Abstract: Look-ahead is one of the most significant functions of modern computer numerical control (CNC) systems and look-ahead algorithms are directly related to the machining efficiency and motion stability of a CNC machine tool. For decades, the look-ahead algorithm based on S-shape acceleration and deceleration (Acc/Dec) has been widely adopted, which however is too complex and time-consuming for real-time requirements. This article proposes a nested adaptive look-ahead algorithm to deal with these weaknesses. Based on the jerk symmetry in S-shape Acc/Dec, a velocity planning model is designed, and an approximation algorithm to the equivalent acceleration profile is proposed to improve model solution efficiency. Analytical solutions are given in the two main stages of velocity curve construction and checking. Taking the machining of continuous micro-segments as an example, the complete fast nested look-ahead algorithm with machining verification is provided. Machining results show that global optimal motion efficiency can be obtained with the proposed algorithm, while computation cost is less than 30% that in the traditional method. This algorithm can meet the real-time requirements to look-ahead in modern CNC systems and even in high-speed high-precision CNC systems. The analytical results given in this article are of general significance for the solution of an S-shape Acc/Dec model, and the algorithm should be applicable to look-ahead planning of continuous micro-segments, conic and even parametric curves such as non-uniform rational B-spline (NURBS).

Key words: look-ahead algorithm, speed control, acceleration/deceleration control, jerk-limited acceleration/deceleration, numerical control systems