Arrangement for measuring the sharpness of shredder blades

Abstract

System for detecting the sharpness of a plurality of knives (48) distributed around the circumference of a chopping drum (22) rotatable for displacing the knives (48) on a path adjacent to a counter-blade (38), comprising: a distance sensor configured to detect the width (d) of a gap between the counter-blade (38) and a circumference described by the cutting edges of the rotating knives (48); a magnetic sensor (54) with a magnet (58) arranged to generate a magnetic flux in the gap between the counter-blade (38) and the cutting edges of the knives (48), and a flux sensor (60) configured to provide an electrical signal (v) representing an influence of the passing cutting edges on the magnetic flux in the gap;and an evaluation unit connected to the distance sensor and the magnetic sensor (54), characterized in that the evaluation unit is configured to calculate a radius (r) representing the sharpness of the cutting edges according to the following equation: r = exp((v + c1 * d + c2) * c3), where c1, c2 and c3 are constants and exp is the exponential function.

Description

The invention relates to an arrangement for detecting the sharpness of shredder blades. State of the art Forage harvesters are used in agriculture to cut or collect crops, shred them into small pieces, and transfer the shredded crops onto a trailer. The shredded crop is used as animal feed or for biogas production. The cutting operation is carried out by a rotating drum with a number of knives distributed around its circumference, which work in conjunction with a counter-blade. The distance (i.e., the gap) between the circumscribed circle of the rotating shredding blades and the counter blade, on the one hand, and the sharpness (i.e., the radius of the cutting edge) of the blades, on the other, are the most important parameters for the cutting quality and the power required for shredding. It is therefore desirable to provide a suitable sensor for detecting the distance between the counter blade and the blades to enable automatic or manual adjustment of this distance. This or another sensor should also be capable of detecting the sharpness of the blades so that they can be sharpened or replaced if necessary, i.e., when they are dull or worn. The distance between the circumscribed circle of the rotating blades and the counter blade is usually adjustable by electric motors that move the counter blade relative to the chopping drum. Inductive sensors have been described for measuring the distance between the counter blade and the chopping blades. These sensors comprise a permanent magnet connected to the counter blade and an induction coil in which an electromotive force (EMF) is induced when the chopping blade passes by. This electromotive force is amplified and then measured. In such an arrangement, described in EP 0 943 888 A2, the induced voltages are subjected to frequency analysis. The ratio of the high-frequency components of the signal spectrum to the low-frequency components is derived. The quotient determined in this way provides information about the distance between the counter blade and the shredding knives. This information can be used to automatically move the counter blade into a suitable position with a sufficiently small gap between the counter blade and the knives. On the other hand, sharpness sensors have been proposed that also use inductive sensors and are based on the assumption that the shorter the pulse induced in the sensor coil by the passing blade, the sharper the blade. Reference is made to the disclosures DD 286 735 A5, DD 286 737 A5, and DE 10 2011 005 317 A1. However, experiments have shown that the amplitude and length of the induced pulses also depend on the distance between the circumscribed circle of the rotating blades and the counter-edge, as this distance influences the location where the blades enter and exit the sensor's sensitive area. Therefore, it was not possible to define a reliable relationship between the sensor's pulse and the sharpness of the shredding blade that would allow a sharpness value to be derived with sufficient accuracy. Task The present invention aims to solve the problem of providing an improved arrangement for determining the sharpness of shredder blades compared to the prior art. Description of the invention According to the present invention as defined in the claims, an improved arrangement for detecting the sharpness of the knives of a shredding drum is provided. The system for detecting the sharpness of a plurality of knives distributed around the circumference of a chopping drum rotatable for displacing the knives on a track adjacent to a counter-blade comprises: a distance sensor configured to detect the width of a gap between the counter-blade and a circumference described by the cutting edges of the rotating knives; a magnetic sensor with a magnet arranged to generate a magnetic flux in the gap between the counter-blade and the cutting edges of the knives, and a flux sensor configured to provide an electrical signal v representing an influence of the passing cutting edges on the magnetic flux in the gap;and an evaluation unit connected to and configured with the distance sensor and the magnetic sensor to calculate a radius r representing the sharpness of the cutting edges according to the following equation: where c1, c2 and c3 are constants and exp is the exponential function.; The electrical signal v, which represents an influence of the passing cutting edges on the magnetic flux in the gap, can be the peak voltage or the length of a pulse induced in the flux sensor by a passing cutting edge. The flux sensor is preferably a coil. The magnetic sensor can be mounted in a bore of the counter-cutting edge or on the counter-cutting edge itself. The distance sensor can be a magnetoresistive sensor, a flux sensor, or an optical sensor. The evaluation unit is preferably connected to a display unit to show the detected radius and / or to send a grinding signal to an operator as soon as the detected radius exceeds a predetermined value. The evaluation unit can also be connected to a grinding device to start a grinding process as soon as the detected radius exceeds a predetermined value. embodiment The drawings depict an embodiment of the invention, which is described in more detail below. Fig. 1 is a schematic left-hand view of a forage harvester on which the system according to the invention can be used. Fig. 2 is a schematic representation of a system according to the invention for detecting the sharpness of a plurality of blades. Fig. 3 is a diagram showing a number of voltage curves as detected by the flow sensor at different cutting edge radii. Fig. 4 is a diagram showing a number of voltage curves as detected by the flow sensor at different cutting gaps. Fig. 5 is a flowchart showing the procedure used by a microprocessor to determine the cutting edge radius. A harvesting machine, shown in Fig. 1 as a self-propelled forage harvester 10, is built on a frame 12, which is supported by front and rear wheels 14 and 16. The harvesting machine 10 is operated from a cab 18, from which a pickup 20 is visible. Material picked up from the ground by the pickup 20, such as grass or the like, is fed to a rotatable chopping drum 22, which is equipped with chopping knives 48 distributed around its circumference. The chopping knives chop the crop into small pieces and feed it to a conveyor 24. The material leaves the harvesting machine 10 through an adjustable discharge spout 26 onto a trailer traveling alongside. A grain processor 28, through which the crop is fed tangentially to the conveyor 24, extends between the chopping drum 22 and the conveyor 24.However, the grain processor is usually removed from the forage harvester 10 during the grass harvest and is only used for maize harvesting. Between the intake 20 and the chopping drum 22, the material is conveyed by lower pre-compression rollers 30, 32 and upper pre-compression rollers 34, 36. The knives 48, distributed around the circumference of the chopping drum 22, interact with a counter-blade 38 to chop the material. The counter-blade 38 is equipped at both ends with adjustment devices 40 designed to move the counter-blade 38 horizontally toward and away from the chopping drum 22 to adjust the width of the cutting gap. A suitable control procedure for the adjustment devices 40 is described in US 7,222,804 A, the contents of which are incorporated into these documents by reference. A grinding device 50 with a grinding wheel is provided for sharpening the knives 48 as needed. Reference is now made to Fig. 2, which shows the counter blade 38, the adjusting device 40, and a knife 48 in more detail. The adjusting device 40 comprises an electric motor arranged to move a connecting rod 52 and thus the counter blade 38 towards and away from the circumscribed circle defined by the rotating knives 48 of the chopping drum 22. The cutting edges of the knives 48 have a radius r, which increases with operating time due to wear and can be reduced by grinding with the grinding device 50. The cutting edges of the knives 48 pass the counter blade 38 at a distance (gap width) that can be varied by the adjusting device 40. The present invention relates to the detection of the distance d and the radius r. For this purpose, a magnetic sensor 54 is arranged in a bore 56 in the counter blade 38. The bore 56 can be located in the surface of the counter blade 38 facing the blades 48, as shown in Fig. 2, or in its second side face (shown on the left in Fig. 2), or on the top or bottom surface of the counter blade 38, or separately from the counter blade 38 but magnetically connected to it. The magnetic sensor 54 comprises a permanent magnet 58 and a magnetic flux sensor 60 in the form of a coil wound around the permanent magnet 58. The permanent magnet 58 could be replaced by an electromagnet. The magnetic flux sensor 60 could be spaced apart from the permanent magnet and, for example, mounted on the bottom surface of the counter blade 38. A Hall sensor could also be used instead of a coil. The permanent magnet 58 generates a magnetic flux within the counter-cutting edge 38, which is made of a magnetically conductive material such as steel. The magnetic flux also extends into the gap between the counter-cutting edge 38 and the blades 48 and is detected by the magnetic flux sensor 60. The blades 48 are also made of a magnetically conductive material, and thus a blade 48 passing through the gap changes the magnetic flux within the gap and also the magnetic flux within the flux sensor 60. This change in magnetic flux induces a voltage in the magnetic flux sensor 60. Fig. 3 shows the time-dependent voltage v provided by the magnetic sensor 60 when a knife 48 passes by, for knives 48 with different radii r of the cutting edges, but with the same gap distance d from the counter-edge 38. The radius r thus clearly influences the peak voltage vSS and the width Δt of the pulse, with increasing width and peak voltage as the radius increases. The peak voltage vSS and the width Δt of the pulse are related by the following formula: where a and b are constants. On the other hand, the voltage v provided by the magnetic flux sensor 60 also depends on the distance d, as shown in Fig. 4 for a number of different distances but constant radii r. The peak voltage vSS and the pulse width Δt increase with increasing distance d. Equation (1) also applies here. The magnetic flux sensor 60 is connected to an input of an amplifier 62, the output of which is connected to the input of an analog-to-digital converter 64. The output of the converter is in turn connected to a microprocessor 68, on which a program for evaluating the output voltage of the magnetic flux sensor 60 is running. The purpose of the microprocessor 68 is to evaluate the digitized signals of the magnetic flux sensor 60 and to determine the radius r of the cutting edges of the blades 48, thereby solving the problem of the dependence of the signals on both the radius r and the distance d, the latter having an even greater influence on the voltage v. The amplifier 62, the analog-to-digital converter 64, and the microprocessor 60 form an evaluation unit for determining the gap d and the radius r. The evaluation unit, including the microprocessor 66, operates according to the diagram shown in Fig. 5. After starting in step 100, the distance 102 is evaluated in step 102. This can be done according to the method described in EP 0 943 888 A2, the disclosure of which is incorporated into the present documents by reference, i.e., by dividing the high-frequency components of the voltage v of the magnetic flux sensor 60 by its low-frequency components (or vice versa) and deriving the distance d from this. Alternatively or additionally, the distance d can be detected by another sensor (not shown), e.g., a magnetoresistance sensor attached to the counter-edge 38, which detects the flux generated by the magnet 58, or by an optical sensor (DE 103 46 412 A1, the disclosure of which is incorporated into the present documents by reference). Another way to determine the distance d is to link the distance measurement with the time of the knife 48 passing the magnetic sensor 54. This is typically a lock-in amplifier approach and provides the best option for improving the noise-to-signal ratio of the sensor 54. The time course, and thus the distance, can be derived solely from the signal of the magnetic sensor 54 (as shown in Fig. 4) or by comparison with a signal from a sensor that detects the respective rotation angle of the chopping drum 22, in order to determine the time at which the knife 48 enters the magnetic field and thus the distance. In the following step 104, the peak voltage vS is derived from the output signal of converter 64. Alternatively or additionally, the pulse width Δt can be derived from the output signal of converter 64 and converted into the peak voltage vS based on equation (1), or vice versa. Then, in step 106, the radius r is calculated using the following formula: where c1, c2, and c3 are constants and exp is the exponential function. The values ​​of the constants c1, c2, and c3 are determined by test measurements. If, in step 104, Δt is determined instead of vSS, equation (1) can be substituted into equation (2). Equation (2) and step 106 thus allow the determination of the radius r of the cutting edge of the knives 48 as soon as vSS or Δt on the one hand and the gap width d of the cutting gap on the other hand are known. The influence of the cutting gap d is thus taken into account and cannot adversely affect the accuracy of the measurement. It should be noted that steps 102 and 104 are typically performed for a sufficiently long acquisition time, and average values ​​for d and vSS are determined and finally used in step 106. During this acquisition time, the rotational speed of the chopping drum 22 should be constant and correspond to a predetermined value. If the rotational speed should change, this is preferably taken into account during the evaluation. After step 106, the distance d and the radius r are displayed on a display unit 68 in the cabin 18, so that the operator can initiate a grinding operation of the grinding device 50 as soon as the radius r exceeds a predetermined threshold value, or this is initiated automatically (steps 108 and 112 in Fig. 5). Having described the preferred embodiment, it is evident that various modifications are possible. For example, the analog-to-digital converter 64 and the microprocessor 66 could be replaced by a purely analog circuit. The result of step 102 can be used to automatically initiate a gap adjustment procedure when the distance d exceeds a predetermined limit. Finally, it should be noted that two or more magnetic sensors 54 can be distributed along the length of the counter blade 38, all of them being connected to the evaluation unit. Drawing text for Fig. 5: 100 Start 102 Distance d 104 Vss 106 Radius r 108 r > Threshold? 110 End 112 Warning on display and / or sharpening

Claims

System for detecting the sharpness of a plurality of knives (48) distributed around the circumference of a chopping drum (22) rotatable for displacing the knives (48) on a path adjacent to a counter-blade (38), comprising: a distance sensor configured to detect the width (d) of a gap between the counter-blade (38) and a circumference described by the cutting edges of the rotating knives (48); a magnetic sensor (54) with a magnet (58) arranged to generate a magnetic flux in the gap between the counter-blade (38) and the cutting edges of the knives (48), and a flux sensor (60) configured to provide an electrical signal (v) representing an influence of the passing cutting edges on the magnetic flux in the gap;and an evaluation unit connected to the distance sensor and the magnetic sensor (54), characterized in that the evaluation unit is configured to calculate a radius (r) representing the sharpness of the cutting edges according to the following equation: r = exp ( ( v + c 1 * d + c 2 ) * c 3 ) ,; where c 1 , c 2 and c 3 constants are and exp is the exponential function. System according to claim 1, wherein v is the peak voltage vSS or the length Δt of a pulse of the flow sensor (60) induced by a passing knife edge. System according to claim 1 or 2, wherein the flow sensor (60) is a coil. System according to one of claims 1 to 3, wherein the magnetic sensor (54) is arranged on the counter blade (38) or in a bore (56) provided therein. System according to one of claims 1 to 4, wherein the distance sensor is a magnetoresistive sensor, the flux sensor (60) is an optical sensor. System according to one of claims 1 to 5, wherein the evaluation unit is connected to a grinding device (50). System according to one of claims 1 to 6, wherein the evaluation unit is connected to a display device (68) for displaying the detected radius and / or a loop signal. Field chopper (10) with a rotatable chopping drum (22) with a plurality of knives (48) distributed around its circumference, which can be set into motion on a path adjacent to a counter blade (38) and a system according to one of the preceding claims.

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