Double Ball Bar Research Articles

Using a double ball bar to measure 10 position-dependent geometric errors for rotary axes on five-axis machine tools

To improve the accuracy of five-axis machine tools, the measurement and compensation of position-dependent geometric errors (PDGEs) for rotary axes are of great importance. A double ball bar-based measuring method is proposed to precisely calibrate PDGEs. For each rotary axis, five measuring patterns are designed, in which the ball bar is sensitive to one direction throughout the measuring process. In the measurement, the ball bar functions as a high-precision displacement sensor with a single degree of freedom. Based on the ball bar readings, analytical solutions for 10 PDGEs, except for two angular position errors, are simply deduced. Simulations are conducted to analyze the influence of PDGEs and setup errors on measuring patterns. The advantages of the proposed method are discussed by comparison with three other measuring schemes. Finally, experimental verification of this approach is conducted on a five-axis machine tool. The results confirm that the method provides precision results of PDGEs for rotary axes.

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.

Using a double ball bar to identify position-independent geometric errors on the rotary axes of five-axis machine tools

A measuring method using a double ball bar (DBB) is proposed for identifying the eight position-independent geometric errors (PIGE) on the rotary axes of five-axis machine tools. Three measuring patterns are used, in which the translational axes are kept stationary and only two rotary axes move to obtain a circular trajectory. In this way, the effects of translational axes are totally excluded, and the measurement accuracy is improved. Motion equations, describing how the A-axis and C-axis move simultaneously to realize a circular trajectory, are presented. The influence of each deviation on the measurement patterns is simulated, and analytical solutions for the eight PIGEs are demonstrated. Finally, the measuring method is verified in a five-axis CNC machine tool. Experimental results confirm that the method provides precision results for the eight PIGEs.

Geometric error measurement and identification for rotary table of multi-axis machine tool using double ballbar

In this paper, comprehensive geometric errors, including linkage errors and volumetric errors, of a rotary table are measured totally by employing a double ballbar and obtained by a two-step identification procedure. The derivations of the center of the ball installed on the table are measured in the error sensitive directions with newly developed serial of two axes controlled circular paths. Hence, there are nine results measured from three mounting positions of the ball at the same rotation angle. These results are used to form the identification model based on the homogeneous transformation. Moreover, a sensitivity analysis method is applied to select the optimum installation parameters of the ballbar to diminish the influence of the inaccuracy of the measurement parameters. As the mounting position errors of the socket on the table are inevitable during the installation of the balls, a new correction procedure is developed as well. Finally, an experiment is conducted on the four-axis machining center. The comparison results between the predicted errors and the measured results are shown to verify the proposed method.

SUM OF SINUSOIDAL PLUS NOISE MODEL TO EXTRACT COMPONENT ERROR FROM A DOUBLE BALLBAR MEASUREMENT ERROR MAP

The double ballbar (DBB) test, which captures actual data from multiple error origins of axes interaction, was defined as sinusoid error map model plus noise. When the number of sinusoids is the same as the number of individual errors of moving axis in the test map, we can extract a single source geometric error value from the DBB error map by modeling the well-known geometric error of linear axis. We considered the “noise” as mixed errors from other sources than geometric errors. This method is applicable to both a full circle and a truncated DBB test path.

Feed Rate Influence on Motion Error Characteristics of CNC Vertical Milling Machine

The increasing number of the Computer Numerical Control (CNC) machines usage in the manufacturing field appears to suggest that they are expected to be in good running condition. Machine tools such as CNC vertical milling machines are meant to produce precise work parts. Motion errors are factors which can directly affect the accuracy of the machine. This study presents an opportunity for interested parties to understand the characteristics of motion errors that exist in the CNC vertical milling machines with different feed rate settings. The purpose of this project is to find out any abnormalities apparent to the machine performance as the feed rate setting is increased. The motion errors of the CNC machines were examined by using a measuring device, namely Double Ball Bar (DBB). This equipment is designed to examine machine performance by measuring the accuracy of its movements. This device, which has two balls of identical size at the end of the bar, was mounted on a spindle and onto a special holder which is mounted on the working table. The machine is programmed to move in a circular motion, clockwise and anti-clockwise in three planes, XY, YZ and ZX. Any deviation from the standard data will represent the imperfection of the machines condition, especially mechanical components such as the slide bearing, spindle bearing or servo motor responses. By knowing the most significant error origin, the corrective countermeasures can then be carried out.

오차행렬을 이용한 5축 공작기계의 오차보정모델 생성 및 실험적 검증

This paper proposes a new model to compensate for errors of a five-axis machine tool. A matrix with error components, that is, an error matrix, is separated from the error synthesis model of a five-axis machine tool. Based on the kinematics and inversion of the error matrix which can be obtained not by using a numerical method, an error compensation model is established and used to calculate compensation values of joint variables. The proposed compensation model does not need numerical methods to find the compensation values from the error compensation model, which includes nonlinear equations. An experiment using a double ball-bar is implemented to verify the proposed model.

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.

Accuracy analysis of machine tools using Elastically Linked Systems

The paper introduces the concept of Elastically Linked Systems (ELS) to directly relate the machine tool positional and static accuracy to the machined part's geometric errors and form deviation. Practical implementation of the ELS concept resulted in a novel test equipment, Loaded Double Ball Bar (LDBB) which is a precision mechatronic device with variable load. The test method based on the device is able to reveal machine tool characteristics not obtainable with existing methods as for instance the variation of stiffness in the entire working space. The LDBB is used to experimentally evaluate the stiffness and the corresponding accuracy of five machine tools.

Geometric error measurement and compensation for the rotary table of five-axis machine tool with double ballbar

This paper proposes a novel measuring method for geometric error identification of the rotary table on five-axis machine tools by using double ballbar (DBB) as the measuring instrument. This measuring method greatly simplifies the measurement setup, for only a DBB system and a height-adjustable fixture are needed to evaluate simultaneously five errors including one axial error, two radial errors, and two tilt errors caused by the rotary table. Two DBB-measuring paths are designed in different horizontal planes so as to decouple the linear and angular errors. The theoretical measuring patterns caused by different errors are simulated on the basis of the error model. Finally, the proposed method is applied to a vertical five-axis machining center for error measurement and compensation. The experimental results show that this measuring method is quite convenient and effective to identify geometric errors caused by the rotary table on five-axis machine tools.

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.

EFFECT OF SERVO SYSTEMS ON THE CONTOURING ERRORS IN INDUSTRIAL ROBOTS

Two important aspects of the performance of a servo system, tracking errors and contour errors, significantly affect the accuracy of industrial robots under high-speed motion. Careful tuning of the control parameters in a servo system is essential, if the risk of severe structural vibration and a large contouring error is to be avoided. In this paper, we present an overview of a method to diagnose contouring errors caused by the servo control system of an ABB IRB 1600 industrial robot by measuring the robot’s motion accuracy in a Cartesian circular shape using a double ballbar (DBB) measurement instrument. Tests were carried out at different TCP (tool centre-point) speed and trajectory radii to investigate the main sources of errors that affect circular contouring accuracy. Results show that radius size errors and out-of-roundness are significant. A simple experimental model based on statistical tests was also developed to represent and predict the radius size error. The model was evaluated by comparing its prediction capability in several experiments. An excellent error prediction capability was observed.

A New Method for Circular Testing of Machine Tools Under Loaded Condition

This paper presents a novel test device for the evaluation of the accuracy of machine tools. The design concept is similar to a double ball bar (DBB) with the difference that an adjustable load generated by the device can be applied between spindle nose and machine tool table. The device, called Loaded Double Ball Bar (LDBB), can be used either as an ordinary double ball bar system with no load applied to the structure, or with a predefined load applied to the structure. The load that is generated by the LDBB is generally not equivalent to real cutting forces. However, from the static deflection point of view the effect of the load on the machine tool structure has similar impact on the static behaviour of the system. For instance, the load can in some cases eliminate existing play in ball screws, plays that under normal machining condition will be eliminated by the effect of cutting forces on the structure. With the help of this test device, not only can the identifiable errors by an ordinary DBB be evaluated but also machine tool elastic deflection in different directions. It is also possible to track different error patterns to the applied load.

DEVELOPMENT AND MANUFACTURING OF A MEASUREMENT APPARATUS FOR NC MACHINE TOOLS DIAGNOSIS BY DOUBLE SHAFT-HOLE BAR (DSB) METHOD

Manufacturing industries rely mostly on machine tools. Product quality produced by these industries depends on machine tools quality used to machine the product. In order to determine the machine tools quality, measurement must be conducted by using measurement apparatus. One of the apparatus is DBB (double ball bar) system. Unfortunately, the price of commercial DBB is quite expensive. To solve the problem, a new measurement apparatus based on Double Shaft-Hole Bar (DSB) method has been developed. The new design uses two precision shaft-hole pairs, instead of two high precision balls, connected by an extension bar. The design resembles ISO 230-1 basic design with some modifications, according to developing countries manufacturing capability. Data acquisition for the measurement system consists of 1µm accuracy LVDT as a position sensor, amplifier, 14 Bit ADC and personal computer. The measurement apparatus capability is measured by comparing apple-to-apple with Renishaw® ball bar system. The results show that an accuracy of 5 µm can be reached in CCW direction. While, in CW direction the accuracy is about 22 µm, due to mechanical clamping system stability, which needs further study. The system, apart from the computer, can be manufactured with a budget less than USD 1,500.

Identification and measurement of geometric errors for a five-axis machine tool with a tilting head using a double ball-bar

Geometric errors are one of the primary potential sources of error in a five-axis machine tool. There are two types of geometric errors: position-dependent geometric errors and position-independent geometric errors. A method is proposed to identify and measure the position-independent geometric errors of a five-axis machine tool with a tilting head by means of simultaneous multi-axis controlled movements using a double-ball bar (DBB). Techniques for identifying position-independent geometric errors have been proposed by other researchers. However, most of these are based on the assumption that position-dependent geometric errors (such as linear displacement, straightness, and angular errors) are eliminated by compensation, once the position-independent geometric errors have been identified. The approach suggested in this paper takes into account the effect of position-dependent geometric errors. The position-dependent geometric errors are first defined. Path generation for circle tests with two or three simultaneous control movements is then carried out to measure the position-independent geometric errors. Finally, simulations and experiments are conducted to confirm the validity of the proposed method. The simulation results show that the proposed method is sufficient to accurately identify position-independent geometric errors. The experimental results indicate that the technique can be used to identify the position-independent errors of a five-axis machine tool with a tilting head.

Study on the Improvement of Motion Accuracy of 5 Axis Control Machining Center (Influence of the Rotating Axis Travelling Range on the Alignment Error)

The demand for five-axis machining centers is rapidly increasing to achieve process integration and high machining accuracy. This paper proposes the development of a five-axis machining center with Table-on-Table structure due to the ease of achieving high accuracy with a large rotary axis travelling range. Table-on-Table type structure can easily achieve a wide travelling range, especially for the first rotating axis. The ease of achieving high accuracy with a large travelling range has been verified by simulating the supposition of double ball bar (DBB) measurement. When the magnitude of alignment error is estimated by using DBB measured data including motion errors of the rotating axis with the same order, it is shown by the simulation analysis that the magnitude of the estimated alignment error depends on the travelling range.

テーパコーンＤＢＢ測定によるテーブル旋回形５軸制御工作機械の幾何誤差の同定と補正

Geometric error, such as a pivot offset of the rotational axis, squareness between axes and so forth, is one of fundamental factors that decrease the machining accuracy for five-axis controlled machine tools. And it is important to identify the geometric errors in order to improve the machining accuracy, so that many methods to identify or diagnose the geometric errors of the rotational axes have been suggested. Meanwhile, the most general method for the accuracy test of five-axis controlled machine tool is the cone frustum machining test, which is prescribed in NAS979. In our previous work, we showed that the geometric errors in five-axis controlled machine tools with a tilting rotary table influence the accuracy of machined cone frustum, and it depends on the location, apex angle, tilt angle or diameter of the cone frustum. It indicates that the geometric errors might be identified with the profiles of cone frustum. In this paper, we suggest a method to identify the geometric errors of linear and rotational axes in five-axis controlled machine tools with a tilting rotary table, which uses the multiple trajectories of the conic motion measured by double ball bar device. Moreover we develop the compensation system of the geometric errors. We also show the result of the compensation of the identified geometric errors by the suggested method.

Tracking Error Transform in Parallel Kinematic Machining

Parallel kinematic machine tools (PKM) are developed to increase dynamic parameters for high speed and high accuracy machining to gain short lead times and high productivity. One of the most important components of machine tools is the numerical controller (NC). Most NCs are organized in a cascaded structure, comprising the position, velocity and current loops. Commonly applied servo controllers generate tracking errors in each axis. These tracking errors are a significant factor that affects machining accuracy, beside geometric machine errors, vibrations, temperature changes and tool errors. In this paper the contour error originated from servo tracking controller in Cartesian kinematic machine tools (CKM) with perpendicular arranged machine axes and PKM is presented. The effects of the forward transformation of the tracking errors in PKM are addressed with experiment and simulation results. The servo tracking effect on trace accuracy is discussed by a tripod mechanism and radial deviations are measured with double ball bar method (DBB).

Kinematic error separation on five-axis NC machine tool based on telescoping double ball bar

The theory and algorithm of the homogeneous transformation matrix (HTM) method are applied in establishing the kinematic error model of five-axis machining tool with two-axis turntable. Based on this model, a new method for the kinematic error separation in five-axis numerical control (NC) machining tool is proposed. In this study, three types of simultaneous three-axis control motions are designed for each rotary axis to identify the deviations. In the measurement, two translational axes and one rotary axis are simultaneously controlled to keep a constant distance between the tool and the worktable. Telescoping double ball bar is used to measure the relative distance between the spindle and the worktable in the motion of NC machining tool. Finally, the value measured by telescoping double ball bar is substituted into the model to obtain kinematic error of NC machining tool. Comparison has confirmed that the proposed method is high precision and can be applied to effectively and conveniently measure the five-axis machining tool.