How to position loads to the micrometer accurately. By Birgit Schulze.
Since 2010, PETRA III has been the most brilliant storage-ring-based X-ray light source in the world and it provides international scientists with excellent experimentation facilities. In particular, this benefits researchers investigating very small samples or those requiring tightly collimated and very short-wavelength X-rays for their experiments. The high-energy radiation of up to and above 100,000 electron volts with high light intensity offers versatile capabilities, for example in the broad field of materials research for the inspection of welded seams, or for the examination of fatigue symptoms in workpieces. In some cases, this involves accurately positioning really heavy loads down to the micrometre. At the heart of the P07 beamline, which delivers the high-energy X-ray radiation required for materials research, is therefore a heavy-duty hexapod. Thanks to its accuracy, it facilitates in-situ measurements of material properties under realistic process conditions.
Hexapods are parallel kinematic positioning systems, available in many versions with travel ranges of up to a few hundred millimetres. With precision below a micrometre, they can position loads weighing from a few kilograms to a few hundred kilograms, or even several tonnes. Their advantages compared with serial, i.e. stacked systems, are that they have much better path accuracy, repeatability and flatness. In addition, the moved mass is lower, enabling better dynamic performance, which is the same for all motion axes. Depending on the geometry of the hexapod, rotations from a few degrees up to 60° and translations of a few millimetres to several centimetres are possible.
Short-wave X-ray radiation
The PETRA III short-wave X-ray radiation penetrates very deeply into the material and thus is also capable of passing through material of greater thickness. This enables welded seams to be inspected and fatigue symptoms in workpieces to be measured as an aid to quantifying the anticipated durability and service lives, or to analyse new metal alloys. Here effects can be proven down to the level of domain or crystal structures.
The opportunities that proceed from this in respect of materials research are leveraged by Helmholtz-Zentrum Geesthacht (HZG) at the High Energy Materials Science Beamline (HEMS), P07, for example when conducting in-situ measurements of the material properties that occur during reshaping processes such as welding, pressing, rolling or stamping. The application of a mechanical load causes tensile and elongation stresses to occur inside the material. The investigation involving X-ray radiation then indicates the chronological sequence of effects occurring within a material at a crystalline level in micrometre-sized domain areas.
At the heart of the described experimentation chamber is a hexapod, developed by Physik Instrumente (PI). Dr Norbert Schell, the scientist in charge of the HEMS Beamline, illustrates the context: “For an increasingly large range of in-situ investigations of ‘real’ processes, that is processes that actually occur in industry (but of course not only there) – associated with the cutting of workpieces, the coating of surfaces for the hardening or improvement of tribological properties, reshaping, welding, heat treatment as well as combinations of these techniques – it is our ultra-rigid hexapod with its tremendous load-bearing capability and micrometre precision positioning that makes it possible for the first time for us to conduct scientifically rigorous examinations of the structural changes that occur at an atomic level. This is highly interesting, and is also important in helping us to understand the processes that are occurring and that, ultimately, enable customised materials to be optimised.”
The parallel-kinematic custom model, the M-850K, delivers micrometre-precision positioning for loads of up to one tonne in every orientation. It stands ±20° approx. 700mm high and has a diameter of 800mm (top platform with large aperture) and 900mm (bottom). The lower platform is installed on a 360° rotation table and the cabling was designed to be dragchain compatible. With its high load capacity of up to one tonne, the hexapod can carry the entire measurement setup including the equipment where mechanical forces are applied. The hexapod positions even these large masses over distances of 400mm to a precision of ±1 micron, and performs rotational motions of ±20° with a resolution to 0.5 µrad. Inside the experimental hutch, this enables complete engine blocks, turbine components, sinter furnaces and cryogenic chambers as well as welding fixtures or other machining units to be aligned precisely for the planned investigations and to be moved accordingly during the analysis. Despite the high forces, the position reached is held in a stable manner; the brushless DC motors integrated in the hexapod struts are equipped with brakes.
For more information visit www.scientistlive.com/eurolab
Birgit Schulze is with Physik Instrumente (PI) in Germany. www.pi.ws
