Tool Center Point Research Articles

Virtual simulation of five-axis machine tool with consideration of CNC interpolation, servo dynamics, friction, and geometric errors

A virtual machine tool (VMT) simulation system which considers tool center point (TCP) interpolation, geometric errors, servo dynamics, and friction effects for a five-axis machine tool is developed in this paper. A novel five-axis interpolation method is proposed to ensure that maximum velocity constraints for each axis can be satisfied. The geometric error model, including lead screw, straightness, angular and squareness errors is built to analyze the volumetric errors within the working space. The model which includes rigid body motion, friction model and servo control loops is utilized to evaluate servo dynamics and non-linear effects. The errors caused by the locations, servo dynamics, and friction effects are integrated into the VMT simulation program. Simulation results of TCP trajectory are represented by small line segments to generate NC codes. Then the NC codes are fed into VERICUT software to perform virtual cutting. To evaluate the interpolation design, cutting experiments are carried out on the five-axis engraving machine with a PC-based controller. The performances using the proposed interpolation method are comparable with the commercial CNC controller from Heidenhain. The effects of different error sources on the surface are demonstrated by cutting the sculpture of a human face. Path overcut caused by servo dynamics is found at sharp corners, and volumetric errors cause obvious tool marks and poor surface roughness. The proposed methodology can serve as a useful tool in evaluating the behaviors of error sources during the design stage.

Tool Path Generation for Turbine Blades Machining With Twin Tool

Blades are essential parts used in thermal and nuclear power generation. Its machining precision is a vital factor that influences the efficiency and life of those industries. Blades are thin-walled parts, which could easily deform under cutting forces, and hence deteriorate the machining precision. In our previous work, a milling process with twin tool for blade is proposed, in which two tools are assigned to machine the basin and dorsal surfaces simultaneously. It is expected that the cutting forces acted on the basin and dorsal surfaces can be counteracted to reduce the deformation of the blade. In this study, a method of twin-tool paths generation is developed. The tool center points and tool axis vectors are generated with consideration of the cutting forces balance, the machine tool kinematics, the surface geometric precision, and the same number of tool paths on basin and dorsal surfaces. Virtual machining, finite element analysis, and trial cutting are carried out and verified that the method which is used for generating the twin-tool paths is successful. The basin and dorsal surfaces have the same number of tool paths and tool contact point coordinates, which guarantees that the two surfaces can be completely machined and can be machined and finished simultaneously. Furthermore, the cutting forces acted on the basin and dorsal surfaces can achieve the balance along the twin-tool paths. Therefore, the deformation of a blade caused by cutting force is obviously reduced compared with a conventional machining process with a single tool.

Virtual Sensor for Kinematic Estimation of Flexible Links in Parallel Robots.

The control of flexible link parallel manipulators is still an open area of research, endpoint trajectory tracking being one of the main challenges in this type of robot. The flexibility and deformations of the limbs make the estimation of the Tool Centre Point (TCP) position a challenging one. Authors have proposed different approaches to estimate this deformation and deduce the location of the TCP. However, most of these approaches require expensive measurement systems or the use of high computational cost integration methods. This work presents a novel approach based on a virtual sensor which can not only precisely estimate the deformation of the flexible links in control applications (less than 2% error), but also its derivatives (less than 6% error in velocity and 13% error in acceleration) according to simulation results. The validity of the proposed Virtual Sensor is tested in a Delta Robot, where the position of the TCP is estimated based on the Virtual Sensor measurements with less than a 0.03% of error in comparison with the flexible approach developed in ADAMS Multibody Software.

A computational method for key-performance-indicator-based parameter identification of industrial manipulators

We present a novel derivative-based parameter identification method to improve the precision at the tool center point of an industrial manipulator. The tool center point is directly considered in the optimization as part of the problem formulation as a key performance indicator. Additionally, our proposed method takes collision avoidance as special nonlinear constraints into account and is therefore suitable for industrial use. The performed numerical experiments show that the optimum experimental designs considering key performance indicators during optimization achieve a significant improvement in comparison to other methods. An improvement in terms of precision at the tool center point of 40% to 44% was achieved in experiments with three KUKA robots and 90 notional manipulator models compared to the heuristic experimental designs chosen by an experimenter as well as 10% to 19% compared to an existing state-of-the-art method.

A generic method to optimize a redundant serial robotic manipulator's structure

Abstract In this paper, an optimization method for a redundant serial robotic manipulator's structure is proposed in order to improve their performance. Optimization was considered in terms of kinematics using the proposed objective function and the non-linear Levenberg-Marquardt algorithm for multi-variate optimization. Range limits of the joints, bounds of the design parameters, and a constrained workspace are enforced in the proposed method. A desired manipulator can be optimized to cover the required task points using dimensional synthesis. This approach effectively optimizes the link lengths of the manipulator and minimizes the position and orientation errors of the tool center point. A commercial heavy-duty hydraulic, underground tunneling manipulator was used to demonstrate the capability of the proposed optimization method. The obtained results encourage the use of the proposed optimization method in automated construction applications, such as underground tunneling, where the confined environment and the required task add challenges in the design of task-based robotic manipulators.

Multi-axis synchronous interpolation feed rate adaptive planning with rotational tool center point function under comprehensive constraints

Real-time nonlinear error caused by rotation axis motion in five-axis interpolation can be compensated by rotational tool center point function. It is well known that reduction of the interpolation nonlinear error can be achieved effectively by increasing the interpolation steps, but what the value of the steps’ number should be has rarely been mentioned in the existing literature. Taking a five-axis interpolation machine tool with B & C swing axis as an example, the real-time planning and controlling for the integrated feed rate of a five-axis interpolation is proposed by analyzing mechanism of the nonlinear error in the interpolation with rotational tool center point function. The nonlinear error and its derivative of the swing axis are derived. Then, a S-shape feed rate real-time planning and control method about curve interpolation is present first, and then considering the restrictions of the tool axis motion by the feed axis’ mechanical properties and the cutting characteristics, the most appropriate interpolation step length in a single interpolation cycle is calculated in real-time. It shows that comparing to approach with no rotational tool center point interpolation error control, the interpolation error acquired by rotational tool center point in proposed approach can be further reduced while ensuring the smooth and safe machining of the machine tool, and the surface quality of workpiece is also greatly improved.

Adaptive learning control for thermal error compensation of 5-axis machine tools

The research presented in this paper shows an adaptive approach for long-term thermal error compensation of 5-axis machine tools (MT). A system of differential equations is used to compute the model based compensation values. The model can predict thermal displacements of the tool center point (TCP) based on changes in the environmental temperature, load-dependent changes and boundary condition changes and states, like machining with or without cutting fluid. The model based compensation of the rotary axis of a 5-axis MT is then extended by on-machine measurements. The information gained by the process-intermittent probing is used to adaptively update the model parameters, so that the model learns how to predict thermal position and orientation errors and to maintain a small residual error of the thermally induced errors of the rotary axis over a long time. This approach not only increases the MT accuracy but also reduces the amount of time spent on preproduction model parameter identification. Additionally an algorithm has been developed to dynamically adjust the length of the on-machine measurement intervals to maintain a high productivity and a constant deviation of the machined parts.Experimental results confirm that the adaptive learning control (ALC) for thermal errors shows a desirable long-term prediction accuracy.

Drive based damping for robots with secondary encoders

Robot machining is a growing field due to the combination of large working envelope with relatively low investment and operating costs, compared to milling machine. Besides, the flexibility, which robot serial kinematics bring along, makes application of robot machining possible for different use cases. However, an occurring drawback of robot based machining systems is low stiffness compared to milling machines and, thus, poor accuracy and low eigenfrequencies with few damping. By that, robots for machining are prone to vibrations, resulting in poor machining results. In this paper the authors therefore present an approach for damping these vibrations, using state-of-the-art drives. Secondary encoders, which are increasingly available on industrial robots, are applied to detect these vibrations. This presented strategy has already been proven applicable on feed drives in milling machines and is now applied on industrial robots. To do so, the robot's vibrational behavior, like eigenfrequencies and eigenmodes, is examined in the whole workspace via measurements on real robots. Additionally the excitation by the milling process has been examined in relation to the occurring oscillations at the robot structure. The results of these research is adduced to estimate the applicability of this approach. Based on these results simulations are carried out to test the applicability of the damping strategy on industrial robots. As compliance of robots results mostly from gearboxes, simulation is carried out using a rigid-body-flexible-joint model. The simulations show that the dynamic behavior of the robots axes can be influenced in a positive way and vibration of the robots tool center point can be reduced significantly. Based on these findings, it is planned to implement this method on real hardware to perform tests, develop it further and optimize it.

Kinematic calibration of a 3-DOF parallel tool head

PurposeThe purpose of this study is to develop a modified parameter identification method and a novel measurement method to calibrate a 3 degrees-of-freedom (3-DOF) parallel tool head. This parallel tool head is a parallel mechanism module in a five-axes hybrid machine tool. The proposed parameter identification method is named as the Modified Singular Value Decomposition (MSVD) method. It aims to overcome the difficulty of choosing the algorithm parameter in the regularization identification method. The novel measurement method is named as the vector projection (VP) method which is developed to expand the measurement range of self-made measurement implements.Design/methodology/approachNewton Iterative Algorithm based on Least Square Method is analyzed by using the Singular Value Decomposition method. Based on the analysis result, the MSVD method is proposed. The VP method transforms the angle measurement into the displacement measurement by taking full advantage of the ability that the 3-DOF parallel tool head can move in the X – Y plane.FindingsThe kinematic calibration approach is verified by calibration simulations, a Rotation Tool Center Point accuracy test and an experiment of machining an “S”-shaped test specimen.Originality/valueThe kinematic calibration approach with the MSVD method and VP method could be successfully applied to the 3-DOF parallel tool head and other 3-DOF parallel mechanisms.

3D Imaging with a Sonar Sensor and an Automated 3-Axes Frame for Selective Spraying in Controlled Conditions

Autonomous selective spraying could be a way for agriculture to reduce production costs, save resources, protect the environment and help to fulfill specific pesticide regulations. The objective of this paper was to investigate the use of a low-cost sonar sensor for autonomous selective spraying of single plants. For this, a belt driven autonomous robot was used with an attached 3-axes frame with three degrees of freedom. In the tool center point (TCP) of the 3-axes frame, a sonar sensor and a spray valve were attached to create a point cloud representation of the surface, detect plants in the area and perform selective spraying. The autonomous robot was tested on replicates of artificial crop plants. The location of each plant was identified out of the acquired point cloud with the help of Euclidian clustering. The gained plant positions were spatially transformed from the coordinates of the sonar sensor to the valve location to determine the exact irrigation points. The results showed that the robot was able to automatically detect the position of each plant with an accuracy of 2.7 cm and could spray on these selected points. This selective spraying reduced the used liquid by 72%, when comparing it to a conventional spraying method in the same conditions.

Dynamic Control of the Bionic Handling Assistant

In the previous decade, multiple approaches for kinematic controllers of continuum manipulators were successfully developed. However, for fast and exact motions of continuum robots that are hardly seen so far, dynamic controllers are necessary—especially for spatial manipulators with multiple sections. Therefore, a dynamic controller in actuator space is developed that both exploits input constraints and decouples the actuators in order to provide a good and intuitive dynamic behavior of the manipulator. Having a pneumatically actuated manipulator with multiple actuators, this can be achieved using a cascaded control approach with a centralized actuator length controller and an underlying decoupled pneumatic controller. For both the outer multiple-input multiple-output (MIMO) controller and the inner single-input single-output (SISO) controllers, model-based controllers are developed based on suitable models using feedback linearization and a combination of feedforward control and linear PD-controllers. The resulting actuator controller can be used for tracking optimized trajectories in actuator space and also for possible extensions towards direct tool center point feedback.

3D Metrology Using a Collaborative Robot with a Laser Triangulation Sensor

Industrial robots are a key element in Smart Manufacturing systems. They can perform many different tasks such as assembly, pick-and-place, or even 3D metrology operations. In order to perform 3D metrology, the robot is equipped with a 2D laser triangulation sensor. The accuracy of the measurements made by this system is dependent of an accurate TCP (Tool Centre Point) calibration and the accuracy of the robot. In this paper, a TCP calibration method is applied to a collaborative robot. The hand-guiding feature of this kind of robots is used to establish a human-robot interaction to obtain the laser sensor TCP using a calibration sphere. Experimental results are presented to validate the procedure and evaluate the quality of the measurements.

Analysis of the Energy Consumption of Fluidic Systems in Machine Tools

Power losses in machine tools are converted into thermal energy warming up components such as rotary table, tool holder, linear guide rails etc., and the machine tool structure. Due to this temperature change, thermo-elastic deformations of the machine tool structure occur which directly influence the position of the tool center point (TCP). Consequently, the accuracy of the machine deteriorates during the production process. The warmed-up parts or components need to be cooled; therefore, fluidic systems, such as cooling system, are installed to prevent this effect. In order to reduce the occurring thermo-elastic deformations and to enhance the production quality it is necessary to minimize the heat input. Previous research projects mainly focused on the energy demand of the machine tool and its main drives, reducing the energy consumption by developing more efficient components, and control strategies. However, the energy consumption of the fluidic systems has not yet been described in detail. Therefore, a detailed analysis of the existing fluidic system structures and their energy demand is necessary in order to ensure a uniform temperature distribution of the machine tool at minimal energy consumption.The main goal of this paper is to analyze the energy consumption of the fluidic system exemplified by two demonstrator machines. This investigation will help obtain the information concerning the energy demand of the fluidic system for these two different machines. This makes it possible to predict the consumed energy and the thermal behavior of the machine tool and its fluidic systems. Furthermore, the Energy consumption can serve as a reference for developing new system structures.Firstly, the paper describes the two different demonstrator machines with a special focus on their fluidic systems. Secondly, the methodology of the measurement to measure the energy consumption of the whole machine and of the fluidic systems is shown. Lastly, with the aid of experimental investigations the energy consumption of each machine is calculated and discussed for a defined process.As a result of the investigation, the energy distribution of the fluidic systems for both machines is determined. This knowledge serves as a reference for further investigations, for example, the cooling system. The first steps of the network-based simulation strategy are illustrated. These network-based models are helpful for future investigations regarding new structuring concepts such as the decentralization of the fluidic systems.

Measuring unbalance-induced vibrations in rotating tools

Unbalances in a tool cause vibrations of the spindle and the machine itself and lead to a waviness of the machined workpiece surface. This paper presents an experimental and analytical procedure for optically measuring the unbalance-induced displacements of the tool centre point (TCP). Therefore, a new method is introduced to determine the dynamic vibrations of a tool by comparing the geometrical profile of the tool with the dynamical profile at a high rotational speed. The necessary steps for measuring the signals and calculating the underlying dynamic vibrations of the tool are presented here. Afterwards, the unbalance-induced vibrations of a milling tool are shown as well as their influence on the eccentricity of the rotation axis. With this newly introduced method it is possible to directly link the displacements of the tool under rotation to the waviness of the workpiece surface and the dynamic stiffness of machine tools.

Optimization of High Speed Machine Tool Spindle to Minimize Thermal Distortion

Thermal errors contribute significantly to the dimensional error on machined components. Various machine design and operating factors are responsible for the thermal error. In the present work, experiments are performed to understand the effect of spindle rotational speed and the chiller unit setting on the temperature distribution in the spindle and consequently the thermal distortion of the Tool Center Point (TCP). A metrology fixture is fabricated and calibrated to measure TCP displacements as a precursor to experimentation. In addition, multiple temperature sensors are affixed at critical points within the spindle. It is seen that the temperature distribution across the outer race of front bearings is non-uniform (> 2.5°C) which in turn indicates that the heat extraction is not uniform across the spindle. Concurrently, significant tilt (yaw and pitch angles of the order of 50μ radian) is visible at speeds greater than 5000rpm. In the second part of the paper, a thermal compensation model is developed in a linear regression framework. To improve robustness of the model and reduce redundancy, only five out of eighteen temperature sensors (two near front bearing, one each near rear bearing and motor coil and an ambient temperature) are chosen through Principal Component Analysis (PCA) and k-means clustering. The predictions of the model agrees well with the measurements taken from a different set of experiments.

Capturing the tacit knowledge of the skilled operator to program tool paths and tool orientations for robot belt grinding

This paper describes a novel methodology for generating grinding tool paths, orientations and grinding parameters based on the captured trajectories of a surface finishing tool operated by the skilled operator. First, a variable frame size moving window principal component analysis is performed on the trajectories. Next, features such as the amplitude, speed of the tool centre point, direction of travel, average position, contact force and orientation of the grinder are calculated. Kernel k-means clustering is applied to the trajectory’s feature vectors for segmentation into the tool path primitives. The robotic tool path primitives would be ordered similar to the manual tool path primitives in the grinding strategy. The robotic tool path primitives are then generated based on the boundaries of the manual tool path primitive in a computer-aided manufacturing software. Anchor points are selected from the corners and at intermediate sections along the robotic tool path to inherit the orientations of the manual tool path. Using the inherited skilled operator tool orientations, spherical linear interpolation and spherical spline quaternion interpolation are used to interpolate the orientations for the robotic tool path points along the cross curves, followed by the flow curves, respectively. A finite element model of the belt grinding process is also introduced to quantify the effect of the contact wheel serrations on the material removal rate. This is to account for the difference in contact wheel design between the handheld and robotic grinding tool. The programmed robotic tool paths generated from the trajectories captured from a skilled operator were proven to be able to grind the workpiece to the desired profile within the desired tolerance in a pre-production environment.

Decentralized structure-integrated spatial force measurement in machine tools

New manufacturing processes and extended movability of modern machine tools, such as five-axis kinematics or hexapods, increase the demand for in-process measurement of spatial forces and torques in up to 6 degrees of freedom (DoF). The approach proposed in this paper is based on the idea of integrating 6 single-axis force sensors into the machine’s structure and converting these sensor forces to spatial forces and torques at the tool centre point (TCP) using a measurement model. This concept is advantageous to costs, ruggedness and available workspace when compared to state-of-the-art 6 DoF force/torque transducers. At the same time, the achievable measuring accuracy is similar and also significantly better than the accuracy of drive current based force evaluation. On the other hand, structure and machine influences have to be addressed by suitable measurement models. This article presents design, parametrization, verification, and characterisation of these measurement models on the example of four integration concepts, two in rigid bar frameworks and two in bar kinematics. Further, experimental results are shown which are classified in comparison to a 6 DoF F/T sensor and drive current based force measurement. Finally, other influences, such as structural design and deformations, as well as the integration of sensors and models into the machine’s control software are discussed.

Analysis, optimization and accuracy assessment of special-purpose portable machines by virtual techniques

This paper proposes a streamlined method to model, analyze and assess the overall performance of small mobile machines that can move along large parts to perform the required machining operations, conventionally called portable machines. The method is based on virtualization techniques and combines a process-force mechanistic model and a reduced machine stiffness model synthesized from virtual and experimental reduced models of subsystems. The model is used to assess and improve the performance of portable machines by examining the time-domain response of the tool center point in representative operations, rather than limiting the study to the frequency domain. A practical application to a particular portable machine is presented and used to conduct the presentation of the work. With the results of the analysis, the accuracy of the use of the portable machines is studied. The procedure also proves to be a useful tool to optimize the machine design to fit particular applications. The method has been experimentally evaluated in a conventional three axis milling machine to ensure the accuracy of the simulations.

Offline simulation of path deviation due to joint compliance and hysteresis for robot machining

Industrial robots offer a cost-effective and flexible machining alternative to the classic machining centers. One disadvantage is the significantly reduced working accuracy that results in considerable path deviation, particularly under load. To maximize the effectiveness of the robot and to derive suitable machining strategies, knowledge of the effects on the working accuracy is required. In this study, a model is presented in joint space that predicts the path deviation on the basis of joint stiffness and reversal error with high accuracy, with respect to the kinematic model and path planning. The identification of the model parameters of individual joints is carried out without disassembling the robot. The model is validated against the hysteresis occurring at the Tool Center Point and through the milling of circular contours. It is shown that the reversal error is mainly caused by hysteresis and not by backlash at zero crossing. The subsequent offline compensation strategy allows a considerable reduction of dimension and form deviations.

TCP-based calibration in robot-assisted belt grinding of aero-engine blades using scanner measurements

Calibration in the robot-assisted belt grinding of complex blades is regarded as one of the key bottlenecks of measurement accuracy. To enhance the accuracy, a TCP-based (tool center point) calibration method is proposed in this paper to calibrate the relationship between the precalibrated 3D laser scanner and the robot end-effector by using criterion spheres as the calibration object. Based on the description of the robot-scanning system from the perspectives of coordinates and scanner measurement modes, the calibration strategies on translational and rotational motions of the robot are provided to determine the translation vector and the orientation matrix. Calibration experiments on the criterion sphere are performed, both the calibration errors (positioning error and orientation error) and sphere fitting error are calculated. A typical case on the robotic belt grinding of 84K-2R1 aviation blade is conducted to validate the calibration results. Finally, the key factors influencing the calibration accuracy are analyzed. It has been demonstrated that the TCP-based calibration method proposed is effective, concise, and time-saving, and can be widely applied in the robot-assisted belt grinding operation.