Sai-Ming Li

Robust Tracking Control Design for Spacecraft Under Control Input Saturation

A continuous globally stable tracking control algorithm is presented for spacecraft in the presence of control input saturation, parametric uncertainty, and external disturbances. The proposed control algorithm has the following properties: 1) fast and accurate response in the presence of bounded disturbances and parametric uncertainty; 2) explicit accounting for control input saturation; and 3) computational simplicity and straightforward tuning. A detailed stability analysis of the resulting closed-loop system is included. It is shown that global stability of the overall system is guaranteed with continuous control even in the presence of bounded disturbances and parametric uncertainty. In the proposed controller a single parameter is adjusted dynamically in such a fashion that it is possible to prove that both attitude and angular velocity errors will tend to zero asymptotically. The stability proof is based on a Lyapunov analysis and the properties of the quaternion representation of spacecraft dynamics. One of the main features of the proposed design is that it establishes a straightforward relationship between the magnitudes of the available control inputs and those of the desired trajectories and disturbances even with continuous control. Numerical simulations are included to illustrate the spacecraft performance obtained using the proposed controller.

Forest fire monitoring with multiple small UAVs

Frequent updates concerning the progress of a forest fire are essential for effective and safe fire fighting. Since a forest fire is typically inaccessible by ground vehicles due to mountainous terrain, small unmanned air vehicles (UAVs) are emerging as a promising means of monitoring large forest fires. We present an effective UAV path planning algorithm utilizing infrared images that are collected on-board in real-time. To demonstrate the effectiveness of our path planning algorithm in realistic scenarios, we simulated the propagation of a forest fire with the EMBYR model. A new cooperative control mission concept is introduced where multiple low-altitude, short-endurance (LASE) UAVs are used for fire monitoring. By employing multiple UAVs, the effectiveness of the mission in terms of information update rate can be improved dramatically.

Autonomous hierarchical control of multiple unmanned combat air vehicles (UCAVs)

In this paper we present a hierarchical control scheme that enables multiple UCAVs to achieve demanding missions in hostile environments autonomously. The objective is to use a swarm of UCAVs for a SEAD type mission: fly the UCAVs in a formation to an enemy territory populated with different kinds of threats, collect enemy information or destroy certain targets, and return to the base, all without human intervention. The scheme is an integration of four distinct components, including: (1) high level Voronoi diagram based path planner to avoid static threats; (2) low level path planner to avoid popup threats; (3) differential flatness based trajectory generator to generate dynamically feasible trajectory; and (4) semi-globally stable formation control algorithm to maintain the formation. The scheme was implemented in Matlab and demonstrated very effective path planning, trajectory generation, and formation flying capabilities. We also developed an interface from Matlab to IWARS, a high fidelity battlefield simulation environment developed by Boeing. This enabled us to study the effectiveness of our scheme under various battle scenarios using IWARS.

GLOBALLY STABLE AUTOMATIC FORMATION FLIGHT CONTROL IN TWO DIMENSIONS

In this paper we address the problem of nonlinear formation flight control design using a leaderfollower configuration. The problem is to design a formation hold autopilot for the follower vehicle such that the relative positions between the leader and the follower are maintained close to their desired values. We first present a nonlinear control algorithm based on the dynamics of the relative positions between the two vehicles. Such a control law results in a locally stable system since the corresponding dynamic model has a singularity in the state space. We next propose a singularity-free formation control algorithm based on a new error formulation in the inertial frame. We prove global stability of the overall system, and demonstrate favorable properties of the proposed formation control algorithm through computer simulations.

Formation flight control design in the presence of unknown leader commands

In this paper we develop stable adaptive formation control algorithms for two-dimensional point-mass models of aircraft dynamics. The algorithms effectively compensate for uncertain leader commands. It is shown that the adaptive formation control design in the body frame of the follower aircraft contains only two adjustable parameters, while in the case of design in the body frame of the lead aircraft there are four parameters that need to be adjusted to assure the stability of the system. In the latter case the information on the bounds on the rates of the commands is also needed to implement the controller. In both cases the adaptive algorithms are based on the variable structure control design technique. A simulation example is included to illustrate the properties of the proposed approach.

Evaluation of the properties of a multiple-model reconfigurable flight controller on a 6 DOF simulation

In this paper we describe the implementation of the multiple model-based FBI-ARC algorithms from [6, 1] in the nonlinear 6 DOF simulation of a Tailless Advanced Fighter Aircraft (TAFA). Due to the nonlinear nature of the simulation, the implementation and tuning of the algorithms is substantially more complicated than of their counterparts in linear simulations. However, the well tuned FDIARC achieved excellent performance despite severe flight-critical variations in the aircraft dynamics.

Initial study of autonomous trajectory generation for unmanned aerial vehicles

Many approaches to trajectory generation for nonlinear systems approximate the feasible set using polytopes. This approximation can be very poor even for simple differentially flat systems. For a large class of flat systems arising in aerospace applications, the feasible flat output set is defined in terms of rational function inequalities. We use this observation and a well-known result from robust control to derive a linear fractional representation (LFR) of the feasible set. It is possible to obtain good convex sufficient conditions for trajectory generation from the LFR as similar problems have been tackled in p-analysis related work. In this paper, we develop a sufficient bilinear problem from the LFR. The global optimization algorithm of Floudas and Visweswaran (1990) can be used to check feasibility though not in polynomial time. We present an example to illustrate the difficulties with polytopic approximations and the advantages of the proposed LFR approach.

On-line failure detection and identification (FDI) and adaptive reconfigurable control (ARC) in aerospace applications

In this paper we describe our recent results related to online failure detection and identification (FDI) and adaptive reconfigurable control (ARC) techniques developed in the context of aerial and space vehicles. Our approach is based on the multiple models, switching and tuning (MMST) methodology and its extensions, and has been demonstrated as an efficient tool for fault tolerant control under subsystem and component failures and structural damage.

Robust supervisory fault-tolerant flight control system

In this paper we present a new supervisory scheme for robust online failure detection and identification (FDI) and adaptive reconfigurable control (ARC) in the presence of multiple control effector failures. The scheme, based on adaptive interacting multiple observers (AIMO), substantially reduces the computational requirements compared to the previous designs, and increases the robustness of the overall system by controlling the plant with a fixed robust controller most of the time. The feasibility of the proposed design was demonstrated through nonlinear simulations of F/A-18C/D aircraft during carrier landing in the presence of battle damage and failures of flight-critical subsystems and components. In particular, we demonstrate the accommodation of a multiple failure that includes 50% left stabilator damage, rudder lock-in-place, and a large external disturbance.

Fault tolerant control of spacecraft in the presence of sensor bias

We develop a stable scheme for bias estimation in the case of attitude tracking. The scheme is based on the design of nonlinear observers for unknown bias identification and state estimation. In the case of gyro bias, our nonlinear observer design, based on the quaternion dynamics, leads to an error model and adjustment laws that result in guaranteed convergence of the unknown bias estimates to their true values. We demonstrate that our scheme results in a stable overall system, and achieves highly accurate pointing in the presence of unknown sensor bias. The properties of the proposed scheme are evaluated through simulations using a generic spacecraft model.