Performance of Energy Absorbing Materials
for Passive Bulk Bin Filling
B. Kliethermes1, W. Messner1, A. Leslie2, T. Baugher2, D. M. Glenn3
R. Rohrbaugh2, J. Koan2,K. Lewis4, P. Heinemann2
1Carnegie Mellon University Robotics Institute
2The PennsylvaniaStateUniversity
3USDA-ARS Appalachian Fruit Research Station
4WashingtonStateUniversity
Abstract
To increase efficiency and allow mechanization of apple harvest, a device is needed to fill bulk bins in the field without causing damage to the fruit. Three experiments were conducted to determine materials and designs suitable for constructing a passive bin filling device. Two concepts for achieving the desired functionality of such a device are (1) grates of granular materials that absorb energy while still allowing fruit to pass through them, and (2) mats of energy absorbing foams. These mats could be used in combination with a grate and would cover any hard materials within the device. In Experiment I, six configurations of energy absorbing grates were tested by dropping apples through them onto a layer of stationary apples. The performance of these grates was evaluated with respect to bruise width, depth, and volume; each configuration proved capable of significantly reducing the chance of bruising. In Experiment II, three different foams were evaluated by dropping apples from various heights onto small samples covering a piece of wood. Bruising was negligible until the threshold height of just under 1 meter (39 inches). The final experiment was a performance analysis for two prototype bin filling systems. The experiment indicated that fruit drop height should be maintained near two inches and that fruit singulation from picking to bin filling yielded the highest amount of bruise free fruit.
Introduction
Labor saving technologies such as full mechanization of harvest or less radical harvest assist technology could greatly increase the efficiency of apple harvest. One major obstacle to any technology for improving efficiency is the need to collect the fruit in a bulk container after it has been removed from the tree without causing significant damage. The standard method of fruit harvest involves having harvest employees fill a picking bag with fruit. Once the bag is full, they carefully empty their bags into a bulk bin. There is potential for substantial economic and ergonomic improvements if picking bags can be removed from the harvest process, and allow for a more continuous process.
The problem lies in the physiology of an apple. Apples are much more fragile and sensitive to bruising under fast loading (Baugher, 2006). If an apple is allowed to fall freely rather than delicately lowered into a bin, it will have a considerable amount of kinetic energy when it either hits the bottom of the bin or other apples. This kinetic energy must be absorbed by something. Without any additional materials present, this energy will be absorbed during a very short period of time in a relatively small area by either the falling apple itself, or the apples already in the bin. If materials are present during the fall that can absorb this energy before the fruit reach the bottom of the bin, bruising can be prevented.
The objective of this study was to determine if a device employing an energy absorbing material, or combination of such materials, could be designed to absorb enough energy of harvested fruit dropped onto it to prevent bruising when the apple struck a layer of apples on a hard surface below it. Construction of a successful prototype would support the possible future development of a passive bin filling device for use in orchards. This device would ideally be used in combination with other harvest assist technology, primarily a harvest platform (Baugher et al., 2009). One system that was modeled, presented in Figure 1, is a granular absorption medium apple distributor design. In this system, the kinetic energy of the fallings is absorbed by cubes (or other shapes) of energy absorbing foam or elastic cords resting on top of or suspended from a grating. The second system that was modeled is a pneumatic self-adjusting design shown in Figure 2.The principle behind the design is to alternate the inflation of two or more sets of cylinders. The cylinders themselves absorb the energy of the dropping apples by keeping the air pressure in the cylinders to a minimum. With these devices, ladders and picking bags could be eliminated all together from the apple harvest, allowing workers to pick continuously from the beginning to the end of a row.
Figure 1. Conceptual model of a granular absorption medium apple distributor design.
Figure 2. Conceptual model of a pneumatic self-adjusting apple distributor.
Materials and Methods
Experiment I: Energy Absorbing Grates
We tested six configurations of energy absorbing grates consisting of variations of three different materials for their ability to passively handle apples without causing damage. The experiment included four replications of six configurations. In each replication eight apples [of 2 ¾ to 3 inch size) passed through that particular configuration. Trials were conducted with Delicious, a variety with low susceptibility to bruising, and Golden Delicious, a variety with high susceptibility to bruising.
We designed an apparatus that allowed the apples to free fall in a semi-random direction onto the uppermost layer of energy absorbing material. Each type of material was arranged in a two-foot square wooden frame. Fastened to a similar frame was a padded ramp at a 15° incline. This ramp (Figure 3) was tapered with padded dividers at one end, which directed the apples in each replicate in a random direction. A rack (Figure 4) allowed the square frames to be inserted at any one of four positions with the ramp on top. The separation between the positions was 13 cm (5 in). The racks were placed every 13 cm from the bottom of the bin so that the materials could be arranged in different configurations. For each experiment, the bottommost frame was on the lowest rack and just above the bottom layer of apples, while the ramp was one rack above the uppermost material.
The three energy absorbing materials were 6.3 cm (2.5 in) diameter hard foam balls strung on elastic (rubber) bands, 7.6 cm (3 in) diameter soft foam balls strung on rubber bands, and rubber bands alone. The rubber bands were stock materials ordinarily used for training apple trees. The foam balls were purchased from toy departments.
The six configurations were as follows: one layer of 48 hard foam balls (Figure 5), one layer of 36 soft foam balls (Figure 6), one layer of each type of ball, two layers of rubber bands (Figure 7), one layer of each type of ball with one layer of rubber bands, and one layer of each type of ball with two layers of rubber bands. In the bottom of the testing apparatus was a single layer of apples onto which the test apples fell after passing through the energy absorbing materials. A control value for bruising was determined by allowing the apples to fall onto the layer of apples without any of the energy absorbing materials present.
Figure 3. Padded ramp with one replicate of apples.
The apples were inspected for previous damage, and any bruises were circled with a permanent marker to distinguish them from bruises resulting from the experiment. The apples were left at room temperature for approximately one hour before the experiment. For each of the four replicates, eight apples were placed in the ramp and allowed to drop all at once. If any apples stopped, the frames were shaken slightly until the apples passed to the bottom. The apples then sat at room temperature overnight until they were inspected for bruising. The skin over the damaged area was peeled back and the bruise diameter was measured. The apples were then cut through the bruise and the bruise depth measured. Levels of downgrading due to bruising were determined based on USDA Grades and Standards (Table 1).
Figure 4. Testing apparatus with multiple racks.
Figure 5. Hard foam balls.
Figure 6. Soft foam balls.
Figure 7. Rubber bands.
Experiment II: Energy Absorbing Foams
Three different types of foams were subjected to a bruise threshold test (Hyde et al., 2003) to determine the ability of each to absorb energy and therefore prevent bruising. Rome and Fuji varieties were used for this experiment. The three types of foam were PORON® brand foams donated by the Rogers Corporation.
Foam Type / Thicknessresilient / 12.7 mm (1/2 in)
energy absorbing / 10.8 mm (~7/16 in)
combination / 19.0 (3/4 in)
Apples were dropped from heights of 15 cm (6 in), 30 cm (12 in), 40 cm (15 in), 60 cm (25 in), 100 cm (39 in), and 160 cm (63 in) onto each type of foam using the same ramp apparatus as before (Figure 8). Apples were also dropped from each height onto a pine board to determine a control value for comparison. These apples were allowed to sit overnight at room temperature. The skin over each bruise was peeled back and the bruise diameter measured. The apples were cut through the bruise and the bruise depth measured.
Figure 8. Bruise threshold test for PORON® foams.
Experiment III – Full Scale Bin Filler Prototypes
Two full scale bin filler prototypes were tested in Experiment III. The first bin filler design was an energy absorbing grate (Figure 9) constructed from nylon bungee cords with a foam base pad. The second system was a pneumatic self-adjusting bin filler design (Figure 10) constructed with inflatable bladders. The study included four replications of three drop height configurations—2.5, 5, and 10 cm (1, 2, and 4 in), respectively. In each replication eight apples (of 2 ¾ to 3 inch size) passed through each particular bin filler configuration. Trials were conducted with Golden Delicious, a variety with high susceptibility to bruising. The testing apparatus was designed to allow the apples to free fall in a semi-random direction onto the uppermost layer of energy absorbing material. Additionally, one incomplete layer of red apples was placed on the bottom of the bin allowing room for the test apples to distribute. The test apples were placed on a padded ramp at a 15° incline with padded dividers to prevent bruising prior to departure from the ramp.
The energy absorbing grate bin filler contained two layers of nylon bungee cords 9 mm (3.6 in) in diameter configured approximately at a spacing of 3.75 cm (1.5 in). The drop height between the two layers of bungee cords was 7.5 cm (3 in) and the drop height to the foam mat was 9 cm (3.5 in).
The pneumatic self-adjusting bin filler contained two layers of 10 cm (4 in) plastic inflated bladders. The pressure in the upper bladder layer was slightly lower than the pressure in the bottom bladder layer. This enabled the system to absorb the largest amount of energy without causing apples to bounce upon impact.
A control value for bruising was determined by allowing the apples to fall onto the incomplete layer of apples without any of the energy absorbing materials present. For the singulated trials, each apple was released individually from the ramp to completely isolate any initial bruising from apple collisions between the end of the ramp and the bin filler.
The apples were inspected for previous damage, and any bruises were circled with a permanent marker to distinguish them from bruises resulting from the experiment. The apples were left at room temperature for approximately one hour before the experiment. For each of the four replicates, eight apples were placed in the ramp and allowed to drop all at once. The apples remained at room temperature overnight until they were inspected for bruising. The skin over the damaged area was peeled back and the bruise diameter was measured. Then the apples were cut through the bruise to measure the bruise depth. Levels of downgrading due to bruising were determined based on USDA Grades and Standards (Table 1).
Table 1. Classification of bruise damage, based on USDA Grades and Standards.
Class / USDA fresh market standard / Bruise specifications1 / “Extra Fancy” / No bruising
2 / “Extra Fancy” / Bruise diameter ≤ 3.2 mm (1/8 in)
3 / “Extra Fancy” / Bruise diameter 3.2 mm (1/8 in) to 6.4 mm (¼ in)
4 / “Extra Fancy” / Bruise diameter 6.4mm (¼ in) to 12.7 mm (½ in) or area of several bruises ≤ 127 mm2
5 / “Fancy” / Bruise diameter 12.7 mm (½ in)to 19 mm (3/4 in)
6 / Downgraded / Bruises larger than the tolerances in “Fancy”
7 / Downgraded / Cuts or punctures of any size
Figure 9. Top view of the energy absorbing grate bin filler.
Figure 10. Pneumatic self-adjusting bin filler with test ramp.
Statistical Analyses
All data were subjected to analysis of variance. Mean separations were conducted using Fisher’s protected least significant difference test at P ≤ 0.05.
Results and Discussion
The results of Experiment I, including the percentage of downgraded fruit, mean bruise width, and mean bruise volume are found in Table 2. All material combinations showed the ability to significantly reduce bruising. However, only the treatments that included rubber bands had 100% Extra Fancy grade fruit. The data suggest that the elasticity of a rubber band is capable of absorbing the energy of falling fruit. In each case, either a foam material fastened to rubber bands, or the bands themselves gripped the apples long enough for the rubber bands to stretch out around them. The rubber bands allowed the fruit to pass through one layer, and they immediately snapped back to their original positions before additional fruit fell.
The results of Experiment II are shown in Figure 9. When a single fruit was dropped onto a wood surface bruising occurred at a drop height as low as 15 cm (38 in). The three foams tested caused no significant bruising until 100 cm (254 in). One foam, which was both resilient and energy absorbent, caused no significant bruising even from our highest drop height of 160 cm (5 ft 3 in). This is noteworthy, as most surfaces, even those used in packing houses, will cause bruising at heights considerably lower than these.
Bruise measurements and corresponding levels of downgraded fruit for all testing configurations in Experiment III are presented in Table 3 and Figure 12. The trend lines for all trials demonstrated that as drop height increases bruise volume increases nearly linearly. The energy absorbing grate reduced bruise volume at all heights compared to the control, which was not surprising based on Experiment I results. The pneumatic bin filler did not perform as well as the grate since the fruit passed more slowly through the air filled bladders, allowing more opportunities for fruit-to-fruit contact. An important finding (Figure 13) was that performance increasessignificantly when fruit are singulated prior to entering the bin filler. The optimal bin filling configuration was asingulated fruit transfer system with a drop height of no more than 5 cm(2 in).
Future Research
The scope of our harvesting research was to design a dry bin filling system that was capable of handling apples within an acceptable level of bruising, and two passive concepts were identified. The three experiments described in this paper helped our project team develop a greater insight into the required design requirements to ultimately develop an in-field bin filling system. In addition, we quantified the significance of maintaining fruit singulation throughout the entire harvesting process from picking to transport to bin filling. Our future efforts will focus on integrating an apple transport system with a bin filler design, so that fruit are singulated upon picking all the way to the bin. This strategy should result in a harvesting system that will improve a fruit harvest worker’s productivity without sacrificing fruit damage.
Acknowledgements
The authors would like to acknowledge the valuable support of Jim Schupp and Terry Salada of the Penn State Fruit Research and ExtensionCenter, Scott Wolford of the USDA-ARS Appalachian Fruit Research Station, David Sherman of the Roger Corporation, Rice Fruit Company, and Bear Mountain Orchards. This project is supported by a USDA Specialty Crop Research Initiative grant titled Comprehensive Automation for Specialty Crops and the Washington Tree Fruit Research Commission.
Literature Cited
Baugher, Tara. 2006. Why apples bruise. Fruit Times 25:1-2.
Baugher, T., J. Schupp, K. Lesser, R.M. Harsh, C. Seavert, K. Lewis, T. Auvil. 2009. Mobile platform increase orchard management efficiency and profitability. ACTA Horticulture 824: 361-364.
Hyde, G.M., R.W. Bajema, J. Varith, and A.L. Baritelle. 2003. Increasing-height multiple-impact measurement of bruise threshold in fruits and vegetables. ACTA Horticulture 599: 409-410.
Table 2. Effects of energy absorbing grates on apple bruising and USDA fresh market grade (percentages based on % of total apples tested).
Treatment / Downgraded to Fancy Grade(%) / Downgraded to No. 1 or Utility Grade
(%) / Bruise width
(mm) / Bruise volume
(mm3)
Hard foam balls / 3 / 0 / 1.1 bz / 10.7 b
Soft foam balls / 3 / 0 / 2.1 b / 12.4 b
1 layer each ball type / 3 / 0 / 1.0 b / 19.2 b
2 layers rubber bands / 0 / 0 / 1.3 b / 8.2 b
2 layers balls + 1 layer bands / 0 / 0 / 0.6 b / 3.6 b
2 layers balls + 2 layers bands / 0 / 0 / 1.6 b / 43.9 b
Control / 28 / 3 / 7.1 a / 195.1 a
z Means, within columns,followed by dissimilar letters are significantly different according to Fisher’s protected least significant difference, P ≤ 0.05.
Table 3. Effects of full scale bin filler prototypes on apple bruising and USDA fresh market grade (percentages based on % of total apples tested).
Treatment / Downgraded to Fancy Grade(%) / Downgraded to No. 1 or Utility Grade
(%) / Bruise width
(mm) / Bruise volume
(mm3)
Energy absorbing grate prototype – 2.5 cm drop / 0 / 0 / 1.3 d / 10.5 de
Energy absorbing grate prototype – 5 cm drop / 3 / 3 / 2.7 cd / 22.5 cde
Energy absorbing grate prototype –10 cm drop / 0 / 9 / 5.6 abc / 62.8 cde
Pneumatic prototype – 2.5 cm drop / 6 / 6 / 5.1 bc / 57.8 cde
Pneumatic prototype – 5 cm drop / 9 / 13 / 6.0 abc / 115.9 bc
Pneumatic prototype – 10 cm drop / 18 / 13 / 6.6 ab / 182.7 b
Energy absorbing grate/singulated – 2.5 cm drop / 0 / 0 / 0.0 d / 0.0 e
Energy absorbing grate/singulated – 5 cm drop / 0 / 0 / 0.3 d / 4.6 e
Energy absorbing grate/singulated – 10 cm drop / 9 / 0 / 1.2 d / 11.5 de
Pneumatic prototype/singulated – 2.5 cm drop / 0 / 0 / 0.2 d / 0.7 e
Pneumatic prototype/singulated – 5 cm drop / 3 / 0 / 0.6 d / 2.4 de
Pneumatic prototype/singulated – 10 cm drop / 6 / 0 / 0.9 d / 5.9 de
Control – 2.5 cm drop / 13 / 6 / 5.5 abc / 111.1 bcd
Control – 5 cm drop / 24 / 13 / 8.7 ab / 187.7 b
Control – 10 cm drop / 34 / 19 / 9.0 a / 289.6 a
z Means, within columns,followed by dissimilar letters are significantly different according to Fisher’s protected least significant difference, P ≤ 0.05.