Tool Center Point Research Articles

Contouring error control of the tool center point function for five-axis machine tools based on model predictive control

In five-axis machining processes, tool center point (TCP) control is often used to adjust the orientation of the ball end milling while maintaining constant tool center point coordinates in the workpiece frame. Therefore, TCP contouring error decreases the quality of the machined surface, but very few efforts have been made so far to minimize the contouring error of five-axis TCP by feedback control. To this end, a model predictive control (MPC) method is developed here for rolling optimization. More precisely, the contouring error model of five-axis TCP function is developed, where a Jacobian matrix is used to approximate the relationship between the contouring error and five drivers’ tracking errors. The MPC algorithm is afterwards derived, where constraints of control signals are considered as well to avoid violation of the power limitation of the drivers. Finally, the proposed MPC method is examined by extensive experiments using testing tool path of ISO 10791-6: 2014 to verify its effectiveness and superiority.

3D Surface Reconstruction of Plant Seeds by Volume Carving: Performance and Accuracies.

We describe a method for 3D reconstruction of plant seed surfaces, focusing on small seeds with diameters as small as 200 μm. The method considers robotized systems allowing single seed handling in order to rotate a single seed in front of a camera. Even though such systems feature high position repeatability, at sub-millimeter object scales, camera pose variations have to be compensated. We do this by robustly estimating the tool center point from each acquired image. 3D reconstruction can then be performed by a simple shape-from-silhouette approach. In experiments we investigate runtimes, theoretically achievable accuracy, experimentally achieved accuracy, and show as a proof of principle that the proposed method is well sufficient for 3D seed phenotyping purposes.

Smooth Geodesic Interpolation for Five-Axis Machine Tools

The smoothness of tool trajectories would be as important as that of axis trajectories for five-axis machining. The present study proposes a geodesic interpolation for five-axis machine tools. The tool positions and orientations are fitted to geodesics, respectively, to achieve smooth accurate motions of the tool center point (TCP), smooth axis motions, and the constant velocities of the TCP position and orientation. Not only the linear and circular geodesic interpolation are proposed, but also the acceleration/deceleration and block connection schemes are considered. The method is illustrated by simulations and experiments on a five-axis machining center, and compared to the interpolator with the TCP control. The real-time implementation of geodesic interpolation is well within the scope of modern CNC systems.

Iterative learning for machine tools using a convex optimisation approach

Abstract Dynamic, quasi-static and motion control deviations lead to nonlinear but systematic tracking errors. It is shown that these errors can be reduced significantly by adjusting the set points using an optimization based iterative learning approach. This method uses either values obtained from internal encoders or alternatively tool center point measurements. The approach is presented, discussed and validated using simulation and measurement results.

Online machining error estimation method of numerical control gear grinding machine tool based on data analysis of internal sensors

This paper presents an online estimation method of cutting error by analyzing of internal sensor readings. The internal sensors of numerical control (NC) machine tool are selected to avoid installation problem. The estimation mathematic model of cutting error was proposed to compute the relative position of cutting point and tool center point (TCP) from internal sensor readings based on cutting theory of gear. In order to verify the effectiveness of the proposed model, it was simulated and experimented in gear generating grinding process. The cutting error of gear was estimated and the factors which induce cutting error were analyzed. The simulation and experiments verify that the proposed approach is an efficient way to estimate the cutting error of work-piece during machining process.

Orientation Smoothing for 5-Axis Machining Using Quasi-Redundant Degrees of Freedom

A new approach for set point generation in the field of 5-axis machining using quasi-redundant degrees of freedom is introduced in this study. In machine tools that possess both rotational and translational axes, no bijective correlation exists between the tool center point and the movement of the machine tool axes based on the manufacturing tolerances. Depending on the manufacturing process, as many as two additional degrees of freedom exist that allow the machine tool axes movement to be optimised within the given manufacturing tolerances with respect to the axes’ inertia. In this study to reduce the mechanical excitation of the machine tool, the jerk of the machine tool axes is minimised. To enhance robustness, the optimisation problem is formulated as a quadratic program with linear constraints. This problem can be solved by using an interior point method. An application example shows that when exploiting quasi-redundancy, the mechanical excitation of the machine tool can be reduced.

Elastodynamic Model-Based Vibration Characteristics Prediction of a Three Prismatic–Revolute–Spherical Parallel Kinematic Machine

Parallel kinematic machines (PKMs) have been proposed as an alternative solution for high-speed machining (HSM) tool for several years. However, their dynamic characteristics are still considered an issue for practice application. Considering the three prismatic–revolute–spherical (3-PRS) PKM design as a typical compliant parallel device, this paper applies substructure synthesis strategy to establish an analytical elastodynamic model for the device. The proposed model considers the effects of component compliances and kinematic pair contraints so that it can predict the dynamic characteristics of the 3-PRS PKM. Based on eigenvalue decomposition of the characteristic equations, the natural frequencies and corresponding vibration modes at a typical configuration are analyzed and verified by numerical simulations. With an algorithm of workspace partitions combining with eigenvalue decompositions, the distributions of lower-order natural frequencies throughout the workspace are computed to reveal a strong dependency of dynamic characteristics on mechanism's configurations. In addition, the effects of the radii of the platform and the base along with the cross section of the limb on lower-order natural frequencies are analyzed to provide useful information during the early design stage. At last, frequency response analysis for the tool center point (TCP) is worked out based on the elastodynamic model to provide primary guideline for cutting chatter avoidance.

Calibration of robot tool centre point using camera-based system

Robot Tool Centre Point (TCP) calibration problem is of great importance for a number of industrial applications, and it is well known both in theory and in practice. Although various techniques have been proposed for solving this problem, they mostly require tool jogging or long processing time, both of which affect process performance by extending cycle time. This paper presents an innovative way of TCP calibration using a set of two cameras. The robot tool is placed in an area where images in two orthogonal planes are acquired using cameras. Using robust pattern recognition, even deformed tool can be identified on images, and information about its current position and orientation forwarded to control unit for calibration. Compared to other techniques, test results show significant reduction in procedure complexity and calibration time. These improvements enable more frequent TCP checking and recalibration during production, thus improving the product quality.

Improving Performance of Turn-milling by Controlling Forces and Thermally Induced Tool-center Point (TCP) Displacement

Improving performance during fine turn-milling operations including accuracy and productivity requires controlling of the cutting forces and the thermally induced displacement of the cutting edge. The objective of this investigation is to determine the thermally induced displacement of TCP during turn-milling and to reduce this displacement by using pressurized cooled air. The forces and tool elongation simulated by FEM are compared to measured values. It was shown that the amount of tool elongation could be 40% of the depth of cut in fine turn-milling, and it is possible to predict the tool elongation by FEM. Furthermore, cooled air can reduce the tool elongation by 65%.

Simulation-based Correction Approach for Thermo-elastic Workpiece Deformations During Milling Processes

Based on the method of adaptive finite elements (FEM) a correction approach has been considered to identify the influence of thermo-elastic workpiece deformations during the production process milling.The paper presents a simulation-based tolerance variation calculation of cutting paths, which is caused by the heat input of the machine tool. Therefore a mathematical method is developed to numerically depict the progress of the miller with different curves A(t), B(t) and C(t). These curves are used to map the state of the milling path during the production process as well as to compare the current workpiece contour and the target workpiece contour. The tool center point (TCP) correction results from mapping of time-dependent deformation fields from the FE simulation. The aim is, on the one hand, to make statements before the production about keeping the tolerance, and on the other hand, to derive other correction approaches for the adaption of the cutting path coordinates.

Design of a Measurement Setup and First Experiments on the Influence of CO2-cooling on the Thermal Displacements on a Machine Tool

The paper investigates the thermal influences of CO2- and oil-cooling on the precision of a machine tool. If CO2 is used for process cooling instead of oil, a different temperature distribution in the machine is observed. Different temperature distributions lead to different tool center point (TCP)-displacements. To measure TCP-displacements under real process conditions with CO2 and oil, a special measurement setup is designed, built and applied. A representative process for the machine is chosen to compare the different process coolings. The results show that the choice of coolant has a crucial influence on the TCP-displacements. It is shown that the thermally induced displacements can be reduced by 15 μm in y-direction, but increase about 4 μm in x-direction, when the coolant is changed from oil to CO2.

Modeling of Heat Fluxes During Machining and Their Effects on Thermal Deformation of the Cutting Tool

The heat fluxes generated by cutting processes lead to thermal deformations in the tools. Particularly, in precision machining it is essential to know the amount of the process heat and its distribution of heat fluxes into tool, workpiece and chips. This paper presents an extended methodology for the calculation of these heat fluxes in machining operations. Additionally, by comparison of experimental results with finite element simulations, the thermally caused tool center point (TCP) displacements in turn-milling operations are discussed. Furthermore, the influence of cooled air on TCP displacements is shown.

Interpolation of Toolpath by a Postprocessor for Increased Accuracy in Multi-axis Machining

The article focuses on the issue of generating points of the toolpath for multi-axis machining. In multi-axis machining, it is possible to control the toolpath using the coordinate transformations of tool center point (TCP) in the control system, or these transformations in the control system are not available and it is necessary to use the transformations in the postprocessor. However, without the use of TCP, the required toolpath tolerance is not respected. Therefore, an algorithm has been proposed in the postprocessor that dynamically generates new toolpath points so as to maintain the required tolerance and to ensure manufacturing accuracy. This algorithm has been verified by implementation in the postprocessor and generation of NC programs for machining of impeller blades. By the postprocessor recalculated tool path meets the required tolerance.

Analysis of the Real-time Compensation for Thermal Error at CNC Milling Machine

This paper focuses on analyzing and discussing thermal errors at CNC milling machines. The thermal affection makes the deformation of machine tools and is the main problem of accuracy error over than 65%. Effectively improving or controlling thermal errors is helpful for the accuracy of machine. The key point of this paper is the position of tool center point. Firstly, 14 pieces of temperature sensors are used for checking the real field of temperature, and then four sensors with better linearity are chosen for real situations. The test bar and 5 pieces of non-contact sensors are utilized for clearly getting the displacement of the tool center point and head during the process. Based on the theory of MRA (Multiple Regression Analysis), the external zero-point is shifted to build the mathematical module. The database is input to the control board and the PLC is used for real-time compensation for machining. Finally, two work pieces (one compensated and one non-compensated) are tested and compensated one presents better precision.

Characteristic Diagram Based Correction Algorithms for the Thermo-elastic Deformation of Machine Tools

This paper presents a characteristic diagram based approach to the correction of thermally induced tool center point displacements of machine tools. The entire process from the introduction of the correction principle to the issues arising in the online capable application of this correction method will be discussed, with special focus on the improvement and optimization of the calculation, input selection and grid structures of the characteristic diagrams.

Enhanced Absolute Accuracy of an Industrial Milling Robot Using Stereo Camera System

In this publication an approach for increasing the absolute positioning accuracy of an industrial milling robot with help of a stereo camera system is presented. To measure the position and orientation of the robot tool center point, a specific adapter with retro reflective markers is mounted on the spindle. The calculation of the transformation of this target holder to the robot tool is part of this paper along with the calibration from the robot to the camera system. These datasets serve as the basis of a static pose control, where the robot absolute accuracy errors will be greatly reduced.

Iterative Learning for Machine Tools Using a Convex Optimisation Approach

Dynamic, quasi-static and motion control deviations lead to nonlinear but systematic tracking errors. It is shown that these errors can be reduced significantly by adjusting the set points using an optimization based iterative learning approach. This method uses either values obtained from internal encoders or alternatively tool center point measurements. The approach is presented, discussed and validated using simulation and measurement results.

GRIBBOT – Robotic 3D vision-guided harvesting of chicken fillets

In Norway, the final stage of front half chicken harvesting is still a manual operation due to a lack of automated systems that are suitably flexible with regard to production efficiency and raw material utilisation. This paper presents the ‘GRIBBOT’ – a novel 3D vision-guided robotic concept for front half chicken harvesting. It functions using a compliant multifunctional gripper tool that grasps and holds the fillet, scrapes the carcass, and releases the fillet using a downward pulling motion. The gripper has two main components; a beak and a supporting plate. The beak scrapes the fillet down the rib cage of the carcass following a path determined by the anatomical boundary between the meat and the bone of the rib cage. The supporting plate is actuated pneumatically in order to hold the fillet. A computer vision algorithm was developed to process images from an RGB-D camera (Kinect v2) and locate the grasping point in 3D as the initial contact point of the gripper with the chicken carcass for harvesting operation. Calibration of camera and robot was performed so that the grasping point was defined using 3D coordinates within the robot’s base coordinate frame and tool centre point. A feed-forward Look-and-Move control algorithm was used to control the robot arm and generate the motion trajectories, based on the 3D coordinates of the grasping point as calculated from the computer vision algorithm. The results of an experimental proof-of-concept demonstration showed that GRIBBOT was successful both in scraping the carcass, grasping chicken fillets automatically and in completing the front half fillet harvesting process. It demonstrated a potential for the flexible robotic automation of the chicken fillet harvesting operation. Its commercial application, with further development, can result in automated fillet harvesting, while future research may also lead to optimal raw material utilisation. GRIBBOT shows that there is potential to automate even the most challenging processing operations currently carried out manually by human operators.

Accurate sensorless lead-through programming for lightweight robots in structured environments

Nowadays, programming an industrial manipulator is a complex and time-consuming activity, and this prevents industrial robots from being massively used in companies characterized by high production flexibility and rapidly changing products. The introduction of sensor-based lead-through programming approaches (where the operator manually guides the robot to teach new positions), instead, allows to increase the speed and reduce the complexity of the programming phase, yielding an effective solution to enhance flexibility. Nevertheless, some drawbacks arise, like for instance lack of accuracy, need to ensure the human operator safety, and need for force/torque sensors (the standard devices adopted for lead-through programming) that are expensive, fragile and difficult to integrate in the robot controller. This paper presents a novel approach to lead-through robot programming. The proposed strategy does not rely on dedicated hardware since torques due to operator's forces are estimated using a model-based observer fed with joint position, joint velocity and motor current measures. On the basis of this information, the external forces applied to the manipulator are reconstructed. A voting system identifies the largest Cartesian component of the force/torque applied to the manipulator in order to obtain accurate lead-through programming via admittance control. Finally an optimization stage is introduced in order to track the joint position displacements computed by the admittance filter as much as possible, while enforcing obstacle avoidance constraints, actuation bounds and Tool Centre Point (TCP) operational space velocity limits. The proposed approach has been implemented and experimentally tested on an ABB dual-arm concept robot FRIDA.

Autonomous path planning solution for industrial robot manipulator using backpropagation algorithm

Here, we propose an autonomous path planning solution using backpropagation algorithm. The mechanism of movement used by humans in controlling their arms is analyzed and then applied to control a robot manipulator. Autonomous path planning solution is a numerical method. The model of industrial robot manipulator used in this article is a KUKA KR 210 R2700 EXTRA robot. In order to show the performance of the autonomous path planning solution, an experiment validation of path tracking is provided. Experiment validation consists of implementation of the autonomous path planning solution and the control of physical robot. The process of converging to target solution is provided. The mean absolute error of position for tool center point is also analyzed. Comparison between autonomous path planning solution and the numerical methods based on Newton–Raphson algorithm is provided to demonstrate the efficiency and accuracy of the autonomous path planning solution.