Electrically driven brake booster

1. An electrically driven brake booster comprising:

a master cylinder for generating a hydraulic pressure to be used for generating a brake force;

a pressing member for controlling a hydraulic pressure generated by said master cylinder;

a shaft member configured to be operated with said pressing member by force operating on a brake pedal to push the brake pedal back toward a release side when the pressing member is not operated, the force changing according to movement of said pressing member;

an electrically driven actuator for moving said pressing member forward and backward;

a sensor to be used for measuring a movement or a position of said shaft member; and

a controller;

wherein:

said controller is configured to control said electrically driven actuator in accordance with an operation of the brake pedal to control a hydraulic pressure to be generated by said master cylinder; and

said controller is configured to perform a learning operation to set a movement reference point for said shaft member, the learning operation comprising:

controlling said actuator to cause a movement of said pressing member,

detecting the movement of said shaft member caused by the movement of said pressing member using said sensor,

judging the presence/absence of an operation of the brake pedal in accordance with the detected movement of said shaft member so as to determine a state that the brake pedal is not operated, and

setting a position of the shaft member as the movement reference point when it is judged that the pedal is not operated.

2. An electrically driven brake booster according to claim 1 , further comprising:

a fixed member mounted on a fixed position; and

a moving member for moving in correspondence with a movement of said pressing member;

wherein said fixed member has a first stopper provided along a movement axis of forward and backward movement of said pressing member;

said moving member has a second stopper moving together with said pressing member; and

in said learning operation, said controller controls said electrically driven actuator to move said moving member in reverse direction of a pressure produced by said pressing member and said controller judges that said moving member has contacted said fixed member.

3. An electrically driven brake booster according to claim 2 , wherein: said controller uses as a standby position a position of said pressing member where said master cylinder does not generate a hydraulic pressure for generating the brake force, and said second stopper is provided at a position more remote from said master cylinder than said standby position; and

when the brake pedal is changed to the state that the brake pedal is not operated, said controller controls said electrically driven actuator to either move said pressing member to the position of said second stopper or make said moving member reach a position of said fixed member.

4. The electrically driven brake booster according to claim 1 , wherein the controller is configured to perform the learning operation under a condition that a vehicle is accelerated.

5. The electrically driven brake booster according to claim 1 , wherein the controller is configured to judge that the operation of the brake pedal is present when the electrically driven actuator drives the pressing member forward and the shaft member immediately follows the movement of the pressing member.

6. The electrically driven brake booster according to claim 5 , wherein the controller is configured to judge that the operation of the brake pedal is absent when the movement amount of the shaft member is delayed until after the pressing member moves past a pressing member reference position.