机械系毕业设计外文翻译原文农机.doc

外文原文Assessment of the Behaviour of Potatoes in a Cup-belt PlanterH. Buitenwerf，W.B. Hoogmoed，P. Lerink and J. Müller.Assement of the Behavior of Potato in a Cup-belt Planter. Biosytems. Engineering, Volume 95, Issue, September 2006: 3541The 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 influence 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 visualize the behavior 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 confined 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.Fig. 1. Working components of the cup-belt planter: (1)potatoes in hopper; (2) cup-belt; (3) cup; (4) upper sheave;(5) duct; (6) potato on back of cup; (7) furrower; (8) roller;(9) release opening; (10) ground levelCapacity 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 coefficient 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 modifications, 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 influence the efficiency 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 distinguished (Du & Sun, 2004; Tao et al., 1995; Zo ¨ dler, 1969). In the Netherlands grading of potatoes is mostly done by using the square mesh size (Koning de et al., 1994), which is determined only by the width and height (largest and least breadth) of the potato. For the transport processes inside the planter, the length of the potato is a decisive factor as well.A shape factor S based on all three dimensions was introduced: (1)where l is the length, w the width and h the height of the potato in mm, with howol. As a reference, also spherical golf balls (with about the same density as potatoes), representing a shape factor S of 100 were used. Shape characteristics of the potatoes used in this study are given in Table 1.2.2. Mathematical model of the processA mathematical model was built to predict planting accuracy and planting capacity of the cup-belt planter. The model took into consideration radius and speed of the roller, the dimensions and spacing of the cups, their positioning with respect to the duct wall and the height of the planter above the soil surface (Fig. 2). It was assumed that the potatoes did not move relative to the cup or rotate during their downward movement.Fig. 2. Process simulated by model, simulation starting when the cup crosses line A; release time represents time needed to create an opening sufficiently large for a potato to pass; model also calculates time between release of the potato and the moment it reaches the soil surface (free fall); r c , sum of the radius of the roller, thickness of the belt and length of the cup; x clear , clearance between cup and duct wall; x release , release clearance;a release , release angle ; o, angular speed of roller; line C, ground level, end of simulation The field speed and cup-belt speed can be set to achieve the aimed plant spacing. The frequency f pot of potatoes leaving the duct at the bottom is calculated as Where vc is the cup-belt speed in ms1 and xc is the distance in m between the cups on the belt. The angular speed of the roller r in rad s1 with radius r r in m is calculated asThe gap in the duct has to be large enough for a potato to pass and be released. This gap xrelease in m is reached at a certain angle release in rad of a cup passing the roller. This release angle release (Fig. 2) is calculated aswhere:rc is the sum in m of the radius of the roller, the thickness of the belt and the length of the cup; and xclear is the clearance in m between the tip of the cup and the wall of the duct.When the parameters of the potatoes are known, the angle required for releasing a potato can be calculated. Apart from its shape and size, the position of the potato on the back of the cup is determinative. Therefore, the model distinguishes two positions: (a) minimum required gap, equal to the height of a potato; and (b)maximum required gap equal to the length of a potato.The time t release in s needed to form a release angle a o is calculated asCalculating t release for different potatoes and possible positions on the cup yields the deviation from the average time interval between consecutive potatoes. Combined with the duration of the free fall and the field speed of the planter, this gives the planting accuracy.When the potato is released, it falls towards the soil surface. As each potato is released on a unique angular position, it also has a unique height above the soil surface at that moment (Fig. 2). A small potato will be released earlier and thus at a higher point than a large one.The model calculates the velocity of the potato just before it hits the soil surface u end in ms .1 . The initial vertical velocity of the potato u 0 in m s .1 is assumed to equal the vertical component of the track speed of the tip of the cup:The release height y release in m is calculated aswhere y r in m is the distance between the centre of the roller (line A in Fig. 2) and the soil surface.The time of free fall t fall in s is calculated withwhere g is the gravitational acceleration (9.8 m s2 ) and the final velocity v end is calculated aswith v 0 in ms .1 being the vertical downward speed of the potato at the moment of release.The time for the potato to move from Line A to the release point t release has to be added to t fall .The model calculates the time interval between two consecutive potatoes that may be positioned in different ways on the cups. The largest deviations in intervals will occur when a potato positioned lengthwise is followed by one positioned heightwise, and vice versa.2.3. The laboratory arrangementA standard planter unit (Miedema Hassia SL 4(6)was modified by replacing part of the bottom end of the sheet metal duct with similarly shaped transparent acrylic material (Fig. 3). The cup-belt was driven via the roller (8 in Fig. 1), by a variable speed electric motor. The speed was measured with an infrared revolution meter. Only one row of cups was observed in this arrangement.Fig. 3. Laboratory test-rig; lower rightpart of the bottom end of the sheet metal duct was replaced with transparent acrylic sheet;upper rightsegment faced by the high-speed cameraA high-speed video camera (SpeedCam Pro, Wein- berger AG, Dietikon, Switzerland) was used to visualise the behaviour of the potatoes in the transparent duct and to measure the time interval between consecutive potatoes. A sheet with a coordinate system was placed behind the opening of the duct, the X axis representing the ground level. Time was registered when the midpoint of a potato passed the ground line. Standard deviation of the time interval between consecutive potatoes was used as measure for plant spacing accuracy. For the measurements the camera system was set to a recording rate of 1000 frames per second. With an average free fall velocity of 2.5ms1 , the potato moves approx. 2.5mm between two frames, sufficiently small to allow an accurate placement registration.The feeding rates for the test of the effect of the speed of the belt were set at 300, 400 and 500 potatoes min -1 (f pot ¼ 5, 6.7 and 8.3s .1 ) corresponding to belt speeds of 0.33, 0.45 and 0.56ms -1 . These speeds would be typical for belts with 3, 2 and 1 rows of cups, respectively. A fixed feeding rate of 400 potatoes min -1 (cup-belt speed of 0.45ms -1 ) was used to assess the effect of the potato shape.For the assessment of a normal distribution of thetime intervals, 30 potatoes in five repetitions were used.In the other tests, 20 potatoes in three repetitions wereUsed.2.4. Statistical analysisThe hypotheses were tested using the Fisher test, asanalysis showed that populations were normally dis-tributed. The one-sided upper tail Fisher test was usedand a was set to 5% representing the probability of atype 1 error, where a true null hypothesis is incorrectlyrejected. The confidence interval is equal to (100-a)%.3. Results and discussion3.1. Cup-belt speed3.1.1. Empirical resultsThe measured time intervals between consecutive potatoes touching ground showed a normal distribution. Standard deviations s for feeding rates 300, 400 and 500 potatoes min -1 were 33.0, 20.5 and 12.7ms, respectively.Fig. 4. Normal distribution of the time interval (x, in ms) of deposition of the potatoes (pot) for three feeding ratesAccording to the F-test the differences between feeding rates were significant. The normal distributions for all three feeding rates are shown in Fig. 4. The accuracy of the planter is increasing with the cup-belt speed, with CVs of 8.6%, 7.1% and 5.5%, respectively.3.1.2. Results predicted by the modelFigure 5 shows the effect of the belt speed on the time needed to create a certain opening. A linear relationship was found between cup-belt speed and the accuracy of the deposition of the potatoes expressed as deviation from the time interval. The shorter the time needed for creating the opening, the smaller the deviations. Results of these calculations are given in Table 2.Fig. 5. Effect of belt speed on time needed to create openingThe speed of the cup turning away from the duct wall is important. Instead of a higher belt speed, an increase of the cups circumferential speed can be achieved by decreasing the radius of the roller. The radius of the roller used in the test is 0.055m, typical for these planters. It was calculated what the radius of the roller had to be for lower belt speeds, in order to reach the same circumferential speed of the tip of the cup as found for the highest belt speed. This resulted in a radius of 0.025m for 300 potatoes min .1 and of 0.041m for 400 potatoes min .1 . Compared to this outcome, a linear trend line based on the results of the laboratory measurements predicts a maximum performance at a radius of around 0.020m.The mathematical model Eqn (5) predicted a linear relationship between the radius of the roller (for r40.01m) and the accuracy of the deposition of the potatoes. The model was used to estimate standard deviations for different radii at a feeding rate of 300 potatoes min .1 . The results are given in Fig. 6, showing that the model predicts a more gradual decrease in accuracy in comparison with the measured data. A radius of 0.025m, which is probably the smallest radius technically possible, should have given a decrease in standard deviation of about 75% compared to the original radius.Fig. 6. Relationship between the radius of the roller and the standard deviation of the time interval of deposition of the potatoes; the relationship is linear for radii r40.01 m, K, measurement data; m, data from mathematical model; , extended for ro0.01 m; , linear relationship; R 2 , coefficient of determination3.2. Dimension and shape of the potatoesThe results of the laboratory tests are given in Table 3. It shows the standard deviations of the time interval at a fixed feeding rate of 400 potatoes min .1 . These results were contrary to the expectations that higher standard deviations would be found with increasing shape factors. 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