POSITION SERVO SYSTEM

57 A servo system (10) (Fig. 1) for positioning a sighting mirror (11) connected to a pulley (16) comprises a drive motor (13) rigidly connected to a drive pulley (15) in turn coupled by a tensioned drive belt to the pulley (16). Angular position of the motor or pulley (15) is measured by sensor (18) and passed by way of phase advance network (19) and amplifier (21) to produce an error signal for the drive motor. The pulley (16) is outside of the servo loop and any oscillations Induced in the tensioned belt at its resonant frequency, possibly originating from electrical noise in the servo loop will be uncontrollable. To avoid such oscillation and retain a simple construction the belt tension is set (25) to a value producing a belt resonant frequency outside the operating bandwidth of the servo system and the feedback loop includes a notch filter (20) tuned to the belt resonant frequency which prevents electrical noise in the system from inducing along-axis oscillation of the drive belt and consequential position jittering of the mirror sightline.