Hybrid active/passive force control strategy for grinding marks suppression and profile accuracy enhancement in robotic belt grinding of turbine blade

Grinding marks and traces, as well as the over- and under-cutting phenomenon are the severe challenges in robotic abrasive belt grinding of turbine blades and it greatly limits the further application of robotic machining technology in the thin-walled blade fields. In the paper, an active force control method consisting of force/positon and PI/PD controller based on six-dimensional force/torque sensor is introduced to eliminate the grinding marks and traces, and a passive force control method including PID controller based on one-dimensional force sensor is proposed to reduce the over- and under-cutting phenomenon in robotic machining system. Then the Kalman filter information fusion methodology is adopted to combine the active and passive force control methods which could improve the controlled force accuracy and efficiency, as well as avoid the control interference. Finally both the test workpiece and turbine blade are employed to examine and verify the reliability and practicality of the proposed hybrid force control method by achieving the desired surface quality and higher profile precision.     

A new calibration method for a dynamic coordinate system in a robotic blade grinding and polishing system based on the six-point limit principle

Automatic robotic grinding and polishing systems have become a developing trend in aerospace parts manufacturing. In a robotic blade grinding and polishing system (RBGPS), the automatic and precise calibration of the dynamic workpiece coordinate frame is the most important process. In this research, a new method that introduces the concept of six-point positioning into the dynamic workpiece coordinate frame calibration process is proposed using a point laser displacement sensor (PLDS). The static coordinate frame calibration process is conducted based on a robot flange and force sensor. The results indicate that the new method can achieve a higher precision calibration result and has improved operational efficiency and cost. Finally, its practicality is verified in the BRGPS, and the results indicate that the polished blade surface after using the new method has good consistency.     

Time-varying isobaric surface reconstruction and path planning for robotic grinding of weak-stiffness workpieces

Severe deformations and vibration usually occur when grinding the weak-stiffness workpieces, then fluctuate the grinding force and damage the surface. In this paper, the time-varying isobaric surface (TVIS) is defined as a virtual surface to generate constant force during robotic grinding. Based on it, a novel robotic grinding method, including contact trial and surface reconstruction, is proposed. In the contact trial process, the robot actively samples the deformation and stiffness of contact point with a force sensor. Then, a TVIS mesh is constructed to replace the original geometry of the workpiece, which is utilized for grinding path planning. Experiments have been conducted to verify the feasibility of this method. The result shows that the proposed method can achieve constant grinding force and is robust to the types of workpieces and the processing techniques. Furthermore, it is considered as an intelligent method for customized robotic machining of the weak-stiffness workpieces.     

Force control-based vibration suppression in robotic grinding of large thin-wall shells

Vibration suppression is a major difficulty in the grinding of low-stiffness large thin-wall shells. The paper proposes that effective workpiece vibration control can be performed by a novel force-controlled end-effector integrated into a robotic grinding workcell. First, a dynamics model is built to capture the characteristics and vibration suppression mechanism of force control-based robotic grinding, then a novel force control-based vibration suppression method is designed for grinding large thin-wall shells, and three robotic grinding tests are conducted to validate the effects of the new method and the grinding performance of the force control-based robotic grinding workcell. The results are: 75% reduction in the amplitude of workpiece vibration; effective suppression of non-tool passing frequency; stable grinding of large thin-wall shells remarkably enhancing grinding depth up to 0.3 mm per pass, grinding depth error less than ±0.1 mm, and significant improvement of the workpiece surface quality up to Ra=0.762 μm.     

发动机叶片机器人精密砂带磨削精度检测技术

当前精密砂带磨削精度检测技术检测准确率低,检测效率差。为了解决上述问题,引用发动机机器人研究了一种新的精密砂带磨削精度检测技术,对其精度数据进行采集,将采集出的系统数据作为基础信息来源,获取叶片零部件的点云信息,处理机器人的工作主坐标系,通过三维激光扫描获取叶片机器人的准确信息,同时配以打磨剖光操作,以PCA算法解析,进一步将数据集简化,根据数据主要分布规律选择合适的算法加工位置与范围,在三维空间中,将点分别对应坐标轴中的点进行匹配,通过对磨削接触面的轮廓以及磨削表面完整性进行分析,以实现对发动机叶片机器人精密砂带磨削精度的检测。实验结果表明,相较于传统检测技术,发动机叶片机器人精密砂带磨削精度检测技术检测精度提高了31.28%,检测误差降低了15.21%。     

A method for grinding removal control of a robot belt grinding system

As a kind of manufacturing system with a flexible grinder, the material removal of a robot belt grinding system is related to a variety of factors, such as workpiece shape, contact force, robot velocity, and belt wear. Some factors of the grinding process are time-variant. Therefore, it is a challenge to control grinding removal precisely for free-formed surfaces. To develop a high-quality robot grinding system, an off-line planning method for the control parameters of the grinding robot based on an adaptive modeling method is proposed in this paper. First, we built an adaptive model based on statistic machine learning. By transferring the old samples into the new samples space formed by the in-situ measurement data, the adaptive model can track the dynamic working conditions more rapidly. Based on the adaptive model the robot control parameters are calculated using the cooperative particle swarm optimization in this paper. The optimization method aims to smoothen the trajectories of the control parameters of the robot and shorten the response time in the transition process. The results of the blade grinding experiments demonstrate that this approach can control the material removal of the grinding system effectively.     

Accurate robotic belt grinding of workpieces with complex geometries using relative calibration techniques

Robotic belt grinding operations are performed by mounting a workpiece to the end effector and commanding it to move along a trajectory while maintaining contact with the belt grinding wheel. A constant contact force throughout the grinding process is necessary to provide a smooth finish on the workpiece, but it is difficult to maintain this force due to a multitude of installation, manipulation, and calibration errors. The following describes a novel methodology for robotic belt grinding, which primarily focuses on system calibration and force control to improve grinding performance. The overall theory is described and experimental results of turbine blade grinding for each step of the methodology are shown.     

Adaptive parameter optimization approach for robotic grinding of weld seam based on laser vision sensor

Automatic robot grinding technology has been widely applied in the modern manufacturing industry. A flexible abrasive belt wheel used to grind the weld can avoid burns on the base material and improve the processing efficiency. However, when the robot grinds a weld seam, the material removal depth does not coincide with the feed depth because of the soft contact and uneven weld height, affecting the weld seam surface uniformity. Given these problems, an adaptive parameter optimization approach for the robotic grinding of a weld seam was proposed based on a laser vision sensor and a material removal model. First, the depth of weld seam removal was obtained by a laser vision sensor based on triangulation in real-time. Then, a macroscopic material removal model considering flexible deformation was established to determine the relationship between the weld height and process parameters, and the model coefficient was experimentally fitted to ensure the accuracy and reliability of the model. In addition, the data of real-time interaction structure between the robot controller and grinding system were obtained and used to unsure that the rotational speed of the belt wheel increased in the convex part and decreased in the concave part, in order to obtain a uniform weld seam surface. Comparative experiments were performed to verify the effectiveness and superiority of the method, and experiments on the surface roughness and weld seam surface height difference were conducted to verify the universality of the method. Experimental results show that the residual height of the weld after grinding can be controlled within 0.2mm, and the maximum removal height difference can be controlled within 0.05mm. The surface roughness Ra of the weld after grinding could reach 0.408 µm.     

Asymmetrical nonlinear impedance control for dual robotic machining of thin-walled workpieces

Impedance control is to provide stable tracking by regulating the impedance response of a robot. In this paper, an asymmetrical nonlinear impedance control (ANIC) is proposed for a dual robotic machining system. The symmetrical linear impedance control (SLIC) is also analyzed as a comparison study. We compared two controllers in terms of the stability and the sensitivity property of the grinding force, as well as the trajectory design. The main advantage of the ANIC is that the grinding force is robust to the environmental disturbances and the variation in thickness of workpieces. In contrast to the traditional control concept, which is devoted to compensate the nonlinear effect of the original system, our design philosophy is to increase the system robustness by introducing an artificial nonlinearity to the system. As a result, the dual robotic system acts as variable stiffness actors to adapt the variation in the thickness of workpieces. Grinding experiments are conducted in the dual robotic machining test rig for both workpieces with the uniform and varied thickness. The experimental results show that the dual robotic system with the ANIC can achieve better grinding quality.     

Precise and smooth contact force control for a hybrid mobile robot used in polishing

Contact force is dominant in robotic polishing since it directly determines the material removal. However, due to the position and stiffness disturbance of mobile robotic polishing and the nonlinear contact process between the robot and workpiece, how to realize precise and smooth contact force control of the hybrid mobile polishing robot remains challenging. To solve this problem, the force tracking error is investigated, which indicates that the force overshoot mainly comes from the input step signal and the environmental disturbance causes force tracking error in stable state. Accordingly, an integrated contact force control method is proposed, which combines feedforward of the desired force and adaptive variable impedance control. The nonlinear tracking differentiator is used to smooth the input step signal of the desired force for force overshoot reduction. Through modeling of the force tracking error, the adaptive law of the damping parameter is established to compensate disturbance. After theoretical analysis and simulation verification, the polishing experiment is carried out. The improvement in force control accuracy and roughness of the polished surface proves the effectiveness of the proposed method. Sequentially, the proposed method is employed in the polishing of a 76-meter wind turbine blade. The measurement result indicates that the surface roughness after mobile robotic polishing is better than Ra1.6. The study provides a feasible approach to improve the polishing performance of the hybrid mobile polishing robot.     

机器人砂带磨削系统作业精度分析与误差补偿

为了更好地反映及提高工业机器人砂带磨削系统的整体性能,通过分析机器人应用系统的特点,详细 描述了工业机器人应用系统“作业精度”的含义及衡量标准．在此基础上,推导了机器人砂带磨削系统作业精度模 型,设计了机器人砂带磨削系统作业误差测量工具及校准系统,建立了实际的机器人砂带磨削系统．通过实际的机 器人磨削实验验证了方法的有效性．     

砂带磨削机器人的灵活性分析与优化

总结了磨削机器人的当前发展和阻碍砂带磨削机器人广泛应用的难点．根据复杂曲面磨削任务对机器 人的实际要求,提出了一种磨削机器人构型．这种机器人属于PPPRRR 构型,具有很高的定位精度和结构刚度．利 用旋量理论中的指数积公式推导了该机器人的运动学正反解．引入了模拟退火算法,分析获得了相对于末端坐标系 描述的砂带磨削机器人的灵活磨削空间,并绘制了灵活磨削空间的横截面图谱．进一步,采用模式搜索法,优化了 磨削机接触轮相对于机器人基坐标系的位移偏移量,获得了最大的灵活磨削空间体积,提高了机器人砂带磨削系统 的灵活性．     

A particle swarm approach for multi-objective optimization of electrical discharge machining process

This paper proposes an experimental investigation and optimization of various machining parameters for the die-sinking electrical discharge machining (EDM) process using a multi-objective particle swarm (MOPSO) algorithm. A Box–Behnken design of response surface methodology has been adopted to estimate the effect of machining parameters on the responses. The responses used in the analysis are material removal rate, electrode wear ratio, surface roughness and radial overcut. The machining parameters considered in the study are open circuit voltage, discharge current, pulse-on-time, duty factor, flushing pressure and tool material. Fifty four experimental runs are conducted using Inconel 718 super alloy as work piece material and the influence of parameters on each response is analysed. It is observed that tool material, discharge current and pulse-on-time have significant effect on machinability characteristics of Inconel 718. Finally, a novel MOPSO algorithm has been proposed for simultaneous optimization of multiple responses. Mutation operator, predominantly used in genetic algorithm, has been introduced in the MOPSO algorithm to avoid premature convergence. The Pareto-optimal solutions obtained through MOPSO have been ranked by the composite scores obtained through maximum deviation theory to avoid subjectiveness and impreciseness in the decision making. The analysis offers useful information for controlling the machining parameters to improve the accuracy of the EDMed components.     

Enhancement and evaluation in path accuracy of industrial robot for complex surface grinding

Lower path accuracy is an obstacle to the application of industrial robots in intelligent and precision grinding complex surfaces. This paper proposes a novel path accuracy enhancement strategy and different evaluation methods for a six-degree-of-freedom industrial robot FANUC M710ic/50 used for grinding an aero-engine blade. Six groups of theoretical tool paths individually planned on this complex surface were obtained using the iso-parametric method and the constant chord height method. Then the actual paths of the robot were dynamically recorded by a laser tracker with a high frequency. A revised Levenberg-Marquardt and Differential Evolution hybrid algorithm was proposed to improve the absolute robotic positioning accuracy by considering the average curvature variation rate, the arc length and the number of cutter contact points on planning paths. The results showed that the maximum positioning error had been drastically reduced from 0.792 mm to 0.027 mm. Based on the redefinition of robotic path accuracy, including position accuracy and shape accuracy in this work, the methods MP-TLD, BP-TPD and MP-TID were proposed to evaluate the enhanced path accuracy. The evaluation results showed that the different path planning methods have almost little effect on path accuracy. Furthermore, the maximum path deviation evaluated by the MP-TLD method was reduced from 0.378 mm to 0.044 mm, evaluated by the BP-TPD method was reduced from 0.374 mm to 0.029 mm, and evaluated by the MP-TID method was reduced from 0.205 mm to 0.026 mm. It is concluded that these evaluation methods are basically valid and the average path accuracy value is about 0.035 mm, for present complex surface grinding with this typical industrial robot. Finally, the robotic grinding experiments of titanium alloy blades are conducted to further validate the effectiveness of the proposed method.     

Automated robotic grinding by low-powered manipulator

A new robotic grinding process has been developed for a low-powered robot system using a spring balancer as a suspension system. To manipulate a robot-arm in the vertical plane, a large actuator torque is required due to the tool weight and enormous gravity effect. But the actuators of the robot system always exhibit a limited torque capacity. This paper presents a cheap and available system for precise grinding tasks by a low-powered robot system using a suspension system. For grinding operations, to achieve position and force-tracking simultaneously, this paper presents an algorithm of the hybrid position/force-tracking scheme with respect to the dynamic behavior of a spring balancer. Material Removal Rate (MRR) is developed for materials SS400 and SUS304. Simulations and experiments have been carried out to demonstrate the feasibility of the proposed system.     

Accuracy analysis in mobile robot machining of large-scale workpiece

Mobile robot machining provides more flexible machining mode compared to the robot machining with a fixed base. However, its machining accuracy is frequently questioned. This paper focuses on the accuracy analysis in mobile robot machining. To evaluate the machining error qualitatively, the tool center point (TCP) error index is defined as the distance between the TCP and the designed machining point. The different error sources acting on the TCP error index are enumerated, and the theoretical accuracy analysis is proposed to eliminate the TCP error. The mobile robot machining strategy is then proposed based on the accuracy analysis. To ensure high machining accuracy, the global measurement system locates the position of the workpiece and the mobile platform. The force-controlled grinding head is used to compensate the TCP error. Experimental results show that the TCP error during mobile robot machining is lower than 40 mm, which mainly introduced by the calibration of the workpiece. The force-controlled grinding head can compensate the TCP error and the fluctuation of the grinding force under the control is lower than ±2 N.     

Robotic grinding of complex components: A step towards efficient and intelligent machining – challenges,solutions, and applications

Robotic grinding is considered as an alternative towards the efficient and intelligent machining of complex components by virtue of its flexibility, intelligence and cost efficiency, particularly in comparison with the current mainstream manufacturing modes. The advances in robotic grinding during the past one to two decades present two extremes: one aims to solve the problem of precision machining of small-scale complex surfaces, the other emphasizes on the efficient machining of large-scale complex structures. To achieve efficient and intelligent grinding of these two different types of complex components, researchers have attempted to conquer key technologies and develop relevant machining system. The aim of this paper is to present a systematic, critical, and comprehensively review of all aspects of robotic grinding of complex components, especially focusing on three research objectives.For the first research objective, the problems and challenges arising out of robotic grinding of complex components are identified from three aspects of accuracy control, compliance control and cooperative control, and their impact on the machined workpiece geometrical accuracy, surface integrity and machining efficiency are also identified. For the second aim of this review, the relevant research work in the field of robotic grinding till the date are organized, and the various strategies and alternative solutions to overcome the challenges are provided. The research perspectives are concentrated primarily on the high-precision online measurement, grinding allowance control, constant contact force control, and surface integrity from robotic grinding, thereby potentially constructing the integration of “measurement – manipulation – machining” for the robotic grinding system. For the third objective, typical applications of this research work to implement successful robotic grinding of turbine blades and large-scale complex structures are discussed. Some research interests for future work to promote robotic grinding of complex components towards more intelligent and efficient in practical applications are also suggested.     

Potential ANN prediction model for multiperformances WEDM on Inconel 718

Neural Computing and Applications - This paper proposes a machining performance prediction approach on multiple performances of wire electrical discharge machining (WEDM) on Inconel 718. Artificial...     

Investigation on chatter stability of robotic rotary ultrasonic milling

In recent years, industrial robots with higher flexibility and lower cost have become a hot topic in the manufacturing field. In terms of practical machining applications, they are mainly employed in the situations with low cutting forces such as deburring, chamfering and polishing. However, the weak stiffness of robot induces milling chatter easily. Severe chatter not only damages the dimensional accuracy of parts, but also decreases machining efficiency and tool life. Thus, it is urgent to seek a new method to suppress robotic milling chatter. In this paper, robotic rotary ultrasonic milling (RRUM) technology is used to restrict machining vibration. Meantime, an analytical model of stability is developed. Robotic milling system is considered as a three degrees of freedom (3-DOF) model. After that, based on analysis of dynamic chip thickness, a linear force model is developed through defining an angle γ affected by ultrasonic vibration. Then, the semi-discretization method (SDM) is applied to obtain stability lobe diagrams. The analysis result indicates that stability region of RRUM is improved by 133% compared with robotic conventional milling (RCM). Finally, verification experiments are carried out to prove the rationality and effectiveness of these stability lobe diagrams.     

Remote robotic underwater grinding system and modeling for rectification of hydroelectric structures

A submersible grinding robot has been designed to automate the dam gate metallic structure repair process. In order to measure and control the amount of material removed during the process, an empirical approach for modeling the material removal rate (MRR) of the underwater grinding application is proposed and presented in this paper. The objective is to determine the MRR in terms of the process parameters such as cutting speed and grinding power over a range of variable wheel diameters. Experiments show that water causes drag and a significant loss of power occurs during grinding. An air injector encasing the grinding wheel has been prototyped, and it is shown that power loss can be reduced by up to 80%. A model, based on motor characterization and empirical relations among system and process parameters, is developed for predicting MRR which will be used for the robotic grinding control system. A validation is carried out through experiments, and confirms the good accuracy of the model for predicting the depth of cut for underwater grinding. A comparative study for dry and underwater grinding is also conducted through experiments and shows that the MRR is higher for underwater grinding than in dry conditions at low cutting speeds.