Advances in Mechanized Harvesting Technologies and Equipment for Chili Peppers

Abstract

Mechanized chili harvesting is essential for improving efficiency, reducing costs, and alleviating labor intensity in production. However, issues such as low harvesting efficiency, high rates of breakage, and contamination continue to severely hinder the development of mechanized chili harvesting. This study provides an overview of global chili production regions and varieties, examining three harvesting approaches: single-pass, multi-stage, and multi-pass approaches. It describes the operational principles of key harvesting mechanisms, including the helical spiral-type, drum finger-type, long-rod comb-type, and belt-mounted comb finger-type mechanisms, and summarizes research progress in major producing countries, such as the United States and China. The paper evaluates both airflow-based and mechanical cleaning–separation devices, highlighting the combined airflow mechanical systems as the most promising approach and reviews their current development status. It also addresses structural challenges in chassis, frameworks, and conveyance systems. Finally, the paper analyzes solutions to the existing challenges, emphasizing the integration of intelligent technologies to resolve mechanical issues, and outlines the future prospects of intelligent development in mechanized chili harvesting.

1. Introduction

1.1. Pepper Planting Status and Variety Classification

Modern agriculture is evolving towards high-efficiency, low-loss, and precision-oriented practices, driving an increasing demand for advanced agricultural equipment technologies, such as high-efficiency seedbed preparation technology [1], high-speed precision seeding technology [2], precision plant protection technology [3], and high-efficiency low-loss harvesting technology [4]. Harvesting is a critical phase in agricultural production; the equipment with high efficiency and low loss plays a role in improving crop quality, reducing costs, promoting the integration of agricultural machinery with agronomic practices, and supporting the sustainable development of agricultural production systems. Chili is the third-largest vegetable crop in the world, following beans and tomatoes [5]. In addition to its direct consumption as fresh produce and use in processed food products, chili holds significant nutritional and functional value and is widely utilized in industries such as cosmetics, pharmaceuticals, and health products [6,7,8,9,10]. According to statistical data from the United Nations Food and Agriculture Organization (FAO) [11], the global area under chili cultivation reached 2.83 million hectares (ha) in 2020 (Figure 1). Over the past two decades, the cultivated area has consistently exceeded 1.59 million ha, and demonstrates a sustained upward trend (Figure 2a,b). Major chili-producing countries include China, Mexico, Turkey, Spain, and Indonesia. Notably, China has the largest chili cultivation area globally, accounting for over 30% of the total global area [12,13]. Chili cultivation in China is characterized by numerous varieties and complex agronomy practices, which, to some extent, reflect global chili cultivation patterns. Taking China as an example, Figure 3a shows that chili cultivation occurs on a large scale in approximately 28 provinces, with diverse varieties and complex distribution. Figure 3b indicates that the total area under chili cultivation in China has steadily increased in recent years, approaching 1.13 million hectares. However, despite this growth, chili yields have not seen a significant increase in production [14]. The stagnation in productivity, despite sustained expansion of cultivated land and improvement of chili varieties, suggests substantial losses during the agricultural production processes. One important reason for this reduction in yield is that while chili harvesting machinery is being widely adopted, its development is still ongoing. Problems such as fruit drop, breakage, and missed harvesting during mechanized harvesting have resulted in lower yields. There are numerous chili varieties globally, with over 180 varieties alone in China.

Table 1. Major chili pepper varieties and harvesting methods.

Name	Picture	Features	Harvesting Methods
Cherry peppers		Chili peppers are round or oblate, resembling cherries or buttons. Varieties vary in their use for dried chili production, fresh consumption, and ornamental purposes.	Dried chilies are harvested using vibratory or air-suction harvesters, such as China’s 4JZ-1 model, while fresh varieties are manually harvested with handheld electric pickers.
Conical pepper		Conical or bullet-shaped chilies, such as Fresno Chili, are primarily for fresh consumption. Certain cultivars, like Serrano Pepper, undergo drying processes.	Fresh chilies are primarily handpicked, while varieties with concentrated ripeness use mechanical harvesters, like the US GH80 model.
Pod pepper		Pod peppers grow upright and are small in size. They can be categorized by single-stage or batch ripening, and are used for fresh consumption and dried chili production.	Pod peppers are mostly handpicked, while central pod peppers are harvested with chili pepper harvesters, such as China’s 4JZ-3600 model.
Bullhorn pepper		Bullhorn peppers are long and thick, with thick skins, and are shaped like cow or ram’s horns. They are mainly used for fresh consumption, e.g., Bulgarian ram’s horns peppers.	Most of the horn peppers are hand-picked due to their inconsistent ripening and primary use for fresh consumption.
Cayenne pepper		Cayenne peppers are linear, thin, slightly curved, and yield high, uniform maturity, making them ideal for dried chili production.	These peppers are primarily harvested for dried chili production using harvesters, such as the 4JZ-3150 model in China.
Bell pepper		Bell peppers are large, thick-fleshed, and lantern-shaped, with a variety of cultivars, primarily used for fresh consumption.	Mechanical harvesting can damage the peppers, so manual picking is predominantly used.

categorizes chili varieties based on their morphological characteristics and provides brief descriptions of their agronomic traits and corresponding harvesting methodologies.

1.2. Existing Pepper Harvesting Methods and Their Characteristics

There are two primary methods for harvesting peppers: mechanical and manual harvesting. For instance, in Xinjiang, China, where the planting area is vast but the labor force is limited, large pepper harvesters are employed. These machines are multifunctional, integrating picking, conveying, sorting, separating, and binning processes, making them suitable for large-scale planting areas (Figure 4). However, the use of such large harvesters is not suitable for small-scale planting areas, such as those in Shandong and Jiangsu provinces. Furthermore, pepper varieties differ, leading to variations in harvesting methods. Peppers are primarily used for fresh consumption or for the production of dried chili. Based on the maturity period, varieties can be classified into those with a single harvest period and those with a batch maturity. The maturity of fresh peppers is inconsistent, making mechanical harvesting unsuitable. Therefore, batch harvesting is more commonly used. For peppers that mature simultaneously, a single harvest method is applied. If the harvested peppers are intended for drying, large-scale drying equipment is used to reduce the moisture content to meet storage standards. However, this drying process increases both labor intensity and harvesting costs.

To address the limitations of single-harvest dried chili production, some regions in China have adopted segmented harvesting methods. Plants are cut in fields using specialized chili-cutting blades for open-air solar drying. When the moisture content of chili peppers has been reduced to the storage standard, the picking and harvesting operations can be conducted. For segmented harvesting, the Xinjiang region uses a large-scale cleaning and separating device in the field to complete the picking, cleaning, and other work. However, manual labor is still required to pick up and support the chili pepper plants fed into the cleaning and separating device. In Shandong, post-drying operations, including collection, bundling, transportation, and storage, rely entirely on manual labor. Both methods significantly increase labor intensity. Xinjiang’s approach results in higher impurity, damage, and loss rates, while Shandong’s method is characterized by prolonged harvesting periods, higher labor costs, and complex storage requirements (Figure 5).

In terms of operational costs, using Xinjiang, China, as a case study for dry chili harvesting, manual harvesting incurs a labor cost of approximately CNY 300 per day. By employing large chili harvesters followed by manual impurity removal, labor expenses are reduced to about CNY 200 per day. Consequently, the current cost of manual harvesting amounts to CNY 9000 per hectare, whereas mechanical harvesting costs are approximately CNY 1500 per hectare, only 1/6 of the manual costs. It can significantly reduce harvesting the potential for significant savings [15]. Thus, mechanized chili harvesting technology plays a crucial role in advancing the development of the chilis industry. Regarding operational efficiency, manual harvesting of fresh peppers is time-consuming, often resulting in missing the optimal harvesting window and a decline in product quality. Health-wise, manual harvesting requires long-term contact with chili peppers, which can cause harm to human eyes, skin, and respiratory system, thereby affecting the well-being of farmers [16].

In summary, current chili harvesting methods lack equipment with high compatibility and superior performance. Enhancing the mechanization of harvesting has become an urgent issue for the development of the chili industry. Given the current situation where China’s pepper cultivation areas are scattered and there are agronomic variations, harvesting machinery must exhibit diverse functionalities to meet regional and agronomic requirements.

1.3. Current Status of Mechanized Chili Harvesting

Research on chili harvesters began earlier abroad and has reached technological maturity. Foreign studies primarily focus on large-scale field operations, developing harvesters with integrated functions, including picking, cleaning, conveying, and collecting, which enable direct chili harvesting. In contrast, China initiated research on chili harvesting technology at a later stage. Initial efforts involved adopting foreign harvester designs to fit Chinese agronomic conditions [17].

According to Chinese standards, harvester performance is evaluated based on four key metrics: impurity rate, damage rate, drop loss rate, and missed-picking rate. The latter two are combined into the total loss rate.

Table 2. Performance criteria for some pepper harvesters.

Standard	Type of Application	Loss Rate	Breakage Rate	Impurity Rate
JBT 12825-2016 [24]	Thread pepper harvester	≤3.0%	≤2.0%	≤18%
DG T 114-2019 [25]	Comb-type chili harvester	≤8%	≤3%	≤20%
Q/ZS206-2017 [26]	Harvester compatible with varieties including thread, Jinta, and American Red chili peppers	≤6%	Thread ≤ 8% Jinta ≤ 10% American Red ≤ 15%	≤20%
Q/321181 SFH 051-2022 [27]	Compact self-propelled chili harvester	≤5%	≤8%	≤15%

provides the national or enterprise standards for the operational performance of selected Chinese chili harvesters. For other countries, relevant standards can be referenced from common guidelines for combine harvesters, such as the British standard BS ISO 8210:2021 [18], the Korean standard KS B ISO 8210-2022 [19], and the international standard ISO 8210:2021 [20], all of which are applicable to chili pepper harvesters. Due to diverse harvester types and operational mechanisms, no unified industry standards exist, meaning that the market chili pepper harvester quality varies and the performance difference is also large. Current harvester designs are structurally stable but underperform, with impurity rates and loss rates exceeding standards, in addition to ongoing issues of missed picking, damage, and limited adaptability to different chili varieties. Globally, medium and large-scale harvesters dominate the market, while small-scale harvesters remain under-researched. Currently, small pepper harvesters are scarce, most of which are small sun cutters designed to cut the pepper plant in preparation for subsequent harvesting (Figure 6). Figure 7 illustrates a Mexican small walk-behind pepper harvester, which can successfully pick peppers but has a high impurity rate. As a result, there is a growing demand for small pepper harvesters in the market. While China has developed medium- and large-sized pepper harvesters for the Xinjiang region, many hilly or small-field pepper-growing areas across the country are unsuitable for such large machines, resulting in a shortage of small-sized pepper harvesters [21,22,23].

2. Pepper Picking Technology and Devices

Research on chili harvesters began in the mid-20th century with single-pass harvesting. The picking mechanism serves as the core component of chili harvesters. Over the years, various types of pepper harvesters have been developed. The most commonly used mechanisms include helical spiral-type, drum finger-type, long-rod comb-type, and belt-mounted comb finger-type picking mechanisms. Other types, such as pneumatic suction-type, vibration-type [28], and high-pressure water jet-type mechanisms, show limited development due to low efficiency or high costs. To address residual stem-attached peppers, re-threshing devices have been designed, including rotary concave plate-type and roller concave plate-type systems. For certain dry chili varieties, a stage harvesting method is adopted, which involves cutting the plants for field drying before collection.

2.1. Current Harvesting Techniques

Fullilove proposed an inclined dual-helix picking mechanism, which was later followed by Creager and Cosimatic’s introduction of a vertical dual-helix design [29]. Similar to the inclined dual-helix system, it consists of a pair of spiral rollers and conveyor belts positioned on both sides of the rollers. The chili stalks are guided into the rotating counter-rotating helical rollers by dividers. The high-speed rotation of the rollers generates friction and collisions with the peppers, causing them to detach and move upward. The tilted rollers then convey the peppers rearward, while the detached peppers are thrown onto side conveyors.

Eaton et al. [30] modified a pea harvester into a drum finger-type chili harvester, with key components including a drum, spring fingers, stalk rollers, and curved guides. The drum is equipped with multiple rows of spring fingers arranged at fixed intervals. During operation, the stalk rollers press the plants downward. The drum rotates clockwise, driving the spring fingers to brush the peppers upward. The peppers are detached by the finger impacts and are guided to the rear conveyors through inertia and curved guides.

Gentry et al. [31] introduced a rod comb-type picking mechanism combining vertical and rotational movements. This system consists of rods, combs, and discs with fixed gaps between the comb teeth. During operation, the harvester advances along the crop rows, with dividers guiding the stalks into the rod gaps. A drive mechanism rotates the discs, causing the rods and combs to brush the peppers upward. The combs throw the detached peppers onto conveyors positioned on both sides [32,33].

Lenker et al. [34] designed a belt comb-type harvester specifically for dry red peppers. The system comprises combs, picking belts, fans, and conveyors. Stalks enter between two picking belts and the combing belt drives the comb teeth to comb bottom-up on both sides of the chili plant. Detached peppers are thrown above the belts, where fans blow them to rear or side conveyors.

As presented in

Table 3. Main types of harvesting techniques, characteristics and adapted varieties.

Type	Working Schematic	Features	Suitable Varieties
Dual-helix picking mechanism		Simple structure and low failure rate. Low damage to peppers due to point contact, resulting in many impurities such as pepper leaves, and needs row-aligned harvesting.	Suitable for pepper varieties with low water content or dry stalks, such as bell peppers.
Roller finger snapping type		Can harvest without row spacing, making it highly adaptable to peppers with varying row distances. Effectively picks up peppers that have fallen to the ground.	Effective for harvesting clusters of peppers and dried peppers with fragile stems.
Rod and bar comb type		Requires harvesting in paired rows, with a continuous bottom-up impact on the peppers. High picking capacity, but causes significant damage to the peppers.	Suitable for chili varieties with concentrated pepper distribution.
Belt-type comb-tooth harvesting device		Requires opposite row harvesting, with two rotating belts moving in opposite directions from bottom to top. Effectively picks up peppers that have fallen to the ground.	Suitable for harvesting larger peppers, such as board peppers (a specific variety of goat horn pepper) and lantern peppers.

, schematic diagrams of four typical picking devices are illustrated, accompanied by concise descriptions of their characteristics and suitable pepper varieties.

To better understand the operating principle of pepper harvesters, this section provides an overview of the principles and current development status of vibration-based and air suction-based methods for harvesting peppers. In 1977, Thomas et al. [35] in the USA applied the vibration principle for harvesting Tabasco peppers but were unsuccessful in achieving effective pepper picking. The principle involves applying an inherent amplitude to the pepper plant through machinery to generate vibrations. When the vibration force is successfully larger, it can successfully detach the peppers. Through experimentation, it was concluded that effective pepper harvesting requires both vibration and striking to be applied simultaneously. Consequently, a pair of helical picking rollers with welded struts was designed to harvest the peppers. To prevent the peppers from falling, an air suction device was incorporated to collect them at the top of the picking mechanism. The working principle of this system is shown in Figure 8. This harvesting method is particularly suitable for small, non-fragile pepper varieties.

2.2. Current Application Status of Harvesting Technologies

2.2.1. Dual-Helix Picking Device

The double-helix picking device is the most commonly used and widely adopted mechanism for pepper harvesting globally. It is particularly suitable for harvesting larger pepper varieties, such as green peppers and red dried chili peppers. For instance, the GH80 pepper harvester (Figure 9a), developed jointly by the American OXBO Corporation and the Yung-Etgar Agricultural Machinery Research Institute in Israel. The representative model in China is the 4JZ-3150 pepper harvester (Figure 9b).

The double-helix picking device attracted significant research attention in developed countries, including the United States and the Soviet Union, after its appearance. In 1976, McClendon Pepper Company in the United States developed the Peter Piper expandable double-helix finger comb pepper harvester. This model replaced helical picking rollers with finger comb picking rollers arranged in a helical pattern [37]. After 1990, the expansion of pepper cultivation and the rising costs of manual harvesting led to increased focus on mechanized pepper harvesting. In 1992, Spain developed a double-helix pepper harvester suited to local conditions. Field tests were conducted in three major production regions of Spain the following year, covering different pepper varieties and harvest periods. The results showed minimal variation in picking rates, demonstrating good adaptability to diverse pepper types and harvest stages. In 1997, Palau et al. [38] designed a hydraulically driven pepper harvester. This harvester was tractor-drawn and only harvested single rows of peppers. Flexible brushes reduced mechanical impact on the peppers, and a fan mounted on the conveyor belt removed small impurities such as leaves during transport.

The double-helix picking device became the most popular harvesting mechanism in the United States. Since the 21st century, many countries have researched its optimal working conditions and core component parameters. Kim et al. [39] designed two small walk-behind pepper harvesters (Figure 10a,b), where the frame, divider, and picking rollers were made from different materials. Field tests compared their picking efficiency, loss rates, and performance across two pepper varieties at varying rotational speeds. Kang et al. [40] used the CH301 harvester to collect peppers planted at different periods, analyzing how spacing affects harvesting outcomes. Kang et al. [41] also designed helical rolls with two and three helixes, testing both the same and opposite rotation and winding directions (Figure 11). At 400 rpm, the helix type and speed affected harvesting efficiency. Picking rates depended on speed but not on helix type, while mechanical damage remained unaffected by either factor. Joukhadar et al. [42] tested six green chili varieties (AZ-1904, Machete, PHB-205, E9, PDJ.7, and RK3-35) from New Mexico using the double-helix picking device. Differences in pepper varieties led to variations in plant structure. Harvesting efficiency (marketable fruit yield as a percentage of total plot yield) ranged from 64.6% to 39.3% across the six varieties, suggesting that pepper harvest efficiency correlates with plant structure and the type of harvesting device used. Gupta et al. [43] developed a finger-comb pepper harvester, replacing helical rods with short welded finger combs arranged in a helical pattern. A test bench was set up in the laboratory to evaluate the picking device, which included the picking mechanism, plant conveying system, control system, and power output system. Efficiency tests were conducted by controlling the rotational speed of the picking device and the plant conveying speed. With the picking device rotating at 177.55 rpm and the conveying speed set to 1.47 km/h, the maximum picking efficiency reached 78.17%, and the minimum damage rate was 2.62%. However, the device requires further improvements before it can be applied to pepper harvesting.

China has also conducted extensive research on expandable helix-type harvesting devices. Liu et al. [44] designed an inclined finger-comb pepper harvester tailored to Guizhou Chaotian pepper cultivation (Figure 12). They developed a double-helix finger-comb picking roller. Through Adams simulation analysis of the pepper harvesting process and verification with experiments, the optimal rotational speed of the spiral roller was found to be 190 r/min. Yuan et al. [45] developed a double-helix counter-roller harvester for pigment pepper harvesting. The design parameters of the spiral roller and the key factors influencing harvesting performance were determined by analyzing the force interactions between the pepper and the spiral roller during the harvesting process. Subsequently, a pigment pepper harvesting test was carried out using harvesting efficiency and breakage rates as performance indicators, with operational speed, spiral roller rotational speed, spacing between the two spiral rollers, and spiral pitch as variables. When the working speed was 2.1 km/h, the rotational speed of the spiral roller was 142 r/min, the spacing between the spiral rollers was 24.3 mm, and the pitch was 10 cm, the harvesting rate was 98.7% and the breakage rate was 3.46%. This provides a valuable reference for the design of pigment pepper harvesters. Wang et al. [46,47,48] developed a small pepper harvester prototype using double-helix rollers and rigid–flexible bionic finger-comb rollers (Figure 13). They investigated the effect of rubber tube thickness on picking performance and conducted EDEM simulations to analyze the impact of roller speed, pepper feeding speed, and roller spacing on loss rates. The rotational speed of the picking roller was 680.41 rpm, the feeding speed was 0.5 m/s, and the distance between the two picking rollers was 12 mm, at which the loss rate of morning glory falling from the ground was 3.526%. Field tests confirmed the optimal operating parameters and demonstrated superior performance of the bionic finger-comb rollers over conventional helix designs. Li et al. [49] designed a self-propelled pepper harvester with adjustable row spacing. It employs a helical mechanism to automatically adjust roller spacing, improving picking efficiency while reducing missed-pick and damage rates. Spiral rollers are the core components of the picking device, and different materials or structures are suitable for picking different varieties of chili peppers. The types and characteristics of common spiral rollers are summarized in

Table 4. Spiral roller types and features.

Spiral Roller Types	Picture	Specificities
Unfolding Spiral Picking Roller		Point contact picking can effectively avoid damaging the peppers and is suitable for picking larger peppers such as bell peppers. Currently the most used picking roller.
Comb Finger Spiral Picking Roller		In comb finger picking, the picking effect is good but it is easy to damage peppers, so it is suitable for picking peppers growing in clusters.
Rigid–Flexible Coupling Comb Roller		Reduces the damage to the peppers by changing the material of the core components or adding flexible materials.

.

Spiral rollers exert small force application ranges during harvesting, effectively reducing stalk breakage and impurity rate. The high combing frequency ensures superior picking rates. The contact between the spiral roller and the pepper is point contact, resulting in a small contact area, which can lower the breakage rate. However, to minimize the missed picking rate, the length of the auger rollers is designed to be longer, which increases the overall length of the pepper harvester. While this design suits large-scale harvesters, it compromises maneuverability in small plots and reduces equipment utilization. Additionally, it requires harvesting in paired rows, which is only compatible with the agronomic practices of certain pepper varieties grown in China.

2.2.2. Roller Finger-Picking Device

The roller-finger picking device emerged relatively recently. Notable models include the 4AZ-2200 chili harvester from China (Figure 14a), and the 4JZ-3600 self-propelled chili harvester (Figure 14b) [50]. Due to factors such as the variety of chili types and agricultural practices, this device has not seen widespread adoption internationally. In 2009, China introduced the VPC II green bean harvester through Longping High-Tech Company for experimental chili line harvesting. The trials revealed several issues, including high loss rates, high impurity levels, and incomplete harvesting. Despite these challenges, the roller-finger chili harvesters proved to be better suited to China’s diverse chili varieties and the complex agronomic conditions of local chili cultivation.

The roller-finger device integrates multiple functions, including pickup, separation, and picking, and is widely used in the harvesting of various crops. Different functions are required for different crops. For instance, when harvesting crops such as wheat and rice, the device performs gathering and feeding, but when harvesting chili peppers, excellent combing performance is needed. Following the introduction of roller-finger picking devices, China has conducted extensive research. Lei et al. [51,52] investigated the picking principles of roller-finger devices, designed key components such as fingers and rollers, built test platforms (Figure 15), and studied the effects of factors such as operating speed, finger spacing, and roller speed on picking efficiency. Duan et al. [53] analyzed the damage characteristics and mechanisms caused by the fingers to chili peppers, identified the damage patterns and causes, and optimized the harvesting device and operational parameters. Kong et al. [54,55,56] developed the 4LZ-3.0 chili harvester, and through experiments, analyzed the trajectory of the flicking motion during chili pepper harvesting, which was found to follow a trochoidal path, and improved the finger structures and layouts. Qin et al. [57,58] created a parameter-adjustable test platform, and designed a mechanism for adjusting drum diameter and flexible tooth spacing, enabling precise regulation of multiple key parameters. Using ADAMS simulations and field tests, they optimized roller speed, forward speed, and circumferential finger velocity based on impurity rates and loss rates. Song et al. [59] established a roller-finger picking model based on chili mechanical properties, designed a corresponding harvesting header, and validated its effectiveness through field trials.

The popping finger is the core working component of the roller-finger picking device. Currently, the fingers used are mainly categorized into metal, nylon, and metal–rubber rigid–flexible coupling types. Metal fingers are resistant to breakage but increase chili damage rates, while nylon fingers reduce damage but are prone to breakage. Metal–rubber hybrid fingers combine some of these advantages but still underperform. Zou et al. [60] replaced metal fingers with nylon fingers, which mimic human fingers (Figure 16) and designed a stem press roller height adjustment mechanism, enabling the harvesting of diverse peppers. Shen et al. [61,62,63] designed nylon fingers and sliding path centerlines based on the oscillating attitude and motion law of elastic teeth (Figure 17), dividing one rotation of the picking drum into three parts: the picking section, the throwing section, and the idle rotation section. To improve the picking effect, different stages require different rotation speeds and angles for the fingers. For instance, during the picking stage, the radial angle between the snap fingers and the drum shaft is reduced to prevent the peppers from falling. The function of the chute is to allow the roller to slide along it while rotating, similar to eccentric rotation, effectively adjusting the attitude of the finger at different stages. Du et al. [64,65] introduced the concept of biomimetics and designed a rigid–flexible coupling finger based on the human finger as a prototype (Figure 14a). The use of flexible materials to cover the rigid finger provides external flexibility and internal rigidity, mimicking the multi-finger pinching action. The concept of multi-finger cooperative arrangement was then introduced to design the arrangement scheme for the elastic fingers on the roller (Figure 18b). Optimizing the finger arrangement on the drum significantly improved the picking effect. Through discrete element calibration, collision simulations, and field tests on erect chili peppers, the model demonstrated superior impact-absorption capability.

Compared to other chili picking devices, the drum finger-type picking mechanism enables non-aligned harvesting. For instance, when plate-shaped and linear chili varieties mature, their plant canopies expand significantly, causing adjacent plants to intertwine irregularly. Under such conditions, traditional row-aligned harvesters experience reduced effectiveness and adaptability. Additionally, the drum finger-type picking mechanism has been widely adopted in Chinese chili harvesters due to its simple structure and versatility in accommodating various chili varieties. However, significant differences in harvesting efficiency are observed when harvesting different chili varieties. Furthermore, the slower drum rotation speed results in delayed chili discharge, while larger finger gaps increase the ground loss rate of harvested chilies. Improving the applicability of the drum finger-type picking device for harvesting different chili varieties and reducing the loss rate of chilies falling to the ground will be the primary focus of future development.

2.2.3. Long-Rod Comb-Tooth-Type Harvesting Device

The long-rod comb-tooth-type harvesting device has undergone extensive development. Its harvesting principle is similar to that of the double-helix comb-finger-type device and is intended for larger green chili peppers.

Following the proposal of the long-rod comb-tooth harvesting principle by Gentry, Urich designed a similar device. This design modification included rubber sleeves on the comb fingers, with the rod tilted at a specific angle relative to the ground, and the disk axis aligned perpendicular to the ground. The motion generated by this configuration ensured that the rubber fingers remained parallel to the ground and perpendicular to the direction of movement, effectively combing the chili plants. Massey Company implemented this mechanism to develop an inclined long-rod rubber comb-finger harvester for harvesting Mexican pepper (Figure 19a) [66]. Eaton’s design featured a disk axis parallel to the ground, with rubber fingers primarily moving vertically upward to harvest chilies from the bottom to the top. McClendon Company developed three-row, four-row, and six-row self-propelled chili harvesters based on the long-rod comb-tooth-type device, integrating cleaning systems (Figure 19b) [67,68]. These machines employed disk drives to enable reciprocating vertical rotation of the harvesting mechanism (Figure 19c). Funk et al. [69,70] conducted harvesting trials on five green chili varieties using five different devices, including the long-rod comb-tooth type. The results showed a harvesting efficiency of 42.8% and a damage rate of 16.6%. The experimental findings indicate that the harvesting efficiency of this device is considerably lower than that of other types of harvesting devices, while the breakage rate of the chili peppers is higher. Currently, this device is mainly used for harvesting dried red peppers and is employed in regions where peppers and cotton are rotated. This type of harvesting device is not well-suited to China’s chili cultivation practices, leading to limited research by Chinese scholars and enterprises.

2.2.4. Belt-Type Comb-Tooth Harvesting Device

The belt-type comb-tooth harvesting device has a relatively short development history. Its harvesting unit, characterized by a wide working width, is suitable for chili varieties planted with wider row spacing. The device operates at high combing speeds and frequencies, enabling it to pick up chili peppers that have fallen to the ground. A notable foreign model is the towed two-row chili harvester produced by Pik Rite Company, as shown in Figure 20a,b. This machine is capable of harvesting different chili types, including using the comb-tooth mechanism for dry red pepper harvesting and replacing the comb-tooth belt with vibration mechanisms for Mexican peppers and bell peppers, enabling high-frequency vibration picking. In 2010, China developed the 4JZ-2.6 self-propelled chili harvester, which incorporated a belt-type comb-tooth harvesting device. However, due to the wide harvesting unit, this design requires wider planting row spacing and faces practical challenges. This type of harvesting device has been less studied abroad. In 1998, Pik-Rite Company collaborated with Bucknell University to retrofit a sweet pepper harvester based on a tomato harvester, adopting the belt-type comb-tooth mechanism for chili harvesting [71]. For linear chili harvesting in Xinjiang of China, Chen [72] designed a prototype harvesting unit with a belt-type comb-tooth device (Figure 21) and constructed a test bench (Figure 22). Through orthogonal experiments, they analyzed the impact of comb spacing, harvesting belt speed, and forward speed on harvesting efficiency, damage rate, and loss rate, identifying an optimized combination of parameters.

2.2.5. Chili Re-Threshing Device

During pepper harvesting, the picking device detaches a portion of the stalk, which is then conveyed along with the peppers to the cleaning unit. When the cleaning and separation unit removes the stems, some peppers that remain attached may be carried away with the stems, resulting in losses. To minimize such losses, a re-threshing device was installed between the cleaning units. This device repeatedly combs and brushes long stems to detach remaining peppers and perform secondary cleaning. As the final process is aimed at reducing pepper loss, optimizing the re-threshing device has attracted significant research interest. Shin et al. [73] developed a four-roller differential-speed re-threshing device. Nam et al. [74] designed a three-drum re-stripping device, where each drum operates at a different speed, significantly improving detachment efficiency (Figure 23a). Zhao et al. [75] created a structure combining moving and fixed teeth, utilizing the relative motion between components to remove peppers. Shen et al. [76] designed a device featuring a combination of three drums and a fixed tooth structure (Figure 23b). This design enables pepper combing between rollers and also between rollers and fixed teeth, enhancing picking efficiency, but it also significantly increases pepper damage rates. Most re-threshing devices for peppers employ a combination of rollers and concave sieves. Their compact structure and small size make them suitable for installation on the cleaning units of self-propelled pepper harvesters. The performance of re-threshing is influenced by multiple factors, such as roller speed and feeding speed, leading to unstable detachment results. Research on pepper re-threshing devices started relatively late globally, with most studies focusing on structural design, leaving considerable room for further improvements. The main direction for future development will be the improvement of the structure and optimization of the core components of compound stripping devices.

2.2.6. Staged Harvesting Device

Staged harvesting is primarily used for the production of dried chili peppers, which requires specific moisture levels for proper preparation. Freshly harvested peppers, collected using mechanical harvesters, often have high moisture content, which increases drying costs. To address this, field drying is commonly adopted in some regions to reduce moisture content prior to harvesting. Staged harvesting consists of four steps: cutting the pepper plants, drying in the sun, plant collection, and pepper harvesting. Research indicates that cutting the plants at the roots for field drying results in the fastest moisture reduction.

Chili peppers are dried to lower their moisture content to below the standard required for dried chili peppers. In Xinjiang, China, cleaning–separation devices are used to directly harvest and clean peppers in the field. This process requires manual collection and feeding into the equipment, leading to high labor intensity, poor quality, elevated impurity rates, and missed harvests. In Shandong Province, before pepper harvesting, manual tasks such as picking, bundling, transportation, and storage are performed. Compared to field harvesting, bundling and transportation are more labor-intensive, costly, and difficult to store.

Pepper harvesting is the most time-consuming and labor-intensive stage in staged harvesting. To address the issues of time and labor associated with hand-picking, some manufacturers have designed field harvesters for dried artichoke peppers (Figure 24). However, these machines face challenges such as large size, difficulty in movement, and low cleaning efficiency. For small plots like those in Shandong, some agricultural machinery manufacturers have developed compact chili-picking devices that use rotating, axial-driven comb teeth to mechanically detach pepper fruits through impact. However, these devices have low efficiency, require manual feeding, and pose safety risks.

Recent advancements in sectional harvesting have prompted manufacturers and researchers to explore segmented chili harvesters. Inner Mongolia Baijiaohui Agricultural Co., Ltd. (E209, Tianfu Hetao Headquarters Base, Linhe District, Bayannur City, Inner Mongolia Autonomous Region, China) has developed a tractor-pulled chili harvester (Figure 25). This machine utilizes pickup belts to collect chili plants through tractor traction, after which the plants are conveyed to an internal picking device for harvesting. Han [77] designed a self-propelled bionic pickup harvester for upright chili varieties (Figure 26). The system includes a bionic pickup device, a lifting unit, a detachment mechanism, a cleaning unit, and a storage bin, performing plant collection, picking, and cleaning. Newfoundland Agricultural Machinery Manufacturing Co., Ltd. (South of Haixiang Road, Haiyang City, Yantai City, Shandong Province, China (in the yard of Kuntai Plastic Steel Profile General Factory)) has designed a self-propelled pepper picker harvester (Figure 27). The operating principle of this harvester involves the rotation of the picker drum at the front, which sets the plants onto a conveyor belt at the rear for backward conveyance. The plants are then fed into the picking device for harvesting, with the picked peppers conveyed to the clearing device for clearing. The cleaned peppers are transferred to a bin for storage. Additionally, the harvester is equipped with an air suction device located behind the picking device, which can collect peppers that have fallen to the ground. This represents the first self-propelled machine specifically developed for sectional harvesting, offering valuable technical insights for future segmented chili harvester designs.

2.3. Development Trends in Picking and Harvesting Technology

The harvesting technology for chili peppers has become relatively standardized, with the four aforementioned methods being the primary approaches. Among these, the unfolding double helix and drum snapping harvesting technologies are the most widely used in chili pepper harvesting machines in foreign countries and China, respectively. Although substantial progress has been made in the research on chili pepper picking mechanisms, unresolved issues such as pepper omission, damage, and high loss rates persist. These challenges still require validation through simulation analysis and extensive experimentation to confirm the structural rationality and parameter optimization of harvesting devices [78,79,80].

The primary cause of pepper loss is the use of rigid materials in the core harvesting components, which generate excessive impact forces, leading to pepper damage. Potential solutions include exploring alternative flexible materials, designing rigid–flexible hybrid harvesting components, or developing contact-free picking devices. Additionally, improper clearance settings in critical components (e.g., the gap in unfolding double helix harvesters) lead to pepper dropouts when harvesting smaller peppers and limit adaptability to various pepper varieties. An optimized solution involves designing adjustable clearance mechanisms that can dynamically adapt to variations in pepper and stem dimensions. Furthermore, altering the picking principle can also reduce the breakage rate of chili peppers. Current devices rely on direct contact with the pepper to complete the harvesting process, but such rigid contact inevitably causes breakage. Thus, developing contact-free picking mechanisms, such as those based on excitation or pneumatic principles, represents a promising avenue for reducing breakage rates. Since chili peppers are highly fragile, similar to tomatoes in their susceptibility to breakage during harvesting, existing tomato harvesting technologies may provide valuable insights for chili pepper harvesting.

The primary reasons for pepper omission are insufficient rotational speed or linear velocity of harvesting components, resulting in reduced impact. However, increasing rotational speed or linear velocity raises the risk of damage. The key technical challenge is to minimize omission rates while maintaining low damage levels. Intelligent solutions could involve the implementation of real-time monitoring systems that dynamically adjust operational parameters based on device status and pepper conditions.

Segmented harvesting offers significant advantages for dry chili production by eliminating transportation and drying processes, thus reducing labor intensity and costs. In China, approximately 45% of chili production is processed, highlighting substantial market potential. However, this technology remains in its early stages, with most cultivation areas still relying on manual harvesting, which is characterized by high labor intensity and costs. Therefore, segmented harvesting equipment holds promising market prospects and represents a crucial development direction for chili harvesting machinery.

3. Pepper Cleaning and Separation Technology and Device

The cleaning and separation device is designed to clean the harvested chili mixture by removing various impurities, such as long and short stems and leaves, while also detaching peppers that remain on the stems. Its operating principle is similar to the cleaning systems used in combine harvesters [81,82,83]. The primary types of cleaning devices include airflow-type, conveyor belt-type, drum-type, and combined configurations. Other variants include finger-type, plate-type, belt-type, and star wheel-type cleaning devices 17. Due to the complex physical properties of the mixed materials, single-type cleaning devices exhibit poor performance. Current solutions primarily rely on combined-type cleaning and separation devices. The following sections describe the most commonly used types of cleaning devices and their working principles.

3.1. Current Technology for Cleaning and Separating Chili Peppers

Airflow-based cleaning and separation device [84]: This device utilizes airflow exclusively to separate mixtures. The mixture is fed via a conveyor belt and thrown down at the top of the belt. An exhaust fan generates airflow to blow lighter impurities upward, while a suction fan removes the impurities through a debris outlet pipe. Chili peppers and heavier stems fall onto a secondary conveyor belt.

Conveyor belt gap-type separation device [85]: This device consists of two conveyor belts and a culm orientation adjustment mechanism. The mixture is conveyed upward by the first conveyor belt. During this process, the orientation mechanism aligns long stems parallel to the belt’s direction of movement. At the upper end of the first belt, peppers fall into the gap between the two belts, while long stems are transferred to the second conveyor belt.

Rotating blade and airflow combined separation device [86]: Key components of this device include a reel, blades, profiled wheels, and fans. The plants are fed by a conveyor belt, and the reel guides them into the grooves of the profiled wheel. The profiled wheel works with high-speed rotating blades to progressively cut the long stems. The mixture (including peppers, short stems, etc.) is conveyed backward via a vibrating inclined plate and falls onto a grid plate. Lightweight impurities are blown away by the airflow, while heavier stems undergo further screening. Short stems (with a smaller diameter than the peppers) pass through the grid gaps onto an impurity conveyor belt, while the cleaned peppers are transported to a collection bin.

Drum-type cleaning and separation device [87]: The core components of this device include an eccentric spiral brush auger, sieve bars, and a rotating drum (counterclockwise). The drum rotates counterclockwise, while the spiral brush rotates clockwise. Materials are fed into one side of the drum, where the spiral brush and drum jointly separate impurities. Short stems and leaves are expelled through the sieve gaps, while the peppers are transported to a collection bin via a rear conveyor belt.

Star wheel and airflow combined separation device [88]: This device consists of a star wheel set, fans, and material conveyors, with the star wheel set mounted above the fan. Materials are fed from the top and the rotating star wheels separate and push the stems backward, while the pepper–leaf mixture falls through the gaps between the star wheels. The fan blows out the leaves and other impurities, while the peppers fall onto the conveyor belt below and are transported to the aggregate box.

As shown in

Table 5. Types and characteristics of cleaning and separation technology.

Type	Schematic Diagram	Characteristics	Applicable Varieties
Pneumatic conveying system		The simple structure facilitates effective separation of pepper leaves but has poor performance in cleaning stems. It is suitable for peppers with larger volumes or higher masses.	Suitable for pepper varieties with fewer stems and more leaves.
Intermittent belt conveyor		Effective at separating long stems, but short stems and leaves tend to fall with the peppers. Excessive clearance increases impurity content, while insufficient clearance leads to blockage.	Suitable for pepper varieties with abundant stems and fewer leaves, particularly effective for small-sized peppers.
Rotary blade-pneumatic integrated system		The entire plant can be fed in for stem–leaf separation. However, the complex structure poses a risk of cutting peppers with blades. Frequent blade wear requires replacement.	Compatible with segmented harvesting methods and pepper varieties that require intricate picking operations.
Cylindrical drum mechanism		Excessive sieve bar clearance causes pepper leakage, while insufficient clearance leads to blockage. The separation efficiency for long stems is poor.	Suitable for cleaning large-sized peppers to prevent small peppers from leaking through gaps.
Star wheel-airflow coordinated device		Provides better clearing effects for both stems and leaves, but the fixed spacing between the star wheels limits its use to peppers of similar size.	Optimal for cleaning small-sized and dried peppers.

, the schematic diagrams of five typical scavenging devices are summarized with a brief description of their features and suitable pepper varieties.

3.2. Current Status of Cleaning and Separating Devices

Existing cleaning and separation devices primarily operate based on two principles: pneumatic and mechanical. Pneumatic devices separate chili peppers and leaves by utilizing their distinct suspension velocities. By adjusting the fan airflow intensity, these devices achieve separation. However, stems and peppers exhibit overlapping suspension velocity ranges, making it difficult for pneumatic devices to separate them. Since stems and peppers differ significantly in physical parameters, mechanical devices offer better performance in stem separation. Consequently, most chili harvesters now adopt combined pneumatic-mechanical configurations.

Research on the picking mechanisms of Chinese self-propelled chili harvesters has advanced rapidly, while the development of cleaning and separation systems lags behind. A representative Chinese model is the 4JZ-3600A self-propelled chili harvester (Figure 28). The harvester uses a star-shaped wheel and an airflow combination for cleaning and separation. The harvested peppers are transported backward through the upper cleaning unit. Peppers and leaves fall through the gaps in the star-shaped wheel into the lower cleaning device, where they are separated from the leaves by a cleaning fan. Long stems are directed to a re-threshing device for repeated brushing. The re-threshed peppers pass through a concave sieve to a secondary cleaning system for impurity removal [89].

Self-propelled chili harvesters are limited by their size and power consumption. Their installed cleaning and separation devices have simple structures and poor cleaning performance, resulting in a high impurity content in the harvested peppers. Large-scale chili cleaning and separation equipment is necessary to re-clean the harvested peppers and reduce the impurity rate. The function of this large equipment is to further clean the chili peppers harvested by the self-propelled harvester. Such equipment is typically large and integrates principles of air blowing, air suction, and mechanical separation, effectively separating peppers, stems, and leaves. Figure 29 shows a large chili separation and cleaning machine produced by Hebei Jilong Agricultural Machinery Co., Ltd. (Yicun, Dalucun, Dalucun Town, Ningjin County, Xingtai City, Hebei Province, China). This equipment can handle multiple chili varieties, including flat peppers, linear peppers, and pod peppers. It consists of four sections: the first section ensures orderly material feeding, while the second section separates long stems and performs re-threshing, using two comb-off rollers to repeatedly brush the stems. Peppers and small impurities fall onto a lower conveyor belt for backward transport, while large impurities such as long stems are discharged via an upper conveyor belt. The third and fourth sections feature pneumatic cleaning devices feature fans operating at different speeds, effectively removing light impurities such as leaves, offering strong cleaning and separation capabilities. However, this device is not suitable for fieldwork due to its size and lack of mobility. It is mainly used for re-cleaning peppers after they are harvested by self-propelled chili harvesters.

To improve cleaning efficiency, numerous studies have focused on separation principles and key components. Yuan [90] designed a star-shaped wheel combined with airflow for linear pepper cleaning (Figure 30). The design of the structure, rotation speed, arrangement, and gap size of the star-shaped wheels was based on the parameters of linear peppers. Virtual simulation tests were then conducted to optimize the structural and operational parameters. The optimal separation effect was achieved with a pitch of 185 mm for the star-shaped wheel, a row distance of 150 mm, for the wheel group, and a roller speed of 160 r/min. Kong et al. [91,92] measured the material properties of harvested linear pepper mixtures in Xinjiang and established suspension velocity models for both linear peppers and leaves. Through single-factor and Box–Behnken experiments, a ternary quadratic regression equation was developed to predict cleaning efficiency, and the optimal parameters for the star-shaped wheel mechanism were identified. Zou et al. [93] improved the structure of the airflow star wheel combination device to adapt it for flat pepper cleaning in Xinjiang (Figure 31a,b). Jo et al. [94,95] designed a card-type separation device similar to the star-shaped wheel. They optimized the arrangement, tilt angle, and rotation speed of the cards, and the test bench results showed that separation efficiency was significantly influenced by the tilt angle and rotation speed. Shin et al. [96,97] built a fan parameter test platform (Figure 32), and conducted factorial experiments on fan speed, air vent number, and tilt angle for cluster pepper cleaning. The optimal fan parameters were determined based on the results. Nam et al. [98] measured physical parameters of Korean red peppers, including Poisson’s ratio, modulus of elasticity, shear modulus, density, coefficient of restitution, and coefficient of friction. These measurements provide a reference for chili pepper parameter analysis and data support for subsequent virtual simulation studies. Zhang et al. [99,100] measured material characteristics of harvested cluster peppers in Guizhou. DEM-CFD coupling simulations were conducted to analyze the motion of mixed materials in airflow fields, and based on these results, a wind suction separation device was designed.

3.3. Development Trend of Scavenging and Separation Technology

Chili cleaning and separation is a critical process in mechanized pepper harvesting, which significantly reduces impurity content and improves harvest quality. During this process, some peppers are inevitably misidentified as impurities and are discharged. Post-cleaning evaluation indicators include impurity content, entrainment loss, damage rate, and device complexity. Existing chili cleaning devices do not adequately meet these requirements. Therefore, the development of a cleaning and separation device that fulfills all or most of these requirements remains an urgent need for advancing mechanized pepper harvesting.

An analysis of the current research status indicates that the focus has shifted from designing and researching mechanized cleaning and separation devices suitable for chili peppers to studying the working principles and parameters of core components. While chili cleaning technology has matured, inherent structural limitations remain. So, optimizing the structure to enhance cleaning quality continues to be the primary direction for the development of current harvesters. The key challenge faced by pepper harvester is the limited cleaning capacity of the cleaning and separating device, which is constrained by the size limitations of the self-propelled pepper harvester. Therefore, reducing the size of the device while ensuring effective cleaning and separation remains crucial for improving cleaning performance.

4. Research on Other Devices of Chili Harvesters

Current harvesting devices often leave pedicels on harvested peppers. M-TEC Corporation developed a device that uses speed differentials between rollers to separate peppers from pedicels through tensile forces [101]. Liao et al. [102] designed a differential roller system employing four speed-varied roller pairs for picking, cleaning, and destemming dried chili peppers, with optimal parameters identified through orthogonal testing. Li et al. [103,104] investigated the physical properties of Xinjiang Red Dragon peppers and designed a pedicel-removal device that utilizes centrifugal force to expel pedicels through mesh screens (Figure 33), but it requires a large space. Wang et al. [105] developed a multi-mode pepper pedicel-removal device that can handle pedicels of various shapes. This machine first removes straight pedicels using rollers, and then cuts bent pedicels with a specialized curved pedicel cutter. Peppers with cut pedicels are transferred to a second roller for residual stalk removal. This destemmer features a bulky structure and is only suitable for post-harvest processing. The device has not yet been integrated into field harvesting machinery.

Both re-threshing and pedicel-removal devices remain in the preliminary stages of development. Their performance directly impacts pepper’s loss and impurity rates, which critically affect harvest quality. Re-threshers primarily assist in separation processes, while the pedicel-removal device focuses on post-harvest processing. Neither technology has achieved optimal performance, and destemming devices have yet to be integrated into harvesters. Optimizing re-threshing mechanisms and pedicel-removal technology could significantly enhance pepper harvesting efficiency.

Chili pepper cultivation is less constrained by topographical factors, with various chili pepper varieties being planted in diverse terrains such as mountainous hills and plains. In China, chili planting areas are widespread. For instance, flat terrains in Xinjiang utilize wheeled chassis harvesters, while hilly regions such as Guizhou use tracked or semi-tracked chassis due to the poor mobility of wheeled chassis. Additionally, chili harvesters suffer from severe vibration, structural damage, and low chassis durability during operation, significantly affecting their performance.

Shooter et al. [71] designed a chili harvester chassis frame and conducted static and modal analyses to identify maximum vibration positions and proposed a vibration-damping solution. Ji et al. [106] designed a crawler-type self-propelled pepper harvester tailored to the hilly landscape, soft soil, and large slopes in Guizhou Province. By analyzing track forces and simulating the harvester’s dynamics under working conditions such as leveling, slopes, and crossing ridges and furrows, and performing kinematic simulations of the chassis using RecurDyn, they verified that the harvester demonstrated good stability and passability driving operation. Wu et al. [107] designed a small walk-behind chili harvester and performed harmonic response analysis on key components using finite element methods. They optimized the frame structure through static stress and prestressed modal analyses (Figure 34). Wang et al. [108] investigated the vibration characteristics of the tracked harvester’s undercarriage through finite element simulations, dynamic modeling, and field tests. Vibration was reduced by optimizing material parameters, providing theoretical guidance for structural design. Li et al. [109,110] modeled the chassis frame of a combine harvester and conducted modal tests to analyze resonance caused by comparing the excitation frequencies of other vibrating components. Xu et al. [111] conducted vibration tests at various points on a combine harvester using a dynamic signal analysis system to analyze excessive vibration causes, providing a reference for vibration optimization. Tang et al. [112] installed four types of prestressed support beams in the frame of a combine harvester’s threshing drum and found that the triangular beam was the most effective in suppressing vibration.

The primary function of the material conveying system is to transport a mixture of harvested peppers and impurities to the cleaning device, and to convey the cleaned peppers to harvested. Challenges faced at this stage include the tendency of the conveyor to clog easily and disordered feeding. Most harvesters utilize conveyor belts with baffles rather than combine harvester-style augers or pneumatic systems as conveyor belts have a simple structure and are less prone to blockages due to poor material flow in chili mixtures. Chen [113] designed a pneumatic-assisted conveyor based on the principle of “fine impurities first, coarse impurities later”, which ensures orderly feeding and improves cleaning efficiency.

Moreover, some pepper varieties have tangled stalks after ripening, which can wrap around the harvesting device when using a pepper harvester. This can lead to reduced harvesting efficiency or cause damage to the equipment. Therefore, it is necessary to design an anti-winding device for the pepper harvester. Xu et al. [114] designed an anti-winding device (Figure 35) that cuts off the stalks through the rotation of a baffle plate, driving the clearing and stationary knives to work in coordination. The anti-winding device is clearly shown in Figure 36a,b.

5. Summary and Outlook

5.1. Current Issues in Chili Harvesting Equipment

The development of chili pepper harvesting equipment in China has a relatively short history, and the level of mechanization remains low. Mechanized harvesting of peppers is only realized in some growing regions. Compared to grain combine harvesters, chili harvesters face multiple challenges, including low picking efficiency, high rates of missed-picking, and other technical limitations of existing chili harvesting equipment. The following section outlines the problems posed by chili pepper harvesting equipment and the proposed solutions.

(1) Poor adaptability of chili harvesters: Chili pepper cultivation in China features a wide range of varieties and diverse terrains, yet current harvesters show limited adaptability to both the different pepper varieties and topographical conditions. Unlike crops like wheat or corn, different chili pepper varieties exhibit significant variations in parameters and plant morphology. For instance, plate-type peppers have large, thick-skinned fruits, thick stems, and multiple branches, requiring double-helix picking devices. In contrast, artichoke peppers are smaller, less fragile, with straight stems and fewer forks, making them better suited for roller finger-snapping picking devices. The need to use different picking devices for different varieties increases production costs. Additionally, optimal picking heights vary among varieties, and the topography of hilly areas causes real-time fluctuations in these heights, exacerbating missed-picking rates and reducing yields with current equipment. Thus, improving machinery adaptability is crucial for reducing costs and increasing production.

(2) High impurity content of chili peppers after cleaning: Self-propelled harvesters often exhibit high impurity rates in cleaned peppers, as spatial constraints limit the size and structural efficiency of cleaning devices. While these systems effectively remove long stems and leaves, short stems are similar in length, diameter, and weight to peppers, so they prove difficult to eliminate. Large pepper scavenging and separating devices consist of a combination of various forms with good scavenging effects, but in order to reduce the cost of harvesting, it is necessary to design a scavenging and separating device specifically suited for chili pepper harvesters.

(3) Entrainment losses in chili harvesting equipment are primarily caused by four factors: First, chili fruits may be dropped during picking operations due to low rotational speeds of picking components or excessive gaps between parts. Second, peppers attached to long stems may remain unharvested and are expelled with the stems after the re-threshing process. Third, poorly designed mechanical cleaning devices misidentify some chili peppers as short stems, leading to their separation. Fourth, poorly designed airflow cleaning devices, such as inappropriate fan parameters or airflow duct structures, generate excessive airflow velocities, which cause peppers to be blown away with the leaves. Entrainment losses represent a major cause of yield reduction.

(4) Severe missed-picking issues: Most chili harvesters in China employ drum finger-type picking devices with limited combing frequency. These devices are restricted by plant morphology and controlled brushing forces to prevent damage, leading to significant missed-picking issues. Notably, missed-picking and fruit damage rates are inversely related. Increasing rotor speeds or reducing operational speeds arbitrarily not only increases energy consumption and reduces efficiency but also exacerbates fruit damage.

(5) Absence of unified technical standards: The current technical specifications for chili harvesters are mainly focused on specific picking methods or particular varieties, lacking comprehensive standards for harvesting equipment. This gap results in inconsistent performance metrics, quality requirements, and safety parameters across products, leading to market quality disparities and hindering industry development. Furthermore, the lack of standardized cultivation practices across regions and varieties complicates improvements in machinery adaptability. Establishing unified technical standards for both harvesting equipment and cultivation practices is essential for sustainable development of mechanization.

5.2. Solutions to Mechanized Harvesting Questions

Addressing the mechanical issues present in chili harvesting equipment is crucial for advancing mechanized harvesting technology. Based on the challenges outlined in Section 5.1, the following solutions are proposed.

(1) Addressing the problem of poor adaptability of pepper harvester to varieties: In the design of new picking devices, we should look for commonalities between different pepper varieties to enhance the applicability of pepper varieties. In the design of interchangeable cutting platforms, harvesting different varieties of peppers can be quickly replaced with the corresponding cutting platform and the use of a picking device to harvest different varieties of peppers through tests used to obtain the specific optimal mechanism and operating parameters, in addition to the use of corresponding parameters when harvesting different varieties of peppers. In order to solve the problem of poor adaptability of the pepper harvester to different terrains, we must reduce the influence of terrain on the harvester’s travel by improving the chassis and travel system. A contour-following mechanism suitable for chili harvesting should also be designed to address the issue of inconsistent pepper heights caused by uneven terrain. Both shape-mimicking mechanisms and picking devices can benefit from bionic principles, such as the bionic-type finger-snapping mechanism, which significantly improves the chili harvester’s picking performance. Therefore, incorporating bionics, by simulating certain organs of animals, may lead to unexpected advancements in chili harvesting equipment.

(2) Reducing impurity rates in cleaned peppers: Efficient and compact cleaning mechanisms are essential. Short stems can be separated from the peppers using vibrating screens with smaller apertures, exploiting the physical differences between the peppers and the stems. Additionally, pedicels (fruit stalks) attached to harvested peppers contribute significantly to impurities. Integrating pedicel-removal mechanisms into cleaning systems will improve the quality of the harvest.

(3) Reducing entrainment losses during chili harvesting: Experimental studies should first investigate the relationship between chili fruit drop and operational parameters of the working components, optimizing these parameters to minimize fruit loss during picking. Effective re-threshing devices should also be designed, as stems and other materials tend to be in a disordered state during re-threshing, which reduces efficiency. Therefore, auxiliary feeding mechanisms should be developed to ensure that long stems are fed into the system in an orderly and directional manner, optimizing the re-threshing process. Furthermore, mechanical cleaning mechanisms should be designed to include vibrating screens to enhance the separation of peppers from impurities. The airflow velocity required for cleaning varies by chili variety; thus, measuring the terminal velocity characteristics of different varieties and adjusting fan parameters accordingly will improve the adaptability of the cleaning system. The airflow duct design plays a crucial role in cleaning efficiency, yet no dedicated duct designs currently exist for chili harvesting. Therefore, research into fan and duct configurations is essential for optimizing the reduction in entrainment losses.

(4) Addressing severe missed-picking issues: A feedback regulation system should be established to balance the rates of missed-picking and damage. Missed peppers can be identified using visual recognition, and the missed rate can be calculated. Sensors can also monitor the breakage rate. By analyzing both rates and balancing them, operational parameters can be adjusted to maintain both rates at low levels, ensuring the picking device operates optimally.

(5) Addressing the lack of relevant standards: Relevant authorities should take the lead in developing and promoting standardized cultivation protocols for different chili varieties, creating favorable conditions for mechanized harvesting. Additionally, unified standards for chili harvesting equipment should be established to enhance its adaptability.

5.3. Intelligent Development Prospects for Chili Harvesting Equipment

Chili harvesting equipment is currently in its foundational development stage, with the primary focus on addressing the mechanical issues outlined earlier. Optimizing the structures and parameters of core components can effectively enhance the performance of the equipment. Advanced grain combine harvesters use hundreds of sensors for real-time operational quality monitoring and incorporate comprehensive intelligent control systems to maintain optimal performance [115,116]. In contrast, chili harvesting equipment exhibits lower intelligence levels and remains in its early stages of development. Nevertheless, the need for intelligent development in chili harvesting equipment is undeniable. Both advancements in intelligence and optimizations of mechanical structures can improve harvesting outcomes. In the future, optimizing mechanical structures and core component parameters while simultaneously advancing intelligent technologies—”advancing both aspects in parallel”—will maximize harvesting efficiency.

Regarding the development of intelligence, the first priority is to improve the intelligent detection and adjustment capabilities of the picking and cleaning–separation devices. This requires breakthroughs in sensor technology to acquire critical operational quality and condition parameters. A crop information perception and harvest quality monitoring system should be established based on these advancements. Sensors will be installed to monitor real-time parameters such as chili moisture content, damage rate, and missed-picking rate. Additional sensors should be installed on picking and cleaning–separation devices to monitor the operational status of the working components and collect relevant parameters. By comprehensively analyzing chili parameters and operational data, adjustments to component parameters can enhance the performance of the picking and cleaning–separation devices.

Second, intelligent decision-making and control must be achieved, which involves two key aspects: adaptive control and path planning. Adaptive control refers to automatically adjusting parameters such as header height based on terrain, chili growth conditions, harvesting environments, and acquired data. Intelligent contour-following mechanisms and systems should be designed to reduce missed picking caused by mismatches between header height and optimal picking height. Path planning involves automatically planning harvesting routes using satellite navigation, map data, and perceptual information. Grain combine harvesters have already made notable advancements in this area [117]. Path planning can effectively reduce redundant operations and missed picking, while improving harvesting efficiency.

Third, the intelligence of the material handling mechanism must be improved. The transport and feeding of materials from picking devices to cleaning–separation systems significantly affect the cleaning–separation process. An automatic monitoring and adjustment system should be established to detect cleaning–separation performance in real time and regulate material feed rates, preventing severe entrainment losses or reduced harvesting efficiency caused by overfeeding or underfeeding. For large-scale chili cleaning–separation equipment, cleaning and separation occur throughout the material conveying process. Intelligently adjusting the speed of each conveying section based on material parameters can reduce costs, improve efficiency, and enhance cleaning quality.

Fourth, automation and unmanned operations must be realized. With the continuous development of autonomous driving technologies, chili harvesters should gradually adopt these technologies to reduce labor intensity. Additionally, chili peppers, which require batch harvesting, are currently manually picked, as large harvesters are unsuitable. Therefore, batch-harvested chili peppers also require intelligent technologies for development, moving toward harvesting robots capable of precise recognition and selective picking of mature peppers. This demands high levels of automation and unmanned operational capabilities.

In general, the current chili harvester still lags significantly behind advanced intelligent combine harvesters in terms of intelligence and low-loss mechanization. Particularly, the foundation of intelligence remains weak. In its development process, intelligent technologies such as machine vision, AI recognition, and so on should be developed in a short period, improving monitoring capabilities, autonomous navigation system, and unmanned operation levels. It is essential that the chili harvester quickly catches up to the intelligent level of the combine harvester and further enhances its performance by optimizing key components.