In agricultural machinery for harvesting crops, rough or uneven terrain causes problems in that the position of the cutting mechanism relative the ground must be continuously controlled. If the cutting mechanism is set too high, an excessive crop loss may result. If the cutting mechanism is set too low, the cutting mechanism may dig into the ground, possibly causing damage to the cutting mechanism and/or causing dirt to be mixed with the cut crop material as it is fed into the machine. With respect to mine plows, it is essential that the plow remain below the ground as it moves over uneven terrain otherwise some mines may be left undisturbed in the field.
Also, it is desirable that the operator have a simple manual control for setting the reference position of the implement and for over-riding or re-establishing automatic control. For example, if the implement is a cutterbar on a harvesting machine it is desirable to be able to terminate automatic position control and raise the implement as the harvester approaches the headlands where the machine is turned around, and to lower the cutterbar and resume automatic position control as the turn is completed and the machine again enters the crop field.
It is known to provide automatic positioning mechanisms wherein one or more sensors sense the position of an implement relative to the ground and, in response to the sensed conditions controlling the positioning of the implement. Lundahl et al. U.S. Pat. No. 4,896,486, Bettencourt et al. U.S. Pat. No. 4,414,792, Thornley et al. U.S. Pat. No. 4,507,910, Kishida U.S. Pat. No. 4,663,712 and Seiferling U.S. Pat. No. 4,573,124 are examples. Some of these devices merely provide automatic "raise/lower" control with no tilting control. Others provide for independently raising or lowering each end of an implement. None of them provide for tilt control of a center-pivoted implement and none provide a simple operator-controlled mechanism for selectively raising or lowering an implement concurrently with terminating or resuming automatic positioning control.
Wynn et al. U.S. Pat. No. 4,641,490 discloses a height and tilt mechanism for a center pivoted harvester header but this system requires at least three height sensors and the tilt made to correct the positioning of one end does not take into account the effect the tilt has on the positioning of the other end.