Development of Electrostatic Levitator at JPL
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DEVELOPMENT OF ELECTROSTATIC LEVITATOR AT JPL WON-KYU RHIM, MELVIN M. SAFFREN AND DANIEL D. ELLEMAN Jet Propulsion Laboratory, California Institute of Technology, Drive, Pasadena, California 91109
4800 Oak Grove
ABSTRACT We report preliminary results of the design and operation of electrostatic levitators we have developed at JPL. Discussion is focussed on the single-axis levitators for groundbased operation and for operation in the reduced gravity environment. Design principles and the performance of final products are described. INTRODUCTION Containerless processing of materials requires a contactless means for their control and manipulation. Two methods for this are currently being developed under the NASA Materials-Processing-in-Space Program. One method utilizes the forces and torques generated by acoustic waves; the other, those generated by RF fields. Each has drawbacks which can be remedied by a third method described in this paper which utilizes electrostatic forces. The acoustic method requires the presence of a gas to sustain the acoustic waves. While gas pressure as low as 2- 10 torr may suffice, the method evidently cannot be used for such processes as purification of molten materials by outgassing into a vacuum. The RF method requires that the material be an electric conductor. Although at sufficiently elevated temperatures all materials are conductors, the RF power required for control and manipulation increases with resistivity. Consequently, as the melt of an intrinsic nonconductor cools and as its resistivity increases, the increasingly intense fields required for control tend to prevent a solidification. When solidification does take place, the RN method fails completely. This method not only causes unavoidable heating, but, because the RF fields induce flows within the melt, it also causes unavoidable stirring as well. In order to alleviate the difficulties of the acoustic and RF methods, our team at JPL has designed levitators which utilize electrostatic fields and has obtained preliminary results verifying some of the basic design principles of the system. In this method the material is electrically charged, forces on the material being produced by electric fields between the electrodeb. Static electric fields cannot, however, provide a stable position for an object, a direct consequence of Earnshaw's theorem that governs the stability of a conductor in an electric field. Stable positioning of the object, therefore, must be accomplished by a feedback system that monitors the position of the object. In this paper we will primarily describe a single-axis system in which the object's position along the vertical axis is actively controlled. Design of a 1 three-axis-control levitator is under way and the results will be published shortly. DESIGN PRINCIPLE The design principle and arrangement of the system will be described in relation to the block diagram shown in Fig. 1. A CCD camera picks up an image of an object inside the chamber with a frame rate - 120 per second. The position and veloc
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