Power plants – Pressure fluid source and motor – Pump means moves motive fluid from one chamber to an...
Reexamination Certificate
2002-11-04
2004-06-22
Look, Edward K. (Department: 3745)
Power plants
Pressure fluid source and motor
Pump means moves motive fluid from one chamber to an...
Reexamination Certificate
active
06751954
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to improved compact hybrid actuators, and, more particularly, to an improved fluid pump that has an electrically-powered solid-state driver arranged to modulate the volume of a fluid chamber, and has a driven valve operatively arranged to control the flows of fluid with respect to the pump chamber. The pump driver and driven valve may be operated at frequencies greater than those attainable by oscillatory pumps with passive valves in the prior art.
BACKGROUND ART
The present invention relates to a new type of hybrid actuator that combines some of the most advantageous features of both hydraulic and electric actuators. In particular, it is well established that electrically-powered actuators (commonly referred to as electro-mechanical actuators, or “EMA”s) can be made small, lightweight and powerful, resulting in very high power density. They are also simple to install, service and replace because the power supply to them comprises only electrical cables. However, when an electrically-powered actuator (e.g., a motor-driven ball-screw, etc.) malfunctions or breaks, it will typically jam in the failed position, resulting in catastrophic failure of the system because of consequent loss of control authority.
On the other hand, hydraulic actuators offer the capability of being able to fail gracefully and predictably. For example, if a hydraulic actuator is used to control an aircraft control surface, a failure may result from leakage of hydraulic fluid, and therefore loss of controllable function, but the actuator will not seize in position. Hence, the aircraft may continue to be “flyable” by means of redundant actuators. Instead of locking in any particular position, the control surface can be made to return to a “neutral” position by the aerodynamic forces acting on it. Its return to neutral and subsequent movement may also be passively damped by the action of pistons in cylinders, and hydraulic fluid remaining in the system.
However, hydraulic systems must be supported by an extensive and complex mechanical infrastructure of central pumps, manifolds and tubing. Because of the large number of joints and fittings typically used, these systems are susceptible to corrosion and leakage, and are therefore maintenance intensive. In addition, maintenance and replacement is complicated by the need to bleed the system every time a hydraulic unit, such as an actuator, is removed from the system for service.
The hybrid actuator broadly comprises an electrical power supply, such as the electrical buss of an aircraft or submarine, power-conditioning electronics, an induced-strain material driven pump, and an output ram. The improved actuator employs a novel, compact, very high frequency pump that is collocated with the actuator to eliminate much of such hydraulic infrastructure. The pump is powered electrically, and the only connections to the actuator are electrical leads. Because the actuator mechanism itself is inherently hydraulic (although fed by a very small local pump in lieu of the previous large, complex central hydraulic system), it has the operational advantages of a conventional hydraulic actuator, discussed above. Because the pump is very small and light, the overall actuator (comprising the pump and output ram) has a very high power density.
Actuators that employ electrically-driven local pumps to directly supply pressurized hydraulic fluid to an actuating ram are known in the prior art. One fairly mature class of such actuators are known as Electro-Hydrostatic Actuators (“EHA”s). These actuators generate pressures using small, very high speed, multi-piston pumps driven by brushless DC motors at speeds up to 20,000 RPM. To control the direction and extent of ram motion, the operation of the motor is reversed to pump fluid to one side or the other of the ram. Typical applications include movement of control surfaces on fighter aircraft. While useful for large-scale large-force actuation, EHAs cannot be readily scaled down to applications below about 5 horsepower, primarily because the miniaturized piston pumps approach the limits of achievable tolerances and manufacturability at this size. Consequently, leakage becomes a larger percentage of total flow, and efficiency falls off unacceptably. In addition, the entire EHA system is relatively complex and has a high part-count when the structure of the electric motor, rotating piston pump and associated power conditioning electronics are considered.
There is increasing demand for small electrical actuators in applications requiring distributed structural control, such as morphing aircraft where the airfoil shape is adjusted to adapt to the operating environment and control demands. Similarly, in unmanned aircraft, there is a need for small electrically-powered actuators
The present invention provides a simpler, lower part-count alternative to such known EHAs, wherein the pump comprises a solid-state electroactive material, such as a piezoelectric material (e.g., lead zirconate titanate), controlling or modulating the volume of a small compression chamber with high-frequency inlet and discharge valve mechanization. Other active, or induced-strain, so-called “smart” materials could be employed, depending on the particular application. For example, a magnetostrictive material, such as Terfenol-D, can be used advantageously when the system must be operated over a wide temperature range. Military aircraft, for example, must use components that remain functional between about −65° F. and about +265° F. Electrostrictive materials, such as lead magnesium niobate, are more suitable for underwater applications, such as submarine rudder control actuators, where the operating temperature range is constrained over a narrow band.
While such materials provide a unique capability of generating very high force displacement in a lower part-count mechanism, the maximum strains obtainable are typically on the order of 1000 microstrains at best. Moreover, the materials themselves are generally quite dense (e.g., lead- and iron-based formulations), and thus must be operated at high frequency (e.g., on the order of 1 kHz-10 kHz) to achieve high power density. In a practical pump, the resulting pulsating flow must be rectified by inlet and discharge check valves. However, higher frequencies in this range are well beyond the capabilities of existing, conventional, passive check valves, which are typically limited to around 50-150 Hz, even with higher performance valves. Accordingly, to implement the invention, entirely novel passive designs must be employed, or the valves must be driven actively at very high frequencies to match the compression cycles in the pump chamber.
Others have proposed to use such induced-strain solid-state “smart” materials to operate an electrically-driven pump. For example, U.S. Pat. No. 4,927,334 (“Engdahl”) discloses various constructions of magnetostrictive rods displacing pistons to produce a pumping device. However, Engdahl generically indicates the necessary check valves by conventional symbols, and does not address their specific construction.
International Patent Application No. PCT/US97/15608 (Publication Number WO 98/11357), assigned to Etrema Products, Inc., discloses a magnetostrictively-driven pumping element in combination with magnetostrictively-driven inlet and exhaust poppet valves, a magnetostrictively-driven four-way directional control valve, and a piston/cylinder actuator. While this device recognizes the need for valve elements that can operate at the same frequency as the pump element, it does not adequately address the problem of providing reasonable valve openings using microstrain actuating elements, other than to suggest that mechanical motion amplifiers might be provided.
A disclosure of the University of South Carolina Office of Technology Transfer, OTT ID No. 97152 by Victor Giurgiutiu, shows a solid-state induced-strain pump with inlet and outlet “valving” elements, termed “fluid diodes”, that depend on the difference in dynamic flow i
Bridger Keith
Cooke Arthur V.
Crowne Frank J.
Lutian Joseph J.
Sewell John M.
Leslie Michael
Look Edward K.
Phillips Lytle LLP
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