Manipulation of the spreader pattern produced by spinning disc fertilizer spreaders, for site-specific applications, requires a continuous change of several spreader parameters, such as orifice position and angular speed of the disc, to keep the working width and the shape of the spreader pattern constant for varying fertilizer rates. To design an efficient control system, a model of the system is developed which takes account of spreader and particle parameters. The model is used for calculations of spreader patterns and forms the basis for further development of a spreader control system. An initial validation comparing measured and simulated spreader patterns, demonstrates that the mathematical model approximates reality quite well. By means of the model, spreader patterns for single- and twin-disc spreaders are calculated and the influence is examined of several spreader characteristics on the distribution pattern. Simulations for variations of angular velocity of the disc as well as the effects of a changing position of the orifice, referred to the driving direction, show a large effect on the spreading width and the shape of the distribution pattern. The shape of the orifice also has a large influence on the shape and the width of the distribution pattern. The simulation results further show that spreading site-specifically, simply by changing the mass flow, leads to a fluctuating spreader pattern, which results in large deviations from the intended application rates. (C) 1996 Silsoe Research Institute.
