Experimental study of contouring accuracy for CNC machines executing curved paths with constant and curvature-dependent feedrates (original) (raw)
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Journal of Intelligent and Robotic Systems, 2000
In this paper, we investigate the servo parameters and axis dynamics influences on the contouring accuracy for practical applications such as contouring control of manufacturing systems (robot, machine tool ...). The analytical formulation of contouring error in the case of straight line, circle and corner crossing is derived using a simplified axis drive model including the main servo parameters and dominating mechanical mode. The effectiveness of the proposed formulation in estimating the evolution of the final contour error is demonstrated experimentally on a two-axis machine tool.
The International Journal of Advanced Manufacturing Technology, 2019
In this paper, a new generalized parametric interpolation method is proposed for optimized five-axis machining with the consideration of both the machine contour errors and the feedrate fluctuation elimination. An analytic processing is presented for linearization of contour errors with respect to feedrate limit. For machine configuration, explicit analytical modeling of the contour error and the tracking error with respect to feedrate is presented; thus, the error constraint problem is nicely converted to a kinematic constraint problem. On this basis, an accurate feedrate upper limit with confined contour errors is further determined by using a shifted Frenet frame with linear computational complexity. With the consideration of both the motion smoothness and machining efficiency, a time-based optimization algorithm is proposed for time-optimal feedrate scheduling. For eliminating the feedrate fluctuation, a new real-time interpolation algorithm is developed for free derivation between the path parameter and the arc length for smoothed five-axis tool path generation. Laboratory testing experiments were conducted for validation and were presented in the paper. And the experimental results indicate that the proposed feedrate interpolation method is capable of confining both the tool tip contour error and tool orientation contour error simultaneously, as well as maintaining a satisfactory interpolation performance in both computation efficiency and accuracy. The presented methods can be used for five-axis machining and the feedrate optimization of complex part machining.
International Journal of Machine Tools and Manufacture, 2012
Experimental results from the implementation of a cross-coupled control scheme on a 3-axis CNC mill governed by an open-architecture software controller are presented. Unlike prior cross-coupling schemes, which depend on osculating-circle approximations to the commanded toolpath for real-time contour error estimation, the proposed scheme is based on essentially exact contour error computations for free-form curved paths. The implementation illustrates the feasibility of precise real-time contour error computation with a modest (300 MHz) CPU and a 1024 Hz sampling frequency. For a P-type controller and paths with strong curvature variation, the new scheme ensures much better diminution of contour error than earlier methods. The contour error is observed to decrease monotonically as the relative gain is increased, up to a critical gain value that incurs instability. For PI-type controllers, the comparative contour error reduction is more modest, due to their effective suppression of steady-state position error along the path.
Machining contour errors as ensembles of cutting, feeding and machine structure effects
International Journal of Machine Tools and Manufacture, 2003
CNC machining has been studied from the perspective of either cutting or feeding. However, machining quality is the outcome of both of these processes. This work investigates the contour errors of a complete CNC machine system. A system model is developed to cover all groups of functions, including trajectory planning, trajectory tracking, cutting process and machine structure. Analysis results reveal the limitations of traditional studies. The dependence of contour errors on trajectory curvature, feed-rate, cutting depth and tracking control is investigated as well. A new model of CNC machining is developed.
Computer-aided Design, 1998
An NC system that machines a curved shape at fixed depth of cut experiences time-varying cutting forces due to the 'curvature effect'-the material removal rate is higher than nominal in concave regions, and lower in convex regions. A curvaturedependent feedrate function that automatically compensates for this effect is formulated, and it is shown that, for Pythagoreanhodograph (PH) curves, the periodic real time computation of reference points in accordance with this function can be analytically reduced to a sequence of root-finding problems for simple monotone functions. Empirical results from an implementation of this variable-feedrate interpolators on an open-architecture CNC milling machine are presented and compared with results from fixed-feedrate interpolators.
CNC Motion Accuracy Analysis Under High Feedrate Values
2010
This study investigates the circular motion error in high feedrate through the Open CNC feedback signal. The motion accuracy is one of the most important factors to ensure the part quality in computerized numerical control (CNC) machine toools. There are several factors that degrade the control system performance, among then the mismatch of control loop gains, different axes dynamic response and servo lags. Regarding the CNC control system and its drives, there appears to be no specific standard tests that consider the influencing of high feedrate values on the control system performance and new tests must be developed. The objective of this study is to investigate the influence of high feedrate values on circular motion accuracy of the control system. In this study the position feedback signal of an open CNC installed on a High Speed Machining Center were measured and analyzed. The results shows that the control system studied provided good behavior for the circular deviation in hi...
Dynamic evaluation of spatial CNC contouring accuracy
Precision Engineering, 2000
An instrument capable of measuring arbitrary, dynamic CNC tool paths through three-dimensional space with micrometer-level accuracy is developed and tested. The instrument uses three Laser Ball Bars simultaneously (i.e., simultaneous trilateration) to allow for dynamic path measurements. The design of the instrument is described. The performance is verified by static repeatability testing, comparison with an independent measurement system, and comparison with the dimensions of machined parts. The instrument is demonstrated to be capable of measurement of arbitrary three dimensional tool paths with near micron level accuracy. Published by Elsevier Science Inc.
Pre-compensation of contour errors in five-axis CNC machine tools
This paper presents an analytical prediction and compensation of contouring errors in five-axis machining of splined toolpaths. The position commands are first fitted to piecewise quintic splines while respecting velocity, acceleration and jerk continuity at the spline joints. The transfer function of each servo drive is kept linear by compensating the disturbance effect of friction with a feed-forward block. Using the analytically represented five-axis, splined toolpath, splined tracking errors and kinematic model of the five-axis machine tool, contouring errors are predicted ahead of axis control loops. The contouring errors are decoupled into three linear and two rotary drives, and the position commands are modified before they are sent to servo drives for execution. The proposed method has been experimentally demonstrated to show significant improvement in the accuracy of contouring five axis toolpaths.
Neural network contour error predictor in CNC control systems
2016 21st International Conference on Methods and Models in Automation and Robotics (MMAR), 2016
This article presents a method for predicting contour error using artificial neural networks. Contour error is defined as the minimum distance between actual position and reference toolpath and is commonly used to measure machining precision of Computerized Numerically Controlled (CNC) machine tools. Offline trained Nonlinear Autoregressive networks with exogenous inputs (NARX) are used to predict following error in each axis. These values and information about toolpath geometry obtained from the interpolator are then used to compute the contour error. The method used for effective off-line training of the dynamic recurrent NARX neural networks is presented. Tests are performed that verify the contour error prediction accuracy using a biaxial CNC machine in a real-time CNC control system. The presented neural network based contour error predictor was used in a predictive feedrate optimization algorithm with constrained contour error.