Injection depth has an important impact on the performance of a shallow manure injection system. Therefore, it should be maintained within narrow margins. For this purpose, an automatic electro-hydraulic depth control system was developed, which minimises the difference between the actual injection, measured by means of an ultrasonic sensor, and the desired injection depth. To minimise this difference the relative position of the shallow slurry injector attached to the four-point linkage at the back of a manure tanker was changed with respect to this tanker.
A grey box model was developed for the conversion of the voltage applied to an electronic control unit into the working depth of a shallow manure injector. A simplified and linearised model based on theoretical considerations was developed. The parameters of this model were estimated by means of frequency-domain identification techniques. This model contained all the dominant dynamics of the system that are relevant for control design. Since the dynamics of the system change during manure application, the worst case with respect to the stability of the depth control system was sought by comparing several extreme situations. The case of an empty tanker with the injector above the ground (minimal soil penetration resistance) was found to be the worst case and the model parameters were determined for this case. The resulting model contains an integrator and is of second order in the numerator, and fourth order in the denominator. Along with the non-linear time delay effect this model was found to be suitable for a model-based design of a stable and accurate injection depth controller.