A Cascaded Adaptive Controller for CNC Lathe Spindle Drives

In modern computer numerically controlled (CNC) lathes the spindle drives often have an infinitely variable speed range to allow constant horsepower cutting. Such spindle drives are typically powered by direct current (dc) servo motors which are conventionally controlled by an analog speed regulator in the form of pseudo-derivative feedback (PDF). The PDF controller contains an integral action to compensate for steady state cutting torques and frictional loads on the spindle. The inclusion of the integrator improves the static servo stiffness (steady state load disturbance rejection capability). However, the tansient response characteristics may still vary considerably as a result of any changes in the spindle operating speed for an otherwise constant set of cutting conditions. To improve the consistency of the transient response an alternative control approach is presented which includes a cascaded model reference adaptive controller (MRAC) as an inner loop. The controller uses a robust on-line estimator to determine any changes in the drive's parameters resulting from the changes in the cutting conditions. The adaptive controller is presented in discrete time, and comparative simulation results are provided for a tapering operation where the cutting torque changes continuously.