (which also can be one algorithm used twice or even more, with different settings) works on an user defined region or regions in the X-ray image. Filter kernels can be chosen from a list given by the system or newly defined be the user. Also inspection regions, image size, sensitivity thresholds as well as tools for analysis and verification are integrated into the user interface.
The YXLON X-ray inspection unit MU69 as shown in figures 1... 3 automatically inspects six different aluminum castings (three part types, left and right hands each) of the type shown in figure 4. Its dimensions are approximately 300 x 250 x 140 mm 3 (L x W x H) and it is inspected in a total of four views. Inspection criteria are set up according to ASTM standard level 2. Overall cycle time for each part is 12 seconds. This includes part handling as well as image processing.
|Fig 4: Inspected part type|
For an application of a similar part using a conventional imaging system with an image intensifier the ADR program needed nine views and the inspection cycle time was 18 seconds.
By using the amorphous silicon array detector it is now possible to detect much smaller flaw sizes formerly thought to be impossible. Due to the lack of blooming and the dynamic range of 16 bit the image processing software is capable of processing both dark and bright regions in the same view (neighboring areas in the specimen of thick and thin cross sections). Additionally featuring an active area of larger size than the input screen of conventional image intensifiers normally in use the flat panel detector enables YXLON to considerably reduce the number of views to inspect a part.
Because of the physical dimensions of the flat panel detector X-ray inspection units can be built more compactly. Using standard PC hardware in combination with the amorphous silicon detector YXLON meets the demands of today's market to build reliable and cost effective X-ray inspection systems.