Electrically driven brake booster

An electrically driven brake booster comprising an input member disposed for back-and-forth movement in operative association with a brake pedal, an output member disposed in operative association with a piston of a master cylinder, drive transmitting means for translating a rotating motion of a motor into a linear motion to be transmitted to the output member, a controller for controlling the motor in accordance with a variable representing an amount by which the brake pedal is maneuvered, a displacement sensor for measuring a value representing a displacement of the input member and inputting the value into the controller, a depressing force sensor for measuring a force of depression on the brake pedal and inputting the value into the controller, a liquid pressure sensor for measuring braking liquid pressure in the master cylinder and inputting the value to the controller and reaction transmitting means for transmitting a reaction from the master cylinder to the output member and the input member at a given proportion, the drive transmitting means comprising a rack formed on the output member and a pinion disposed in operative association with the motor and in meshing engagement with the rack, the output member and the input member being axially displaceable relative to each other and the controller, based on the input signals from the displacement, depressing force and liquid pressure sensors, controls the torque output of the motor so that the force of depressing the pedal which is transmitted from the input member and a propelling force from the output member is balanced against an opposing force from the master cylinder.