An agricultural combine is a large machine used to harvest a variety of crops from a field.  During a harvesting operation, a header at the front of the combine cuts ripened crop from the field.  A feederhouse supporting the header transfers the crop material into the combine.  Threshing and separating assemblies within the combine remove grain from the crop material and transfer the clean grain to a grain tank for temporary holding.  Crop material other than grain exits from the rear of the combine.  An unloading auger transfers the clean grain from the grain tank to a truck or grain cart for transport, or to another receiving bin for holding. 
Platform headers and draper headers are header types commonly used when harvesting crops such as small grains, peas, lentils, and rice.  During a harvesting operation with these header types, it is desirable to maintain a cutting height as low as possible to the ground in order to collect substantially the entire ripe crop from the field.  To accomplish this, combines typically employ a header float system or a terrain following system to enable the header to follow the ground over changing terrain without gouging or digging into the soil. 
Manufacturers have developed a number of such systems over the years.  U.S. Pat.  Nos.  3,717,995, 3,623,304, and 4,724,661 disclose examples of header float systems using resilient means to suspend the header, thereby reducing the apparent weight of the header, allowing it to lightly skid across the ground over changing terrain.  U.S. Pat.  Nos.  3,597,907, 4,622,803 and 5,471,823 disclose examples of similar float systems, but using dynamic means to suspend the header.  U.S. Pat.  Nos.  5,577,373, 6,041,583 and 6,758,029 B2 disclose examples of terrain following systems using dynamic means to position the header, thereby sensing and changing the vertical position of the header to follow changing terrain.