Precision-limit positioning of direct drive systems with the existence of friction

摘要

This paper discusses the issue of precision-limit positioning of a direct drive system with the existence of friction. By precision-limit positioning it means that, in addition to the common sense of sub-micro, nano, or pico precision, the precision error equals the sensor resolution. This means a zero-count precision error when a digital type position sensor is used in the feedback loop. Precision-limit positioning also enforces 100% repeatability as precision is concerned. Of course, this is the limit of the precision that a real control system can achieve. In the past, this kind of performance was not considered possible mainly for lacking accurate and enough information about friction, especially the static friction. However, as more and more knowledge of the pre-sliding motion has been revealed, it is shown in this research that it is possible to achieve such kind of performance. It also shows that, from the dynamic point of view, to achieve precision limit an integral type control should be included in the final stage of positioning. A simple and explicit criterion is given for the integral control gain to guarantee asymptotic stability. Some practical issues of a digital precision-limit positioning system are also addressed.