Double Ball-bar Measurements Research Articles

6축 로봇의 공구중심점 교정 및 틸팅-로터리 테이블의 기하학적 오차 측정

Tool-center-point (TCP) calibration and geometric error identification procedures are proposed to improve the accuracy of a 6-axis manipulator with a tilting rotary table. The accuracy of a 6-axis manipulator is affected by the accuracy of TCP calibration. In general, TCP calibration of the 6-axis manipulator uses a conical fixture provided by the manufacturer. However, since a TCP cannot be accurately positioned to the tip of the conical fixture repeatedly, a large positional deviation occurs at the calibration depending on the worker proficiency. Thus, accuracies of TCP calibration and the 6-axis manipulator are reduced. In this paper, a 3-DOF measuring device, consisting of a nest with three dial gauges and a precision ball, is developed to calibrate the TCP and to improve the accuracy of the 6-axis manipulator. Then, geometric errors of a tilting rotary table are identified via double ball-bar measurements according to the ISO 10791-6 with TCP initial alignment using an extension fixture. Finally, proposed TCP calibration and geometric error identification procedures are validated experimentally, and they show improvements in positional accuracy by 55 and 90%, respectively.

New identification method for computer numerical control geometric errors

To address the problems of the low accuracy of geometric error identification and incomplete identification results of the linear axis detection of computer numerical control (CNC) machine tools, a new 21-item geometric error identification method based on double ball-bar measurement was proposed. The model between the double ball-bar reading and the geometric error term in each plane was obtained according to the three-plane arc trajectory measurement. The mathematical model of geometric error components of CNC machine tools is established, and the error fitting coefficients are solved through the beetle antennae search particle swarm optimization (BAS–PSO) algorithm, in which 21 geometric errors, including roll angle errors, were identified. Experiments were performed to compare the optimization effect of the BAS–PSO and PSO and BAS and genetic particle swarm optimization (GA–PSO) algorithms. Experimental results show that the PSO algorithm is trapped in the local optimum, and the BAS–PSO is superior to the other three algorithms in terms of convergence speed and stability, has higher identification accuracy, has better optimization performance, and is suitable for identifying the geometric error coefficient of CNC machine tools. The accuracy and validity of the identification results are verified by the comparison with the results of the individual geometric errors detected through laser interferometer experiments. The identification accuracy of the double ball-bar is below 2.7 µm. The proposed identification method is inexpensive, has a short processing time, is easy to operate, and possesses a reference value for the identification and compensation of the linear axes of machine tools.

Path generation and optimization for DBB measurement with continuous data capture

In this paper we present a systematic approach to generating a path for continuous Double Ball Bar(DBB) measurement. The generated path is feasible, free of collision, jerk-optimized and sensitive to the perturbation in kinematic parameters. The approach could be automated and be applied not only to parallel kinematic machines(PKM) but also to serial mechanism. We will discuss about the properties of continuous DBB measurement and conclude that the condition number of identification Jacobian is a proper index to describe the sensitivity of a continuous path to the perturbation in kinematic parameters. The sensitivity of the generated path will be experimentally validated on a Stewart-Gough platform with comparison to two otherwise designed paths. Measurement shows that the generated path is also suitable for time-demanding tasks.

Diagnosis of machine tools: assessment based on double ball-bar measurements from a population of similar machines

The presented work is toward population-based predictive maintenance of manufacturing equipment with consideration of the automatic selection of signals and processing methods. This paper describes an analysis performed on double ball-bar measurement from a population of similar machine tools. The analysis is performed after aggregation of information from Computerised Maintenance Management System, Supervisory Control and Data Acquisition, NC-code and Condition Monitoring from a time span of 4 years. Economic evaluation is performed with use of Monte Carlo simulation based on data from real manufacturing setup.

Volumetric accuracy evaluation for five-axis machine tools by modeling spherical deviation based on double ball-bar kinematic test

Abstract This paper presents a new spherical deviation measurement method to evaluate the volumetric accuracy of five-axis machine tools based on double ball-bar measurements. Nine circular test paths with three kinds of tool axis directions in XY, YZ and ZX planes are planned to construct a typical sphere. The radial deviations measured by ball-bar are used to calculate the spherical deviation, which is defined as the maximum radial range of deviations around a least-squares sphere. Compared to the circular deviation described in ISO10791.6, which could only reflect the errors in one test path with one kinds of tool axis direction separately, the proposed method can reflect not only each test path's error, but also the offset errors between different test paths. And it could comprehensively quantify the effect of accuracies of the five axes together in three-dimensional space. The proposed method for five-axis machine tool can not only reflect the tool center position errors caused by three linear axes, but also indicate the tool axis direction errors caused by two rotary axes. The proposed method is verified through simulations and experiments on a five-axis machine tool.

Accuracy design of high precision machine tools using error sensitivity analysis methodology

Geometric accuracy is a crucially important performance factor for machine tools. Theoretically, the effects of source errors on pose accuracy (positional and angular accuracy) of 3-, 4- or 5-axis machine tools cannot fully be compensated by software, and only those pose errors associated with the permission motions are compensatable by means of error compensation. Therefore, the uncompensatable pose errors should be strictly guaranteed in the processes of design and manufacture. In this paper, after the geometric error model is established, the source errors affecting the uncompensatable pose accuracy are identified out of all the source errors. In order to enhance the understanding of which source errors have more influences on the pose accuracy, a probabilistic sensitivity analysis method is proposed, and the global sensitivity index is defined to evaluate the influence in the overall workspace. According to the sensitivity analysis results, the uncompensatable pose accuracy index is allocated to each source error. And then, assembly accuracy acceptance criteria are proposed as a guideline for machine assemblers. As an application example, the presented approaches are applied to the accuracy design and manufacture of a 4-axis machine tool, and double ball bar measurement and machining test are carried out to check the accuracy of the designed machine tool.

Compensation of position-independent and position-dependent geometric errors in the rotary axes of five-axis machine tools with a tilting rotary table

We describe a method to separate, measure, and compensate for position-independent and position-dependent geometric errors in the rotary axes of a five-axis machine tool with a tilting rotary table using double ball-bar measurements. The measurement paths require control of a single rotary axis, and they are not affected by the accuracy of the linear axes. The positions of the balls of the double ball-bar are modeled as the nominal positions, with geometric errors considered via a kinematic analysis to represent the measurement configuration. The method is applied to measure both position-independent and position-dependent geometric errors, and the results are verified via measurements with compensation for the geometric errors. Furthermore, the possible range of the measured geometric errors is determined by analyzing the contributors.

Circular tests for accurate performance evaluation of machine tools via an analysis of eccentricity

Circular tests to determine the circular/spherical deviation of machine tools based on double ball-bar measurements are proposed and applied to machine tools for accurate performance evaluation. In current circular tests, eccentricity is mathematically removed from the measured data to evaluate performance. However, eccentricity is affected not only by set-up error of the double ball-bar, which is trivial, but also by geometric error, which this test is intended to determine. The relationship between eccentricity and geometric error was investigated during circular deviation measurements using a simple example. The results demonstrated that the measured data must be evaluated as they are, without any mathematical modifications. Additionally, the circular test must be performed while ensuring negligible set-up error. The proposed approach was applied to three machine tools to investigate the effect of eccentricity when circular/spherical deviation appears.

The Use of DBB Measurement in Horizontal Machine Tool

The double ballbar(DBB) is an instrument to measure roundness error of the machine tools. Traditionally, DBB is used in the vertical machine tool which has a vertical spindle, so the DBB can be easily installed to measure without additional jigs. But the condition in the horizontal machine tool is different. Because the horizontal machine tool has no vertical spindle, additional jigs are needed for the measurement. Not only the differences in the using DBB but also in the forming circular trajectory exist. Center of the circular trajectory in the vertical machine tool is stationary, but not in the horizontal machine tool. The trajectories of two balls of the DBB are not same in the two kinds of the machine tools. The method that the DBB measurement in the horizontal machine tool has the same effect comparing with the measurement in the vertical machine tool is presented.

A technique for accuracy improvement of squareness estimation using a double ball-bar

Squareness measurement and its compensation are important for improving the performance of machine tools. A double ball-bar is widely used to measure squareness because of its simple, convenient setup and measurement procedures. This study addresses two important issues affecting the accuracy of squareness estimation when a simplified model of the geometric errors is used and estimation is carried out over constrained local ranges of the moving axes. A simplified model is frequently applied to geometric error estimation using a double ball-bar to avoid singularity problems while solving the double ball-bar equation. However, the use of a simplified error model significantly affects the accuracy of the squareness estimation. In this article, a squareness estimation technique using double ball-bar measurements and a laser interferometer is proposed. In addition, a convenient squareness estimation method involving only single double ball-bar measurements is suggested for the full travel range utilizing the straightness error data. The validity of the proposed method is verified by comparison with laser interferometer measurement results using an optical square on the XY-plane of a three-axis machine tool.

An Improvement Geometric Error Identification Method of CNC Machine Tool Based on Double Ball Bar

The double ball bar is widely used because it can quickly, easily and cost-effectively detect and evaluate the accuracy of CNC machine tools. But since the error recognition algorithm based on the double ball bar ignores the quadratic item, its recognition accuracy would be reduced. In this paper, an improved CNC verticality error and position error identification formula, combined with the machine tool error model to deduce a new error recognition model of double ball bar measurement is proposed. It can be drawn that the accuracy of the model are better than the existing methods because it keeps the second item in the derivation process of the model.

Accuracy evaluation of machine tools by modeling spherical deviation based on double ball-bar measurements

In this study, the accuracy of a machine tool was evaluated by modeling the spherical deviation based on double ball-bar measurements under unloaded conditions. Circular measurement paths on the XY-, YZ-, and ZX-planes were planned, and three linear axis drives were commanded to follow the paths describing a nominal sphere. The spherical deviation, defined as the maximum radial range of deviations around a least-squares sphere, is affected by the accuracies of the three linear axes together. Therefore, the spherical deviation represents the accuracy of machine tools by quantifying the effect of the accuracies of three linear axes, whereas the circular deviation only quantifies the accuracies of two linear axes among the three linear axes. In this experimental study, spherical deviations of vertical/horizontal machine tools were measured and analyzed under various nominal lengths of a double ball-bar for various feed rates. The measurement uncertainty of the measured spherical deviation was investigated to determine the confidence interval.

Extracting Single Source Geometric Error Value from a Double Ballbar Measurement Error Map

The double ballbar (DBB) test is a well-known way to check the geometric error of axis interaction. The DBB test captures actual data from multiple error origins. Here, we define the DBB measurement result as the sinusoid error map model plus noise. Using this concept, we extract a single source geometric error value from the DBB error map by LS fitting. We considered the “noise” as mix error from other sources. To ensure the quality of a numerical fitting, we used a sinusoid model of each geometric error that was generated by simulation of axis movement based on homogeneous transformation matrices (HTMs) as general best-fit curve. To verify the proposed method, we extract a well-known geometric error of linear axes and compare it with the result from a commercial measurement system. This method is applicable to both a full circle and a truncated DBB test path. Then, we use the method to estimate the geometric error of axis interaction between linear and rotary axes in a five-axis machine. A sequence of DBB tests is arranged based on linear-linear and linear-rotary simultaneous motions. The tests contain seven DBB test runs with two setups, and are able to identify eleven geometry errors of interaction of axes in less time, and with less human “intervention” error.

Double Ballbar Measurement for Identifying Kinematic Errors of Rotary Axes on Five-Axis Machine Tools

This paper proposes a novel measuring method which uses double ballbar (DBB) to inspect the kinematic errors of the rotary axes of five-axis machine tool. In this study, kinematic error mathematical model is firstly established based on the analysis of the rotary axes errors which originated from five-axis machine tools. In the simulation, working conditions considering different error origins are simulated to find the relationship between the DBB measuring patterns and the kinematic errors. In the measuring experiment, the machine rotary axes move simultaneously along a specified circular path while all the linear axes are kept stationary. The original DBB measuring data are processed to draw the measuring patterns in the polar plots which can be employed to observe and identify the kinematic errors. Rotary error compensation is implemented based on the function of external machine origin shift. Both the simulation and the experiment results show the convenience and effectiveness of the proposed measuring method as well as its operability as a calibration method of five-axis machine tools.

Identification of Motion Error Sources on Five-axis Machine Tools by Ball-bar Measurements (1st Report)

The inclusion of the application of the double ball bar (DBB) measurement to the accuracy calibration of five-axis machine tools into the revision of ISO standards is currently under the discussion. This paper presents the modified DBB measurement device, referred to as “DBB5” in our study, where master balls are supported from the 45°direction to the spindle axis. It can perform all the circular tests on XY, YZ, and ZX planes without changing the setup. This paper first presents the classification of motion error components of a five-aixs machine tool into location and component errors. Experimental application examples of the DBB5 to the calibration of location and component errors assciated with rotary axes on a five-axis machine tool are then presented.

Modeling and Diagnosis of Motion Error of Multi-Axis Machines Using a Ball Bar Test

The contouring accuracy of a multi-axis machine has a critical effect on the quality of many advanced technology products. One of the best approaches to assessing the contouring performance of machine tools is through double ball bar measurement. During circular interpolation motion, the machine traverses, with two axes at a time, a circular trajectory, with each axis subject to sinusoidal changes in acceleration, velocity, and position. The motion error is measured by detecting the relative distance between a point on the spindle nose and another point on the machine table and plotting this distance in polar coordinates. The present paper derives mathematical models and diagnosis procedures for first and second-order motion error resulting from the active degrees of freedom of a multi-axis machine. The theoretical results are verified by both computer simulation and double ball bar testing experiments.