An electromagnetic shaker based on a linear induction motor has been developed for use in flight flutter testing of aircraft. The shaker is a wing-tip-mounted unit capable of delivering root-mean-square forces as large as 140 lb (623 N) at frequencies from 10 to 60 Hz.

Advanced flight flutter testing methods that are undergoing development show promise for significantly improving the process of expansion of the flight envelope. However, these methods will require airborne equipment that can rapidly, repeatedly, and vigorously excite all of the critical vibrational resonances of an aircraft with precise, carefully crafted forcing functions, the time dependencies of which can be measured. These capabilities are well beyond those of previously developed shakers.

This Linear Induction Motor (LIM) is designed to satisfy a requirement for high force, plus tight constraints on weight and size.

During the development of the present electromagnetic shaker, a linear induction motor (LIM) was combined with a field-oriented (vector) controller that enables the use of the LIM as a servo. The LIM can therefore be made to track an analog signal that represents the desired force as a function of time. As the LIM produces the commanded force, it accelerates and decelerates the mass of the moving portion of the LIM. The resulting reaction force on the nominally stationary portion (that is, the portion attached to a wing-tip accessory rail) is delivered to the wing. The field-oriented controller utilizes feedback from a position sensor and from phase current sensors, in order to control the force produced by the LIM.

A requirement for high force, plus tight constraints on weight and size necessitated a novel LIM design. In particular, as shown in the "disassembled" view in the figure, the LIM features two end primary windings, two secondary "ladder" conductors (each double-sided), and a middle double-sided primary winding in a multiple-airgap arrangement. The magnetic flux that emerges from an end primary winding must pass through the two secondary conductors, the middle double-sided primary winding, and the four airgaps before reaching the other end primary winding. Consequently, only the end primary windings must be provided with iron return paths, so that the shaker can be made very compact. In addition, designing the secondary conductors to be stationary and the primary windings to move enables limitation of the stationary portion of the mass of the shaker to 10.5 lb (4.8 kg).

In tests, the force that the LIM can apply was found to be limited by heating. For example, if heating were not a consideration, then the LIM could produce a force in excess of 1,000 lb (4.4 kN) at electrical breakdown while operating at a magnetic field near saturation, with ohmic losses of 10 kW. On the other hand, the LIM can produce a force of 200 lb (0.9 kN) at a more reasonable level of losses (1.0 kW) at lower fields. The foregoing results imply that at low duty cycles, one can achieve forces much greater than those achievable in continuous operation. Tests also demonstrated the ability of the shaker to follow sinusoidal force commands at frequencies up to 90 Hz.

The test results indicate the feasibility of using this and other LIM-based reaction-mass actuators for both vibration-cancellation and shaker applications. The long stroke of the LIM is particularly advantageous in that it enables the LIM to generate large forces at frequencies down to <10 Hz. It might even be possible to use actuators like the present one to replace pneumatically powered hammers in some applications.

This work was done by Fred Flynn, Ron Ghofrani, Jim Goldie, Kevin Leary, John Swenbeck, Dick Hockney, Scott Makseyn, Steve Armstrong, Gita Rao, Geoff Lansberry, Dick Satter, and Cynthia Paine of SatCon Technology Corp. under a NASA SBIR contract and Leonard Voelker of Dryden Flight Research Center.For further information, access the Technical Support Package (TSP) free on-line at under the Machinery/Automation category,or circle no. 156 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

SatCon Technology Corporation
161 First Street
Cambridge, MA 02142-1221
(617) 349-0821

Refer to DRC-98-04, volume and number of this NASA Tech Briefs issue, and the page number.