Because a conventional electric parking brake device (please see Patent Literature 1 or 2, etc.) operates a motor under open loop control, it is impossible to control a clamp position between brake pads and a brake disk, a clamp speed, etc. Moreover, in the case that a battery goes flat once and then restores, position information of a motor disappears, and it is impossible to distinguish between on state and off state of an electric parking brake.
Therefore, there is an idea to control a press amount of brake pads, a pressing speed, etc. by detecting a rotation frequency, etc. of a motor with a rotational angle detecting means such as a rotary encoder, a resolver, etc. installed into an electric parking brake device. However, a rotary encoder cannot withstand a severe environment in a vicinity of a vehicle wheel. Moreover, a conventional resolver (please see Patent Literature 3 or 4, etc.) is large in size, in particular, coil parts of it are thick. Thus, when a conventional resolver is installed into an electric parking brake device, the device is larger in size as a whole and is not suitable for a practical use. The fact is that an electric parking brake device is not delicately controlled except for an on/off control.
USPTO Backgrounds
USPTO Backgrounds