A new accurate curvature matching and optimal tool based five-axis machining algorithm

Free-form surfaces are widely used in CAD systems to describe the part surface. Today, the most advanced machining of free from surfaces is done in five-axis machining using a flat end mill cutter. However, five-axis machining requires complex algorithms for gouging avoidance, collision detection and powerful computer-aided manufacturing (CAM) systems to support various operations. An accurate and efficient method is proposed for five-axis CNC machining of free-form surfaces. The proposed algorithm selects the best tool and plans the toolpath autonomously using curvature matching and integrated inverse kinematics of the machine tool. The new algorithm uses the real cutter contact toolpath generated by the inverse kinematics and not the linearized piecewise real cutter location toolpath.     

A methodology for systematic geometric error compensation in five-axis machine tools

To enhance the accuracy, an efficient methodology was developed and described for systematic geometric error correction and their compensation in five-axis machine tools. The methodology is capable of compensating the overall effect of all position-dependent and position-independent errors which contribute to volumetric workspace. It was implemented on a five-axis grinding machine for error compensation and for the check of its effectiveness. Error compensation algorithm was designed, and a routine was written in Matlab software. The developed technique and software are based on an error table which interprets the function of axis through cubic spline technique and synthesis modeling of a machine tool. Recursive compensation methodology was used to remove the machine errors from the actual tool path and inverse technique was implemented to find the corrected positions of prismatic and rotary joints. Moreover, it can convert the corrected tool paths into practical compensated NC codes. The generated, corrected and modified NC codes directly fed to the controller of a five-axis machine tool. Validation of the technique was preceded by repeated experimentation of measurement and through machining of typical standard workpieces with some additional specific features. Experimental results exhibit effective compensation and remarkable improvement in the parametric and volumetric-workspace accuracy of the five-axis machine tool.     

Iterative learning control scheme for manipulators including actuator dynamics

This paper presents the iterative learning control for the industrial robot manipulators including actuator dynamics. Motivated by human learning, the basic idea of iterative learning control is to use information from previous execution of a trial in order to improve performance from trial to trial. This is an advantage, when accurate model of the system is not available as friction and actuator dynamics, though present in the system, are not modeled to reduce the computational complexity. In this paper different aspects of ILC including the design schemes and control algorithms are covered. The learning control scheme comprises two types of control laws: a linear feedback law and a feed-forward control law. In the feedback loop, the fixed gain PD controller provides stability of the system and keeps its state errors within uniform bounds. In the feed-forward path, a learning control rule/strategy is exploited to track the entire span of a reference input over a sequence of iterations. Algorithms are verified through detailed simulation results on a two DOF robot manipulator.     

STEP-NC控制器及其刀轨规划方法研究

为实现产品模型数据交换标准-数控程序在传统数控机床上的应用，提出了产品模型数据交换标准-数控程序的刀轨规划方法。首先，概述了产品模型数据交换标准-数控程序控制器的类型及其3大核心功能模块，分析和研究了刀轨规划过程中坐标系转换问题，给出了利用产品模型数据交换标准-数控程序实体索引映射表规划加工特征刀轨的方法和步骤，着重分析了平面和型腔两个加工特征的加工策略，并基于上述方法给出了具体的规划方法流程图。最后，结合一个基于平面、孔和型腔3个加工特征的产品模型数据交换标准-数控程序实例，将所规划出的G代码程序在配备开放式数控系统HITCNC的3轴数控铣床上进行实际加工，实验结果证明，该刀轨规划方法是正确可行的，并具有一定的通用性。     

Experimental implementation of Taylor series expansion error compensation on a bi-axial CNC machine

In this paper, a new model-based Taylor series expansion error compensation (TSEEC) strategy is proposed to improve the contouring accuracy for computer numerically controlled (CNC) machines. In TSEEC, the contour error compensation problem is formulated as a Taylor series expansion problem, in which the value of the contour error is expanded around the reference points and the compensation components are calculated as the deviations from the reference points. Simulations show that, with perfect knowledge of the axial dynamics, zero contour errors can be achieved with TSEEC for both linear and circular contours. Due to modeling errors, external disturbances, and measurement noise, some modifications and experimentation need to be made to determine suitable parameters for implementation of the TSEEC scheme on a real machine. These measurements include a low-pass filter, a choice of a compensation target, and a compensation gain. Experimental results show the effectiveness of TSEEC in reducing contour errors and demonstrate the superiority of TSEEC over inverse feedforward compensation and cross-coupled control in improving the contouring accuracy.     

新型3T2R龙门式混联机床动力学模型

提出了一种以能实现沿X、Z轴平移和绕X、Y轴转动的新型并联机构2-RPU2-UPS作为主体,辅以能实现主体Y轴滑动的直线导轨来共同实现五坐标联动加工的一种新型龙门式混联机床的机构设计方案。运用螺旋理论分析了该机床实现3T2R运动原理,计算出该机床的自由度,进行输入选取与论证,并进行机床机构奇异分析,提出减少奇异的参数设计条件。建立该机床的位置反解数学模型,推导出了该机床的速度雅可比矩阵和加速度分析表达式,求解了该机床的位置反解、速度和加速度。基于虚功原理和该机床的运动学模型建立该机床的动力学模型,并采用Adams软件对该机床动力学进行模拟仿真,模拟结果表明理论结果完全正确。     

Contouring accuracy improvement using a tangential contouring controller with a fuzzy logic-based feedrate regulator

In contour-following tasks, contour error reduction is an issue of much concern. Generally speaking, contour error is caused by the mismatched dynamics between each axis. To reduce the contour error, many previous studies have focused on developing proper controllers and/or more accurate contour error estimation algorithms. An alternative method for reducing contour errors is to exploit the idea of desired feedrate adjustment. This paper proposes using the approximate contour error information to develop a fuzzy logic-based feedrate regulator, which adjusts the value of the desired feedrate. Moreover, to further reduce contour error, an integrated motion control scheme is also developed. This scheme consists of a position loop controller with velocity command feedforward, a tangential contouring controller (TCC), a real-time contour error estimator, and the proposed fuzzy logic-based feedrate regulator. Several experiments on free-form contour-following tasks are conducted to evaluate the performance of the proposed approach. The experimental results clearly demonstrate the effectiveness of the proposed approach.     

High-speed cornering by CNC machines under prescribed bounds on axis accelerations and toolpath contour error

To exactly execute a sharp corner in the toolpath, the feedrate of a CNC machine must instantaneously drop to zero at that point. This constraint is problematic in the context of high-speed machining, since it incurs very high deceleration/acceleration rates near sharp corners, which increase the total machining time, and may incur significant path deviations (contour errors) at these points. A strategy for negotiating sharp corners in high-speed machining is proposed herein, based upon a priori toolpath/feedrate modifications in their vicinity. Each corner is smoothed by replacing a subset of the path that contains it with a conic “splice” segment, deviating from the exact corner by no more than a prescribed tolerance ?, along which the square of the feedrate is specified as a Bernstein-form polynomial. The problem of determining the fastest traversal of the conic segments under known axis acceleration bounds can then be formulated as a constrained optimization problem, and by exploiting some well-known properties of Bernstein-form polynomials this can be approximated by a simple linear programming task. Some computed examples are presented to illustrate the implementation and performance of the high-speed cornering strategy.     

A new high-efficiency error compensation system for CNC multi-axis machine tools

To enhance the accuracy of CNC machines for the request of modern industry, an effective static/quasi-static error compensation system composed of an element-free interpolation algorithm based on the Galerkin method for error prediction, a recursive software compensation procedure, and an NC-code converting software, is developed. Through automatically analyzing the machining path, the new error prediction method takes into consideration the fact that the machine structure is non-rigid, and can efficiently determine the position errors of the cutter for compensation without computing a complex error model on-line. The predicted errors are then compensated based on a recursive compensation algorithm. Finally, a compensated NC program will be automatically generated by the NC-code converting software for the precision machining process. Because of the advantage of the element-free theory, the error prediction method can flexibly and irregularly distribute nodal points for accurate error prediction for a machine with complex error distribution characteristics throughout the workspace. To verify the algorithm and the developed system, cutting experiments were conducted in this study, and the results have shown the success of the proposed error compensation system.     

数控机床热误差的在线测量与补偿加工

对数控机床热误差的产生原因及特点进行了综合分析,简单介绍了应用激光干涉仪对其进行在线测量的基本原理,对数控机床热误差补偿的具体实施过程进行了详细阐述。利用PLC抗干扰能力强、可靠性高、运算速度快及良好的接口性能,给出了数控机床热误差补偿系统的硬件实现方法。应用PLC技术可以提高误差补偿精度,对误差补偿的硬件实施具有一定参考意义。     

A postprocessor for table-tilting type five-axis machine tool based on generalized kinematics with variable feedrate implementation

Improvements in the machine tool and the machining process technologies increased the need for generic postprocessors in order to exploit the capabilities of the machine tools. Contrary to conventional machining approach, next-generation machining technologies such as force-based feedrate scheduling and toolpath optimization requires the implementation of the variable feedrate during toolpath which constitutes the aim of this article. Therefore, this paper introduces a postprocessor for table-tilting type five-axis machine tool based on generalized kinematics with variable feedrate implementation. Furthermore, a practical yet effective method for avoiding kinematic singularities by spherical interpolation and NC data correction is presented as well. Proposed approach is validated for various five-axis machine tools with different kinematic configurations via virtual machine simulation module. Results of the verification tests show that presented postprocessing approach can accurately convert the cutter location information into NC codes and it is demonstrated that integrated virtual simulation module can simulate toolpaths with large number of blocks.     

Adaptive tracking control of two-wheeled welding mobile robot with smooth curved welding path

This paper proposes an adaptive controller for partially known system and applies to a two-wheeled Welding Mobile Robot (WMR) to track a reference welding path at a constant velocity of the welding point. To design the tracking controller, the errors from WMR to steel wall is defined, and the controller is designed to drive the errors to zero as fast as desired. Additionally, a scheme of error measurement is implemented on the WMR to meet the need of the controller. In this paper, the system moments of inertia are considered to be partially unknown parameters which are estimated using update laws in adaptive control scheme. The simulations and experiments on a welding mobile robot show the effectiveness of the proposed controller.     

Design methodology for fully dynamic-controlled polynomial profiles and reduced tracking error in CNC machines

High speed and high precision on computer numerically controlled (CNC) machines are major issues for most industrial automation processes. Among the most important factors for precise motion control are the control law, the controller implementation, and the profile reference. The effects of the control law and controller implementation on the tracking error in CNC machines have been widely studied, while the issue of profile reference has not received the required attention. To improve the dynamics provided by the conventional profile reference, several techniques have been proposed, focusing on peak jerk limitation. The novelty of this work is to develop a generalized methodology designed for producing dynamically constrained higher degree piecewise polynomial profiles, without limits in maximum polynomial degrees. They lead to define in an arbitrary way the dynamics shape, while the peak dynamics values constrained are maintained in order to improve the tracking error on CNC systems and giving the possibility to design fully dynamic-controlled trajectories. For experimentation purposes, a two-axis proportional–integral–derivative controller and profile reference generator are implemented in a low-cost field programmable gate array. The experimentation demonstrates the efficiency of the proposed methodology, showing the peak jerk and tracking error reduction, compared against recent reports from literature.     

Design of a rotary fast tool servo for ophthalmic lens fabrication

We have developed a novel fast tool servo and associated prototype diamond- turning machine for the production of plastic spectacle lenses. Our fast tool servo carries the cutting tool on a rotary arm and, thus, on a circular path, as opposed to straight line paths in conventional designs. The actuator, sensors, and bearings are standard elements that together allow experimentally demonstrated 500 m/s² instantaneous accelerations at the tool tip over a 3-cm range of cutting depth. We also describe in this paper new approaches we have developed for toolpath generation and calibration. The paper also presents associated control algorithms, because the controller must supply very high dynamic stiffness to the tool servo axis at multiples of the spindle frequency. This stiffness is achieved by means of repetitive control techniques. The new fast tool servo is shown to have great promise for machining asymmetric surfaces with large amplitude asymmetries.     

感应电机神经网络逆控制的数字实现

提出了一种感应电机神经网络逆控制的数字实现方法.证明了感应电机的可逆性,给出了感应电机神经网络逆控制的实现结构.详细阐述了采用数字信号处理器（DSP）实现神经网络逆控制系统的硬件组成和软件设计方法.实验结果表明,系统具有良好的动、静态控制性能.     

Efficient high-speed cornering motions based on continuously-variable feedrates. II. Implementation and performance analysis

A novel high-speed cornering strategy for piecewise-linear motions is proposed, based on the G ² continuous Pythagorean–hodograph (PH) rounding segments and continuously-variable feedrates described in Part I of this two-part paper. This strategy employs an acceleration-limited approach to feedrate scheduling, including smooth deceleration and acceleration profiles along linear segments entering and leaving the rounded segments. A G-code part program parsing software package has been developed, that automatically identifies toolpath corners and inserts appropriately-sized PH quintic corner rounding segments, with associated feedrate suppression ratios. The method has been tested on a three-axis CNC mill with an open-architecture controller incorporating real-time interpolators that realize smoothly varying feedrates along the PH corner curves. Tests on a representative selection of toolpaths yield substantial savings (up to 40 %) in execution times, compared to the traditional “full stop” strategy for unmodified sharp corners.     

Tool path optimization in postprocessor of five-axis machine tools

Generally, tool path is generated in a computer-aided manufacturing software considering only the geometry of machining parts. It is converted into numerical control (NC) codes in the postprocessor based on the particular machine kinematics. For some special types of five-axis machine tools, e.g., non-orthogonal five-axis machine tools, the generated NC codes may produce unqualified parts because of the existence of the non-linear error. Conventional commercialized postprocessors usually do not have the function of non-linear error checking. Observing that the tool path is a non-smooth trajectory full of corners and a series of connected line segments, cubic spline interpolation is applied to smooth the tool path at regular points in this study. The cutter tip center points are computed by the cubic spine interpolation, while the cutter posture vectors are obtained via linear interpolation. At the splines (for regular points) and the line segments (feature points), more points are chosen to be converted into NC codes to reduce the non-linear error, which is called data densification. Using the cubic spline to smooth the tool path and the data densification to reduce the non-linear error, a novel tool path optimization algorithm in postprocessor is proposed. Experiments were carried out on an inclined rotary spindle axis non-orthogonal five-axis machine tool. It shows that the proposed tool path optimization provides improved accuracy and surface quality.     

Precision tracking control of a horizontal arm coordinate measuring machine in the presence of dynamic flexibilities

One of the major sources that affect measurement accuracy and limit the use of high motion speeds in coordinate measuring machines (CMM) is the position error. In fact, static and dynamic probe errors are more direct factors in measuring machine accuracy, but are not the subject of this research. However the accuracy of acquisition of component position errors using a CMM in motion is also of importance, hence the dynamics of a CMM need to be considered. Therefore, this research aims to model the dynamics of a horizontal arm CMM by considering drive flexibility at joints and evaluates the characteristics of the system for fine motion control purposes. Design of a precision tracking controller (PTC) to perform superior tracking for enhancing the measurement accuracy and the probing speed in providing less inspection time at high motion speeds is carried out. A dynamic model for the CMM is developed including drive flexibilities represented with lumped springs at the joints. Due to the non-collocated nature of the control scheme in the flexible CMM dynamics, a non-minimum phase system is observed in the proposed CMM model. Using the derived CMM model with joint flexibilities, tracking motion control simulations are conducted at different probing speeds for the cases where a PI controller and a feedback PTC are employed. A comparison of the PI controller with the feedback PTC is also performed. Results demonstrate that the effects of joint flexibilities on the contour error and probing speeds are significant and the PI controller is not capable of providing good accuracy during challenging tasks such as corner tracking. However, the simulation results indicated that by using the proposed feedback precision tracking controller, contour errors in corner tracking that are caused by joint flexibilities can be reduced effectively .     

开放式数控系统在机检测模块实现方法

为了实现数控系统在机检测功能,提出了一种基于开放式数控系统在机检测模块实现方法。开放式数控系统采用Windows操作系统,以工控机为上位机,以运动控制器为下位机的结构形式。在机检测系统由开放式数控系统、在机测头、机械系统、驱动系统等硬件组成;采用Visual C++软件基于面向对象思想开发了模块化在机检测系统软件。对测量误差进行了分析并介绍了相应误差补偿方法。实际应用表明,该在机检测模块能够完成复杂曲面的检测,实现了数控系统在机检测功能,有利于节约生产成本、提高生产效率。     

Implementation of robust adaptive control for robotic manipulator using TMS320C30

A new adaptive digital control scheme for the robotic manipulator is proposed in this paper. Digital signal processors are used in implementing real time adaptive control algorithms to provide an enhanced motion for robotic manipulators. In the proposed scheme, adaptation laws are derived from the improved Lyapunov second stability analysis based on the adaptive model reference control theory. The adaptive controller consists of the adaptive feedforward and feedback controller and PI type time-varying control elements. The control scheme is simple in structure, fast in computation, and suitable for implementation of real-time control. Moreover, this scheme does not require an accurate dynamic modeling, nor values of manipulator parameters and payload. Performance of the adaptive controller is illustrated by simulation and experimental results for a SCARA robot.