Voice Coil Actuator for Active Vibration Isolation in Microgravity

Many microgravity experiments require very low levels of acceleration which cannot be achieved on the International Space Station due to the residual vibration. A vibration isolation system is then usually devised between the experiment and the space station to obtain the desired accelerations at the experiment level. The very low frequency threshold required by the isolation specifications makes passive solutions for the isolation difficult to implement. This is mainly due to the practical impossibility of achieving high values of compliance of the elastic suspension. Furthermore, the unavoidable connections of uncertain characteristics between the experiment and the space station makes the problem even more difficult to be addressed. Disturbance reduction can be performed by means of active vibration isolation, based on magnetic suspension technology acting both at rack and at scientific experiment levels. The stiffness and damping of the active suspension can be tuned by the control loop to minimise the acceleration of the payload. The mechatronic design of an active magnetic suspension for vibration isolation in microgravity has been performed by resorting to the so-called voice-coil configuration, after a preliminary trade-off analysis of the available magnetic actuators and materials. The optimisation of the actuator layout was developed with respect to the design airgap and force density (N/kg of actuator) and force resolution requirements, by demonstrating that the configuration based on Lorentz magnetic force is more suitable for the above application in terms of stability, bi- directionality of the actuation, cross coupling effects and linearity of the force. The aim of the design was the maximisation of the actuation force/mass ratio. The FEM analysis of the voice coil allowed to investigate the flux leakage and the cross coupling effects between the actuation forces along the three principal directions of the active device. A procedure for the numerical data fitting has been found in order to model the magnetic force path within the airgap. Voice coil electric parameters have been also designed by checking the correspondence between the expected and actual values on a small preliminary prototype. The experimental validation of the FEM models included the characterisation of the magnets, the construction of the first magnetic actuator and the design of the power amplifier prototype. The comparison between numerical and experimental results showed a good agreement for both two-dimensional and three-dimensional FEM models, at least for what the static behaviour of the actuator is concerned. The power amplifier design was particularly difficult because of the specifications of low power dissipation, low noise, output current and voltage, related to the maximum actuation force and its resolution, added to the requirements imposed by the control systems. Some results of the final tests on a ground test rig simulating the space equipment to be isolated are reported. The system performed as required in open and closed loop in dynamic operating conditions