# The Development of Rubber Tapping Machines in Intelligent Agriculture: A Review
## 1 Introduction  
Natural rubber is an important industrial raw material and strategic resource, the products of which are important in the fields of transportation, national defense, and the military industry [1]. The excellent resilience, electrical insulation properties, wear resistance, airtightness, and flexibility of natural rubber make it widely used in people's daily life, in the areas of medicine, hygiene, and so on. There are two types of rubber currently used in industry [2]. One is natural rubber, produced by natural plants, and the other is synthetic rubber, synthesized by chemical processing. Driven by industrialization, the use of synthetic rubber has developed rapidly, accounting for more than half of the total production capacity in the world [3]. Synthetic rubber is superior to natural rubber in terms of some of its special properties, such as chemical resistance and grease resistance [4]. However, the high impact resistance, puncture resistance, and tear resistance of natural rubber are unmatched by synthetic rubber. This facilitates metal bonding, which makes it irreplaceable, especially for aircraft tires and truck tires used in service under complex load conditions [5]. For instance, the manufacturing of truck radial tires requires 50% natural rubber, off-the-road (OTR) tires need to be made of 90% natural rubber, and aero tires needto be made of 100% natural rubber. According to statistics, there are more than 70,000 types of products around the world that are made from natural rubber. As shown in Figure 1, natural rubber is obtained from the laticiferous vessels of the rubber tree via regular tapping [6]. Tapping is the process of creating a path that must lie 1.2-2.0 mm and at an appropriate angle above the cambium, which is the tissue between the wood and laticiferous vessels [7]. The thickness consumed in the vertical direction during each tapping is defined as the bark consumption, and this is required to be 1.4-2.1 mm [8]. Thus, it is a skill-oriented job [9]. During 2:00-6:00 a.m., the latex output is higher due to the expansion pressure reaching its highest point, so this operation is usually performed in the early morning [10; 11; 12]. Tapping operations may also lead to latex allergy and occupational disease [13; 14; 15; 16]. With the combination of the development of urbanization and industrialization, as well as great skill requirements and high labor intensity of manual tapping, the problem of dwindling rubber farmers has become increasingly prominent [17]. With the aim of responding to the above problems, emerging information technologies are important in transforming traditional farming methods into a new type of precision agriculture [19; 20; 21; 22; 23]. For example, CITEVEA Technology Company Limited, Ningbo, China put forward an automatic intelligent rubber tapping system [24]. This system sends tapping instructions from a remote-controlled terminal and receives data signals through a secondary base station. Then the base station in the field transmits instructions to the machines to complete the operation of rubber tapping. This example preliminarily reveals the application of agricultural information technologies in the production management of rubber plantations. Nevertheless, a critical limitation of the system is the inconvenience of the maintenance process, due to the need to connect each rubber tree in the orchard to a machine. Thus, improving the automation and intelligence level of rubber tapping machines plays an important role in increasing the output of natural rubber [25]. Research in the area of rubber tapping has followed several avenues. Early work by [PERSON] [26], [PERSON] et al. [27], and [PERSON] et al. [28] was concerned with the effect of tapping frequency and tapping environment (the living tree under completely aseptic conditions and in the absence of oxygen) on rubber yield, based on biology (symptoms of bark scaling, cracking, drying, necrotic streaking, and browning of the internal bark, leading to the decay of internal tissues). [PERSON] et al.