Dual-stage fast tool servo cascading a primary normal-stressed electromagnetic stage with a secondary piezo-actuated stage

The dual-stage actuation effectively extends the dynamic response and tracking accuracy of conventional fast tool servos (FTS). In this study, a normal-stressed electromagnetic stage is deliberately designed as the primary stage to cascade with a secondary piezo-actuated stage, constructing a new medium-range dual-stage FTS. Analytical models for both the normal-stressed actuator and the flexure mechanisms are established. The mechanical and electromagnetic parameters are selected and verified by finite element simulation, targeting the desired motion stroke and the maximized natural frequency. The prototype using the designed parameters is tested to have an actual stroke of 80 & mu;m and a first resonant frequency around 814 Hz. The dual-stage FTS employs a decoupled control strategy to track the desired trajectory precisely. As for the main controller, an additional disturbance observer is designed to work with the typical PID controller to compensate for both the internal nonlinear dynamics and external disturbances for each stage. The practical result demonstrates that the closed-loop bandwidth can be improved from 220 Hz to 500 Hz by cascading the secondary stage. As for the tracking of a complex trajectory with multiple frequencies, a motion error of around & PLUSMN;0.8 & mu;m was achieved, which is less than 30% of that obtained by the primary stage alone.