Multi-sensor tomography – Two-tube measuring system now also with sub-microfocus tube

The sub-microfocus tube increases the resolution of the two-tube measuring system by an order of magnitude, achieving a resolution of less than one micrometre. The 300 kV microfocus tube enables the measurement of large or dense workpieces with resolutions in the range of a few micrometres. This makes the machine suitable for applications with widely varying resolution requirements and for different workpiece sizes and materials. Almost all computed tomography measurement tasks can now be solved with a single machine. By combining it with region-of-interest computed tomography (ROI-CT), several small details can also be measured on comparatively large workpieces (patent). For this purpose, the overview CT is measured with the microfocus tube and the ROI with the submicrofocus tube at a higher magnification.

Powerful duo

The microfocus tube achieves voltages of up to 300 kV with a maximum power of 80 W and focal spot sizes of a few micrometres. The transmission target enables such small focal spot sizes even at maximum power. This means that large or dense workpieces can be measured quickly and with high resolution.

The sub-microfocus tube has a maximum voltage of 160 kV and up to 50 W power. Thanks to an additional focussing unit, it achieves a small focal spot size of 0.5 μm. The characteristic of a CT machine is often specified as the resolution in the radiographic image, which usually does not correspond to the resolution in the volume, or only a voxel size is specified, which can be chosen to be almost arbitrarily small. Neither of these is a measurement of the structure size that is actually resolved. With the Werth sub-microfocus tube, the resolution in the radiographic image is 0.5 μm.

The effective resolution of a CT coordinate measuring machine results mainly from the combined effect (convolution) of the resolution functions of the focal spot, rotary axis and detector, taking the magnification into account. For this reason, the machine is also equipped with a highly accurate rotary axis with radial run-out errors of less than 0.2 μm. This means that the practical volume structure resolution of the two-tube measuring system with sub-microfocus tube is approx. 1 μm. Applications of the sub-microfocus tube include measurements of the fibre structure of filter materials, the measurement of the penetration depth of particles with colour-coded display and the measurement of particle sizes.

Requirements

Even a drift of less than one micrometre leads to a significant loss of resolution. It is therefore necessary to capture and correct the drift during the tomography scan. Werth has many years of experience in this field thanks to its highly accurate multisensor coordinate measuring machines. For the sub-microfocus tube, the process for detecting drift during the tomography scan was optimised and a new method for checking the volume structure resolution was developed. This made it possible to prove that the theoretically achievable resolution is actually reached and that influences such as drift are negligible according to correction.
A two-tube measuring system can be installed in the TomoScope® L, XL and XL NC machines. Other sensors such as the Chromatic Focus Point sensor for optical measurements can be added, for example for the rapid capture of measurement points in parts or for roughness measurements.

Application examples

A two-tube measuring machine can be used to solve measuring tasks with widely varying resolution requirements and different workpiece sizes and materials. In plastic injection moulding with fibre composites, the geometrical characteristics can be measured with the microfocus tube, while fibre orientation, length and diameter can be measured with the sub-microfocus tube. Die-cast workpieces can be analysed for pores, voids or cracks. Other applications can be found in research and development, for example.