INTEGRATED RADAR SYSTEMS FOR MACHINE TOOLS
Until recently, distance and position measurement in machines tools was dominated by glass scales or laser-based sensors. The development of innovative, high-precision radar sensors is, however, a flexible and cost-effective alternative that opens up new possibilities in the machine tool industry.
High precision through high frequencies and large bandwidths
In the past, established measurement systems such as glass scales and laser-based sensors were far superior to radar-based solutions both in terms of measurement precision and price. In the last few years, however, radar techniques – primarily driven by the automotive industry – made up for lost ground and are now also firmly established in other sectors. Thanks to the latest developments, modern radar systems are now a serious alternative to existing measurement systems in the machine tool industry. At present, silicon-germanium technologies with critical frequencies above 300 GHz, which allow cost-effective and mass market utilization of the upper millimeter wave range, form the technological basis. One of the technologies used by Fraunhofer FHR to realize radar systems with frequencies of up to 240 GHz and bandwidths greater than 50 GHz is Infineon's B11HFC SiGe BiCMOS technology. The small free space wavelength of just a few millimeters at these frequencies combined with high bandwidths allow high-resolution measurements with micrometer precision.
Versatile alternative to existing technologies
Given that glass scales and laser-based sensors have been the established standard in machine tools for many years already, why is it necessary to introduce radar-based sensors in this area? The specialist area micro manufacturing offers a very plausible answer: when small work pieces are subjected to high-precision processing in large machine tools, there is a very long mechanical lever arm – due, for example, to a glass scale in the drive axle – between the processing point on the work piece, the tool center point (TCP) and the position measurements. This causes vibration- and torsion-induced precision loss. Sophisticated and expensive machine constructions, often weighing several tons, is necessary to counter this effect. One alternative is position measurement directly on the work piece near the TCP. In principle, this is possible using laser-based sensors, but the optical measurement is greatly influenced or even impossible due to chips and coolant mist produced during processing. Here, radar-based solutions can display their full potential and allow reliable and precise position measurement near the processing point even under zero visibility conditions caused by dense coolant mist and flying chips.
Flexible application possibilities and new machine concepts
The flexible application capabilities of compact radar sensors for distance and position measurements open up new possibilities for the machine tool industry. The Priority Programme 1476 "Small Machine Tools for Small Parts" of the German Research Foundation (DFG), in which Fraunhofer FHR also participated, focused on the development of new concepts for modular, configurable machine tools for micro manufacturing. The aim was to create a uniform, flexible platform on which various tool, drive and positioning modules can be mounted depending on the part that is to be processed using standardized electromechanical interfaces. Due to the light and compact design, radar sensors have clear advantages when position measurement is required near the work piece in confined areas. Due to the reflection properties of metallic surfaces, the alignment of the radar sensor is much less critical compared to optical, laser-based systems with the result that retooling in a modular set-up is possible with very little effort. The absence of optical lens elements which are vulnerable to soiling also allows utilization in harsh and dirty environments without special protection measures.
Robust design and cost-effective systems
Due to the high frequencies, integrated antenna structures which allow a particularly compact and cost-effective design are used. As no complex high frequency technology is needed outside of the chip, the design can be realized with inexpensive board material on an epoxy resin basis which, in combination with the chip technology that is known from the automotive industry, can be produced in large quantities for the mass market. Radiation through a Teflon lens element also facilitates the utilization of a hermetically sealed, pressure-tight sensor with high mechanical stability which can also cope with the harshest industrial conditions.