Differential valve

A differential valve system that maintains a predetermined pressure difference between an inlet pressure P1 and an outlet pressure P2 and that maintains a controlled pressure difference between said outlet pressure P2 and an exhaust pressure P3, comprising:

• a housing having a passageway, an inlet port that receives fluid at said pressure P1 and that is connected to a rear portion of said passageway, an outlet port that carries fluid at said pressure P2 and that is connected to a middle portion of said passageway, and an exhaust port at said pressure P3 that is connected to a rear portion of said passageway;
• first and second poppets that are each slideable along an axis in said passageway and that each has a rear surface exposed to fluid at said inlet port at pressure P1 and a front surface exposed to fluid at said outlet port at pressure P2;
• said second poppet has a front end that forms a first valve seat and said first poppet is moveable forward off the first valve seat to flow fluid from said inlet port at pressure P1 to said outlet port at pressure P2;
• a spring that biases said first poppet rearward to prevent opening said first valve seat unless the pressure difference P1−P2 exceeds a predetermined value;
• said housing forms a second valve seat, said second poppet has a front portion that is moveable rearward off said second valve seal, and said outlet port opens to said front portion of said passageway that is forward of said second valve seat, so fluid in said passage that lies rearward of said second valve seat flows through said second valve seat toward said exhaust port;
• a spring that biases said first poppet rearward to prevent opening said first valve seat unless the pressure difference P1−P2 exceeds a predetermined value, the rearward force of said spring being transmitted through said first poppet and said first valve seat to said second poppet.