Abstract
In this paper, a voice coil motor (VCM) actuated fast tool servo (FTS) system is developed for diamond turning. To guide motions of the VCM actuator, a crossed double parallelogram flexure mechanism is selected featuring totally symmetric structure with high lateral stiffness. To facilitate the determination of the multi-physical parameters, analytical models of both electromagnetic and mechanical systems are developed. The designed FTS with balanced stroke and natural frequency is then verified through the finite element analysis. Finally, the prototype of the VCM actuated FTS is fabricated and experimentally demonstrated to achieve a stroke of ±59.02 μm and a first natural frequency of 253 Hz. By constructing a closed-loop control using proportional–integral–derivative (PID) controller with the internal-model based resonant controller, the error for tracking a harmonic trajectory with ±10 μm amplitude and 120 Hz frequency is obtained to be ±0.2 μm, demonstrating the capability of the FTS for high accuracy trajectory tracking.
Highlights
Fast tool servo (FTS) is a promoting servo axis that is widely engaged in combination with diamond turning that oscillates the diamond tool several times in each rotation of the workpiece clamped on the spindle to fabricate optical freeform surfaces, which have broadly received increasing demands in aerospace, defense, optical industries, and many more [1]
With the fast tool servo (FTS) system, it mainly consists of three subsystems, including the actuator to drive the cutting tool, the guiding mechanism to guide the motion of the actuator, and the feedback control system to guarantee the motion accuracy
An axis-symmetry cross-section view of the magnetic circuit of the designed voice coil motor (VCM) is illustrated in Figure 3, where all structural parameters are represented with lumped circuit elements
Summary
Fast tool servo (FTS) is a promoting servo axis that is widely engaged in combination with diamond turning that oscillates the diamond tool several times in each rotation of the workpiece clamped on the spindle to fabricate optical freeform surfaces, which have broadly received increasing demands in aerospace, defense, optical industries, and many more [1]. With respect to the major of commercially available VCMs, the center part is fulfilled by soft magnetic material to construct the magnetic circuit [4,5] It is convenient for the VCM design, but using VCM as actuating element in FTS there is a challenge for placing the sensors to capture the motion of the tool. The design of the flexure mechanism mainly focused on the stiffness along the desired motion direction, and the lateral stiffness was mainly ignored which is especially important for FTS turning considering the cutting force in the free space. Another issue related to the VCM actuated FTS would be the tracking accuracy of the trajectory. The resonant controller is applied to achieve a high accuracy for high-frequency trajectory tracking
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