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Active damping of machine tools with magnetic actuators

University of British Columbia

Flexible parts and tools are often found in machining operations, such as boring large cylinders, or turning long and slender shafts. The excessive flexibility of such tool or shaft may cause static deflection, forced vibration, and even chatter vibrations, which result in poor surface finish, tool breakage, and even damage to the machine, and thus become the main constraints in achieving higher productivity. This thesis presents an active damping solution to such problems, by using a novel three degrees of freedom linear magnetic actuator, which can increase the damping and stiffness of flexible structures in machining. The actuator is comprised of four identical magnetic actuating units; the magnetic force output of each actuating unit is linearized with regard to the input current by biasing magnets. Fiber optic sensors are integrated into the actuator to measure the displacements of the structure during machining. The magnetic actuator is used for three purposes: active damping of boring bar, increasing its static stiffness, and monitoring cutting forces based on the control current signals and fiber optic displacement sensor signals. The active damping is achieved by controlling the magnetic force as a function of measured vibrations. Three different types of controllers (loop shaping controller, Derivative-Integral controller, and H∞ controllers) have been developed to actively damp the displacements of a flexible boring bar during machining tests. The actuator can deliver 248 N force up to 850 Hz, and 107 N force up to 2000 Hz which is limited by the current amplifier used in the experimental setup. The cutting force is estimated through a Kalman filter, which was experimentally verified to be effective up to 550 Hz. Both the dynamic stiffness and static stiffness of the boring bar have been increased considerably with the designed magnetic actuator, leading to a significant increase in the chatter-free material removal rates. Although the proposed magnetic actuator is demonstrated for active damping of a slender boring bar in the thesis, the proposed magnetic actuator principle can be applied to suppress vibrations of rotating shafts, long boring bars and flexible structures in machine tools and other machineries.

Text

2014-12-17

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/51547

Mechanical Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/