This paper presents a theoretical and experimental study on controller design for the AMBs in a small-scale flywheel energy storage system, where the main goals are to achieve low energy consumption and improved rotordynamic stability. A H-infinity optimal control synthesis procedure is defined for the permanent-magnet-biased AMB-rotor system with 4 degrees of freedom. Through the choice of design weighting functions, notch filter characteristics are incorporated within the controller to reduce AMB current components caused by rotor vibration at the synchronous frequency and higher harmonics. Experimental tests are used to validate the controller design methodology and provide comparative results on performance and efficiency. The results show that the H-infinity controller is able to achieve stable rotor levitation and reduce AMB power consumption by more than 40% (from 4.80 to 2.64 Watts) compared with the conventional PD control method. Additionally, the H-infinity controller can prevent vibrational instability of the rotor nutation mode, which is prone to occur when operating with high rotational speeds.

A new concept for a smart drill spindle with active magnetic thrust bearing has been developed to enhance the monitoring and control capabilities of robotic drilling systems. This article reports on the realization and evaluation of an integrated thrust-force sensing technique for the spindle unit. The proposed technique uses a model-based state estimation scheme with calibrated nonlinear model of the spindle bearing forces. The method requires only one axial displacement sensor for real-time tool-force estimation and position control. A speed-dependent disturbance model is further defined and incorporated within the estimation scheme to compensate for spindle rotation effects. A method of using robot end-effector acceleration measurements to decouple the force estimate from both robot motion and gravity effects is also shown. Experiments are performed with a 7-axis lightweight industrial manipulator to assess the system's ability to provide accurate thrust force information under varied drilling conditions, while simultaneously controlling the motion of the drilling tool relative to the robot end-effector. The proposed technique provides new and important capabilities for drill-state recognition and penetration control, as well as the implementation of force-based control strategies.

This paper describes an exact linearizing control approach for a distributed actuation magnetic bearing (DAMB) supporting a thin-walled rotor. The radial DAMB design incorporates a circular array of compact electromagnetic actuators with multi-coil winding scheme optimized for supporting thin-walled rotors. A distinguishing feature is that both the x and y components of the radial bearing force are coupled with all four of the supplied coil currents and so a closed form solution for the linearizing equations cannot be obtained. To overcome this issue, a gradient-based root-finding algorithm is proposed to solve the linearizing equations numerically in real-time. The proposed method can be applied with any chosen constraints on current values to achieve low RMS values while avoiding zero-current operating points. The approach is implemented and tested experimentally on a rotor system comprising two radial DAMBs and a uniform cylindrical shell rotor. The results show that the method achieves more accurate reproduction of demanded bearing forces, thereby simplifying the rotor suspension control design and providing improved stability and vibration control performance compared with implementations based on operating point linearization.