In a method of subordinating the orientation of a grab on a Public Works machine to the angle of rotation of the turret three directions are chosen, the first two being determined by the desired orientation of the grab in the loading and unloading zones, respectively, and the third passing through the axis of rotation of the turret 2, the angle of rotation of the turret starting from the third direction is measured and compared with the angle between a straight line, passing through the point of intersection of the first two directions and the axis of rotation of the turret, makes and the third direction, the desired angle of the grab with respect to the boom in the loading or unloading zones is determined and the angle that the grab actually makes with the boom is measured and compared with the appropriate desired angle, the results of this comparison being used to control rotation of the grab to reduce the difference to zero.
A method and apparatus for controlling the horizontal motion of a double articulated crane comprises determining the angle about a common axis between a base member pivotally mounted on a crane post and a head member carrying a load application point on an outer end thereof. The angle can vary from 0.degree. to 360.degree.. A desired movement direction value is then set on a direction selector movable on a direction indicator. The value corresponds to a target position for the load application point. The desired movement direction value is adjusted by small increments to reach the target however. The desired movement direction value is broken up into two components, one aligned with the axis of the head and the other aligned with a perpendicular axis. The values of these two compartments are calculated as a function of the sin or cos of an angle of misalignment between the longitudinal axis and the direction of the selector. The pivotal speed of the base member about the crane post and the head member about the common axis between the base and the head members is then calculated and utilized to drive the equipment for rotating the members.
