Research On Space Voltage Vector Control Model Of Universal Rotating Magnetic Field

Endoscopic capsule robot with embedded permanent magnet driven by external rotating magnetic field not only realizes the active control,but also has the advantages of stable,improved efficiency,safety and reliability in gastrointestinal examination.It has become a research hot spot for many scholars because of its great clinical application value.Our research group uses triaxial orthogonal square Helmholtz coils to generate the space universal rotating magnetic field to control the capsule robot to adjust its attitude and motion.In order to improve the precision and control flexibility of the rotating magnetic field,study on the power driving technology of the load coils is carried on and a space voltage vector control model of universal rotating magnetic field is proposed in this paper.Firstly,the superposition principle of the universal rotating magnetic field is introduced.Based on the magnetic field modeling at any spatial point inside the uniaxial coil,the result is extended to the triaxial coils by coordinate transformation and superposition.The accuracy of the magnetic field in the rotation plane at different time points in a rotation period is calculated and the magnetic field distribution of the triaxial coils is analyzed.The trajectory of the voltage space vector applied on the coils is derived and the trajectory characteristics are analyzed by the differential geometry principle.It is concluded that the trajectory is a plane curve in space,and its shape is a generalized rotation ellipse with the same frequency as the rotating magnetic field.The corresponding vector of the semi-major axis and semi-short axis and modulus length of the major axis and short axis are derived.The discussion on the trajectory lays the foundation for the design of the space voltage vector control model.Secondly,a space voltage vector control model is proposed.The three-phase six-leg power circuit topology is proposed for the spatial structure and load characteristics of the traxial orthogonal Helmholtz coils.The combination of each bridge arm state is listed and all the corresponding basic voltage vectors and their spatial distribution are calculated.The segmentation plane is designed to divide the three-dimensional space to form a plurality of spatial solid sectors according to the basic voltage vector distribution characteristics.The sector judgment condition in which the reference vector is located is designed.The basic voltage vector duty cycle calculation matrix is derived based on the volt-second balance principle,and the periodic timing design method of the basic voltage vector sequence is proposed.Then,harmonic analysis is performed on the universal rotating magnetic field generated by the designed space voltage vector control model.Based on the principle of Fourier series,the control periodic timing series waveform is expanded,and the mathematical analysis model of the harmonic error component of the rotating magnetic field is derived.The error evaluation indicators are established.The advantages of the proposed space voltage vector control model in reducing the magnetic field harmonic error component are verified compared with the harmonic component error of the bipolar sinusoidal pulse width modulation driving Helmholtz coils.Finally,simulations are carried out to verify the space voltage vector control model proposed in this paper.The results show the effectiveness of the control method.At the same time,the harmonic error mathematical analysis model of the universal rotating magnetic field under the space voltage vector control model and the bipolar sinusoidal pulse width modulation are simulated and the correctness of the model is proved,which lays a foundation for the research and application of the space voltage vector control method of the capsule robot.