Abstract: A forage harvester having a feed mechanism for transporting crop material along a feed path to a rotating cutterhead is provided with a first detector for detecting metal objects in the crop material and a second detector for detecting hard objects. The feed mechanism includes a hollow feed roll rotatable about a fixed shaft and both detectors are supported by the fixed shaft inside the feed roll. The first detector is a metal detector mounted inside a housing supported by the fixed shaft. The second detector is a thin film microphone for sensing air-borne sound waves produced when hard objects impact the feed roll. The second detector may be mounted on the housing or, alternatively, mounted on a support member attached to the fixed shaft. In either arrangement, a vibration-absorbing pad is provided to isolate the microphone from mechanical vibrations occurring in its supporting structure.

Abstract: A thin film highly directional audible sound sensor is mounted within a rotatable feed roll of a forage harvester to detect the presence of stones and other hard foreign objects in a crop material being fed past the feed roll to a cutter mechanism. Air-borne sound waves produced by the impact of foreign objects on metal parts of the harvester are detected by the sensor which produces a signal for stopping the crop feed mechanism before the foreign objects reach the cutter mechanism. The sensor is mounted on a stationary support within the rotatable feed roll. Output signals from the sensor control a stop mechanism for the crop feed mechanism via conductors extending through a stationary support shaft for the feed roll, thereby avoiding the need to commutate output signals from the detector.