Electromagnetic solenoids having controlled reluctance

1. An apparatus, comprising:

a housing;

a solenoid coil disposed within the housing;

a pole member; and

an armature configured to move from a first position to a second position when the solenoid coil is energized, a contact surface of the armature spaced apart from a contact surface of the pole member by a first distance when the armature is in the first position, the contact surface of the armature spaced apart from the contact surface of the pole member by a second distance when the armature is in the second position, the contact surface of the pole member defining an outer diameter that is between an outer diameter defining the contact surface of the armature and an inner diameter defining the contact surface of the armature,

the housing, the pole member and the armature collectively defining a flux path characterized by a first reluctance when the armature is in the first position and a second reluctance when the armature is in the second position, the difference between the first reluctance and the second reluctance is less than about thirty percent of the value of the first reluctance.

2. The apparatus of claim 1 , wherein the contact surface of the armature is tapered.

3. The apparatus of claim 1 , wherein the contact surface of the armature and an axis along which the armature moves define an acute angle.

4. The apparatus of claim 1 , wherein a portion of the contact surface of the armature and a portion of the contact surface of the pole member define an air gap area within the flux path, the pole member and the armature configured such that the air gap area decreases as the armature moves from the first position to the second position.

5. The apparatus of claim 1 , further comprising:

a retainer configured to retain the solenoid within the housing, a portion of the armature configured to move within the retainer, the portion of the armature and the retainer being included in the flux path.

6. The apparatus of claim 5 , wherein:

the portion of the armature includes a sliding surface; and

the retainer includes a surface, the sliding surface of the armature and the surface of the retainer define an air gap area within the flux path, the retainer and the armature configured such that the air gap area decreases as the armature moves from the first position to the second position.

7. The apparatus of claim 1 , further comprising:

a retainer configured to retain the solenoid within the housing,

a portion of the contact surface of the armature and a portion of the contact surface of the pole member define a first air gap area within the flux path, the pole member and the armature configured such that the first air gap area decreases as the armature moves from the first position to the second position,

a sliding surface of the armature configured to move within the retainer, the sliding surface and a surface of the retainer define a second air gap area within the flux path, the retainer and the armature configured such that the second air gap area decreases as the armature moves from the first position to the second position.

8. An apparatus, comprising:

a housing;

a solenoid coil disposed within the housing;

a pole member;

a retainer configured to retain the solenoid coil within the housing; and

an armature configured to move from a first position to a second position when the solenoid coil is energized, a first surface of the armature spaced apart from a contact surface of the pole member by a first air gap when the armature is in the first position, the first surface of the armature in contact with the contact surface of the pole member when the armature is in the second position, a second surface of the armature spaced apart from a surface of the retainer portion by a second air gap, an outer diameter defining the contact surface of the pole member is between an outer diameter defining the first surface of the armature and an inner diameter defining the first surface of the armature,

the housing, the pole member, the armature and the retainer collectively defining a flux path including the first air gap and the second air gap, a portion of the first surface of the armature and a portion of the contact surface of the pole member defining a first air gap area within the flux path, the pole member and the armature configured such that the first air gap area decreases as the armature moves from the first position to the second position.

9. The apparatus of claim 8 , wherein the flux path is characterized by a first reluctance when the armature is in the first position and a second reluctance when the armature is in the second position, the difference between the first reluctance and the second reluctance is less than about thirty percent of the value of the first reluctance.

10. The apparatus of claim 8 , wherein the second reluctance is within a range of about seventy percent, fifty percent, about forty percent, about thirty percent, about twenty percent or about ten percent of the first reluctance.

11. The apparatus of claim 8 , wherein the second surface of the armature is configured to move within the retainer.

12. The apparatus of claim 8 , wherein the second surface of the armature and the surface of the retainer define a second air gap area, the retainer and the armature are configured such that the second air gap area decreases as the armature moves from the first position to the second position.

13. The apparatus of claim 8 , wherein:

a reluctance of the first air gap is reduced when the armature moves from the first position to the second position; and

a reluctance of the second air gap is increased when the armature moves from the first position to the second position.

14. A method, comprising:

coupling a retainer to a housing to retain a solenoid coil within the housing;

disposing a pole member within the housing, the pole member including a contact surface defined by an outer diameter;

disposing an armature within the housing, the armature configured to move from a first position to a second position when the solenoid coil is energized, a first surface of the armature is spaced apart from the contact surface of the pole member in the first position, where the outer diameter of the pole member is between an outer diameter defining a contact surface of the armature and an inner diameter defining the contact surface of the armature, the housing, the pole member, the retainer, and the armature collectively defining a flux path having a first reluctance when the armature is in the first position and a second reluctance when the armature is in the second position, the difference between the first reluctance and the second reluctance is less than about thirty percent of the value of the first reluctance.

15. The method of claim 14 , wherein the contact surface of the pole member and an axis along which the armature moves define an acute angle.

16. The method of claim 14 , wherein a portion of a contact surface of the armature and a portion of the contact surface of the pole member define an air gap area within the flux path, the pole member and the armature configured such that the air gap area decreases as the armature moves from the first position to the second position.

17. The method of claim 14 , wherein a portion of the armature is configured to move within the retainer, the portion of the armature and the retainer being included in the flux path.

18. The method of claim 17 , wherein:

the portion of the armature includes a sliding surface; and

the retainer includes a surface, the sliding surface of the armature and the surface of the retainer define an air gap area within the flux path, the retainer and the armature configured such that the air gap area decreases as the armature moves from the first position to the second position.

19. The method of claim 14 , wherein:

a portion of the contact surface of the armature and a portion of the contact surface of the pole member define a first air gap area within the flux path, the pole member and the armature configured such that the first air gap area decreases as the armature moves from the first position to the second position,

a sliding surface of the armature configured to move within the retainer, the sliding surface and a surface of the retainer define a second air gap area within the flux path, the retainer and the armature configured such that the second air gap area decreases as the armature moves from the first position to the second position.

20. A method, comprising:

energizing a solenoid coil of a pump assembly to move an armature from a first position to a second position within a solenoid housing, the armature coupled to a pump element, the solenoid housing, a pole member, the armature and a retainer collectively defining a flux path, a first surface of the armature spaced apart from a contact surface of the pole member by a first air gap when the armature is in the first position, the first surface of the armature in contact with the contact surface of the pole member when the armature is in the second position, the contact surface of the pole member defining an outer diameter that is between an outer diameter defining the contact surface of the armature and an inner diameter defining the contact surface of the armature, a second surface of the armature spaced apart from a surface of the retainer portion by a second air gap, a portion of the first surface of the armature and a portion of the contact surface of the pole member defining the first air gap area within the flux path, the pole member and the armature configured such that the first air gap area decreases as the armature moves from the first position to the second position; and

deenergizing the solenoid to move the armature from the second position to the first position within the solenoid housing.