Precision ground armature assembly for solenoid actuator and fuel injector using same

A solenoid actuator includes a hard guide piece and a soft flux piece. The hard guide piece has a stop surface ground to create a final air gap distance between the soft flux piece and a stator assembly when the stop surface on the guide piece is in contact with the stator assembly. The final air gap is set by grinding the stop surface on the guide piece so that the distance between the stop surface on the guide piece and a top surface on the soft flux piece along an axis of the guide bore is equal to the final air gap. The step of grinding the armature assembly may be done after attaching the guide piece and the flux piece together. In an exemplary embodiment, the step of grinding the stop surface and associated guide surface(s) are performed in a single chucking.

TECHNICAL FIELD

The present disclosure relates to the field of solenoid actuators, and more particularly, to the field of solenoid air gap features in electronically controlled fuel injectors.

BACKGROUND

People skilled in the art recognize the goal to mass produce a solenoid actuator having smaller initial and final air gaps with improved parallelism between a stator assembly and an armature in a cost efficient manner. Even though it may be possible to produce a solenoid actuator assembly having a very small air gap and where the armature is parallel to the stator assembly, those in the art recognize there are significant costs involved in mass producing such assemblies.

Typical solenoid actuated fuel injectors include an armature connected to a valve member that controls the flow of fuel and/or pressure through the fuel injector. By having the armature connected to the valve member, the movement of the armature within the stator assembly may be compromised. By moving the armature with the valve member coupled thereto, the armature might travel at reduced speeds due to the increased mass, and any attempts to improve parallelism with the stator assembly were also hindered due to the tolerance stack ups that invariably increase during production with more connected parts. Moreover, in the past, some armature assemblies included a hard guide piece that was part of, or drove a fuel injection valve member, and a soft armature piece that served to enhance the magnetic forces acting on the armature. In order to improve parallelism and maintain a predetermined initial and final air gap, manufacturers used various category parts that took into account the inaccuracies that existed in the dimensions of the solenoid actuator assembly despite establishing very tight tolerances during mass production.

When the coil of the solenoid is energized, the armature moves towards the stator assembly, moving the valve member, and thereby controlling the fluid flow and/or pressure in the fuel injector. When the coil ceases to be energized, a mechanical spring or other bias forces the armature away from the stator assembly, causing the valve member to return to its original position and thereby controlling the fluid flow and/or pressure in the fuel injector again. It is known in the art that the time taken for the solenoid actuator, and hence the control valve of a fuel injector, to move from a first position to a second position and back again is a function of the highest possible forces acting on the armature over the shortest possible travel distance. It is desired by those in the art to reduce the time taken for the armature to travel from the initial air gap position to the final air gap position and back to the initial air gap position.

The magnetic forces acting on the armature are functions of the electromagnetic properties of the armature, the initial and final air gap between the armature and the stator assembly and the parallel orientation of the armature with reference to the stator assembly, including others. It is well known in the art that a magnetic field in a solenoid has the greatest force when the armature is parallel to the stator assembly and the air gap between them is as small as possible. Having a larger initial air gap will translate to the armature having a lower initial attraction force and maybe a larger travel distance, hence increasing the time taken to travel from the initial air gap position to the final air gap position. Having a smaller final air gap will allow for a smaller initial air gap and also allow a stronger magnetic force to act on the armature, hence increasing the speed at which the armature travels from the final air gap position to the initial air gap position and back. A lack of parallelism can create side forces leading to imbalance and increased wear at guide interfaces.

There has been an ongoing effort to improve parallelism in prior references, while striving to achieve the smallest final air gap. One prior art reference, U.S. Patent Application US2006/0138374 A1 teaches the use of an adjustable spacer coupled between the armature housing and the stator. The spacer is adjusted depending on the tolerance variation of the assembled parts. U.S. Pat. No. 6,550,699 teaches the use of plating a hard film layer on the armature as a spacer. The prior art, although geared towards achieving some of the goals this disclosure aims to achieve, have been met with limited success.

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

SUMMARY

In one aspect, a method for assembling a solenoid actuator includes the steps of attaching a soft flux piece to a hard guide piece. A stop surface is ground on the guide piece relative to the top surface on the flux piece so that a final air gap is at a predetermined distance when the stop surface is in contact with a stator assembly.

In another aspect, a solenoid actuator assembly includes an armature assembly and a stator assembly. The armature assembly comprises a soft flux piece attached to a hard guide piece, which has a stop surface ground on it. The stator assembly defines a guide bore through which the guide piece is slidably received. The guide piece moves between a first position where the stop surface on the guide piece is in contact with the stator assembly, and the second position where the stop surface is out of contact with the stator assembly. Also, a final air gap is defined between a bottom surface on the stator assembly and a surface on the flux piece when the guide piece is in the first position.

In yet another aspect, a fuel injector assembly comprises an armature assembly. The armature assembly is made of a soft flux piece attached to a hard guide piece, which includes a stop surface. The guide piece moves between a first position where the stop surface on the guide piece is in contact with the stator assembly, but the guide piece is out of contact with a valve member. When moved to a second position, the stop surface is out of contact with the stator assembly, but the guide piece is in contact with the valve member.

DETAILED DESCRIPTION

Referring toFIG. 1, a fuel injector10includes an electronically controlled valve assembly60and a valve nozzle92that is opened and closed by a valve needle90. The electronically controlled valve assembly60includes a solenoid actuator assembly20, a valve member61, a first spring56having a first pre-load and a second spring58having a second pre-load. The solenoid actuator assembly20includes a stator assembly21and an armature assembly40. The stator assembly21and armature assembly40are both made from various assembled parts. Valve needle90includes a closing hydraulic surface66exposed to fluid pressure in a needle control chamber67. Energizing and de-energizing solenoid actuator assembly20moves valve member61to change pressure in needle control chamber67(via fluid connections not shown) to allow valve needle90to open and close valve nozzle92in a conventional manner.

Referring now toFIGS. 2 and 3, the stator assembly21includes an outer pole piece25attached to an inner pole piece24, such as via welding them together at the weld joint30. In other embodiments, other attachment mechanisms and locations may be used to attach the inner pole piece24to the outer pole piece25. The pole pieces24and25may have co-planar bottom surfaces. As the pole pieces24and25are attached to each other, in this embodiment they share the same bottom surface, which is referred to as the planar bottom surface26. In one embodiment, a coil29is carried on a bobbin28inside a cavity formed within the pole pieces24and25. The remainder of the space between the pole pieces may be filled with plastic filler27. Inner walls of the inner pole piece24form a pole bore23through which a guide sleeve31is attached. In one embodiment, the guide sleeve31may be press fitted through the pole bore23so that it fits snugly along the inner walls of the inner pole piece24. Other embodiments may contemplate other ways of attaching the guide sleeve31to the inner walls of the inner pole piece24, such as a weak press fit accompanied by a weld. The guide sleeve31has an inner diameter surface32, which defines a guide bore33. The guide bore33has a longitudinal axis35that is perpendicular to the planar bottom surface on the pole piece26. The guide sleeve31has a stop surface77, which is the bottom surface on the guide sleeve31and in one embodiment, it may be flush with, or be considered part of the bottom surface26. In one embodiment of the disclosure, the bottom surface26on the entire stator assembly21is machined to form a planar bottom surface on the entire stator assembly21. Those skilled in the art will recognize that guide sleeve31and pole pieces24and25may be made from the same or different materials. For instance, pole pieces24and25may be chosen for their magnetic flux channeling capacities, but the guide sleeve material may be chosen more for wear characteristics in guide bore33and stop surface77.

Referring now toFIG. 4, the armature assembly40includes a guide piece43made of a hard material which exhibits impact resistant properties and a flux piece45made of a soft material which exhibits high magnetic properties. The flux piece45may be attached to the guide piece43at a weld joint53. In many embodiments, the pieces may be attached by welding the pieces together, press fitting them or using a combination of a light press fit and a weld, among other attachment strategies. The guide piece43includes at least one guide surface36and37, an enlarged diameter portion44and a stop surface75located on the portion44. In the embodiment shown inFIG. 4, the guide piece43has a first guide surface36, a second section or guide surface37and a reduced diameter section38. By reducing the diameter on the guide piece43in section38, the armature assembly40has a lower mass and therefore, requires a smaller force to displace the armature assembly40. In one embodiment, the outer surface on the guide piece, including the first guide surface36and second guide surface37, may be ground after attaching the guide piece43to the flux piece45in such a manner that when the guide piece43is received in the guide bore33, the guide clearance along the inner diameter surface32on the guide sleeve31, and hence the guide sleeve31itself, is so small resulting in a much improved parallelism between the top surface50on the flux piece45and the planar bottom surface26. Thus, armature assembly40may be guided through the guide bore33via an interaction between the guide piece43and the guide sleeve31. Furthermore, the stop surface75on the guide piece43will not be planarly flush with a top surface50on the flux piece45in an exemplary embodiment. The distance between the stop surface75on the guide piece43and the top surface50on the flux piece45along the axis35of the guide bore33is a predetermined final air gap70. In one embodiment of the disclosure, a final air gap of about 0.05 mm can be achieved on a consistent basis while maintaining efficient operating costs. The term “about” means that when the number is rounded to a like number of significant digits, the numbers are equal. Thus, both 0.045 and 0.054 are about 0.05.

One other aspect of the disclosure teaches the step of grinding the stop surface75on the guide piece43to be performed after the flux piece45is attached to the guide piece43. Conventional wisdom in the art focuses on producing pieces with ever increasing tightened tolerances so that after attachment, the tolerance stack-ups would not amount to substantial variations. This disclosure resolves the problems faced by others in the art by allowing parts to be manufactured under less stringent tolerances, attaching the pieces together and then grinding the surfaces on the pieces in a single chucking. This produces an armature assembly40that compensates for the tolerance variations in the geometric dimensions of each individual piece while producing a much more accurate orientation between the guide piece43and the guide sleeve31. The grinding step may be performed by grinding a stop surface75on the shoulder of the guide piece43, such that the stop surface75is parallel to the flux piece45of the armature assembly40and is at a distance equivalent to the final air gap70. Also, the grinding step can include grinding the guide surfaces36and37of the guide piece43and grinding the stop surface75on the guide piece43in a single chucking. This will allow a more improved orientation of the guide piece43into the guide bore33and also allow the guide piece43to have an orientation that is perpendicular to the flux piece45, improving the parallelism between the flux piece45and the bottom planar surface26.

InFIGS. 1,2and3, the armature assembly20is shown in a first position. In the first position, the coil29is energized causing the solenoid actuator20to apply a pulling force on the armature assembly40bringing stop surface75of the armature assembly40in contact with the stop surface77, which is part of the planar bottom surface26of the stator assembly21. The armature assembly40may have a larger travel distance than the valve member61in order to be decoupled from the valve member61. In this position, armature assembly40is out of contact with the valve member61, resulting in a gap71between the armature assembly40and valve member61. The stop surface75on the guide piece43, however, comes into contact with the stop surface77on the guide sleeve31. A final air gap70is formed between the planar bottom surface26and the top planar surface50on the flux piece45. Furthermore, the first spring56remains in contact with the guide piece43and exerts a first pre-load bias force on the guide piece43in a direction away from stator assembly21. The second spring58exerts a second pre-load bias on the valve member61forcing the valve member61to move from the lower valve seat64toward upper valve seat65in a conventional manner.

The armature assembly40moves toward a second position when the coil29is de-energized. The stop surface75on the guide piece43moves out of contact with the stop surface77on the guide sleeve31. The guide piece43, however, is in contact with valve member61and valve member61moves into contact with lower seat64under the action of first spring56. Furthermore, the first spring56now has a greater pre-load than the pre-load of the second spring58so that valve member61will move to its lower seat when coil29is de-energized. The distance between the planar bottom surface26and the top planar surface50on the flux piece45along the longitudinal axis35of the guide bore33is equivalent to an initial air gap.

By decoupling the action of solenoid assembly20from valve member61slight misalignments between an axis of valve member61and guide axis35can be tolerated with altering performance. In addition, the speed of the valve member61moving between seats64and65are determined primarily by respective pre-loads on springs56and58, which may be set precisely with respective spacers80and81. Seats64and65may be considered as first and second stops for valve member61. The decoupled solenoid assembly20can now function with greater precision and may allow for a smaller initial and final air gap69and70. Furthermore, by decoupling the armature assembly40and the valve member61, the armature assembly40will function independently of the valve member61as long as the armature assembly40travels faster than the valve member61. This also desensitizes the valve member61from any misalignments that may occur due to construction tolerance variances and any lateral shifting in the armature assembly40in order to improve parallelism between the armature assembly40and the stator assembly21.

INDUSTRIAL APPLICABILITY

The present disclosure finds potential application in any solenoid assembly in any machine. Although this particular embodiment of the disclosure is directed towards an electronically controlled valve assembly for use in a common rail fuel injector, the disclosure is not limited to fuel injectors and could find applicability in a much broader array of industries that use solenoid actuators. The present disclosure finds particular application to fuel injectors used in compression ignition engines. Other fuel injector applications include, but are not limited to, cam and/or hydraulically actuated fuel injectors. Electronically controlled valve assemblies may be used to control the flow of fluids and/or pressure through a fuel injector. In the present disclosure, the valve assembly performs repeated cycles of movement at an extremely high rate over many millions of cycles.

The solenoid actuator20has two states. An off or de-energized state, which corresponds to the second position of the armature assembly40and an on or energized state, which corresponds to the first position of the armature assembly40. In the off state, the solenoid actuator20is switched off and no current is passing through the coil29of the solenoid actuator20. As there is no current passing through the coil29, there are no magnetic forces produced within the stator assembly21. The first spring56exerts a force on the armature assembly40and the valve member61causing them to be pushed away from the stator assembly21to stop when valve member61contacts lower seat64. The second spring58exerts an opposite force on the valve member61and the armature assembly40towards the stator assembly21but the force is not great enough to overcome the force exerted by the first spring56. Therefore, the net resulting force from the two springs56and58causes the valve member61to assume a second stop position in contact with the valve seat64that corresponds to either an open or a closed position which in turn controls the flow of fluid and/or pressure through the fuel injector10depending on the configuration of the valve assembly60. The armature assembly40is positioned away from the planar bottom surface26and the distance from the planar surface50of the flux piece45to the planar bottom surface26of the stator assembly21along the longitudinal axis35of the guide bore33is the initial air gap.

As the solenoid actuator20is switched to its on state, the armature assembly40moves from its second position to its first position. Switching the solenoid actuator20on energizes the coil29. The coil29produces a magnetic field around the stator assembly21and creates a magnetic force in the surrounding region. The force of the magnetic field is strong enough to pull the armature assembly40towards the stator assembly21. This force is greater than the force of the spring56hence causing the armature assembly40to move towards the stator assembly21. In addition, when the armature assembly40is pulled towards the stator assembly21, the armature assembly40may be pulled faster than the valve member61is pushed upward by the second spring58. This allows the armature assembly40to lose contact with the valve member61. The valve member61moves from the second stop position to a first stop position that corresponds to either an open or a closed position which in turn controls the flow of fluid and/or pressure through the fuel injector10depending on the fluid configuration of the valve assembly60. The guide piece43moves up the guide bore33of the stator assembly21maintaining a guide clearance with the guide sleeve31. The guide piece43stops moving when the stop surface75on the guide piece43comes in contact with the stop surface77on the guide sleeve31. A top surface49on the guide piece43remains in contact with the first spring56. The distance between the planar bottom surface26of stator assembly21and the top surface50on the flux piece45is at its smallest distance, corresponding to the final air gap70, and may be equal to the distance between the stop surface75on the guide piece43and the top surface50on the flux piece45. When the armature assembly40is in the first position, the first spring56exerts a bias force on the guide piece43. However, as long as the coil29is energized, the magnetic force is exerted on the armature assembly40and the armature assembly40remains in the first position. Depending on the fluid connections, fuel injection events may be initiated and ended by energizing and de-energizing solenoid actuator20in a known manner.

Finally, the solenoid actuator20is turned off again and the coil29is de-energized. The coil29no longer provides a magnetic force therefore allowing the net resulting force of the springs56and58to force the armature assembly40to move from the first position to the second position again. The first spring56exerts a force on the top surface49on the guide piece43. The stop surface75on the guide piece43loses contact with the stop surface77on the guide sleeve31, while the bottom impact surface48on the guide piece43comes back in contact with the valve member61pushing the valve member61back to its original position, and thereby allowing the valve member61to control the fluid flow and/or pressure through the fuel injector10again. The armature assembly40finally stops when it reaches the second position, wherein the distance between the flux piece45and the planar bottom surface26is equal to the initial air gap69.

The armature assembly40continues to move from the second position to the first position and back as long as the solenoid actuator20is turned on and turned off. This continuous process demonstrates why it may be important for the impact surfaces of the guide piece43to be made of a hard, impact resistant material. The continuous pounding of the bottom surface48and the stop surface75of the guide piece43with member61and the guide sleeve31, respectively, cause wear and tear on the surfaces on the guide piece43possibly requiring the impact surfaces of guide piece43to be made of a material able to withstand these impacts over extended periods of use. It is known to those in the art that the flux piece45should be made of a soft material possessing superior magnetic properties in order to move between the first and second position with less force than might otherwise be needed. With the structure shown, the travel distance of valve member61will inherently be smaller than the travel distance of armature assembly40.

This disclosure provides numerous ways to reduce the initial and final air gap of solenoid actuators and improve parallelism between the top surface50on the flux piece45and the bottom surface26on the stator assembly21. Grinding the stop surface75on the guide piece43, after attaching the armature assembly40, may permit smaller geometric variations than in the past. Grinding the surface75after the attaching step eliminates the need to develop parts with ever increasingly tightened geometric tolerances because the grinding step after attachment allows parts with larger geometric variations to be ground to the same predetermined dimensions. Furthermore, when the armature assembly40is ground (guide surfaces36,37and stop surface75) in a single chucking, the guide piece43and the flux piece45are oriented more accurately than if ground in more than a single chucking. This produces an improved, more geometrically aligned stop surface75on the guide piece43and better parallelism between the top surface50on the flux piece45and the planar bottom surface26of the stator assembly21.