Advanced Active-Magnetic-Bearing Thrust-Measurement System

Stennis Space Center

Monday, September 01 2008

Automatic multipoint calibration and a fringing model are used to increase accuracy.

An advanced thrust-measurement system utilizes active magnetic bearings to both (1) levitate a floating frame in all six degrees of freedom and (2) measure the levitation forces between the floating frame and a grounded frame. This system was developed for original use in measuring the thrust exerted by a rocket engine mounted on the floating frame, but can just as well be used in other force-measurement applications.

This system offers several advantages over prior thrust-measurement systems based on mechanical support by flexures and/or load cells:

• The system includes multiple active magnetic bearings for each degree of freedom, so that by selective use of one, some, or all of these bearings, it is possible to test a given article over a wide force range in the same fixture, eliminating the need to transfer the article to different test fixtures to obtain the benefit of full-scale accuracy of different force-measurement devices for different force ranges.
• Like other active magnetic bearings, the active magnetic bearings of this system include closed-loop control subsystems, through which the stiffness and damping characteristics of the magnetic bearings can be modified electronically.
• The design of the system minimizes or eliminates cross-axis force-measurement errors. The active magnetic bearings are configured to provide support against movement along all three orthogonal Cartesian axes, and such that the support along a given axis does not produce force along any other axis. Moreover, by eliminating the need for such mechanical connections as flexures used in prior thrust-measurement systems, magnetic levitation of the floating frame eliminates what would otherwise be major sources of cross-axis forces and the associated measurement errors.

Overall, relative to prior mechanical-support thrust-measurement systems, this system offers greater versatility for adaptation to a variety of test conditions and requirements.

The basic idea of most prior active-magnetic-bearing force-measurement systems is to calculate levitation forces on the basis of simple proportionalities between changes in those forces and changes in feedback-controlled currents applied to levitating electromagnetic coils. In the prior systems, the effects of gap lengths on fringing magnetic fields and the concomitant effects on magnetic forces were neglected. In the present system, the control subsystems of the active magnetic bearings are coupled with a computer- based automatic calibration system running special-purpose software wherein gap-length-dependent fringing factors are applied to current- and magnetic-flux-based force equations and combined with a multipoint calibration method to obtain greater accuracy. All of the inputs required for calibration can be obtained from the control subsystems of the active magnetic bearings (and from magnetic-flux sensors if they are used). Tests have verified that force accuracies characterized by errors or <5 percent of full-scale readings are achievable when using current-based force equations or by errors <0.5 percent of full-scale readings when using flux-based equations.

This work was done by Joseph Imlach of Innovative Concepts In Engineering LLC and Mary Kasarda and Eric Blumber of Virginia Polytechnic Institute and State University for Stennis Space Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to:

Innovative Concepts In Engineering LLC
2142 Tributary Circle
Anchorage, AK 99516
(907) 337-8954

Refer to SSC-00177-1/8-1, volume and number of this NASA Tech Briefs issue, and the page number.