High-resolution velocity estimation for all-digital, AC servo drives

Because the position transducers commonly used in AC servo drives (optical encoders and electromagnetic resolvers) do not inherently produce a true, instantaneous velocity measurement that can be used for full closed-loop velocity and position control, some signal processing technique is generally used to estimate the velocity at each sample instant. This estimated signal is then used as the velocity feedback signal for the velocity loop control. The authors present an analysis of the limitations of such approaches and offer an approach that optimally estimates the velocity at each sample instant. The approach is shown to offer a significant improvement in command driven systems and to reduce the effect of quantized angular resolution, which limits the ultimate performance of all digital servo drives. The noise reduction is especially relevant for AC servo drives due to the high current loop bandwidths required for their correct operation. Improved measurement performance over a classical DC tachometer is demonstrated.>