Crank drive apparatus for power assisted bicycle

A power transmission apparatus configured to drive a wheel, the power transmission apparatus comprising:

• a shaft, wherein an axial direction of the shaft is extended along an X axis, and the shaft is capable of rotating around the X axis;
• a propelling plate disposed on the shaft, capable of rotating around the X axis with the rotating of the shaft, wherein the propelling plate has a first surface and at least one first block, the first block is disposed on the first surface, the first surface is parallel to a plane formed by a Y axis and a Z axis, the X axis, the Y axis, and the Z axis are perpendicular to each other, and the first block has a propelling surface;
• a first driven plate disposed on the shaft, the first driven plate capable of moving along the X axis relative to the shaft;
• wherein the first driven plate has a second surface and at least one second block, the second block is disposed on the second surface, the second surface is parallel to the plane formed by the Y axis and the Z axis, the second surface is opposite to the first surface, the second block has a driven surface, and the driven surface at least partially touches against the propelling surface;
• a sensor device disposed close to the first driven plate, wherein the sensor device detects a moving distance of the first driven plate along the direction of the X axis;
• a second driven plate disposed on the shaft and at a side of the first driven plate opposite to the propelling plate and driven to rotate by the first driven plate, wherein the second driven plate is fixed relative to the shaft in the direction of the X axis, and coupled to the wheel;
• and
• a resilient member disposed between the first driven plate and the second driven plate;
• wherein when the first driven plate moves toward the second driven plate along the direction of the X axis, the resilient member is compressed to provide the first driven plate a restoring force toward the propelling plate on the direction of the X axis;
• wherein when the shaft does not rotate together with the propelling plate, the restoring force compels the first driven plate to touch against the propelling plate to reveal a first distance between the sensor device and the first driven plate;
• when the shaft rotates together with the propelling plate, the propelling surface compels the driven surface to correspondingly rotate, an interaction force generated by a contact between the propelling surface and the driven surface has an X axis force component on the direction of the X axis, the X axis force component overcome the restoring force, so as to compel the first driven plate to move away from the propelling plate toward the second driven plate along the direction of the X axis to reveal a second distance between the sensor device and the first driven plate, the first distance is different from the second distance, the second driven plate is driven to rotate by the first driven plate, and the second driven plate rotates to drive the wheel to rotate.