Chimpalthradi R. Ashokkumar

Trajectory Transcriptions for Potential Autonomy Features in UAV Maneuvers

If a mission operation in a battlefield environment is complex, unmanned aerial vehicles (UAVs) and unmanned combat air vehicles (UCAVs) are expected to assist human pilots and ground personnel so that the threats to life are mitigated. Although these vehicles are controlled remotely by humans through ground commands or through another vehicle, possibilities to engage the onboard computer commands to respond to an environment and control the vehicle are not ruled out. Such systems are referred as autonomous UAVs and UCAVs. Following the traditional control technique adopted in aircraft motion technology, this paper assumes that autonomy and decision based controls for unmanned aircraft maneuvers are the features associated with the trajectory transcription techniques when the linear controllers at the inner loop are reconfigured. Hence in this paper, a tutorial on the switched linear controllers resulting from a non-zero equilibrium state from where an unmanned aircraft maneuver is required are presented. These evolving maneuvers instantaneously demanded in pitch plane are formulated using linear functional controllers of a micro air vehicle model. Linear functional controllers are the multiple input state feedback controllers which retain desired closed loop eigenvalues at fixed locations but the infinite eigenvector options that these controllers offer are transcripted for various ascent and descent angles of unmanned aircraft maneuvers.

UAV Control and Simulation Using Trajectory Transcriptions

A practice adopted in aircraft control and simulation is to design a linear time-invariant model based controller for each operating point and then schedule these controllers on-line at the inner loop to associate the operating points, which are also referred as equilibrium or trim points. At each of these trim points, the aircraft maintains specific orientation with respect to an inertial frame and offer a flight control mode (FCM) such as an ascent, descent, altitude hold, etc. In accomplishing various mission operations anticipated from an unmanned air vehicle (UAV), either remotely or autonomously, it is often required to fly the UAVs by using several of these FCMs whose nonlinear aircraft trajectories experience transcriptions. Accordingly the controllers are switched. One of the objectives in transcription is to preserve stability while the FCMs transit. In this paper, admissible controller sequence that can be scheduled in order to integrate FCMs through stable trajectory transcriptions is presented. FCMs resulting from static state and output feedback controllers are integrated to accommodate the principal objectives of gain scheduling, control allocation, reconfigurable control, etc. These evolving maneuvers instantaneously demanded in pitch plane are formulated to respond to the requirements originating from situational awareness of a typical UAV.

Aircraft Navigation with Uncertain Aerodynamics

In assignments such as navigation, nonlinear aircraft is controlled through the linear time-invariant model based stabilizing controllers (or simply referred as controllers). When the aircraft model is poorly known due to an unpredictable atmospheric conditions, one of the concerns in aircraft control is that the controller becoming fragile to uncertainties. Hence, design of controllers for uncertain aircraft navigation has been categorized as a challenging problem. In this paper, uncertainties such as plant and control parameter variations, sensor and actuator faults, etc., are considered and fragile definition [5] is applied to separate admissible uncertainties for which aircraft navigation using a controller would be valid. In linear systems framework, first it is shown that the uncertainties at which a certain matrix becomes singular are all the contributing factors of fragile aircraft dynamics with respect to the controller. A procedure to determine these singular points is presented. Then the admissible parametrically dependent stable Jacobian matrix as a function of state trajectory variables is derived as a sufficient stability condition for nonlinear uncertain aircraft. It is again posed as a matrix singularity problem to analyze potential eigenvalue bifurcations in the right half plane of the complex plane when a trajectory for navigation varies. A micro air vehicle model in pitch plane is considered to illustrate navigation of the fragile aircraft dynamics using control when uncertainties as real parameter variations are present.

Fault Tolerant Margins for Unmanned Aerial Vehicle Flight Safety

Consider an unmanned aerial vehicle (UAV) operation from the time it is launched until the time it is put into an autonomous flight control mode. The control input u(t) during this time duration is modeled σu(t) and assumed healthy with σ = 1. In practice, however, the control inputs are less effective with a σ-value in the interval [0, 1) (Fan et al. 2012; Jayakumar and Das 2006; Hu et al. J. Guid. Control. Dyn. 34(3), 927–932, 2011). Complete damaged condition is inferred from σ = 0. Given a stabilizing controller, a range of σ-values in the interval [0,1) for which the closed loop system would remain stable is referred as the fault-tolerant margin (FTM). Flight operations with control effectiveness beyond the FTM are catastrophic. Further, when aerodynamic parameter variations due to an uncertain atmospheric condition in an UAV are present, it is shown that the FTMs are extremely sensitive to these parameter perturbations. In this paper, the FTMs are presented. The effects of control effectiveness factor on the nonlinear UAV when it is in the stable range are investigated. An UAV model in Ashokkumar and York (2016) is considered to illustrate the FTMs as a threshold to guarantee the safe operation.

Sensor fusions for constant thrust aircraft navigation in pitch plane

Constant thrust aircraft navigation is an altitude hold mode in air transportation systems, air refueling systems, traffic alert and collision avoidance system and automatic dependent surveillance broadcast based collision avoidance, as well as in surveillance operations of unmanned aerial vehicles. In pitch plane, determination of the elevator input options available to maneuver the aircraft from its altitude hold mode using constant thrust is an important problem. Such inputs are useful to avoid time critical obstacles or collisions sensed in the horizontal plane. In this paper, it is shown that sensor fusions in feedback path are particularly attractive to generate such elevator inputs. Given the nonlinear aircraft’s linear time-invariant model based controllers at the inner loop, constant thrust aircraft navigation requires these controllers derived for sensor fusions. That is, in state feedback format, usually an identity matrix is considered for all state variables as measurements whereas in the present paper, particular combinations of the sensors in the feedback path are considered. A procedure to derive the stabilizing controllers that accommodates the sensor fusions is presented in partial pole placement framework. Then the nonlinear aircraft embedded with such controllers is simulated with capabilities to avoid the time critical obstacles. Examples by using a micro air vehicle model are illustrated.

Data science for decision aiding UAV control

The dynamics of a human operated nonlinear unmanned aerial vehicle (NUAV) with a given controller at the inner loop is governed by the admissible nonzero initial conditions and the pilot inputs. The movements of the joystick connected to the throttle and the control surfaces in a way are the indications of the pilot decision points to operate the stabilized NUAV. If these decisions fall short to avoid an obstacle or a denied air space, data science to generate the admissible control inputs through appropriate initial conditions becomes an important problem. Such control inputs at the autonomous decision points (decisions not made by the human pilot) complement the pilot inputs and may be used to avoid the obstacles and the denied airspace. In this paper, the rules to choose these control inputs at the autonomous decision points are discussed. A three degree of freedom aircraft in pitch plane is considered for illustrations.

Unmanned Aerial Vehicle Flight Control Evaluations Under Sensor and Actuator Faults

Nonlinear aircraft controlled by its linearized model based controller will be asymptotically stable if its trajectories originating from various initial conditions are all contained in the stability region (or the region of attraction). In such a stable aircraft, suppose a sensor or an actuator fault occurs. Depending upon the fault size, the stability region is modified. As a result, the trajectories for a potential failure to contain in the modified stability region could lead to flight control degradation. In this paper, first, a stability margin is determined to separate an acceptable fault size from an unacceptable fault. Secondly, with an acceptable fault size, the aircraft will remain stable in distorted stability regions. In this paper, admissible control inputs which generate the perturbed trajectories in the distorted stability regions is presented for safe unmanned aerial vehicle flight control evaluations. A three degree of freedom aircraft in pitch plane is considered to illustrate the stability margins and control inputs that are safe to operate the damaged aircraft.

Time response bounds in nonlinear UAV control

One of the intriguing problems in nonlinear unmanned aerial vehicle (NUAV) control using one of its linearized model based controllers at the inner loop is generating admissible control inputs for which the aircraft trajectories are confined to its stability region. It is an initial condition selection problem that is not necessarily applied at initial time but can be applied at any part of the flight duration where a bifurcation is needed, say, to avoid a collision with an obstacle. So, valid initial condition options at a given time instant can generate a stable constant speed maneuver tree that can be augmented to an existing autopilot and enhance autonomous characteristics at the decision points where changes in flight path directions are needed. In this paper, a procedure to select these initial conditions is presented. The time response bounds of the NUAV in its stability regions are developed. Both zero input response without pilot inputs and total response with pilot inputs and initial conditions are analyzed. A three degree of freedom NUAV in pitch plane is considered to illustrate the hierarchical initial conditions for which the time responses are bounded.

Adaptive fault tolerant control for reconfigurable systems

Reconfigurable systems are the safe dynamical systems wherein the health of the control input and/or sensor output variables are continuously monitored and then they are reconfigured, if necessary, in an event of an unexpected fault detected online. This idea is applied, in particular, at the reconfigurable system where the number of control variables is more than the state variables considered to define its motion. In this paper, an adaptive pole placing controller is developed for use in such reconfigurable systems. Adaptive pole placing controller, in theory, is adaptive to take a fault size and retain the closed loop poles it assigns at fixed locations in the open left half plane of the complex plane. As a result, when the controller is not adaptive, it means that the pole perturbations could lead to catastrophes. However, to design adaptive pole placing controller for a reconfigurable system, fault size is required. Hence, the primary objective of the paper is to present a simple procedure that determines the fault size for tuning in the adaptive pole placing controller. Both direct and full-order Luenberger observer based state feedback adaptive pole placing controllers are presented. In situations where the adaptive pole placing controller is not possible, the fault size for which the reconfigurable system with a non-adaptive pole placing controller would transit from a stable to unstable system (referred as fault tolerant margin with respect to the non-adaptive pole placing controller) is also presented. Some of these features of this paper are illustrated by using the reconfigurable aircraft model available in the literature.

Nonlinear cooperative UAV maneuvers in pitch plane

Flight formations of unmanned aerial vehicles may require coordinated motion in pitch for such tasks as terrain tracking. They maintain a constant altitude over varying terrain elevations and may assist collision avoidance where an altitude change is needed rather than a lateral change. In these maneuvers, controller ability to adjust relative altitude positions (as attractions and repulsions) of the aircraft subject to stability constraints that ends up in a formation shape should be demonstrated. The ascent and descent flight control mode combinations of each unmanned aerial vehicle participating in the formation generally offer the attractions and repulsions. In this paper, centralized and decentralized controller abilities to develop a cooperative formation with flight control modes made of transients and steady states are presented by using two and more than two homogenous unmanned aerial vehicles. A challenge in presenting such a formation by using a centralized controller is its design itself. Generally, eigenstructure properties depict flight control mode variations. That is, variations in closed-loop poles offered by a structurally varying centralized controller compatible to its reconfigurable communication patterns are sufficient to capture the flight control mode options. Hence, a procedure to design such a controller using real parameters is presented.