fig . shows a schematic side view of a self - propelled forage harvester 1 . the forage harvester 1 is equipped with , inter alia , working assemblies such as a front attachment 14 , an intake assembly 15 having an assigned sharpening device 8 , a conditioning device 16 , a post - accelerator 17 , a transfer device 18 and a driver &# 39 ; s cab 19 for accommodating an operator . during the harvesting operation , the forage harvester 1 is driven over a field and cuts plants from the field using the front attachment 14 , in order to feed them to the chopping assembly 2 by way of the intake assembly 15 . the chopping assembly 2 comprises a chopper drum 3 that is equipped with a plurality of knives and rotates opposite a stationary shear bar 4 . the design and mode of operation of the chopping assembly 2 are explained in greater detail in the following in association with fig2 and 3 . the crop is fragmentized as it passes through the chopping assembly 2 , is subsequently processed in the conditioning device 16 and accelerated in the post - accelerator 17 . the accelerated crop is then ejected out of the forage harvester 1 by way of a conveyor chute disposed behind the driver &# 39 ; s cab 19 and by way of the adjoining transfer device 18 . in the harvesting operation depicted in fig1 , the chopper drum 3 rotates in the counterclockwise direction in order to chop crop in interaction with the stationary shear bar 4 . the crop is conveyed from the intake assembly 15 as a pre - pressed crop mat in the direction of the chopper assembly 2 . since the knives ( not depicted in fig1 ) become worn during the harvesting operation , they should be sharpened as needed . the sharpening is carried out using the sharpening device 8 to remove material from the knife edges that have become dull . since material has been removed , the distance between the shear bar 4 and the edge of the knife that is effective in the chopping process has changed after the sharpening procedure . the inventive method and device provide for improved state detection of the chopper assembly 2 , more particularly , for the improved detection of the clearance position of the shear bar 4 with respect to the chopper drum 3 . improved detection of the state of the knives and the shear bar also is realized by the inventive device and method . fig2 shows a chopper assembly 2 , in a schematic side view that is installed in a forage harvester 1 according to fig1 . the chopper assembly 2 comprises a chopper drum 3 , which rotates about a rotational axis 24 , and a shear bar 4 which is adjusted relative to the chopper drum 3 . as an example , a cutting blade 7 attached to the chopper drum 3 is shown , which belongs to a plurality of further cutting blades ( not shown ) distributed around the circumference . when the chopper drum 3 rotates about the rotational axis 24 , the cutting blades 7 sweep over the outer enclosing circle of the chopper drum 3 , as indicated by the dashed lines . the radius of the enclosing circle is influenced by the sharpened state of the cutting blades 7 . the shear bar 4 comprises a shear bar carrier 25 on which a counterblade bar 26 is mounted . the shear bar carrier 25 is supported by pivot heads 27 with respect to a fixed ( e . g . on the frame of a forage harvester ) cross member 28 . the shear bar 4 is therefore able to swivel about a swivel axis that extends parallel to the rotational axis 24 , wherein the clearance position of the counterblade bar 26 relative to the cutting cylinder 3 is changeable by swiveling the shear bar 4 . fig3 shows , inter alia , in a schematic view , the chopper assembly 2 according to the line a - a shown in fig2 from the front . a first and a second vibration sensor 11 and 12 , respectively , are mounted at the shear bar carrier 25 at the two opposing end regions thereof . since fig2 is a side view from the left ( relative to the direction of travel of the forage harvester ), it only shows the first vibration sensor 11 , which is located on the left side of the machine . in the view depicted in fig3 , only four cutting blades 7 are shown of the cutting blades belonging to a plurality of further cutting blades ( not shown ) distributed around the circumference . the fig3 depiction also shows that the cutting blades 7 are fastened to the chopper drum 3 in two rows in a v - shaped configuration . fig2 and 3 show that the shear bar 4 is coupled to a control unit 20 , by way of which the clearance position of the shear bar 4 relative to the chopper drum 3 can be changed . the control device 20 comprises two actuators in the form of electric actuating motors 21 , 22 , each of which is assigned to one side of the shear bar 4 and coupled thereto by a lever arm 29 in each case . for this purpose , a lever arm 29 which extends transversely to the rotational axis 24 of the chopper drum 3 is attached at each end of the shear bar carrier 25 , laterally on the outside of the chopper drum 2 in each case . each lever arm 29 is equipped with a friction brake 30 . to this end , a passage is therefore formed at each lever arm 29 , which , as indicated using an oval shape in fig2 , is larger than the cross section of a fixed brake pin , which extends therethrough . the lever arm 29 moves , therefore , with an amount of play predetermined by the passage , in order to allow adjusting motions of the shear bar 4 . the play of the lever arm 29 is braked by a spring preload ( cf . fig3 ) applied to the brake pin . each of the dual - sided lever arms 29 is adjusted using a screw mechanism that is driven by a first actuating motor 21 and a second actuating motor 22 , respectively . each output axle of the motors 21 , 22 is designed as a threaded spindle which engages in a spindle nut 23 mounted at the end of the lever arm 29 facing away from the shear bar 4 . the clearance position of the shear bar 4 relative to the chopper drum 3 is therefore adjustable by actuating the motors 21 , 22 . since the motors 21 , 22 on both sides of the shear bar 4 are actuated independently of one another by way of a control device 13 , it is possible to freely adjust , within mechanical limits , the clearance position of the shear bar 4 . that is , the clearance as well as the orientation of the shear bar 4 ( the angle thereof relative to the chopper drum 3 ) is adjustable . the control device 13 also is connected to a sharpening device 8 for the knives 7 , which is known per se and will therefore not be described in greater technical detail . the control device 13 initiates and terminates sharpening procedures by way of appropriate signaling . the two vibration sensors 11 and 12 are connected via signal lines to a signal processing device in the form of a digital signal processor 5 . digital signal processor 5 is a microcontroller that operates at high frequency and which is suitable for filtering and analysis of input signals with a high sampling rate . that is , the signal processor 5 is suitable for comparing signals received from the vibration sensors 11 , 12 with one another , wherein , more particularly , transit time differences between signals from the first sensor 11 are determined with respect to signals from the second sensor 12 and / or temporally sequential signals from the same sensor 11 , 12 can be compared to one another . the mode of operation of the device depicted in fig3 is described as follows . in order to set a desired clearance between the shear bar 4 and the chopper drum 3 for the harvesting operation , for example , after a sharpening procedure has been carried out , the control device 13 activates , either automatically or manually by the machine operator , the actuating motors 21 , 22 in order to move the shear bar 4 toward the rotating chopper drum 3 . the actuating motors 21 , 22 are preferably operated simultaneously for this purpose . since the distance between the shear bar 4 and the chopper drum 3 continuously decreases , the counterblade bar 26 of the shear bar 4 eventually impacts at least one ( possibly several or even all ) of the rotating knives 7 of the chopper drum 3 . the contact induces a vibration in the shear bar 4 , the vibration source 6 of which is indicated , for example , by two dashed curves in fig3 . as shown therein , the contact occurs at a region of the counterblade bar 26 that faces the second sensor 12 ( on the left side in the illustration ). the vibration caused by the contact propagates from the vibration source 6 outward into the shear bar 7 as a structure - borne acoustic vibration having a certain velocity . since , in the case shown in fig3 , the vibration source 6 is located much closer to the sensor 12 than to the sensor 11 , the sensor 12 detects the vibration sooner than does the sensor 11 . for that matter , sensor 11 may not even detect the vibration at all if it has already been damped too much on the long path of propagation . the common signal analysis device 5 , which is ready to receive signals from both sensors 11 , 12 , deduces information about the origin of the vibration on the shear bar 4 on the basis of the presence of the signal from only the sensor 12 . in the simplest case , the signal analysis device 5 simply allocates the vibration to one of the two sides of the shear bar 4 , i . e ., in the present case , to the side of the sensor 12 where the presence of a vibration is detected , since the side of the sensor 12 strikes the chopper drum 3 . in order to correct the tilted position of the shear bar 4 , which was determined as described , with respect to the chopper drum 3 , the control device 13 connected to the signal analysis device 5 actuates the actuating motors 21 , 22 in such a way that a parallel position of the shear bar 4 is attained . to this end , for example , the motor 22 could be halted while the motor 21 continues to operate until a contact signal also is received from the sensor 11 . the receipt of signals from both sensors 11 , 12 therefore serves as an indicator for a parallel position of the shear bar 4 with respect to the chopper drum 3 . the signal analysis device 5 derives information about the origin of the vibration on the shear bar 4 from the points in time at which signals arrive from the sensors 11 , 12 , which are in different locations . by carrying out a suitable evaluation , it is therefore possible to perform a quantitative position determination of the vibration source that goes beyond a merely qualitative allocation to a particular side . to this end , the signal analysis device 5 determines a time offset in the sense of a transit time difference between the signals from the two sensors 11 , 12 . since the position of the sensors 11 , 12 and the design - related sound transit time in the shear bar 4 are known , the signal analysis device 5 is capable of quantitatively calculating the position of the vibration source 6 on the shear bar 4 on the basis of the transit time difference that was calculated . the invention thereby precisely controls the actuating motors 21 , 22 in order to rapidly and accurately reach a parallel clearance position with respect to the chopper drum 3 . in deviation from the example shown , a vibration source could lie in another region of the shear bar 4 . for example , if vibrations were caused in the central region of the shear bar 4 , the vibration would occur at both sensors 11 , 12 nearly simultaneously . the signals transmitted to the signal analysis device 5 would be simultaneous in this case . alternatively or in addition to a comparison of the time of arrival , it is also possible to analyze the signals that are received with respect to the duration thereof . for example , on the basis of relatively long signals that are caused by the tilted position of the knives 7 on the chopper drum 3 it can be deduced that contact takes place not only at one point , but also that the shear bar 4 and chopping blades 7 are in contact across a broader region . the signal analysis device 5 may be used to initially filter out the contact signals caused by different knives 7 during rotation of the chopper drum 3 and to then compare them with one another in order to deduce therefrom the knife state and / or the state of the shear bar . to simplify the assignment of contact signals to individual knives 7 , the chopper assembly 2 can be connected to a ( not shown ) rotational position sensor that transmits a rotational position of the chopper drum 3 to the signal analysis device . if different signals ( amplitude , signal length ) at the knives 7 are detected during a pause in chopping , for example , this can be an indicator that a sharpening procedure is required since individual knives 7 obviously have geometries that differ from one another . such a sharpening procedure could be triggered automatically , for example , by the control device 13 activating the sharpening device 8 . however , if signals that are nearly the same ( comparable amplitude , signal length ) are detected at different knives 7 after a sharpening procedure , this is an indication that a sharpening procedure can be terminated since the knives 7 now have the same geometry once more . if a signal interruption is measured at the same point in time at all knife contacts after a sharpening procedure that cannot be eliminated by further sharpening , this is an indication that the shear bar 4 itself is worn . in general , an output device 9 connected to the signal processor 5 can be provided , which outputs all types of states , which have been determined by the signal processor 5 , to the machine operator in an acoustic and / or visual form . the following list of reference signs of various elements mentioned above is included ( as follows ), for ease of explanation : as will be evident to persons skilled in the art , the foregoing detailed description and figures are presented as examples of the invention , and that variations are contemplated that do not depart from the fair scope of the teachings and descriptions set forth in this disclosure . the foregoing is not intended to limit what has been invented , except to the extent that the following claims so limit that .
a method for state detection of a cutting device for agricultural crop is described . the cutting device is formed with a cutting tool set into rotation with respect to a shear bar operates such that a clearance position of the shear bar is changeable with respect to the cutting too . the method includes assigning at least two interspaced sensors for detecting vibrations to the shear bar caused by the cutting tool upon contact with the shear bar , transmitting signals generated by the sensors to a common signal analysis device and , the common signal analysis device comparing the signals and determining a state of the cutting device .