Apparatus is provided for fast-adjustment of the position of a plant in a servo system in which a servo output indicative of the plant position is provided to a positioning system for positioning the plant in dependence on the servo output. The apparatus includes an adjustment signal generator, operable in a fast-correction mode and a shock-compensation mode, for generating an adjustment signal for supply to the positioning system to effect fast positional adjustment of the plant. The apparatus also has an adjustment controller for receiving the servo output. The adjustment controller is adapted to detect from the servo output occurrence of a disturbance affecting positioning of the plant and, in response, to initiate the shock-compensation mode of the adjustment signal generator. The adjustment controller is further adapted to initiate the fast-correction mode of the adjustment signal generator in response to indication of an error in positioning of the plant. On initiation of each mode, the adjustment controller supplies an input signal dependent on the servo output to the adjustment signal generator. The adjustment signal generator processes the input signal to generate the adjustment signal such that, in the fast-correction mode the adjustment signal effects adjustment of the plant position to correct the positioning error, and in the shock-compensation mode the adjustment signal effects adjustment of the plant position to counter the effect of the disturbance.