**1. Introduction**

Harvesting is an important element of orchard production since it has a brief window period, high labor intensity, and high labor volume. The high labor cost in the harvesting stage limits the fruit industry's development. With this backdrop, fruit-picking robots have become a hotspot for study in related fields [1,2]. Researchers have completed several projects and made significant progress in important technologies such as robot perception and positioning [3,4], system integration [5], and efficient harvesting end effector design.

As a critical step in robotic harvesting, grasping determines the picking effect directly. During harvesting, the traditional robotic rigid clamping mechanism has issues: high requirements for fruit positioning [6] and easy damage to the apple pericarp [7,8]. In practical applications, it not only required the grippers to be dexterous, light, stable, and reliable to grasp but also to ensure that the appearance of the fruits is not damaged, to prevent harming commerciality. As a result, research on non-destructive harvesting end grippers for safe, reliable, and stable gripping is an important topic for harvesting robots with a promising application.

**Citation:** Chen, K.; Li, T.; Yan, T.; Xie, F.; Feng, Q.; Zhu, Q.; Zhao, C. A Soft Gripper Design for Apple Harvesting with Force Feedback and Fruit Slip Detection. *Agriculture* **2022**, *12*, 1802. https://doi.org/10.3390/ agriculture12111802

Academic Editors: Jacopo Bacenetti and Tao Cui

Received: 27 July 2022 Accepted: 27 October 2022 Published: 29 October 2022

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To lower the fruit damage rate, the soft gripper technology is attracting more and more researchers' attention. Some researchers [9–11] used soft materials on the surface of the fingers to increase the gripper flexibility and, hence, prevent damage to grabbed objects. However, due to the rigid support of the fingers' main body, it is also easy to cause different degrees of damage to the fruit pericarp. Furthermore, the structure is more complex, and the grasping stability is insufficient.

The soft structure gripper has a high adaptability, wide range of variability, and excellent working ability for gripping objects that are susceptible to damage [12,13].

Shepherd et al. [14] proposed the PneuNet (pneumatic mesh) structure, a bending multi-cavity pneumatic soft actuator. The soft gripper [15–17] designed by Whiteside's group has the characteristics of minimal pressure bearing, large deformation, and flexible movement. However, the end contact force is limited, and the stability is insufficient when grasping objects. A vision-equipped six-finger soft harvesting gripper [18] can identify the type and maturity of fruits and vegetables, and it can softly grab fruits and vegetables based on their shape but only for tiny fruits. Muscato et al. [19] created a soft citrus harvesting gripper out of spirally organized rubber sheets that had a strong wrapping capacity for gripping things but that lacked rigidity.

German bionics researcher Leif Kniese accidentally discovered the "Fin Ray effect" in 1997 [20], which was later widely employed in the study of robotic soft grippers [21,22]. Fin Ray soft fingers are highly compliant and can take greater loads than other soft constructions. Thanks to its superior grabbing stability, the Fin-Ray-effect-inspired grippers have received extensive attention from researchers.

However, the basic finger structure is not optimal for soft grippers, and studies have recently increased the gripping force by improving the finger structure [23–26]. Crooks et al. [23] proposed a multi-material structure gripper with a higher grabbing weight, but the fabrication method for this multi-material structure is quite tricky. Basson et al. [24] varied the slope and curve of the cross beams in a Fin Ray finger and analyzed the stress and displacement on the improved finger through simulation. However, the effects of other variables have not been fully tested. Shin et al. [25] analyzed the changes in stress and displacement when the finger touched an object by varying the number of cross beams, the front beam slope, and the slope of the cross beams. Elgeneidy et al. [26] developed a soft finger that could handle fragile objects by varying the angle and number of cross beams. Nevertheless, whatever structure maximizes the Fin-Ray finger gripping force while causing no damage to the object has yet to be determined.

Although it can greatly avoid fruit damage due to grasping by using the soft fingers, it is not sufficient to rely solely on the soft structure to ensure the gripper's lossless grasping. The gripper's lack of a force feedback system makes it unable to collect the contact state information between fingers and gripping items, which may cause damage due to excessive gripping force or slippage owing to insufficient gripping force.

Some researchers added force sensors to the fingers of soft grippers [27–31]. The sensing system is simple, but the sensor deforms with soft fingers, which has a great influence on the accuracy. When directly embedding force sensors through the manufacturing process but the cost is large and the universality is low due to its sophisticated driving scheme and manufacturing method [32,33]. Some researchers [34–36] estimated the contact force by substituting the force perception model from finger deformation by vision. Belzile et al. [37] used the quasi-static analysis method to calculate the contact force generated by the gripper, which realizes the internal force perception without the use of additional force sensors, but the solution process and control algorithm are complex.

In addition to preventing fruit damage due to excessive gripping force, slip detection is also an important factor due to the rough surface of the fingers [38,39]. Some studies use multi-axis or more force sensors to monitor the static friction coefficient between the finger surface and the object [40,41] or to detect vibration caused by sliding between the two contact surfaces using piezoelectric phenomenon [42], time–frequency conversion technique [43], or filtering [44] to accomplish slip detection. However, the sensors are dependent

on the working environment, and utilizing more sensors to gather more tactile information would not only dramatically raise the cost but will also place a significant load on the gripper structure and control system. Some recent studies employ tactile data for training, and neural networks can predict item sliding [45,46], as well as physical parameters such as temperature, electromagnetism, light intensity, and acceleration to predict slippage [47]. Liu et al. [48] introduced a novel design of the GelSight Fin Ray gripper, which used a vision-based tactile sensor for tactile reconstruction, orientation estimation, and slip detection. But it is difficult to grasp heavier objects due to the design of its hollowed-out finger. Nonetheless, these technologies are rarely used on harvesting grippers.

To solve the above problems, this work proposes a novel soft harvesting gripper with flexible adaptive envelope, force feedback, slip detection, and other features. To design the Fin-Ray finger structure in such a way that the gripping force is high enough to ensure it is sufficient to successfully separate fruits from stems, the influence of various parameters of the Fin Ray structure on the gripping force and deformation of the finger was investigated through simulations, as the basis for the design of the soft gripper structure. The following are the main contributions:


In addition, to provide a theoretical basis for the design of the gripper, some mechanical properties of apples are given in the experiments.

**Remark 1.** *It should be clarified that the force feedback system and slip detection are two main contributions in this paper. To provide a stable mechanical design of the gripper as a study basis for these two points, we also analyze the structural parameters of Fin Ray fingers by the finite element analysis method.*

#### **2. Structural Design of a Soft Gripper with Three Fingers**
