IEEE TRANSACTIONS ON MAGNETICS, VOL. 40, NO. 3, MAY2004 1601 Magnetic Force Control Based on the Inverse Magnetostrictive Effect Toshiyuki Ueno, Jinhao Qiu, and Junji Tani Abstract—We describe a novel magnetic force control method. The method employs a mechanical stress applied to a magne- tostrictive material to control the attractive force between fixed and movable members of a magnetic circuit that includes a permanent magnet. The method has the advantage over electro- magnet control that a constant force can be maintained without energy consumption. We discuss the variation of the magnetic force with compression of several magnetostrictive materials. The experimental results agree with theoretical predictions of magnetic force based on analysis of an equivalent magnetic circuit and the piezomagnetic properties of the magnetostrictive materials. Index Terms—Inverse magnetostrictive effect, magnetic forces, magnetostrictive material. I. INTRODUCTION E LECTROMAGNETS are essential devices for converting electric energy to mechanical energy and for controlling magnetic forces in various devices. Electric motors are the most widely used devices for electromechanical energy conversion, using electromagnets for generation and control of the torques acting on the rotors. The magnetic force is based on the magne- tomotive force on the coils, which is proportional to the number of the turns of the windings and the currents. As a result of im- provements in magnetic materials, most of the electrical energy input to the electromagnets is converted to mechanical energy, but part of it is inevitably dissipated as Joule heating in the coils as a result of their resistance. In controlling magnetic forces during long periods of operation, Joule heating can represent a significant loss of energy. Conventional magnetic levitation sys- tems, which balance the gravitational forces on a ferrous object with magnetic forces, are a typical example, since they require a continuous supply of electrical energy, most of which is dissi- pated as Joule heat in the coils. To overcome this problem, we have proposed a method of controlling magnetic forces that does not use coils or currents [1]. This method is based on the inverse magnetostrictive effect of magnetostrictive materials, in which variation of the magne- tization of magnetostrictive materials with compressive stress is converted to variations of magnetic force by magnetic cir- cuits. In this method, no energy is required to maintain con- trol of a constant force. We have previously demonstrated the variation of magnetic force using a giant magnetostrictive mate- Manuscript received July 10, 2003; revised January 5, 2004. T. Ueno is with Department of Precision Machinery Engineering, the Uni- versity of Tokyo, Tokyo 113-8656, Japan (e-mail: ueno@intellect.pe.u-tokyo. ac.jp). J. Qiu and J. Tani are with the Institute of Fluid Science, Tohoku University, Sendai 980-8577, Japan. Digital Object Identifier 10.1109/TMAG.2004.826626 Fig. 1. Magnetic circuit with (a) free stress and (b) compressive stress on magnetostrictive material. rial [2], [3]. The giant magnetostrictive material is a Tb–Dy–Fe alloy with a large piezomagnetic constant and small perme- ability, which produces an adequate magnetic force when un- stressed, and also allows large variations. This paper derives the expected variation of magnetic force by analysis of an equiva- lent magnetic circuit. Several giant magnetostrictive materials made by different manufacturing processes and with different dimensions are tested to verify the validity of the analysis and to clarify the relation between piezomagnetic properties and the behavior of the variations in the magnetic force. II. MAGNETIC FORCE CONTROL BY THE INVERSE MAGNETOSTRICTIVE EFFECT A. Principle and Formulation of Magnetic Force A basic magnetic circuit for converting a mechanical load to variation of a magnetic force is shown in Fig. 1(a). The mag- netic circuit consists of fixed and movable yokes, a permanent magnet, and a magnetostrictive material (MM) which exhibits a positive magnetostriction in a magnetic field. Neglecting leakage, two sets of flux loops arise: one consists of the magnet and MM (labeled 1), and the other includes the magnet and the gap (labeled 2). The magnetomotive force of the magnet is used to magnetize the MM accompanied by a magnetostriction and generate a magnetic force between the fixed and movable yokes. For a fixed gap, the sum of the fluxes in two paths is approximately conserved, so that the magnetic force is varied by the magnetization of the MM (flux in path 1), which can be controlled by mechanical stress. Application of a compressive stress to a suitably biased MM changes the flux distribution as shown in Fig. 1(b); the magnetization of MM decreases by the inverse magnetostrictive effect, and conversely the flux in path 2, and the resulting magnetic force, increases. 0018-9464/04$20.00 © 2004 IEEE