A standard harvesting machine such as a field chopper has a pickup head that cuts a standing crop or picks up a windrow of previously cut crop, for instance corn, wheat, or soy, and a conveyor mechanism that feeds a stream of the cut crop back through treatment devices that typically crush and comminute it. The conveyor mechanism normally includes at least two rollers that are oppositely driven.
In order to protect the machinery from hard objects such as stones that might be entrained in the crop stream, one of the rollers is normally mounted so that it can be deflected against a spring force away from the other roller. Thus when a hard object is drawn in, the one roller will deflect laterally and allow it to pass without damage to the conveyor rollers.
While such a system protects the conveyor rollers themselves from harm, it does not protect the downstream equipment which is equally likely to be damaged by contact with the hard object. Therefore systems are provided that allow the equipment to be stopped or shut down when a hard object is encountered by the intake conveyor.
In East German patent 110,413 and 116,553 of Wehsely mechanical feelers are provided between the feed rollers that detect hard objects and operate mechanism for sorting out any hard object. Such systems require a relatively slow feed rate that greatly reduces throughput of the machine. In addition they are expensive and failure prone.
Another solution proposed in East German patent 114,893 of Wehsely proposes directing a bright light through the crop stream upstream of the feed rollers, since the crop is normally somewhat translucent and most hard objects are solidly opaque. This system requires that the light be very bright, so that when used with a dry crop like straw there is a severe danger of fire, making the system unsuitable for much field use.
In East German patent 111,534 also of Wehsely the pickup head's transverse conveyor is provided with a plurality of electronic sensors that if physically contacted by a hard object emit a signal that is used to stop the system and even eject the object. The problem with this system is that the foreign object is often masked in the crop stream so that it does not contact the sensors which, instead, remain in contact with the relatively soft crop stream.
U.S. Pat. No. 4,353,199 of Chow describes a harvesting-machine stone detector wherein the feed rollers are provided on their inner surfaces with piezoelectric acoustic sensors that produce a characteristic signal when the roller is struck by a hard object such as a stone. The resultant electrical signal is processed so that when a stone is detected the machine can be shut down or the crop stream can be temporarily diverted out of the machine to eject the stone. The orbiting piezoelectric sensors must be connected through failure-prone commutator rings with the processing circuit so that there is frequent failure of transmission of the signal to the circuit. Furthermore when the stone is well cushioned in the crop stream the sensors will not detect it.
German utility model 296 16 473 of Weiss describes a system where one of the intake rollers can be deflected transversely to the crop stream against the force of a fluid-type shock absorber. A pressure sensor in the absorber can detect the sudden change in pressure caused when a stone wedges between the rollers and forces them abruptly apart. This pressure sensor produces an output that can be processed to trigger stopping of the machine or ejection of the stone when the output of the sensor exceeds a predetermined threshold, indicating that a stone is trying to get between the rollers. The problem with this system is that it is only applicable to high-end machines with pneumatically or hydraulically damped feed rollers. Furthermore it is possible for a sudden pressure peak to be created by something other than a potentially dangerous stone, so that the system gives frequent false alarms.
Further such systems are described in European patent publications 0,217,417 and 0,217,418 both of Weiss which measure the acceleration of the feed rollers. The signal is processed, however, identically as in the above cited utility model, producing the disadvantage that, when the crop stream is moving slowly for fine chopping of the crop, a stone can get past the rollers while when the crop is moving rapidly there are frequent false alarms.