Large Free-form Surface Research Articles

Multi-robot coverage path planning for dimensional inspection of large free-form surfaces based on hierarchical optimization

Multi-robot coverage path planning for dimensional inspection of large free-form surfaces based on hierarchical optimization

Five-Axis Trochoidal Sweep Scanning Path Planning for Free-Form Surface Inspection

Freeform surface inspection is a vital process in manufacturing, and the newly emerged five-axis continuous sweep scanning technology is one of the most efficient and accurate means for free-form surface inspection. The key in employing the five-axis inspection technology is to plan an effective and efficient sweep scanning path respecting both the inspection surface and the properties of the inspection device. Current sweep scanning paths suffer from the oscillating pattern that forces the high-speed stylus of the five-axis inspection device to swing back and forth frequently, which imposes excessive kinematic loading on the probe head, and in turn, undermines the inspection stability and efficiency. In this paper, we present a new trochoidal sweep scanning path for freeform surface inspection. The proposed method is novel in that the generated inspection path is a smooth trochoidal-shaped curve on the surface without causing any oscillation during a sweep scanning process. The kinematic performance of the probe head is significantly improved, which enables much better inspection stability and efficiency. Both computer simulation and physical inspection experiments of the proposed method have been conducted, whose results convincingly validate the feasibility and superiority of the proposed trochoidal sweep scanning path over the traditional oscillating sweep scanning path in terms of inspection stability and efficiency. Note to Practitioners—This work is motivated by the high requirements on the stability and efficiency of the newly emerged five-axis sweep scanning technology for freeform surface inspection. A novel trochoidal sweep scanning path is generated to make the original heavy-loaded rotary axes of the five-axis inspection machine move smoothly without needing frequent acceleration and deceleration as suffered from a traditional five-axis oscillating sweep scanning path. The kinematic loads of the five-axis inspection device are significantly reduced based on the proposed sweep scanning path; thus, the inspection machine could afford a higher sweep velocity of inspection, and the inspection stability and efficiency could be improved considerably as well. Compared with a traditional oscillating sweep scanning path, in our tests, the proposed method can improve the inspection efficiency by more than 20%, while the maximal kinematic loads (e.g., the velocity and jerk of the C-axis) can be reduced by 55%-85%. With superior inspection efficiency and stability, the proposed trochoidal sweep scanning path can be utilized for accurate and efficient inspection of large freeform surfaces like those in the aerospace and automotive industries.

A 3-D Reconstruction Method for Large Freeform Surfaces Based on Mobile Robotic Measurement and Global Optimization

Automatic measurement and 3-D reconstruction are critical to the robotic manufacturing of large surface parts. The existing approaches have limitations in terms of measurement range, automation degree, and reconstruction accuracy. To achieve the automatic measurement and reconstruction of large freeform surface parts with high accuracy and high efficiency, this article proposes a measurement approach based on a mobile industrial robot and a 3-D reconstruction method based on multi-viewpoint cloud registration. First, to perform the automatic measurement and obtain the locally measured point clouds of the large freeform surface part, a mobile measurement robot is employed. And the initial global measurement poses, estimated by the robot’s pose information and the light detection and rangings (LiDAR’s) location result, are used to coarsely stitch the local multi-viewpoint clouds. Second, an adjacency judgment approach based on point cloud collision detection is proposed, with which the local measurement pose constraints are constructed. Finally, a global optimization approach based on improved pose graph optimization is proposed to achieve accurate global measurement poses estimation and high-accuracy reconstruction of multi-viewpoint clouds. Experiments were carried out on measuring and reconstructing a standard test blade and a real wind turbine blade. The results demonstrate that the proposed approach reaches the mean accuracy of 0.835 mm for a measuring range of about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.5\times2.5\,\,\text{m}^{2}$ </tex-math></inline-formula> , which outperforms the previous methods in terms of measuring range, accuracy, and practicality.

Assessing surface imperfections of freeforms using a robotic light scattering sensor

Light scattering-based characterization techniques are fast, sensitive, and robust, which can be exploited for roughness measurements or homogeneity and defect assessment. However, only compact scattering instruments enable even large freeform surfaces to be inspected close to fabrication or even within fabrication processes. The combination of a light scattering sensor with an industrial robot, as well as its application for full surface evaluation of aspheriscal and freeform surfaces, is addressed. In addition to the technical considerations regarding the sensor, the robotic handling system, or the sample curvature, examples of application to different optical components fabricated at the Fraunhofer Institute for Applied Optics and Precision Engineering using state-of-the-art techniques are presented. The results include the discussion of anisotropic surface structures induced by diamond turning or polishing as well as the analysis of the influence of a protection layer.

Large scale layering laser surface texturing system based on high speed optical scanners and gantry machine tool

Abstract A layering laser surface texturing system for large freeform surface workpiece was realized with a reconfigurable structure of usual mechanical gantry machine tool plus laser processing head, which is composed of dual high speed galvanometer optical scanning axes and a mechanical linear axis. The principle of layering processing ability of laser focus point and the method to divide freeform surface into partitions and layers was analyzed. The kinematic of five axes machine tool with three axes laser processing head was derived. A novel computer aided laser texturing software named WnloLaserRobots was designed on visual C++ platform, which provides full function of 3D model data analyzing and real time processing control. The laser head position, the orientation data, and the layering scanning paths for processing a freeform surface were generated through importing and analyzing the universal initial graphics exchange specification (IGES) model file. In the control module, the software sends command to five axes gantry machine tool to position laser head normal to the processing surface partition and control laser processing head to accomplish layering laser texturing. This proposed system is particular suitable for laser ablation of film layer from multilayer materials on large scale free form surface. It has been used in practical application and satisfactory results were obtained.

Surface modeling of workpiece and tool trajectory planning for spray painting robot.

Automated tool trajectory planning for spray-painting robots is still a challenging problem, especially for a large free-form surface. A grid approximation of a free-form surface is adopted in CAD modeling in this paper. A free-form surface model is approximated by a set of flat patches. We describe here an efficient and flexible tool trajectory optimization scheme using T-Bézier curves calculated in a new way from trigonometrical bases. The distance between the spray gun and the free-form surface along the normal vector is varied. Automotive body parts, which are large free-form surfaces, are used to test the scheme. The experimental results show that the trajectory planning algorithm achieves satisfactory performance. This algorithm can also be extended to other applications.

Development and Polishing Process of a Mobile Robot Finishing Large Mold Surface

□ A novel self-determination polishing robot finishing large mold free-form surface is developed, and the finishing process method is researched. Contrary to traditional approaches, our premise is that a large mold surface can be polished by using a small robot. This robot system is mainly composed of a polishing robot part, a computer system and a visual positioning system. A type of robot with four uniform distribution wheels was designed, which has two driving wheels and two driven wheels. Active compliant control of the polishing tool was provided by a pneumatic servo system, and a new special compliant abrasive tool was proposed on the basis of robot characteristics. The process planning steps consisted of subdividing the free-form surface, choosing an abrasive tool, planning the polishing path and optimizing machining parameters. Based on the orthogonal experiment and the grey relational analysis method, the optimal parameter combination was obtained for polishing force, tool speed and feed rate. Aiming to polishing times, the surface roughness method and polishing efficiency method were studied in detail. The polishing experiments were carried out in the robot using process parameters obtained by the efficiency method. These research results provided significant theory foundation and experimental data for a mobile robot planning polishing to realize intelligible process parameter selection.

High speed non-contact on-machine profile measurement for ultrahigh precision machining of large free-form optical surface

High speed non-contact on-machine profile measurement for ultrahigh precision machining of large free-form optical surface

Study on Symmetrical Machining Technology of a Ship Model Surface

Because a ship model surface (SMS) is a large symmetrical freeform surface (LSFS), the machining efficiency and precision of current handcraft method is very low. According to the shape characteristics of SMS, this paper adopted symmetrical machining (SM) technology to manufacture SMS, solved the collision of cutters or headstocks in SM process, proposed the machining solutions for symmetrical machining residuals (SMR), divided the manufacturing process to SM stage and SMR stage, and planed the tool path respectively. The real machining results of a twin-skeg SMS, which was machined by a 4-axis SM machine tool, conforms that the SM can reduce the SMS machining time from 60 workdays needed for handcraft method to 7 hours, and the machining precision and surface fairness are also improved obviously.

Timoshenko beam-based stability and natural frequency analysis for heavy load mechanical spindles

The heavy load mechanical spindle is an important functional component in a 5-axis computer numerical control (CNC) machine tool, which is used to process large and complex free-form surfaces. It is necessary to obtain the natural frequency and analyze the spindle stability for improving the machining precision. In this paper, Timoshenko beam theory is introduced to model the mechanical spindle shaft, where the centrifugal force and gyroscopic effects are considered. Stability of the heavy load mechanical spindle shaft is analyzed, and the buckling load of the spindle shaft is obtained under different rotational speeds. The natural frequency of spindle is investigated in a freedom and restraint state, respectively. Comparing the proposed method with the simplified hollow cylinder and shaft prototype in the freedom state, the results show that they are highly correlated with experimental results. For the restraint state, the axial load, rotational speed, gyroscopic effect, and centrifugal force are discussed, and all of these parameters affect the natural frequency. The proposed modeling approach can be used for spindle design and optimization in a given machining process and can be easily extended to other spindle design.

Regular mesh measurement of large free form surfaces using stereo vision and fringe projection

The paper presents a new approach to measure 3D points into a single intrinsically structured regular mesh. The developed system merges stereo vision and fringe projection to measure large area free form surfaces with a freely definable density of points expressed in a global coordinate system. Fringe projection is used to measure dense and regular three-dimensional point clouds using novel algorithms that can determine the 3D points positions, with pre-defined X and Y values, and homologous points simultaneously. Experimental evaluations, using geometric standards and known free form surfaces, demonstrate the feasibility and the advantages of the proposed methods.

Analytical Modeling of the Spindle Shaft of Heavy Load Deflection Angle Milling Head Dynamics Using Timoshenko Beam Model

The spindle shaft of heavy load deflection angle milling head is an important functional component of 5-axis CNC machine tool, which has characteristics of high torque and high power, used for large and complex free-form surface processing. The spindle shaft is modeled as Timoshenko’s beam by including the centrifugal force and gyroscopic effects. The effects on inherent characteristic of the spindle shaft of heavy load deflection angle milling head are analyzed owing to axial force and spindle shaft speed. Simultaneously, numerical solution of spindle shaft is gotten. And, the numerical results are well agree with the exact modeling, which illustrates that the theoretical modeling of spindle shaft is correct and the exact dynamic characteristics of spindle shaft is obtained. The analytical modeling results of spindle shaft are effective and helpful for design and improvement of spindle shaft of heavy load deflection angle milling head.

Experimental Research of the Elastic Abrasive Tool Used for Finishing Mould Surface

A compliant abrasive tool is investigated in this paper. By the polishing experiment of this abrasive tool, the technique of polishing the large mould free-form surface is acquired. The polishing experiments about the elastic abrasive tool are carried out, which indicates that the abrasive tool has the advantages of the high efficiency, the elasticity and compliant performance. The relation of the deflection and polishing force is investigated and it is concluded that they is approximately linearity. The effect of polishing pressure on the surface roughness is researched by experiment. Polishing quality for the edge of abrasive tool was investigated and presented. These experiment results provide significant foundation for planning polishing path and controlling process parameters to achieve good surface roughness and high efficiency in the next research.

A Novel Small Mobile Robot System for Polishing Freeform Surface of Large Mold

Making large automatic device for polishing freeform surface of large mold has many disadvantages. We propose a new thinking of polishing large freeform surface with small mobile robot, and develop a novel small mobile polishing robot which has good maneuverability for moving on and polishing curved surface continuously. By analyzing its kinematics model, algorithm for tracking polishing path is designed, in which discrete points along planed polishing path on surface are taken as robot’s navigation points. And virtual robot view methods are designed in the tracking algorithm to avoid the oscillate phenomena. Both the simulation and field experiments prove that this type robot can satisfy the requirement of polishing process.

Study on Profile-Adaptive Tool System for Small Robots in Large Freeform Surface Polishing

An adaptive tool system of polishing robots for large freeform surface which is installed discoid tool is proposed. To improve processing efficiency, the mechanical of adaptive tool system adjust to curvature changing of large freeform surface is designed. Adaptability of the processing is analyzed and simulated, and kinematic equation of tool system is established. The polishing process of large freeform surfaces with tool system is analyzed.

On Obtaining Dynamic and Static Characteristics of Heavy Load Deflection Angle Milling Head by CAE and Experiment Techniques

The heavy load deflection angle milling head is an important functional component of 5-axis CNC machine tool, which is used for large and complex free-form surface processing. The characteristics of this kind of milling head are implementing multiple degrees of freedom motion with revolution and deflection angle simultaneously. Obtaining milling head dynamic and static characteristics is a pre-requisite problem to resolve for the development and research of heavy load deflection angle milling head independently. Dynamic characteristics are obtained through combination with CAE (Computer-aided Engineering) technique and experiment validation. For CAE analysis, the main parts of milling head are glued together considering milling head operating at a specific position. Experimental validation is carried through with initial fixing constraint style, which validates the correctness of CAE technique. Experimental and CAE analysis results are compared and analyzed. Therefore, exact dynamic characteristics of milling head and unsubstantial segments and its influence factors are gotten. Static characteristic is obtained considering milling head operating in different posture by CAE technique. The analytical method of three points at half of cutter are loaded is introduced. Calculated results indicate that the deformation of tool tip is the largest when milling head operating in 45° posture, which corresponds with the deformation situation of the milling head operating in workshop. The analytical results of dynamic and static characteristics of milling head provide theoretical and experimental support for structure optimization and improving of heavy load deflection angle milling head.

Key techniques of symmetrical machining

To improve the machining efficiency of large symmetrical freeform surface, this paper proposes a new machining method: symmetrical machining (SM). Based on the concept of collision avoidance plane (CAP), collisions among cutters or headstocks are avoided, and the area of machining residue is controlled effectively. Machining residue is eliminated due to proper design of symmetrical tool in mechanism and control project. Taking 5-axis symmetrical machine with columnar headstocks and flat cutters as example, this paper proposes collision avoidance algorithm. Finally, using a twin-skeg ship model surface as an example, the method is applied to avoiding collisions in original tool path. Simulation and real machining results show that the scheme is practical. Furthermore, the machining efficiency of SM is improved 85% more than that of the traditional single cutter technology.

Task-oriented programming of large redundant robot motion

Large robots are a new domain of advanced robotics. Examples of their application fields are tasks like operations on large free-form surfaces, especially aircraft cleaning and removing paint from hulls. They are equipped with a programmable robot control comparable to a control system used for industrial robots. However, conventional teach-in methods are not able to manage the complexity of programming large redundant robot operation on free-form geometries. The Fraunhofer IPA has developed an innovative off-line programming system that allows the creation of robot motion programs which satisfy time and energy optimization criteria. This system helps to avoid collisions within the workspace and to fulfill conditions that arise from the robot kinematics and dynamics. This advanced programming system has been successfully used to generate motion programs for the world’s largest mobile robot, the aircraft cleaning manipulator SKYWASH. In this context offline programs for eleven different types of aircraft have been developed.