Electronic rear derailleur

1. An electronic rear derailleur, comprising:

a movable portion comprising a main drive gear meshing with a first driven gear and a second driven gear;

a support frame disposed on the movable portion and for the main drive gear, the first driven gear, and the second driven gear to perform positioning, the support frame comprising a first support hole;

a drive unit configured to drive, by using a transmission assembly, the main drive gear to rotate;

a fixing portion fixed to a rear frame of a bicycle and having a first connecting portion connected to the first driven gear, to cause the first connecting portion to be moveably connected to a first end of the movable portion;

a chain guide portion having a second connecting portion connected to the second driven gear, to cause the chain guide portion to be moveably connected to a second end of the movable portion, the chain guide portion comprising at least one pulley meshing with a chain of the bicycle; and

a positioning system comprising: a main board, a processing unit and a position sensor disposed on the main board, and a positioning shaft disposed in the first support hole of the support frame and connected to the first driven gear, the positioning shaft comprising a magnetic field generation unit facing the position sensor.

2. The electronic rear derailleur according to claim 1 , wherein the first driven gear comprising a first recess in a direction facing the support frame, a center of the first driven gear comprising a first shaft hole, the positioning shaft comprising a convex portion assembled with the first recess, a convex pillar extending from the convex portion toward a direction of the first shaft hole, and the convex pillar being assembled with the first shaft hole of the first driven gear.

3. The electronic rear derailleur according to claim 1 , wherein the positioning shaft comprises an accommodating space, the magnetic field generation unit being disposed in the accommodating space.

4. The electronic rear derailleur according to claim 1 , wherein the positioning shaft is mounted in the first support hole of the support frame through a first bearing.

5. The electronic rear derailleur according to claim 1 , wherein the drive unit comprises a control module to control a number of drive cycles and a rotational speed of the drive unit, the control module being electrically connected to the processing unit.

6. The electronic rear derailleur according to claim 5 , wherein the main board further comprises a memory unit electrically connected to the processing unit, the memory unit storing a comparison table comprising comparison information between the positioning information and a position of the electronic rear derailleur.

7. The electronic rear derailleur according to claim 1 , wherein the position sensor is a Hall effect sensor, and a side of the magnetic field generation unit facing the position sensor has at least an N pole and an S pole.

8. The electronic rear derailleur according to claim 2 , wherein the positioning shaft is a bolt, the convex pillar has an external thread, and the first shaft hole comprises an internal thread for the convex pillar to be screwed and for the positioning shaft to rotate with the first driven gear.

9. The electronic rear derailleur according to claim 1 , wherein the position sensor and the drive unit are disposed on two opposite sides of the main board, respectively.

10. The electronic rear derailleur according to claim 1 , wherein the position sensor and the magnetic field generation unit are disposed with no contact.