A controllable variable length link device is disclosed. The device includes a first half-link having a first end connected to a driving system and a second end, a second half-link having a first end connected to a driven system and a second end, a rotary displacement member including a rotator shaft and a rotary head cooperating with the second ends of the half-links to bring about opposite movement of the half-links, a bearing unit in which the rotator shaft is mounted, respective guiding units which guide the two half-links in translation in at least two parallel directions, the guiding units being mechanically connected to the bearing means; and a driving unit which drives the shaft in rotation in both directions in accordance with a predetermined relationship.