1. In a system of the character described, a carriage movable on a structure; a wire feeding mechanism on said carriage; a differential system including a first and a second axle gears, a ring gear, spider gear means journaled for rotation on said ring gear and meshing with said axle gears; means for moving said carriage on said structure; means responsive to the speed of said carriage on said structure and coupled to said first axle gear; gear reduction means coupled between said first axle gear and said ring gear to produce a speed of rotation of said second axle gear commensurate with the speed of said first axle gear; means responsive to the speed at which said wire is being laid by said wire feeding mechanism on said structure when said carriage moves on said structure and producing rotation of an element in accordance with the last-mentioned speed; and means comparing the related speed of said element and said second axle gear and producing an output when said relative speed exceeds a nominal value; and means receptive to said output and restoring said relative speed to said nominal value in accordance with said output.

4. In a system of the character described in which a carriage traveling with respect to a structure mounts a wire feeding mechanism; differential means including a first and a second shaft; means for moving said carriage on said structure; means driving said first shaft at a speed representative of the speed of said carriage; means driving said second shaft at a speed which differs from the speed of said first shaft by a constant amount; means driving a third shaft at a speed representative of the speed at which wire is being laid on said structure; and means comparing the speeds of said third shaft with said second shaft to obtain a comparison; and means controlling said wire feeding mechanism in accordance with said comparison.

5. In a system of the character described wherein a carriage moves around a circular structure when and as wire is being wound under tension around said structure; a carriage; means for moving said carriage on said structure; first sensing means on said carriage sensing its speed relative to said structure; wire feeding means on said carriage; second sensing means sensing the speed at which said wire is fed by said wire feeding means to said structure; comparing means controlled by said first and second sensing means and producing an output representative of the difference between said carriage speed and wire feeding speed, said comparing means including a differential system having a pair of axle gears coupled by spider gears, the spider gears being on a ring gear, said first sensing means including a rotatable element being also coupled to said ring gear through an adjustable transmission.

7. In a system of the character described wherein a wire gripping means and a carriage on which said gripping means is mounted travel around a medium on which wire is laid under tension, means producing relative movement between said carriage and said medium, means monitoring the lineal speed of said relative movement and producing a first quantity representative of the difference in speed between said carriage and said medium, means monitoring the lineal speed at which wire is being laid and producing a second quantity representative of the speed at which said wire is being laid on said medium, means responsive to one of said quantities and producing a third quantity representative of a desired nominal value of the other of said quantities, means comparing said third quantity with said other quantity and obtaining from such comparison deviations from said nominal value, and means responsive to said deviations and restoring said other quantity to a value substantially equal to said nominal value, the comparing means including a servo system and the means responsive to said deviations includes a differential speed mechanism coupled to and controlled by said servo system and having an input shaft coupled to said movement producing means and an output shaft coupled to said wire gripping means.