Active vibration control using an adaptronic smart platform for high precision cutting machines

Vibration control is gaining an increasing competitive advantage in machine tool design. In particular for Ultra High Precision Machining, the goal of machine tool design is to guarantee high accuracy, specified performances, and to maintain them over life cycle time. Micromilling operations have to generate outputs characterized by very close tolerances, high precision and surface finishing. During the machining process (e. g. milling), the contact between the cutting tool and the work-piece surface at the tool tip point generates chattering vibrations. Any vibration is recorded on the workpiece surface, directly affecting its roughness, which leads to poor surface finishing, unacceptable in high precision milling. In this paper the active vibration control of an innovative mechatronic subsystem (Smart Platform), which can be installed, in a modular way within ultra high precision micromilling machines is presented. The smart platform includes two main parts: the fixed platform, which is directly constrained to the machine tool ram, and the mobile platform, which is constrained to the housing of the spindle. Three piezoelectric actuators-sensors units are connected the two platforms in axisymmetrical configuration, with 120 degrees difference angle. The design idea is based on the incorporation of three high performance piezoelectric displacement actuators, each equipped with a collocated, piezoelectric force sensor, in an axisymmetrical configuration within a mechanical holding structure, using suitable mechanical and electronic interfaces. Every piezo actuator is connected to an innovative flexural joint, designed to avoid torsional and shear stresses to the piezo elements. Furthermore, two flexural springs are positioned close to every actuator, which have been designed to connect the two platforms (fixed and mobile). Every spring is characterized by high torsional and radial stiffness, but free to move in axial direction. The proposed active control techniques of the smart platform aims to improve the high precession machining operation by using a broadband AVC strategy, which could dynamically compensate for the vibrations of the tool tip, by actuating in three degrees of freedom: one axial and two bending. Firstly, a dynamic identification and modelling of the Smart Platform is presented. Secondly, an integrated mechatronic model able to predict in closed-loop the active damping and vibration-suppression capability of the integrated system is presented and simulation results are discussed. Finally, the effectiveness of the Smart Platform for active vibration control is experimentally illustrated on a dedicated experimental setup. According to the experimental results, the used active vibration control techniques is able to effectively damping the vibration of the tool tip especially around the resonance frequency, successfully damping all of the three resonance modes, between 100 and 170 Hz, with a reduction factor up to 75%, also it is able to improve the dynamic stiffness of the tool tip for a wide range of operating frequencies.