A New Type Of CNC System And Its Interpolator Based On Digitizer/Player Architecture

The computer numerical control(CNC)system carries out important tasks such as automatic control and human-machine interaction for CNC machines.It becomes more and more complex and heavy with the growing demands and the advances of hardware technology.Meanwhile,the number and computational loads of online tasks during machining are also increasing.The conflict between the demands and the real-time capability of the CNC system,which represents the ability to finish online tasks in a critical period of time,becomes increasingly prominent.As the tool for converting geometrical descriptions of paths into motor position commands,the interpolator in the CNC system generates the relative movement between the tool and the workpiece which determines the machining accuracy,surface finish and the life cycle of the machine.The CNC system which uses parametric curves to represent the tool paths costs less memory,offers more geometric features,results in lower impact of the motion and obtains better surface finish than that which uses the short line segments.In this dissertation,according to the abovementioned background,solutions are given to address the problems arising from the existing CNC systems and their interpolators.The innovations are presented as follows:First,to solve the conflict between the growing functional requirements and the real-time performance,a new type of CNC system based on the digitizer/player architecture is proposed.By using the proposed system,the real-time computational load can be reduced.The cooperation between multiple machines can be achieved by connecting a server with several light-weighted customers in a distributed NC system.Second,the feedrate fluctuation problem,which is caused by computational errors during curve parameter computations in the curve interpolation,is studied.The augmented Taylor's expansion(ATE)method,which can be used in the existing parametric curve interpolators,is proposed and compared with existing curve parameter computation methods.The ATE method is able to obtain an accurate parameter in each step in a short time similar to the second-order Taylor's expansion method with less memory,and to avoid the unexpected halt of machining due to an out-of-domain curve parameter.Third,to interpolate different types of curves in a unified manner in an accurate speed,an interpolator using the digitizer/player is developed.Based on the augmented Taylor's expansion method,the parametric curves are converted into a sequence of proximity points and the differences between adjacent proximity points are encoded as motion bitstreams offline.The bitstreams are decoded to generate motor position commands during machining.The interpolation of a curve containing cusps with zero derivatives can be carried out.The feedrate fluctuation due to real-time curve parameter computations is avoided.The commanded feedrate can be adjusted during machining.The interpolations of different types of curves are unified and therefore the real-time interpolator is simplified.Fourth,to overcome the problem of inaccurate computations of the velocity limits around tiny corners by existing velocity limit profile generation methods,the unit arc length increment scanning method is proposed to avoid unnecessary accelerations and decelerations to reduce the time of movement and improve the efficiency.Finally,to interpolate parallel curves with synchronized arc lengths for multi-axis EDM,the digitizer/player is generalized to support the interpolation of parallel curves in both forward and backward directions.A CNC system is built on an industrial personal computer and a motion controller for a 6-axis electrical discharge machine and the feasibility of the digitizer/player architecture is validated by the machining of the channels of a shrouded blisk which is a key component in the aerospace engines.