John D. Schierman

Integrated Adaptive Guidance and Control for Re-Entry Vehicles with Flight Test Results

To enable autonomous operation of future reusable launch vehicles, reconfiguration technologies will be needed to facilitate mission recovery following a major anomalous event. The Air Force’s Integrated Adaptive Guidance and Control program developed such a system for Boeing’s X-40A, and the total in-flight simulator research aircraft was employed to flight test the algorithms during approach and landing. The inner loop employs a modelfollowing/dynamic-inversion approach with optimal control allocation to account for control-surface failures. Further, the reference-model bandwidth is reduced if the control authority in any one axis is depleted as a result of control effector saturation. A backstepping approach is utilized for the guidance law, with proportional feedback gains that adapt to changes in the reference model bandwidth. The trajectory-reshaping algorithm is known as the optimum-path-to-go methodology. Here, a trajectory database is precomputed off line to cover all variations under consideration. An efficient representation of this database is then interrogated in flight to rapidly find the “best” reshaped trajectory, based on the current state of the vehicle’s control capabilities. The main goal of the flight-test program was to demonstrate the benefits of integrating trajectory reshaping with the essential elements of control reconfiguration and guidance adaptation. The results indicate that for more severe, multiple control failures, control reconfiguration, guidance adaptation, and trajectory reshaping are all needed to recover the mission.

Adaptive Terminal Guidance for Hypervelocity Impact in Specified Direction

The problem of guiding a hypersonic gliding vehicle in the terminal phase to a target location is considered. In addition to the constraints on its final position coordinates, the vehicle must also impact the target from a specified direction with very high precision. The proposed 3-dimensional guidance laws take simple proportional forms. The analysis establishes that with appropriately selected guidance parameters the 3-dimensional guided trajectory will satisfy these impact requirements. We provide the conditions for the initial on-line selection of the guidance law parameters for the given impact direction requirement. The vehicle dynamics are explicitly taken into account in the realization of guidance commands. To ensure high accuracy in the impact angle conditions in an operational environment, we develop closed-loop nonlinear adaptation laws for the guidance parameters. We present the complete guidance logic and associated analysis. Simulation results are provided to demonstrate the effectiveness and accuracy of the proposed terminal guidance approach. I. Introduction Recent interests in developing on-demand global-reach payload delivery capability have brought to the forefront a number of underlying technological challenges. Such operations will involve responsive launch, autonomous entry flight, and precision terminal maneuvers. In certain scenarios the mission requirements call for the payload to impact the target location from a specific direction with supersonic speed. One example is to impact the target in a direction perpendicular to the tangent plane of the terrain at the target. The terminal guidance system will be responsible for directing the vehicle to the target and achieving the desired impact direction. The impact precision requirements under the scenarios considered are very high and stringent. For instance, the required Circular Error Probable (CEP) of the impact distance is just 3-meter. 1 The errors of the impact angles are desired to be within 0.5 deg. The very high speeds throughout the terminal phase only make it considerably more difficult to achieve these levels of precision. Yet cost considerations dictate that the terminal guidance algorithm should be relatively simple and computationally tractable for real-time operations. While a number of guidance methods can guide the vehicle to the target, not many address the unique need for impact from a specific direction. One method that can is the so-called “dive-line” guidance approach in Ref. 2. In this method one or more lines intersecting the Earth are established. The final dive-line intersects the target, and its direction can be set to the desired direction. The vehicle’s velocity vector is

ADAPTIVE GUIDANCE WITH TRAJECTORY RESHAPING FOR REUSABLE LAUNCH VEHICLES

Reusable launch vehicle (RLV) designs are influenced by weight and other constraints that rarely allow for significant effector redundancy. Therefore, reconfiguration of the inner-loop control system and outer-loop guidance functions can be necessary to compensate for significant control effector failures, aerodynamic uncertainties, and disturbances such as strong winds. Trajectory reshaping is also important in some situations to judiciously manage vehicle energy on-line to meet the flight objectives. This paper presents an Optimum-Path-To-Go (OPTG) algorithm, which is a general framework to perform the on-line trajectory-command generation task. The methodology was applied to Lockheeds X-33 RLV for the approach- and-landing phase of flight, and a Monte-Carlo simulation analysis was used to demonstrate the benefits of the approach. Random increments to the vehicle drag were inserted in the simulation at various downrange locations. Such variations are representative of mismodeled aerodynamics, speedbrake failure, or significant winds. The X-33 implementation of the OPTG was able to reshape the trajectories to result in a safe landing for greater than 93% of the cases. Without the OPTG capability, safe landing was accomplished for only 29% of the drag variations simulated.

On-Line Trajectory Command Reshaping for Reusable Launch Vehicles

To enable autonomous operations in future Reusable Launch Vehicles (RLVs), onboard trajectory command reshaping will be required to facilitate recovery of the mission following a major anomalous event such as an effector failure. The Optimum-Path-To-Go (OPTG) on-line trajectory-reshaping algorithm is presented. In the OPTG methodology, a trajectory database is precomputed off-line covering all variations under consideration. Then, polynomial-based networks are generated which map these variations to basis function coefficients that describe the shape of the trajectories. The networks are then interrogated on-line, and the resulting coefficients are used to generate trajectory commands. Thus, based on the current state of the system, the algorithm will reshape the commanded trajectory to give the best remaining path to the end of the mission segment. For this study, the commanded trajectory was reshaped on-line due to a severe multiple control surface failure. Without reshaping, the vehicle is lost, even with control reconfiguration and guidance adaptation. With trajectory reshaping, the mission is recovered.

In-Flight Entry Trajectory Optimization for Reusable Launch Vehicles

A novel in-flight trajectory command generation approach for Reusable Launch Vehicles was developed. This new approach has the ability to reshape the vehicle’s commanded trajectory onboard, in real-time for significantly changed energy conditions due to, for example, control effector failures. The approach also has the ability of retargeting the trajectory to abort to more appropriate alternative landing sites. The key feature of the new approach is an innovative optimization method: by describing decision variables in terms of appropriate basis functions, the trajectory optimization problem can be reformulated to find the relatively few basis function coefficients that characterize the desired trajectory. This significantly reduces the search domain, enabling rapid convergence to feasible solutions. An entry trajectory optimization problem was formulated, incorporating real-world vehicle constrains such as heating and dynamic pressure boundaries. It was demonstrated that the trajectory could be reshaped in-fight for both low and high energy cases. Furthermore, the approach was demonstrated to retarget to two defined alternate landing sites. The nominal landing site was defined to be Kennedy Space Center in Florida, while the two alternate landing sites were Atlanta and Houston. For a standard desktop computer, the time to realize a solution was reduced from one to two minutes using a traditional optimization method to only approximately 10 seconds for the new formulation. The ability to rapidly and accurately reshape trajectory commands to perturbations in energy, and the ability to retarget the trajectory to alternate landing sites was successfully demonstrated.

Flight Test Results of an Adaptive Guidance System for Reusable Launch Vehicles

For next generation Reusable Launch Vehicles (RLVs), reconfigurable control, adaptive guidance, and on-line trajectory-command reshaping will often be required to recover the mission in the face of a major anomalous event such as an effector failure. An adaptive guidance system that works in conjunction with a reconfigurable controller and an autonomous trajectory command reshaping algorithm is presented. The guidance law utilizes a backstepping architecture to generate pitch rate commands that drive the inner-loop control system. Under extreme failure conditions the control surfaces can saturate in an attempt to meet commanded moments. In these cases, the guidance feedback gains are reduced to preserve stability margins in the guidance loops. In addition, simulation and flight test results of the complete reconfigurable control/adaptive guidance/trajectory reshaping system are presented for a simulated X-40A RLV. The Total In- Flight Simulator research aircraft was utilized to flight test the X-40A system under a variety of failure conditions. This work was completed in conjunction with the Air Force Research Laboratorys Integrated Adaptive Guidance & Control (IAG&C) program. Both simulation and flight test results indicate the major benefits of the new system. With on-line trajectory reshaping, the vehicle is able to achieve a safe touchdown, whereas the vehicle is lost without trajectory reshaping.

Run-Time Assurance for Advanced Flight-Critical Control Systems *

New and emerging mission and operational capabilities, such as micro air vehicles, morphing wings, cooperative flight, and automated aerial refueling call for ever-increasing levels of complexity and autonomy. While fundamental controls research has made great progress in addressing these needs, advances in verification and validation (V&V) practices have failed to keep pace. Most V&V is still based exclusively on evidence generated through exhaustive testing. As systems become increasingly complex and involve more system-ofsystems interactions, this level of exhaustive testing will become increasingly infeasible due to the number of interactions that must be exercised. Since V&V practices have remained essentially unchanged, many compelling solutions offered by controls research cannot currently be realized, producing a widening gap between realized system capability and desired system capability. There is clear, pressing need for new V&V techniques that can deliver strong safety guarantees for advanced systems while controlling V&V costs. This paper presents a new run-time assurance approach to provide safety to systems employing advanced control solutions that cannot be certified with today’s V&V technologies. The approach employs a monitor that continually checks that the system lies within safe operating bounds. If uncertified bounds are imminent, then the system is switched to a reversionary, certified control system that can, at least, provide “return-to-base” capabilities. A number of experiments have been completed through both desktop and realtime, hardware-in-the-loop simulations that demonstrate the benefits of this approach.

New Verification and Validation Methods for Guidance/Control of Advanced Autonomous Systems

Approaches such as autonomous, intelligent, and adaptive control algorithms have shown signiflcant promise for improving the safety and performance, and expanding the capabilities of air vehicles, but V&V has been a major obstacle to their implementation in ∞eet aircraft. This paper presents a V&V aware control architecture intended to help overcome these hurdles and facilitate the ∞ight certiflcation of advanced control approaches. The architecture segregates high risk components of the control system, such as the adaptive and intelligent algorithms, and employs run-time safety monitors to perform real-time checks on the behavior of these components. In the event that a problem is detected, the safety monitors trigger a switch from the high-risk components to a failsafe control mode. The failsafe mode is fully verifled and validated at design time to provide a safe return to base capability, though performance and mission capabilities will typically be reduced in this mode. The run-time architecture was demonstrated in a batch simulation of a VTOL UAV with an indirect adaptive control law performing a shipboard landing task. The approach greatly increased the percentage of safe landings in the case of an intentionally poorly tuned on-line parameter identiflcation algorithm in the indirect adaptive control law.

Intelligent Guidance and Trajectory Command Systems for Autonomous Space Vehicles

This effort represents continued developments of an integrated reconfigurable control, adaptive guidance, and onboard trajectory command reshaping system that was successfully flight tested in 2003. The purpose of these advanced algorithms is to recover the mission in the face of severe off-nominal conditions and control effector failures. In the flight test program, the system was developed for the missions final flight phase known as approach and landing. The current effort is furthering the technology with application to other flight phases such as re-entry and Terminal Area Energy Management (TAEM). The guidance law utilizes a backstepping architecture to generate attitude rate commands that drive the inner-loop control system. Under certain control surface failure conditions, the bandwidth of the inner-loop control system is purposely reduced to lessen the commanded moments. In these cases, the guidance feedback gains are adapted on-line to preserve stability margins in the guidance loops in the face of degraded maneuvering capabilities. During the course of the flight test program, it was shown that failure scenarios that significantly alter the energy management of the vehicle will require the commanded trajectory to be reshaped in order to achieve an acceptable touchdown - even with reconfigurable/adaptive control and guidance. The onboard trajectory reshaping algorithm is known as the Optimum-Path- To-Go (OPTG) approach. OPTG results from the flight test program will be reviewed. However, new results of trajectory command reshaping during the TAEM guidance flight phase will also be presented. In this flight phase, the altitude and velocity must be brought to acceptable values at the start of the final approach. Further, the Heading Alignment Cone, or HAC turn, is flown to align the vehicles heading with the runway centerline. It will be shown that the OPTG algorithm can successfully reshape the HAC turn due to significant changes in the vehicles lift and drag. These changes may come about due to a control effector failure or significant head or tail winds. It will be shown that the mission is able to achieve an acceptable TAEM/final approach interface with trajectory reshaping.

Run-Time Verification and Validation for Safety-Critical Flight Control Systems

Mission and safety requirements for next-generation aerospace vehicles have given rise to flight software with an ever-increasing level of complexity and autonomy. “Intelligence” is built into new and emerging designs through the development of novel algorithms that detect, learn, adapt, switch modes, coordinate, plan, etc. As the complexity of flight controllers grows, so does the cost associated with Verification and Validation (V&V). Current-generation controllers are already reaching a level of complexity that pushes the envelopes of existing V&V approaches, with little hope for affordable certification of nextgeneration intelligent systems under current V&V practices. One possible solution is to combine run-time monitors for advanced components with simple backup modules that provide a safe reversionary mode if undesirable behavior is detected. Such an architecture allows the V&V to be partitioned into design-time V&V (for the relatively simple monitor and fail-safe subsystems), and run-time V&V (for the full complex controller). A prototype run-time monitoring approach for flight-critical systems has been developed and demonstrated in batch and real-time simulations for a UAV system. Software faults were seeded into advanced control algorithms to test the runtime monitoring systems. In all experiments, it is shown that without a runtime V&V system, the vehicle either fails to accomplish the mission, or worse, is lost due to ensuing instability. With the runtime V&V system, the vehicle is saved and can either continue the mission or return to base.