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INTRODUCTION
In recent years there have been significant advances in the development of remotely piloted aircraft systems (RPAS) (mainly multirotor), becoming a stable and reliable technology, applied in agriculture (Davila et al., 2017; Thibbotuwawa et al., 2020).
RPAS can provide information with a much higher resolution than sensorial data from satellites and manned aircraft. These vehicles, used in agriculture, provide high-resolution spatial images, used to monitor crops on a millimeter scale (Christiansen et al., 2017). Therefore, they constitute a real time application and low-cost alternative to classic manned aerial photogrammetry.
In recent years, different platform designs have been proposed, showing a strong development for autonomous flight, control and landing, with global positioning devices (GPS), inertial measurement sensors IMU (Inertial Measurement Unit) and processing systems (Galimov et al., 2020). These devices make up the flight control system, allowing complex calculations to apply control strategies to stabilize the position and achieve a flight path for the vehicle (Lara et al., 2017; Pei et al., 2019).
Considering the necessary characteristics that aircraft must have for use in precision agriculture, the objective of this work was to design, build and evaluate a quadcopter type RPAS with a system that allows autonomous and stable flights to obtain georeferenced images in a sequential way, through the instrumentation of a RGB sensor.
MATERIALS AND METHODS
RPAS Design
To carry out the optimal and competitive design of the RSAP, the parameters recommended by Orna et al., (2017), Nistal, (2017) and Dündar et al., (2020) were considered, which are:
The aerodynamic analysis which allows determining the required power according to the operating conditions.
The mechanical design proposing a structure that resists the loads to which the equipment is submitted.
The control system (controller) which allows communication (telemetry and radio control), geopositioning and autonomous flights.
Tables 1, 2 and 3 list the parameters used for the design of RPAS, considering that it is for agricultural photogrammetry and its weight should not exceed 2 kg to comply with the Mexican standard Secretaría de Comunicaciones y Transportes: NOM-107-sct3-201(2019).
TABLE 1 Aerodynamic design parameters
| Aerodynamic Design Parameters | ||
|---|---|---|
| Parameter | Values | Comments |
| Maximum load (Own weight + Payload) (Power) | Own weight <1.5 kg (Own weight + Payload) <2kg | The payload to be used is 200 g (Hawkeye Firefly 8S camera) |
| Time of flight range | 10 minutes | At a height of 20 m, with a speed of 1m/s, using telemetry and a camera |
TABLE 2 Mechanical design parameters
| Mechanical Design Parameters | ||
|---|---|---|
| Parameter | Values | Comments |
| Structural stress | Safety factor >2 | Stress simulation on a commercial chassis (f330) (Solid Works). |
| Number of engines | 4 engines (quadcopter) | X-structure, "Cross Style" configuration, (Best mechanical simplicity) |
TABLE 3 Control system parameters
| Control System Parameters (photogrammetric use) | |
|---|---|
| Parameter | Features |
| Controller |
Autonomous flight. (Flight missions with "waypoints").Geopositioning (GPS). Different flight modes for good control (Loiter, RTL, Alt Hold, Stabilize, Auto) Configuration of flight parameters (speed, PID, acceleration). A system of free use, stable, documented, and easy to use for the incorporation of different sensors. A system that allows different communications between the pilot and the controller (Rc, telemetry, Bluetooth,WIFI). |
| Communication |
911 MHz telemetry. (vehicle information during the flight) 8-channel radio control (RC 2.4 GHz) Optional (video) |
Because the six- and eight-engine multirotors are structurally larger and therefore heavier than the three- and four-engine ones, it was determined to use the design model of a quadcopter with an additional lifting capacity of 200 g (payload), which guarantees good maneuverability and stability (Fernandez et al., 2016).
Power Calculation
Nistal, (2017) mentions that, starting from the lifting requirement of a RPAS according to its weight and payload, the vertical thrust calculation is carried out. This force refers to the capacity of the engines with their respective propellers, to sustain the flight, which implies a uniform distribution of the total force generated among the number of engines in the vehicle (Equation 1) (Fernandez et al., 2016).
Where: Et: Total thrust (kgf), Nm: Number of motors on the multirotor, f e: Thrust force of each engine with a specific propeller (kg).
In this study, 4 engines of the brand "Racer Star (2212)" where selected, each one providing a thrust of 910g with eight-inch propellers and an attack angle of five inches. Therefore, having 4 engines, the quadcopter would be able to lift a total weight of 3,640g (running at full power). Knowing that the total weight of the quadcopter is 1600 g, it deduced that, it will not be necessary to use the maximum power of the engines to fly.
Electronic Speed Controller
The selection of the ESC was based on the maximum current that is supplied to the electric motor and the amperage the ESC must supply, which is 19 A. It was also considered the recommendation of Bonney et al., (2020), which indicate that the ESC values are at least 30% above the maximum consumption value of the motors. Therefore, ESCs of 30 (A) were chosen because they ensure the necessary supply amperage.
Battery Selection
The batteries used in the RPAS were "LiPo" type, since, this type of batteries provide a great amount of power in a reduced period, besides of being light, compared to the typical lead or Nickel-Cadmium batteries (Nistal, 2017 and Dündar et al., 2020). Moyano (2014) mentions that, to select a battery it is necessary to consider:
Loading and Unloading Constant
This parameter is used to identify the storage capacity (charge) in the battery and to indicate the current at which it can be discharged, so that it is not damaged (Fernandez et al., 2016). The mathematical expression that defines this constant is Equation 2:
Where: Q: Battery capacity in (mAh), C: Loading/unloading constant in mAh/A
With the battery charge/discharge constant, the maximum current that the battery can supply can be calculated (Equation 3).
Where: Current max : Maximum current capacity that can be supplied by the battery (A), Ct: Constant discharge intensity (non-dimensional).
The discharge rate of the battery used is 50 C, which indicates that the maximum current supply is 200 A. Therefore, the battery can supply the power (ESC), control, communication, and sensors systems, (as long as they do not exceed 200 A current).
Calculation of the Time of Flight
To calculate the flight time, the consumption of the engines is considered, since the rest of the components do not consume too much current (Fernandez et al., 2016) and it is considered that the vehicles will be at 50% of the maximum consumption, in horizontal forward flights, 20% in upward axial flights and 20% in fixed point flights (Serrano and Perez, 2017). Equation 4 was used to calculate the flight time.
Where: T: Estimated time of flight (h), Cap: Battery storage capacity (mAh), Pt: Total consumption of the motors (W), Vol: Battery voltage (v).
According to the calculation of the flight time and the maximum current that the battery can supply, a 400mAh 3S battery (3 cells) was chosen, with a nominal voltage (11.1 v) and a constant discharge current of 50C, which generates a flight time of 7 minutes (theoretically) and a discharge current of 200(A), sufficient to supply the motors, the controller and the camera.
Mechanical Design
One factor that is related to weight is the mechanical resistance and fatigue of the chassis materials, although there are a lot of cheap and low weight materials, many of them are not appropriate, because their mechanical resistance is not adequate (Nistal, 2017).
The chassis or “frame” is the structure in which all the components are placed. The center of the frame consists of elements that support the electronic components such as the controller, the receivers (RC, telemetry), GPS, the battery and the camera. On the other hand, the ESC speed controllers, motors and propellers are mounted on the arms (Fernandez et al., 2016).
The factors used to choose the correct chassis are the size of the propeller and the space occupied by the different electronic modules (Navarro, 2019). The chassis structure must have a distance between engines equal to the diameter of the propeller, plus a safety distance, which prevents the propellers from colliding (Bonney et al., 2020).
Structures of different configurations, sizes and materials are available in the market, which have different mechanical properties. (Fernandez et al., 2016). Analyzing the required dimensions and costs, the f330 chassis was chosen, because it allows the placement of engines with 8-inch (20.32 cm) propellers, ensuring that the propellers do not collide with each other (Figure 1). The f330 chassis, allows placing components, such as the controller, GPS, telemetry module and camera, in addition, it is configurable with the Cross Style flight mode (for photography) (Bonney et al., 2020).
Stress Analysis
The mechanical analysis was carried out to ensure the structural resistance of the aircraft according to the weight it will support. Figure 2 shows the chassis drawn in the Solid Works software, which was subjected to stress simulation analysis. The material chosen for the simulation was "Nailo 101", and with a 0.5 mm mesh.
A force of 4.9 N was applied at each end of the chassis arms, considering that the maximum weight is 2 kg (the mass was multiplied by the acceleration of gravity, resulting in 19.62 N), this force was divided among the 4 engines and applied in the position corresponding to each one. Section (A) of Figure 2 was set during the simulation, to find the Von Mises stress on each arm (Figure 3)
With the stress simulation in the chassis, the minimum and maximum tensions of Von Mises were obtained (Figure 3), a physical magnitude proportional to the distortion energy (Serrano and Pérez 2017), calculated by Equation 5.
Where: 𝜎VM: Stress of Von Mises (N/m2), 𝜎𝑥𝑥, 𝜎𝑦𝑦, 𝜎𝑧𝑧: Main stresses of the tensioner at one point of a deformable (N/m2).
With the values of the maximum stress of 7.551e+06 N/m2, obtained from the simulation, and the material's elastic limit of 6.000e+07 N/m2, the safety factor (FS) was calculated (FS=
Center of Mass
With the RPAS prototype model in Solid Works (CAD), the center of mass was calculated, which allows, to identify anomalies of the assembly and to rectify the position of the components. A center of mass above the horizontal axis of the propellers will cause an unbalance in flight and will need a better adjustment in the PID control, but if a center of mass is below, in the PID control adjustment the values will have a wider range of adjustment (Bonney et al., 2020). In Table 4 shown, the values obtained from the center of mass calculation in Solid Works
TABLE 4 RPAS Mass Properties
| Mass properties of selected components | |
|---|---|
| Mass | Center of mass |
| 1495.27 g |
X = 1.7 mm (relative to the geometric center) Y = -1.9 mm (with respect to the geometric center) Z = 165.45 mm (with respect to the base) |
Vargas-Fonseca (2015) mentions that for the RPAS to fly properly, the center of mass must be in the center of the structure. As shown in Table 4, the center of mass on the "X" and "Y" axis is not geometrically centered, due to the location of the devices, GPS, RGB sensor and battery, so it was necessary to adjust the PID control of the RPAS (Berra, 2016).
Control System
The control system has the function of continuously evaluating the status of the RPAS and is responsible for managing the navigation and control functions. It is considered the brain of the UAV, since it exercises direct control over its behavior. It is responsible for the stabilization and navigation of the aircraft, in automatic flight mode and in manual flight mode. (Santana, 2017).
In Figure 4, the diagram of the RPAS control system is shown with the necessary characteristics to fulfill the proposed objectives, such as the use of sensors for geolocation (GPS, IMU, barometer and compass) and communication (telemetry and radio control).
The controller used was the Pixhawk, based on Ardupilot's hardware free independent project, a high-quality flight controller at the lowest possible price (Nistal, 2017). This controller has all the mentioned functions of geopositioning, control and communication (Pei et al., 2019).
Construction of the RPAS
To make the construction of the RPAS, the chassis was mounted, to have the structure where all the components were assembled, and the engines were mounted on the ends of the arms (Figure 5).
The Pixhawk flight controller was placed in the center of the chassis, so that the accelerometers and gyros would work properly (Fernandez et al., 2016), oriented with the arrow pointing towards the front of the vehicle (Figure 6) (Bonney et al., 2020). The controller was mounted with vibration-damping foam pads.
ESC drives were placed in the chassis arms. The GPS was located on the right side of the flight controller, holding it with screws (Figure 6).
The camera was attached to an anti-shake base with bearings to reduce motion and prevent distortion in the pictures (Figure 7).
Figure 8 shows the quadcopter, assembled with all the components: controller (Pixhawk), GPS, battery, variable speed drives (ESC), engines, propellers and RGB sensor (camera). A housing covering was used on the top, to prevent the components from being in contact with the dust generated by the movement of the propellers.
Assembly of the Electronic System
This section details how the entire electrical and electronic connection of the developed quadcopter was made and how the energy and signals of the flight controller are distributed.
As it can be seen in the general connection diagram (Figure 9), the battery is responsible for supplying energy to the controller and the ESCs, by means of the power module. In addition, the power module distributes the input power from the battery to the 4 speed controllers (ESC).
The flight controller feeds the radio control receiver, GPS, buzzer, camera, telemetry module and RGB led. The controller has specific ports for the components, in which, the device to be connected is mentioned and each device contains a connector with the correct number of pins, to be fitted.
The ESC control cables were placed in the controller's PWM ports (1-4, respectively), the camera's auto-trigger cable was connected to port 51, and the radio control receiver was connected to the RC port (Figure 10). The upper part corresponds to the mass (-), the middle part to the power supply (+5v) and the lower part to the PWM signal sent or received by the controller.
The ESC number, corresponds to the engine number of the Figure 11, the PWM signal cables of the ESC, were placed according to the engine number in the ports 1-4 of the flight controller.
The ESCs supply power to the motors by means of a three-phase signal (Figure 12) (Fernandez et al., 2016). The ESC and the motors have 3 cables L1, L2 and L3, which are connected to each other (regardless of order or color). The motors are capable of rotating both clockwise (CW) and counterclockwise (CCW), the correct rotation of each is shown in Figure 11. To adjust the rotation of the motors, L1 and L2 of the motors were exchanged, which go to the ESC.
CONCLUSIONS
The RPAS type quadcopter was designed and built, using an open-source controller, and complying with the RPAS regulation standards in Mexico, with a software that does not allow the RPAS to fly beyond a horizontal distance of 457 meters from the pilot and a maximum height of 122 meters, with a weight of 1.7 kg (less than 2 kg), and with a system that allows autonomous flights and georeferenced photography, through the instrumentation of an RGB sensor.
The RPAS was drawn in 3D, using a "CAD" system, which allowed the modeling of the center of mass and stresses caused by the weight of the vehicle with a maximum stress of 7.551e+06 N/m2, obtaining a safety factor of 7.95.
