1. Introduction
Small unmanned aerial vehicles (UAVs) are very well known for their long-flight endurance. This capability of the fixed-wing aircraft is further extended through various techniques which use the structure of the aircraft to extract energy from the surrounding wind, and achieve even longer flights [
1,
2]. Unlike the multirotor UAV, the fixed-wing aircraft enjoy the luxury of a bigger surface area. The latest work reports the deployment of light weight solar panels on to the most of the available surface area of the fixed-wing UAV to generate power and enhance its flight time [
3]. Fixed-wing UAVs are also used to quickly and securely deliver life-saving medicines and products to the distant and hilly areas [
4,
5]. Some other mature applications include the terrain mapping [
6,
7], collection of data using obstacle avoidance techniques [
8,
9], forestry and coastal surveys [
10,
11] and telecommunication [
12,
13,
14].
The small fixed-wing UAVs are usually more susceptible to wind gusts. The sensitivity to wind gusts or the random pressure changes in the environment is mainly associated with the size of the main wing, specifically when its size is smaller than the wind disturbance envelope [
15]. Consequently, a harsh flight environment has an adverse impact on all of the axes governing the stability of the airplane, especially the roll axis [
16]. The poor roll control leads to significant flight deviations along with altitude fluctuations. In the case of small and high-frequency disturbances, the actuators’ working bandwidth limitation is also the main cause of the degraded performance of aircraft even if it is using the novel turbulence sensing techniques [
17,
18]. As far as the structure of a fixed-wing UAV is concerned, the literature reports many attempts to enhance the performance of the fixed-wing UAV. Examples include the airplane with segmented control surfaces [
19,
20,
21], flexible-wing design [
22,
23,
24,
25,
26] blended wing body design [
27] and split-aileron wing [
28]. By the design itself, the segmented aileron control surfaces are explored in [
19,
20], demonstrating their practicality and advantages. The segmented surfaces have couple of aerodynamic advantages as well such as improvement in active wing lift distribution profile and reduction in drag force by working in favor of rudder and pitch control surfaces [
19]. The segmented ailerons have also been used in in-flight load distribution by incorporating optical fibers as wing strain sensors [
29]. Various segments are selectively actuated to recover from wing fluttering which is linked with poor flight performance. Modern commercial aircraft also utilize the segmented ailerons to mitigate the wing fluttering, which leads to more comfortable and safe air travel [
30]. For instance, multiple ailerons are integrated in the Airbus 380 and are engaged in different ways depending on the speed of the aircraft [
31]. Although the physical designs for the small fixed-wing aircraft with many aileron segments may exist in the literature, a thorough system identification study and a control architecture has not yet been developed and analyzed for such an aircraft. However, for ordinary small fixed-wing UAV with single aileron pair, i.e., one aileron control surface on each side of the main wing, several advances and optimization are reported in the literature. Notable work includes the use of PID controller [
32,
33,
34], sliding mode controller [
35,
36], model predictive controller (MPC) [
37,
38], fuzzy controller [
39] and back-stepping based control in [
40,
41].
PID controllers continue to be the most widely used because of their straightforward structure and practical ease of implementation [
42]. Additionally, they do not require a lot of computational resources. The ability to modify controller parameters without comprehension of a plant’s model is one of the benefits of PID controllers. Online experimental testing is another way to fine-tune the PID controller parameters for unmanned aerial vehicles [
43]. These attributes have led to the adoption of PID controllers in the multitude of commercially available autopilot flight controllers today, such the Acro Naze32, Mikrokopter, Ardupilot, Paparazzi auto-pilot and Micropilot, etc. Furthermore, PID controllers’ instant parameter adjustment and seamless implementation make them ideally suited for cascade control architecture. For the dynamically complicated systems, an optimum control strategy is attained using a cascaded PID control architecture [
44]. In other words, larger and complicated system can be divided into smaller, independent subsystems, and then PID controllers are asynchronously tuned exclusively based on the dynamics of each deconstructed subsystem [
45,
46]. One of the most significant challenges when developing the controller for a UAV is tuning the controller parameters. The UAV control system’s cascade control structure makes the tuning process even more arduous, especially when the UAV’s nonlinear mathematical model is incorporated.
This study presents the detailed process of the system identification and the roll attitude control design of the fixed-wing UAV with multiple aileron control surfaces. For the experimental purposes, the main and severely disturbed axis in the turbulence, i.e., the roll axis, is taken into consideration. For a cascade control system of a fixed-wing UAV with multiple segmented ailerons, an automatic tuning method is suggested. In a supervised experimental environment, closed-loop experiments are conducted in order to automatically acquire a time-invariant model of the aircraft’s roll dynamics. Following that, PID controllers are designed for the cascade control system using the frequency points. The aircraft undergoes the relay feedback experiment to proceed with the autotuning methodology. To recursively approximate the closed-loop fundamental frequency, responses from relay feedback tests are used [
47]. Additionally, the PID controllers are tuned using the tuning guidelines suggested in [
48] when the appropriate frequency points on the Nyquist plot are determined for the given aircraft. Finally, a cascade PID control system is enacted and validated using experimental results. Following is a summary of this paper’s novel contributions:
- 1.
A robust roll attitude control system of the fixed-wing UAV with multiple aileron segments is designed utilizing autotuning methodology. At first, this UAV is subjected to the process of system identification via relay instrument to acquire frequency response points. The process of system identification from scratch is thorough as it captures not only the complex system’s actual dynamics, but also those which might impact the closed-loop operation of control system such as computation delays, sensors, and actuators. Afterward, the acquired frequency points are directly used to design and autotune the PID controllers based on simplistic sensitivity functions and beta values as discussed later on. The suggested autotuning methodology is straightforward and does not require any prior information on the plant;
- 2.
In order to efficiently deal with the complexity of the system, the proposed control system is designed to have cascade. As discussed later on, unique controllers are handling the actuation of the inner and outer aileron segments through a cascade control system by considering each aileron pair as an independently manipulated variable. Moreover, a novel error-threshold control technique is proposed and incorporated to firmly reject the severe turbulence and other external disturbances. The experimental results have shown that such method of the controller tuning and multiple aileron control leads to highly stable and pleasant flight.
- 3.
A complete hardware of the fixed-wing UAV with multiple aileron segments along with a custom flight control board is designed to test and validate the efficacy of the autotuing methodology and multi-segment design against turbulence mitigation. All the experiments documented in this work we performed in a professional wind tunnel environment situated in RMIT University. The acquired results demonstrate that the fixed-wing UAV with multiple aileron segments can easily perform in-flight switching between a conventional and multi-segmented UAV, whereas asserting that the multi-segment configuration exhibits stronger disturbance rejection characteristics during a hostile flight environment.
The paper is structured as follows.
Section 2 details the hardware specifications of the multi-segment fixed-wing UAV along with the environment in which the experiments have been conducted. The conventional non-linear mathematical model of a common fixed-wing UAV is also explained.
Section 3 describes the relay experiment which is utilized to discover unknown dynamics of the system. This section also explains the step by step process of frequency response extraction and controller design.
Section 4 provides the numerical explanation of inner and outer loop controllers design for cascade system by utilizing two frequency points.
Section 5 presents the cascade control structure implementation for the roll axis and the related hardware validation experiments.
Section 6 provides the novel technique to control all the segments based on the roll angle deviation to enhance the stability of the aircraft.
Section 7 summarizes the research findings and future directions.
5. Hardware Validation of Autotuned Controllers in Cascade System
When the inner and outer loop controllers are ready for both inner and outer segments, cascade configuration is deployed to integrate the controllers as shown in
Figure 12 where
is the desired reference angle input,
is angle output,
is aileron input to the inner segments and
is aileron input to the outer segments.
Initially, two separate experiments were conducted to test the performance of aircraft. First, the cascade loop configuration is applied only on the inner segments, while the outer segments are kept inactive, i.e., at 0 degrees. Afterwards, the PID controllers designed for outer segments are cascaded to test the performance of outer segments while keeping inner segments inactive. It should be kept in mind that the performance of the inner and outer segments is tested in two test flight environments, i.e., in laminar and turbulent airflow.
In laminar flow or smooth airflow, the UAV encounters practically no substantial external disturbance other than native turbulence intensity of the wind tunnel test section, which was around
–
. However, to test the aircraft’s performance in the presence of external disturbances, a turbulent environment is created.
Figure 13 shows a special box placed upright in the path of incoming airflow to generate turbulence inside the wind tunnel’s test section. The flow of experiments conducted in the wind tunnel is presented in
Figure 14.
5.1. Performance Evaluation: Inner Aileron Segments
The step response of the roll angle is shown by
Figure 15a when only inner segments are activated in laminar air flow. Whereas, roll rate response of the UAV under similar conditions is presented in
Figure 15b. It is evident from
Figure 7 and
Figure 11 that inner segments have lower gain and bandwidth than the outer ones; hence, there is always a trade-off between rise time and overshoots. By studying the control signal presented in
Figure 16a,b, it can be observed that the inner segments are struggling to keep up with reference tracking, especially in the event of turbulence. While at
and 26 s, the control signal reaches the saturation limits because of the large step changes, as shown in
Figure 16a, during the laminar airflow, the situation is concerning during turbulent airflow.
Figure 16b shows that the control signal spends most of the time at the extremes of the graph. This is a result of the inherent lower gain of the plant, i.e., the inner segments, as discussed earlier.
The experimental evaluation is repeated in a turbulent environment. When the UAV works in an environment that has wind gusts changing suddenly, the inner segments are found to be less effective due to a slower response time.
Figure 17a,b show the roll angle and rate output during turbulent airflow. It must be noticed that the outer aileron pair is kept at
during these experiments. The closed-loop responses from the experiments performed in laminar and turbulent airflow demonstrate that the inner aileron segments have the ability to control the roll motion of the UAV during step changes in the reference signal. While the experimental results show that roll angle tracking in a turbulent environment is not as effective as in laminar flow, the inner segments are still capable of maintaining the closed-loop stability.
5.2. Performance Evaluation: Outer Aileron Segments
In order to better analyze performance difference, the outer aileron segments were tested under a similar experimental setup as the one used in the inner aileron segments. The step response generated while the UAV relying on only outer segments is presented in
Figure 18a,b. These roll angle and roll rate responses were recorded when the aircraft was subject to laminar airflow. During the laminar airflow, the outer segments perform almost identically to the inner segments, except for lesser overshoots being observed at
s.
Figure 19a presents the control signal behavior when the outer segments are working in normal flight conditions. An important change to notice here is that the the control signal is mostly working within the range of
, whereas for inner segments, the control signal is in the range of
. Moreover, the control signal
has quickly reached the saturation limits at every step change, while the outer segments control signal
as zero taps on saturation points during the whole experiment. This behavior suggests that the outer segments, in comparison with inner ones, have good potential for tackling the big reference step changes or incoming disturbances.
However, when the UAV confronts irregular airflow, the outer segments are also able to reject externally induced disturbance and sustain the closed-loop stable operation. The closed-loop roll angle and roll rate responses are depicted in
Figure 20a,b, respectively, when only the outer segments are working in a turbulent airflow. Further analysis into the control signal reveals that the outer segments are not using their full capacity to achieve the goal of reference tracking even in the presence of turbulence intensity similar to that of inner segments experiments.
Figure 19b depicts that the control signal for the outer segments has the longest stay at saturation limit, i.e.,
within the range of
s and that is because of the huge step-change in the reference signal which is from
to
, i.e., of
.
6. The Error-Threshold-Based Approach to Control of Segmented Surfaces
The detrimental impact of a windy and turbulent environment on a typical fixed-wing UAV is well acknowledged. Therefore, increasing the aileron size will require larger actuators that will affect the UAV’s reaction time and battery power when tackling severe turbulence. As a result, there will be additional difficulties that must be overcome, for instance, introducing the requirement for high-performance digital processors and expensive sensors. This aircraft deploys error-threshold-based aileron segment control in order to solve the problem. As was previously mentioned, the UAV in question has independent actuation capabilities on both its inner and outer aileron control surfaces. It is emphasized that only one pair of the ailerons should be active for reference tracking and rejection of insignificant disturbances in order to conserve battery energy during regular operation. The other pair of ailerons, in this case, the inner segments, is reserved as redundant ones. It is only active while rejecting powerful disturbances such as severe turbulence or improving reference following.
The control algorithm that determines whether the redundant pair of ailerons should be active or not relies on the intensity of external disturbance or the deviation of the UAV from given set-point. Since the external disturbance is directly linked with measured feedback error, i.e., the difference between reference roll angle denoted by
and the UAV’s actual roll angle
, the large feedback error will call for involvement of all the aileron pairs to better stabilize the aircraft. Proceeding with this concept, the activation signal for multiple segments is formulated as,
For the UAV used in this work, outer aileron segments are acting as primary actuators while inner ailerons are working as redundant ones. If the actuation threshold is defined by the pilot to be
, then the error
will lead to
, resulting in deactivation of inner ailerons. In the situation where error is
the resulting
will be equal to 1, which will make inner segments active and work in support of outer aileron segments.
The value of
is user-selected and can vary based on an estimate of how turbulent an environment the UAV is going to fly in. All the experiments performed in this work are based on
. Since the value of
is independent of PID controller values, depending upon user preferences, multiple thresholds such that
can be programmed and triggered remotely using a flick of a switch. The independently tuned and deployed controllers for the inner and outer segments ensure that the lower the threshold, the better the disturbance rejection performance.
Figure 21 represents the block diagram of the cascade controllers incorporating the error-threshold-based control. The switch before
can be treated like
, hence, deciding whether the inner segments should be active or not.
The error-threshold-based control was tested in two flight environments.
Figure 22a,b present the roll angle and roll rate outputs of aircraft while operating using the aforementioned control in the laminar airflow. The performance in this scenario resembles that of the outer segments since they are acting predominantly during the whole experiment. A slight improvement in the rise time can be noticed when looking at the points where step changes occur. This is because of the fact that the inner segments are active during the transition of the reference signal as
and working in conjunction with outer segments to minimize the error as soon as possible. The working sequence of the inner and outer ailerons is made clear while looking at the control signals given in
Figure 23. It is immediately observed from
Figure 23a that the outer segments are normally active all the time within the average working capacity of
. When compared to the individual performance of the outer segments where the overshoots were within the range of
(see
Figure 19), the same ailerons are exhibiting lower overshoots at the reference step changes, i.e., within the range of
because of the assistance acquired from the inner segments.
Figure 23c shows the error angle signal outlining the threshold band of
.
Figure 23b presents the active and inactive time of the inner ailerons (
). By comparing
Figure 23b,c, the error-threshold-based control can be observed in action such that the inner ailerons are only energized when the error signal has crossed the threshold limits, whereas, for rest of the time, they are held constant at
.
The impact of the error-threshold-based control becomes clearer on the attitude performance as we proceed with experiments in a more challenging test environment.
Figure 24 presents the attitude performance of the UAV in terms of the roll angle and rate while working in a turbulent airflow. It is noticed that the aircraft is able to sustain a smoother performance as compared to the previous experiments where only outer segments were active. A clear reduction in overshoots is observed, along with the proper tracking of roll angular rate, as shown in
Figure 24b. The control signals and error signal behavior are depicted in
Figure 25. The difference in working methodology of the inner segments is perceived in
Figure 25b when compared to that of the laminar flow experiment. By analyzing the error signal given by
Figure 25c, it is observed that the inner segments are active whenever the condition
becomes true. This methodology ensures that there is no additional load on the limited onboard battery energy of the aircraft in a normal flight, and the improved performance is available at the discretion of the user or the challenging flight environment.
The mean squared error (MSE), which is calculated in order to better quantify the boost in the closed-loop performance of the hardware experiments conducted, is defined as
where the number of samples is denoted by
M. The MSE for the three distinct control configurations is depicted in
Table 6. It can be inferred that the roll angle control response of the error-threshold based approach is superior by
when compared to the independent performance of the outer ailerons only during laminar flow. Under similar conditions, the outer ailerons excel by a percentile of
when compared to the independent working of the inner segments. However, the performance of the error-threshold-based control is much different from the individual actuation of either inner or outer segments in the presence of turbulence. The MSE analysis shows that the error-threshold-based control can offer a performance improvement of
to
while flying in a hostile environment.