Enhanced Servo-Control Performance of Dual-Mass Systems

This paper provides systematic analysis and controller design methods for high-performance two-mass servo drives with an appraisal of the effects of supplementary filtering elements associated with practical systems. Implementation issues and the resulting performance achievable from proportionalintegral, proportional-integral-derivative (PID), and resonance ratio control (RRC) controllers with regard to both closed-loop robustness and control of the process variable (load velocity), in response to a step reference speed or load-side disturbance, are presented. It is shown that the high-frequency gain of the controllers is a critical design variable for determining the resulting robustness of the closed-loop system when subject to unmodeled resonant modes, high-frequency noise from the derivative of quantized sensor signals, and process perturbations, and is strongly influenced by the location and number of filters present in the various feedback loops and, importantly, the ratio of their time constants. A complete design methodology is also presented to assign the time constants of the various loop filters, and their location, using a single user-definable variable, thereby reducing the time-consuming trial-and-error approach commonly employed using conventional tuning procedures. The technique employs both time- and frequency-domain design tools to address the conflicting requirements of robustness and control performance (overshoot, bandwidth, etc.). It is also shown that, since the PID and RRC controllers are closely related, they are theoretically able to impart identical closed-loop input-output dynamics. However, by virtue of the different feedback mechanisms employed, RRC is shown to provide superior closed-loop robustness. This paper demonstrates, and practically validates, the proposed techniques by showing significant performance enhancements from a commercial off-the-shelf servo-drive test platform.