Abstract
Wind field and sound field characteristics are the key indexes for unmanned aerial vehicle. Therefore, in this study, the wind field and sound field characteristics of a quad-rotor unmanned aerial vehicle are investigated. First, the experimental platform was set up based on quad-rotor unmanned aerial vehicle. Second, the experiments were performed on the wind field and the sound field characteristics of the unmanned aerial vehicle at different working currents. Then, the experiment results were analysed. Meanwhile, the experimental results showed that the working current has a large impact on the wind field and the wind intensity increases as working current increases; as the working current increases, the sound field is enhanced and a linear relationship exists; within a certain distance range of the unmanned aerial vehicle, as distance increases, sound intensity dramatically decreases. The presented methods and results can not only be used to evaluate the performance of the electric multi-rotor unmanned aerial vehicle but also provide references for the further improvement of the performance of the unmanned aerial vehicle.
Introduction
Unmanned aerial vehicle (UAV) refers to a drone or vehicle operated under remote wireless control or autonomous flight with no driver in the vehicle and the vehicle is reusable.1–3 In recent years, communication technology, electronic technology and the Internet of things (IOT) have progressed rapidly leading to the development of a variety of electronic products including the UAV. The multi-rotor UAV is smaller in size, more lightweight and more visible than the fixed-wing UAV and can also achieve low-altitude suspension and vertical take-off and landing (VTOL). Therefore, the multi-rotor UAV has been widely used in military and civil engineering applications as a portable and stable platform. In military applications, the multi-rotor UAV is used for reconnaissance, air strikes, and search and rescue operations.4–6 Civil engineering applications include agricultural plant protection, express deliveries, weather detection, aerial photography and traffic monitoring.7,8 Due to the wide applicability of multi-rotor UAV, extensive research on multi-rotor UAV-related technologies is currently being carried out.
At present, the main focus of UAV research is on the design of control systems for field applications. Zhao and Wang 9 proposed a multi-rotor UAV system using the Arduino Mega 2560 micro-controller platform and completed flight simulations and wall barrier experiments in both the AltHold and Loiter flight mode. With the aim of applying the multi-rotor UAV universally, Wang et al. 10 introduced an airframe design method and combined rational design with the meteorological survey data of an UAV. Moreover, Liao et al. 11 prioritized the design of an STM32 controller for a quad-rotor UAV to acquire farmland information and consider the degree of low-altitude disturbance and adopt a double closed-loop proportional–integral–derivative (PID) control method. Thus, a stable rotor control system with good anti-interference ability was realized and step response experimenting and flight testing in a farmland environment were successfully performed. Yue et al. 12 used a distributed system to identify the appearance and the approximate position of unwelcome drones using a wireless acoustic sensor network and machine learning algorithms.
Numerous studies on the design of UAV control systems can be found in the literature;13–16 however, few studies have been carried out on the field application and characteristics of UAVs. Therefore, in this study, a quad-rotor UAV platform is used to investigate the wind field and acoustic field characteristics of UAV and to provide a theoretical basis for the further development and widespread application of UAV in the future.
The remainder of this article is organized as follows. Experimental setup and design are briefly introduced in section ‘Experimental setup and design’. In section ‘Analysis of experimental results’, analysis of experimental results is given. Finally, section ‘Conclusion’ presents the overall conclusions.
Experimental setup and design
UAV system experimental platform
The UAV system experiment platform comprised a Pixhawk flight controller, electronic governor, brushless motor, 6s lithium battery cells, control receiver and additional elements such as a carbon fibre frame, as shown in Figure 1. The control terminal consisted of a wireless remote controller and computer terminal (ground station).
UAV experiment platform.
The Pixhawk flight controller (PX4 2.4.8.) was used as the control centre of the UAV and consisted of a Cortex-M4 processor and gyroscope, electronic compass, barometric altimeter, accelerometer and emergency handler. The main function of the flight controller was to collect parameter data, such as GPS information and UAV working state information via a wireless data transmission module and deliver instructions from the ground station or remote-control receiver, which were sent to the electronic governor and used to control the motor by setting the required working conditions.
The use of a brushless motor has the advantages of quick start up and free speed regulation after start-up. In this study, a JFRC u3508 brushless motor was used and the control signal was received by the motor from the electronic governor. The waterproof motor exhibited low power consumption and a wide working temperature range (−30°C to +85°C).
The 3DR Wireless data transmission module acted as the hub of communication between the UAV and ground station. Two types of transmission module exist, 433 and 915 MHz, and the 433 MHz module was used in this study with a baud rate of 57,600 bps and maximum transmission distance of 800 m. This particular type of transmission is low cost, small in size, offers a wide communication range, operates on open-source firmware and supports an Android OTG connection.
The electronic governor (HobbyWing X-Rotor 40A) received signals from the flight controller and based on the particular signal, controlled the operation and speed of the motor. The throttle response speed was extremely fast; moreover, the throttle signal line was a twisted pair, which made flight more stable by effectively reducing the crosstalk that can be caused by transmitting signals through a copper wire.
Ultra-low internal resistance MOSFET was used because of its stronger resistance to flow, compatibility with various flight controllers and support of throttle signals up to 621 Hz.
The WFLY 7 Remote control (WFT07S) was used and a 7-channel 2.4 GHz communication frequency band was adopted. The required communication distance was relatively far for realizing remote control of UAV flight. Therefore, a bus data transmission was used, which offers high sensitivity and supports a 4.8 v-6v battery, low voltage design, high-end spread spectrum, and frequency-hopping technology to ensure strong anti-jamming capabilities.
The external electronic compass used in experiments was a high-performance positioning module designed for an M8N GPS Pixhawk. To further improve GPS performance, the built-in compass of the Pixhawk controller was also used and the UAV was able to receive data quickly to enable stable and accurate positioning.
Finally, nylon propellers (1245 MR) were used with a 12-in diameter and 4.5-in pitch. The propellers were located 6 mm in front of the centre hole, 9 mm in the centre hole and 6 mm in the centre.
Experimental apparatus and design
In this study, sound intensity measurements were performed using a digital psophometer (SMART SENSOR AR854), which comprised capacitor sensors with a measurement range of 30–130 dB, measurement accuracy of +1.5 dB and frequency response of 20 Hz–8 kHz. Wind velocity measurements were taken using an anemometer (SMART SENSOR AS836) with a wind speed measurement range of 0.3–45 m/s and measurement error of ±3% and ±0.1 dgt. Time was measured using a stopwatch (Shimano PC2810). Other experiment instruments included a stent, tape measure and well-defined wind field plate.
Wind field experiments
To avoid the influence of the ambient wind field and ground feedback wind field on the actual wind field of the propeller, experiments were performed in an empty room with no wind. A stent was used to maintain the height of the UAV and the distance between the propeller and ground was 57 cm. Measurements were taken at a distance of either 20 or 30 cm from the propeller and the working current varied between 5 and –11 A.
To carry out the wind field experiments, the following procedure was performed:
The working current of the UAV was varied by changing the throttle size via remote control of the throttle rocker.
Using the control variable method, all other parameters remained unchanged except working current. Different working power flows were experimental down the 30 cm wind field below the UAV. Each working current was measured three times and each test was repeated three times.
All parameters were held constant except the measured distance from the propeller and once the working current of the UAV reached 5 A, it remained unchanged. The wind field at a distance of 20 and 30 cm at the bottom of the oar was measured three times for each high wind field.
During the experiments, data were collected, and the errors were removed. Finally, average value of the three measurements for each working current was calculated.
Sound field experiments
To reduce the error caused by external noise, an outdoor environment with a slight breeze was used as the experimenting environment with an average ambient noise level was 39 dB. The working current was 5–11 A.
Sound field experiments were performed according to the following steps:
The working current of the UAV was varied by remotely changing the throttle size of the throttle rocker.
The throttle was increased until the UAV began to turn and within 1–2 s, the working current reaches the fixed current value and remained constant.
Using the control variable method, all parameters were held constant except working current. The sound field of the UAV was experimented within 10 m for different working currents. Cantered on the UAV and at a radius of 10 m in three different directions, the sound field was measured three times for each working current and each experiment was repeated five times.
The results were collected, and the errors were removed. Finally, average values of three measurements for the each working current were calculated.
Analysis of experimental results
Rotation speed in different working currents
The rotor rotation speed of the motor is a key index of its performance. In order to study the relationship between the working current and the rotation speed, different working currents were selected to obtain the average speed of the four motors. The experiment was repeated five times while varying the working current and average values were calculated from the repeated measurements. The curve as shown in Figure 2 was obtained by analysing the results.
Rotation speed of UAV motors under different working currents.
As can be seen from Figure 2, the rotation speed of the UAV motor increases with the continuous increase of working current. That is to say that the greater the current, the greater the rotation speed. After curve fitting, there is an obvious linear relation between working current of UAV and rotor rotation speed.
Wind fields under different working currents at the same height
The horizontal wind field with a vertical height of 30 cm is illustrated in Figure 3, composed on the horizontal plane of the four propeller centres. Analysing a single propeller, the circular wind speeds of a circular radius of approximately 6 cm are generally greater than other area. The diameter of the maximum wind speed is similar to that of the propeller, and farther away from the centre, wind speed decreases. Analysing the wind field composed of all propellers under the same working current, the wind field of each propeller varies and whereas the shape of the wind fields is similar, wind intensity varies. Two propellers close together generate small wind speeds. It can clearly be seen that the intensity of the wind field increases as the working current increases.
Schematic diagram of wind field under different working currents: (a) 5 A, (b) 8 A and (c) 11 A.
Based on the analysis, formation of the wind field for a single propeller is at a circular distance formed by the propeller centre and wind speed is faster farther away from the centre; moreover, wind speed decreased rapidly with increasing distance. Compared with the wind field of a single propeller, interference between the wind fields of different propellers does not occur and the wind field in the cross sections between two adjacent wind fields is not disturbed. Due to the battery deployment, the UAV’s centre of gravity is inclined to the upper right corner. Therefore, the wind field at the lower left motor is less intense than the other wind fields. Because of the offset of gravity centre, the flight controller of the UAV is used to control the speed of different motors in order to maintain balance. Meanwhile, from Figure 3, we can get that as the working current increases, the wind field of four motors gradually increases. It is easy to know that the increasing working currents lead to an increase in hot area and a change in location.
Wind fields at under same working current with varying vertical height
Wind fields at the same working power for single propeller at a vertical height of 20 and 30 cm are shown in Figure 4. At the centre with coordinate (15, 15), the wind field is clearly suppressed, as evidenced by low wind speeds, and the same results are demonstrated in Figure 4(a) and (b). This suggests that the centre point of the propeller does not reach the maximum wind speed during rotation. Comparing Figure 4(a) and (b), the wind speed of the entire horizontal wind field decreases with increasing vertical distance from the propeller centre.
Relationship between distance and wind field: (a) distance of 20 cm from propeller and (b) distance of 30 cm from propeller.
Sound field analysis with different distance
The same work current experiment was repeated three times. A rectangular coordinate system was constructed with the UAV as the origin. The direction is the first quadrant, bounded by the positive X-axis and positive Y-axis, and the maximum measurement distance was 10 m. The experiment was repeated five times while varying working current and average values were calculated from three repeated measurements. Based on the analysis, the relationship between sound intensity and distance for different electric flows was described, as shown in Figure 4. The specific fitting equation can be defined as
where S is sound intensity, x is distance and A, B are the coefficients. The curve fitting coefficients R2 of different working current are more than 0.95. In other words, there is a logarithmic relationship between the sound intensity and distance.
From Figure 5, it is easy to get that the sound intensity would decrease with increasing distance under different working current conditions. When the distance is less than 3 m, the slope of the curve is large and sound intensity greatly varies with distance. At distances greater than 3 m, the slope of the curve decreases, and the sound intensity slightly decreases with distance. It can be seen that sound intensity increases with increasing working current at same distance, as shown in Figure 5.
Sound intensity under different working currents.
Conclusion
To address the lack of literature related to the mechanical performance of UAVs, the wind field characteristics and acoustic field characteristics of a multiple UAV rotorcraft were studied. An experimental platform was built to investigate UAV mechanical properties and a four-rotor UAV underwent wind and sound field experimenting. Data were collated and analysed to provide necessary supporting information for the practical and wide application of the UAV platform in the future. Based on the results, the following conclusions can be made:
There is an obvious linear relation between working current of UAV and rotor rotation speed. The UAV wind field is clearly affected by working current. Larger currents lead to a stronger wind field and greater wind speeds at the same position. For a horizontal plane parallel to the propeller, at a closer distance, the wind field will be stronger. Wind fields of four adjacent rotor blades simultaneously rotating will overlap resulting in interference.
As the working current increases, the UAV sound field is enhanced, and a linear relationship between sound and current is observed. Moreover, the logarithm of sound intensity varies with distance, and as the UAV moves farther away, the log of sound intensity decreases. Within a certain distance range of the UAV, as the distance increases, sound intensity dramatically decreases.
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This work was supported by Featured innovation projects of Guangdong province (2019GKTSCX075), open fund project of Guangdong province key laboratory of computer network in 2019 (CCNL201906), and Guangzhou Science and Technology Program (No. 201804010427).
