Kinematic Error Compensation Research Articles

A feedforward kinematic error controller with an angular positioning deviations model for backlash compensation of industrial robots

The accuracy of industrial robots, typically on the order of several millimeters, has inhibited their adoption in advanced aerospace manufacturing applications, such as robotic machining. For this reason, extensive investigations have been conducted to develop solutions to improve their accuracy. These solutions can be categorized into offline and online compensation strategies, both of which have advantages and limitations. Offline compensation has been shown to improve the accuracy of industrial robots by two to three orders of magnitude; however, the sensitivity of these solutions to environmental changes and external disturbances can degrade their performance. In contrast, online compensation, which utilizes real-time measurement and compensation, is robust to these changes; however, the performance of these solutions is limited by the causality of time, preventing them from compensating fast changing errors such as backlash. In this work, a novel kinematic modeling strategy, which models the robot’s angular positioning deviations to predict when backlash will occur, is integrated into a novel online kinematic error compensation framework to leverage the advantages of both solutions. By integrating the proposed kinematic model into the online compensation framework, backlash errors can be predicted and preemptively compensated to improve performance. The performance of the combined system was evaluated using ball bar measurements, where it was shown to improve the circularity of the ball bar motion by 25 %.

Experimental Analysis on the Effectiveness of Kinematic Error Compensation Methods for Serial Industrial Robots

Based on the established serial 6-DOF robot calibration experiment platform, this paper aims to analyze and compare the effects of four error compensation methods, which are pseudotarget iteration-based error compensation method with three different forms and the Newton–Raphson-based error compensation method. Firstly, the pose error model of the serial robot is established based on the M-DH model in this paper. The calibration results show that the accuracy of the Staubli TX60 robot has been greatly improved. The average comprehensive position accuracy is increased by 88.7%, and the average comprehensive attitude accuracy is increased by 56.6%. Secondly, the principles of the four error compensation methods are discussed, and the effectiveness of the four error compensation methods are compared through experiments. The results show that the four error compensation methods can achieve error compensation well. The compensation accuracy is consistent with the identification accuracy of the kinematic model. The pseudotarget iteration with differential form has the best performance by the comprehensive consideration of accuracy and computational efficiency. Error compensation determines whether the accuracy of the identified model can be achieved. This paper presents a systematic experimental validation research on the effectiveness of four error compensation methods, which provides a reliable reference for the kinematic error compensation of industrial robots.

A Kinematic Error Controller for Real-Time Kinematic Error Correction of Industrial Robots

Industrial robots are being used more and more for manufacturing applications that require accuracy beyond what can be obtained from joint measurement. While offline calibration techniques such as volumetric error compensation can be used to correct robot kinematic error, these methods are unable to compensate for robot deformations caused by changing tool loads during the manufacturing operation. This paper explores the use of a real-time robot kinematic error compensation technique where an external high-precision feedback sensor (in this case a laser tracker) directly measures the robot kinematic error and corrections are implemented during processing. A robot kinematic error model is constructed to describe the difference between the programmed trajectory of the robot’s last link and the actual trajectory based on the laser tracker measurement of the 6DoF sensor attached to the last link of the robot. The compensation scheme developed in this paper requires the synchronization of the robot encoder and laser tracker sensor measurements, which is accomplished in the Robot Operating System (ROS). The previously developed kinematic error observer is briefly discussed and a kinematic error compensation that adjusts the robot’s reference path in real time is created in this paper. A discussion of the system (i.e., Yaskawa/Motoman MH180 industrial robot and Automated Precision Inc. laser tracker) hardware components and the software architecture utilized in the experiments conducted in this paper are provided. Initial experimental studies are conducted to determine delays in the feedback measurement, which was 30 ms, explore the effects of controller gain on the system performance, and characterize the noise in the feedback measurement. The controller had an overdamped response with a settling time of 8.758 s. Subsequent experiments investigated the effect of robot velocity on tracking and the ability of the controller to reject a constant force disturbance. It was found that the kinematic error increased linearly as the robot velocity increased, and the controller was able to reject a constant force disturbance of 45 lb within the designed settling time.

A low-cost camera-based tracking theodolite for large-scale metrology applications

ABSTRACT A wide spectrum of Large-Scale Metrology instruments for measuring 3D positions of mobile targets with different levels of accuracy, operational ranges and corresponding cost exist. However, future assembly paradigms and further industrial applications would benefit from a system with ultra low-cost targets and an accuracy in the order of magnitude of 1 mm, which is currently not available. Therefor the authors propose a camera-based tracking theodolite as a novel instrument for measuring 3D positions of printed visual markers. The approach is implemented as a prototype composed of two rotation stages with built-in encoders, a state-of-the-art industrial CMOS-camera and lens with fixed focal length in conjunction with printed chessboard targets. According mathematical models for the calculation of the targets’ position, kinematic error compensation and measurement uncertainty estimation are elaborated. Competitive experiments against a laser tracker show that the position of the marker can be measured with an absolute error below 1 mm and a statistical uncertainty close to 1 mm in a radius of 5 m, confirming the applicability of the developed concept for a novel Large-Scale Metrology instrument.

Kinematic error compensation of a double swivel head in five-axis abrasive water jet machine tool

With the improvement of manufacturing technology, five-axis machine tools have solved most of the linear error problems. And the rotating kinematics error problems have come to the front. Rotation tool center point (RTCP) function is one of the most effective solutions. Based on the RTCP function, a double swivel cutting head used in abrasive water jet cutting machine has been selected and its kinematic errors have been investigated through a double ball bar (DBB). And further, a new error mapping method has been built to compensate the geometric error. To verify the effectiveness of the method, a series of experiments have been carried out. The results of the experiments showed that, by applying this method, the precision of the tool cutting path could be improved greatly.

Improving the accuracy of a profile grinding wheel grooving robot attachment for a CNC grinding machine

The purpose of this paper is to present a novel profile grinding wheel grooving robot attachment and to demonstrate a simple, economical method of measuring and compensating for the kinematic error of the robot. Grooved grinding wheels have many advantages over conventional grinding wheels, but existing grooving methods are incapable of producing grooves on profile grinding wheels. The authors, therefore, developed a unique robot grinding wheel grooving attachment for a CNC grinding machine that can produce grooves on profile grinding wheels. The accuracy of the robot is critical to future investigations of grooved profile grinding wheels. As a result, a simple and economical method of measuring and compensating for the kinematic errors of the robot was developed that employs a single dial gauge and the digital readout of the grinding machine. This method was validated experimentally by measuring the contour error of two different grinding wheel profiles formed by the robot. The kinematic error compensation method reduced the contour error of the grinding wheel profile to within a range of ± 10 μ m: a reduction of up to 50%. It was concluded that this range of contour error was acceptable for grooving profile grinding wheels, but more work is needed to compensate for other sources of machining error.

Motion error compensation of multi-legged walking robots

Existing errors in the structure and kinematic parameters of multi-legged walking robots, the motion trajectory of robot will diverge from the ideal sports requirements in movement. Since the existing error compensation is usually used for control compensation of manipulator arm, the error compensation of multi-legged robots has seldom been explored. In order to reduce the kinematic error of robots, a motion error compensation method based on the feedforward for multi-legged mobile robots is proposed to improve motion precision of a mobile robot. The locus error of a robot body is measured, when robot moves along a given track. Error of driven joint variables is obtained by error calculation model in terms of the locus error of robot body. Error value is used to compensate driven joint variables and modify control model of robot, which can drive the robots following control model modified. The model of the relation between robot’s locus errors and kinematic variables errors is set up to achieve the kinematic error compensation. On the basis of the inverse kinematics of a multi-legged walking robot, the relation between error of the motion trajectory and driven joint variables of robots is discussed. Moreover, the equation set is obtained, which expresses relation among error of driven joint variables, structure parameters and error of robot’s locus. Take MiniQuad as an example, when the robot MiniQuad moves following beeline tread, motion error compensation is studied. The actual locus errors of the robot body are measured before and after compensation in the test. According to the test, variations of the actual coordinate value of the robot centroid in x-direction and z-direction are reduced more than one time. The kinematic errors of robot body are reduced effectively by the use of the motion error compensation method based on the feedforward.

Higher order approximation of the generalized kinematic error compensation model for robots

This article presents a higher-order approximation of the “generalized” kinematic error compensation model to enhance position accuracy and repeatability of robotic manipulators. The “generalized” model originally proposed by Driels and Pathre is successfully extended to include non-linear coupling effects among all error parameters. A pertubration technique is used in which second-order error terms are retained for improvement to the Denavit-Hartenberg A and T matrices. The model is called “generalized” in a sense that it incorporates ten error parameters (all six possible errors and four link parameter errors) per manipulator's joint axis. The second-order terms in the model become important when considering large robot structure size or when input kinematic errors increase in their magnitudes. The formulation of the generalized Jacobian matrix is also presented including second-order error terms in the analysis.

Robot manipulator kinematic compensation using a generalized jacobian formulation

AbstractThe kinematic error compensation of robot manipulators is at present being attempted by improving the precision of the nominal robot kinematic parameters. This paper addresses the problem of kinematic compensation using a new mathematical joint model proposed to account for shortcomings in existing methods. The corrected manipulator transformation is formulated in terms of “generalized Jacobians”: relating differential errors at the joints to the differential change in the manipulator transformation. The details of application are discussed for a particular industrial manipulator.