Abstract: To avoid skidding of the driving wheels of a tractor (1) which runs on tires and which is fitted with hydraulic lifting devices (3, 4) on which are mounted working tools (35, 45) which are buried in the ground (in this case ploughs), the apparatus includes detector structure for determining the force for forward movement of the tractor, comparing that force to a reference value and producing pulses of less than one second, for alternately raising the two lifting devices (3, 4), the amplitude of the pulses being proportional to the excess of the forward movement force, up to a limit amplitude of centimeters approximately. The pulses are too short for the tools (35, 45) to lift with respect to the ground and the amplitude of the raising movement appears as squashing of the tires, which increases grip in order to overcome the excess of forward movement force.

Abstract: Apparatus for controlling the frequency of vibrations imparted by the vibrating element of a compacting machine to the ground which comprises a vibration pick-up responsive to the amplitude of vibrations of a mass composed of the vibrating element and the ground being vibrated which produces a vibration signal; a position sensor sensing the angular position of an eccentrically loaded rotary shaft on which the vibrating element is mounted, which produces a position signal; and a phasemeter which receives the vibration and position signals and provides an output signal related to the phase difference therebetween. This phase difference may be displayed in which case the motor for the rotary shaft can be controlled manually in accordance therewith or it may be fed to an automatic control system in which it is compared with a set signal to produce an error signal for automatically regulating the speed of motor turning the rotary shaft.

Abstract: A direction and speed control system for a steerable locomotive having independent speed controls for two driving rollers, in which, where C.sub.D and C.sub.G are speed control signals for the two rollers, .alpha. is a steering angle of a steerable support, which may be a mere skid or a driving roller and C.sub.max is a maximum speed control value, there are three operational networks which generate signals F.sub.1, F.sub.2 and F.sub.3 given by C(1-K.sub.1 tan .alpha.), C(1+K.sub.1 tan .alpha.) and C.sub.max (1-K.sub.1 .vertline. tan .alpha..vertline.)/(1+K.sub.1 .vertline. tan .alpha..vertline.), comparators which receive C.sub.max and F.sub.1 and C.sub.max and F.sub.2 respectively and output terminals fed in parallel, through gating circuits controlled by the comparators, by the first and third networks and by the second and third networks respectively.