The present invention relates to metal detection systems such as those used in forage harvesters.
In a forage harvester it is well known to provide upper and lower front feed rolls and upper and lower rear feed rolls for compacting a crop material and feeding it to a cutter head assembly which cuts it into smaller pieces. The bulk of the crop material passing between the upper and lower feed rolls varies. To assure compaction and feed of the crop material, Shriver U.S. Pat. No. 4,278,211 proposes mounting the upper feed rolls on an assembly which is biased under spring tension toward the lower feed rolls.
It is also well known to provide a metal detector in a forage harvester to detect tramp metal in the crop material, the detector producing an output signal to stop the feed rolls before the cutter head can be damaged by the crop material. Garrott U.S. Pat. No. 3,959,953 discloses such a metal detector located within the lower front feed roll of a forage harvester. In normal operation the metal detector inherently exhibits a certain level of output due to the rotation of metallic machine elements in its sensing field. This output is commonly referred to as noise, and its major contributors are the rotating feedrolls: lower front, lower rear and upper front and upper rear. The detection threshold of the metal detector is typically set high enough to prevent stopping the feedrolls in response to this noise. Entry of a piece of tramp metal into the sensing region of the metal detector induces a signal which, when added to the aforementioned noise increases the level of output so as to exceed the detection threshold causing the feed rolls to stop.
With the advent of tensioned upper feed rolls, the contribution of noise by the upper feed rolls has become variable over a wider range than before as the upper feedrolls can be displaced further from the metal detector as the volume of crop entering the harvester increases. Upward displacement of the upper feedrolls moves them farther from the metal detector thereby diminishing their contribution to the noise. Consequently, with the diminished noise, a large signal is needed from the tramp metal to exceed the detection threshold. Or, stated otherwise, a piece of tramp metal which resulted in marginally reaching the metal detector detection thershold while the upper feedrolls were in their lowermost position, would fall short of the detection threshold with diminished noise when the upper feedrolls are displaced by a larger volume of crop material. Thus it is seen that the ability of the metal detector to respond to smaller pieces of tramp metal is compromised as larger volumes of crop material displace the tensioned upper feed rolls upwardly, reducing their noise contribution, and thereby decreasing the liklihood of detection.