Piezoelectric bender with increased activating force, and valve using same

A piezoelectric actuator includes at least one piezoelectric bender that is deformable between a first arcuate shape and a second arcuate shape that have different curvatures. A resistance device includes a moveable portion in contact with the piezoelectric bender throughout its deformation from the first shape to the second shape. In addition, the resistance device is operable to resist the deformation, preferably by resisting a change in the transverse dimension of the piezoelectric bender through a friction interaction. By resisting transverse dimension changes in the piezoelectric bender, the axial force produced by the actuator can be increased, and the same structure can compensate for wear that could otherwise undermine performance after many deformation cycles. The piezoelectric actuator finds particular application in fast valves, such as those associated with pressure switching in fuel injection systems.

TECHNICAL FIELD

The present invention relates generally to piezoelectric bender actuators, and more particularly to a valve with a piezoelectric bender actuator that includes actuating force enhancement features.

BACKGROUND

Piezoelectric devices alter their shape in response to an applied electric field. An electric field applied in the direction of polarization affects an expansion of the piezoelectric material in the same direction, while a voltage applied in the opposite direction of polarization will cause a contraction of the material in that same direction. Piezoelectric devices, such as thermally pre-stressed benders, use the bending action of the bender to convert electrical energy into mechanical energy. In other words, when a voltage is applied across a piezoelectric bender, the bender flattens and produces an axial force. When an opposite voltage is applied, the piezoelectric bender increases its curvature, and produces an axial force in an opposite direction.

Engineers have observed that the magnitude of an axial force from a given piezoelectric bender is related to the stiffness of the bender. Stiffness can be adjusted by restricting the transverse expansion and contraction of the piezoelectric bender, such as by using a peripheral clamp of a type generally described in co-owned U.S. Pat. No. 6,376,969 to Forck. Forck seeks to increase stiffness of a piezoelectric bender by clamping around its peripheral edge. In addition, Forck compensates for temperature changes by including a temperature responsive element in its clamp, so that the piezoelectric bender operates similarly across a range of temperatures. Forck in essence teaches a static clamping load around the periphery of the piezoelectric bender. This static clamping load varies with temperature due to the inclusion of a temperature responsive element in the clamp.

More recently, engineers have observed that, due to geometrical changes occurring at the peripheral edge of the piezoelectric bender, that the clamp may not remain clamped to the piezoelectric bender throughout its deformation. The consequence of this phenomenon is a change in the stiffness of the piezoelectric bender at different points in its deformation, which results in decreased actuating forces from the piezoelectric bender, especially at the extreme deformation. In other words, as the piezoelectric bender flattens due to the application of an electrical voltage, the profile of the piezoelectric bender at the peripheral edge becomes slightly thinner due to that increasing flatness. That slight change in the profile thickness of the piezoelectric bender can be sufficient to cause one side or the other of the piezoelectric bender to lose contact with a clamping surface, resulting in a decreased resistance to the deformation, and hence a reduced actuating force from the piezoelectric bender.

Another potential problem associated with piezoelectric benders relates to wear at the surfaces where the peripheral edge of the piezoelectric bender comes in contact with a clamp surface or other housing surface. Over time, wear at these surfaces can also alter the geometry in the peripheral region, and hence alter the stiffness and performance of the piezoelectric bender. In extreme cases, the wear can become so severe that the piezoelectric bender becomes substantially unclamped over its entire deformation range, resulting in a relatively drastic reduction in the actuating force available. Thus, without some means for compensating for wear, the actuating force produced by a piezoelectric bender could gradually decrease over its operational lifetime, potentially undermining the operation of the device to which the piezoelectric bender is coupled. For instance, if the piezoelectric bender is used in conjunction with a valve, the loading of the valve member on a valve seat could gradually decrease over the lifetime of the piezoelectric bender, eventually resulting in valve seat loading that drops below that necessary to keep the valve seated.

The present invention is directed to overcoming one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect, a method of increasing an axial force from a piezoelectric bender includes a step of resisting a change in a transverse dimension of the piezoelectric bender. This resistance is accomplished at least in part by following deformation of the piezoelectric bender with a deformation following portion of a resistance device. The resistance device might be a clamp, but could have other functionally equivalent features different from that produced by a clamp.

In another aspect, a piezoelectric actuator includes at least one piezoelectric bender that is deformable between a first shape and a second shape. A resistance device includes a moveable portion in contact with the piezoelectric bender throughout its deformation from its first shape to its second shape. The resistance device is operable to resist the deformation of the piezoelectric bender.

In still another aspect, a valve includes a moveable valve member at least partially positioned in a valve body that includes at least one fluid passage. A piezoelectric actuator is operably coupled to the valve member. The piezoelectric actuator includes at least one piezoelectric bender and a deformation following resistance device.

DETAILED DESCRIPTION

Referring now toFIG. 1, a valve10includes a piezoelectric actuator12that is operably coupled to move a valve member16that is positioned in valve body14. Valve member16is normally biased downward into contact with a lower valve seat24by a pre-stress in piezoelectric benders60or by application of a negative voltage. When a sufficient voltage is applied across piezoelectric benders60, valve member16is lifted to an upward position in contact with upper valve seat26. In the illustrated example, the valve member travel distance is on the order of about 100 microns. Valve10is illustrated as a three way poppet type valve, but it could be any type known in the art, including a ball valve member or a spool valve member. In addition, the valve could be a two way valve or a multi-way valve without departing from the present invention. In the illustrated example, when valve member16is in contact with lower seat24, a high pressure passage28is fluidly connected to a control pressure passage32. When valve member16is in contact with upper seat26, control pressure passage32is blocked from high pressure passage28, but open to low pressure passage30.

The valve body14includes an upper seat component18separated from a lower seat component22by a valve lift spacer20. Valve lift spacer20is preferably a category thickness part. This allows the travel distance of valve member16to be tightly controlled by choosing an appropriate thickness for lift spacer20. As already discussed, high pressure passage28, low pressure passage30and control pressure passage32are all disposed within valve body14.

Piezoelectric actuator12preferably includes a housing42with an attached male electrical connector43that includes electrical contacts58, which are arranged to produced an electric voltage across piezoelectric benders60. Piezoelectric actuator12includes a deformation following resistance device40, which in the illustrated example embodiment is a clamp41. The deformation following portion of the clamp41includes a ceramic ring washer54that is biased into contact with a substrate layer of piezoelectric bender60via a ring shaped wave spring44. An opposite side of a substrate layer of the bottom piezoelectric bender60bears against a steel ring62, which is preferably ceramic coated. The clamping load produced by wave spring44can be preset by advancing ring62into a steel can portion56of housing42until a predetermined load is produced. At that point, ring62is preferably attached to can56, such as by an annular weld at corner location64. A fastener52is preferably attached to piezoelectric benders60via a pair of ceramic washers46and48, and attached to valve member16via a threaded engagement. Thus, depending upon the polarization of the electrical voltage applied across piezoelectric bender60, fastener52, and hence valve member16, will be driven with an axial force along a centerline50. When this occurs, the diameter of piezoelectric benders60slightly change in a transverse direction, which is perpendicular to centerline50. In the illustrated example embodiment, four piezoelectric benders60are stacked upon one another as part of actuator12. Nevertheless, those skilled in the art will appreciate that depending upon the application, any number of one or more piezoelectric benders could be utilized without departing from the present invention. In addition, the piezoelectric benders60are shown as having a circular shape with an arcuate cross section that curves upward. Depending upon the application, the curvature of the arcuate shape could be adjusted and the perimeter shape of the piezoelectric benders could have shapes other than a circle, such as a rectangle or oval shape.

Referring now toFIGS. 2 and 3, schematic views show valve10in its first and second positions respectively. In addition, these figures are simplified in that only one piezoelectric bender is shown. In the illustrated example, each piezoelectric bender60includes an electrically conductive layer70, which is preferably copper; an electrically active layer71, which is preferably an electrically active ceramic material; and a substrate layer72, which is also preferably an electrical conductor such as stainless or carbon steel. A voltage is applied across electrically active ceramic layer71by attaching electrical leads onto respective surfaces70and72. These layers are preferably attached to one another via a suitable adhesive, and pre-stressed using known temperature or mechanical techniques to produce the arcuate shape shown. Various components are preferably ceramic or otherwise insulated, or coated with an insulated material in order to isolate valve member16and the valve body from the electricity supplied to piezoelectric bender60. For instance, fastener52preferably has a ceramic coating or insulating sleeve, at least in the area of its top portion ordered to facilitate this electrical isolation. Nevertheless, those skilled in the art will appreciate that various other techniques could be utilized in order to isolate the electricity supplied to the piezoelectric bender from other components.

In the illustrated structure, clamp41preferably acts to distribute a clamping load around the outer perimeter75of the substrate layer72, while preferably avoiding contact with the electrically active ceramic layer71. When assembled, the piezoelectric benders60are preferably pre-stressed in order to provide a pre-loading force on valve member16against lower seat24. Thus, when the valve is in the condition shown inFIG. 2, the outer edges of piezoelectric bender60produce upward forces on ring54in order to provide the downward pre-load force of valve member16on lower seat24. Those skilled in the art will recognize that the outer upward force produced by piezoelectric bender60is less than the downward force produced by wave spring44. If the force of wave spring44were insufficient, the piezoelectric bender would assume its rest arcuate shape and lift ring54against the action of wave spring44, such that the load of valve member16onto seat24would be equal to the force from wave spring44alone. When the piezoelectric bender60is energized, its curvature tends to flatten and pull valve member16upward into contact with seat26as shown in FIG.3. This flattening of piezoelectric bender60is resisted by the clamping force produced by clamp41. This resistance to the change in the transverse dimension of piezoelectric bender60adds stiffness and hence increases the force of valve member16against seat26. Thus, those skilled in the art will appreciate that the pre-load on wave spring44, and hence the clamping load, should be chosen with some care in order to have an adequate seating load when valve10is in the configuration ofFIG. 2, and also have a desired seating load when the valve is in the configuration of FIG.3. In other words, the clamping load produced by wave spring44should be sufficiently high to prevent piezoelectric bender60from assuming its rest arcuate shape, yet not so high that the piezoelectric bender60is prevented from changing its transverse dimension when undergoing deformation.

Referring now toFIG. 4, an enlarged and greatly exaggerated illustration is useable in explaining how the deformation following clamp41interacts with the outer perimeter75of piezoelectric bender60. The solid lines show the location of piezoelectric bender when valve10is in the configuration ofFIG. 2, and the dotted lines show an exaggerated view of how the piezoelectric bender is shaped when valve10is in the configuration of FIG.3. In this illustration, it should be noted that in the illustrated embodiment, housing42and ring62remain fixed in position when piezoelectric bender60undergoes its deformation. The resistance to deformation by piezoelectric bender60in either direction is accomplished by a friction interaction between corner80of substrate layer72with a friction surface81on ceramic ring washer54, and a friction interaction between corner84of steel ring62with a friction surface85on substrate layer72. This friction force resistance to deformation changes in piezoelectric bender60is maintained throughout that deformation since ceramic ring washer54follows the deformation by having the ability to move a distance d when piezoelectric bender60is moving between its first and second shapes. Although not necessary, the deformation following resistance device40is preferably organized and structured such that contact with electrically active ceramic layer71is avoided throughout the deformation process. This can avoid potentially damaging stresses to the relatively brittle ceramic material. Nevertheless those skilled in the art will appreciate that in other applications it might be desirable to accomplish the deformation resistance task through at least a partial contact interaction with the electrically active ceramic layer71. Preferably, wave spring44is chosen such that the load it produces is relatively constant even when ring54moves through distance d.

Referring now in addition toFIG. 5, the axial force produced by a piezoelectric bender can generally be described by the following formula:
F=Kx
where F is the axial force, K is a stiffness factor, and x is the axial displacement of the piezoelectric bender at centerline50. The present invention seeks to manipulate the stiffness factor K, which can also be thought of as the ratio of force to displacement, so that K remains relatively constant over the deformation of piezoelectric bender60. InFIG. 5, K1is reflective of the present invention since clamp41provides a relatively uniform resistance to transverse dimension changes in piezoelectric bender60. The second curve showing a stiffness factor K2that changes with deformation represents an example curve when a static clamping load is utilized. When a static clamping load is utilized, the stiffness factor K can be relatively high when a small amount of deformation has occurred such that the clamp remains in contact with the piezoelectric bender, but as deformation continues, the clamping load continues to decrease, and hence the stiffness factor K2decreases as the deformation of piezoelectric bender60increases.

INDUSTRIAL APPLICABILITY

The present invention finds potential application in any electrical actuator that utilizes one or more piezoelectric benders. The present invention finds particular application as an electrical actuator associated with fluid valves. In particular, the present invention has been illustrated in the context of a three way pressure switching valve, which is particularly applicable for use in fuel injection systems.

Although the present invention has been illustrated in which the deformation following resistance device40is a clamp that produces a clamping load parallel to the axial force produced by the piezoelectric bender60, those skilled in the art will appreciate that other equivalent strategies could be used to resist changes in the transverse dimension of the piezoelectric bender. For instance, the geometry and orientation of the various friction surfaces80,81,84and85(FIG. 4) could be reoriented in any suitable manner to influence the stiffness factor in a desired manner throughout the deformation of the piezoelectric bender60. In addition, the friction forces which cause the resistance to transverse dimension changes can be adjusted by both geometrical adjustments to the friction surfaces and/or by adjusting the smoothness of the materials as well as the materials themselves.

In addition, a different strategy might include the use of a ring mounted around the outer perimeter75of substrate layer72as a substitute for clamp41. In such a case, the ring would be elastic and resistant to changes in its diameter, but would be sufficiently elastic that the piezoelectric bender could change in its transverse dimension. Nevertheless, the illustrated deformation following resistance device40is preferable since it has the ability to resist transverse dimension changes in piezoelectric bender60when deforming in either direction.

The structure of the deformation following resistance device40of the present invention is also preferable in that it has the ability to compensate for wear that is likely to occur at friction interaction surfaces80,81,84and85of FIG.4. As these surfaces wear, wave spring44will maintain a desired clamping load by adjusting the position of ring44to maintain contact with substrate layer72. Thus, the illustrated structure should have the ability to compensate for wear even after undergoing millions of deformation cycles with little or no degradation in performance.

Those skilled in the art will appreciate that other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.