Development of Soft Type Metering Device for Garlic Planter and Performance Analysis in Accordance with Design Parameters

Abstract

The main goal of a garlic planter is to plant garlic cloves individually with even spacing, requiring minimization of missed or multiple plantings. This study adopted an air-suction metering device to increase planting speed. To handle irregularly shaped garlic cloves effectively, soft silicone suction holes were fabricated and attached to the metering device. Performance was assessed by varying suction hole diameters (8, 11, 14, and 17 mm) and material hardness levels (1.4, 16.7, and 27.2 HA Shore A) at multiple-metering rates. The optimal metering rate of 98.2% was achieved with a 14 mm suction hole diameter and 16.7 HA hardness. This success was attributed to the soft suction hole effectively conforming to the garlic clove surface. The findings revealed a critical limitation: when metering rates exceeded 90%, multiple-metering rates increased linearly, indicating inherent constraints of air-suction techniques for single-seed metering. These results provide valuable insights into design parameters needed to improve reliability, operational efficiency, and mechanization performance of garlic planters. The metering device type is expected to be applicable to autonomous or unmanned agricultural machines, advancing agricultural mechanization capabilities.

1. Introduction

Garlic is a spice vegetable cultivated worldwide because it is one of the most common cooking ingredients, with global production estimated to reach 28.7 million tons in 2023. The largest garlic producers recently reported are China and India, expected to produce 20.7 million tons and 3.3 million tons, respectively, in 2023, accounting for approximately 72% and 11% of global production [1]. Despite this, garlic cultivation still relies heavily on human labor rather than mechanization. In particular, the mechanization rate for garlic cultivation in Korea remains low, at 14.8% in 2023 [2]. Mechanization of most plant cultivars, including garlic, involves soil tillage, planting, fertilizer application, pesticide application, and harvesting. For planting, which involves inserting the seeds into the ground and covering them with soil, mechanization is difficult due to the shape of the garlic clove. The development of a planter requires securing technologies such as clove metering, planting spacing, and depth control [3]. Clove metering refers to picking up a clove of garlic from a hopper filled with a large quantity of garlic. Precise clove metering is especially difficult in garlic planting, where multiple or missed metering events commonly occur [4]. Autonomous or unmanned agricultural machines have been introduced in recent years [5,6,7,8], and this technological trend requires work units with precise performance and electronic control capabilities. This study aims to enable the application of clove metering technology to autonomous or unmanned agricultural machines.

The accuracy of single-clove metering is a crucial performance metric for garlic planters, as it directly affects the effectiveness of planting operations and the quality of mechanized garlic cultivation [9,10]. A metering device, which is the core component of garlic planters, quantitatively meters garlic cloves and discharges them individually. In this process, the missed metering rate, that is, the percentage of garlic that was either not properly selected or planted, becomes an important criterion for evaluating the performance of the planter. A high missed metering rate results in reduced yields and increased supplementary manual work in missed areas [11]. In addition, multiple metering, where two or more garlic cloves are planted simultaneously, leads to competition among the cloves, resulting in smaller garlic bulbs and unmarketable shapes [12].

Metering devices, such as bucket-type (bucket), string-type (string), and gripper (gripper) planters, have been developed and applied [12,13,14,15]. However, bucket-type garlic planters, which use buckets the size of a single garlic clove and are mounted on a drum or chain to meter the garlic by repeatedly rotating within the hopper, have drawbacks. They do not operate if the planter is not level, and their metering performance significantly degrades at high speeds [13,14]. The string-type garlic planter operates by unwinding a paper-based string to which garlic cloves are attached at regular intervals and planting them as the string is released; a major drawback of this method is the need for extra labor involved in the pre-processing step of attaching garlic cloves to the string [15]. The gripper-type garlic planter, which picks up garlic cloves as grippers open and close while passing through the hopper, is a technology that has been successfully commercialized by addressing these problems. However, the gripper type also has the limitation of a significant drop in metering performance at high speeds, making it difficult to increase the operational speed [12]. Therefore, it is necessary to develop a technology that enables high-speed planting without being affected by the level of the planter.

Vacuum-based methods that use air suction have been studied for high-speed planting. Vacuum methods have demonstrated a metering performance of 94–97% for spherical and lightweight seeds, such as soybeans and cottonseed [16,17]. Recent advances in air-suction systems have further improved this performance, achieving over 99% seed-picking efficiency for corn at high speeds [18]. The evaluation of vacuum-type units for crops such as cotton and maize has focused on optimizing the accuracy of seed spacing by addressing issues such as misses and multiple indices [19]. When vacuum metering technology was used for large spherical seeds, such as Paeonia suffruticosa seeds, it was confirmed that a high vacuum pressure of 13–20.5 kPa was required [19]. In previous studies, suction holes for seed adhesion were typically fabricated from rigid materials, which complicated the process of their application to irregularly shaped cloves, such as garlic. Furthermore, unlike soybeans and corn seeds, garlic cloves lack a hard coat and are easily damaged when handled using rigid tools. This damage can lead to reduced growth owing to pests or fungi, even if planting is successful. This study aimed to achieve high-speed planting while minimizing damage to garlic cloves. Additionally, vacuum metering technology can be implemented to operate independently of the PTO or wheel rotation of agricultural tractors, and electronic control of metering speed is possible, making it suitable for autonomous or unmanned agricultural machinery.

In this study, an air-suction soft-metering device was developed to enable rapid garlic planting, while minimizing damage to garlic cloves. Ring-shaped soft suction holes were fabricated using a silicone material and applied to an air-suction metering disk. The design parameters of the soft suction holes were the hole diameter and hardness of the material. In this experiment, the effects of these differences in shape and material properties on metering performance were comparatively analyzed.

2. Materials and Methods

2.1. Design of the Air-Suction Soft-Metering Device

The air-suction soft-metering device consists of a garlic hopper to hold garlic cloves, a metering disk that rotates inside the hopper and picks up garlic cloves, a motor to rotate the metering disk, air holes for inserting a soft suction hole that facilitate garlic clove pick-up, a fan motor to generate the air-suction effect, and an air-suction tunnel connecting the air holes and fan motor. The diameter of the metering disk was 300 mm, and six air holes were provided at 60° intervals. The configurations of the device at the front and back are shown in Figure 1a and Figure 1b, respectively.

The configurations of the soft suction hole and the metering disk are illustrated in Figure 2. The metering disk consists of a base plate of aluminum and cover plate of transparent acrylic. Twelve air holes on the base plate were manufactured, but only six air holes were utilized due to combining the cover plate. Additional air holes in the base plate were not used in this study, as they are reserved for future experiments. Two primary design parameters were considered: the diameter of the suction hole and the hardness of the material. Garlic is characterized by having a variety of surfaces, including round, flat, edgy, and conical [20]. The proposed suction hole was fabricated from silicone and featured a ring-shaped design to ensure uniform adhesion regardless of the surface type of the garlic clove. The use of a soft material enables the suction hole to deform upon contact, allowing it to conform to the garlic clove surface, thereby enhancing suction effectiveness. Similarly, related studies investigated soft suction structures capable of simultaneously grasping flat and spherical objects, further supporting the efficacy of deformable suction mechanisms [21]. A prototype of the developed metering device is shown in Figure 3.

2.2. Operation Principle

Air-suction pressure is generated when the fan motor operates. This pressure provides a suction force to the suction holes through the air-suction tunnel. The rear configuration of this system, which is not fully depicted in Figure 4, is shown in Figure 1. The metering disk is rotated counterclockwise, as shown in Figure 4. The suction force is not applied to all the suction holes simultaneously; rather, it is activated only in the suction holes that enter the air-suction area, as indicated by the dotted line in Figure 4a. The system was designed such that a maximum of two suction holes simultaneously entered the air-suction area to minimize the required suction pressure. When a suction hole on the rotating metering disk enters the air-suction area, a garlic clove from the hopper attaches to the suction hole because of the suction pressure, as illustrated in Figure 4b. Garlic attached to the suction hole is shown in red, whereas the unattached garlic that remains in the hopper or falls to the ground is depicted in brown. As the metering disk continues to rotate, the attached garlic is lifted within the hopper. Simultaneously, as the next suction hole enters the air-suction area, a second garlic clove is attached, as shown in Figure 4c. With further rotation of the metering disk, the suction hole carrying the attached garlic clove moves out of the air-suction area. Consequently, the suction pressure is released, causing the garlic to detach from the suction hole and fall, thereby completing the metering process, as shown in Figure 4d. The metered garlic then falls onto the soil and is covered with a covering device, thereby completing the planting process.

2.3. Design Elements and Experimental Methods

Two design elements were selected for the soft-metering device: diameter of the soft suction hole and hardness of the soft suction hole. The suction hole diameter was set to four levels, 8 mm, 11 mm, 14 mm, and 17 mm, depending on the size of the garlic, and these were fabricated (Figure 5). The size of garlic cultivated in Korea was investigated to be 25–49 mm in length, 10–24 mm in width, and 12–27 mm in thickness [22]. The smallest hole size, 8 mm, is the condition to prevent any gaps between the hole and the garlic, and the largest hole size, 17 mm, is the maximum size to prevent most garlic from passing through the hole. To eliminate the effect of garlic size on the metering performance, garlic cloves of similar sizes were selected. The size of garlic that passes through a 19 mm standard sieve but is filtered out by a 16 mm standard sieve was selected for the experiment, which is approximately the medium size of cultivated garlic.

The suction hole hardness was set to three levels, soft, moderately soft, and relatively firm, and fabricated accordingly. The hardness of the three suction holes was measured as 1.4 HA, 16.7 HA, and 27.2 HA, respectively, based on Shore A hardness, according to ASTM D2240 [23] (SHORE 10A, TMTeck, China).

The experiments were conducted using garlic cloves. The rotational speed of the metering disk was adjusted to 10 rpm for a discharge rate of 1 clove/s. At this metering speed, the metering result could be accurately confirmed and calculated with the naked eye. The air-suction pressure was measured between 88.6 kPa and 94.0 kPa (Testo 552, Testo, Germany). When garlic cloves were attached to all suction holes, the pressure was low, and conversely, when the garlic clove was discharged from the hole, the pressure was slightly high. This vacuum pressure is four times higher than previous study on the peony seeds [19]. This is because when the vacuum metering method is applied to garlic, a void is created between the suction hole and the garlic due to the shape of the garlic, which increases the pressure loss. Garlic metering tests involving 30–80 cloves were conducted to evaluate garlic discharge performance. The success rate of garlic discharge is defined as the metering rate (MR), and the rate at which two or more cloves are discharged is defined as the multiple-metering rate (MMR), which was calculated using Equations (1) and (2), respectively. The experiment was repeated five times for each design element. Images illustrating metering failure, single metering, and multiple metering observed during the experiments are shown in Figure 6a, Figure 6b, and Figure 6c, respectively.

$$\mathrm{M}\mathrm{R}={\displaystyle \frac{n_s+n_m}{N}}$$

(1)

$$\mathrm{M}\mathrm{M}\mathrm{R}={\displaystyle \frac{n_m}{n_s+n_m}}$$

(2)

where N is the number of suction holes passing through the hopper, and $n_s$ and $n_m$ are the numbers of single discharged garlic cloves and multiple discharged garlic cloves during the experiment, respectively.

2.4. Statistical Methods for Performance Analysis

Statistical methods were used to analyze the metering performance on the design parameters, which are the diameter and hardness of the air-suction hole. First, the average MR and MMR were calculated according to the diameter and hardness of the suction hole. These results show the individual performance for each design parameter. Second, ANOVA was used to analyze the statistical significance of the effect of the suction hole diameter on the metering performance. ANOVA was performed using R version 4.5. Third, to determine the effect of the hardness of the suction hole on the metering performance, the average and distribution of the metering rate were calculated. Lastly, the relationship between MR and MMR was confirmed through regression analysis.

3. Results and Discussion

3.1. Analysis of Metering Rate Depending on Suction Hole Diameter

When the hardness of the suction hole material was 1.4 HA, the metering rate ranged from 74.9% to 96.5%. As the suction hole diameter increased, both the metering rate and multiple-metering rate also showed an increasing trend, as shown in Figure 7a. When the metering rate was 90% or higher, the multiple-metering rate was confirmed to be high, ranging from 22.5% to 28.4%. For the softest suction hole material (1.4 HA), if the suction hole diameter was larger than 8 mm, it effectively enveloped and adhered to the garlic. Simultaneously, an increasing tendency was observed for multiple garlic cloves attached to a single suction hole.

When the suction hole material was moderately soft (hardness of 16.7 HA), the highest metering rate of 98.2% was observed for a suction hole diameter of 14 mm. The multiple-metering rate was 29.2%. Subsequently, as the diameter increased to 17 mm, the metering rate and multiple-metering rate decreased slightly to 95.8% and 27.3%, respectively. Similar sensitivity to material and diameter combinations has also been noted in vacuum-disk-based seeders, where increased stiffness or reduced suction area led to reduced single-seed pickup efficiency [24].

For the material 27.2 HA hardness, the highest metering rate of 94.5% was observed for a suction hole diameter of 11 mm, as shown in Figure 7c. The multiple-metering rate was also the highest at 31.4%. However, as the diameter increased, the metering rate decreased to less than 90%, which is a different trend from that in the case of low hardness. This can be interpreted to be a result of the reduced effect of the soft suction hole enveloping the irregular shape of the garlic as the material hardness increases.

Overall, the metering rate increases when the hardness of the soft suction hole decreases. The optimal diameter was different for each suction hole. When the metering rate increased, an increasing trend in the multiple-metering rate was also observed. Contact deformation of the soft material suction hole by garlic shape was visually observed in the softest and moderately hard materials. In case of the hardest one, although contact deformation was not observed, the effect of preventing damage to the garlic surface can be expected.

A statistical analysis of the relationship between the suction hole diameter and metering rate showed that for the suction hole with a hardness of 1.4 HA, a difference was observed at the 8 mm suction hole diameter compared to the other diameters. In other words, there was no difference in the metering rates at 11 mm, 14 mm, and 17 mm diameters (

Table 1. Relation between the diameters of the soft suction hole and metering rate with 1.4 HA.

Hole Diameter	Mean	Standard Deviation	F Value	Multiple Comparisons
8 mm	74.9%	0.374%	20.72 ***	8 ≠ 11, 14, 17 mm ***
11 mm	95.4%	0.084%
14 mm	92.5%	0.471%
17 mm	96.5%	0.066%

*** p < 0.001.

). For the suction hole with a hardness of 16.7 HA, a difference in metering rate was observed between the 8 mm suction hole diameter and the other diameters, and a difference in metering rate was also observed between the holes of the 11 mm diameter and those of 14 mm and 17 mm diameters. For holes with 14 mm and 17 mm diameters, no difference in metering rate was observed (

Table 2. Relation between the diameters of the soft suction hole and metering rate with 16.7 HA.

Hole Diameter	Mean	Standard Deviation	F Value	Multiple Comparisons
8 mm	80.1%	0.048%	31.54 ***	8 ≠ 11 mm ** 8 ≠ 14, 17 mm *** 11 ≠ 14, 17 mm **
11 mm	88.3%	0.072%
14 mm	95.6%	0.118%
17 mm	95.6%	0.113%

** p < 0.01, *** p < 0.001.

). For the suction hole with a hardness of 27.2 HA, a minimal difference in the metering rate with respect to the suction hole diameter was observed, although a significant difference with low probability was found between the metering rates for the 8 mm and 11 mm diameters, as indicated in

Table 3. Relation between the diameters of the soft suction hole and metering rate with 27.2 HA.

Hole Diameter	Mean	Standard Deviation	F Value	Multiple Comparisons
8 mm	82.7%	0.608%	3.549 *	8 ≠ 11 mm *
11 mm	96.3%	0.073%
14 mm	85.8%	0.617%
17 mm	89.3%	0.634%

* p < 0.05.

.

3.2. Analysis of Metering Rate Depending on Suction Hole Hardness

The change in the metering rate depending on the hardness of the suction hole material was analyzed for the 11 mm and 14 mm diameters, where the highest metering rate was observed in Figure 8. When the suction hole diameter was 11 mm, high metering rates were observed at hardness levels of 1.4 HA and 27.2 HA. By contrast, when the diameter of the suction hole was 14 mm, a high metering rate was observed for a hardness of 16.7 HA. At the average metering rate, a distinct relationship between the material hardness and suction hole diameter was not apparent. However, the distribution of the metering rate increased with the suction hole diameter. When the suction hole hardness was 16.7 HA, the difference between the maximum and minimum metering rate values increased from 6.3% to 9.1% when the diameter increased from 11 mm to 14 mm. However, when the hardness of the suction hole was 1.4 HA and 27.2 HA, the difference increased significantly from 7.5% to 17.3% and from 6.2% to 19.7%, respectively. These results are attributed to the air-suction volume per unit area for picking up garlic decreasing as the diameter of the suction hole increased. Furthermore, even when the garlic adheres to the suction hole, the empty space between the garlic and suction hole may not form an optimal seal and reduce the suction force, thereby diminishing the effectiveness of adhesion and subsequent detachment.

3.3. Analysis of the Relationship Between Metering Rate and Multiple-Metering Rate

The relationship between the metering rate and multiple-metering rates, depending on the hardness of the suction hole, is shown in Figure 9. The blue, red, and green data points represent the metering rates at suction hole hardness values of 1.4 HA, 16.7 HA, and 27.2 HA, respectively. The multiple-metering rate, regardless of the suction hole diameter or hardness, was approximately 6% when the metering rate was below 80% and approximately 10% when the metering rate was below 90%. The relationship between MR and MMR could be described in Equation (3), with 0.923 of $R^2$. This shows that when the MR is below 90%, the MMR increases slowly, but when the MR is above 90%, the MMR increases rapidly. This trade-off between achieving a high metering rate and minimizing multiple-metering rates is a recognized challenge in vacuum-type precision metering systems, particularly for high performance at increased operational speeds [21]. This behavior often reflects the common limitation of vacuum-based planters. For instance, efforts to ensure a high metering rate through increased suction may lead to a higher incidence of multiple-metering rates.

$$\mathrm{M}\mathrm{M}\mathrm{R}=0.0153e^{0.0766\times MR}$$

(3)

3.4. Performance Indexes and Optimization

In this study, two performance indexes, the MR and MMR, were evaluated and their relationship was found. In fact, it is hard to find optimal parameter conditions with these limited experiments. The optimal parameter conditions can be derived through analysis of the relationships among various performance indexes. Performance indexes of the vacuum-type metering device for garlic include missed metering rate, vacuum pressure, and metering speed. Missed metering rate is the opposite of the MR. Metering speed is proportional to the rotating speed of the metering disk. Vacuum pressure can be lowered by supplying more power to the fan motor. These performance indexes are interrelated. If the metering speed is increased by rotating the metering disk faster, the time that the garlic in the hopper meets the suction hole will be shorter, which will reduce the MR. If the vacuum pressure is then lowered, the MR may increase again. However, the MMR can also rise as a side effect. Ideal performance would include increasing both the MR and metering speed while reducing the MMR and fan motor power. Based on these relationships, it is necessary to find the optimal combination for the best performance of the metering device.

3.5. Future Work and Application

The air-suction soft-metering device developed in this study operates by rotating the metering disk. This allows the metering speed to be increased by rotating the metering disk faster. Using a metering disk with more suction holes also allows for faster metering speeds. Future research will explore the relationship between metering speed and rate, proposing methods to maintain the metering rate while increasing speed. Since this device will be used outdoors at the end, it is necessary to evaluate the impact of environmental conditions such as humidity and dust on its performance using an environmental chamber. The suction disk’s single-axis rotational design enables straightforward parallel integration of multiple metering units, facilitating systematic evaluation of planting performance and operating principles to achieve optimized planting efficiency. The metering device discharges the garlic cloves by natural gravity. To apply this metering device as a planter, additional devices are needed to more reliably remove the garlic and drop it where desired. Additionally, it is necessary to study the shape of the hopper and the appropriate level of garlic contained in the hopper. While the garlic cloves attached to the suction holes move among the many other garlic cloves in the hopper, the attached cloves may fall off due to the other garlic.

The air-suction soft-metering device will be used as a core component of a garlic planter, which is expected to be attached not only to conventional agricultural tractors but also to autonomous or unmanned agricultural machines. It will be tested in a variety of driving environments, including uneven terrain or hillsides. The following are possible applications of the metering device developed in this study.

• Garlic planting implements for conventional agricultural tractors;
• Electric control garlic planters for autonomous or unmanned agricultural vehicles;
• Automatic garlic planting devices for conveyor planting systems in nurseries;
• Planting machines for shallots, tulips, etc., which have a shape like a garlic clove.

4. Conclusions

This study was conducted to develop an air-suction soft-metering device for garlic planters and evaluate its performance. The diameter and hardness of the suction hole were selected as key design parameters. The highest MR of 98.2% was observed when diameter and hardness of suction hole were 14 mm and moderate (16.7 HA), respectively. Through statistical analysis, the effect of the diameter and hardness of suction hole on the metering rate was confirmed. In particular, it was found that when the MR increases by more than 90%, the MMR also increases simultaneously regardless of design parameters. This remains a limitation of the air-suction method and will be a challenge for future research. Furthermore, design parameter optimization will be conducted through an analysis of relationships among the performance indexes, including not only the MR and MMR, but also the missed metering rate, vacuum pressure, and metering speed. The next stage of the research is to develop a garlic planting machine with the air-suction metering device adapting soft suction holes and will be used not only on existing agricultural tractors but also on autonomous or unmanned machines currently under active development.