Patent Application
20090134240
The present disclosure relates generally to piezoelectrically actuated devices, and relates more particularly to making such a device by elastically and plastically deforming different portions of a casing therefor to set an actuator preload at a target preload.
Piezoelectric devices such as actuators have been the subject of much attention in recent years, due to the promises they hold for improved precision, robustness and reliability in various applications. In the art of fuel injectors, piezoelectric actuators are commonly coupled with a control valve to control the timing, duration and rate shape of fuel injection events. In general terms, a piezoelectric actuator will include one or more piezoelectric elements which, when subjected to an electrical potential, experience a conformational change. This phenomenon is leveraged to relatively precisely control the position of a component of interest coupled with the actuator, in the case of a fuel injector a control valve as mentioned above. Despite the heightened interest in piezoelectric actuators, they have yet to achieve widespread commercial use in fuel injectors. Issues relating to manufacturing, assembly and operation of piezoelectric elements used in such actuators continue to challenge fuel system manufacturers.
One problem with piezoelectric actuators relates to setting the “preload” on the piezoelectric element used therein. As is well understood by those familiar with piezoelectric actuators, the piezoelectric element must typically be held in compression for it to respond predictably and reliably to a voltage potential. Many proposals for piezoelectric actuators couple the actuator with a relatively small, low-flow control valve which is moved rapidly to control pressure and/or flow of a larger volume of fuel within a fuel injector. In such instances, the need for predictability and reliability will be readily apparent. Where preload on the piezoelectric element of the actuator is too high or too low, however, the piezoelectric element may not behave as desired. A wide variety of preloading mechanisms and assembly/manufacturing strategies for piezoelectric actuators have been proposed over the years, only relatively few of which have seen success.
One known piezoelectric actuator device is set forth in U.S. Pat. No. 7,145,282 B2 to Oakley et al. (“Oakley”). In several designs proposed by Oakley, preloading of a stack of piezoelectric disks is purportedly achieved via elasticity of a housing for the actuator. This would appear to offer the advantage of not needing a separate spring to apply the preload. The strategy Oakley sets forth for preloading the piezoelectric disks during actuator assembly, however, suffers from certain disadvantages. In particular, Oakley suggests stretching the housing by urging a top seal downward against the stack, then welding the seal to a collar engaging a lip on the housing while the seal compresses the disks. While assembly of the actuator in Oakley may be relatively simple, stretching the housing and preloading the actuator together may make it relatively difficult to achieve a prescribed target preload.
In one aspect, a method of making a piezoelectrically actuated device includes the steps of compressing a piezoelectric element toward a compression state which is based on a target preload for the piezoelectric element, and elastically deforming a casing for the piezoelectric element toward a spring state which is also based on the target preload for the piezoelectric element. The method further includes the steps of placing a subassembly which includes the piezoelectric element within the casing, and plastically deforming the casing about the subassembly when the piezoelectric element and the casing are at their respective compression state and spring state to set a preload of the piezoelectric element at the target preload.
In another aspect, an actuator includes a subassembly having a movable element for adjusting a position of a valve, and a piezoelectric element configured to actuate the movable element. The actuator further includes a casing in which the subassembly is at least partially disposed, the casing having an elastic segment with a spring force and an inelastic segment, the casing being coupled with the movable element and holding the subassembly in compression via the spring force to preload the piezoelectric element. The inelastic segment further comprises a plastically deformed segment of the casing trapping the subassembly therein.
In still another aspect, a fuel injector includes an injector body having an injection valve and a control valve assembly for the injection valve disposed therein. The fuel injector further includes an actuator for the control valve assembly which includes a piezoelectric element and a casing, the casing having an elastic segment with a spring force and an inelastic segment coupled with the movable element and holding the subassembly in compression via the spring force to preload the piezoelectric element, wherein the inelastic segment further includes a plastically deformed segment of the casing trapping the piezoelectric element therein.
Referring to
Casing 40 may be generally cylindrical, and may be made at least in part by roll forming steel sheet material, for example. Casing 40 may further have a plurality of segments, including a first segment 54, a second segment 56, a third segment 58 and a fourth segment 60. One or more of these segments may comprise an elastic segment having a spring force. In one embodiment, segment 58 comprises an elastic segment, including at least one corrugation 59 formed in casing 40 and imparting elasticity to casing 40. In the illustrated embodiment, two corrugations 59 are shown, each being circumferential of casing 40, however in other contemplated embodiments fewer than two corrugations, greater than two corrugations, or some other features imparting elasticity to casing 40 such as slots might be used. Moreover, the relative positioning of corrugations 59 might differ in other embodiments. Corrugations 59 might be located adjacent to first segment 54, for example, or spaced from one another. It may be desirable to form corrugations 59 during roll forming casing 40, however, they might be created later in the manufacturing process in certain embodiments. In the embodiment shown, corrugations 59 are exvolute, extending outwardly of outer diameter 42 and having a maximum width greater than a width of adjoining segments of casing 40. In other embodiments, corrugations 59 might be involute, and have a maximum width less than that of adjoining segments of casing 40, or a combination of involute and exvolute corrugations might be used. It is emphasized that elastic segment 58 might include a variety of features or combinations thereof which impart elasticity to casing 40.
Casing 40 may further have one or more inelastic segments, including segments 54, 56 and 60, for example. In one embodiment, segment 60 may comprise a plastically deformed end segment which is coupled with closure member 16 and holds subassembly 12 in compression via the spring force of elastic segment 58. As casing 40 is typically cylindrical, segment 60 may comprise a closed circular perimeter and extend radially inwardly, although alternatives are contemplated wherein inwardly extending tabs or some other feature which does not comprise a closed perimeter is used. Plastically deformed segment 60 can trap subassembly 12 within casing 40 in a state at which the spring force of elastic segment 58 preloads piezoelectric element 30, as further described herein. It may also be noted that closure members 14 and 16, and the other components of subassembly 12 have a uniform width, W, which is approximately equal to or greater than about one-fourth a length L of casing 40. In other embodiments, these relative dimensions might differ, or be non-uniform.
Casing 40 may still further comprise a mounting flange 55 which is located in first segment 54. In one embodiment, segment 54 may comprise another plastically deformed segment which extends radially outwardly to form flange 55. A radially inwardly extending retaining plate or ring 50 may be coupled with flange 55, such as by welding, to trap closure member 14 within casing 40. Retaining plate 50 may further include an aperture 52 which accommodates the portion of closure member 14 whereupon electrical connector 32 is located.
Turning now to
Referring now to
Turning to
Assembly of actuator 10 from the state shown in
Once actuator 10 is properly held in a desired orientation, a force producing device 70 may be positioned against contact button 18 and actuated to apply a compressive force to piezoelectric element 30. In one embodiment, force producing device 70 may comprise a linear actuator such as a press, while in other embodiments, a rotational actuator such as threaded components rotatable relative to one another to generate compressive force on element 30, might be used. In any event, piezoelectric element 30 may be compressed toward a compression state which is based on a target preload for piezoelectric element 30. As used herein, the term “target preload” should be understood to refer to a compression state at which piezoelectric element 30 is considered to have a desired behavior upon activation, e.g. extension length or extensive force. It should be appreciated that force producing device 70 need not necessarily compress element 30 to its target preload. Instead, device 70 might compress element 30 to a compression state less than or more than its target preload, as elastic segment 58 will also contribute to the preload state of element 30 once assembly and preparing of actuator 10 is completed. It should thus be appreciated that a compression state for element 30 which is based on its target preload might include a range of compression states. Bearing surface 73 may react the compressive force applied via device 70, which defines a first force vector having a direction illustrated with arrow A in
Prior to, during or after piezoelectric element 30 has been compressed to the desired compression state, casing 40 may be elastically deformed toward a spring state which is also based on the target preload for piezoelectric element 30. A gripping device 74 may be used to clamp to end 47 of casing 40 and apply a force thereto defining a second force vector having a direction opposed to that of the force vector illustrated with arrow A, and shown with arrows B in
As mentioned above, casing 40 may be elastically deformed toward a spring state which is based on the target preload for element 30 prior to placing actuator 10 within injector body 84. Similar to compressing element 30, the subject spring state of casing 40 may lie within a range of spring states. When element 30 is compressed, it may have a tendency to return toward an uncompressed state, whereas when casing 40 is elastically deformed it may have a tendency to return toward a relaxed state. The tendency for casing 40 to relax, and the tendency for element 30 to decompress will result in opposing forces when segment 60 traps subassembly 12 within casing 40. Accordingly, these opposing forces may be balanced to result in element 30 being held at a target preload by varying one or both of the extent to which element 30 is compressed with device 70 and the extent to which casing 40 is elastically deformed with gripping device 74. The presently described method of making actuator 10, 110 or other actuators described herein, thus provides two independent variables which can each be manipulated to preload element 30. This strategy contrasts with many earlier designs, such as Oakley, described above, wherein preloading a piezoelectric element is attempted by way of a single assembly step intended to stretch a housing and compress a piezoelectric element at the same time. By implementing the present disclosure, achieving a target preload is expected to be relatively easier than in such earlier designs.
The present description is for illustrative purposes only, and should not be construed to narrow the breadth of the present disclosure in any way. Thus, those skilled in the art will appreciate that various modifications might be made to the presently disclosed embodiments without departing from the full and fair scope and spirit of the present disclosure. For example, while much of the foregoing description emphasizes actuators in the context of fuel injectors, the present disclosure may find applications outside of fuel systems. Piezoelectric actuators for other purposes such as controlling delivery of relatively minute quantities of fluid in laboratory or manufacturing settings might be designed and constructed according to the teachings set forth herein. Piezoelectric actuators used in vibratory devices might also be made in accordance with the present disclosure. Other aspects, features and advantages will be apparent upon an examination of the attached drawings and appended claims.
