A deployable device includes a supporting structure, a deployable structure capable of switching from a configuration wound around a first axis to a configuration deployed according to a second axis substantially at right angles to the first axis, by being deployed by a length defined in a frame of reference of the supporting structure, wherein it further comprises a deployed length control device comprising a sensor and a computer linked to the sensor, a first conductive track electrically linked to the sensor extending on the deployable structure, substantially along the perimeter of the deployable structure, and forming one or more turns of a first coil in the wound configuration, and in that the sensor is configured to measure an electrical parameter from the first electrically conductive track which varies with the deployed length of the deployable structure and the computer is configured to determine the deployed length of the deployable structure.