Position error reduction of tool center point in multi-tasking machine tools through compensating influence of geometric deviations identified by ball bar measurements

2021 ◽  
Author(s):  
Yan Yao ◽  
Yuki Itabashi ◽  
Masaomi Tsutsumi ◽  
Keiichi Nakamoto
Keyword(s):  
Machine Tools ◽  
Error Reduction ◽  
Position Error ◽  
Center Point ◽  
Ball Bar ◽  
Geometric Deviations

A Universal Postprocessor of General Table-Tilting Type of Five-Axis Machine Tools Without Rotational Tool Center Point Function for Actual NC Code Generation

2018 ◽  
Author(s):  
Guoqiang Fu ◽  
Hongli Gao ◽  
Tengda Gu
Keyword(s):  
Code Generation ◽  
Machine Tools ◽  
Kinematic Chain ◽  
Rotary Axes ◽  
Nc Code ◽  
Rotation Axes ◽  
Cutting Test ◽  
Center Point ◽  
Rtcp Function ◽  
Five Axis

The postprocessor is essential for machining with five-axis machine tools. This paper develops one universal postprocessor for table-tilting type of five-axis machine tools without rotational tool center point (RTCP) function. Firstly, positions of two rotary axes and the workpiece in the machine coordinate system (MCS) are introduced into the kinematic chain of the five-axis machine tools. The uniform product of exponential (POE) formula of the tool relative to the workpiece is established to obtain the universal forward kinematics. On this basis, the postprocessor of table-tilting type of five-axis machine tools is developed. The calculation of rotation angles of rotation axes is proposed in details, including the calculation of double solutions, the determination of rotation angles of C-axis and the selection principle of the shortest path of rotation angles. Movements of linear axes are calculated with rotation angles of rotary axes. The generated movements of all axes are actual positions of all axes relative to their zero positions, which can be used for machining directly. The postprocessor does not rely on RTCP function with positions of rotary axes and the workpiece in MCS. Finally, cutting test in VERICUT and real cutting experiments on SmartCNC500_DRTD five-axis machine tool are carried out to verify the effectiveness of the proposed postprocessor.

A Complete, Continuous, and Minimal Product of Exponentials-Based Model for Five-Axis Machine Tools Calibration With a Single Laser Tracker, an R-Test, or a Double Ball-Bar

2019 ◽  
Vol 141 (4) ◽  
Author(s):  
Peng Xu ◽  
Benny C. F. Cheung ◽  
Bing Li
Keyword(s):  
Machine Tool ◽  
Machine Tools ◽  
Rigid Bodies ◽  
Error Model ◽  
Laser Tracker ◽  
Calibration Model ◽  
Transformation Matrices ◽  
Double Ball Bar ◽  
Ball Bar ◽  
Five Axis

Calibration is an important way to improve and guarantee the accuracy of machine tools. This paper presents a systematic approach for position independent geometric errors (PIGEs) calibration of five-axis machine tools based on the product of exponentials (POE) formula. Instead of using 4 × 4 homogeneous transformation matrices (HTMs), it establishes the error model by transforming the 6 × 1 error vectors of rigid bodies between different frames resorting to 6 × 6 adjoint transformation matrices. A stable and efficient error model for the iterative identification of PIGEs should satisfy the requirements of completeness, continuity, and minimality. Since the POE-based error models for five-axis machine tools calibration are naturally complete and continuous, the key issue is to ensure the minimality by eliminating the redundant parameters. Three kinds of redundant parameters, which are caused by joint symmetry information, tool-workpiece metrology, and incomplete measuring data, are illustrated and explained in a geometrically intuitive way. Hence, a straightforward process is presented to select the complete and minimal set of PIGEs for five-axis machine tools. Based on the established unified and compact error Jacobian matrices, observability analyses which quantitatively describe the identification efficiency are conducted and compared for different kinds of tool tip deviations obtained from several commonly used measuring devices, including the laser tracker, R-test, and double ball-bar. Simulations are conducted on a five-axis machine tool to illustrate the application of the calibration model. The effectiveness of the model is also verified by experiments on a five-axis machine tool by using a double ball-bar.

Utilizing the Nonlinearity of Tendon Elasticity for Compensation of Unknown Gravity of Payload

2018 ◽  
Vol 30 (6) ◽  
pp. 873-879
Author(s):  
Chao Shao ◽  
Junki Togashi ◽  
Kazuhisa Mitobe ◽  
Genci Capi ◽  
◽  
...  
Keyword(s):  
Low Cost ◽  
Cost Effective ◽  
Force Sensor ◽  
Error Reduction ◽  
Position Error ◽  
Gravity Compensation ◽  
Robot Arm ◽  
Positioning Control ◽  
State Position ◽  
Significant Cost Reduction

This paper discusses the positioning control of an elastic tendon-driven robot arm under gravity. The robot is driven by rubber string tendons and winding drums attached on the outside frames. Low-cost rubber strings that are available commercially are used as tendons. The goal is to utilize the nonlinear nature of the rubber materials to control a low-cost and soft robot arm. Theoretically, a mathematical model with accurate parameters and accurate measurement of the payload weight is necessary for rigorous gravity compensation. However, the necessity for the information of the robot parameters is hindering easy adaptability, versatility, and cost-efficiency. This paper presents an iterative estimation and compensation method for unknown payloads based on the steady-state position error and the nominal stiffness coefficient. Owing to the nonlinearity of the actual rubber strings, the position error remains after a single operation of the gravity compensation. However, experiments indicate that the error reduces by a simple iteration of the same compensation operation. Considering the nonlinearity in rubber strings, the mechanism of the error reduction is analyzed theoretically. Although the iterative process is time consuming, the method requires less prior information. In addition, it is cost effective because a sophisticated force sensor is not required. As the mechanism of error reduction applies to typical rubber string materials, it is useful for significant cost-reduction and reconfigurable robotics.

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