Electronic parking brake system and method for controlling clamping force thereof

1. An electronic parking brake system, comprising:

an electronic parking brake (EPB) comprising a pair of brake pads disposed on both sides of a brake disc rotating with a rear wheel of a vehicle, a piston provided to press the pair of brake pads, a nut member provided to press the piston, a spindle member provided to move the nut member, and an electric motor configured to rotate the spindle member;

a wheel speed sensor configured to detect a wheel speed of the rear wheel;

a G sensor configured to detect a longitudinal acceleration of the vehicle;

an accelerator pedal sensor configured to detect an operation of an accelerator pedal of the vehicle;

an EPB switch configured to receive a parking apply command or a parking release command from a driver; and

a controller configured to control the electric motor,

wherein the controller is configured to determine whether a residual clamping force is present in the EPB based on a rear wheel speed or the longitudinal acceleration at a time of departure of the vehicle after parking is released, and when the residual clamping force is present, release the residual clamping force through an additional parking release control.

2. The electronic parking brake system of claim 1 , wherein the controller is configured to determine that the residual clamping force is present in the EPB, when it is determined that the rear wheel speed is low in comparison with an amount of an accelerator pedal operation by the driver at the time of departure of the vehicle.

3. The electronic parking brake system of claim 1 , wherein the controller is configured to determine that the residual clamping force is present in the EPB, when it is determined that a shake of the vehicle occurs from the longitudinal acceleration at the time of departure of the vehicle.

4. The electronic parking brake system of claim 1 , wherein the controller is configured to perform the additional parking release control for a preset period of time.

5. The electronic parking brake system of claim 1 , further comprising:

a warning part configured to warn of presence of the residual clamping force, and

wherein the controller is configured to warn the driver through the warning part, when the residual clamping force is present after the additional parking release control is performed.

6. A method for controlling a clamping force of an electronic parking brake system configured to generate or release the clamping force in an EPB, the method comprising:

determining whether a residual clamping force is present in the EPB based on a rear wheel speed or a longitudinal acceleration at a time of departure of a vehicle after parking is released; and

when the residual clamping force is present, releasing the residual clamping force through an additional parking release control.

7. The method of claim 6 , wherein the determining of whether the residual clamping force is present comprises detecting an amount of an accelerator pedal operation by a driver at the time of departure of the vehicle, and when it is determined that the rear wheel speed is low in comparison with the detected amount of the accelerator pedal operation, determining that the residual clamping force is present in the EPB.

8. The method of claim 6 , wherein the determining of whether the residual clamping force is present comprises determining that the residual clamping force is present in the EPB, when it is determined that a shake of the vehicle occurs at the time of departure of the vehicle from the longitudinal acceleration.

9. The method of claim 6 , wherein the releasing of the residual clamping force comprises performing the additional parking release control for a preset period of time.