Advanced Active-Magnetic-Bearing Thrust-Measurement System

Tag : thrust bearings

An advanced thrust-measurement system utilizes active magneticbearings to both (1) levitate a floating frame in all six degreesof freedom and (2) measure the levitation forces between thefloating frame and a grounded frame. This system was developed fororiginal use in measuring the thrust exerted by a rocket enginemounted on the floating frame, but can just as well be used inother force-measurement applications.
This system offers several advantages over prior thrust-measurementsystems based on mechanical support by flexures and/or load cells: The system includes multiple active magnetic bearings for eachdegree of freedom, so that by selective use of one, some, or all ofthese bearings, it is possible to test a given article over a wideforce range in the same fixture, eliminating the need to transferthe article to different test fixtures to obtain the benefit offull-scale accuracy of different force-measurement devices fordifferent force ranges. Like other active magnetic bearings, the active magnetic bearingsof this system include closed-loop control subsystems, throughwhich the stiffness and damping characteristics of the magneticbearings can be modified electronically. The design of the system minimizes or eliminates cross-axisforce-measurement errors. The active magnetic bearings areconfigured to provide support against movement along all threeorthogonal Cartesian axes, and such that the support along a givenaxis does not produce force along any other axis. Moreover, byeliminating the need for such mechanical connections as flexuresused in prior thrust-measurement systems, magnetic levitation ofthe floating frame eliminates what would otherwise be major sourcesof cross-axis forces and the associated measurement errors.
Overall, relative to prior mechanical-support thrust-measurementsystems, this system offers greater versatility for adaptation to avariety of test conditions and requirements.
The basic idea of most prior active-magnetic-bearingforce-measurement systems is to calculate levitation forces on thebasis of simple proportionalities between changes in those forcesand changes in feedback-controlled currents applied to levitatingelectromagnetic coils. In the prior systems, the effects of gaplengths on fringing magnetic fields and the concomitant effects onmagnetic forces were neglected. In the present system, the controlsubsystems of the active magnetic bearings are coupled with acomputer- based automatic calibration system runningspecial-purpose software wherein gap-length-dependent fringingfactors are applied to current- and magnetic-flux-based forceequations and combined with a multipoint calibration method toobtain greater accuracy. All of the inputs required for calibrationcan be obtained from the control subsystems of the active magneticbearings (and from magnetic-flux sensors if they are used). Testshave verified that force accuracies characterized by errors or<5 percent of full-scale readings are achievable when usingcurrent-based force equations or by errors <0.5 percent offull-scale readings when using flux-based equations.
This work was done by Joseph Imlach of Innovative Concepts InEngineering LLC and Mary Kasarda and Eric Blumber of VirginiaPolytechnic Institute and State University for Stennis SpaceCenter.
Inquiries concerning rights for the commercial use of thisinvention 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.