Measurement of the Spatial Rotation Angle in the Ball Joint

Precision spherical joints are widely employed in parallel mechanisms, but its rotation orientation and angle can not be known in its passive motion. Measurement of multi-dimensional angular displacement will be very of great significance in the prediction, feedback and control of motion errors of parallel mechanism. Based on the magnetic effect, a new method to identify the orientation and rotation angle with Hall sensors array matched by permanent magnet is proposed. The basic idea is embedding a permanent magnet(PM) in the ball head and several hall sensors are distributed in the ball socket. When the ball head carrying the permanent magnet rotates together, the Hall sensors array will detect the variation of magnetic induction intensity, and then the angle can be calculated by mathematics model. An embedded prototype is developed after finishing the theoretical modeling, simulation calculation and feasibility experiment, the real-time measurement of the spherical hinge rotation angle has been realized and improved by optimizing the inverse algorithm and the signal redundancy calculation. The experiment results indicated that the average error of single axis is about 0.21°in the range of ±10°, and the average error is about 0.39°in ±20°. The main error source has been smelled out from the experiment data, and a new way to further improve performance of the spherical joint has been found. The measuring accuracy is very likely to reach or exceed 15" in nearly future, which will be helpful for the measuring method to apply in the precision engineering.