## 1 _Introduction_  
In orchard management, orchard plant protection is the key link to ensure stable and increased fruit production, and its workload accounts for about 25% of the total workload of orchard management [1]. Although frequent chemical control can effectively control the occurrence of pests and diseases, it also causes many problems such as excessive application dosage, serious pollution, and pesticide residues [2]. The realization of on-demand, accurate, and intelligent pesticide application technology and equipment guarantees the improvement of the pesticide utilization rate, and it is also an important measure to ensure food safety and reduce farmers' labor intensity, which was the focus of the current research [3]. The vast majority of citrus is planted in mountainous and hilly areas [4] with complex terrain and scattered planting. The orchard row spacing is generally 4.0-5.0 m and the seed spacing is 3.0-4.0 m. The canopy is mostly of the open-centered type with dense leaves and thick branches. Due to the lack of consideration of mechanized operations in the establishment of orchards, it is easy to form interlaminar closures [5]. At present, orchard pesticide application technology mainly consists of pipeline spraying [6, 7, 8, 9], air-assisted spraying [10, 11], variable-rate spraying [12, 13, 14, 15], electrostatic spraying [16], recycling spraying [17, 18], and aerial spraying [19, 20]. In pesticide spraying, it is expected that the droplets of the pesticide liquid should be deposited and attached to the foliage as much as possible [21]. Among these pesticide application technologies, the pipeline spraying technology is widely used in orchards, and compared with the vehicle-mounted spraying technology,it saves more time and labor. However, this kind of technology still has some problems such as frequent pipe explosions caused by uneven distribution of the pipeline pressure, corrosion by the residual pesticide liquid in the pipeline, and easy poisoning of operators who are applying a pesticide in a closed orchard. Spraying machinery based on air-assisted spraying, electrostatic spraying, variable-rate spraying, recycling spraying, and other pesticide application technologies are mostly connected to the rear of the tractor in a pull type or suspension type. In addition, these spraying technologies can be combined to further improve the deposition rate of the pesticide liquid and reduce the spray drift. Among them, compared with an air-assisted quantitative sprayer, the laser target variable-rate sprayer takes the canopy as the spray target, which reduces spray drift in the air by 70% to 100% and reduces spray volume by 47% to 73% [22]. It promotes reductions in both pesticide wastes and pollution of the environment by pesticides. However, due to the small row distances of fruit trees and a lack of transfer agricultural machinery road in hilly orchards, a conventional spraying machine is too large to move in such orchards. Based on aerial application technology, plant protection UAV is flexible in pesticide application and is not limited by terrain and crop growth, but its droplet deposition distribution is uneven and the droplet penetration is poor [23]. All of the above orchard-spraying technologies spray from the outer layer to the inner layer of the canopy. With a gradual increase in leaf density, the deposition rate in the canopy will gradually decrease [24] and the deposition rate on the back of leaves will be much lower than that on the front of leaves. The current spraying machines are all aimed at the entire canopy in spraying operations, but the spray actuator cannot reach into the canopy. With flexible kinematic performance in complex working environments [25; 26], the cable-driven flexible robot has shown a good capability for obstacle avoidance in narrow spaces [27] by dexterously avoiding the trunk and going deep into the pest-infested and diseased areas within the canopy, and is suitable for hilly orchards with closed rows and dense leaves.