Outer Position Loop Research Articles

Aerodynamic Feedforward-Feedback Architecture for Tailsitter Control in Hybrid Flight Regimes

This paper presents a guidance-control design methodology for the autonomous maneuvering of tailsitter transitioning unmanned aerial systems (t-UASs) in hybrid flight regimes (i.e., the dynamics between VTOL and fixed-wing regimes). The tailsitter guidance-control architecture consists of a trajectory planner, an outer-loop position controller, an inner-loop attitude controller, and a control allocator. The trajectory planner uses a simplified tailsitter model, with aerodynamic and wake effect considerations, to generate a set of transition trajectories with associated aerodynamic force estimates based on an optimization metric specified by a human operator (minimum time transition). The outer-loop controller then uses the aerodynamic force estimate computed by the trajectory planner as a feedforward signal alongside feedback linearization of the outer-loop dynamics for 6DOF position control. The inner-loop attitude controller is a standard nonlinear dynamic inversion control law that generates the desired pitch, roll, and yaw moments, which are then converted to the appropriate rotor speeds by the control allocator. Analytical conditions for robust stability are derived for the outer-loop position controller to guarantee performance in the presence of uncertainty in the feedforward aerodynamic force compensation. Finally, both tracking performance and stability of the control architecture are evaluated on a high-fidelity flight dynamics simulation of a quadrotor biplane tailsitter for flight missions that demand high maneuverability in transition between flight modes.

Control Parameters Design of Spraying Robots Based on Dynamic Feedforward

The positioning and velocity accuracy of spraying robots determine the quality of the coating, and the influence of the robotic dynamic characteristics on control precision is significant. This paper presents a method of linearizing dynamic characteristics into feedforward coefficients and designs a dual-loop control system consisting of an inner velocity loop and an outer position loop. The system is divided into three sections: a cascaded section, a feedback section, and a feedforward section. The cascaded section eliminates the nonlinear characteristics of the system; the feedback section ensures the stability of the system; the feedforward section compensates for the internal errors of the system. The main innovation of this paper lies in proposing an offline parameter tuning method, which avoids online parameter adjustments and significantly enhances the real-time performance of the control system. Additionally, this method does not require specific physical information of the system, thus avoiding the cumbersome process of parameter adjustment. The experimental results demonstrate that when facing different high-speed trajectories, the proposed control system exhibits a significant improvement in control accuracy compared to other advanced control schemes.

Three-Dimensional Path Tracking of Over-Actuated AUVs Based on MPC and Variable Universe S-Plane Algorithms

Autonomous Underwater Vehicles (AUVs) are widely used for the inspection of seabed pipelines. To address the issues of low trajectory tracking accuracy in AUV inspection processes due to uncertain ocean current disturbances, this paper designs a new dual-loop controller based on Model Predictive Control (MPC) and Variable Universe S-plane algorithms (S-VUD FLC, where VUD represents Variable Universe Discourse and FLC represents Fuzzy Logic Control) to achieve three-dimensional (3-D) trajectory tracking of an over-actuated AUV under uncertain ocean current disturbances. This paper uses MPC as the outer-loop position controller and S-VUD FLC as the inner-loop speed controller. The outer-loop controller generates desired speed instructions that are passed to the inner-loop speed controller, while the inner-loop speed controller generates control input and uses a direct logic thrust distribution method that approaches optimal energy consumption to distribute the thrust generated by the propellers to the over-actuated AUV, achieving closed-loop tracking of the entire trajectory. When designing the outer-loop MPC controller, the actual control input constraints of the system are considered, and control increments are introduced to reduce control model errors and the impact of uncertain external disturbances on the actual AUV model parameters. When designing the inner-loop S-VUD FLC, the strong robustness of the variable universe fuzzy controller and the easy construction characteristics of the S-plane algorithm are combined, and integral action is introduced to improve the system’s tracking accuracy. The stability of the outer loop controller is proven by the Lyapunov method, and the stability of the inner loop controller is verified by simulation. Finally, simulations show that the over-actuated AUV has fast tracking processes and high tracking result accuracy under uncertain ocean current disturbances, demonstrating the effectiveness of the designed dual-loop controller.

Optimal formation strategy for interconnected quadrotor drones: Theory and experiment

The formation control for a quadrotor swarm with the energy consumption constraint and/or the regulation performance constraint is challenging, where both the guaranteed-performance formation strategy and the guaranteed-cost formation strategy cannot realize the optimal control. Especially, the flying experiment of the optimal time-varying formation is very difficult to realize. A new control protocol with an optimization index is proposed to realize the optimal time-varying formation and the optimization index is integrated as a Kronecker product form containing the Laplacian matrix of the communication topology of a quadrotor swarm. Moreover, an analytic criterion for optimal time-varying formation achievability is proposed, and explicit expressions of the formation center function and the minimum value of the optimization index are presented, respectively. Furthermore, an optimal formation achievability algorithm is proposed and a formation flying experiment is performed, where the outer-loop position control input is converted into the inner-loop attitude control and an Euler-angle loop controller and an angular velocity loop controller are introduced.

A solution using only electromagnetic coils for relative pose control in satellite docking

This paper proposes an electromagnetic coil topology and its control strategy, which can be incorporated into the electromagnetic docking device to achieve relative pose control in satellite docking. The target satellite has a main coil; the chaser satellite possesses a main coil of the same size accompanied by six and four evenly arranged secondary coils inside and outside the main coil, respectively. The coil on the target satellite is DC energized, while the currents in the coils of the chaser satellite are regulated. To remove the coupling between the pitch/yaw torque and translational force, the internal and external secondary coils of the chaser satellite interact with the main coil of the target satellite to perform the control of relative pitch/yaw and relative translation, respectively, so relative pose control can be achieved. The torque and force vectors exerted by the secondary coils of the chaser satellite are synthesized onto the pitch and yaw axes of the body frame. According to their spatial composition relationship, the formulas are proposed, which obtain the magnetic moment vectors of the coils from the set torques and forces. The controllers regulating pitch/yaw, translation, and distance utilize a three-loop cascaded structure that consists of an outer position loop, a middle velocity loop and an inner current loop. The control strategy is verified by dynamics simulation.

Computed torque control and force analysis for mechanical leg with variable rotation axis powered by servo pneumatic muscle

It is difficult for a humanoid leg driven by two groups of antagonistic pneumatic muscles (PMs) to achieve a flexible humanoid gait, and its inherent strong coupling nonlinear characteristics make it hard to achieve good tracking performance in a large range of motion. Therefore, a four-bar linkage bionic knee joint structure with a variable axis and a double closed-loop servo position control strategy based on computed torque control are designed to improve anthropomorphic characteristics and the dynamic performance of the bionic mechanical leg powered by servo pneumatic muscle (SPM). Firstly, the relationship between the joint torque, the initial jump angle and the bounce height of the mechanical leg is established, and then we design a double-joint PM bionic mechanical leg containing a four-bar linkage mechanism of the knee joint. Secondly, a cascade position control strategy is developed, which consists of the outer position loop and the inner contraction force loop, and the mapping relationship is designed between joint torque and antagonistic PM contraction force. Finally, we further project bounce action timing of mechanical leg to realize the periodic jumping movement of the mechanical leg, and simulation and physical experiments of the real-style machine platform have been provided to demonstrate the effectiveness of the designed SPM controller.

Robust Control for UAV Close Formation Using LADRC via Sine-Powered Pigeon-Inspired Optimization

This paper designs a robust close-formation control system with dynamic estimation and compensation to advance unmanned aerial vehicle (UAV) close-formation flights to an engineer-implementation level. To characterize the wake vortex effect and analyze the sweet spot, a continuous horseshoe vortex method with high estimation accuracy is employed to model the wake vortex. The close-formation control system will be implemented in the trailing UAV to steer it to the sweet spot and hold its position. Considering the dynamic characteristics of the trailing UAV, the designed control system is divided into three control subsystems for the longitudinal, altitude, and lateral channels. Using linear active-disturbance rejection control (LADRC), the control subsystem of each channel is composed of two cascaded first-order LADRC controllers. One is responsible for the outer-loop position control and the other is used to stabilize the inner-loop attitude. This control system scheme can significantly reduce the coupling effects between channels and effectively suppress the transmission of disturbances caused by the wake vortex effect. Due to the cascade structure of the control subsystem, the correlation among the control parameters is very high. Therefore, sine-powered pigeon-inspired optimization is proposed to optimize the control parameters for the control subsystem of each channel. The simulation results for two UAV close formations show that the designed control system can achieve stable and robust dynamic performance within the expected error range to maximize the aerodynamic benefits for a trailing UAV.

Nonlinear Robust Fault-Tolerant Tracking Control of a Tri-Rotor UAV against Actuator’s Abnormal Behavior

In this paper, a new nonlinear robust fault-tolerant tracking control method is proposed for a tri-rotor unmanned aerial vehicle (UAV) under unknown abnormal actuator behaviors together with unknown external disturbances. The actuator anomalies are modeled as time-varying multiplicative parameters to improve the model accuracy. The control system is decoupled into two parts, including the inner-loop attitude control and the outer-loop position control. The radial basis function neural network (RBFNN) is utilized in the outer loop to estimate the actuator anomalies and external disturbances, and then the state feedback controller is employed for the position tracking of the UAV. Then, the robust integral of the signum of the error (RISE) controller is designed for the inner loop to compensate for actuator anomalies and external disturbances. The composite stability of the closed-loop system and the asymptotical tracking performance are proved via a Lyapunov-based stability analysis. Numerical simulations based on the proposed fault tolerant control (FTC) scheme as well as the comparison results with a sliding mode-based FTC method validate the effectiveness and better performance of the proposed control design.

High Precision Sinusoidal Position Tracking of a Voice-Coil Linear Servomotor Using Resonant Control

This paper presents a new sinusoidal position-tracking control scheme with a resonant controller for linear motor drive systems. The sinusoidal tracking controller is designed without any added algorithm for system identification and requires only approximate values of the mechanical parameters. Therefore, the controller is simple and robust to parameter variations. The proposed sinusoidal tracking resonant-based controller (STRC) is designed to track reference positions using a cascade control structure with an inner current/force control with hysteresis current control followed by a speed control loop with a resonant controller, and an outer position loop with a proportional and velocity-feedforward controller. The stability of the cascade feedback scheme and its parameter tuning are analyzed using the Routh–Hurwitz criterion. The performance of the proposed control scheme is validated using simulations and experiments on a voice-coil linear stage. The proposed STRC strategy is characterized by ease of implementation and shows excellent performance with fast response and high accuracy at different frequencies with a maximum error of 0.58% at 0.25 Hz.

Anti-disturbance Trajectory Tracking Control for a Quadrotor UAV with Input Constraints⋆

This paper presents the trajectory tracking control for a quadrotor unmanned aerial vehicle(UAV) with disturbances and input constraints. The UAV's system is decomposed into an outer position loop and an inner attitude loop. The position tracking control is achieved by a class of saturated proportional-derivative(PD) control laws. Assuming that the unknown disturbances mainly effect on the translational dynamics, an adaptive control term is investigated to compensate the effects of the disturbances. The position tracking control force implies a coupled-attitude system that connects the inner attitude loop with the outer position loop. An intrinsic PD control law, which is directly designed on the special orthogonal group SO(3), is proposed to track the desired attitude. In order to reduce the calculational difficulty of the attitude tracking control law, an observer-based control law is further developed while the control law and observer can be designed separately. Simulation results complete this work.

Aggressive Formation Tracking for Multiple Quadrotors Without Velocity Measurements Over Directed Topologies

This paper addresses the aggressive formation tracking problem of quadrotors without velocity measurements over directed interaction topologies. A distributed formation control scheme is proposed for a multiple quadrotors system, which includes an outer-loop position controller to achieve the desired formation and an inner-loop attitude controller to regulate the attitudes. First, a second-order auxiliary system is designed to overcome the unmeasurable linear velocity. Next, the posotion controller is presented using only the position information under the directed topologies. Then, the geometric attitude controller is developed to carry out aggressive rotation maneuvers. Furthermore, an auxiliary system based on Lie algebra is proposed to eliminate the unreliable angular velocity feedback caused by disturbances and bias perturbations. Using Lyapunov stability theory on a cascaded system, it is rigorously proven that the proposed formation controller is asymptotically stable. Finally, the applicability and performance of the proposed control scheme are demonstrated by the numerical simulations.

Trajectory tracking control for autonomous underwater vehicles based on dual closed-loop of MPC with uncertain dynamics

This paper presents the trajectory tracking problem for constrained autonomous underwater vehicles (AUVs). More specifically, a novel control framework of model predictive control based on dual closed-loop is designed enhancing the tracking performance of the AUV. On the basis of the dynamic characteristics of the AUV, the initial optimization problem is transformed into the design of an outer-loop position controller and an inner-loop velocity controller. The contraction constraint devised in the control problem of two loops ensures the convergence of the AUV motion trajectory. The recursive feasibility and closed-loop stability of the system are demonstrated. In addition, the inner-loop controller design is supplied with robustness through a combination of robust model predictive control and backstepping control. Despite the existence of model dynamic uncertainties and external disturbances of the ocean currents, the tracking errors of the closed-loop system can converge to a small neighborhood of zero. Finally, the simulation experiments verify the effectiveness of the proposed scheme.

Current sensorless position-tracking control with angular acceleration error observers for hybrid-type stepping motors

This paper exhibits an advanced observer-based position-tracking controller for hybrid-type stepping motors with consideration of parameter and load uncertainties. As the main contribution, a current sensorless observer-based pole-zero cancellation speed controller is devised for the outer loop position-tracking controller including the convergence rate boosting mechanism. The features of this study are summarized as follows; first, the pole-zero cancellation angular acceleration error observer for the inner loop speed controller, second, the pole-zero cancellation speed control forcing the order of the controlled speed error dynamics to be 1, and, third, the outer loop position control incorporating the first-order target tracking system with its convergence rate booster. The resultant effectiveness is verified on a 10-W stepping motor control system.

Distributed Output-Feedback Formation Tracking Control for Clustered Quadrotors

In this article, we study the cooperative formation tracking problem of clustered quadrotors without velocity and angular velocity measurements. In particular, the clustered quadrotors are required to track a reference trajectory while preserving a well-defined formation shape. However, the information interaction among the quadrotors is local and the reference trajectory information is only known to partial quadrotors. To overcome the aforementioned dilemmas caused by measurement incompleteness and interaction restriction, a distributed output-feedback control algorithm is proposed based on the cascaded framework. To be specific, by introducing two auxiliary dynamics with saturation constraints on their states, a distributed command force with a saturation attribute is first developed in the outer position loop. Next, in terms of a self-contained selection criterion of the control parameters, a bounded applied thrust and a nonsingular command attitude are extracted from the developed command force. Whereafter, an applied torque resorting to another auxiliary dynamics is developed in the inner attitude loop for the purpose of tracking the command attitude. It is shown that, despite the absence of the velocity and angular velocity information of clustered quadrotors, the concerned cooperative formation tracking objective is fulfilled by the proposed distributed control algorithm. Finally, simulations are performed to validate the built theoretical results.

Sliding mode disturbance observer and sliding mode controller for quadrotor UAV

The dynamics of quadrotor UAV is described considering internal uncertainty and external disturbance. The control system of quadrotor UAV is divided into inner loop attitude control system and outer loop position control system, In order to solve the problem of underactuated character. Three virtual control variables are introduced to decouple the position dynamics. Three sliding mode disturbance observers (SMDOs) are designed to estimate the total disturbances in the position dynamics. Then three sliding mode controllers (SMCs) are designed to drive the trajectory tracking errors to zero asymptotically. The simulation example is carried out to test the effectiveness of SMDOs and SMCs. Simulation results validate that the designed SMDOs and SMCs provide both fast exact tracking capability and strong disturbance rejection capability for the quadrotor UAV.

Sliding Mode Controller Based on the Extended State Observer for Plant-Protection Quadrotor Unmanned Aerial Vehicles

Owing to the complex dynamics of quadrotor unmanned aerial vehicles (UAVs) and their susceptibility to unknown interferences in an actual working environment, the flight control accuracy of UAVs is extremely high. Moreover, their anti-interference ability is particularly important. This study designed a sliding-mode controller based on the extended state observer. The position control was obtained through the outer-loop position controller. The attitude control was determined through the inner-loop attitude controller. The input of the UAV system was obtained through the controller. The boundary-layer function was used to weaken the oscillatory response of the system, and the traditional extended state observer was improved to improve the response speed, robustness, and tracking accuracy of the controller. For the entire process, the input and output state information of the system and total internal and external disturbances were estimated in real-time through the extended state observer. A sliding-mode control law was designed to compensate for the estimated disturbance in real-time to realize attitude control. Finally, Lyapunov theory was used to confirm the stability of the system. The simulation results demonstrated the improved anti-interference and tracking ability of the designed controller.

Adaptive Fault-Tolerant Distributed Formation Control of Clustered Vertical Takeoff and Landing UAVs

This article investigates the formation control issue of clustered vertical takeoff and landing (VTOL) unmanned airborne vehicles (UAVs) subject to undesired perturbations composed by actuator faults, parametric uncertainties, and external disturbances. In particular, a cluster of VTOL UAVs fly together in a given velocity with a well-specified formation configuration. For this purpose, an adaptive fault-tolerant distributed control algorithm via local information interaction is proposed under a hierarchical framework. Specifically, a distributed command force resorting to projection-based adaptation is first synthesized in the outer position loop. Such a command force is of a saturation attribute in order that a boundeddesired thrust and a nonsingular command attitude can be extracted in terms of appropriate control parameters. Next, in the inner attitude loop, a desired torque also with the help of projection-based adaptation is synthesized. Given that the underlying undirected topology is connected, it is shown based on the Lyapunov theory that the proposed adaptive fault-tolerant distributed control algorithm ensures the accomplishment of the concerned formation control objective of the clustered VTOL UAVs in spite of the undesired perturbations. Simulations validate and further highlight the effectiveness of the proposed distributed control algorithm.

A Fast and Efficient Attitude Control Algorithm of a Tilt-Rotor Aerial Platform Using Inputs Redundancies

Overactuated multirotor unmanned aerial vehicles (UAVs) usually consist of multiple tiltable thrust actuators. The controllers are mostly designed by regarding the thrust forces and actuator tilting angles as inputs of outer-loop position and attitude controllers, while formulating an inner-loop controller for each actuator to track the thrust and angle as required by the outer-loop. This hierarchical control strategy separates the complicated combined dynamics into two relatively simple systems, and thus simplifies the control design. However, the interaction between the two systems is neglected and therefore the control performance will be degraded when the inner-loop dynamics are not sufficiently fast. This letter investigates the capability of a new overactuated multirotor UAV configuration, where regular quadcopters are passively hinged onto the frame as tiltable thrust actuators. Apart from the thrust force and tilting angle, each actuator has additional auxiliary torque inputs, which exhibit fast responses as they are not subject to the inner-loop actuator tilting angle dynamics. In this letter, an add-on attitude compensation control is designed exploiting the auxiliary inputs to reduce the tracking and disturbance-rejection errors from the nominal control loop. The effectiveness is demonstrated in simulation and verified by experiment.

Geometric Formation Tracking of Quadrotor UAVs Using Pose-Only Measurements

In this brief, the problem of formation control of a swarm of quadrotor UAVs with a group leader using only position and attitude measurements is addressed. A distributed formation controller is proposed for the quadrotor team, which consists of an outer-loop position controller to achieve the desired formation without linear velocity measurements, and an inner-loop attitude controller to regulate the attitudes without angular velocity measurements. Two auxiliary dynamic systems driven by proper inputs respectively are designed to overcome the lack of linear and angular velocities. The position controller derived from the intermediary translational control inputs guarantees the boundedness of the actual thrust force inputs. The attitude controller is developed directly using rotation matrices to avoid the singularities of Euler-angles and the ambiguity of quaternions. The stability analysis is presented using Lyapunov stability theory on cascade systems, and the effectiveness of the proposed control strategy is demonstrated through numerical simulations.

Nonlinear cascade control of an electro‐hydraulic actuator with large payload variation

AbstractElectro‐hydraulic actuators have been widely used in industrial production, but the unknown variable payload seriously affects its position control accuracy. Therefore, a radial basis function neural network disturbance observer is designed to estimate the lumped disturbance force through strong online learning ability in the absence of force sensor. Besides, a nonlinear cascade controller with double loop structure is proposed in this paper. A global fast terminal sliding mode control method is firstly applied in the outer loop position system, which can eliminate chattering and improve convergence speed comparing to traditional sliding mode control. The inner loop force system adopts a backstepping control method to calculate the actual input of the whole system. Theoretical analysis indicates that the proposed controller is stable even if existing time‐variant disturbance. Moreover, three comparative controllers are designed and tested in both simulations and experiments. Comparative results show that the developed method has absolute average errors of 1.14 and 0.49 mm in different position tracking, which means more satisfactory tracking performance compared to the contrast controllers.