A large air gap magnetic levitator for precise positioning of a clinical microcatheter: nonlinear modelling and control

This paper details the application of nonlinear modelling and control strategies to a large air gap magnetic levitator. Specifically, this levitator uses four electromagnets in a heteropolar active magnetic bearing (AMB) configuration to control the tip position of a flexible microcatheter for applications in interventional medicine. The internal AMB diameter is 10 mm, which is considerably larger than the 1.5 mm outer diameter of the microcatheter and the 2 mm outer diameter of the ferromagnetic collar mounted to its tip. Because the air gap between the collar and the electromagnet bearing is large, the system exhibits strongly nonlinear behaviour associated with excessive magnetic leakage flux around the collar and mutual flux through the collar. A sliding mode controller (SMC) is designed using a finite element analysis (FEA) model of the magnetic field as a function of catheter position. Simulated and experimental results are compared to PID control. The SMC is able to track a time-varying trajectory with a mean error of 0.0747 mm as compared to PID control, which achieved a mean tracking error of 0.2168 mm. When given a 1.5 mm horizontal step reference, the SMC settles to a steady-state error of − 0.018 mm in 2.46 s. PID is unable to stabilize the system given the same stepped reference. These results demonstrate the effectiveness for FEA-based nonlinear modelling and control strategies to control large air gap magnetic levitators of this type.