Abstract
Precision spherical joint is a spherical motion pair that can realize rotation with three degrees of freedom. This joint is widely used in robots, parallel mechanisms, and high-end medical equipment, as well as in aerospace and other fields. However, the rotation orientation and angle cannot be determined when the joint is in passive motion. The real-time determination of the rotation orientation and angle is crucial to the improvement of the motion control accuracy of the equipment where the joint is installed in. In this study, a new measurement method that utilizes eddy current sensors is proposed to identify the special features of the joint ball and realize angle measurements indirectly. The basic idea is to manufacture the specific shape features on the ball without affecting its movement accuracy and mechanical performance. An eddy current sensor array is distributed in the ball socket. When the ball head rotates, the features on the ball opposite to the sensor, as well as the output signal of every eddy current sensor, change. The measurement model that establishes the relationship between the output signal of the eddy current sensor array and the rotation direction and angle of the ball head is constructed by learning and training an artificial neural network. A prototype is developed using the proposed scheme, and the model simulation and feasibility experiment are subsequently performed. Results show that the root mean square angular error of a single axis within a range of ±14° is approximately 20 min, which suggests the feasibility of the proposed method.
Highlights
Spherical joints support 3D rotation motions, and are extensively utilized in robots, parallel mechanisms, automobiles, and medical devices due to their smooth motion, compact structure, and high load capacity
In the error acquisition training, learning, and modeling were completed in MATLAB, the measurement model in the experiment, the biaxial platform rotated at a certain step and provided the standard angle value, MATLAB script needed to be connected to the measurement software
The difference experiment, the biaxial platform rotated at a certain step and provided the standard angle value, and between the calculated and standard angles provided by the biaxial platform is regarded as the the measurement model calculated the results using the outputs of the three sensors
Summary
Spherical joints support 3D rotation motions, and are extensively utilized in robots, parallel mechanisms, automobiles, and medical devices due to their smooth motion, compact structure, and high load capacity. The real-time and accurate detection of the orientation and rotating angle of spherical joints is beneficial for achieving the real-time and closed-loop control of equipment and improving the control accuracy. The methods for the real-time determination of the rotation angle of spherical joints have attracted the attention of local and international researchers. Kok-Meng Lee et al measured the angle of rotation of a variable-reluctance spherical motor through the visual measurement method. The measurement resolution was dependent on the grid density. The rotation angle was obtained by identifying the grid coding information through visual measurement
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