**1. Introduction**

Grapes occupy an important place in the world; their annual output has reached about 79 million tons [1]. Grape cultivars can be categorized into four main groups for food usage: table grapes, wine grapes, sweet juice grapes, and raisin grapes [2]. Among these, table grapes occupy an important place in global fresh cluster fruit production. Because of their high quality, attractiveness, and numerous nutritional facts [3], more than 65% of

**Citation:** Faheem, M.; Liu, J.; Chang, G.; Abbas, I.; Xie, B.; Shan, Z.; Yang, K. Experimental Research on Grape Cluster Vibration Signals during Transportation and Placing for Harvest and Post-Harvest Handling. *Agriculture* **2021**, *11*, 902. https:// doi.org/10.3390/agriculture11090902

Academic Editor: Francesco Marinello

Received: 28 July 2021 Accepted: 14 September 2021 Published: 19 September 2021

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grapes produced are consumed as a fresh eating fruit [4]. The yield of fresh eating table grapes is enhanced by the development of the grape industry [5].

Table grapes are not a climacteric fruit, and in the process of harvesting and postharvest operations such as storage, packaging, transportation, and logistics, table grapes undergo serious mechanical loads due to impact, collision, and long-term vibration. These mechanical loads cause the berry falling of clusters. Therefore, it is of great significance to analyze the berry drop mechanism of grape clusters during post-harvest operations to improve the quality and shelf life of grape clusters and reduce the economic loss of the producer. The harvest and post-harvest handling of fresh eating table grapes are in the form of clusters or bunches until they reach the supermarkets. As to manual or robotic handling during the harvesting and post-harvest of fresh grape clusters, the mechanism and phenomenon of damage are totally different from single stem fruits [6]. Usually, grape clusters are clamped and cut from the main rachis. The hanging cluster, after cutting, needs to be transported to a basket or box, then unloading and placed into a basket or bulk bin to complete the on-site transportation. Additionally, for long-distance transportation or post-harvest operations, grape clusters need to be handled several times [7–15]. So the probability of berry drop greatly increases. The loss caused by the berry drop and decay of fruit grain is up to 20% to 30%. The integrity of fresh cluster fruit and non-destructive evaluation are two major quality criteria of grape clusters [16]. The problem of berry fall seriously affects their shelf life and marketability [10,17,18], which has become a serious problem that has plagued the table grape industry chain for a long time. Additionally, it has become a key obstacle to the development and control of machinery and robotic equipment for grape cluster post-harvest handling. Therefore, basic research on grape cluster vibration signals can help to predict and grasp the berry dropping mechanism during robotic transportation and placing. This study will help suggest an effective means of control that has important scientific significance and commercial value.

Mechanical loads have been known for many years as a major factor causing postharvest losses and damage to many single stem fruits. The dynamic impact of collision is the main cause of single stem fruit damage. Many studies have been carried out on the impact damage of various kinds of single stem fruits all over the world. The most common methods to determine impact loading damage are as follows: (1) drop tests [19–23]; (2) pendulum action, either by attaching a fruit to the pendulum [24,25] or by hitting the fruit with a pendulum tipped with a specific shape impactor [26,27]; (3) electronic fruit or impact recording devices [28–30]. However, the main problem with these methods is that vibrations make it difficult to accurately record force and deformation during impact due to shorter time periods to observe the mechanism. Therefore, high-speed cameras are more frequently used to observe quality and damage for impact and vibratory research into different fruits [8,21,30–34]. Impact damage is mainly caused by the factors such as the type of packaging surface onto which the single stem fruit drops, drop height, and the velocity at the moment of collision [35–40]. However, impact damage for single fruits is totally different from cluster fruits because cluster fruits are gripped and cut from the main rachis. Therefore, vibration transmissions during transportation and excitation transmissions due to the impact of packaging surfaces on the cluster fruits are totally different from single stem fruits.

The post-harvest operations affect the quality of table grapes through direct contact with packaging materials and machine components. A large quantity of table grapes is wasted just because of damage such as berry fall and fruit decay. Berry fall or decay is mostly caused by impact loads during the mechanical handling, packaging, storage, and transport of table grapes [41]. During fresh fruit transport and handling, dynamical loads cause, by far, the most fruit decay and shatter damage because these loads are higher in incidence and magnitude than static loads [42,43]. The berry drop (shatter) of the grape cluster during and after harvest is related to its physiological process and physical function. Vibration and impact in each operation can lead to fruit stalk detachment. There are three categories of grape berry drop: (1) berry shatter, which consists of a detachment of berries

from the main rachis due to the fragile tissue structure of the stalk; (2) wet drop, that is, berries are sloughed from the stems and attached to the pedicel because of the short and thin berry brush [44,45]; (3) dry drop or abscission, which is caused by the formation of an abscission zone (AZ) in the grape, which develops at the junction between the pedicel and berry [4].

At present, chemical methods are widely used in the grape industry to solve the problem of grape berry drop and berry decay [46–49]. However, in recent years, more and more attention has been paid to non-chemical methods. Researchers have been trying to explore the relationship between mechanical harvesting and berry damage to optimize mechanical handling methods for the reduction of berry fall and berry damage. For instance, Pezzi et al. used an electronic fruit to investigate the collision of fresh grapes during mechanical harvesting and transportation [50,51], and Yue et al. found that the drop impact of grape berries has significant effects on physiological quality during storage and transportation [52]. Bian et al. studied the influence of drop height on the dielectric properties of red globe grapes [53], and Vinokur et al. found that the berry fall rate is directly proportional to the free-fall height [41]. Jung et al. evaluated the effect of vibration stress on the quality of packaged grapes by simulated transportation [2], and Vallone et al. measured the effect of mechanical harvesting of grapes using an instrumented sphere [54]. Deng et al. developed a mathematical model that predicts grape berry drop during storage [4], and Fischer et al. determined the critical frequencies for grape and strawberry fruit shattering during transportation for distribution [55,56]. Lu designed tests, such as an emergency stop test in vertical fall, to observe the fruit collision of grape clusters with a piezoelectric film [57], and Hao et al. found that the greater the vibration acceleration, the greater the damage to Kyoho grapes during storage and road transportation [58]. Demir et al. calculated the natural frequency of grape and berry drop during simulated transportation [59]. The above studies deal with the collision between single berries and placing surface and the effect of drop height on berry damage. However, the impact of mechanical load on harvest and post-harvest quality and berry drop of fresh grape clusters in term of vibration has still many research gaps.

Grape cluster vibration plays a vital role in the process of mechanical harvesting and post-harvest handling of table grapes because it will cause berry fall and berry damage. To explore the influence of excitation on vibration, a simulation modeling method is very important, in addition to experimental methods. Simulation can help us to study the transmission route of the excitation and vibration of the cluster. Additionally, simulation can help us to realize the effect of mechanical handling on the grape cluster. To find out the vibration range under different conditions, Kondo et al. designed a low-speed tomato vibrating test system and modeled the panicle tomato [60], and Liu et al. found that acceleration and deceleration are the reasons for vibration in grape fruit clusters; they also found the relationship between the angle deviation of the grape cluster and the excitation transmission through a high-speed camera [61]. Liu et al. designed a compound mechanical model of grape clusters and carried out simulation and experimental analyses under different excitations in horizontal transportation to observed the swing angle of each berry [8]. Faheem et al. found the relationship of the swing angle of clusters with hanging force during linear robotic transportation of the whole grape cluster at different excitations [62]. No specific studies have been done on the impact of packaging materials and the effect of the cluster's vibration on the berry drop mechanism during robotic vertical transportation and placing of the whole grape cluster.

The damage due to the impact and vibration of the grape cluster has a significant effect on control losses during robotic post-harvest handling on the industrial as well as farm level. In this context, the main purpose of the study is to analyze the berry dropping mechanism during vertical transportation and the placing of the whole grape cluster. The behavior of the grape cluster's berries and the hanging force signals (the force that bears the weight of the gripped grape cluster against gravity during robotic transportation) under different speed and acceleration excitations were observed and analyzed. The effect of different packaging materials on the berries from the top and bottom sides of the cluster was analyzed using a high-speed video camera. Additionally, the relationship of the cluster's mass with forces before and after the impact was analyzed to understand the berry drop mechanism. Overall, this study provides theoretical support to the industries by optimizing the berry falling loss of different cluster fruits during robotic post-harvest handling and suggests a safe packaging material and excitation at which the cluster vibrates with less magnitude. Hence, berry drop will be reduced.

#### **2. Materials and Methods**

#### *2.1. Structure of Fresh Grape Clusters*

Cluster fruits have some special features compared to single-stem fruits. Fresh table grapes develop as clusters (bunches), with each berry attached to the pedicel through rachis and sub rachis, which contain vascular bundles (also known as the cap stem). The stem unites the berry with the rachis, as shown in Figure 1. This union is very important to avoid loss of berries (dropping or shattering) [62].

**Figure 1.** Structure of table grape cluster with and without berries. (**a**) Table grape cluster; (**b**) stalk fruit structure.
