A spherical joint is a commonly used mechanical hinge with the advantages of compact structure and good flexibility, and it becomes a key component in many types of equipment, such as parallel mechanisms, industrial robots, and automobiles. Real-time detection of a precision spherical joint clearance is of great significance in analyzing the motion errors of mechanical systems and improving the transmission accuracy. This paper presents a novel method for the micro-clearance measurement with a spherical differential capacitive sensor (SDCS). First, the structure and layout of the spherical capacitive plates were designed according to the measuring principle of capacitive sensors with spacing variation. Then, the mathematical model for the spatial eccentric displacements of the ball and the differential capacitance was established. In addition, equipotential guard rings were used to attenuate the fringe effect on the measurement accuracy. Finally, a simulation with Ansoft Maxwell software was carried out to calculate the capacitance values of the spherical capacitors at different eccentric displacements. Simulation results indicated that the proposed method based on SDCS was feasible and effective for the micro-clearance measurement of the precision spherical joints with small eccentricity.

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Due to the flexible and compact structures, spherical joints are widely used in parallel manipulators and industrial robots. Real-time detection of the clearance between the ball and the socket in spherical joints is beneficial to compensate motion errors of mechanical systems and improve their transmission accuracy. This work proposes an improved capacitive sensor for detecting the micro-clearance of spherical joints. First, the structure of the capacitive sensor is proposed. Then, the mathematical model for the differential capacitance of the sensor and the eccentric micro-displacement of the ball is deduced. Finally, the capacitance values of the capacitive sensor are simulated with Ansoft Maxwell. The simulated values of the differential capacitances at different eccentric displacements agree well with the theoretical ones, indicating the feasibility of the proposed detection method. In addition, the simulated results show that the proposed capacitive sensor could effectively reduce the capacitive fringe effect, improving the measurement accuracy.

Calibrators can effectively improve the calibration accuracy of articulated coordinate measuring machine (AACMM). However, the traditional calibration method of a calibrator is usually cumbersome. To study the influence of sampling strategies of various calibration methods on the calibration effect of AACMM, we design a new multifunctional calibrator for AACMM kinematics parameters, and propose three calibration methods with high accuracy and high efficiency. In the multifunctional calibrator, 84 different calibration points are provided by changing the rotation angle of calibration bar and the inclination angle of calibration plate. Compared with previous calibrators, the calibrator can obtain more positions and directions. The length calibration, multi-point calibration and virtual center distance calibration methods are proposed to calibrate kinematic parameter of AACMM. In addition, the sampling strategy for the three calibration methods is explored. Finally, the consistency of single-axis coordinate measurement is verified based on the three calibration methods. The results show that the three calibration methods can effectively improve the measurement accuracy of AACMM. Especially, the length calibration method has the highest accuracy of the three methods. The multi-point calibration method can significantly improve the calibration efficiency.