Accuracy Of Industrial Robots Research Articles

Investigation of positioning accuracy of industrial robots for robotic-based X-ray computed tomography

In this research work we investigated the accuracy of a standard industrial robot. We wanted to find out, how accurate an X-Ray Computed Tomography (CT) scan can be performed when using such a robot as a manipulator. The accuracy was measured using a laser-interferometer. The measured deviations were used to run an X-Ray simulation via Fraunhofer EZRT’s Scorpius X-Lab. Metrological analysis was performed as a measure for the quality of the simulated CT-scan. The metrological deviations reflect the feasible accuracy of a CT-scan in a real CT-setup.

Improving the Accuracy of Industrial Robots via Iterative Reference Trajectory Modification

In this paper, a novel repetitive control (RC) scheme is presented and discussed. The general framework is the control of repetitive tasks of robotic systems or, more in general, of automatic machines. The key idea of the proposed scheme consists in modifying the reference trajectory provided to the plant in order to compensate for external loads or unmodeled dynamics that cyclically affect it. By exploiting the fact that uniform B-spline trajectories can be generated by means of dynamic filters, the trajectory planning phase has been integrated within an RC scheme that is able to modify in real time the reference signal in order to nullify the tracking errors occurring at the desired via-points. Because of this mechanism, the control scheme is very suitable for the application to industrial plants with off-the-shelf, unmodifiable controllers. Experimental results obtained with a standard industrial manipulator both in joint space and in workspace show the effectiveness of the proposed method.

Constructive methods to reduce thermal influences on the accuracy of industrial robots

Abstract Due to their kinematics and their mobility, industrial robot systems offer a flexible, adaptable basis. An influencing variable, which is particularly relevant for processes with long process times, is the thermal heating and the associated thermal drift of the tool center point. The maximum deviation from the actual nominal position can reach up to 1.5 mm. To counteract these displacements, a cooling strategy or a targeted preheating can be installed. Both possibilities were evaluated under constructive, energetic and economic aspects and implemented on at least one axis of an industrial robot. Furthermore, a structural thermal decoupling and optimized design of the respective components were carried out. The results can help to reduce the influence of thermal heating and the associated thermal drift of the TCP by structural support without using cost-intensive measures with additional hardware and software on external computers for compensating the errors.

Kinematic calibration for industrial robots using articulated arm coordinate machines

To improve the position accuracy of industrial robots, a novel kinematic model and calibration method using articulated arm coordinate machines (AACMM) is proposed in this paper. The end of the industrial robot is connected to the probe of an AACMM, thus forming a closed kinematic chain. The coordinate systems of the double arms were established, based on which the mapping of the joint angles and the position of the end were derived as well as the nominal value of the kinematic parameters of the industrial robot. A two-step search method was presented for kinematic parameter identification of the industrial robot. The identified kinematic parameters were used to compensate the errors of the nominal kinematic parameters in the controller of the industrial robot. Experiments were conducted to verify the calibration method, which show that after calibration, the average position errors of the industrial robot were decreased greatly.

Online pose correction of an industrial robot using an optical coordinate measure machine system

In this article, a dynamic pose correction scheme is proposed to enhance the pose accuracy of industrial robots. The dynamic pose correction scheme uses the dynamic pose measurements as feedback to accurately guide the robot end-effector to the desired pose. The pose is measured online with an optical coordinate measure machine, that is, C-Track 780 from Creaform. A root mean square method is proposed to filter the noise from the pose measurements. The dynamic pose correction scheme adopts proportional-integral-derivaitve controller and generates commands to the FANUC robot controller. The developed dynamic pose correction scheme has been tested on two industrial robots, FANUC LR Mate 200iC and FANUC M20iA. The experimental results on both robots demonstrate that the robots can reach the desired pose with an accuracy of ±0.050 mm for position and ±0.050° for orientation. As a result, the developed pose correction can make the industrial robots meet higher accuracy requirement in the applications such as riveting, drilling, and spot welding.

A spatial vector projection based error sensitivity analysis method for industrial robots

Precision design is an important approach to guarantee the accuracy of industrial robots, where error sensitivity analysis lays a vital foundation. Intuitively, if the pose error vector caused by one error component is nearly perpendicular to the sum vector that is caused by all error sources, this error component will have little influence on the accuracy of end-effector even though its norm is large. Unimportant errors may unreasonably be regarded as vital errors when their direction characteristics are ignored. In this paper, the local sensitivity index (LSI) is defined based on the projection theory of spatial vector to evaluate the contribution of an error component in a definite pose. Both the direction and norm characteristics of pose error vector are considered in the novel definition of LSI, which is quite different from the previous researches. According to the definition of expectation and variance, the average sensitivity indices (ASI) and variance sensitivity indices (VSI) of error sources are established. Finally, the precision design of a 6-DOF (degrees of freedom) industrial robot is conducted in numerical simulation based on the proposed error sensitivity analysis method. The results show that the pose accuracy of end-effector can greatly be increased by improving very few error components obtained via error sensitivity analysis. The proposed error sensitivity analysis method is quite effective, which can greatly improve precision design effects.

The problem in Accuracy Compensation of Industrial Robot

Industrial robots are increasingly being used in automated production line with their high flexibility and low cost However the poor features of repeat positioning accuracy and absolute positioning accuracy of industrial robots have been a bottleneck restricting its application in higher accuracy situation Many scholarshave studied methods to improve the accuracy of robot But there is congenital defect for industrial robots when their accuracy is further improved by compensation and few people pay attention to this In this article the backlash of industrial robot joint is discussed and its impact on accuracy compensation is analyzed Finally the method of eliminating the backlash and further improving the robot s accuracy based on joint feedback is proposed

Design of a Two-Step Calibration Method of Kinematic Parameters for Serial Robots

Serial robots are used to handle workpieces with large dimensions, and calibrating kinematic parameters is one of the most efficient ways to upgrade their accuracy. Many models are set up to investigate how many kinematic parameters can be identified to meet the minimal principle, but the base frame and the kinematic parameter are indistinctly calibrated in a one-step way. A two-step method of calibrating kinematic parameters is proposed to improve the accuracy of the robot’s base frame and kinematic parameters. The forward kinematics described with respect to the measuring coordinate frame are established based on the product-of-exponential (POE) formula. In the first step the robot’s base coordinate frame is calibrated by the unit quaternion form. The errors of both the robot’s reference configuration and the base coordinate frame’s pose are equivalently transformed to the zero-position errors of the robot’s joints. The simplified model of the robot’s positioning error is established in second-power explicit expressions. Then the identification model is finished by the least square method, requiring measuring position coordinates only. The complete subtasks of calibrating the robot’s 39 kinematic parameters are finished in the second step. It’s proved by a group of calibration experiments that by the proposed two-step calibration method the average absolute accuracy of industrial robots is updated to 0.23 mm. This paper presents that the robot’s base frame should be calibrated before its kinematic parameters in order to upgrade its absolute positioning accuracy.

Estimation of uncertainty of laser interferometer measurement in industrial robot accuracy tests

The subject of this article is the assessment of measurement uncertainty of the Renishaw XL80 laser interferometer in MOTOMAN HP20 industrial robot inaccuracy test. The paper presents the methodology for estimating the measurement uncertainty of the system used in tests. Estimates of standard and extended uncertainty were calculated for the given research method. These uncertainties are based on the information included in the device calibration certificate (method B) but also on the basis of measurements and statistics (method A). The authors proposed their own research methodology, taking into account measurement capabilities of the applied system and the specific character of the robot work. Calculations employed universal computing systems based on standard algorithms. The results obtained from the research and calculations precisely defined key uncertainties allowing for objective evaluation of industrial robot errors identified by the Renishaw XL80 system.

Concept of an offline correction method based on historical data for milling operations using industrial robots

Industrial robots for machining applications have been researched to achieve an alternative, flexible and less expensive machining technology against conventional machine tools. The previous work on industrial robots indicates that industrial robots are only applicable without requirement of high accuracy. The low accuracy and instability under various configurations of industry robot are still major barriers for deploying robots for machining in industries. In this paper, a new method, namely historical data based correction (HDC), is presented to improve the accuracy of industrial robots for milling operations. First, the HDC method is able to calculate the corrected values to compensate the systematical errors during milling operations. Consequently, new tool paths of robots with corrected values are generated. In addition, the HDC method is able to fulfill the requirement of large-scale production. With the proposed method, the surface quality of the machined parts can be improved by more than 60 % without using any additional measuring systems.

Positional error similarity analysis for error compensation of industrial robots

The purpose of this paper is to propose an error compensation method with error similarity analysis to improve the absolute positional accuracy of industrial robots. The positional error similarity is proposed with the analysis of the error model established by robot kinematic parameters, and is quantified with semivariogram function. Then an error compensation method is proposed base-on positional error similarity. The ordinary Kriging method is used to calculate the positional errors of the robot TCP. The error compensation is performed by modifying the position coordinates in the robot controlling commands. Experimental verification showed that the maximum positional error of the robot TCP was reduced by 75.36% from 1.2912mm to 0.3182mm.

Industrial Robot Accuracy Testing with QC20-W Ballbar Diagnostic System

An important characteristic of industrial robots is their accuracy. It is of particularly importance in high-precision tasks, e.g. mounting or machining of elements. In order to meet the requested quality demands for products, as well as appropriate working conditions, it is absolutely essential to regularly inspect the technical condition of robots, e.g. their accuracy.The following paper aims at identification of accuracy errors of an industrial robot MOTOMAN HP20. The selected measuring method was the roundness test with the use of the telescopic, kinematic QC20W - Ballbar. The paper presents the methodology of experimental tests. The influence of the radius of the interpolation circle, as well as the influence of the set motion speed on the value of chosen accuracy errors was determined. The results were presented graphically and analysed.

Provision of Controlled Motion Accuracy of Industrial Robots and Multiaxis Machines by the Method of Integrated Deviations Correction

There is a developed method of correction of the integrated motion deviations of industrial robots and multiaxis machines, which are caused by the primary geometrical deviations of their segments. This method can be used to develop a control system providing the motion correction for industrial robots and multiaxis machines.

The repeatability positioning analysis of the industrial robot arm

Purpose – The purpose of the following work was to work out the dependency to allow for the determination of the repeatability positioning error value of the robot at any given point in its workspace, without the necessity of conducting time-consuming measurements while routing a precise surface of repeatability positioning. Design/methodology/approach – The presented dependency permits for the possibility to determine, even at the planning phase, the optimal connection point in the workspace, ensuring the best parameters for the process of machine assembly, without needless overestimation of precision of the utilized equipment. To solve the task the sequential quadratic programming (SQP) method implemented in the MATLAB(R) environment was used. To verify the hypothesis of the compatibility of the empirical distribution with the hypothetical distribution of the robot’s positioning error, the Kolmogorov test was used. Findings – In this paper, it has been demonstrated theoretically and experimentally that the industrial robot accuracy can vary over a very wide range in the workspace. This provides an additional opportunity to increase reliability of the assembly process through the appropriate choice of the point of parts joining. The methodology presented here allows the designer of assembly workstations to rapidly estimate the repeatability of robot positioning and to allocate at the design stage of assembly process the optimal position in the robot workspace to ensure the required precision, without unnecessarily high accuracy of equipment used and, therefore, without inflated costs. Originality/value – An alternative solution to the stated problem can be the proposed method for determining the robot’s positioning errors, requiring a much smaller amount of measurements to be taken that would be necessary to determine the parameters of the random variable errors of the joint coordinates of the robot and for their verification by the repeatability of positioning in randomly selected points in the workspace. Additionally discussed in the study, the methodology of identifying connection place was designed for typical combinations of machine parts, most frequently encountered in assembly process and was taken into account, typical limitations occurring in actual manufacturing conditions.

Deflection model for robotic friction stir welding

Purpose – This paper aims to present a deflection model to improve positional accuracy of industrial robots. Earlier studies have demonstrated the lack of accuracy of heavy-duty robots when exposed to high external forces. One application where the robot is pushed to its limits in terms of forces is friction stir welding (FSW). This process requires the robot to deliver forces of several kilonewtons causing deflections in the robot joints. Especially for robots with serial kinematics, these deflections will result in significant tool deviations, leading to inferior weld quality. Design/methodology/approach – This paper presents a kinematic deflection model, assuming a rigid link and flexible joint serial kinematics robot. As robotic FSW is a process which involves high external loads and a constant welding speed of usually below 50 mm/s, many of the dynamic effects are negligible. The model uses force feedback from a force sensor, embedded on the robot, and predicts the tool deviation, based on the measured external forces. The deviation is fed back to the robot controller and used for online path compensation. Findings – The model is verified by subjecting an FSW tool to an external load and moving it along a path, with and without deviation compensation. The measured tool deviation with compensation was within the allowable tolerance for FSW. Practical implications – The model can be applied to other robots with a force sensor. Originality/value – The presented deflection model is based on force feedback and can predict and compensate tool deviations online.

Elasto-geometrical modeling and calibration of robot manipulators: Application to machining and forming applications

This paper proposes an original elasto-geometrical calibration method to improve the static pose accuracy of industrial robots involved in machining, forming or assembly applications. Two approaches are presented respectively based on an analytical parametric modeling and a Takagi–Sugeno fuzzy inference system. These are described and then discussed. This allows to list the main drawbacks and advantages of each of them with respect to the task and the user requirements. The fuzzy logic model is used in a model-based compensation scheme to increase significantly the robot static pose accuracy in a context of incremental forming application. Experimental results show the efficiency of the fuzzy logic model while minimizing development and computational resources.

Theory and Experiment of Industrial Robot Accuracy Compensation Method Based on Spatial Interpolation

With the development of technology of robot,industrial robot is used more and more widely in the industrial field.The industrial robot usually has a high repeat positioning accuracy and a low absolute positioning accuracy.According to Denavit-Hartenberg kinematics model,the robot coordinate system is established,meanwhile the geometric error model is also established by taking account of errors introduced in joint parameters.The inner relationship between positioning accuracy of two adjacent points is discussed by using the error model,and therefore the concept of positioning errors similarity is proposed.Above on this,a method of robot accuracy compensation based on spatial interpolation is proposed.A KUKA robot is introduced to invalidate the proposed method,and the results show that by using the accuracy compensation method the maximum value of the robot positioning error is 0.386 mm,and the mean value is 0.156 mm,which are much more better than the previous values 1~3 mm,thus the method is feasible and effective.

Design and Implementation of Service Robot Lightweight Dual-Arm Based on CAN Bus

Because high positioning accuracy of industrial robots requires high stiffness at the price of high robot mass and energy consumption relative to the payload of the robot, this paper describes a kind of service robot dual-arm constructed by lightweight reconfigurable modules. After analyzing the composition characteristics of service robot dual-arm, the type selection of robot arm modules is accomplished, and the hardware platform of robot dual-arm composed by modular joint is builded. According to the modularization characteristics of arm joints, the distributed control system is designed.

Validation of iGPS as an external measurement system for cooperative robot positioning

External metrology systems are increasingly being used in modern manufacturing to improve the accuracy of industrial robots. In this paper, the problem of achieving absolute accuracy in the positioning and movement of cooperating robots is addressed using the indoor GPS (iGPS) technology as an external position measurement system for real-time feedback and control. This metrology system is presented as an introduction to the iGPS-based 3D Pose Detector and a new concept using generalised measurement systems inspired by iGPS. Attached to the robot end-effectors, the receivers allow coordinate frame measurements to provide spatial information on the robot poses in six degrees of freedom. Experimental results show a strong correspondence between iGPS measurements of cooperating robot end-effector positioning and the control measurements obtained from a double ballbar. Ballbar measurements are further used to determine the relative accuracy between state-of-the-art cooperating manipulators. The iGPS system is validated as an external measurement system using a ballbar device, and its use in the external control of basic robotic tasks is demonstrated. The predicted accuracy achievable for the robots when being controlled or compensated is determined to be at least within 0.3 mm, subject to improvements with continuing research and refinements.

Ball Bar Measurement on Machine Tools with Rotary Axes

A ball bar is a very convenient device for measuring the motion accuracy of machine tools. Some trials have also been done for measuring motion accuracy of industrial robots. Nowadays, multi-axis machines such as five-axis machining centers are very popular, and therefore, there is increased demand for checking their accuracy. This paper introduces an idea for checking the motion accuracy of five-axis machining centers and diagnosing error sources by reviewing trial measurements on articulated industrial robots. There are two problems. The first problem is that the ball bar can measure only distances, and the second problem is that the ball bar is a linear device and therefore not suitable for the rotary axis motion of 5-axis machines and articulated robots. Finally, the test conditions for the measurement of the motion accuracy of a machine tool showing conical motion, by using the ball bar and ISO/DIS 10791-6 (which is currently being edited) are reviewed and verified.