Since most modern agricultural vehicles employ some form of hydraulic steering
system, recent developments in automatic steering controllers are merely modifications
to the existing hydraulic system. As an example, Van Der Lely (1985) holds
a patent for an automatic guidance system that uses fluid power to actuate the
steering linkage. Laine (1994) analyzed E:H control techniques for a parasitic
steering valve. US equipment manufacturers primarily offer electrohydraulic (E:H)
steering systems for agricultural vehicles.