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A novel Servo Piezo Tool SPT400MML for free form surfaces machining with few nanometres resolution

In the world of the machining process, the holy grail is high speed, high precision and high quality at the same time taking into account the cost factor as low as possible. While the precision is related to the quality, the speed always represents the limitation in providing with high precision during the machining process and so a problem to lead to high quality of the machining parts. This trade off between speed and precision or speed and quality is even harder to cope with especially if the machining parts are of complex shape.

In order to keep pace with the demand of high quality and speed machining process coming from many industries (optics and automotive), Cedrat Technologies SA (CEDRAT), a French company involved in mechatronic engineering and components manufacturing, developed a novel and versatile Servo Piezo Tool, so called SPT400MML.
The SPT400MML is a fast tool servo driven by piezoelectric actuator for high speed and precise diamond turning and Lathe machining. The SPT is particularly used in applications where unsymmetrical surfaces have to be machined such as oval piston and non spherical optics. The SPT400MML uses the patented Amplified Piezo Actuators (APA) from CEDRAT because of their high stiffness (in the transverse and actuation axes) and then high achievable bandwidth and acceleration to get a fast motion of the diamond tool with long stroke. The APA is used in a new counter-balanced configuration to limit the impact of the vibrations during the machining process, to improve the accuracy of the tooling system and to obtain a high quality surface.

To overcome the inherent hysteretic and drift nonlinearity effects while keeping the powerful performances of the piezoelectric actuator, a closed-loop control system is established using position feedback based on a high resolution Eddy current sensor integrated in the casing, which has compact structure and avoid any interference in the machining.

The UC75 controller includes a National Instrument Core based on Compact RIO@NI (based on native parallelism of the FPGA component) and the power of the Labview@NI Libraries (based on simple graphic code) to control any system with fast ticks and deterministic time. This real time controller with fast sampling rate up to 100kS/s permits to tune the regulator to obtain a high bandwidth and high accuracy control of the tool holder with a robust regulator relying on the classic PID topology. As a consequence, the UC75 allows the SPT400MML to follow the complex trajectory motion in oval piston machining and complicated free form surfaces in optics machining.

The SPT400MML displays a 450µm stroke and a bandwidth up to 500Hz (see table 1), thanks to the powerful linear amplifier LA75C-1 available from the standard range of Cedrat piezoelectric driver.

Various tests were performed, including some trials to assess its performances when coupled with standard lathes, on aspheric and elliptical form machining. The surface roughness obtained with a "Contour fine tooling" diamond tool is 10 nm rms. The SPT400MML prototype was developed under the FP6 MasMicro project.