Tri-stable solenoid-operated valve

A tri-stable solenoid-operated valve, comprising:

• a) body means defining an internal chamber,
• b) an outlet passageway leading away from the chamber,
• c) a valve seat surrounding the outlet passageway,
• d) a primary piston mounted within the chamber for reciprocating movement toward and away from said valve seat, between a first and a second position,
• e) closure means supported by said primary piston, the closure means being adapted to close against said valve seat when the primary piston is at its first position, and to be spaced away from the valve seat when the primary piston is at its second position,
• f) an inlet passageway opening into said chamber such that it is denied communication with said outlet passageway when the primary piston is at its first position, and is in communication with said outlet passageway when the primary piston is at its second position,
• g) a secondary piston mounted to the primary piston for reciprocating movement with respect thereto between a first and a second position, the secondary piston having a portion which projects into the outlet passageway to cause the outlet passageway to be occluded to a predetermined degree when the primary piston is at its second position and the secondary piston is at its first position;
• said portion of said secondary piston causing the outlet passageway to be occluded to a degree less than said predetermined degree when both the primary and secondary pistons are at their respective second positions,
• h) biasing means urging both said pistons toward their respective first positions,
• i) solenoid means which, when energized to a first predetermined level, overcomes said biassing means to move both pistons to their respective second positions, and which, when energized to a second predetermined level lower than said first predetermined level, partly overcomes said biassing means to move only the primary piston to its second position, whereby the lower energization of said solenoid causes the valve seat to be opened to a degree intermediate between closure and that achieved by the first level of energization.