Apparatus for positioning and attitude stabilising an inert mass 9 pivotably mounted on a movable base includes at least two electromagnetic clutches or brakes 7 which are controllable by means of a closed control loop for delivery of opposite controlled positioning moments to the inert mass. The closed loop controller receives as reference inputs desired values of one or more quantities 4 from a main stabilising controller 3. At least one controller characteristic is varied in dependence on at least one measurable quantity of the control system as a whole. IMAGE
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