Actuator of electronic brake system

An actuator of an electronic brake system comprising:

• a drive unit generating a power and including a motor having a stator and a rotor;
• a piston unit operated by the drive unit and including a piston reciprocating in a bore chamber provided in a piston housing to contain a fluid;
• a power transmission unit having one end coupled to the rotor and the other end coupled to the piston to convert a rotational motion of the rotor into a linear motion of the piston;
• a sensing magnet coupled to the power transmission unit to rotate along the motor;
• and
• a motor position sensor disposed coaxially with and spaced apart from the sensing magnet to sense a position of the motor based on the rotation of the sensing magnet,
• wherein the power transmission unit includes a spindle coupled to the rotor to rotate together with the rotor, the spindle including a magnet holder provided to extend into the bore chamber, and the sensing magnet is provided at an end of the magnet holder and located in the bore chamber containing the fluid.