Abstract: A servo control system comprising, in a feedback loop, means for providing an error signal representing the difference between desired and actual conditions of a controlled system, and means responsive to said error signal to produce a control signal for the controlled system; the control system also comprising means for injecting a predetermined disturbance signal into the loop, and means responsive to a comparison of said disturbance signal and a loop response signal for varying gain in said loop.

Abstract: A multiplier circuit arrangement comprises two parallel paths for the same signal, one of the paths being associated with signal level adjusting means; and means responsive to inbalance of d.c. components including any in said parallel paths, the inbalance being in accordance with a d.c. signal in a third signal path, to produce a control signal for varying the signal level adjusting means to reduce the imbalance, so that a.c. components in said parallel paths will be unbalanced to an extent proportional to the d.c. signal in said third path.

Abstract: A multiplier circuit arrangement comprises two parallel paths for the same signal, one of the paths being associated with signal level adjusting means; and means responsive to imbalance of d.c. components including any in said parallel paths, th imbalance being in accordance with a d.c. signal in a third signal path, to produce a control signal for varying the signal level adjusting means to reduce the imbalance, so that a.c. components in said parallel paths will be unbalanced to an extent proportional to the d.c. signal in said third path. Such multipliers are particularly suited to resolving signals in a first set of axes into signals in a second set of axes where other signals are available which are combinable to give trigonometric functions required to translate between the first and second sets of axes.