The invention provides a locking device for preventing movement between two elements (11, 12) that are mounted to move relative to each other, the locking device including a lock (31) mounted to rotate relative to one of the elements in order to present successive angular positions for locking and for release in which the lock alternates between preventing and allowing relative movement between the two elements, the lock being constrained to rotate with a selector (55) of an angular indexing mechanism (50) actuated by a pulse-controlled actuator (70, 71) arranged to push the selector against a spring member (58) in order to cause it to turn on each pulse and thereby cause the lock to pass from one angular position to the other.