Research On Local Smoothing And Contour Control Strategy Of Multi-Axis CNC Machining Trajectory

Complex surfaces have wild applications in aerospace,shipbuilding,transportation,mold,energy,astronomy,and biomedical fields.Multi-axis CNC(Computer-Numerical-Control)machining,as the main manufacturing method for complex surface parts,is constantly developing towards the trend of high speed and high precision.The improvement of this manufacturing technology level is of great significance to the development of the national economy and the modernization of national defense.As one of the main command path forms for multi-axis(three-axis and five-axis)simultaneous CNC machining of complex surface parts,the linear tool path is first-order discontinuous at the corners.This leads to poor acceleration and deceleration performance of the tool tip of CNC machining equipment(CNC machine tools and industrial robots)during multi-constrained coupled feed rate scheduling,which is easy to cause shock and vibration,thereby reducing machining efficiency and surface machining quality.Additionally,due to the limited bandwidth of the multi-axis CNC servo feed system and the mismatch of dynamic characteristics between axes,there is a deviation(i.e.,contour error)between the actual motion position of the tool and the commanded motion path,which is more obvious in the process of high-speed machining,and then affects the machining accuracy of parts.The above two problems have become the bottleneck of multi-axis linkage high-speed and high-precision CNC machining,which urgently needs to be solved to improve the level of the national manufacturing technology.Therefore,aiming at improving the high-speed and high-precision ability of multi-axis linkage NC machining,this paper carries out the following research:Aiming at the problems of low continuity of linear tool path and multi constraint coupling in feed rate scheduling,the research on the local smoothing theory of linear tool path and trajectory feed rate scheduling method of multi-axis CNC machining is carried out.Aiming at the problem of significant contour error caused by the mismatch of dynamic characteristics of multi-axis CNC servo control system,the research on multi-axis linkage contour error estimation and compensation technology is carried out.The specific research contents are as follows:(1)The research on local smoothing of three-axis machining trajectory and its look-ahead interpolation is carried out.Firstly,sufficient conditions for the three-axis machining trajectory to be C~2 continuous concerning time are derived.Accordingly,from the perspective of the geometric path,a local smoothed path with a cubic B-spline is constructed.On the premise of meeting the preset smoothing tolerance,the curvature continuity of the smoothed path and the analytical expression of the curvature extreme value are realized by analytically calculating the coordinates of control points of the B-spline.From the perspective of trajectory motion,an improved bi-directional scanning algorithm of feed rate curve and iterative S-shaped feed rate scheduling method are proposed.This method takes into account the chord error and kinematic constraints,analytically realizes the continuity of feed speed/acceleration curve,and reduces the trajectory residue caused by interpolation cycle rounding.It has the advantages of strong real-time and high-order smoothing of command.Then,combined with the above-mentioned methods,a look-ahead interpolation strategy for a three-axis machining linear path is proposed,which is suitable for real-time processing of a large number of code segments,including an NC interpreter module,a real-time look-ahead module,and an interpolation point calculation module.Finally,the simulation and experiment are carried out on the three-axis CNC machining platform.The results show that the proposed local smoothing and look-ahead interpolation algorithm can not only control the smoothing error but also improve the rough machining efficiency of complex surfaces by more than30%compared with the linear trajectory of traditional three-axis machining.(2)The research on decoupling local smoothing interpolation of five-axis machining trajectory is carried out.Firstly,sufficient conditions for the C~2 continuity concerning the time of the five-axis machining trajectory in the workpiece coordinate system are derived.Then,at the geometric path level,a decoupled local smoothing algorithm for five-axis machining linear paths is proposed.Two decoupled cubic B-splines are constructed respectively to complete the C~2 continuous local smoothing of the tool tip path and tool orientation path,ensuring that the smoothing error is analytically controllable,and the analytical optimal curvature extreme value is obtained.Secondly,at the trajectory level,a time-synchronized feed rate scheduling method based on finite impulse response(FIR)filters is proposed.This method adaptively adjusts the time parameters of FIR filter,taking into account the tangential/normal kinematic constraints of tool tip trajectory/tool orientation sub trajectory and the kinematic constraints of the machine tool drive axis,realizes the motion time synchronization of tool tip/tool axis vector sub trajectory,and ensures the continuous motion speed/acceleration of each axis of machine tool,so as to obtain the high-performance five axis machine tool motion.Finally,the machining simulation and experimental results of the five-axis machine tool show that the proposed five-axis machining trajectory decoupling local smoothing interpolation method is suitable for a variety of linear path segments with different lengths.On the premise of controllable smoothing error,compared with G01 interpolation,it can improve the finishing efficiency of the complex surface by more than 14%and obtain a smoother machining surface.(3)The research on the local smoothing of the machining trajectory of the de-redundant robot is carried out.For the de-redundant six-axis universal robot,the transformation between the de-redundant robot’s tool poses and the cutter location data in the workpiece coordinate system is established.Then,the proposed five-axis linear path decoupling local smoothing algorithm is extended,and a de-redundant robot machining path decoupling local smoothing method is proposed.In the workpiece coordinate system,two decoupled quintic B-splines are constructed by analytical calculation in this method,which constraints the smoothing error intuitively and accurately,so that the C~3 continuous local corner smoothing of the tool tip path/tool orientation path concerning the arc length parameter/radian parameter is realized.Secondly,a time-optimized synchronization feed rate scheduling method based on a fourth-order FIR filter is proposed.Taking into account various coupling kinematic constraints,the high-efficiency and smooth trajectory motion of the robot is obtained,with continuous jerk curves of each joint.Finally,simulation and experiments are carried out on the robot processing platform.The results show that compared with the robot processing trajectory obtained by the point to point path,the trajectory after the local smoothing can reduce the acceleration/jerk during the movement process by about 30%,and improve the processing efficiency by about 5%,while the final machined surface roughness is reduced by about 15%.(4)The research on online estimation and compensation of multi-axis local smoothing trajectory contour error is carried out.Firstly,for the mixed trajectory composed of straight line and parametric curve after local smoothing,an online estimation strategy of contour error based on motion time synchronization is proposed.This strategy searches for the closest point to the actual tool tip point in the commanded interpolation path segment as the tool tip contour point.And based on time synchronization,the tool orientation contour vector matching the tool tip contour point is found.Fast and high-precision estimation of contour errors in multi-axis machining is achieved by the obtained tool tip contour points and tool orientation contour vectors.Secondly,the corresponding contour error proportional compensators are designed in the position loop,velocity loop,and current loop of each axis servo controller respectively.Feedback compensation is performed after decoupling the obtained multi-axis trajectory contour error estimates.The stability of the control system is analyzed,and the determination method of proportional compensation parameters is given.Finally,the simulation and experimental results of the multi-axis control platform show that for the multi-axis local smoothing trajectory with parameter synchronization and time synchronization,the proposed online estimation and compensation method of contour error can effectively suppress the contour error,which has good universality.