A traveling gantry comprising:
- (a) a bridge portion including:
- (1) a platform having an upwardly directed face, said face defining a normally horizontal plane,
- (2) a plurality of elongated carrier arms, and
- (3) a plurality of hinge means securing respective longitudinal end portions of said carrier arms to said platform for pivoting movement in a plane parallel to and adjacent to said horizontal plane about respective pivot axes spaced from each other and extending transverse to said planes,
- (4) each said carrier arm having a free longitudinal end portion remote from the associated said hinge means;
- (b) at least one road wheel secured to said free longitudinal end portion of each said carrier arm for movement about a horizontal extending axis of rotation;
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- (c) steering means for turning said wheel about an upright steering axis transverse to said axis of rotation;
- (d) said carrier arms being freely rotatable about said respective pivot axis;
- (e) said steering means comprising a steering motor control means for controlling said steering motor, said control means including a superimposing circuit for providing a combined steering signal;
- (f) a primary signal generator, said superimposing circuit having a first input connected to said primary signal whose output signal represents a desired angular position of the road wheel with respect to the associated carrier arm about said steering axis;
- (g) a comparator, a reference signal generator, a sensor, said superimposing circuit having a second input connected to the output of said comparator, one input of said comparator being connected to said reference signal generator delivering a signal representing a desired angular position of said associated carrier arm with respect to said platform about said pivot axis, a further input of said comparator being connected to said sensor indicating the actual angular position of said carrier arm with respect to said platform about said pivot axis so that the output of said comparator provides a deviation signal representing the deviation of said actual angular position from said desired angular position of said carrier arm with respect to the platform;
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- (h) signal processing means being provided between the output of said comparator and said superimposing circuit, said signal processing means delivering in response to the deviation signal a correction signal to said superimposing circuit, which correction signal represents an alteration of said desired angular position of said road wheel with respect to its associated said carrier arm, which alteration is necessary for moving said carrier arm from its actual angular position to its desired angular position with respect to said platform.