Spindle rotary position estimation for fast tool servo trajectory generation

In diamond turning of non-rotationally symmetric surfaces, fast tool servo (FTS) trajectory is generated in real time based on the spindle rotation angle. Essentially, a digital rotary encoder updates its output (samples the spindle angle) at fixed rotary angle interval. In time domain, the encoder sampling frequency is different from that of the FTS control computer. Without proper spindle position estimation, this asynchronous nature between the encoder spatial sampling and the computer temporal sampling can cause significant amount of aliasing noise in the generated trajectory. In this paper, we first analyze this aliasing phenomenon using discrete-time signal processing theories. Based on this analysis, a spindle rotary position estimator is designed to improve both accuracy and resolution of the spindle angle feedback. Its effectiveness and the associated trajectory generation are experimentally verified in diamond turning of sinusoidal surfaces with ultra-FTS.