A Novel Long Stroke Planar Magnetic Bearing Actuator

There is a growing demand for magnetic suspension, combined with electromagnetic propulsion, especially in the field of high accuracy twoor three-dimensional positioning. No mechanical wear, dust free operation, and (ultra) high vacuum compatibility are strong motivations to at least consider the application of magnetic bearings. This paper presents a novel Planar Active Magnetic Bearing [PAMB] implying reluctance forces for accurate suspension combined with Lorentz forces for fast propulsion. The principle combines power economic performance, fast, sub-micrometer accuracy positioning, and large obtainable strokes both linear as angular. The paper will deal with all aspects of the design, including the working principle, the dimensioning of the magnetic circuitry, the mechanical design and the controller aspects. Results will be presented of experiments on a first test set-up, which is capable of handling a planar stroke of 160 mm x 160 mm. Power dissipation during suspension can be as low as 0.8 mW for the total set-up, while accelerations up to 2 ms are achieved. HISTORY, MOTIVATION AND APPROACH The research on linear active magnetic bearings started at the Laboratory for Micro Engineering with the Ph.D. research of F. Auer [1]. Main accomplishment was the development of the Suspension and Propulsion Unit [SPU], which combined reluctance and Lorentz forces, see figure 1. Experimental set-ups have proven a negligible influence from the propulsion task to the suspension task one to the other and vice-versa. Application of 3 SPUs led to an XYΦ positioning table with the following specifications: FIGURE 1: Principle of the Suspension and Propulsion Unit [SPU] by Auer [1] combining active magnetic suspension with direct Lorentz propulsion with little cross coupling. • 10 mm x 10 mm stroke in the horizontal plane, • a dynamical stiffness of 4 N/μm, • 0.72 N maximum propulsion force on a platen of 3 kg, • a relative accuracy of one micrometer, and • 86 W power dissipation during steady state suspension. The first author continued this project in June 1995. The goal of this continuation is roughly expansion of knowledge on linear active magnetic bearings. By a study on possible applications it was concluded that the following disadvantages must be prevented or overcome in a new design, before magnetic bearings can become a serious alternative in the field of precision engineering tasks like scanning and positioning. 1. Firstly, the SPU principally offers either a small stroke or a heavy suspended object ('rotor'), and, hence limited propulsion force to mass ratio. 2. Secondly, the generation of bias flux by electromagnets causes generation of a lot of heat in the stator while the attainable induction remains low (below 0.2 T). 3. Thirdly, the multipolar E-shaped stator makes commutation of the secondary coil indispensable, when strokes need to be achieved larger then the 10 mm 'leg' separation of the E shaped core. 4. Finally, magnetic hysteresis and possibly Eddycurrents may result from the multipolar shape. Still, the main advantages of the SPU principle should be kept, like direct Lorentz force type propulsion in the plane of the center of mass, and double usage of the magnetic induction in the air-gap that already exists for suspension by controlled reluctance. DESIGN CONSIDERATIONS Permanent magnet biasing There are only two principally different configurations possible for a linear magnetic bearing which combines permanent magnets [PMs] for biasing and electromagnets [EMs] for control called series (coplanar) and parallel (non-coplanar) respectively, see figure 2. Both can be used to overcome the power dissipation of electromagnetic biasing. Other advantages are increased force-slew-rate [11], and, in case of the SPU, higher propulsion forces. By earlier research we concluded that the non-coplanar configuration was favorable in several ways [9]. Power dissipation decreased to below 1 W by the Non-Coplanar Suspension Unit [NCSU] while flux density increased 5–20 times compared with the SPU.