HITCH CONTROL SYSTEM

A hydraulic rockshaft actuator (24) operates a rockshaft (12) to raise and lower a hitch in accordance with mixed position control set by a lever (28) and draft control set by a lever (32). The set points established by these levers are combined mechanically with actual rockshaft position sensed mechanically by a ramp (14) on the rockshaft and actual draft load sensed hydraulically by a load sensor (34) and communicated to the mechanical combining arrangement (16) via a load control valve (17). In order to provide this known system with engine speed control (to avoid labouring the engine) and/or wheel slip control (to avoid excessive wheel slip), an electronic control unit (46) processes signals from corresponding transducers (54, 58, 60) and derives an electrical control signal. A pressure control valve (40) in the hydraulic line between the draft load sensor (34) and the load control valve (17) enhances the hydraulic load signal so that reduced engine speed and/or increased wheel slip emulates increased draft load and therefore leads to the hitch being raised. To this end the control valve (40) has electrical pilot control (44) responsive to the electrical control signal in parallel with hydraulic pilot control (42) responsive to the pressure from the draft load sensor (34). The draft control lever (32) sets the proportions of the mixed control and, within the control unit (46) the electrical control signal is gain-scaled in dependence upon a signal (A(6)) from a transducer (52) on the lever (32) so that the effect of the electrical control signal is the greater the greater the proportion of draft load control in the mixed control.