Original papers
Sensor and control for consistent seed drill coulter depth

https://doi.org/10.1016/j.compag.2016.07.029Get rights and content

Highlights

  • The novel position system detected the high-frequency drill coulter depth vibrations.

  • By coulter down pressure control, the low-frequency depth variations were minimised.

  • The system provided a mean depth deviation from the desired coulter depth of ±1.2 mm.

  • A three-position control system was found to be the best, cost-efficient solution.

Abstract

The consistent depth positioning of seeds is vital for achieving the optimum yield of agricultural crops. In state-of-the-art seeding machines, the depth of drill coulters will vary with changes in soil resistance. This paper presents the retrofitting of an angle sensor to the pivoting point of a drill coulter, providing sensor feedback to a control system that via an electro-hydraulic actuator delivers a constant coulter depth. The results showed a strong correlation between the angle of the coulter and the coulter depth under static (R2 = 1.00) and dynamic (R2 = 0.99) operations, verified by a sub-millimetre accurate positioning system (iGPS, Nikon Metrology NV, Belgium) mounted on the drill coulter. At a drill coulter depth of 55 mm and controlled by an ordinary fixed spring loaded down force, the change in soil resistance reduced the mean depth by 23 mm. By dynamically controlling the spring loaded down force based on the angle sensor, the mean depth was independent of the seedbed resistance change as shown from tests in soils ranging from sand to gravel. The PID controller was most effective because it providing a mean depth deviation from the target depth of −0.17 mm and +0.08 mm for sand and gravel, respectively. The most cost efficient control function was found to be the three-position control system, resulting in a mean depth deviation from the target depth of −0.89 mm and −1.18 mm for sand and gravel, respectively. A Fast Fourier Transform (FFT) analysis of the coulter depth measurements showed that the control system also provided a damping effect on the coulter depth variations. The research showed that it is possible to minimise the low-frequency drill coulter depth variations and provide a consistent coulter depth independent of soil conditions by using the developed sensor system and control system.

Introduction

A consistent seeding depth is vital for achieving the optimum yield of agricultural crops. In conventional seeding, the seedbed is prepared through the tillage operations, which ensure the correct ratio between temperature and moisture content to accelerate the seed germination (Forbes and Watson, 1992). The primary tillage can be executed by ploughing or deep harrowing (low-till) to an approx. 20–25 cm depth, and thereby removing residues and loosening the soil (Håkansson et al., 2002). Secondly, as part of the tillage for seedbed preparation, the seedbed is slightly compacted, and the top soil is loosened to a depth defined by the seed size and crop species, which in all creates a smooth and loose top soil horizon with the required aggregate size (Henriksson, 1989, Håkansson et al., 2002). Following the tillage, the objective is to place the seeds evenly and exactly at the bottom of the created seedbed in order to provide the optimal conditions for a quick germination (Håkansson et al., 2002, Karayel, 2008). By this placement, the seeds are able to absorb moisture from the compacted soil below, and independently of weather conditions and heat from above, which is transferable through the loose top horizon (Chang et al., 2004, Forbes and Watson, 1992). Research has shown that the seeding depth has a significant effect on germination and that the yield decreases with inaccurate seeding depth (Brennan and Leap, 2014, Kinsner et al., 1993, Morrison and Gerik, 1985). Crop emergence decreases with increased depth variations and a delay in germination generates the adverse effect of the crop having to compete with the other plants or weeds (Brennan and Leap, 2014). Different machines for drilling seeds in a variation of seedbed conditions have been studied and the conclusion is that large depth variations persists (Brennan and Leap, 2014, Morrison and Gerik, 1985). A seedbed most often inhabits irregular and inconsistent soil resistance, which causes drill coulters to generate vibrations and inconsistent seeding depths due to the static coulter down force. In this study, it was assumed that the correct tillage operations have been performed as the focus was on the seeding operation and the development of a system to control and maintain a consistent drill coulter depth, independently of irregularities in the seedbed and variations in soil resistance. However, a depth control system is also highly relevant for direct seeding.

The state-of-the-art seed drill depth control concept has been based on springs or weight for decades, creating a static down force. The springs are usually adjusted before seeding and no sensor feedback exists. Depth guide wheels in the proximity of the drill coulter avoid seed placements that is too deep but do not prevent the overly shallow seeding and depth variations of more than ±10 mm (SD) that are experienced (Hörner and Pütz, 2013). Recent innovations consist of sensors constantly measuring the pressure on the depth guide wheels. If the pressure on the depth guide wheels changes because of a change in speed or soil resistance, the automatic coulter pressure system responds immediately. The central hydraulic coulter pressure adjustment is used to ensure that the reconsolidation pressure of the depth-guide wheel always is constant, provided that the depth wheel is designed to always stay on top of the seedbed (Lemken Gmbh & Co., Germany). Coulters with or without depth guide wheels are affected by irregularities in the soil and operation speed (Hörner and Pütz, 2013). In addition, the design of the coulters has different influence on the seed trajectory depending on seed shape, velocity, angle and soil conditions/movements immediately after the seed is released from the coulter, which also leads to a undesirable inconsistent seed placement and emergence (Bufton et al., 1974). Recent studies of a coulter depth control system was developed and tested using multiple sensing inputs, detecting the soil surface and coulter positions (Suomi and Oksanen, 2015). The technology was able to maintain the desired working depth within a tolerance ±10 mm (SD) at a driving speed of 10 km/h.

The aim of this study was to demonstrate and evaluate the proof of concept for an applicable low cost depth control system for a seed drill coulter. The first objective was to examine and analyse the depth variations and vibrations by detecting the angle of the Suffolk drill coulter. The second objective was to design and implement a control system to manage coulter depth adjustments. The hypothesis was that angle detection of the drill coulter and a known seeder frame depth can be used for control and thereby obtain an even coulter depth, reducing the low-frequency vibrations, independently of soil resistance.

Section snippets

Materials and methods

The research was carried out at Research Centre Foulum in Denmark (9°34′40.22″E 56°29′47.617″N), using a rotating soil bin. The instrumentation for the experiments is shown in Fig. 1. The setup consists of a Compact Rio as an embedded controller (National Instruments Ltd., USA), programmed in LabVIEW interfaced by a laptop. For the reference positioning of the coulter position, the high-accuracy independent iGPS system was used (Nikon Metrology NV, Belgium). The prototype seeder system is

Results and discussion

For the coulter depth sensing, the sensor’s non-linearity of ±0.5% did not generate a considerable impact on the precision, even though it was the most dominating precision error in the system. It was determined in the error propagation (Eq. (10)) that the theoretical vertical distance precision (dε) for the long and short coulter resulted in ±3.3 mm and ±2.1 mm, respectively. This precision can be minimised by changing the sensor pre-determined sensitivity operating interval from 30° to 15°,

Conclusion

A novel coulter depth control system for maintaining a consistent drill coulter depth independent of soil resistance was developed and examined in a soil bin. By the ordinary fixed down force spring, the mean depth decreased by 23 mm due to change in soil resistance, when the seedbed was changed from sand to gravel. The dynamic down force control system of the coulter showed significant improvements and achieving the desire coulter depth independently of the soil resistance. The system requires

Acknowledgements

The authors acknowledge financial support from Aarhus University, Faculty of Science and Technology, Department of Engineering for the research presented. Thanks to Jens Kristian Kristensen and Peter Storegaard Nielsen for practical guidance and support during the experimental sessions. Thanks to the engineers at the Kongskilde R&D department and Agro Intelligence ApS for technical discussions.

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