Dynamic controlled milling process for bone machining

Optimal control of the machining process in orthopedic surgery can not only increase productivity but also ensure safety during tool usage. The authors have developed a technology for a force control system. The system has two modes of operation: the “air-cutting mode” and the “force control mode.” In the air-cutting mode, tool feed is scheduled by predicting the air and bone-cutting zones from the CAD/CAM system. In the force control mode, the software monitors the cutting force and the cutting temperature, and it controls the feed override according to the difference between the real and the desired cutting force. The software is installed on a robot controller, and its effectiveness is evaluated with a urethane bone.     

Feedrate scheduling for indexable end milling process based on an improved cutting force model

In CNC machining, an optimal process plan is needed for higher productivity and machining performance. This paper proposes a mechanistic cutting force model to perform feedrate scheduling that is useful in process planning for indexable end milling. Indexable end mills, which consist of inserts and a cutter body, have been widely used in the roughing of parts in the mold industry. The geometry and distribution of inserts compose a discontinuous cutting edge on the cutter body, and tool geometry of indexable end mill varies with axial position due to the geometry and distribution of inserts. Thus, an algorithm that calculates tool geometry data at an arbitrary axial position was developed. The developed cutting force model uses cutting-condition-independent cutting force coefficients and considers run out, cutter deflection, geometry variation and size effect for accurate cutting force prediction. Through feedrate scheduling, NC code is optimized to regulate cutting forces at given reference force. Experiments with general NC codes show the effectiveness of feedrate scheduling in process planning.     

高精度数控机床进给伺服系统的模糊自适应PID控制

数控机床进给伺服系统是一个复杂的机电耦合系统。由于具有较强的时变参数特性、负载扰动和电机的非线性,很难给出控制系统的精确模型。基于数控机床进给伺服系统的数学模型,提出一种模糊自适应PID控制器的设计方法。将该控制器应用于数控机床进给伺服系统控制,可获得良好的控制性能。仿真结果表明：该方法不仅具有无静态失真,而且响应速度快、超调小。这种模糊自适应PID控制器具有较高的稳定性和精度。     

基于Filtered-X LMS自适应算法的车削颤振控制系统设计与实验研究

基于Filtered-X LMS自适应滤波算法,设计了一种压电作动器来控制刀具与工件的切削力位移,并进行了数值仿真和实际的切削实验.研究结果表明:在相同的切削条件下,基于Filtered-X LMS自适应滤波算法控制的刀具加工精度明显优于传统刀具的加工精度,能有效的减少车刀的振动量,验证了该控制系统的有效性,对于提高车床的加工精度具有重要的意义.     

三维椭圆振动切削系统模糊自适应滑模控制

三维椭圆振动切削被认为是目前最具潜力的机械加工方式,但是加工过程中的控制问题还未被很好的解决,特别是加工过程中对于外界干扰的自适应问题,为了在三维椭圆振动切削过程中实现控制方法的鲁棒性,根据所研制的一种三维椭圆振动切削装置独特的结构方式,首先分析了各个运动之间的串扰情况,并根据装置柔性铰链的特征建立了三维椭圆振动切削装置的动力学模型。提出了三维椭圆振动切削模糊自适应滑模控制的滑模函数与滑模控制律,并通过李雅普诺夫稳定性条件证明了所设计的滑模控制器系统稳定性,采用正弦信号数字实现模糊自适应滑模控制,位置跟踪在3 s内误差控制在0.005范围内,速度跟踪在0.4 s内控制在±0.05之内,能够达到满意的精度,系统模糊自适应滑模控制的模糊逼近也能在0.2 s内收敛,证明了三维椭圆振动切削系统采用模糊自适应滑模控制可以实现较强的鲁棒性。     

NURBS interpolator for constant material removal rate in open NC machine tools

Conventionally used linear or circular interpolators are undesirable for the precision machining of 3D free-form surfaces for the following reasons: the transmission errors due to the huge number of point data, discontinuity of curve segmentation, and unsmooth motion speed. In this regard, modern CNC machine tools are designed with a function for machining arbitrary parametric curves. However, these systems do not consider controlling feedrate adaptively, which dominates the quality of the machining process. This paper proposes a NURBS interpolator based on the adaptive feedrate control for the constant material removal rate. This is accomplished by varying feedrate using the curvature of a surface. The curvature-compensated feedrate system has important potential applications in ensuring part accuracy and protecting the cutting tool. The simulated and experimental results show it is applicable to real machining.     

Adaptive cutting force control for a machining center by using indirect cutting force measurements

This paper presents an adaptive cutting force controller for the milling process, which can be attached to most commercial CNC machining centers in a practical way. The cutting forces of x, y and z axes are measured indirectly from the use of currents drawn by a.c. feed-drive servo motors. A typical model for the feed-drive control system of a horizontal machining center is developed to analyze cutting force measurement from the drive motor. The pulsating milling forces can be measured indirectly within the bandwidth of the current feedback control loop of the feed-drive system. It is shown that indirectly measured cutting force signals can be used in the adaptive controller for cutting force regulation. The robust controller structure is adopted in the whole adaptive control scheme. The conditions under which the whole scheme is globally convergent and stable are presented. The suggested control scheme has been implemented into a commercial machining center, and a cutting experiment on face milling process is performed.     

数控机床刀具寿命管理系统的研究

在数控加工过程中,利用数控系统厂商提供的刀具寿命管理功能有效地实现了刀具的自动化管理。但针对不同实际生产情况,会存在一定的局限性。设计一个PMC子程序,解决了配备FANUC数控系统的加工中心刀具寿命管理系统中出现的刀具寿命次数设置不能超过10 000次、不能实现预报警等问题,为类似的数控系统功能设计和改进提供参考。     

Development of a modular dynamometer for triaxial cutting force measurement in turning

Accurate and reliable measurement of cutting forces in turning is essential for tool geometry, tool trajectory and cutting parameters optimization, as well as for tool condition monitoring and machinabilty testing. In this work, an innovative dynamometer for triaxial cutting force measurement in turning, specifically designed to be applied at a milling-turning CNC machine tool endowed with an indexable head, is presented. The device is based on a piezoelectric force ring integrated into a commercial toolshank, and its modular design allows the easy change of the cutting insert without altering sensor preload. The prototype device was assembled and experimentally tested by means of static calibration and dynamic identification, which evidenced good static and dynamic characteristics. Eventually, the sensor was tested in operating conditions by machining a benchmark workpiece.     

Dynamic stiffness enhancement of direct-driven machine tools using sliding mode control with disturbance recovery

This paper presents a disturbance adaptive discrete sliding mode controller for feed drives equipped with linear motors. The control law is expressed as a function of friction and cutting force disturbances which are estimated from the motor force and control states. The accurate prediction of disturbance forces is used to actively compensate low frequency machine tool structural modes which are within the bandwidth of the controller. The proposed control system is experimentally demonstrated on a high performance linear drive, which exhibited high bandwidth and significant increase in dynamic stiffness compared to classical cascaded control methods.     

A fast tool servo design for precision turning of shafts on conventional CNC lathes

A technique for precision turning of shafts on conventional CNC turning centers is presented. The shaft is semi-turned on a conventional CNC lathe. The precision finish turning operation is delivered by a piezoelectric based fast tool servo which is mounted on the same CNC lathe's turret. The precision tool tip motion is delivered by a proposed sliding mode controller which is robust to changes in the cutting process and hysteresis in the piezo actuator. Sliding mode controller is also quick to compensate the cutting force disturbances, and keeps the tool tip at the desired location within the displacement measurement sensor resolution (±0.1 μm). The fast tool servo system is packaged in a PC, and its effectiveness is demonstrated on a bearing location turning.     

Bandsawing. Part II: detecting positional errors, tool dynamics and wear by cutting force measurement

This article presents a method for measuring individual cutting forces during bandsawing. Methods for detecting errors in cutting edge positions, tool dynamics during machining and geometry changes due to wear have also been developed. The experimental studies that we have conducted show that the cutting forces vary during the tool engagement. These variations in force were then quantified using a previously developed cutting force model for multi-tooth cutting processes including effects of positional errors, tool dynamics and wear.     

Real-time surface interpolator for 3-D parametric surface machining on 3-axis machine tools

The conventional approaches for surface machining use a sequence of straight lines to approximate the part surface. These straight lines are subsequently translated into linear g-codes by a post processor, and then sent to the CNC system. This process limits the surface machining capability of CNC systems. For example, the CNC machine tool can be controlled only to follow these approximated straight lines since the machine loses the surface information. In contrast, this research utilised a real-time approach for 3-D parametric surface machining on 3-axis CNC machine tools. This real-time approach, called CNC surface interpolator, can read surface g-codes and perform surface machining interpolation. The input to the CNC surface interpolator is the surface g-code, which contains geometric information, such as the coefficients of the parametric surface, as well as cutting conditions. This results in better machining feedrate controls, interpolates more precise machining commands, and requires less machining time compared to that produced by off-line approaches.     

Improvement of a delayed velocity reference control (DVRC) for machining operations

This paper deals with a non-time-based controller for machining operations, suitable to control the interaction force between the tool and the workpiece.In the traditional approach usually the tool feed velocity (on which the force depends) is predetermined: it is chosen on the basis of some important parameters, such as tool dimensions, shape and material, the depth of cut, the workpiece material, etc. In this traditional approach, the reference function is calculated off-line, and during the task execution, at each instant, the control module is required to track the input reference; the drawback of this approach is that the system is not able to modify its velocity if unpredictable events occur or if side conditions change; this could produce high forces that could damage the tool or the workpiece.This paper presents a non-time-based controller for machining operations. Basically, in this control, the desired input reference is calculated on-line according to the measured force signal in such a way to improve the interaction. The controller allows to set the optimum interaction force F_max. In addition to this the same control is able to move the tool, during free motion paths, with a constant customizable velocity V_max, thus implementing a velocity control. As a third feature, it is possible to set the maximum allowed acceleration A_max of the end effector during the motion. This control scheme, called delayed velocity reference control (DVRC) belongs to the category of non-time-based controllers, since the reference signal is not directly related to time.The effectiveness of the controller has been proven by means of its implementation on a three-axes CNC center for EPS foam cutting.     

Computer-aided accuracy enhancement for multi-axis CNC machine tool

A computer-aided error compensation scheme has been developed to enhance the accuracy of multi-axis CNC machine tools by compensating for machine geometric and thermal errors in software way. Stationary geometric errors including the coupling effect of linkage errors between machine slides are calibrated off line. Dynamic thermal errors are predicted on line by an artificial neural network model. Because machine errors are variant with the cutting time and slide positions, a PC based compensation controller has been developed to upgrade commercial CNC controllers for real-time error compensation. The real-time compensation capability is achieved by digital I/0 communication between the compensation controller and CNC controller without the need of any hardware modification to the machine servo-drive loops. The compensation scheme implemented on a horizontal machining center has been proven to improve the machine accuracy by one order of magnitude using a laser interferometer and cutting test.     

基于BP神经网络数控机床切削能耗的研究

数控机床的能耗来源于工作时的电动机空载和切削过程中的负载消耗。分析切削过程中的切削速度、进给速度、切削深度等切削参数对数控机床能耗的影响;基于BP神经网络搭建数控机床能耗与切削参数的模型,简化了经验公式繁琐的计算过程;利用遗传算法对切削参数进行优化。对比试验表明:用优化后的参数进行加工,能明显地降低能耗,为加工过程能耗控制提供了一个良好的方案。     

Hybrid adaptive control based on the characteristics of CNC end milling

In this paper, hybrid adaptive control algorithm that controls feedrate is proposed to regulate spindle current. For this purpose, variation of steady state spindle current, time constant and time delay were examined through experiments based on step end milling under various cutting condition. The developed hybrid adaptive control algorithm is composed of adjustable proportional feedback control (adjustable P control), fine control carries out detailed control, and entry feedrate control reduces the peak current produced when a tool makes contact with a workpiece. The adjustable P control was verified through comparisons between calculate P gains and experimentally obtained P gains. The hybrid adaptive control algorithm was applied to various cutting conditions and it showed global stability as well as excellent applicability behavior.     

A new cyber-physical adaptive control system for drilling of hybrid stacks

A new cyber-physical adaptive control system (CPACS) is presented for drilling hybrid stacks. A generalized approach for features recognition, and learning, of spindle power signals was developed for real-time detection of tool wear level. Subsequently, a high fidelity model predicts the drilling forces and the damage at the layers and interfaces of the stacked panel. The CPACS uses these predictions to maximize the feedrate while maintaining the damage-free force limit at each layer. The system communicates with the CNC machine controller to continuously update the drilling conditions. Validation test showed improved productivity and extended tool life while preserving part quality.     

数控机床位置伺服系统的无模型自适应迭代学习控制

数控机床位置伺服系统在加工过程中受负载、摩擦和电路系统响应特性等因素影响,很难精确建立其加工过程动力学模型。针对批量零件加工过程中的重复执行过程,设计了一种数据驱动的无模型自适应迭代学习控制方案。该方案借助沿迭代轴的动态线性化方法,将数控机床位置伺服系统加工动力学过程等价转化成一个虚拟的迭代数据模型,并根据设计的迭代学习控制律和参数估计律构建数控机床位置伺服系统的无模型自适应迭代学习控制方案。仿真结果表明:该迭代学习控制方案基于数控机床重复运行的特点,仅利用位置和电机电流信息,完成了对零件加工过程的改善,提高了加工精度。     

CNC机床指示输入及其系统

CNC机床指示输入是依据机械加工人员的操作习惯而设计的加工程序自动生成模式，CNC机床操作人员用加工任务指示器指示机床的运动部件和刀具，在完成加工过程空运转及试切的同时自动生成数控码，文章论述了实现这种方法的系统构成，运动参数的输入与处理。