The present invention relates to rotary fluid pressure devices of the type including electromagnetic valves, and more particularly, to a method of controlling the net-displacement of such rotary fluid pressure devices. 
Although the present invention can be used in connection with various pump and motor configurations, which contain various types of fluid displacement mechanisms, including but not limited to an axial piston type, a radial piston type, a cam lobe type, and a vane type, it is especially advantageous when used with fluid motors having fluid displacement mechanisms of the gerotor type.  Therefore, the present invention will be discussed in connection with fluid motors having fluid displacement mechanisms of the gerotor type without intending to limit the scope of the invention. 
Fluid motors of the type utilizing a gerotor displacement mechanism to convert fluid pressure into a rotary output are widely used in a variety of low speed, high torque commercial applications.  Typically, in fluid motors of this type, the gerotor mechanism includes a fixed internally toothed member (ring) and an externally toothed member (star) which is eccentrically disposed within the ring and orbits and rotates relative thereto.  This relative orbital and rotational movement defines a plurality of volume chambers in the gerotor mechanism that sequentially expand and contract.  Typically, fluid is communicated to these volume chambers through conventional valving means, such as spool and disc.  These conventional valving means provide fluid communication between the fluid inlet, the fluid outlet, and the volume chambers.  During the sequential expansion and contraction of the volume chambers, the fluid inlet is in fluid communication with the expanding volume chambers, while the fluid outlet is in fluid communication with the contracting volume chambers. 
In U.S. Pat.  No.  4,767,292, a different valving means was described.  In the '292 patent, electromagnetic valves provided fluid communication between the fluid inlet and the expanding volume chambers and the fluid outlet and the contracting volume chambers.  Therefore, the invention described in the '292 patent utilizes the same sequential pattern of valving as employed by the conventional valving means. 
Although valving means which employ this sequential pattern of valving are quite effective and successful in many commercial applications, one of the problems with this type of valving is that it leads to variations in output torque and output speed at constant fluid conditions.  In order to improve the workability and comfort during the operation of various off-highway construction and agriculture vehicles, including but not limited to skid-steer loaders, mini-excavators, and air seeders, many manufacturers of such vehicles are now requesting fluid motors which are capable of providing torque and flow outputs with minimal variations at constant conditions.