# Sensorless Posture Detection of Reluctance Spherical Motor Based on Mutual Inductance Voltage
## 4 Conclusions  
Aiming at the main problems existing in the 3-DoF position detection in the current research and taking lessons from the basic principle of sensorless position detection of a traditional rotating electrical motor, the reluctance spherical motor with a nonlinear magnetic circuit and iron core is taken as the object to make full use of its doubly salient characteristics and study the position of the sensorless detection method for the spherical motor. Starting from the influence of the structure of the spherical motor on the numerical and spatial variation range of the stator coil inductance and using a mathematical analysis, a computer simulation, and experimental verification methods, a sensorless position detection method is realized to detect the rotor's spatial position through the coil mutual inductance voltages. The experimental results show that the detection method has a good on-line detection effect, and the population standard deviation is within 1.8\({}^{\circ}\) Therefore, the developed technique can be used for replacing the position detection method with sensors. This paper aims to achieve a breakthrough in the key technology of 3-DoF position detection of spherical motors through systematic research and promote the development and the practical process of multi-DoF motors. Following the research presented in this paper, we plan to further improve the detection accuracy while simplifying the detection circuit and algorithm to speed up the detection speed, thereby improving the applicability of the proposed sensorless detection method.  
In addition to realizing sensorless 3-DoF position detection, the following conclusions can be drawn by summarizing the research process of this paper:  
1. Sensorless position detection is often carried out by using the salient pole effect or nonlinear saturation characteristics of the motor. In order to avoid excessive reluctance torque, permanent magnet spherical motors usually adopt a coreless design; thus, there are no convex polarity or nonlinear saturation characteristics that can be utilized. Compared with permanent magnet spherical motors, it is easier for the reluctance spherical motor to realize sensorless position detection.
2. The design structure of the spherical motor has great influence on the sensorless position detection effect. For example, the periodicity of the rotor structure corresponds to the periodicity of the detection result. From the perspective of the detection range, the number of rotor magnetic poles should be as small as possible. Therefore, whether from the perspective of subsequent drive control or position detection, the influence should be considered at the early stage of motor design.  
Conceptualization, J.X. and R.Z.; funding acquisition, Q.W., R.Z. and L.J.; investigation, L.J.; methodology, J.X.; project administration, Q.W. and G.L.; software, R.Z.; validation, J.X. and S.Z.; writing--original draft, J.X.; writing--review and editing, Y.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the key project of the China National Natural Science Foundation, grant number 51637001 and was also funded by Anhui Natural Science Fund Project, grant number 1908085 QE236, 1908085 ME168 and 2008085 ME156.  
The authors declare no conflict of interest.