Piezoelectric Fluid Pump

Abstract

A piezoelectric fluid pump is provided comprising: a main housing, a fluid reservoir located within said main housing, a piston head movably mounted within said main housing, a piezoelectric stack, a biasing mechanism arranged to couple the piston head and the piezoelectric stack to maintain said piezoelectric stack in a compressed state, an outlet plate statically mounted within said main housing adjacent said piston head, wherein adjacent surfaces of said outlet plate and piston head form a pump chamber, an inlet disc valve arranged to permit unidirectional flow of fluid from said fluid reservoir to said pump chamber, and an outlet disc valve arranged to permit unidirectional flow of fluid from said pump chamber. The combination of preloading the piezoelectric stack in a compressed state and the use of an inlet disc valve may allow the above described piezoelectric pump to provide substantial improvements in pressure and flow capabilities compared to other piezoelectric pumps.

Description

[Technical field]

[0001] The present application relates to an improved design for a piezoelectric fluid pump that may be used, for example, as a hydraulic pump in an aircraft system. [Background technology]

[0002] Modern aircraft, even small personal aircraft, have many hydraulically actuated systems, such as wing flap actuators, landing gear actuators, etc. To date, a central hydraulic pump is provided to provide a supply of pressurized hydraulic fluid to each system. Each system may have its own dedicated pump, or multiple pumps, or alternatively, all hydraulic systems are served by the same pump. This centralized location has many disadvantages, such as weight and number of parts (e.g., hydraulic pipes, connectors, and valves) that are subject to wear.

[0003] To mitigate the shortcomings of centralized hydraulic systems for aircraft, electrohydraulic actuators (EHAs) can be used, where each actuator has its own associated, often integrated, electrically driven hydraulic pump. By powering each actuator around the aircraft electrically, rather than hydraulically, this reduces weight and part count.

[0004] Conventional electrohydraulic actuators have a hydraulic pump driven by a separate electric motor. These separate parts can be replaced by a piezoelectric pump, thereby resulting in a further reduction in weight and number of parts subject to wear. The basic principle of a piezoelectric pump is that a stack of piezoelectric elements is driven by an alternating current, thus causing the stack to alternately expand and contract in a reciprocating motion, so that the volume of a fluid pump chamber alternately increases and decreases, thus allowing a volume of fluid to be pumped in and out of the chamber. Summary of the Invention [Problem to be solved by the invention]

[0005] However, piezoelectric pumps typically have low pressure and low flow capabilities making them undesirable for use in electro-hydraulic actuators in aircraft applications. [Means for solving the problem]

[0006] Thus, there is provided a piezoelectric pump comprising: a main housing; a fluid reservoir located within the main housing; a piston head movably mounted within the main housing; a biasing mechanism coupling a piezoelectric stack and arranged to maintain the piezoelectric stack in compression; an outlet plate statically mounted within the main housing adjacent the piston head, wherein adjacent surfaces of the outlet plate and piston head form a pump chamber; an inlet disc valve arranged to allow unidirectional flow of fluid from the fluid reservoir to the pump chamber; and an outlet disc valve arranged to allow unidirectional flow of fluid from the pump chamber.

[0007] The combination of preloading the piezoelectric stack in compression and the use of an inlet disk valve allows the above-described piezoelectric pump to offer substantial improvements in pressure and flow capabilities compared to other piezoelectric pumps.

[0008] The piezoelectric stack may be located between the piston head and the base plate, and the biasing mechanism may comprise a spring element arranged to exert a force biasing the piston head and the base plate towards each other.

[0009] Maintaining the piezoelectric stack in compression has the advantage of avoiding undesirable tensile loads being applied to the stack during operation.

[0010] The piston rod may be coupled to the piston head, the piston rod extending from the piston head through the fluid reservoir and the base plate and coupled to the base plate. The piston rod may be coupled to the base plate by one or more retaining elements, the spring element being located between the retaining elements and the base plate. The spring element may comprise one or more Belleville washers.

[0011] Alternatively, the spring element may be disposed around the outside of the piezoelectric stack and is connected to the piston head and base plate.

[0012] The piezoelectric stack may have an internal cavity with a fluid reservoir, which has the advantage that the pumping fluid in the reservoir acts as a coolant to prevent excess heat build-up within the piezoelectric stack.

[0013] The piezoelectric pump may further include a fluid inlet in fluid communication with the fluid reservoir.

[0014] One or more fluid inlet passages can be formed in the piston head to provide fluid communication between the fluid reservoir and the pump chamber, and the inlet disc valve is positioned to prevent flow of fluid from the pump chamber to the fluid inlet passages.

[0015] Similarly, one or more fluid outlet passages can be formed in the outlet plate to provide fluid communication from the pump chamber to the fluid outlet chamber, with the outlet disc valve positioned to prevent flow of fluid from the fluid outlet chamber to the fluid outlet passages. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 shows a cross section of a piezoelectric pump. [Diagram 2] FIG. 2 shows an enlarged portion of the pump of FIG. 1 at a different point in the pumping cycle. [Diagram 3] FIG. 3 shows an enlarged portion of the pump of FIG. 1 at a different point in the pumping cycle. [Figure 4]FIG. 4 shows an enlarged portion of the pump of FIG. 1 at a different point in the pumping cycle. [Diagram 5] FIG. 5 shows an enlarged portion of the pump of FIG. 1 at a different point in the pumping cycle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Piezoelectric pumps offer the aforementioned advantages of reducing the parts count and wear surfaces of traditional EHA solutions. However, substantial improvements in pressure and flow capacity are needed to compete with traditional EHAs. However, there are many challenges that must be addressed to increase the pressure and flow capacity of piezoelectric pumps.

[0018] The high frequency operation required to accumulate a very small pumped volume into a significant flow rate results in the need for a responsive valve capable of operating at such high frequencies to control the flow of fluid to and from the pumping chamber. Operating a piezoelectric stack at high frequencies generates a significant amount of heat in the stack, thereby necessitating increased heat dissipation from the piezoelectric stack. Also, the piezoelectric material that makes up the individual elements of the piezoelectric stack is significantly less capable of resisting tensile loads than compressive loads. Unless mitigated against, operating a piezoelectric stack at high frequencies will result in high tensile loads being applied to the stack, and a method of preloading the piezoelectric stack to ensure that the tensile loads on the piezoelectric stack are limited is desirable.

[0019] Hydraulic fluids do exhibit some compressibility, but it is small. This arises, for example, from entrapped air in the oil and from the inherent properties of hydraulic fluids. As a result, the magnitude of motion provided by the piezoelectric element is small, so the volume of the pump chamber must be minimized to ensure that compressibility effects do not reduce the overall pressure capability of the pump.

[0020] As with all pumps, sealing of the pump chamber is important to ensure that lost flow is minimized.

[0021] Pump integration to accommodate high frequency valves, how to preload and seal while maintaining the required low pump chamber volume all present technical problems that need to be addressed.

[0022] FIG. 1 illustrates a cross-section of a piezoelectric pump according to one embodiment of the present invention. The pump 100 has a main housing 102. An outlet plate 104 is located inside the main housing in such a manner that the outlet plate cannot move relative to the housing. The outlet plate 104 divides the interior of the main housing 102 into two sections. A stack of piezoelectric elements is disposed on a first side of the outlet plate 104 (the left side as shown in FIG. 1). The piezoelectric stack 106 is configured such that the piezoelectric stack 106 can reciprocate within the main housing 102 when driven by an appropriate electrical signal. In a preferred embodiment, the piezoelectric stack is substantially cylindrical, although other geometric shapes may be utilized. In the particular embodiment illustrated in FIG. 1, the stack 106 has an outer sheath of low friction material that is disposed to slide against a stack liner 110 as the piezoelectric stack moves. The outer sheath and stack liner 110 combine to keep the piezoelectric stack 106 centered in the main housing 102, while the stack liner 110 also functions to define the height (horizontal length as shown) of the chamber in which the piezoelectric stack is disposed. However, in other embodiments, the outer sheath and / or stack liner may be omitted.

[0023] The piezoelectric stack 106 is hollow, i.e., formed with an internal cavity 112, which allows pumped fluid to flow through the piezoelectric stack 106. Located between the piezoelectric stack 106 and the outlet plate 104 is a piston head 114. A piston rod (or tie rod) 116 extends from the piston head through the internal cavity of the piezoelectric stack and passes through a base plate 118 of the stack. The base plate 118 is arranged to be fixed relative to the main housing 102. The piston rod 116 is arranged to reciprocate with the piezoelectric stack through the base plate. The end of the piston rod 116 that protrudes beyond the base plate 118 within the housing 102 has one or more nuts 120 threaded thereon. One or more resilient elements 122, such as Belleville washers, are secured by the nut between the nut 120 and the base plate 118. The resilient element 122 is held in compression against the base plate 118 by a nut, which in turn exerts a biasing force on the piezoelectric stack 106 via the piston rod and piston head. This biasing force preloads the piezoelectric stack such that the stack is permanently held in compression.

[0024] The base plate 118 has one or more fluid passages 124 formed therein to allow the flow of fluid into the internal cavity 112 within the piezoelectric stack 106. In use, a fluid supply is provided to an inlet in the main housing (not shown).

[0025] The space between the opposing faces of the piston head 114 and the outlet plate 104 defines a pump chamber 126 (more easily seen in later figures). The piston head 114 has one or more fluid inlet passages 128 providing fluid communication between the internal cavity 112 of the piezoelectric stack and the pump chamber 126. An inlet disc valve 130 is fixed to a face of the piston head to define the pump chamber and is configured to control the flow of pumping fluid through the fluid inlet passages 128 from the internal cavity 112 to the pump chamber 126. The outlet plate 104 also has one or more fluid outlet passages 132 providing fluid communication between the pump chamber 126 of the pump 100 and a fluid outlet chamber 134 from which pressurized fluid is provided in use. An outlet disc valve 136 is fixed to a face of the outlet plate 104 opposite the face defining the pump chamber, i.e., the face of the outlet plate adjacent the fluid outlet chamber 134. An outlet disc valve 136 is positioned to control the flow of pumping fluid through the fluid outlet passage 132 from the pump chamber 126 to the fluid outlet chamber 134 .

[0026] The operation of pump 100 will now be described with reference to FIGS. 2-5, which illustrate an enlarged portion of the pump illustrated in FIG.

[0027] FIG. 2 shows an enlarged view of a portion of the piezoelectric pump shown in FIG. 1, centered on the pump chamber 126. FIG. 2 represents the pump 100 at a point in the pumping cycle when the piezoelectric stack 106 is fully extended and therefore the piston head 114 is at its closest point to the outlet plate 104. As a result, the volume of the pump chamber 126 is at a minimum. The inlet disc valve 130 and the outlet disc valve 136 are more easily visible in FIG. 2. In the illustrated embodiment, both disc valves comprise planar annular sections of resilient material, such as spring steel. The inlet disc valve 130 is secured to the piston head 114 about its center by a retaining screw 202 or other suitable retention mechanism. The inlet disc valve extends radially from its center a sufficient distance to span each of the fluid inlet passages 128. By action of the retaining screw and the resilient properties of the disc valve material, the inlet disc valve is biased against the piston head 114, sealing the fluid inlet passages 128 from the pump chamber 126. The outlet disc valve 136 is secured in a similar manner to the outlet plate 104 with a retaining nut 204. The outlet disc valve also extends radially across each of the fluid outlet passages 132 and is biased against the outlet plate to seal the fluid outlet passages from the fluid outlet chamber 134. The disc valves may also have additional "damping" holes formed in them to allow fluid to flow through the valve when the valve is open and to allow fluid to effectively flow around the valve. Such damping holes would be positioned in the disc valve to allow fluid to pass through the valve when the valve is closed, but to seal against the piston head 114 or outlet plate 104 when the valve is closed. The disc valves may also have additional springs, such as a second disc of small diameter spring steel, to stiffen the inner portion of the main valve body. In FIG. 2, the inlet and outlet disc valves are both shown closed, thus sealing their associated fluid passages from no fluid flow through the pump. However, it will be appreciated that the opening and closing of these disc valves is not a direct correlation with the position of the piezoelectric stack, but rather depends on factors such as the spring stiffness, mass, and frequency of operation of the pump.

[0028] 3 depicts the pump 100 as the piezoelectric stack 106 is partially retracted, partially through the intake stroke of the pump. Relative movement of the piston head 114 toward the base plate 118 increases the volume of the pump chamber and reduces the pressure therein. The pressure difference between the fluid in the interior cavity 112 and the underpressure within the pump chamber is sufficient to overcome the bias of the inlet disc valve 130, as shown, allowing fluid to flow through the inlet passage 128, through the deformed disc valve, and into the pump chamber 126. During at least a portion of the intake stroke, the outlet disc valve 136 will be closed, as illustrated in FIG. 3.

[0029] 4 illustrates the pump 100 with the piezoelectric stack 106 fully retracted (compressed). In this position, pumping fluid does not flow through the inlet passage 128 into the pump chamber 126, so that the biasing force of the inlet disc valve 130 is sufficient to return the valve to its natural position flat against the piston head 114.

[0030] FIG. 5 shows the pump 100 when the piezoelectric stack is partially extended, i.e., when the stack is driven in the opposite direction to FIG. 3. This represents the pump midway through its exhaust stroke. Relative movement of the piston head 114 toward the outlet plate 104 reduces the volume of the pump chamber 126 and increases the pressure therein, thereby forcing fluid to flow from the pump chamber through the outlet passage 132, as indicated by the arrows. The pressure difference between the fluid in the pump chamber and the underpressure in the fluid outlet chamber 134 is sufficient to overcome the bias of the outlet disc valve 136, as shown, allowing fluid to flow through the deformed disc valve and into the fluid outlet chamber 134. During this time, the inlet disc valve 130 will tend to close, thereby preventing fluid from flowing back from the pump chamber through the inlet passage 128 into the internal cavity 112 of the stack.

[0031] Movement of the piezoelectric stack 106 between the positions shown in Figures 2-5 (and back to the position of Figure 2) constitutes a complete work cycle of the pump. As previously mentioned, to achieve a desired fluid flow rate, such as 0.5 liters / minute, the piezoelectric stack must be driven at a relatively high frequency, such as 1000Hz-1400Hz. In some circumstances, the piezoelectric stack may be driven up to 2000Hz. At these operating frequencies, the stack is likely to generate undesirable amounts of heat (due to inherent energy conversion losses in the piezoelectric material). However, by utilizing a hollow piezoelectric stack as shown, the flow of fluid through the interior of the piezoelectric stack during operation of the pump provides some cooling.

[0032] Also, as mentioned above, it is undesirable to operate the piezoelectric stack under tensile load at the frequencies mentioned above. This is overcome by the use of Belleville washers 122, which allow the piezoelectric stack 106 to be constantly held in compression between the piston head 114 and the base plate 118 while allowing the stack to expand and contract. Although Belleville washers can be replaced with any other suitable elastic elements, such as coil springs, Belleville washers have the advantage of providing a relatively high spring force for their overall size and displacement. Preloading the piezoelectric stack with Belleville washers can also increase the pressure capacity of the pump by operating within the region of the Belleville washer spring curve characteristic where the force is constant over displacement. However, other mechanisms for preloading the piezoelectric stack can be provided, for example, a spring or bellows around the outside of the stack and connected between the piston head 114 and the base plate 118, such that the spring tension acting on the piston head and base plate exerts a compressive force on the intervening stack elements.

[0033] To avoid any compressibility effects arising from the fluid being pumped, and because the displacement of the piezoelectric stack is inherently small (on the order of less than a millimeter), the volume of the pump chamber is kept to a minimum. For example, the pump chamber volume may be on the order of 0.7 mL and 1 mm in length. The total length of the piezoelectric stack in such a pump chamber volume would be on the order of 60-70 mm. The above dimensions are provided purely as an aid to understanding the scale of the pump, and are not necessarily the desired or preferred dimensions.

[0034] The high frequency operation required to accumulate significant flow from a very small pump volume requires a responsive valve. The dynamic capabilities of the inlet and outlet disc valves meet this requirement. Furthermore, incorporating a low profile inlet disc valve onto the piston head, as discussed above, minimizes pump chamber volume, resulting in higher pressure capabilities.

[0035] The combination of these features (the flow of fluid routed through the hollow piezoelectric stack for cooling, the preloading of the piezoelectric stack, and the use of inlet and outlet disk valves) allows the above-described piezoelectric pump to provide substantial improvements in pressure and flow capabilities as compared to other piezoelectric pumps. As a result, possible aircraft applications for such improved piezoelectric pumps include, but are not limited to, landing gear uplocks, lock stays, gear door actuators, brake and steering actuators, engine bleed valves, and aircraft environmental systems.

Claims

1. In a piezoelectric hydraulic actuating fluid pump, the piezoelectric hydraulic actuating fluid pump includes a main housing, a fluid reservoir located within the main housing, a piston head movably mounted within the main housing, a piezoelectric stack, a biasing mechanism arranged to connect the piston head and the piezoelectric stack and maintain the piezoelectric stack in a compressed state, an outlet plate fixedly mounted within the main housing adjacent to the piston head, wherein an adjacent surface of the outlet plate and the piston head forms a pump chamber, an inlet disk valve arranged to allow one-way flow of fluid from the fluid reservoir to the pump chamber, and an outlet disk valve arranged to allow one-way flow of fluid from the pump chamber. The piezoelectric stack has an internal void portion that includes the fluid reservoir, the piezoelectric hydraulic actuating fluid pump.

2. The piezoelectric stack is located between the piston head and a base plate, and the biasing mechanism includes a spring element arranged to apply a force that biases the piston head and the base plate towards each other, the piezoelectric hydraulic actuating fluid pump according to claim 1.

3. A piston rod is connected to the piston head, the piston rod extends from the piston head through the fluid reservoir and the base plate, and is connected to the base plate, the piezoelectric hydraulic actuating fluid pump according to claim 2.

4. The piston rod is connected to the base plate by one or more retaining elements, and the spring element is located between the retaining elements and the base plate, the piezoelectric hydraulic actuating fluid pump according to claim 3.

5. The piezoelectric hydraulic actuating fluid pump according to claim 4, wherein the spring element comprises one or more Belleville washers.

6. The piezoelectric hydraulic actuating fluid pump according to claim 2, wherein the spring element is disposed around the outside of the piezoelectric stack and is connected to the piston head and the base plate.

7. The piezoelectric hydraulic actuating fluid pump according to claim 1, further comprising a fluid inlet in fluid communication with the fluid reservoir.

8. One or more fluid inlet passages are formed in the piston head to provide fluid communication between the fluid reservoir and the pump chamber, and the inlet disk valve is arranged to prevent fluid flow from the pump chamber to the one or more fluid inlet passages. The piezoelectric hydraulic actuating fluid pump according to claim 1.

9. One or more fluid outlet passages are formed in the outlet plate to provide fluid communication from the pump chamber to the fluid outlet chamber, and the outlet disk valve is arranged to prevent fluid flow from the fluid outlet chamber to the fluid outlet passages. The piezoelectric hydraulic actuating fluid pump according to any one of claims 1 to 8.

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