APS is one of the top model of steering valves:
The steering unit is part of the hydrostatic power steering system. Usually the steering unit is connected to the steering wheel through a steering column. During the steering, the steering wheel moves a gear with outer cogs which has an orbital movement respect to the outer toothed gear. Inside the steering unit, the rotor, which has the function of volumetric dosing unit, releases an amount of oil that is proportional to the turning angle made by the steering wheel.