It is well known in the art to provide agricultural harvesting machines, such as forage harvesters, with an apparatus for detecting foreign material in the stream of crop material which is being fed to a crop processing unit, such as a rotating cutterhead cooperating with a stationary shearbar. Such apparatus may comprise a metal detector sensor as described in U.S. Pat. No. 4,433,528. The signal generated by this sensor is fed to feeder arrest means, which provoke an immediate stop of the means feeding the crop material to the cutterhead. Thus is prevented that stray metal objects, which were picked up from the field, reach the cutterhead and cause serious damage to the knives and the shear bar. Portions of damaged knives might even get detached and cause even worse damage to other sections of the crop processing unit or the rest of the harvester. Small metal parts would be comminuted by the knives and mixed with the crop. When ingested by cattle, they cause serious harm to the stomachs and intestines and even cause death of the animal. The use of a foreign object detector and quick stop apparatus prevents such injuries.
Most metal detector sensors comprise a plurality a magnets providing magnetic flux lines extending into the crop feed path. Changes to the magnetic field are sensed by coils or Hall effect sensors which are arranged in the magnetic field, usually adjacent the magnets. The sensor may have an array of permanent magnets, some of which having a north pole and others a south pole extending towards the crop, as described in U.S. Pat. Nos. 4,433,528 and 5,343,676. Herein most flux lines flow from the one magnet to the other. In this manner the detection field is limited substantially to the area above the magnets. However, the sensitivity of the sensor decreases quickly with the vertical distance from the magnets, as the flux lines become less dense. As a result a small foreign object in the upper layer of the crop may pass undetected and reach the cutterhead.
A higher detection field can be realised by using magnets that all have the same polarity, as in U.S. Pat. No. 5,504,428 and DE-A-199 12 407. Such sensors are capable of detecting foreign objects in the top layers of the crop flow. However, it has been experienced that they also are less reliable because they frequently generate false detection signals in response to vibrations or movements of components of the harvester. For instance ferrous rollers or augers rotating in the vicinity of the sensor cause a substantial magnetic noise, which may include peak values exceeding the threshold value for triggering the quick stop apparatus. The operator may react to frequent false detections by raising the threshold value. As a result the sensor arrangement also becomes less sensitive to genuine signals caused by stray metal in the crop.
Hence, there is a need for a detector apparatus that is sufficiently sensitive to the presence of foreign objects in the crop flow, but is not lightly disturbed by the surrounding components of the harvester.