**3.1 Equipment and payload subsystem**

The equipment of a multirotor aircraft depends primarily on the mission to be performed, which affects the selection of components and parameters of other subsystems. In addition to standard applications where multirotor UAVs are used in data *Autonomous Aerial Robotic System for Smart Spraying Tasks: Potentials and Limitations DOI: http://dx.doi.org/10.5772/intechopen.103968*

**Figure 4.** *DJI Agras representation [41].*

collection missions, mainly using different types of cameras, they can also be used in special applications. Since the paper considers the application in precision agriculture in smart spraying tasks, the payload of the aircraft is divided into two segments. The first segment consists of the equipment in charge of distributing and spraying the chemical under pressure. The essential parts are a set of hoses and manifolds, sprinklers, nozzles, and pump assembly. It is mounted on the existing aircraft frame, mainly on the landing gear or propulsion arms. The second segment consists of a tank containing a chemical that has a variable mass since it is deployed during the mission.

One of the most widely used commercial aircraft for agricultural purposes is the DJI Agras MG-1, an electric motor multirotor UAV with protection against dust and water. It is designed for applications in a variety of environments and terrains and can be used in fields, terraces, orchards, or other areas. It uses a microwave radar located on the underside of the aircraft that in combination with an altitude stabilization system maintains the aircraft at the desired height above the plants in order to ensure optimal spraying. The volume of the tank is 10 liters, and according to the manufacturer's specifications, it can cover an area of 7–10 acres per hour. The spray mechanism consists of four sprinklers located on two sides of the aircraft. The diameter of the aircraft is 1520 mm, and the configuration consists of eight rotors (octorotor) placed in one plane as shown in **Figure 4** [41].

#### **3.2 Electric energy subsystem**

As already mentioned, multirotor UAVs are characterized by high energy consumption as they use rotating wings (propellers) to move in 3D space. The energy subsystem must provide sufficient energy to the aircraft to perform the intended missions and must be compatible with the components of the propulsion subsystem. When selecting the components and parameters of the energy subsystem, the energy requirements of the propulsion subsystem must be taken into account, which in turn depends on the mass and size of the aircraft and the number of propulsion units. The energy subsystem consists of one or more lithium polymer (LiPo) batteries and energy distribution elements. LiPo batteries consist of one or more electrochemical cells in which lithium ions transfer charge between electrodes. They are characterized by high energy density and high discharge rate, which allows higher power and consistent energy flow to the propulsion subsystem. The main parameters of LiPo batteries are their mass, capacity, discharge rate (C), and the number of cells that determine the operating voltage (S).


#### **Table 1.**

*Typical characteristics of high voltage (12S) LiPo batteries [43].*

Batteries are the heaviest elements of the aircraft system and have the greatest impact on aircraft dynamics, so it is advisable to place them as close as possible to the aircraft center of gravity. Battery capacity also plays an important role as the flight time of the aircraft depends on it. Hence, the ratio of mass and capacity of the battery is one of the key data when designing a multirotor UAV system. The parameters of the considered Gens ace commercial high-voltage (12S) batteries are listed in **Table 1**. In addition to batteries, the energy subsystem consists of sophisticated circuits for energy distribution and measurement of electrical parameters of the battery.

#### **3.3 Electric propulsion subsystem**

The propulsion subsystem of a multirotor UAV is determined by the parameters of the geometric arrangement of the configuration and the characteristics of the propulsion units that make it up. All designs of the propulsion subsystem (configurations) have in common that they consist of N propulsion units (rotors) that generate the necessary forces and moments for the movement of the aircraft in 3D space. Conventional multirotor configurations generally consist of an even number of equal rotors symmetrically arranged in one or more parallel planes. Each pair consists of CW and CCW rotors for the purpose of canceling the reactive moment about the vertical axis of the aircraft. The required performance of the aircraft depends on the type and profile of the mission such as payload, flight duration, power consumption, or other specific requirements. The choice of the propulsion configuration and the type of propulsion units is the key step in the design of the multirotor type of UAV because the flight performance depends on it. **Figure 5** shows the configurations on the same scale of the six-rotor configuration considered in this paper and the eight-rotor configuration that makes up the propulsion subsystem of the DJI Agras commercial aircraft.

The considered electric propulsion units (EPUs) enable precise and fast regulation of control forces and moments that directly affect the position and orientation of the aircraft. The EPU consists of an electronic unit (driver) and a mechanical motor assembly on whose rotor a fixed-pitch propeller is mounted. The brushless DC (BLDC) motor is the central part of the EPU for which there are mostly detailed manufacturer specifications with relevant collocation of driver and propeller. There are EPU components on the market with a very wide choice of motor power, so they can be used in a wide range of multirotor applications, including precision agriculture missions such as smart spraying tasks where carrying a heavier payload is required. The motor speed is controlled by an integrated power inverter, the so-called electronic speed controller (ESC), which generates the switching sequence of the motor phases for the desired RPM specified by the control unit. The rotor of the propulsion unit on which the propeller with fixed pitch is mounted creates aerodynamic forces and moments necessary for the movement of the aircraft. BLDC motor is defined with motor velocity constant

*Autonomous Aerial Robotic System for Smart Spraying Tasks: Potentials and Limitations DOI: http://dx.doi.org/10.5772/intechopen.103968*

**Figure 5.** *Conventional multirotor UAV configurations.*

**Figure 6.** *Considered EPU characteristics [44].*

(back EMF constant) Kv. Motors of low power, small dimensions, and large motor constants are used mainly to power micro and small aircraft intended for entertainment or sports (drone racing). On the other hand, high-power and large-dimensions motors with small motor constants are intended for heavy equipment and loads (heavy lift).

In this study, for the needs of the aerial robotic system concept, five combinations of EPUs are considered, which are combined with a high-voltage (12S) energy subsystem setup. Based on the specification of the propulsion components manufacturer, the characterization of EPUs intended for heavy payloads was performed. Selected BLDC motors have a low motor velocity constant (Kv <200), which means that they have lower speeds, so in combination with larger-diameter propellers, they achieve higher torques. **Figure 6** shows the thrust force and efficiency of EPUs as a function of electrical power for the five considered setups. Propeller designations indicate geometry where the first two numbers indicate the propeller diameter in inches, e.g., a propeller marked G32x11 has a diameter of 32″. The next two numbers indicate the pitch of the propeller, also in inches, as the distance that propeller advances during one revolution.

#### **3.4 Control subsystem**

The basic task of the control subsystem is to guide the multirotor UAV in 3D space according to the given input variables. In addition, it takes care of the functioning of the entire system and is a kind of interface between the multirotor and the docking facility. The control subsystem primarily consists of a flight controller (FC), state estimation sensors, telemetry, and a remote control receiver. Since the multirotor type of UAVs is characterized by inherent instability, the key component of the aircraft is FC, and it can be freely said that it represents the brain of the aircraft. To control the aircraft concept that would be used in precision agriculture, Pixhawk open-source FC is being considered. The control algorithm generates control signals that it sends to the propulsion units in order to achieve the desired movement in 3D space, i.e., to perform the mission. Orientation sensors are integrated into the Pixhawk FC, and as for the position of the aircraft, it is obtained using a peripheral compatible GPS.

From the aspect of system design, the control subsystem is very demanding because, in addition to the choice of hardware, it is necessary to design a software solution. The considered control unit has already been used in the research so far, and certain segments of code have been tested. **Figure 7** schematically shows the custom firmware that is planned to be used in the future to control the aircraft in precision agriculture.

A series of experiments were conducted to primarily verify the motor mixer subsystem for different aircraft configurations. This will be extremely important for implementation on a prototype aircraft as configurations with different geometric arrangement parameters and with different propulsion unit characteristics have

#### **Figure 7.**

*Schematic representation of custom firmware main subsystems.*

**Figure 8.** *Attitude control experiment for custom quadrotor.*

*Autonomous Aerial Robotic System for Smart Spraying Tasks: Potentials and Limitations DOI: http://dx.doi.org/10.5772/intechopen.103968*

**Figure 9.** *Attitude control experiment for custom octorotor.*

been tested. The first series of experiments was done with a small custom-made quadrotor with x-arrangement. **Figure 8** shows the experimental results of reference attitude tracking. In the next series of experiments, a configuration consisting of eight rotors in a + arrangement, so-called octorotor, was tested (**Figure 9**).
