For example, in a hybrid vehicle, a By Wire type of braking device for generating braking forces through an electric system has been adopted in addition to a conventional braking device for generating braking forces through a hydraulic system. In the related braking device having a By Wire system, an operation quantity of the brake pedal by a driver is converted into an electric signal to apply the electric signal to an electric actuator for driving a piston of a slave cylinder (hereinafter referred to as “motor cylinder device”).
Then, the piston driven by the operation of the electric actuator generates a hydraulic pressure in the motor cylinder. The hydraulic pressure generated as described above is applied to wheel cylinders to generate braking forces for the vehicle (for example, see JP2009-227023A).
In the by-wire type of braking device according to JP2009-227023A, to avoid to provide an operation feeling during depressing a brake pedal, for example, a stroke simulator is used for generating a reaction force characteristic in accordance with an operation quantity of brake paddle by a driver (brake stroke quantity) using a plurality of springs in combination.
According to the stroke simulator of the braking device disclosed in JP2009-227023A, the reaction force characteristic can be created in accordance with the operation quantity of the brake pedal by the driver.
In the stroke simulator of the braking device according to JP2009-227023A, it is desirable to set such a reaction force characteristic that, when the operation quantity of the brake pedal is small, a smaller reaction force is generated, and when the operation quantity of the brake pedal is larger, a larger reaction force is generated. Further, though the operation force of the braking pedal is extremely large, it is preferable to have a reaction force characteristic which does not give the driver such a used-up feeling that a remaining stroke of the brake pedal is used up.
However, to set the above-described reaction force characteristic the stroke simulator, it is required to have a precise mechanical structure in which a plurality of spring members having different spring constants are precisely assembled. Accordingly it is difficult to provide the above-described reaction force characteristic. Further, the request for down-sizing the stroke simulator may be a factor hindering the reaction force characteristic from being realized. Accordingly, when the operation force of the brake pedal has a certain magnitude, such a case may occur that the brake pedal comes in a used-up status in which the remaining stroke of the brake pedal is used up. In such a case, for example, if a target hydraulic pressure is calculated on the basis of the operation quantity of the brake pedal, a hydraulic pressure becoming a maximum value may be set when the operation force of the brake pedal has a certain magnitude.
In such a case, a maximum hydraulic pressure is frequently set as the target. Accordingly, there will be frequent chances of the braking device becoming fully operating by performing the braking ability to an upper limit. As a result, the braking device may become an overheat state as well as durability of the components of the braking device may be decreased.