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Research on giant magnetostrictive micro-displacement actuator with self-adaptive control algorithm

Giant magnetostrictive micro-displacement actuator has some unique characteristics, such as big output torque and high precision localization which can be in the nanometer scale. Because the relation between input magnetic field and output strain of giant magnetostrictive micro-displacement actuator exhibits hysteresis and eddy flow, the actuator has to be controlled and used in low input frequency mode or in static mode. When the actuator is controlled with a high input frequency (above 100 Hz), the output strain will exhibit strong nonlinearity. This paper found hysteresis and nonlinearity dynamic transfer function of the actuator based on Jiles-Atherton hysteresis model. The output strain of Jiles-Atherton hystersis model can reflect real output of actuator corresponding to the real input magnetic field, and this has been verified by experiment. Against the nonlinearity generated by hysteresis and eddy flow in this paper, the output strain of actuator is used for feedback to control system, and the control system adopted self-adaptive control algorithm, the ideal input and output model of actuator is used for a reference model and a hysteresis transfer function for the actuator real model. Through experiment, it has been verified that this algorithm can improve the dynamic frequency of the giant magnetostrictive micro-displacement actuator and guarantee high precision localization and linearity between the input magnetic field and output strain of the actuator at the same time.


Introduction
Giant magnetostrictive material Tb 0.23 Dy 0.75 Fe 1.95 is a kind of rare earth compound, and has strong magnetostriction effect. It has some unique features, such as large strain =1000-1500ppm , high magnetic-mechanical coupling coefficient 0.73 ,high frequency response and large output torque. A giant magnetostrictive micro-displacement actuator which is developed with giant magnetostrictive material and giant magnetostriction effect changes electromagnetic energy into mechanical energy and has large output torque, high resolution of displacement and frequency response, can be universal, used in micro-processing, ultra-precise micro-dive and focus control etc.
The magnetic field of the giant magnetostrictive micro-displacement actuator is composed of a magnetic field produced by an electric solenoid and magnetization of the giant magnetostrictive material. If the actuator works in static or low frequency alternating magnetic field, hysteresis and eddy flow of the actuator may be ignored, and the output displacement and input current of the actuator can present linearity. But if the actuator is applied in focus control which needs high frequency response, the hysteresis and eddy flow may induce the output and input of the actuator appear non-linear. So the non-linearity model of the actuator is important for improving the actuator application. Calkins [2] created a strain and quadratic hysteresis model of magnetization intensity based on the Jiles-Atherton [2] ferromagnetic hysteresis model and magnetic domain magnetization model, and this model is a low order differential equation. This model only needs six physical parameters, so parameters can be determined. But this model doesn't involve dynamic performance of the system and can't inverse the working state of the actuator in high frequency response. In this paper, the non-linearity dynamic equation of the actuator has been presented, and this equation is composed of the Jiles-Atherton model and the dynamic equation of the actuator. Moreover, the closed loop system of the actuator adopts a self-adaptive control Algorithm to improve output accuracy and dynamic performance.

Principle of actuator
The Structure of the giant magnetostrictive micro-displacement actuator shown as figure 1, an electric solenoid produces magnetic field which can drive the giant magnetostrictive rod. A bolt supplies a pre-load for the system, while a permanent magnet supplies bias magnetic field. A flexible hinge changes the strain of giant magnetostrictive into output displacement.  (2) Where M is mass of system, C is damping coefficient of system, K is rigidity coefficient of system; F u 0 are separately output torque, displacement, pre-load, cross-sectional area of bar. Integrating formula (1) and formula (2), using Laplace transform and neglecting irreversible magnetization intensity r A irr M , the ideal dynamic transfer function of system is obtained:

Self-adaptive control Algorithm
Because of the effect of hysteresis and eddy flow, the magnetic field of the actuator will alter non-linearly as the frequency of input current changes. Integrating the Jiles-Atherton hystersis model and corresponding control method is an ideal method for solving the non-linearity of the giant magnetostrictive micro-displacement actuator.
As shown in figure 3, the self-adaptive control method is adopted in the actuator system. The dynamic non-linearity model of the actuator is taken as an ideal reference model. In formula 3, X m (s) is the ideal distance output without hysteresis and eddy flow disturbance. X o (s) is the bar's actual output. The error e(s) and de(s)/dt are inputs of fussy arithmetic. The experiment uses some proper fussy control regulation and fussy illation to regulate parameters K i , K p and K d of PID controller on line, and then alter the bar's output to get the ideal static value of distance change.
The Dynamic system of the actuator is shown as figure 2. It is composed of a bar and a load, so its model parameters are made up of these two parts. M=M t +M f =1.058Kg, where M f is load weight; K=6.256×10 7 N/m rigidity coefficient; C=10×10 3 Ns/m damping coefficient; s =1300×10 -6 saturation magnetostriction coefficient; M s =7.7×10 5 A/m magnetization intensity.
Basing on parameters listed above, experiment and simulation are done as follows. As shown in figure 4, when the input current of the actuator is given a unit phase step, the output of the actuator has some different results. When it is not self-adaptive, and the cure is actual line'-' in figure 4, for the output to be steady state needs a long time .When adopting self-adaptive, and the curves are dotted line '--'and'-*' in figure 4, the system needs 60us to reach steady state and system won't oscillate.

Conclusion
The giant magnetostrictive material Tb 0.27 Dy 0.73 Fe 2 is a sort of novel ferro-magnetic material, and the giant magnetostrictive micro-displacement actuator is developed with ferro-magnetic performance and magnetostrictive theory. It has a big output torque, high frequency response, and no creep deformation. In this paper, the Jiles-Atherton hysteresis model and dynamic model of the actuator have been integrated, and adopted a self-adaptive control Algorithm in the closed loop of the system. From the Experiment and simulation, the method has been verified to solve the non-linearity phenomenon of the giant magnetostrictive micro-displacement actuator, when the actuator has high input frequency.