Michael J. Gerver

Passive damping and velocity sensing using magnetostrictive transduction

Magnetostrictive Terfenol-D transducers are an attractive alternative to viscoelastic dampers, and electrodynamic and piezoelectric actuators for damping and self-sensing. These advantages include high stiffness and primary load carrying capability, high power density, low voltages, and low temperature sensitivity. Terfenol-D converts 50 percent of the transducer strain energy into magnetic field energy. Because the Terfenol-D transducer is a primary load carrying member, large amounts of structural energy are converted into magnetic field energy. This magnetic field energy is converted into electric energy by a surrounding coil and dissipated in a resistor to provide damping. The voltage developed in the surrounding coil is proportional to the strain rate of the magnetostrictive material, thus producing a velocity signal. This velocity signal can be used for colocated active damping by controlling coil current based on coil voltage induced by transducer velocity. Experiments using a Terfenol-D actuator capable of 65 microns motion and 1,000 N force showed modal loss factors to 0.22 (relative damping to 0.11) and velocity sensing scale factors to 183 volts/(meter/sec). Room temperature tests of a transducer designed for 77 degree(s)K use showed only 20 percent reductions in damping and velocity signals. Magnetic modeling supports the damping and sensing observations.

Composite Terfenol-D sonar transducers

A sonar transducer, 28 mm in diameter and 40 mm long, has been built using composite Terfenol-D, consisting of grains of Terfenol-D embedded in an epoxy and magnetically aligned while the epoxy is setting. The transducer has been tested in air, where it has a resonant frequency of 18 kHz, and Q equals 18 at low amplitudes. In water it is expected to have Q equals 4.5, an acoustic output power of 48 watts, a power efficiency of 32 percent, and a maximum duty cycle of 6 percent. Surprisingly, hysteresis losses appear to be negligible when the bias field is greater than 800 oersteds, and 90 percent of the power dissipation is due to eddy currents, with 10 percent due to ohmic losses in the coil. The anomalously high eddy currents, still much lower than in monolithic Terfenol-D, can be understood in terms of the arrangement of Terfenol-D grains in the composite. At this time we have no explanation for the anomalously low hysteresis loss. It should be possible to greatly reduce the eddy currents, increasing the power efficiency to 76 percent, the output power to 69 watts, and the maximum duty cycle to 60 percent. Composite Terfenol-D should be superior to both monolithic Terfenol-D and PZT in transducers for sonar arrays operating in the 20 to 30 kHz range.

Observations and theory of Terfenol-D inchworm motors

SatCon is developing linear and rotary motors that rely on the peristaltic motion of a Terfenol-D element along a tight-fitting channel. Magnetostrictive inchworm motors offer extended or unlimited travel, as well as those attributes normally associated with magnetostrictive actuators: high force and torque densities, quick response, and fine resolution motion. Unlike Kiesewetters cylindrical design, the Terfenol-D element is a rectangular slab placed between two tight-fitting plates that are spring-loaded to maintain proper contact in spite of wear. Also, the excitation coils do not enclose the Terfenol-D element, allowing extension of the concept to rotary motors. A model for inchworm performance has been developed, based on observations of a prototype linear inchworm motor. Speed is approximately the product of peak magnetostrictive strain and phase velocity of the magnetic field, but is reduced by the finite element of the contact zone between the Terfenol- D and the plates. As a result, speed drops with load, since magnetostrictive strain is reduced and the contact zone grows longer with increasing applied load. The speed was limited by the skin effect present in the Terfenol-D element. A second prototype employs composite Terfenol-D with its high resistivity, in order to permit operation at higher speeds. SatCon is also developing a rotary motor and making improvements to the design of linear motors.

Nonlinear modeling for control of Terfenol-D-based actuators

Nonlinear modeling and control methods can be used to increase the usable range of operation of Terfenol-D. Presently, in dynamic applications the usable range of Terfenol-D is often limited to approximately 850ppm. This limitation is imposed by harmonic distortion, spurious vibration, and/or tracking error considerations. These nonlinear effects are due to large variations in the magnetoelastic parameters and hysteresis. The preliminary results of this program indicate that a large performance advantage may be gained through proper control of the nonlinearities. As an example, a recently designed reaction mass actuator that weighs 1.125lbm can produce peak forces as high as 125lbf. However, to limit the open-loop total harmonic distortion to less than 2 percent requires that peak forces be limited to roughly 65lbf. To determine the magnetoelastic parameters, quasi-static experiments were performed with a specially designed apparatus. The research included modeling and simulation based on the static nonlinear magnetoelastic equations. Under assumptions of quasi-static magnetoelastic behavior, a fourth-order linear model was extended with the static nonlinearities. The model is compared with preliminary experiments. These types of models will allow nonlinear control strategies to be developed for Terfenol-D based actuators, thus extending the harmonic-free operating range.

High force-to-volume Terfenol-D reaction mass actuator: modeling and design

A high force to volume ratio magnetostrictive reaction mass actuator has been designed and developed. The actuator operates as a resonant device allowing the stored strain energy to be utilized. A discussion of the design issues associated with this actuator are presented. In addition, preliminary data is presented for this actuator. This data includes a linear analysis, evidence of parameter variation, and preliminary small signal tests intended to explore this behavior.

Integrated actuation system for individual control of helicopter rotor blades

The unique configuration of the rotorcraft generates problems unknown to fixed wing aircraft. These problems include high vibration and noise levels. This paper presents the development and test results of a Terfenol-D based actuator designed to operate in an individual blade control system in order to reduce vibration and noise and increase performance on Army UH- 60A helicopter. The full-scale, magnetostrictive, Terfenol-D based actuator was tested on a specially designed testbed that simulated operational conditions of a helicopter blade in the laboratory. Tests of actuator performance (strike, force moment, bandwidth, fatigue life under operational loading) were performed.

Acoustic panels using magnetostrictive Metglas

Passive barriers to transmission of sound waves at frequencies below 500 Hz require large masses. Active noise cancellation systems, on the other hand, are complicated and expensive. We are developing a method for noise control, using an array of panels of magnetostrictive Metglas, which combines the low mass and flexibility of active noise control with the relatively low cost and simplicity of passive noise control. The method is based on the well known fact that an acoustic panel with a reaction mass, resonant at the frequency of the sound wave, will completely reflect that wave, simulating an infinite mass. By wrapping a coil around each Metglas panel, and terminating the coil in an impedance, the stiffness of the Metglas, and hence the resonant frequency of the panel, can be controlled by varying the terminal impedance. By choosing a terminal impedance which is itself frequency dependent, the panel can be made to resonate, and hence to have effective infinite mass, at all frequencies (over some fairly large range) simultaneously. This generally requires negative impedance, which can be produced by a simple circuit with an amplifier and feedback loop. In effect, the Metglas acts like both microphone and speaker in an active noise control system. Preliminary experimental results will be presented.

Magnetostrictive structural elements fabricated from metallic glass

Magnetostrictive adaptive materials have many benefits over competing materials such as piezoelectric types. These advantages include very low power and voltage, high toughness and strength, and the ease of composite manufacture. The amorphous metal Metglas was chosen for fabrication of tubular and bimorph samples of magnetostrictive composites. Metglas composites produced magnetostrictive strains greater than 50 ppm in tests. The composite Youngs modulus was 7.8 X 106 psi, which is 78 percent of aluminum. Specific stiffness is 48 X 106 in, which is 47 percent of aluminum. This specific stiffness is favorable because the composite replaces heavy actuator components, such as high density piezoelectric materials, as well as supplying primary load carrying capability. Very low power requirements are anticipated because of the 90 percent conversion efficiency from magnetic to mechanical energy.