New predictive techniques: control axis solutions

Abstract

Improving performances of machine-tools can be achieved with predictive control laws instead of classical PID controllers. Previous studies dealing with generalized predictive control (GPC) have proved that a better efficiency is possible, coupled to a simple implementation due to autotuning procedures. But the limitations of these techniques may be reached with strong specifications of very high speed machining. This paper presents new gradual solutions to the cascade control of CNC machines with very severe objectives. First a multirate predictive algorithm, attractive to benefit of the different dynamics of the loops, can be considered as a positive answer to an increase of rapidity, of possible use if a cascade structure is required. The second solution develops, in case of very small sampling periods, a version of GPC using the delta operator instead of the time shift operator, to avoid oversampling and sensitivity to the accuracy of the transfer function coefficients. The possibilities of these two versions are analysed on an experimental benchmark and compared to results obtained with "classical" GPC algorithms.<>