An improved magnetostrictively actuated pump has been developed to satisfy a need for a small, low-pressure, high-flow-rate fluid pump that contains few moving parts and can run reliably for long periods without maintenance. The pump could be used, for example, to circulate water in the portable life-support system worn by a firefighter or a chemical worker or in any setting where reliability is important and maintenance is difficult. The pump is designed primarily for water as the pumped fluid, but it could also be used with other fluids, including cryogenic ones.

The Improved Magnetostrictive Pump is designed to take maximum advantage of the small stroke of the magnetostrictive actuator. Because this stroke is so small, great care must be exercised in design and assembly to optimize the bias stress on the magnetostrictive components, and to maximize the rigidity of all components except for the required degrees of compliance of the bellows and the springs (not shown here) in the check valves.

The figure shows a meridional cross-section of the pump. The bottom part contains a magnetostrictive actuator, including an annular permanent magnet that provides a constant (bias) magnetic field, and an electromagnet coil that generates the variable magnetic field needed for actuation. The magnetostrictive material is the alloy Tb0.27Dy0.73Fe2 (commercially available under the trade name "Terfenol-D").

The unusual aspect of the actuator lies in a two-stage design that approximately halves the actuator length needed to obtain a given stroke. There are two pieces of magnetostrictive material, each 1.5 in. (3.81 cm) long: a central rod 0.75 in. (1.9 cm) in diameter, and a surrounding sleeve of the same volume as that of the rod. The upper end of the sleeve pushes against the lower end of the rod via a stainless-steel connector, so that the rod telescopes out from the sleeve and the magnetostrictive strain of the rod is added to that of the sleeve to obtain nearly the same total strain as that of a 3-in. (7.6-cm)-long, 0.75-in. (1.9-cm)-diameter rod of the magnetostrictive material. The connector is designed to undergo very little strain, relative to the magnetostrictive strain at the anticipated actuation loads.

The diameter of the two-stage actuator is greater than it would be with a single stage, but this increase in diameter does not increase the overall diameter of the pump, because the piston that effects the pumping action has a greater diameter. In addition, power consumed by the two-stage actuator is only slightly greater than it would be for an equally capable single-stage actuator.

Above the actuator is a hydraulic stroke amplifier that includes an outer and an inner drive bellows. This stroke amplifier multiplies the actuator stroke by about a factor of 7.5 [from 2 to 15 mils (0.05 to 0.38 mm)] while dividing the actuator force by a factor of 10 in driving the piston. About 75 percent of the work done by the actuator goes into the output of the stroke amplifier; the remaining 25 percent is consumed in compression of the hydraulic fluid and strain energy of the bellows.

The stroke amplifier drives the piston, the periodic motion of which draws water into a chamber through an intake valve and pushes the water out of the chamber through an outflow valve. These are lightweight, fast-response, spring-positioned check valves. These valves are positioned to make the water flow circumferentially around the chamber to obtain a centrifugal effect that makes trapped air bubbles accumulate at the center of the chamber, where they flow out. The accumulated air must be vented because the pump stroke is so small that even as little as a few milliliters of trapped air greatly impedes performance, and more than that amount can totally block the pumping action.

Above the pump chamber in which the piston operates there are two compensation bellows - one on the intake side and one on the outflow side. These bellows smooth out the flow, reducing the pulsations that occur at the pump operating frequency, which is about 24 Hz. If the pulsations were not smoothed out, they would give rise to huge forces (water hammer) that would build up in the water tubes connected to the pump and thereby prevent the pump from operating.

The pump is designed to have a flow rate of 30 milliliters per second and a pressure of 5 psi, and to consume about 25 W of electric power.

This work was done by Michael J. Gerver, Robert Ilmonen, Frank Nimblett, and John Swenbeck of SatCon Technology Corp. for Johnson Space Center. MSC-22890

Motion Control Tech Briefs Magazine

This article first appeared in the December, 1999 issue of Motion Control Tech Briefs Magazine.

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