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.     

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.     

Calibration of a hand/eye matrix and a connection matrix usingrelative pose measurements

The problem investigated in this paper is an extension of the robotic hand/eye calibration problem. The system consists of a robot, a gimbal, and a three-dimensional (3D) sensor. It is assumed that the sensor, the gimbal, and the robot are calibrated in advance; therefore, their inaccuracy is negligible. The task is to determine the hand/eye matrix that relates the sensor coordinate frame to the first gimbal coordinate frame and the connection matrix that relates the last gimbal coordinate frame to the robot tool frame. The paper focuses on a special case of this problem. The robot is an x-y-z type and the gimbal has two rotary joints. Linear and iterative methods are presented, along with the discussion on issues such as solution uniqueness and parameter observability. Simulation and experimental results are presented to demonstrate the feasibility of the method     

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.     

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.     

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%。     

Position-based force tracking adaptive impedance control strategy for robot grinding complex surfaces system

As a key technology of robot grinding, force control has great influence on grinding effects. Based on the traditional impedance control, a position-based force tracking adaptive impedance control strategy is proposed to improve the grinding quality of aeroengine complex curved parts, which considers the stiffness damping environmental interaction model, modifies the reference trajectory by a Lyapunov-based approach to realize the adaptive grinding process. In addition, forgotten Kalman filter based on six-dimensional force sensor is used to denoise the force information and a three-step gravity compensation process including static base value calculation, dynamic zero update and contact force real-time calculation is proposed to obtain the accurate contact force between tool and workpiece in this method. Then, to verify the effectiveness of the proposed method, a simulation experiment which including five different working conditions is conducted in MATLAB, and the experiment studying the deviation between the reference trajectory and the actual position is carried out on the robot grinding system. The results indicate that the position-based force tracking adaptive impedance control strategy can quickly respond to the changes of environmental position, reduce the fluctuation range of contact force in time by modifying the reference trajectory, compensate for the defect of the steady-state error of the traditional impedance control strategy and improve the surface consistency of machined parts.     

Advanced teleoperation and control system for industrial robots based on augmented virtuality and haptic feedback

There are some industrial tasks that are still mainly performed manually by human workers due to their complexity, which is the case of surface treatment operations (such as sanding, deburring, finishing, grinding, polishing, etc.) used to repair defects. This work develops an advanced teleoperation and control system for industrial robots in order to assist the human operator to perform the mentioned tasks. On the one hand, the controlled robotic system provides strength and accuracy, holding the tool, keeping the right tool orientation and guaranteeing a smooth approach to the workpiece. On the other hand, the advanced teleoperation provides security and comfort to the user when performing the task. In particular, the proposed teleoperation uses augmented virtuality (i.e., a virtual world that includes non-modeled real-world data) and haptic feedback to provide the user an immersive virtual experience when remotely teleoperating the tool of the robot system to treat arbitrary regions of the workpiece surface. The method is illustrated with a car body surface treatment operation, although it can be easily extended to other surface treatment applications or even to other industrial tasks where the human operator may benefit from robotic assistance. The effectiveness of the proposed approach is shown with several experiments using a 6R robotic arm. Moreover, a comparison of the performance obtained manually by an expert and that obtained with the proposed method has also been conducted in order to show the suitability of the proposed approach.     

Sensor-based force decouple controller design of macro–mini manipulator

A robotic polishing system includes a force-controlled end-effector (mini manipulator) and a position-controlled industrial robot (macro manipulator). This combination mode has a fast response and a large workspace. However, the force-controlled axis component of the macro motions and the geometric of the workpiece surfaces will affect the contact force response rate and tracking accuracy due to the coupling dynamics between the macro and mini, limiting system performance. A new dynamic decoupling method employing dual force sensors (DFSs) is proposed to address these problems. One of the force sensors installed between the endpoint of the macro and the fixed platform of the mini realizes the dynamic decoupling of the macro and mini. The other one is added at the endpoint of the mini to obtain the interaction force in contact with the environment and feed it back to the control loop. When the disturbances produced by the macro trajectories and the uncertainties coming from the workpiece are introduced into the system, the proposed method can improve force response rate and tracking accuracy without knowing the dynamic models and parameters of the macro and the geometric of the workpiece surface. Several experiments are carried out under various conditions. Experimental results indicate that the contact force response rate and tracking error of DFSs are better than those of the conventional force-controlled and impedance matching methods, proving the proposed method’s effectiveness. In addition, the last comparison experiment verifies that the DFSs method applies to different kinds of end-effectors with various dynamics.     

Robotic sanding system for new designed furniture with free-formed surface

In this paper, a sanding system based on an industrial robot with a surface following controller is proposed for the sanding process of wooden materials constructing furniture. Handy air-driven tools can be easily attached to the tip of the robot arm via a compact force sensor. The robotic sanding system is called the 3D robot sander. The robot sander has two novel features. One is that the polishing force acting between the tool and wooden workpiece is delicately controlled to track a desired value, e.g., 2 kgf. The polishing force is defined as the resultant force of the contact force and kinetic friction force. The other is that no complicated teaching operation is required to obtain a desired trajectory of the tool. Cutter location (CL) data, which are tool paths generated by a CAD/CAM system, are directly used for the basic trajectory of the handy tool attached to the robot arm. The robot sander can be applied to the sanding task of free-formed curved surface with which conventional sanding machines have not been able to cope. The effectiveness and promise are shown and discussed through a few experiments.     

A new force-depth model for robotic abrasive belt grinding and confirmation by grinding of the Inconel 718 alloy

Robotic abrasive belt grinding has been successfully applied to the grinding and polishing of aerospace parts. However, due to the flexible characteristics of robotic abrasive belt grinding and the time-varying characteristics of the polishing contact force, as well as the plastic and difficult-to-machine material properties of Inconel 718 alloy, it is very difficult to control the actual removal depth and force of the polished surface, which brings great challenges to robot automatic polishing. Therefore, the relationship between the grinding force and the grinding depth in the robotic abrasive belt grinding is analyzed in detail, the robot machining pose error model considering the deformation of the grinding head is established, and the Inconel 718 alloy machining experiment of the robotic abrasive belt grinding is designed. The mapping relationship between the grinding force and the grinding depth is obtained, and the grinding force ratio in the downgrinding and upgrinding mode is discussed. The experimental and theoretical comparisons results show that with the increase of the grinding depress depth, both the grinding depth and the grinding force show an irregular increasing trend, and the increasing trend of the grinding force (increases by about 344.44%–445.45%) is obviously greater than that of the grinding depth (increases by about 52.94%). When the grinding depress depth is large (greater than 3 mm), the feed direction force and the normal force appear obvious secondary pressure peaks at the beginning and end of grinding, which has not been seen in previous studies. In addition, regardless of whether it is downgrinding or upgrinding, the grinding force ratio decreases with the increase of the depress depth, and the grinding force ratio of downgrinding (average 0.668) is smaller than that of upgrinding (average 0.724). This study provides a reference for robotic abrasive belt grinding, and the surface quality of Inconel 718 alloy of robotic abrasive belt grinding can be further improved through the optimization of force and depth.     

点云下地平面检测的RGB-D相机外参自动标定

目的 RGB-D相机的外参数可以被用来将相机坐标系下的点云转换到世界坐标系的点云,可以应用在3维场景重建、3维测量、机器人、目标检测等领域。 一般的标定方法利用标定物（比如棋盘）对RGB-D彩色相机的外参标定,但并未利用深度信息,故很难简化标定过程,因此,若充分利用深度信息,则极大地简化外参标定的流程。基于彩色图的标定方法,其标定的对象是深度传感器,然而,RGB-D相机大部分则应用基于深度传感器,而基于深度信息的标定方法则可以直接标定深度传感器的姿势。方法 首先将深度图转化为相机坐标系下的3维点云,利用MELSAC方法自动检测3维点云中的平面,根据地平面与世界坐标系的约束关系,遍历并筛选平面,直至得到地平面,利用地平面与相机坐标系的空间关系,最终计算出相机的外参数,即相机坐标系内的点与世界坐标系内的点的转换矩阵。结果 实验以棋盘的外参标定方法为基准,处理从PrimeSense相机所采集的RGB-D视频流,结果表明,外参标定平均侧倾角误差为-1.14°,平均俯仰角误差为4.57°,平均相机高度误差为3.96 cm。结论 该方法通过自动检测地平面,准确估计出相机的外参数,具有很强的自动化,此外,算法具有较高地并行性,进行并行优化后,具有实时性,可应用于自动估计机器人姿势。     

质心坐标系的牛顿-欧拉动力学方程

在机器人的运动学和动力学分析中,杆的附件坐标系通常是建立在关节轴线上的.本文采用在机器人各杆的质心建立杆附体坐标系的方法,推导了机器人的牛顿-欧拉动力学递推方程.并以平面两杆机械手为例给出这种方法的结果.     

移动机器人足目标定方法

为构建完整的移动机器人视觉导航系统,提出了一种移动机器人足目标定方法．该方法以一个带有数个已知标记点的平面反射镜为中介,分别标定出平面镜坐标系与摄像机坐标系的欧氏变换关系、镜像坐标系与摄像机坐标系的欧氏变换关系、镜像坐标系与平面镜坐标系的欧氏变换关系及机器人坐标系与平面镜坐标系的欧氏变换关系,从而确定摄像机坐标系和移动机器人车体坐标系的欧氏变换关系,实现了移动机器人足目标定．实验结果表明,该方法简单、有效,无需昂贵的标定装置和复杂的标定过程,且可实现现场标定．     

加速度传感器动态校准系统不确定度评估方法的研究

针对加速度传感器动态校准获取其工作频带的过程中,因校准系统存在测试不确定度导致传感器校准不准确的问题,设计了某型号加速度传感器的动态校准试验,以试验数据为依据提出了系统不确定度评估方法。通过对冲击试验台为冲击激励源构成的动态校准系统测试过程中几个主要误差源的分析,结合文中提出的系统不确定度评估方法计算出校准系统的测试标准不确定为1.77%,扩展不确定度为3.54%。对比测量仪器特性评定相关指标可知该校准系统符合工程校准需要,而该评估方法的提出对校准系统的不确定度评估有一定的现实指导意义。     

Compliant motion using a mobile manipulator: an operational space formulation approach to aircraft canopy polishing

The operational space formulation provides a framework for the analysis and control of robotic systems with respect to interactions with their environments. In this paper, we discuss its implementation on a mobile manipulator programmed to polish an aircraft canopy with a curved surface of unknown geometry. The polishing task requires the robot to apply a specified normal force on the canopy surface while simultaneously performing a compliant motion keeping the surface of the grinding tool tangentially in contact with the workpiece. A human operator controls the mobile base via a joystick to guide the polishing tool to desired areas on the canopy surface, effectively increasing the mobile manipulator's reachable workspace. The results demonstrate the efficacy of compliant motion and force regulation based on the operational space formulation for robots performing tasks in unknown environments with robustness towards base motion disturbances. The mobile manipulator consists of a PUMA 560 arm mounted on top of a Nomad XR4000 mobile base. Implementation issues are discussed and experimental results are shown.     

Simulation of micro-patterns engraved by grinding process with screw shaped wheel

Grinding is an important means of realizing precision and ultra-precision machining of workpiece surface. The surface patterning of workpiece directly affects its mechanical properties such as friction, wear, contact stiffness, fitting property. Therefore, prediction of the geometry of the workpiece surface is very important to evaluate the workpiece quality to perform mechanical function accurately. In this paper using MATLAB simulation, prediction for the geometry pattern of the workpiece according to the developed shape of the grinding wheel dressed by thread cutting was studied. The model for the geometry of the grinding wheel surface was first developed and subsequently, a new simulation model for surface pattern by grinding process was established. The simulation results could be used to optimize the grinding process and to improve the workpiece surface quality or predict the surface pattern by given grinding parameters.     

基于目标检测的机器人手眼标定方法

智能协作机器人依赖视觉系统感知未知环境中的动态工作空间定位目标,实现机械臂对目标对象的自主抓取回收作业。RGB-D相机可采集场景中的彩色图和深度图,获取视野内任意目标三维点云,辅助智能协作机器人感知周围环境。为获取抓取机器人与RGB-D相机坐标系之间的转换关系,提出基于yolov3目标检测神经网络的机器人手眼标定方法。将3D打印球作为标靶球夹持在机械手末端,使用改进的yolov3目标检测神经网络实时定位标定球的球心,计算机械手末端中心在相机坐标系下的3D位置,同时运用奇异值分解方法求解机器人与相机坐标系转换矩阵的最小二乘解。在6自由度UR5机械臂和Intel RealSense D415深度相机上的实验结果表明,该标定方法无需辅助设备,转换后的空间点位置误差在2 mm以内,能较好满足一般视觉伺服智能机器人的抓取作业要求。