Seat lifter

1. A seat lifter comprising:

an inner link for changing a height position of an inner lower arm with respect to a floor of a vehicle, the inner lower arm being disposed on an inner side in a width direction of the vehicle;

an outer link for changing a height position of an outer lower arm with respect to the floor, the outer lower arm being disposed outside the inner lower arm in the width direction;

a coupling rod that couples the inner link to the outer link so as to cause the inner link and the outer link to rotate together;

a planetary gear mechanism connected to the coupling rod and capable of causing the coupling rod to rotate relative to the inner lower arm and the outer lower arm;

a drive unit capable of driving the planetary gear mechanism; and

a power transmission rod that transmits power of the drive unit to the planetary gear mechanism,

the planetary gear mechanism being disposed between the inner lower arm and the outer lower arm, and

the drive unit being disposed outside the inner lower arm in the width direction or outside the outer lower arm in the width direction.

2. The seat lifter according to claim 1 , wherein

the planetary gear mechanism includes

a sun gear connected to a tip end of the power transmission rod,

a planetary gear that rotates around the sun gear while meshing with the sun gear,

a fixed internal gear having internal teeth that mesh with the planetary gear, and fixed to one of the inner lower arm and the outer lower arm so as not to be rotatable relative to the inner lower arm and the outer lower arm,

an inner movable internal gear having internal teeth that mesh with the planetary gear, and rotatable together with the inner link relative to the fixed internal gear, and

an outer movable internal gear having internal teeth that mesh with the planetary gear, and rotatable together with the outer link relative to the fixed internal gear,

the fixed internal gear is disposed between the inner lower arm and the outer lower arm,

the inner movable internal gear is disposed between the fixed internal gear and the inner lower arm, and

the outer movable internal gear is disposed between the fixed internal gear and the outer lower arm, and rotates together with the inner movable internal gear.

3. The seat lifter according to claim 2 , wherein

the planetary gear includes

a fixed gear portion that meshes with the fixed internal gear,

an inner gear portion having a same number of teeth as the fixed gear portion, and that meshes with the inner movable internal gear, and

an outer gear portion having a same number of teeth as the inner gear portion, and that meshes with the outer movable internal gear, and

the inner movable internal gear and the outer movable internal gear have a different number of teeth than the fixed internal gear.

4. The seat lifter according to claim 2 , wherein

the outer lower arm is connectable to a lower end portion of an outer side frame of a pair of side frames included in a back frame, the outer side frame being located closer to a belt insertion portion through which a seat belt is inserted,

the inner movable internal gear is connected to an outer end portion of the coupling rod in the width direction, and

the outer movable internal gear is connected to the outer lower arm from an inner side in the width direction.

5. The seat lifter according to claim 4 , wherein

the drive unit is disposed outside the outer lower arm in the width direction.

6. The seat lifter according to claim 4 , wherein

the inner link is fixed to the coupling rod, and

the outer link is fixed to the outer movable internal gear.

7. The seat lifter according to claim 4 , further comprising:

an inner slide rail that causes the inner lower arm to slide with respect to the floor; and

an outer slide rail that causes the outer lower arm to slide with respect to the floor, wherein

the inner slide rail includes

an inner lower rail fixable to the floor, and

an inner upper rail slidable with respect to the inner lower rail,

the outer slide rail includes

an outer lower rail fixable to the floor, and

an outer upper rail slidable with respect to the outer lower rail,

the inner link is coupled to the inner upper rail so as to be rotatable with respect to the inner upper rail, and

the outer link is coupled to the outer upper rail so as to be rotatable with respect to the outer upper rail.

8. The seat lifter according to claim 7 , wherein

an outer end portion of the drive unit in the width direction is located inside an outer end portion of the outer slide rail in the width direction.