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Development of a novel mechatronic system for mechanical weed ...

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State <strong>of</strong> the art<br />

1998). This device consists <strong>of</strong> a 30 cm diameter rotating disk that rotates on an<br />

axis parallel to both the ground and direction <strong>of</strong> travel. Two knives, attached<br />

with springs opposite to each other on the perimeter <strong>of</strong> the disk, can stay<br />

retracted inside the disk area or they can fold out depending on the intensity <strong>of</strong><br />

the rotational speed, due to the balance between the centrifugal <strong>for</strong>ce and the<br />

spring <strong>for</strong>ce. The disk is actuated by a hydraulic motor whose exact rotational<br />

speed is controlled by a hydraulic controller. The controller adjusts the rotational<br />

speed value according to the detected plant position. The detection unit<br />

consists <strong>of</strong> a combination <strong>of</strong> three infrared transmitters and receivers placed<br />

above each other on a gantry on different plant height levels. A weighted<br />

summation <strong>of</strong> the signals from three sensors is combined to a single signal<br />

which is a base <strong>for</strong> processing.<br />

During the <strong>weed</strong>ing operation the motor rotates permanently at 850 rpm with<br />

knives extended to the working position, until the detection <strong>system</strong> does not<br />

detect a beet plant. After a beet has been recognised the rotational speed<br />

decreases to 700 rpm with almost immediate fold-in <strong>of</strong> the knives.<br />

At Cranfield University in Silsoe, UK, a rotating disk <strong>weed</strong>er has been<br />

developed (Dedousis et al. 2006). It consists <strong>of</strong> a thin rotating disc with a cut-out<br />

sector and a bevel cut back at its circumference (see Figures 2.5 and 2.6). The<br />

disc acts in a horizontal plane and operates at a fixed distance parallel to the<br />

plant row. This allows the disc to work the ground in both the intra- and inter-<br />

row areas. As the disk rotates, the <strong>weed</strong>s are cut by the sharp edge along the<br />

perimeter. The bevelled cut-out section allows <strong>for</strong> one plant per revolution to<br />

stay in the protected area without the contact with the disc. Assuming a plant<br />

distance <strong>of</strong> 300 mm, the ideal disc geometry was calculated to have a 175 mm<br />

diameter and 130° cut-out section. A bevel shape can be modified on the disk to<br />

achieve different kinds <strong>of</strong> <strong>weed</strong> cutting. Also, the inclination angle and working<br />

depth are adjustable depending on the plant type and soil conditions.<br />

Preliminary tests have shown that up to 60 % <strong>of</strong> <strong>weed</strong>s within an 80 mm radius<br />

can be eliminated, and up to 80% <strong>of</strong> <strong>weed</strong>s at greater radii. In a test in<br />

September 2006, 77 % <strong>of</strong> <strong>weed</strong>s were judged to be removed and only one plant<br />

out <strong>of</strong> 24 showed damage. Angular error <strong>of</strong> the disc was also quite low,<br />

normally less than 10°. A disadvantage <strong>of</strong> this tool is that it is limited to plant<br />

24

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