BACKGROUND OF THE INVENTION

The present invention relates to magnetostrictive actuators, more particularly to magnetostrictive actuators that seek to produce a linear force output and/or a uniform magnetic bias flux in the magnetostrictive elements.

Magnetostrictive actuators offer great promise for applications that require a high force output over a wide bandwidth. However, magnetostrictive materials have two characteristics that limit their use. The first limiting characteristic is the inherent nonlinear material response in strain to magnetic flux density. Many applications require a linear force output that has not been achieved by the current art for magnetostrictive actuators. The second limiting characteristic is the relatively low permeability of magnetostrictive materials insofar as it makes it difficult to achieve a uniform magnet bias down the length of the magnetostrictive element due to flux leakage.

U.S. Pat. No. 5,451,821 to Teter et al., incorporated herein by reference, teaches a method of compensating for magnetic flux leakage using magnets outside the drive coils to apply a magnetic field perpendicular to the desired bias direction. Teter et al.'s method has proven to be effective and has been widely adopted in the design of magnetostrictive actuators. However, the perpendicular magnets required by Teter et al. are relatively large, thus increasing the size and cost of an actuator using this method. The perpendicular magnets also cause large magnetic fields external to the actuator, which are not acceptable in many applications. These external fields cannot be effectively shielded by the usual method of surrounding the entire actuator with a ferromagnetic case, because doing so would short out the perpendicular magnet flux.

U.S. Pat. No. 6,891,286 to Flanagan et al., incorporated herein by reference, teaches large axially polarized disk magnets at each end of a magnetostrictive rod to achieve a uniform magnetic flux down the length of the rod. However, this approach of Flanagan et al. does nothing to address the inherent nonlinear material response, requires large magnets, and has large external magnetic fields.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention to provide a magnetostrictive actuator having linear force output and uniform magnetic biasing in the magnetostrictive elements.

An exemplary embodiment of the present invention is a magnetostrictive actuator that uses parallel bars so as to produce an at least substantially linear force output concomitant with compensation for flux leakage, thereby allowing for enhanced length as compared with the current state of the art. Exemplary inventive practice achieves linear force output and uniform magnetic biasing through inventive implementation of a relatively small bias magnet in a closed flux loop without significant external fields. An exemplary parallel bar magnetostrictive actuator in accordance with the present invention produces a linear force output using a center bias magnet, and provides compensation for flux leakage using a ferromagnetic shunt or a tapered permanent magnet.

U.S. Pat. No. 5,587,615 to Murray et al., incorporated herein by reference, teaches a method to linearize the output of a magnetic actuator with force generated across air gaps. Murray arranges two air gaps with the total actuator force equal to the difference of the forces across them, and then establishes (i) magnetic bias flux in opposite directions in the two air gaps and (ii) coil flux in the same direction in the two air gaps. Therefore, as coil flux increases it tends to cancel the bias flux in one gap and add to the bias flux in the other gap.

The inherent force generated across an air gap is quadratic with respect to the total flux across the gap. If the bias flux is ψbiasand the coil flux is ψcoil, then the force in one gap can be written as F=k (Φbias±Φcoil)2, where k is a proportionality constant dependent on the geometry. The net force in the two gaps can be written as Fnet=k [(Φbias+Φcoil)2−(Φbias−Φcoil)2]. Simplifying this equation yields Fnet=4kΦbiasΦcoil. Thus, the net output force is linear with respect to the coil flux.

By doping iron with gallium, U.S. Navy researchers created Galfenol, a material that has higher magnetic permeability than other giant magnetostrictive materials and that can withstand high tensile stress that would cause other materials to fail. Tensile stress allows longer magnetostrictive elements to be used in an actuator, as compared to compressive stress, because tension eliminates buckling as a mode of failure.

The present inventors recognize that the higher magnetic permeability of Galfenol enables an actuator with a parallel bar arrangement in which magnetic flux from surrounding coils is conducted up one Galfenol bar, across a ferromagnetic link, down a parallel Galfenol bar, and then across another ferromagnetic link back to the first bar in a complete loop. The parallel bar arrangement eliminates the need for a separate dedicated magnetic flux return, making the resulting actuator more compact. However, current art for magnetic biasing of parallel bar Galfenol-type actuators is to use external magnets with the attendant size, cost, and external magnetic field problems. Also, the maximum actuator length, although greatly enhanced, is still limited by flux leakage.

The present inventors also recognize that the response of a magnetostrictive material such as Galfenol is substantially quadratic with respect to magnetic flux through the material up to the flux level at which it begins to saturate. The present invention uses this quadratic response characteristic to produce a linear net output force in a manner analogous to the method of Murray. In accordance with exemplary inventive practice, a center bias magnet drives flux through series magnetostrictive bars in opposite directions, while surrounding drive coils apply flux in the same direction through the bars. A second parallel set of magnetostrictive bars completes the flux path. The force output connection is between the series bars and therefore the net output is the difference of the force generated in them. The output force is linear over the flux range for which the magnetostrictive material response is quadratic.

As exemplarily practiced, the present invention compensates for flux leakage between the parallel bars by either of two inventive methods. The first inventive method for flux leakage compensation includes implementation of ferromagnetic shunts between the magnetostrictive bars that conduct flux parallel to the bars at the same rate as the leakage flux. Therefore, there is no net change in bar flux down the length the bar. The second inventive method for flux leakage compensation includes implementation of tapered permanent magnets between the bars polarized perpendicular to the bars. The magnets inject flux into the bars at the same rate as the leakage flux. Again, there is no net change in bar flux down the length of the bar.

Inventive application of either flux leakage compensation method is optional, depending on the magnetic permeability of the material used and the distance between the parallel bars as compared to the length of the bars (e.g., wherein each bar has the same length). As a general rule for Galfenol, acceptable performance may be inventively achieved without flux leakage compensation, as long as the bar length is less than about four times the bar separation distance.

Various preferred modes of practicing the present invention include what are referred to herein as a “first” mode of inventive practice, a “second” mode of inventive practice, a “third” mode of inventive practice, and a “fourth” mode of inventive practice.

The first mode of inventive practice uses ferromagnetic links between series magnetostrictive bars to conduct flux from the permanent magnet into the ends of the bars, and uses the ferromagnetic shunt method of flux leakage compensation.

The second mode of inventive practice uses the same ferromagnetic link and series bar arrangement as the first inventive mode, but uses the tapered permanent magnet method of flux leakage compensation.

The third mode of inventive practice uses continuous magnetostrictive bars, rather than series magnetostrictive bars with a ferromagnetic link therebetween. The flux from the permanent magnet enters the side of the bar at the center. This third inventive mode uses the same ferromagnetic shunt method of flux leakage compensation as the first inventive mode.

The fourth mode of inventive practice uses the same continuous magnetostrictive bar arrangement as the third inventive mode, and the same tapered permanent magnet method of flux leakage compensation as the second inventive mode.

This United States patent application is related to U.S. patent application Ser. No. 15/717,658, filed 27 Sep. 2017, now U.S. Pat. No. 10,930,838, issue date 23 Feb. 2021, hereby incorporated herein by reference, entitled “Magnetostrictive Actuator with Center Bias,” sole inventor John E. Miesner.

