Assessment of the Behaviour of Potatoes in a Cup-belt PlanterH. Buitenwerf; W.B. Hoogmoed; P. Lerink;J.MullerFarm Technology Group, Wageningen University, P.O Box. 17, 6700 AA Wageningen, The Netherlands;e-mail of corresponding author: willem.hoogmoedwur.nlKrone GmbH, Heinrich-Krone Strasse 10, 48480 Spelle, GermanyIB-Lerink, Laan van Moerkerken 85, 3271AJ Mijnsheerenland, The NetherlandsInstitute of Agricultural Engineering, University of Hohenheim, D-70593 Stuttgart, Germany(Received 27 May 2005; accepted in revised form 20 June 2006; published online 2 August 2006)The functioning of most potato planters is based on transport and placement of the seed potatoes by a cup-belt. The capacity of this process is rather low when planting accuracy has to stay at acceptable levels. The main limitations are set by the speed of the cup-belt and the number and positioning of the cups. It was hypothesised that the inaccuracy in planting distance, that is the deviation from uniform planting distances, mainly is created by the construction of the cup-belt planter.To determine the origin of the deviations in uniformity of placement of the potatoes a theoretical model was built. The model calculates the time interval between each successive potato touching the ground. Referring to the results of the model, two hypotheses were posed, one with respect to the effect of belt speed, and one with respect to the inuence of potato shape. A planter unit was installed in a laboratory to test these two hypotheses. A high-speed camera was used to measure the time interval between each successive potato just before they reach the soil surface and to visualise the behaviour of the potato.The results showed that: (a) the higher the speed of the cup-belt, the more uniform is the deposition of the potatoes; and (b) a more regular potato shape did not result in a higher planting accuracy.Major improvements can be achieved by reducing the opening time at the bottom of the duct and by improving the design of the cups and its position relative to the duct. This will allow more room for changes in the cup-belt speeds while keeping a high planting accuracy.1. IntroductionThe cup-belt planter (Fig. 1) is the most commonly used machine to plant potatoes. The seed potatoes are transferred from a hopper to the conveyor belt with cups sized to hold one tuber. This belt moves upwards to lift the potatoes out of the hopper and turns over the upper sheave. At this point, the potatoes fall on the back of the next cup and are conned in a sheet-metal duct. At the bottom, the belt turns over the roller, creating the opening for dropping the potato into a furrow in the soil.Capacity and accuracy of plant spacing are the main parameters of machine performance. High accuracy of plant spacing results in high yield and a uniform sorting of the tubers at harvest (McPhee et al., 1996; Pavek & Thornton, 2003). Field measurements (unpublished data) of planting distance in The Netherlands revealed a coefcient of variation (CV) of around 20%. Earlier studies in Canada and the USA showed even higher CVs of up to 69% (Misener, 1982; Entz & LaCroix, 1983; Sieczka et al., 1986), indicating that the accuracy is low compared to precision planters for beet or maize.Travelling speed and accuracy of planting show an inverse correlation. Therefore, the present cup-belt planters are equipped with two parallel rows of cups per belt instead of one. Doubling the cup row allows double the travel speed without increasing the belt speed and thus, a higher capacity at the same accuracy is expected.The objective of this study was to investigate the reasons for the low accuracy of cup-belt planters and to use this knowledge to derive recommendations for design modications, e.g. in belt speeds or shape and number of cups.For better understanding, a model was developed, describing the potato movement from the moment the potato enters the duct up to the moment it touches the ground. Thus, the behaviour of the potato at the bottom of the soil furrow was not taken into account. As physical properties strongly inuence the efciency of agricultural equipment (Kutzbach, 1989), the shape of the potatoes was also considered in the model.Two null hypotheses were formulated: (1) the planting accuracy is not related to the speed of the cup-belt; and (2) the planting accuracy is not related to the dimensions (expressed by a shape factor) of the potatoes. The hypotheses were tested both theoretically with the model and empirically in the laboratory.2. Materials and methods2.1. Plant materialSeed potatoes of the cultivars (cv.) Sante, Arinda and Marfona have been used for testing the cup-belt planter, because they show different shape characteristics. The shape of the potato tuber is an important characteristic for handling and transporting. Many shape features, usually combined with size measurements, can be