Analysis And Compensation Of Universal Rotating Magnetic Field Errors Caused By The Coils Assembly Applied In Capsule Robot

Capsule robot is a medical device for the diagnosis and treatment of gastrointestinal diseases.Its control accuracy directly affect the effect of diagnosis and treatment.At present,the external magnetic field driven method has been widely accepted and is expected to achieve precise control of capsule robot.Therefore,the accuracy of the external magnetic field has become one of the key research issues in this research field.Our research group proposed a control method of capsule robot based on the space universal rotating magnetic vector.The spatial universal rotating magnetic vector is generated by a tri-axis Helmholtz coils.The research shows that the assembly errors of the tri-axis Helmholtz coils can affect the precision of magnetic vector.Therefore,according to different forms of assembly errors,the corresponding compensation methods of magnetic vector errors are proposed in this paper.Firstly,the main forms of coils assembly errors are analyzed.The assembly errors of the tri-axis Helmholtz coils are mainly composed of the assembly errors of single Helmholtz coils and the assembly errors of three sets of Helmholtz coils.The assembly errors of single Helmholtz coils refers to the magnetic vector error caused by the non-parallel and non-coaxial of the two coils.The assembly errors of three sets of Helmholtz coils refers to the magnetic vector error caused by axis intersection offset errors and axis non-orthogonal errors.Secondly,aiming at the compensation of the assembly errors of single Helmholtz coils,based on Biot-Savar law,the mathematical analysis model of the error magnetic vector is derived,and the influence of error parameters on the magnetic vector is analyzed by Matlab software.The assembly errors of three sets of Helmholtz coils are divided into coils axis intersection offset errors and coils axis non-orthogonal errors.Aiming at the compensation of coils axis intersection offset errors,according to the uniformity magnetic field characteristics of single Helmholtz coils,the influence of the offset of the axis intersection on the uniform magnetic field is analyzed.Aiming at the compensation the non-orthogonal errors of coil axis,based on the principle of spatial universal rotating magnetic vector superposition,the mathematical model of errors is established.The non-orthogonal errors of the axis are analyzed and the end trajectory of the magnetic vector is changed from circle to ellipse owing to the non-orthogonal errors of the axis.The influence of error parameters on the magnetic vector are analyzed by Matlab software.Finally,according to the scalar calibration method,the linearization errors parameters model is derived.According to the principle of least squares,the error parameters are identified.And the assembly errors are compensated by adjusting the loading currents of the tri-axis Helmholtz coils.Simulation results show that the method can effectively compensate the assembly errors of the tri-axis Helmholtz coils.Through adjusting the loading currents of the tri-axis Helmholtz coils to compensate the errors of magnetic vector,the method can effectively improve the control precision of the capsule robot,which lay a more solid foundation for the clinical application of the capsule robot.