Computer Numerical Control System Research Articles

From Open CNC Systems to Cyber-Physical Machine Tools: A Case Study

The aim of next-generation Computer Numerical Control (CNC) is shifting from an open architecture, which has better flexibility, adaptability, versatility and expansibility, to a cyber-physical model, which offers real-time monitoring and control of the machining processes. This paper introduces a real case study to demonstrate such tendency from Open CNC systems to Cyber-Physical Machine Tools (CPMT) based on a low-power embedded platform. Firstly, a new open CNC architecture is presented, which is able to achieve high-precision, high-efficiency, and low-power consumption. Secondly, the open CNC architecture is extended to a CPMT by using Wireless Sensor Networks (WSN), where WSN is utilized to enable monitor and control the machining processes, and the integrated development platform is termed as CPMT. Finally, a case of health monitoring system for CPMT is designed and its system testing is carried out.

Communication methodology between machine tools using MTConnect protocol

Development of smart manufacturing led to a trend of developing autonomous, accessible and cooperative numerical control equipment, which needs a communication subsystem that follows a universal accepted industrial standard. MTConnect has been widely considered as an efficient and sufficient protocol for data collection since its appearance. In this paper, a networked information exchange methodology between computer numerical control systems based on MTConnect technology is proposed, which includes the theoretical architecture of the information exchange module and the implementation of the module based on a CNC systems developed on the TwinCAT platform. A communication system according to the MTConnect protocol for CNC systems was designed, which is able to collect processing data from controllers and transmits those data between controllers by agent and client embedded in the information exchange module. The agent and client which were defined in the protocol were developed through C# language and MTConnect Managed SDK. The proposed methodology was implemented and tested on actual CNC system.

Electromagnetic pulse-assisted incremental drawing forming of aluminum alloy cylindrical part and its control strategy

This paper explores a compound forming method of the cylindrical drawing by using the servo press forming combined with the electromagnetic pulse-assisted incremental drawing. The method comprises four steps: the punch press pre-drawing, electromagnetic pulse-assisted incremental drawing, punch shaping, and the final continuous punch drawing. By applying the four steps sequentially, the good drawing performance was achieved, and the maximum drawing height was of 3.4 times compared with that of the conventional punch drawing process, without changing the diameter of the punch. During the electromagnetic pulse-assisted incremental drawing process, the drawing coils, the corner coils, and the upper and bottom auxiliary coils generate the electromagnetic pulse force to help achieve the drawing of the metal sheet. The electromagnetic pulse-assisted incremental drawing step and the punch press deforming step were alternately carried out. In the electromagnetic pulse drawing step, by applying a two-time discharge using 13 and 14 kV in turn, the height of the drawing was improved, and the thinning rate of sheet metal was reduced. The application of the servo press and the computer numerical control system in the electromagnetic pulse-assisted incremental drawing process can effectively improve the processing efficiency and provides technical basis for the intelligent control of the electromagnetic pulse-assisted incremental drawing process.

An optimal curvature-smooth transition algorithm with axis jerk limitations along linear segments

Linear toolpath is generated by commercial computer-aided manufacturing (CAM) systems and it is most popular in computer numerical control (CNC) systems. Considering the discontinuity between two neighbor linear segments, corner feedrate is restricted and axis accelerations and jerks always exceed the given limitations, leading to low efficiency and poor accuracy, respectively. To overcome these two drawbacks, a novel curvature-smooth optimal transition algorithm and a jerk-continuous feedrate-scheduling scheme with axis jerk limitations are proposed in this paper. Firstly, based on the theoretical feedrate constraints with bounded axis accelerations and jerks, a quintic B-spline curve is adopted to generate curvature-smooth toolpath. To improve machining efficiency, corner feedrate is regarded as the optimal objective to determine the transition B-spline curve. Afterwards, considering the curvature-smooth toolpath, a corresponding five-phase jerk-continuous feedrate profile is provided to achieve higher machining precision. Finally, compared with curvature-continuous toolpath in simulations and experiments, the proposed algorithms can bound the axis kinematic parameters as expected and have advantages on improving machining precision especially under high kinematic limitations.

Synergistic real-time compensation of tracking and contouring errors for precise parametric curved contour following systems

The tracking and contouring errors are inevitable in real computer numerical control contour following because of the reasons such as servo delay and dynamics mismatch. In order to improve the motion accuracy, this paper proposes a synergistic real-time compensation method of tracking and contouring errors for precise parametric curve following of the computer numerical control systems. The tracking error for each individual axis is first compensated, by using the feed-drive models with the consideration of model uncertainties, to enhance the tracking performances of all axes. Further, the contouring error is estimated and compensated to improve the contour accuracy directly, where a high-precision contouring-error estimation algorithm, based on spatial circular approximation of the desired contour neighboring the actual motion position, is presented. Considering that the system structure is coupled after compensation, the stability of the coupled system is analyzed for design of the synergistic compensator. Innovative contributions of this study are that not only the contouring-error can be estimated with a high precision in real time, but also the tracking and contouring performances can be simultaneously improved although there exist modeling errors and disturbances. Simulation and experimental tests demonstrate the effectiveness and advantages of the proposed method.

Servo-controlling structure of five-axis CNC system for real-time NURBS interpolating

NURBS (Non-Uniform Rational B-Spline) is widely used in CAD/CAM (Computer-Aided Design / Computer-Aided Manufacturing) to represent sculptured curves or surfaces. In this paper, we develop a 5-axis NURBS real-time interpolator and realize it in our developing CNC(Computer Numerical Control) system. At first, we use two NURBS curves to represent tool-tip and tool-axis path respectively. According to feedrate and Taylor series extension, servo-controlling signals of 5 axes are obtained for each interpolating cycle. Then, generation procedure of NC(Numerical Control) code with the presented method is introduced and the method how to integrate the interpolator into our developing CNC system is given. And also, the servo-controlling structure of the CNC system is introduced. Through the illustration, it has been indicated that the proposed method can enhance the machining accuracy and the spline interpolator is feasible for 5-axis CNC system.

A new high-performance open CNC system and its energy-aware scheduling algorithm

Computer numerical control (CNC) systems are shifting to a direction of open architecture which has better flexibility, adaptability, versatility, and expansibility. Existing CNC systems tend to have a high level energy consumption. This paper introduces a new open CNC system based on the low-power embedded platform, named open and high-performance CNC (OHP-CNC). OHP-CNC is able to achieve high precision, high efficiency, and low power consumption by making use of international standards, open components such as hardware and software, and an energy-aware real-time scheduling algorithm. The proposed algorithm for mixed tasks, including periodic and aperiodic tasks, is divided into two phases. Firstly, the slack time and utilization are calculated on each processor and tasks are assigned to the processor according to the load. Secondly, because there is a trade-off between the energy-saving and the response times of the aperiodic task, the scheduling server is used to schedule aperiodic tasks in order to meet the response time constraints of aperiodic tasks. Meanwhile, periodic tasks recycle the slack time with dynamic voltage scaling technology to achieve low power consumption. Experiment results show that the energy-aware real-time algorithm yields high-performance and effective machining processes.

Parallel acceleration/deceleration feedrate scheduling for computer numerical control machine tools based on bi-directional scanning technique

The acceleration/deceleration feedrate scheduling is one of the most important techniques in computer numerical control systems. Along with this technique, the bi-directional scanning technique is always employed. The bi-directional scanning technique consists of a backward scanning process followed by a forward scanning process. The two scanning processes in the conventional methods are executed in a serial manner by scanning through all the scheduling blocks one by one. Consequently, the feedrate scheduling will suffer from a heavy computational burden when there are massive blocks to be scanned, which deteriorates its real-time performance for computer numerical control machining. To alleviate the computational burden, a parallel acceleration/deceleration feedrate scheduling approach is proposed in this article. With this method, the scheduling blocks are splitted into several scheduling units and the feedrate for each of them is scheduled simultaneously. The feasibility of the proposed approach is validated through the feedrate scheduling for two widely used butterfly and helix paths. For a constructed example of feedrate scheduling, a significant acceleration ratio about 3.7 on a personal computer with a quad-core central processing unit is achieved.

Performance of an ultrasonic-assisted drilling module

The ultrasonic-assisted drilling (UAD) process is a technology which applies high frequency vibrations (>20 kHz) at amplitudes of 2–20 μm in the feed direction to the drill tip in order to improve machining conditions and productivity. Past work in this field is reviewed, noting the valuable foundation provided by laboratory-based research systems. This work is aimed at the development of a robust UAD module, fully compatible with computer numerical control (CNC) systems, and able to function in a production environment, describing results of drilling tests on 4340 steel. The UAD module, designed using FEA methods, with a 16-mm carbide drill, attached by shrink fit, is used to study the effects of ultrasonics on thrust force, torque, surface quality, hole size, and the microstructure of drilled 4340 steel. In order to provide a better understanding of the performance of this module, two different test scenarios were designed and the results for each were compared with the results obtained from baseline settings for conventional drilling. The results show that ultrasonic vibrations not only can reduce the average values of thrust force and torque but also can improve the surface quality of the hole. It is also found that a major increase in productivity could be achieved by using ultrasonic vibrations. Finally, the effects of ultrasonic vibrations on hardness and microstructure of the drilled hole is assessed and the results show that high frequency vibrations do not have significant effect on dimensional accuracy or on the microstructure of the drilled holes.

A universal velocity limit curve generator considering abnormal tool path geometry for CNC machine tools

Non-linear parametric curves, such as B-spline curves, are becoming increasingly available in modern CNC (Computer Numerical Control) systems. The smoothness of parametric curves offers higher order of continuity and thus invokes less vibration in machines as compared to short line segments. Nevertheless, the computations for velocity limit curves, velocity profiles and interpolation points are quite complicated and time-consuming and therefore approximation methods are applied. Unnecessary accelerations and decelerations, which cost additional time of motion, can be caused by inaccurate computations of the velocity limits around tiny corners. To overcome the problem, the unit arc length increment scanning method (UALISM) is proposed to reduce the time of movement and to improve the efficiency. The scanning interval is fixed at 1 BLU (basic length unit) which is irrelevant to the type, size and shape of the tool path curve. The constraints of chord height errors and axis accelerations are considered and the velocity limit for the specified scanning point is computed using the coordinates of multiple scanning points near the specified scanning point. Simulation results show that the unnecessary accelerations and decelerations can be avoided and thus the total motion time can be reduced by UALISM.

A locally optimal transition method with analytical calculation of transition length for computer numerical control machining of short line segments

Line segments, or G01 codes, generated by computer-aided manufacturing softwares are the most widely used toolpath format for computer numerical control systems. The linear toolpath normally consists of thousands of short line segments due to the high-accuracy requirement of the machined parts. Due to the tangential and the curvature discontinuities at the junction of two segments, the feedrates at the start and the end points of line segment have to be slowed down. In order to increase the feedrates along short line segments, a locally optimal transition method is proposed, which uses a two-step strategy to generate a blended toolpath composed of cubic Bezier curves and line segments. In the first step, the optimal proportional coefficient is represented as the function of the angle between two adjacent line segments, which can be employed to minimize the curvature variation energy of the cubic Bezier curve. In the second step, the local optimization model with the aim to minimize the sum of two curvature extrema is established to determine the optimal transition length. The transition length can be analytically obtained under the constraint of the approximation error and the constraints of the line segment lengths. The simulation and experiment results demonstrate that, by comparison with the conventional transition methods, the proposed method can significantly decrease the machining time but does not increase the contouring error.

Research on surface morphology of the ruled surface in five-axis flank milling

Due to the complexity of ruled surface blade in five-axis flank milling and the limited interpolation capacity of Computer Numerical Control system, the continuous tool location trajectory can only be approximated as a combination of a series of micro linear segments in flank milling process. As a result, the smooth ruled surface is decomposed into many micro planes. In this paper, the generation principle of surface morphology for the micro plane is studied without considering the tool vibration, the workpiece vibration, and shape and position errors of the tool in the milling. In the surface morphology simulation of each micro plane, a fixed plane I which is stationary and perpendicular to the micro plane is firstly defined, then the intersection distribution between each cutter tooth and the fixed plane I is studied when the cutter has no speed in the direction that is vertical to the tool axis, and then the intersection distribution can turn to the groove lines of surface morphology when the speed vertical to the cutter axis exists. For the other part of morphology besides groove lines, tool’s trochoid motion along the workpiece is approximated as arc motion, so the ridges and grooves are taken shape combined with the entry angle and the exit angle of the tool. The simulation result proves the correctness of the model.

Self-adaptive stress accelerated life testing scheme

The initial stress of step-up stress accelerated life tests is commonly too low which is not very efficient when used in accelerated life testing. Though the efficiency of step-down stress accelerated life tests has improved in general, researchers and engineers may have to deal with the failure mechanism changing due to the high starting stress. In this paper, we develop a new accelerated life test scheme, called self-adaptive stress accelerated life tests (SAS-ALT), with considerations of the relative stress loading criterion. We also develop algorithm steps for implementation of SAS-ALT. We formulate and discuss an optimization function of the proposed SAS-ALT that minimizes the variance of the system life time using the maximum-likelihood method, Fisher matrix, and accelerated model. Some factors of the SAS-ALT, such as stress and failure number, were chosen depending upon the optimum function. We also discuss an application of the computer numerical control system based on real failure data to illustrate step-by-step reliability accelerated life test design policy based on our proposed SAS-ALT scheme.

Stock of downstream complementary assets as a catalyst for product innovation during technological change in the U.S. machine tool industry

Research summary: We investigate the effect of incumbents' stock of downstream complementary assets on their product innovation during a disruptive technological change. We theorize that a firm's stock of downstream complementary assets, by providing critical information about shifting demand conditions, will play a catalytic role in firm adaptation during such a change. Using the advent of disruptive computer numerical control machine tools in the U.S. machine tool industry during the 1970s and 1980s as the context, we find that firms with greater stocks of downstream complementary assets are likely to be product innovation leaders during such a change.Managerial summary: Disruptive changes are challenging firms across industries. We concentrate on the U.S. machine tool industry during the 1970s and 1980s when Japanese manufacturers with disruptive computer numerical control systems challenged the U.S. manufacturers. We find that, under the threat of disruption, the greater the stock of downstream complementary assets a U.S. machine tool manufacturer has, the more likely it is to be the product innovation leader with the disruptive technology. Our findings provide novel insights for managers in companies that face disruptive changes and can help them avoid the consequences of such changes as predicted by prior research. Copyright © 2016 John Wiley & Sons, Ltd.

Piecewise modelling and parameter estimation of repairable system failure rate.

Lifetime failure data usually presents a bathtub-shaped failure rate, and random failure phase dominates its life cycle. Relatively large errors always existed when single distribution model is used for analysis and modelling. Therefore, a bathtub-shaped reliability model based on piecewise intensity function was proposed for repairable system. Parameter estimation was studied by using maximum likelihood method in conjunction with two-stage Weibull process interval estimation and Artificial Bee Colony Algorithm. The proposed model and estimation approach fit the failure rate under bathtub curve very well, adequately reflect the duration of random failure phase, and guarantee the feasibility of quantitative analysis. Moreover, the estimated critical point of bathtub curve could be calculated. At last, the proposed model was applied in the reliability analysis of a repairable Computer Numerical Control system, and the validity of the proposed model and its parameter estimation method were verified.

Trajectory planning for a planar macro-micro manipulator of a laser-cutting machine

Purpose In the industry, there is always a demand to shorten the task completion durations to maximize the efficiency of the operation. This work focuses on making use of a special type of kinematic redundancy, macro–micro manipulation, to minimize the task completion duration. The purpose of this paper is to develop the most convenient trajectory planner to be integrated with industrial computerized numerical control (CNC) systems to resolve kinematic redundancy for task duration minimization. Design/methodology/approach A special type of kinematic redundancy is devised by using two kinematically different mechanisms that have different advantages, which are named as macro and micro mechanisms. In this case, the control design including the trajectory planning should be devised taking into account the distinct advantages of both mechanisms. A new trajectory planning algorithm is designed and used for the constructed planar laser-cutting machine, and some benchmark pieces are cut. Findings Offline method has practical limitations for employment in a real case scenario such as assuming infinite jerk limits for each axis motion. This limitation was removed by using an online trajectory generation technique. Experimental test results indicate that the online trajectory planning technique developed for the macro–micro mechanism to shorten the task duration was successful. Practical implications Although the new trajectory planning algorithm is implemented for a laser-cutting machine, it can also be used for other manufacturing systems that require higher acceleration and accuracy levels than the conventional machines. The new algorithm is compatible with the commercially available CNC systems. Originality/value In this work, a new approach to reducing the task duration for planar machining operations was introduced by making use of macro–micro manipulation concept. The core novelty of the work is devising trajectory planning algorithms to get the most efficiency in terms of acceleration limits from a macro–micro manipulation while making these algorithms deployable to most of the CNC systems.

Evaluating the accuracy of three-dimensional reconstruction of the intercuspal position for dentition casts aided by a mechanical appliance

To develop a aided mechanical appliance for rapid reconstruction of three-dimensional(3D)relationship of dentition model after scanning and evaluation of its accuracy. The appliance was designed by forward engineering software and fabricated by a high precision computer numerical control(CNC)system. It contained upper and lower body, magnetic pedestal and three pillars. Nine 3 mm diameter hemispheres were distributed equally on the axial surface of each pedestal. Faro Edge 1.8m was used to directly obtain center of each hemisphere(contact method), defined as known center. A pair of die-stone standard dentition model were fixed in intercuspal position and then fixed on the magnetic pedestals with low expansion ratio plaster. Activity 880 dental scanner was used to scan casts after the plaster was completely set. In Geomagic 2012, the centers of each hemisphere were fitted and defined as scanning centers. Scanning centers were aligned to known centers by reference point system to finish the 3D reconstruction of the intercuspal occlusion for the dentition casts. An observation coordinate system was interactively established. The straight-line distances in the X(coronal), Y(saggital), and Z(vertical)between the remaining 6 pairs of center points derived from contact method and fitting method were measured respectively and analyzed using a paired t-test. The differences of the straight-line distances of the remaining 6 pairs of center points between the two methods were X:(-0.05±0.10)mm, Y:(0.02±0.06)mm, and Z:(0.01 ± 0.05)mm. The results of paired t-test showed no significant differences(P>0.05). The mechanical appliance can help to reconstruct 3D jaw relation by scanning single upper and lower dentition model with usual commercial available dental cast scanning system.

Development of a feature-based open soft-CNC system

Modern computer numerical control (CNC) is intended to be more flexible, interoperable, adoptable, open, and intelligent. In realisation of such a CNC system, an ISO standard known as STEP for numeric control (STEP-NC) or ISO 14649 was developed in 2004 to alleviate a number of challenges associated with the widely used CNC standard (ISO 6983). Implementation of STEP-NC was initially carried out on some commercial CNC systems via an indirect STEP-NC programming approach. However, this approach failed to meet all the requirements of a modern CNC system, due to the translation of data from the high level to low level, which is vendor-specific and machine tool dependent, thus detrimental to the basic resource independent philosophy of STEP-NC. This paper presents a new generation of CNC systems, based on the Open Architecture Control (OAC) technology supported by STEP-NC data model to fundamentally solve these challenges. The developed system employs a new set of techniques for STEP-NC data interpretation, graphical verification, execution, monitoring, and report generation that supersede the exiting techniques in scope and capability. Using these techniques, the proposed system provides a flexible, intelligent, and adaptable manufacturing platform that can provide unprecedented levels of scalability and resource allocation agility in modern manufacturing enterprises. A prototype implementation of the proposed model is based on STEP-NC interpretation, 3D simulation, machine motion control, and live video monitoring with automatic document generation modules has been used to verify and validate the system in conjunction with industrially inspired test components.

A feedforward controller for tuning laser cladding melt pool geometry in real time

In this paper, a feedforward proportional-integral (PI) controller is developed to effectively control and tune the laser cladding melt pool geometry in real time. Width setpoint is included in the computer numerically controlled (CNC) programming, making possible its instantaneous change in relation to the position and time, as opposed to conventional controllers that do not have real-time information about these variables. The new concept of variable setpoint at different positions applied to laser cladding represents a great improvement in its use for changeable geometry applications such as blade fabrication. Several experiments are performed to characterize the behaviour of the system, revealing some key factors from monitoring system and image processing crucial for the controller. Laser power is selected as the input control variable, and the clad width is chosen as the output. The width of the melt pool is obtained based on measurements of CMOS camera images and an in-house image processing software algorithm. Closed-loop parameters are identified from the experimental data and Matlab simulations. The architecture of the controller consists on a conventional PI feedback loop and a feedforward module that shows low overshoot and fast response times. Instantaneous connections between laser, CNC, and PC systems allow for knowing the relationship among the exact position and real and setpoint melt pool values. The performance of the controller is verified in the fabrication of cladded parts with variable widths and in real time.

The Study of S-Shaped Acceleration and Deceleration Curve and the Trajectory Planning Strategy Analysis

The invention of the Numerical Control (NC) technology have brought the new revolution of manufacturing since 20th century intermediate stage. Computer Numerical Control (CNC ) technology is the foundation of the modern manufacturing technology, integrating the traditional manufacturing technology, modern control technology and modern optical-electromechanical technology.[1,2]As the core of CNC technology, CNC system is requesting higher precision and higher speed as it develops. So it is necessary to meet the requirements of the machining surface quality and avoid the occurrences of the processing speed decline or even stall, which depends on the good controls of the acceleration and deceleration of the feed shaft [3].This article briefly analyzes some existing strategies of the acceleration and deceleration, then mainly introduces the s-shaped acceleration and deceleration curve method and the trajectory planning strategy of it.