An assembly structure of a mobile objective above an optical sensor assembled on a support board includes a cover assembled on the support board with an opening facing the sensor, this cover having a conical external surface with an axis orthogonal to the support board. The structure also includes a frame having first and second elements, shiftable with respect to each other under the action of an electric motor. The first frame element has a conical internal surface capable of mating with the conical external surface of the cover to ensure its positioning and to put resilient connection blades of a fixed portion of the motor in contact with pads formed on the support board, and the second frame element supports the objective and a shiftable portion of the motor.