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Fraunhofercover story

Inaccurate machining with industrial robots is a thing of the past. A separate compensating actuator for the milling head compensates for inaccuracies measured in real time during machining. The compensation kinematics, which was developed including the actuators and mechanics at the Fraunhofer IPA in Stuttgart, is based on piezo actuators of the company Physik Instrumente.

Machine tools or machining centers are usually used for precision machining. High costs and the workpiece sizes limited by their geometry must be accepted. It would be much cheaper and more flexible if you could work with industrial robots instead. This failed so far, however, because of their inaccuracy. It is caused by the long serial kinematic chain with only low rigidity. Now there is another approach. For this purpose, robots were by no means newly developed, but standard versions combined with an external actuators, thus increasing the absolute accuracy.PI2

In order to achieve the accuracy required for machining with an industrial robot, a separate compensating actuator for the milling head has been developed at the Fraunhofer IPA. The inaccuracies occurring at the robotic milling machines are measured online during processing and compensated for in real time by the compensation factor right where they occur. The advantages of the method are obvious: The component is not firmly clamped on a table as it is when working with CNC machines. Instead, the robot grips it and guides it through the milling head during machining. For handling and processing only one machine is required; the investment costs are reduced, the flexibility increases and, in addition, such a robot solution can be used well within fully automated production lines.

Piezoactuators and solid-state joints

The compensation kinematics, which was developed including the actuators and mechanics at the Fraunhofer IPA in Stuttgart, is based on piezo actuators from Karlsruhe-based Physik Instrumente (PI). There were several reasons for this choice. The piezo actuators work without wear and friction and without slippage. In addition, they can be accelerated up to 10g and are suitable for the high frequencies required to compensate for inaccuracies in the robot path.

PI3Since piezoelectric actuators inherently work only with small strokes, the IPA combined them with solid-state joints. In this way, paths up to 690 μm can be realized in the described application. The travel accuracy of the kinematics in all three axes is in the nanometer range. The solid-state joints for force and motion transmission also work wear-free and low maintenance. In addition, they are lighter, quieter, stiffer, more dynamic and more accurate than conventional balancing mechanisms. Piezo actuators have proven once more that they are driving forward technical development. The controlled robot solution opens up new possibilities for the milling of metals and plastics.PI4

The piezo effect

Already the end of the 19. In the 19th century, Jacques and Pierre Curie discovered that mechanical pressure in quartz crystals generates electrical charges. They called this phenomenon "piezo effect" after the Greek word "piezo" for "pressure" or "pressing". Later, they discovered that electric fields can deform piezoelectric materials. This is called the "inverse piezoelectric effect". While the direct piezo effect can be used for sensor applications, the inverse piezo effect is particularly suitable for the realization of actuators. Piezo actuators achieve travel ranges of up to about one millimeter at resolutions down to the nanometer range with high dynamics with frequencies up to several kilohertz. Because the movement is based on crystalline effects, there are no rotating or rubbing parts; As a result, piezo actuators are maintenance and wear-free, and since no lubrication is necessary, they are also suitable for vacuum. You can move large loads and build very compact.

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