Several specific attempts have been similarly made in the elimination of contour. To provide a direct and effective elimination of deviation , orientation and tracking-lag that are main concerns in five-axis tool-path tracking control, a new tool-path control scheme was presented by Lo. The control system, consisted of a real-time transformation between the drive-coordinate and the workpiece-coordinate bases, an model for calculation of the deviation, etc.
and control laws to eliminate them. The proposed control system constructs a ‘global and coupled’ loop to achieve an effective control of the overall performance in terms of the deviation,the orientation,and the tracking lag. The deviation,etc. are components defined in the workpiece coordinate basis. In contrast, the fed back position signals (Pd) and the control signals (Ud) sent to the axial drives are both defined in the drive-coordinate basis. Coordinate transformations were introduced to the proposed control system.
The proposed tool-path control method was evaluated and compared with the conventional method on the basis of on their capability in elimination of the deviation, the orientation and the tracking-lag. The deviation was found to be significantly reduced by the proposed method for all the three surface profiles tested.
The orientation was found to be significantly reduced for the cylindrical surface and only slightly for the canonical and ruled surfaces. Tracking was, however, not significantly reduced.In a significant breakthrough to the design of axis control systems to minimise contour, Koren proposed a cross-coupled controller (CCC) for biaxial control of machine tool axes. The first such system was developed by Sarachik and Ragazzini in 1957 with a storage device f(x) that provides the desired value of y as its output for an input x. The Y-axis has a closed loop control while the X-axis has an open-loop with the Y-axis as input to the X-axis. Although this non-symmetrical system was found to be suitable for some contouring operations, it was not found to be practical under most circumstances.
In a sampled-data type CNC system, each axis is controlled independently in a loop closed by software within the computer. The axis is driven by a servomotor with an encoder feedback as shown in Fig. 12. The control program compares the two types of input namely the reference number R proportional to the required speed of the axis and the feedback signal F proportional to the actual speed of the axis. The difference represents the speed. In the CCC, the control program is fed by two reference numbers R1 and R2 proportional to the required speeds of axes 1 and 2 as shown in Fig. 13. The operation of the CCC is based on providing corrections proportional not only to the individual axial s but to the contour as well.