This paper explores a robust μ-synthesis control scheme for structural resonance vibration suppression of high-speed rotor systems supported by active magnetic bearings (AMBs) in the magnetically suspended double-gimbal control moment gyro (MSDGCMG). The derivation of a nominal linearized model about an operating point was presented. Sine sweep test was conducted on each component of AMB control system to obtain parameter variations and high-frequency unmodeled dynamics, including the structural resonance modes. A fictitious uncertainty block was introduced to represent the performance requirements for the augmented system. Finally, D-K iteration procedure was employed to solve the robust μ-controller. Rotor run-up experiments on the originally developed MSDGCMG prototype show that the designed μ-controller has a good performance for vibration rejection of structural resonance mode with the excitation of coupling torques. Further investigations indicate that the proposed method can also ensure the robust stability and performance of high-speed rotor system subject to the reaction of a large gyro torque.

AMB Vibration Control for Structural Resonance of Double-Gimbal Control Moment Gyro With High-Speed Magnetically Suspended Rotor

This paper presents a semiactive method for stiffness control of electromechanical systems. The proposed method consists of intermittently connecting a piezoelectric element on an oscillating electrical network. It is demonstrated that such a nonlinear treatment allows an effective control of the stiffness, while requiring less power than the classical methods. Experimental measurements carried out show that such a technique allows controlling the stiffness in a wide range of value, which validates the theoretical predictions.

Stiffness Tuning Using a Low-Cost Semiactive Nonlinear Technique

To isolate the mounted object from the vibration sources, it is required that the qualities of the actuators in an active vibration isolation system are highly promising. Therefore, it is unreasonable to ignore actuator nonlinearity to realize the excellent performances of the vibration isolation. In this paper, the problem of unknown actuator backlash in the vibration isolation system is investigated. To handle the lines–segments backlash nonlinearity, an adaptive smooth backlash inverse is employed to compensate the deleterious effect. Moreover, a promising controller is proposed to ensure that the motion of the moving platform converges to desired values. Based on Lyapunov’s second method, all of the involved signals in the closed-loop system are proved to be stable. Finally, comparative studies involving active vibration isolation with actuator backlash are constructed and carried out to validate the effectiveness of the proposed control scheme.

Actuator Backlash Compensation and Accurate Parameter Estimation for Active Vibration Isolation System

3D Gradient coil design – Toroidal surfaces

This paper presents the development of an energy-regenerative damper consisting of a mass-spring system coupled with a permanent-magnet dc machine, power electronics converter, and a battery. The vibration energy induced in the mass-spring system is converted into battery charge through the dc machine and a power electronics controller. The latter is a pulse-width-modulated boost-type converter, which enforces a pseudoresistive behavior between its input terminals through a feedback control scheme developed in this study. Introducing this pseudoresistive behavior across the input terminals of the dc machine produces the same effect as a mechanical damper but with an energy-regenerative function. The dynamics of the mass-spring system coupled with the linear machine are used to obtain a condition for extracting maximum electric power from mechanical vibrations. Experimental results are presented that demonstrate the performance of the regenerative damper using a small-scale suspension system test bed.

A Boost-Type Power Converter for Energy-Regenerative Damping

Lorentz-force-induced vibrations in MRI systems cause significant acoustic noise levels during scanning, the main acoustic noise source being the vibrating gradient coil. In this paper, a novel active vibration control technique is presented to reduce vibrations of the gradient coil, and hence, achieve a reduction of acoustic noise during scanning. The active vibration control technique uses seismic masses that are actuated by means of piezo actuators to create forces on the gradient coil counteracting its vibrations. Using four seismic mass actuators, a vibration reduction of 3-8 dB at resonance frequencies is achieved, giving an overall vibration reduction of 3 dB for a typical field gradient (FE)-echo planar imaging (EPI) gradient sequence, as substantiated by measurements. Using eight actuators, an overall vibration reduction of 5 dB is predicted for this sequence.

A.H. Koevoets

Papers

Active Vibration Control of Gradient Coils to Reduce Acoustic Noise of MRI Systems