This study introduces a straightforward and cost-effective method to enhance the positional accuracy of a 6-axis serial robot using a double ball-bar (DBB). Kinematic errors, a primary source of inaccuracies in offline programming, are estimated and calibrated through circular tests. The kinematics of the robot are modeled using the Denavit-Hartenberg (D-H) convention, and a mathematical relationship between radial deviation and kinematic errors is established. To avoid singularities, identifiable parameters are selected using singular value decomposition. The method involves three steps: measuring the tool center point (TCP) with the DBB, estimating key kinematic parameters, and verifying the calibration results. Redundant or less significant parameters are excluded to concentrate on the most impactful ones. During the process, the robot is commanded to trace a circular path while radial deviations are recorded. This data is then utilized to estimate and adjust the kinematic model. After recalculating and executing the circular path with the calibrated model, a notable reduction in deviation is achieved. This proposed approach requires no additional equipment and provides a quick, affordable solution for improving the accuracy of industrial robots while lowering maintenance costs.

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.

ISO 10791-6 specifies test conditions, BK1 and BK2, including circularly interpolated motions by simultaneous control of two linear axes and a rotary/tilting axis, for five-axis machine tools with a tilting-rotary table. Eccentricities of measured motions are used to identify position-independent geometric errors of the rotary/tilting axis. However, time-consuming alignments of measurement devices are required to execute the circular motions due to large geometric errors of the tilting axis. In this paper, a simple method is proposed to align an initial position of a tool-center-point (TCP) relative to the actual tilting axis of five-axis machine tools for application of ISO 10791-6. A ball at the tool nose with an extension fixture, supplied commercially by a double ball-bar manufacturer, is used to measure positional deviations of a ball on workpiece table at 90° command angle of a tilting axis. An alignment error of a TCP is identified simply by using a geometric relationship of the TCP and measured deviations. Then, identified alignment errors are used to calculate initial position of a TCP for fine measurements of position-independent geometric errors specified in ISO 10791-6. The proposed method is applied to a five-axis machine tool and verified experimentally.