1. Field of the Invention
This invention relates to a brake booster device which is provided in a brake system of, for example, an automobile and which is operated to increase an operating force (or control force) applied from a brake pedal, and transmit the increased operating force to a master cylinder.
2. Description of the Related Art
FIG. 5 is a diagram schematically illustrating the construction of an example of a conventional speed density type control system for an automotive internal combustion gasoline engine. As shown in this figure, an injector 2 for injecting fuel, and an ignition coil 3 for generating sparks are attached to an engine body 1. Moreover, an intake (or inlet) manifold 4 is connected to the engine body 1. Further, part of the gas flowing from the engine body 1 toward the intake manifold 4 is fed back to the intake manifold 4 through a recirculation pipe 5. An exhaust gas recirculation (EGR) valve 6 is provided at a midpoint on the recirculation pipe 5.
A throttle valve 8, which is opened and closed mainly by operating an accelerator pedal (not shown), is provided in an air suction pipe 7 for introducing air from an air cleaner into the intake manifold 4. Further, a bypass pipe 9 for connecting upstream and downstream portions of the air suction pipe 7, which are upstream and downstream to the throttle valve 8 respectively, with each other is attached to the air suction pipe 7. Moreover, a bypass air valve 10 is provided in the bypass pipe 9. The injector 2, the ignition coil 3, the throttle valve 8 and the bypass air valve 10 are controlled by a control section (namely, a computer) 11.
A brake booster body 14 for increasing operating force applied from the brake pedal 12 and transmitting the increased operating force to the master cylinder 13 of a brake unit is connected with the intake manifold 4. A negative-pressure (or vacuum) chamber 14a which is communicated with the intake manifold 4 through a negative-pressure suction pipe 15, and an atmospheric pressure chamber 14b into which atmospheric air is introduced when pressing the brake pedal 12, are provided in the brake booster body 14.
Furthermore, a booster pressure sensor 16 which is operative to detect a pressure in the negative-pressure chamber 14a and to output a signal representing the detected pressure to the control section 11, is connected with the brake booster body 14. A manifold pressure sensor 17 which is operative to detect a pressure in the manifold 4 and to output a signal representing the detected pressure to the control section 11, is provided in the manifold 4. The amount of fuel injected from the injector 2 is calculated by the control section 11 according to a signal output from the manifold pressure sensor 17. Further, an atmospheric pressure sensor 18 for detecting atmospheric pressure is provided in the control section 11.
FIG. 6 is a partially sectional diagram showing the construction of the brake booster body 14 in FIG. 5. Similar construction of a brake booster is illustrated in, for example, the May 1996 issue of "Automobile Engineering" (Jidosha-Kogaku). As shown in FIG. 6, a power piston 22 capable of moving in lateral directions (as viewed in this figure) is enclosed in a booster casing (or case) 21. A diaphragm 23 is securely fixed onto the power piston 22. Circumferential edge portions of the diaphragm 23 are fixed to the booster casing 21. Thus, the inner space of the booster casing 21 is partitioned into the negative pressure chamber 14a and the atmospheric pressure chamber 14b.
A poppet valve assembly 24 is attached to the central portion of the power piston 22. The poppet valve assembly 24 is operated by the brake pedal 12 through a valve operating rod 25. Namely, when the brake pedal 12 is not depressed, the negative pressure chamber 14a and the atmospheric pressure chamber 14b are communicated with each other and are shut off from the outside. Further, when depressing the brake pedal 12, the atmospheric pressure chamber 14b is shut off from the negative pressure chamber 14a, while at the same time air is introduced into the atmospheric pressure chamber 14b.
The movement of the power piston 22 is transmitted to the master cylinder 13 through a push rod 26. The power piston 22 is pushed by a diaphragm return spring 27, which is disposed in the negative pressure chamber 14a, toward the atmospheric pressure chamber 14b.
Next, the operation will be described. When the brake pedal 12 is not depressed, the negative pressure chamber 14a and the atmospheric pressure chamber 14b are communicated with each other and are shut off from the outside by the poppet valve assembly 24. Therefore, a pressure Pa in the negative pressure chamber 14a is equal to a pressure Pb in the atmospheric pressure chamber 14b (namely, Pa=Pb=a negative pressure).
In contrast, when depressing the brake pedal 12, the atmospheric pressure chamber 14b is shut off from the negative pressure chamber 14a and air is introduced into the atmospheric pressure chamber 14b. Consequently, Pa&lt;Pb=atmospheric pressure. Thus, a differential pressure between the internal pressure of the negative pressure chamber 14a and that of the atmospheric pressure chamber 14b is generated. Furthermore, the power piston 22 is moved toward the negative pressure chamber 14a against the force of the diaphragm return spring 27 by the differential pressure. Thus, the pushing force of the power piston 22 is transmitted to the master cylinder 13 through the push rod 26.
The pushing force of the push rod 26 due to the differential pressure is obtained by multiplying the differential pressure (Pb-Pa) by the area of the diaphragm 23. Thus, when depressing the brake pedal 12, the force obtained by adding the pushing force caused by the differential pressure to the brake pedal depressing force is transmitted to the master cylinder 13.
Thus, in the case of this conventional brake booster device, the operating force to be exerted on the master cylinder 13 is increased by utilizing the differential force between the (internal) negative pressure of the negative pressure chamber 14a and the atmospheric pressure. Consequently, there is the necessity of securing the aforementioned differential pressure at all times. Therefore, the internal pressure of the negative pressure chamber 14a is detected by the booster pressure sensor 16. In addition, the atmospheric pressure is detected by the atmospheric pressure sensor 18, and the differential pressure is monitored by the control section 11. Further, in the case that the differential pressure is insufficient, the throttle valve 8 or the air bypass valve 10 is closed. Thus, the internal pressure of the negative pressure chamber 14a communicated with the intake manifold 4 is reduced to thereby regulate the differential pressure.
In the case of the conventional brake booster device constructed as described above, since the internal pressure of the negative pressure chamber 14a is detected by the booster pressure sensor 16 which is communicated with the negative pressure chamber 14a, there is a need to provide the atmospheric pressure sensor 18 in the control section 11 to detect the differential pressure between the internal pressure of the negative pressure chamber 14a and the atmospheric pressure. This results in an increased manufacturing cost of the device. Moreover, there are variations among individual booster pressure sensors 16 and atmospheric pressure sensors 18. Consequently, measurement errors are increased.
This invention is provided to solve the aforementioned problems of the conventional device.