**1. Introduction**

China is a major producer of vegetables in the world, with its production scale and export scale ranking first in the world [1]. As a vegetable favored by the world, broccoli's planting scale is increasing yearly. At present, the total planting area in China is about 100,000 hectares, and the output accounts for 50% of the global total [2,3]. With the increase in planting scale, traditional manual harvesting is time-consuming and laborious; therefore, the development of picking equipment is urgently needed [3,4].

The design of a manipulator in vegetable-picking equipment is very important and requires a stable structure, accurate picking, fast response, and minimal damage to the crop [5,6]. Developed countries began to study picking robots in the 1960s, and the research and development of vibrating, suction, shear, and other manipulators promoted the mechanization of picking equipment, but the actual efficiency was not high [7–9]. At present, fruit and vegetable picking is accomplished by designing a manipulator with a stable structure and coordinating with vision [10–12]. Kinugawa et al. [13] designed an underactuated manipulator for circular plates, which is not applicable to plates with other shapes. Xiong et al. [14] improved the flexibility of the end-effector for the difficult task

**Citation:** Xu, H.; Yu, G.; Niu, C.; Zhao, X.; Wang, Y.; Chen, Y. Design and Experiment of an Underactuated Broccoli-Picking Manipulator. *Agriculture* **2023**, *13*, 848. https:// doi.org/10.3390/agriculture13040848

Academic Editors: Cheng Shen, Zhong Tang and Maohua Xiao

Received: 25 March 2023 Revised: 6 April 2023 Accepted: 6 April 2023 Published: 11 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of picking in an unstructured environment and carried on a two-arm cooperative robot to complete strawberry picking. However, due to the lack of flexibility of the end-effector, the success rate of picking was 70%. Arad et al. [15] developed a sweet pepper-picking robot with six degrees of freedom, but its picking efficiency could not be guaranteed owing to poor algorithm recognition. Although the research and development of picking equipment started late in China, it is developing rapidly now. Various kinds of picking equipment, such as pneumatic apple picking, hand-held tea tender shoot picking, cooperative kiwi picking, and under-driven grape picking, have emerged one after another [16–19]. The underactuated manipulator has good grasping stability, a strong envelope, and high flexibility [20–22]. Yang et al. [20] proposed a new underactuated manipulator, which has three degrees of freedom and ensures the stability of grasping but is not applicable to crops. The humanoid finger mechanism designed by Yin et al. [19] ensures flexible grasping by installing torsional springs with different stiffness coefficients at the knuckle limit, but the structural strength of the mechanism cannot be determined. Ma et al. [23] designed a Y-type underactuated manipulator according to the growth characteristics of sweet pear. Although the picking rate of the device is high, the degree of damage caused by the picking of this manipulator cannot be judged.

Broccoli flower balls are big, have an unequal diameter, and have a thick stem. Therefore, few robot hands are suitable for enveloping flower balls. At present, the mechanized harvesting equipment of cabbage uses high-horsepower tractors loaded with harvesters to complete one-time harvesting. Although highly efficient, it has no selective harvesting function [24]. Blok [25] et al. developed a broccoli image recognition system to solve the problem of selective harvesting. Although selective harvesting can be achieved, the harvesting efficiency is too low. Lapalmeagtech Co., Ltd. combined Sami4.0 [26] with a broccoli harvester to develop a fully automatic harvesting robot. Although the harvester is highly efficient, its large fuselage is not suitable for use in hilly areas. In China, Shandong Hualong Agricultural Equipment Co., Ltd. (Qingzhou, China) developed a broccoli-harvesting and loading machine, which relies on human labor to harvest broccoli and place the flower balls into the harvesting conveyor belt, but a whole mechanized harvesting operation has not been realized [27]. Yu et al. [28] designed a hanging bucket for broccoli harvesting, which can reduce the labor force. However, the actual nature of the device is similar to that of Shandong Hualong's broccoli-harvesting device, which relies on human resources. However, the broccoli-harvesting equipment of China Agricultural University [29] and Jiangsu University [30] is still in the research and development stage.

In this paper, an underactuated broccoli-picking manipulator was designed to solve problems in the manual picking of mature broccoli, such as big flower balls, thick stems, and huge time and labor consumption. The underactuated broccoli-picking manipulator will be designed to have a compact underactuated structure and a simple control system to realize the stable picking of flower balls. First, physical damage tests were carried out to determine the range of flower ball surface pressure and stem-cutting force. Then, the mechanical structure and control system of the manipulator was designed from the view of the statics of the picking mechanism. Finally, a picking test bed was set up to carry out the indoor picking test.
