CROP MACHINE WITH AN ELECTRONICALLY CONTROLLED HYDRAULIC CYLINDER FLOTATION SYSTEM

In a crop harvesting machine (100) there is provided a pair of hydraulic float cylinders (116, 117) for a header (104) relative to a vehicle (102), where a float pressure to the cylinders (116, 117) is directly controlled by an electronic control (28) supplying a variable control signal to a PPRR valve arrangement (18, 20) to maintain the float pressure at a predetermined value. At the set pressure a predetermined lifting force is provided to the header (104). A position sensor (120, 121) is used to generate an indication of movement and/or acceleration. The electronic control (28) is arranged, in response to changes in the sensor signal, to temporarily change the control signal to vary the lifting force and thus change the dynamic response of the hydraulic float cylinder (116, 117). In order to reduce static friction so that the system can react quickly, an arrangement is provided for causing relative reciprocating movement in an alternating wave pattern between the piston and cylinder.