Two stage servomotor with smooth actuation sequence

Claim 1

In a power braking system having a servomotor with a wall moved by a pressure differential acting thereacross for supplying an operational force to activate the wheel brakes in response to an input force in a first mode of operation, control means for modifying the operational force in response to an input force in a second mode of operation, said control means comprising: a housing having a bore therein with a first diameter section connected to a reservoir by a first port and a second port and a second diameter section connected to said wheel brakes; first piston means connected to said wall and extending through said first diameter section to establish an output chamber in the second diameter section, said output chamber being connected by an axial flow path to the first diameter section through a first axial passage and a second axial passage, said axial flow path having a control chamber located between the first axial passage and the second axial passage; second piston means concentric to and located between a first stop and a second stop on the first piston means for separating the first diameter section into a pressurizing chamber and a relief chamber; resilient means connected to the first piston for urging said second piston means toward said first stop; first valve means located with said axial flow path for controlling communication between the pressurizing chamber and the output chamber; second valve means located within said axial flow path for controlling communication between the pressurizing chamber and the relief chamber; first actuator means located within the axial flow path and moved away from the first valve means when the first pressurizing force moves the second piston means against the second stop in opposition to the resilient means during the first mode of operation, said first pressurizing force opening said first valve means to supply the wheel brakes with an actuation force; and second actuator means responsive to said input force in the second mode of operation to move the second valve allowing said first pressurizing force present in the pressurizing chamber to proportionally escape into the relief chamber as a function of the input force and allow the pressure differential force acting on the wall to move the first piston in the output chamber to develop a second pressurizing force for activating the wheel brakes in the secoNd mode of operation, said resilient means moving said second piston toward said first stop upon termination of said second mode causing said first actuator means to open said first valve means and establish unhampered communication between the output chamber and the pressurizing chamber.