# A New Rotor Position Measurement Method for Permanent Magnet Spherical Motors  
[PERSON]  
1 School of Electronics and Information Engineering, Anhui University, Hefei 230601, China; [EMAIL_ADDRESS] (Y.L. ); [EMAIL_ADDRESS] (Y.H. ); [EMAIL_ADDRESS] (C.Z.) 2 Power Quality Engineering Research Center of Ministry of Education, Hefei 230601, China; [EMAIL_ADDRESS]  
3 National Engineering Laboratory of Energy-Saving Motor and Control Technology, Hefei 230601, China  
4 China Electronics Technology Group No.38 Research Institute, Hefei 230601, China  
5 Faculty of Science, Engineering and Technology, Swinburne University of Technology, Melbourne 3122, Australia; [EMAIL_ADDRESS]  
6  
[PERSON]  
2 Power Quality Engineering Research Center of Ministry of Education, Hefei 230601, China; [EMAIL_ADDRESS]  
3 National Engineering Laboratory of Energy-Saving Motor and Control Technology, Hefei 230601, China  
4 China Electronics Technology Group No.38 Research Institute, Hefei 230601, China  
5 Faculty of Science, Engineering and Technology, Swinburne University of Technology, Melbourne 3122, Australia; [EMAIL_ADDRESS]  
7 Correspondence: [EMAIL_ADDRESS]; Tel. : +86-158-5511-51157  
Rotor position measurement (RPM) in spherical motors is necessary when considering rotation angles in three directions. It is much more complicated than the RPM in traditional single-DOF motors when only one rotation angle needs to be calculated. Many multi-DOF RPM methods have been proposed, which are generally divided into contact type and non-contact type methods. The contact type method adds a mechanical detection mechanism to the rotor [9; 10; 11; 12; 13; 14]. This type of the RPM can achieve high-precision results, but the heavy structure of the RPM system increases the moment of inertia to the rotor and brings huge friction resistance to the bearings. In order to avoid extra moment of inertia and friction resistance, the non-contact type method has been put forward to measure rotor position. A non-contact RPM method based on a photoelectric sensor has been proposed by [PERSON] et al. [14; 15; 16; 17]. [PERSON] et al. have proposed a non-contact RPM method based on machine vision [18]. Both methods provide high-precision results by increasing the density of the grid pattern, but it is difficult to ensure clear grid pattern on the spherical shell when the spherical motor is moving. A non-contact laser-based orientation RPM method has been proposed by [PERSON] et al. [19]. This method is capable of achieving high-precision results, but the bulky structure makes it difficult to be installed in spherical motors. Hall-effect sensors have been used to measure the rotor position for spherical motors [20; 21; 22; 23; 24; 25; 26; 27; 28], however the magnetic field varies so slowly that the signal induced in the Hall-effect sensor cannot be used to differentiate different rotor positions with high precision. In addition, the terrestrial magnetic field may influence the Hall-effect sensor, leading to a large error in the RPM. Given the limitations and trade-offs observed from the existing techniques, a novel high-precision non-contact RPM method based on machine vision is proposed for PMSMs in this paper. A LED light spot generation module (LSGM) was installed at the top of the rotor shaft in a spherical motor to form three optical feature points (OFPs). A high-speed camera was used to obtain the images of these three OFPs to compute rotation angles in three directions through image processing, obtaining the rotor position of PMSMs. Compared with other non-contact RPM methods, the proposed method provided reliable and accurate RPM with a simple structure. It was not affected by the environmental field or the moving surface of spherical rotors and, as such, is suitable for other types of spherical motors. The remaining content of this paper is organized as follows. Section 2 presents the structure of a PMSM. Section 3 introduces the composition of the measurement device, and the principle of the proposed RPM method.