High frequency high amplitude magnetic field driving system for magnetostrictive actuators

Abstract

Magnetostrictive actuators are the focus of many research and development activities, especially for high frequency applications. A key component of these actuators is the magnetic field driving system. In this research work, magnetic field drivers for magnetostrictive actuators are studied and a detailed methodology to build a low cost, power efficient, High Frequency High Amplitude (HFHA) magnetic field driver system is developed. The detailed study of magnetic field drivers suitable for magnetostrictive actuators revealed that the design of the driver must be done at a system level. Changes in geometry of the magnetostrictive material and the mechanical structure have significant effect on the efficiency of the driver. For the purposes of the research work, the magnetostrictive material was limited to two forms, viz. bulk rod and thin film. First, a suitable mechanical structure must be selected based on the amplitude and frequency requirements. For purposes of this work, a cantilever beam was chosen so that large amplitude can be achieved with proper sizing. Simple vibration analysis was used to size the beam so that the designed actuator frequency is within the third bending mode. Second, the magnetostrictive actuator was designed so that a mechanical bias and a magnetic bias can be applied to maximize the output of the actuator. The driver design primarily consists of a coil, a least resistant magnetic return path, a power source, and a command signal generator. Power drawn by the magnetic field driver increases with the frequency of operation due to increases in its inductance, skin effect losses, and eddy current losses. The magnetostrictive material with its low resistivity, when used as a core, therefore requires system level approach in designing the driver. Methodology developed to drive a magnetostrictive actuator includes determining optimal values of diameter of the magnetic wire, number of turns, length of the coil, and overall geometry. This methodology also accounts for the skin effect losses in the driving coil wire and the eddy current losses in magnetostrictive material. The power consumption is further reduced by creating an electrical resonance at the desired operating frequency by adding a capacitor in series with the driver. The least resistance magnetic return path is ensured by using a high magnetic permeability material and custom geometry permanent magnets. In this research work, response of the system comprising a well known model of cantilever beam with mirror and 1-d model of actuator with nonlinear magnetostrictive rod is measured empirically. The developed methodology is validated experimentally using a custom fabricated magnetostrictive mirror deflector for rapidly tunable laser system. The fabricated mirror deflector is capable of producing a deflection of 6.1 mrad at 5.28 kHz with a power consumption of 0.8 W and a deflection of 3.8 mrad at 10.8 kHz with 0.65 W. The electrical resonance circuits used have further reduced the power consumption by as much as 32%. The methodology is extended to MEMS thin film mirror deflectors. The work reported here is the first step towards building a comprehensive model of High Frequency High Amplitude driver for Magnetostrictive actuators. This needs more work to include the true nonlinear behavior of the magnetostrictive rod and the integrated system