Magnetostrictive Micropositioner for Cryogenic Applications

NASA's Jet Propulsion Laboratory, Pasadena, California

A magnetostrictive linear-translation mechanism has been designed to function as a micropositioning device at any temperature from ambient down to the temperature of liquid helium (about 4 K). Still undergoing development at the time of reporting the information for this article, this magnetostrictive micropositioner is a prototype of micropositioners for a variety of room-temperature and low-temperature applications in which there are requirements for high stiffness, increments of motion < 1 µm, and long travel (through concatenated multiple small increments). Such micropositioners could be used to make fine position adjustments in diverse scientific and industrial instruments; for example, they could be used to drive translation stages in scanning tunneling microscopes or to move optical elements that must be located at long but precise distances from each other (as in telescopes and interferometers).

A Magnetostrictive Inertial-Reaction-Motor Rotary Drive is combined with a threadless rotary-to-linear clutch to obtain axial motion of the shaft, with very small steps, self-braking, and capability of operation in a cryogenic system.

Magnetostrictive micropositioners that act in "inchworm" fashion were reported in "Magnetostrictive Actuators for Cryogenic Applications" (NPO-19218), NASA Tech Briefs, Vol. 20, No. 3 (March 1996), page 84. Magnetostrictive micropositioners that exploit a combination of stick/slip and inertial effects were reported in "Magnetostrictive Inertial-Reaction Linear Motors" (NPO-20153), NASA Tech Briefs, Vol. 22, No. 6 (June 1998), page 6b. The present magnetostrictive micropositioner shares some characteristics with the inchworm and inertial-reaction types; like an inchworm actuator, it is nonbackdriveable and self-braking (it retains its position when power is not applied), and like an inertial-reaction actuator, it exploits a combination of stick/slip and inertial effects. However, the present magnetostrictive micropositioner features a distinct design that addresses major issues of lubrication and energy efficiency that arise in a cryogenic environment.

The prime mover in this micropositioner is a linear actuator that comprises (a) a single crystal of the magnetostrictive rare-earth alloy Tb_0.74Dy_0.26surrounded by (b) high-temperature-superconductor solenoid. The superconductivity of the solenoid minimizes electric power dissipation, thereby contributing to energy efficiency and to reduction of waste heat (which must be removed to maintain a cryogenic environment).

The reason for choosing a magnetostrictive (instead of, say, a piezoelectric) actuator to obtain small increments of motion is that magnetostrictive actuators function throughout the desired temperature range and even work better as temperature decreases, whereas piezoelectric actuators tend to become inoperable in cryogenic environments. Tb_0.74Dy_0.26 was chosen because it exhibits a large magnetostrictive effect in the intended operating-temperature range; for example, application of a magnetic flux density of 1,000 G to a 20-mm-long Tb_0.74Dy_0.26 crystal produces a stroke as large as 0.1 mm. The use of a single crystal of magnetostrictive material contributes further to energy efficiency and reduction of waste heat, in that relative to polycrystalline mass, a single crystal undergoes much less heating when magnetostrictively flexed.

The magnetostrictive crystal is connected to a linear-to-rotary clutch: The solenoid is driven with a sawtooth signal, causing the crystal to repeatedly extend slowly and snap back rapidly. The motion of the crystal drives a pendulum that is lightly spring-loaded against a drive shaft. The slow extension of the crystal causes the shaft to rotate through a small increment of angle in one direction. However, the force of the snap-back acceleration is greater than the force of friction between the pendulum and the drive shaft, so that the shaft does not rotate in the opposite direction. The cycle then repeats, producing another increment of shaft rotation. Of course, the shaft can be made to rotate in repeated increments in the opposite direction by reversing the polarity of the drive waveform.

By use of a little known but highly reliable rotary-to-linear clutch, the rotary motion of the drive shaft is used to obtain lengthwise motion of the shaft. The rotary-to-linear clutch includes six small bearings that are spring-loaded against the drive shaft in two groups of three bearings each. The axes of the bearings are skewed slightly from the axis of the shaft, so that each incremental rotation of the shaft causes the shaft to advance lengthwise by an amount that depends on the skew angle and the diameter of the shaft (2 ¼m of advance per degree of rotation in the present design). The rotary-to-linear clutch provides the desired self-braking and nonbackdriveability, and the spring loading affords compliance needed to tolerate changes in temperature.

At the time of reporting the information for this article, there were no lubricants suitable for long-term cryogenic sliding mechanical contacts like those of lead screws in conventional linear actuators. The use of rolling-contact bearings in the present magnetostrictive micropositioner obviates the issue of lubrication of sliding contacts. The rolling contacts are lubricated with molybdenum disulfide, which is a proven low-temperature solid lubricant for ball bearings.

In a test at room temperature, this magnetostrictive micropositioner was found to be capable of producing linear position increments of about 1 µm. With further refinement, it should be possible to achieve increments as small as 0.1 µm. The overall travel is limited only by the length of the drive shaft; a typical overall travel of 10 cm is easily achieved.

This work was done by Robert Chave of Caltech for NASA's Jet Propulsion Laboratory.

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