Research On Dynamic Characteristics Of A Magnetic-actuated Double-spin Spherical Capsule Robot During Fixed-point Posture Adjustment

Compared with the traditional flexible cable endoscope,the wireless capsule endoscopy is safe,comfortable and non-invasive,and has obvious advantages in the diagnosis of gastrointestinal(GI)diseases.However,the existing capsule endoscopies lack active locomotion and posture control functions,can only realize passive movement by GI peristalsis or gravity.Its diagnostic effect is better in small intestine,but it has a high miss rate in three-dimensional spacious environment such as stomach and colon.At the same time,the existing passive capsule can’t achieve the future extended functions such as drug delivery,biopsy and minimally invasive surgery.Therefore,it is urgent to expand the scope of capsule endoscopy to three-dimensional spacious environment such as stomach and colon,and realize the active control.For this reason,this paper has carried out the structural design,dynamic modeling,posture dynamic characteristics analysis and nonlinear dynamic characteristics research of the new type of magnetically controlled active capsule.The specific research contents are as follows:To realize the active movement and posture control of the capsule,a dual-spin spherical capsule robot(DSCR)driven by the spatial universal uniform rotating magnetic field(SURMF)is proposed,along with an orthogonal transformation control method of the SURMF.The structure of the DSCR is characterized in that the rotating upper hemisphere and the unrotating lower hemisphere form a dual-spin body,and it has good orientation ability and posture adjustment ability.The DSCR uses the coupling magnetic moment of the embedded radial magnetization permanent magnet and the SURMF as the medium.By controlling the orientation of the SURMF,the effective control of posture and position of the DSCR can be achieved,and the problem of separation and conversion between the fixed point posture adjustment and the rolling locomotion of the DSCR can be solved.In order to realize the dimensional decoupling of the SURMF and the flexible control of the posture of the DSCR,the three-phase current superposition formula with the azimuth angle and altitude angle as the independent control variables is derived according to the principle of orthogonal transformation.The orientation of the SURMF can be adjusted independently along the azimuth and altitude directions.In order to realize the steering rolling of the DSCR in curved intestine,the relationship between the pixel coordinates of the centroid of the dark zone and the bending direction of the intestine is established,and the steering rolling strategy based on the monocular vision of the DSCR is proposed.By controlling the orientation of the SURMF,the DSCR can realize the functions of the fixed-point posture adjustment in passive mode,the rolling locomotion in active mode,and the steering rolling in curved intestine.Therefore,it provides a new and reliable platform for the traversing inspection of three-dimensional spacious environment such as stomach and colon.In order to realize the precise magnetic control of the DSCR,the posture stability and precession characteristics of the system are studied.The coupled magnetic moment,the viscoelastic friction moment and the gravity moment of the DSCR are analyzed.Based on Euler dynamics,the posture dynamic equation describing the electro-magnetic-mechanic-hydraulic coupling behavior of the DSCR is established.Three motion forms of the DSCR for the fixed-point posture adjustment are obtained,which are asymptotic stable motion,periodic motion and chaotic motion.Based on the Floquet theory,the stability of the equilibrium point and the periodic motion of the DSCR are studied.The equilibrium stability region of the system is obtained by using the interval state transition matrix algorithm,and the periodic motion stability region is obtained by using the HSU method.At the same time,the precession characteristic of the DSCR is also studied,and the influence of the parameters such as the offset of the center of gravity,the viscoelastic damping coefficient,the rotation speed of the magnetic field and the magnetic flux density on the precession characteristic of the DSCR are analyzed.Taking the system mean orientation error and the system precession amplitude as the optimization objectives,the genetic algorithm is used for multi-objective optimization,a set of optimal control parameters of the system are obtained.In order to investigate the global nonlinear dynamic response law of the system when the control parameters vary widely,the bifurcation and chaotic dynamic characteristics of the DSCR are studied by the incremental harmonic balance(IHB)method and the Runge-Kutta method.The semi-analytical periodic solution of the posture dynamic equation is obtained,and the stability of periodic solution is analyzed by using the Floquet theory.With the magnetic flux density and the rotation speed of the magnetic field as the bifurcation control parameters,the bifurcation diagram,the time domain diagram,the phase diagram,the Poincare map diagram and the Lyapunov exponent diagram are used as analysis methods to study the global nonlinear dynamic response of the system.Finally,the change law of the global topology structure and the mechanism of chaos generation of the system are obtained when the magnetic flux density and the rotation speed of the magnetic field vary in a large range.The experiment platform of the SURMF system is built to verify and analyze the comprehensive performance of the DSCR.Theoretical and experimental studies show that the DSCR can reliably realize the fixed-point posture adjustment function in the passive mode,the rolling locomotion function in the active mode,and the mutual conversion function between the active and passive modes.It also has good posture stability and strong anti-interference ability.When the DSCR adjusts the posture at a fixed point,increasing the viscoelastic damping coefficient and reducing the offset of the center of gravity is beneficial to reduce the deviation between the polar axis of the DSCR and the target orientation,thereby improving the control accuracy of the system.Increasing the rotation speed of the magnetic field and decreasing the magnetic flux density will improve the stability of the system and reduce the chattering degree of the polar axis of the DSCR.