1. Field of the Invention
The present invention relates to a method of adjusting piezoelectric valve components of a fuel injector device to establish and maintain a precise interface between a piezoelectric actuator and a movable valve plunger, and more particularly to, a process of establishing a matched interface between the valve components.
2. Description of Related Art
Piezoelectric devices are desirable for use as valve, actuators for several reasons. One being that the devices allow for precise metering and control of small quantities of pressurized fuel. Another desirable feature is that piezoelectric actuators have reliable characteristics when calibrated properly and precisely. However, in a fuel injection valve, the amount of displacement of a piezoelectric element necessary to move the valve element through its valve stroke is very small. Therefore, it is necessary to take into account the small amount of displacement when calibrating, making precise calibration difficult.
Moreover, establishing an accurate interface between a piezo actuator and movable valve element can be difficult and costly due to small strokes and large forces associated with piezoelectric actuators. Stack-up tolerances due to the assembly of various components also make it difficult to create a match or flush interface between the actuator and valve element.
In U.S. Pat. No. 5,205,147, an adjustment screw is rotated for course adjustment of the axial position of the actuator. Secondly, a tapered wedge is used to make fine adjustments in the actuator position before being secured with a pin and second adjusting screw. However, the coarse and fine adjustments are made to adjust the position of the entire actuator, not to adjust the compressive preload.
U.S. Pat. No. 6,326,717 discloses an adjustment nut which engages the external threads of an extension bolt disposed on a top plate of the actuator body to adjust the pre-stressing forces of a pair of spring bands. The adjustment nut is in series with the piezoelectric element stack and end plate, i.e., the operational load path. An adjustment results in an equal movement of the tappet thereby making fine adjustments in the axial position of the tappet very difficult.
Thus, there is a need for an accurate and low cost method of adjusting the piezoelectric actuator and valve components to allow for a precise offset therebetween.
An object of the present invention is to solve the above problems by providing a simple, effective process of setting, including adjusting, the position of valve components.
Another object of the present invention is to provide a method for matching the interface between a valve plunger and an actuator rod of a fuel injector, thereby enabling more precise control of preselected fuel delivery amounts.
Still another object of the present invention is to provide a method for accomplishing a more fine adjustment of a piezoelectric actuator's position relative to a valve plunger by increasing or decreasing the preload of a piezoelectric stack of elements.
Another object of the present invention is to provide a method which corrects for the inability to assemble all injector and actuating components without stack-up tolerances thereby providing a method which to compensate for these stack-up tolerances. In one embodiment, the actuator rod and actuator housing are precisely ground in the same plane and/or a shim is provided to place the valve plunger and valve housing in a common plane, which upon assembly, creates a common plane for all surfaces.
In establishing these and other objects of the present invention, there is provided a method for adjusting piezoelectric valve components to establish and maintain a precise interface between a piezoelectric actuator and a movable valve plunger, the method including the steps of providing a valve body and valve plunger. The valve body has a first surface, and the valve plunger has first and second opposed ends. A first difference between the first surface of the valve body and the first end of the valve plunger is measured. The first measured difference of the valve components is then substantially eliminated, or compensated for, such that the first surface of the valve body and the first end of the valve plunger are in a first common plane. A piezoelectric actuator rod movably disposed in an actuator housing is provided. The piezoelectric actuator rod has opposed first and second ends and the actuator housing has an end surface. A second common plane is established between the end surface of the actuator housing and the second end of the actuator rod. A fine adjustment is conducted by rotating at least one nut disposed axially with the actuator rod to adjust the axial position of the actuator rod.
The step of initially compensating for, or substantially eliminating, the first measured difference can comprise machining the first surface of the valve body and the first end of the valve plunger in the first common plane. The step of initially compensating for the first measured difference may alternatively comprise positioning a shim within a recess formed between the first surface of the valve housing and the first end of the valve plunger, the shim having a width equal to the first measured difference. The step of establishing the second common plane can comprise machining the end surface of the actuator housing and the second end of the actuator rod to position these surfaces in the second common plane.
The at least one nut may include an inner adjustment nut mounted on the actuator rod. The position of the piezoelectric actuator rod can be adjusted by rotating the inner adjustment nut of the piezoelectric actuator to perform a fine adjustment of the preload.
As shown in
Referring to
The initial, coarse adjustment also includes eliminating any differences between end surface 28 of actuator housing 38 and second end 34 of the actuator rod. In the preferred embodiment, this is accomplished by machining the surfaces such that a common plane is established across the surface 28 and second end 34. It should be appreciated that although second end 34 of the rod is shown to extend from surface 28 of the actuator housing, second end 34 can be inward of surface 28. In this situation, a common plane would preferably still be formed by machining the surfaces.
It should be appreciated that other matching means are contemplated which would result in a common plane between the surfaces to provide for an initial course adjustment of the components of the control valve. For example, the initial, coarse adjustment can be accomplished by measuring the height h1 of a plurality of stock of valve plungers 22 and bodies 20. Likewise, a plurality of actuator rods 30, piezo element assemblies 42 and housings 38 can be assembled and the various heights h2 (
It should be appreciated that the term “matched” or “matching” used herein means that, when the valve components are assembled, the surfaces are positioned immediately adjacent one another to within about as small as a clearance therebetween as possible, if any, and/or height h1 equals h2, within about as small of a tolerance as possible.
During normal operation of the fuel injector nozzle, piezoelectric element 42 changes dimensions with the imposition of an electric field, either expanding or contracting to move the actuator rod and hence the valve plunger 22 to open or close, respectively, a valve port 23 to control fuel flow from, for example, a control volume 25 (
The second step of the method of the present invention functions to provide a precise, fine adjustment to the axial position of the actuator rod by making slight adjustments in the actuator preload assembly. Referring to
Piezoelectric discs 44 are preloaded by spring washers 36 by rotation of outer nut 54 during the initial assembly of the piezo actuator assembly 40. During rotation of outer nut 54, inner nut 52 does not rotate. After the initial coarse adjustment and assembly of the injector, the fine adjustment step of the matching process of the present invention is accomplished by loosening or tightening nut 52 to axially move the actuator rod 30 slightly relative to valve plunger 22. Specifically, for fine adjustment, inner nut 52 may be rotated relative to nut 54 so as to loosen nut 52 and move rod 30 downwardly in
Importantly, adjustment nut 52 is positioned in series with the load path of the preload force, not parallel to the load path, thereby permitting controlled fine adjustment using the resistance of the preload. Also, because there is a difference in compressibility between spring washers 36 and stack 42, i.e. spring washers 36 being more compressible, when the inner nut is rotated, most of the axial movement is absorbed by the spring washers 36 with little axial movement at the outer end 34 of the rod, thereby enhancing the fine adjustment. The threaded portions of nut 52 and the outer nut 54 (
The two-step, i.e., coarse and fine adjustments, method of the present invention allows for the mating surfaces of the actuator assembly and the control valve to be easily matched to, i.e. achieve an interface having a clearance of, about a 1-5 micron. First, the initial coarse adjustment can be accomplished by either using a shim and/or machining the components to provide for a common plane between the valve plunger/body and the actuator rod/housing resulting in a matched interface having approximately a 5-10 micron clearance. Fine adjustment can then be made during a functional test of the complete injector assembly to achieve the 1-5 micron clearance and possibly even less than 1 micron. This second step of the process makes very slight adjustments in the axial position of the rod by increasing or decreasing the actuator's preload through rotation of the inner adjustment nut 52 to establish a precise matched interface between rod 30 and valve plunger 22. If a gap exists at the interfaces of the surfaces, then there is an undesirable transition time or delay between actuation and opening of the valve. The present invention incorporates a fully accessible, operational adjustment nut that permits fine adjustment in the rod position.
Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. The present invention therefore is defined by the appended claims and legal equivalents.