Real-Time Jerk Limited Feedrate Profiling and Interpolation for Linear Motor Multiaxis Machines Using NURBS Toolpaths

In this article, a NURBS toolpath feedrate profile generation algorithm for a biaxial linear motor control system is presented. High achievable velocities and accelerations of linear motor machines present new computational challenges in implementing feedrate generation and toolpath interpolation algorithms in real-time controllers. The proposed algorithm is capable of online generation of the feedrate profile with axial acceleration and jerk constraints. Each stage of the feedrate profiling algorithm is described with attention being given to both performance and implementation aspects. Furthermore, an alternative to the commonly used Taylor series interpolation method is also tested to ensure minimal interpolation errors. The feedrate profiling and interpolation algorithms’ implementation in a PC-based controller with real-time Linux kernel is described. Experimental results are presented that confirm that the algorithm is capable of limiting acceleration and jerk in the machine’s axes and it is low computation time enables real-time on-line operation in a PC-based CNC controller

A review of dynamics design methods for high-speed and high-precision CNC machine tool feed systems

With the development of CNC machine tools toward high speed and high precision, the traditional static design methods can hardly meet the demand. Hence, in this paper, the dynamics matching design methods of existing CNC machine tool feed systems were investigated and analyzed. Further, sub-system coupling mechanisms and optimization design studies were carried out for each sub-system. First, the required kinematic indexes must be achieved when designing the feed system dynamics of high-speed, high-precision CNC machine tools. Second, the CNC machine tool feed systems generally have four sub-systems: motion process, control system, motor, and mechanical structure. The coupling effect between the sub-systems should also be considered in the design. Based on the dynamics design, each sub-system should be optimized to maximize the system dynamic performance with minimum resource allocation. Finally, based on the review, future research directions within the field were detected

From computer-aided to intelligent machining: Recent advances in computer numerical control machining research

The aim of this paper is to provide an introduction and overview of recent advances in the key technologies and the supporting computerized systems, and to indicate the trend of research and development in the area of computational numerical control machining. Three main themes of recent research in CNC machining are simulation, optimization and automation, which form the key aspects of intelligent manufacturing in the digital and knowledge based manufacturing era. As the information and knowledge carrier, feature is the efficacious way to achieve intelligent manufacturing. From the regular shaped feature to freeform surface feature, the feature technology has been used in manufacturing of complex parts, such as aircraft structural parts. The authors’ latest research in intelligent machining is presented through a new concept of multi-perspective dynamic feature (MpDF), for future discussion and communication with readers of this special issue. The MpDF concept has been implemented and tested in real examples from the aerospace industry, and has the potential to make promising impact on the future research in the new paradigm of intelligent machining. The authors of this paper are the guest editors of this special issue on computational numerical control machining. The guest editors have extensive and complementary experiences in both academia and industry, gained in China, USA and UK

Reparameterization of ruled surfaces: toward generating smooth jerk-minimized toolpaths for multi-axis flank CNC milling

This paper presents a novel jerk minimization algorithm in the context of multi-axis flank CNC machining. The toolpath of the milling axis in a flank milling process, a ruled surface, is reparameterized by a B-spline function, whose control points and knot vector are unknowns in an optimization-based framework. The total jerk of the tool's motion is minimized, implying the tool is moving as smooth as possible, without changing the geometry of the given toolpath. Our initialization stage stems from measuring the ruling distance metric (RDM) of the ruled surface. We show on several examples that this initialization reliably finds close initial guesses of jerk-minimizers and is also computationally efficient. The applicability of the presented approach is illustrated by some practical case studies.RYC-2017-2264

Novel control approaches for the next generation computer numerical control (CNC) system for hybrid micro-machines

It is well-recognised that micro-machining is a key enabling technology for manufacturing high value-added 3D micro-products, such as optics, moulds/dies and biomedical implants etc. These products are usually made of a wide range of engineering materials and possess complex freeform surfaces with tight tolerance on form accuracy and surface finish.In recent years, hybrid micro-machining technology has been developed to integrate several machining processes on one platform to tackle the manufacturing challenges for the aforementioned micro-products. However, the complexity of system integration and ever increasing demand for further enhanced productivity impose great challenges on current CNC systems. This thesis develops, implements and evaluates three novel control approaches to overcome the identified three major challenges, i.e. system integration, parametric interpolation and toolpath smoothing. These new control approaches provide solid foundation for the development of next generation CNC system for hybrid micro-machines.There is a growing trend for hybrid micro-machines to integrate more functional modules. Machine developers tend to choose modules from different vendors to satisfy the performance and cost requirements. However, those modules often possess proprietary hardware and software interfaces and the lack of plug-and-play solutions lead to tremendous difficulty in system integration. This thesis proposes a novel three-layer control architecture with component-based approach for system integration. The interaction of hardware is encapsulated into software components, while the data flow among different components is standardised. This approach therefore can significantly enhance the system flexibility. It has been successfully verified through the integration of a six-axis hybrid micro-machine. Parametric curves have been proven to be the optimal toolpath representation method for machining 3D micro-products with freeform surfaces, as they can eliminate the high-frequency fluctuation of feedrate and acceleration caused by the discontinuity in the first derivatives along linear or circular segmented toolpath. The interpolation for parametric curves is essentially an optimization problem, which is extremely difficult to get the time-optimal solution. This thesis develops a novel real-time interpolator for parametric curves (RTIPC), which provides a near time-optimal solution. It limits the machine dynamics (axial velocities, axial accelerations and jerk) and contour error through feedrate lookahead and acceleration lookahead operations. Experiments show that the RTIPC can simplify the coding significantly, and achieve up to ten times productivity than the industry standard linear interpolator. Furthermore, it is as efficient as the state-of-the-art Position-Velocity-Time (PVT) interpolator, while achieving much smoother motion profiles.Despite the fact that parametric curves have huge advantage in toolpath continuity, linear segmented toolpath is still dominantly used on the factory floor due to its straightforward coding and excellent compatibility with various CNC systems. This thesis presents a new real-time global toolpath smoothing algorithm, which bridges the gap in toolpath representation for CNC systems. This approach uses a cubic B-spline to approximate a sequence of linear segments. The approximation deviation is controlled by inserting and moving new control points on the control polygon. Experiments show that the proposed approach can increase the productivity by more than three times than the standard toolpath traversing algorithm, and 40% than the state-of-the-art corner blending algorithm, while achieving excellent surface finish.Finally, some further improvements for CNC systems, such as adaptive cutting force control and on-line machining parameters adjustment with metrology, are discussed in the future work section.It is well-recognised that micro-machining is a key enabling technology for manufacturing high value-added 3D micro-products, such as optics, moulds/dies and biomedical implants etc. These products are usually made of a wide range of engineering materials and possess complex freeform surfaces with tight tolerance on form accuracy and surface finish.In recent years, hybrid micro-machining technology has been developed to integrate several machining processes on one platform to tackle the manufacturing challenges for the aforementioned micro-products. However, the complexity of system integration and ever increasing demand for further enhanced productivity impose great challenges on current CNC systems. This thesis develops, implements and evaluates three novel control approaches to overcome the identified three major challenges, i.e. system integration, parametric interpolation and toolpath smoothing. These new control approaches provide solid foundation for the development of next generation CNC system for hybrid micro-machines.There is a growing trend for hybrid micro-machines to integrate more functional modules. Machine developers tend to choose modules from different vendors to satisfy the performance and cost requirements. However, those modules often possess proprietary hardware and software interfaces and the lack of plug-and-play solutions lead to tremendous difficulty in system integration. This thesis proposes a novel three-layer control architecture with component-based approach for system integration. The interaction of hardware is encapsulated into software components, while the data flow among different components is standardised. This approach therefore can significantly enhance the system flexibility. It has been successfully verified through the integration of a six-axis hybrid micro-machine. Parametric curves have been proven to be the optimal toolpath representation method for machining 3D micro-products with freeform surfaces, as they can eliminate the high-frequency fluctuation of feedrate and acceleration caused by the discontinuity in the first derivatives along linear or circular segmented toolpath. The interpolation for parametric curves is essentially an optimization problem, which is extremely difficult to get the time-optimal solution. This thesis develops a novel real-time interpolator for parametric curves (RTIPC), which provides a near time-optimal solution. It limits the machine dynamics (axial velocities, axial accelerations and jerk) and contour error through feedrate lookahead and acceleration lookahead operations. Experiments show that the RTIPC can simplify the coding significantly, and achieve up to ten times productivity than the industry standard linear interpolator. Furthermore, it is as efficient as the state-of-the-art Position-Velocity-Time (PVT) interpolator, while achieving much smoother motion profiles.Despite the fact that parametric curves have huge advantage in toolpath continuity, linear segmented toolpath is still dominantly used on the factory floor due to its straightforward coding and excellent compatibility with various CNC systems. This thesis presents a new real-time global toolpath smoothing algorithm, which bridges the gap in toolpath representation for CNC systems. This approach uses a cubic B-spline to approximate a sequence of linear segments. The approximation deviation is controlled by inserting and moving new control points on the control polygon. Experiments show that the proposed approach can increase the productivity by more than three times than the standard toolpath traversing algorithm, and 40% than the state-of-the-art corner blending algorithm, while achieving excellent surface finish.Finally, some further improvements for CNC systems, such as adaptive cutting force control and on-line machining parameters adjustment with metrology, are discussed in the future work section

Smart machining system platform for CNC milling with the integration of a power sensor and cutting model

Novel techniques and strategies are investigated for dynamically measuring the process capability of machine tools and using this information for Smart Machine System (SMS) research. Several aspects of the system are explored including system integration, data acquisition, force and power model calibration, feedrate scheduling and tool condition monitoring. A key aspect of a SMS is its ability to provide synchronization between process measurements and model estimates. It permits real time feedback regarding the current machine tool process. This information can be used to accurately determine and keep track of model coefficients for the actual tooling and materials in use, providing both a continued improvement in model accuracy as well as a way to monitor the health of the machine and the machining process. A cutting power model is applied based on a linear tangential force model with edge effect. The robustness of the model is verified through experiments with a wide variety of cutting conditions. Results show good agreement between measured and estimated power. A test platform has been implemented for performing research on Smart Machine Systems. It uses a commercially available OAC from MDSI, geometric modeling software from Predator along with a number of modules developed at UNH. Test cases illustrate how models and sensors can be combined to select machining conditions that will produce a good part on the first try. On-line calibration allows the SMS to fine tune model coefficients, which can then be used to improve production efficiency as the machine learns its own capabilities. With force measurements, the force model can be calibrated and resultant force predictions can be performed. A feedrate selection planner has been created to choose the fastest possible feedrates subject to constraints which are related to part quality, tool health and machine tool capabilities. Monitoring tangential model coefficients is shown to be more useful than monitoring power ratio for tool condition monitoring. As the model coefficients are independent of the cutting geometry, their changes are more promising, in that KTC will increase with edge chipping and breakage, while KTE will increase as the flank wearland expands

Modelling and optimization of multi-axis machining process considering CNC motion limitations

The importance of multi-axis machining processes is increased over the years, especially for industries such as automotive, aerospace, dies and molds, biomedical where the parts have complex surfaces. As the demand for products is increased from these industries, it became crucial to minimize the cycle time to overcome the demand and also reduce the production costs while maintaining or enhancing the part quality. In order to achieve this the dimensional tolerances and a desired surface quality should be inside the acceptance limit while increasing productivity. The properties of the machine tool such as its own structure, axis drives, drivetrain, axis control limits and axis motor maximum capabilities can be regarded as boundary conditions of the process. The limits for the drives cannot be used at full capacity constantly as the machining process is a highly variable and flexible operation. For instance, sharp maneuvers on the tool path may not be realized at high feedrate values. In some cases, the required motion exceeds the motion capability of the axis drives, i.e. jerk, acceleration and velocity limitations. In those cases, the CNC unit slows down the motion to synchronize machine axes to keep up within geometrical limits of the required tool path. On the other hand, sometimes the commanded feed rate may not be achieved at some instances of a cycle involving short distances due to limited jerk and acceleration of the axes. These problems reduce the productivity of the operation as well as the quality of the final product. This thesis presents a new feed-rate optimization algorithm which re-adjusts the rotary axis motions to stay in the acceleration and jerk limits as well as to obtain a better surface quality for the final product in multi-axis machining. All measured velocity, acceleration and jerk limits are given to the algorithm to re-calculate the tool axis vector, such as lead and tilt angles, for minimizing the cycle time and enhancing part surface quality. As the current studies do not rely on the drive limits for choosing the tool orientation in multi-axis machining, for the first time, the algorithm represented in this thesis optimizes the tool’s lead and tilt angles at each Cutter Location (CL) point. The technique used in the study optimizes the tool orientation vector for minimizing the cycle time by observing the acceleration and jerk limits of the axis drives of the machine tool. The unnecessary motions between CL points generated by commercial software can be eliminated by the algorithm and this increases the productivity of the process. The feasibility of the algorithm and the models in this thesis is presented on an industrial part geometry where the productivity and machined surface quality improvements are demonstrated

Tool path generation for milling of free form surfaces with feed rate scheduling

Upotreba slobodnih (skulptorskih) površina u procesu projektovanja proizvoda raste po eksponencijalnom nivou kako iz funkcionalnih tako i iz estetskih razloga. U procesu projektovanja i izrade slobodnih površina neizostavna je upotreba CAD/CAM softvera. Dok su geometrijski aspekti projektovanja relativno dobro pokriveni, problemi i dalje ostaju kada je u pitanju stvarna proizvodnja slobodnih površina. Glavni problemi su povezani sa određivanjem odgovarajuće putanje alata koja bi obezbedila zahtevan kvalitet obrađene površine, minimizaciju ukupnog vremena obrade, kontrolu intenziteta sile rezanja itd. U radu je prikazan algoritam za generisanje putanje alata zasnovan na kriterijumu održanja sile rezanja na konstantnu unapred definisanu vrednost za procese 3-osne obrade loptastim glodalom. U tu svrhu je razvijen model za predikciju sile rezanja koji je uključen u algoritam za generisanje putanje alata i softver koji je kompatibilan sa svim CAD/CAM sistemima. Eksperimentalno je potvđeno da predloženi algoritam ima brojne prednosti u odnosu na strategije obrade komercijalnih CAD/CAM softvera.The use of freeform (sculptured) surfaces in the product design process is accelerating at an exponential rate driven by functional as well as esthetics demands. CAD/CAM software is a must in their design and manufacture. While the geometric aspects of the design are relatively wellcovered, issues still remain when it comes to the actual manufacture of freeform surfaces. The major issues are related to the generation of the proper toolpaths that would assure the required surface quality, the minimization of the total maching time, the control of the magnitude of the cutting forces, etc. This paper presents an algotithmic procedure for tool path generation based on the criterion of maintaing the cutting forces at a constant pre-defined level for 3-axis ball end milling processes. To this end, a model for cutting force prediction is formulated and incorporated into the tolpath generation algorithm and software that is compatible with all CAD/CAM systems. It has been experimentally confirmed that the proposed algorithm offers a number advantages over the machining strategies used in commercial CAD/CAM software

Tool path generation for milling of free form surfaces with feed rate scheduling

Upotreba slobodnih (skulptorskih) površina u procesu projektovanja proizvoda raste po eksponencijalnom nivou kako iz funkcionalnih tako i iz estetskih razloga. U procesu projektovanja i izrade slobodnih površina neizostavna je upotreba CAD/CAM softvera. Dok su geometrijski aspekti projektovanja relativno dobro pokriveni, problemi i dalje ostaju kada je u pitanju stvarna proizvodnja slobodnih površina. Glavni problemi su povezani sa određivanjem odgovarajuće putanje alata koja bi obezbedila zahtevan kvalitet obrađene površine, minimizaciju ukupnog vremena obrade, kontrolu intenziteta sile rezanja itd. U radu je prikazan algoritam za generisanje putanje alata zasnovan na kriterijumu održanja sile rezanja na konstantnu unapred definisanu vrednost za procese 3-osne obrade loptastim glodalom. U tu svrhu je razvijen model za predikciju sile rezanja koji je uključen u algoritam za generisanje putanje alata i softver koji je kompatibilan sa svim CAD/CAM sistemima. Eksperimentalno je potvđeno da predloženi algoritam ima brojne prednosti u odnosu na strategije obrade komercijalnih CAD/CAM softvera.The use of freeform (sculptured) surfaces in the product design process is accelerating at an exponential rate driven by functional as well as esthetics demands. CAD/CAM software is a must in their design and manufacture. While the geometric aspects of the design are relatively wellcovered, issues still remain when it comes to the actual manufacture of freeform surfaces. The major issues are related to the generation of the proper toolpaths that would assure the required surface quality, the minimization of the total maching time, the control of the magnitude of the cutting forces, etc. This paper presents an algotithmic procedure for tool path generation based on the criterion of maintaing the cutting forces at a constant pre-defined level for 3-axis ball end milling processes. To this end, a model for cutting force prediction is formulated and incorporated into the tolpath generation algorithm and software that is compatible with all CAD/CAM systems. It has been experimentally confirmed that the proposed algorithm offers a number advantages over the machining strategies used in commercial CAD/CAM software

Toolpath interpolation and smoothing for computer numerical control machining of freeform surfaces : a review

Driven by the ever increasing demand in function integration, more and more next generation high value-added products, such as head-up displays, solar concentrators and intra-ocular-lens, etc., are designed to possess freeform (i.e., non-rotational symmetric) surfaces. The toolpath, composed of high density of short linear and circular segments, is generally used in computer numerical control (CNC) systems to machine those products. However, the discontinuity between toolpath segments leads to high-frequency fluctuation of feedrate and acceleration, which will decrease the machining efficiency and product surface finish. Driven by the ever-increasing need for high-speed high-precision machining of those products, many novel toolpath interpolation and smoothing approaches have been proposed in both academia and industry, aiming to alleviate the issues caused by the conventional toolpath representation and interpolation methods. This paper provides a comprehensive review of the state-of-the-art toolpath interpolation and smoothing approaches with systematic classifications. The advantages and disadvantages of these approaches are discussed. Possible future research directions are also offered