Magnetostrictive actuators that would hold their displacements with power turned off have been proposed for use at temperatures from about 10 to about 77 K. Such "set and hold with power off" actuators are attractive for effecting fine position adjustments in cryogenic scientific instruments, wherein significant amounts of heat would be released and could disturb instrument operation if it were necessary to apply power continually to maintain actuator displacements.

Figure 1. This Cryogenic Magnetostrictive Actuator would hold its position when power was not applied, by virtue of the persistent current flowing circumferentially in the high-temperature-superconductor cylinder.

The essential components of a magnetostrictive actuator are a rod of magnetostrictive material and an electromagnet coil to control the magnetic field applied to the rod. In an actuator of the proposed type, the magnetostrictive material would be single-crystal Tb0.6Dy0.4Zn, which exhibits a saturation magnetostrictive strain of as much as 0.5 percent at a preload stress of only 13.8 MPa and an applied magnetic field of about 500 Oersted ( ≈4 × 104 A/m). The magnetostrictive rod would be surrounded by an electromagnet coil in the form of a monolithic cylinder made of a high-temperature superconductor (see Figure 1), which could be either YBa2Cu3O7–x or Bi2Sr2CaCu2O8+δ (aka BSCCO 2212). The persistent magnetic field needed to maintain a constant magnetostrictive stain (in order to hold a constant displacement with power off) would be generated by an electrical current flowing circumferentially in the superconductive cylinder.

The superconductive cylinder would be charged with the desired current by applying a suitable pulsed current to a normally conductive (e.g., copper-wire) solenoidal electromagnet coil surrounding the superconductive cylinder. The pulse would be tailored so that, at first, the superconductor would first be brought to its critical state (which occurs at the maximum superconducting current), and then additional magnetic field would be applied, all at nearly constant temperature. In the critical state, magnetic flux could be made to move freely into or out of the cylinder (flux could be pumped) with some attendant dissipation that would cause a momentary small increase in temperature. The increase in temperature would be too small to disrupt the superconductivity.

A circuit to implement this charging scheme is depicted in Figure 2, wherein the normally conductive charging coil is represented by inductor L1, the superconductive cylinder is represented by inductor L2. The momentarily dissipative nature of the superconductive cylinder is represented by resistor R, which limits the current in L2 to no more than the critical current, and which is zero when the current in L2 is smaller than the critical current.

In preparation for generating a pulse of charging current, switch S1 would be closed to charge the capacitor from the power supply. Once the capacitor was charged, switch S1 would be opened and switch S2 would be closed. Assuming a high coefficient of coupling between L1 and L2, the current in L1 would rise rapidly immediately following closure of S2 because, at that time, the combination of L1 and L2 would constitute a transformer with a short-circuited secondary winding. The increase in current in L1 would be balanced by an opposite change in the current in L2. Once the current in L2 reached the critical value, it would remain steady at that value and the load presented to the capacitor would suddenly become inductive, slowing down the rate of increase of current in L1. The circuit would then behave like an ordinary capacitor-and-inductor system with a characteristic oscillation period.

Figure 2. This Pulse/Transformer Circuit would charge the superconductive cylinder to the current needed to maintain the desired actuator displacement.
Once the current in L1 reached its maximum value and started to decay, the state of the superconductor would depart from criticality; therefore, the inductive load would suddenly disappear and the circuit would once again become a short circuit, with no voltage but large current. With no driving voltage, the current in L1 would decay very rapidly, without oscillations. The final result would be that the energy in the capacitor, minus the energy dissipated in L1, would be transferred to the superconductor.

The charging process as described thus far could be repeated until the current in the superconductive cylinder had increased to the desired value (but not more than the critical value). The current in the superconductive cylinder could also be made to decrease in a discharging process, which would be the identical to the charging process except that the capacitor would be charged in opposite polarity.

This work was done by Garnett C. Horner of Langley Research Center, Leslie Bromberg of Massachusetts Institute of Technology, and J. P. Teter of the Naval Surface Warfare Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Physical Sciences category. L-17837


NASA Tech Briefs Magazine

This article first appeared in the May, 2000 issue of NASA Tech Briefs Magazine.

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