Background

L1 Adaptive Control is a novel theory for the design of robust adaptive control architectures using fast adaptation schemes.

The key feature of L1 adaptive control is the decoupling of the adaptation loop from the control loop, which enables arbitrarily fast adaptation without sacrificing robustness. In fact, in L1 adaptive control architectures, the rate of the adaptation loop can be set arbitrarily high, subject only to hardware limitations (computational power and high-frequency sensor noise), while the tradeoff between performance and robustness can be addressed through conventional methods from classical and robust control. This separation between adaptation and robustness is achieved by explicitly building the robustness specification into the problem formulation, with the understanding that the uncertainties in any feedback loop can be compensated for only within the bandwidth of the control channel. From an architectural perspective, this modification of the problem formulation leads to the insertion of a bandwidth-limited filter in the feedback path, which ensures that the control signal stays in the desired frequency range.

On one hand, fast adaptation allows for compensation of the undesirable effects of rapidly varying uncertainties and significant changes in the system dynamics. Fast adaptation is also critical to achieve predictable transient performance for system’s both signals, input and output, without enforcing persistency of excitation or resorting to high-gain feedback. On the other hand, the bandwidth-limited filter keeps the robustness margins bounded away from zero in the presence of fast adaptation. To this extent, the bandwidth and the structure of this filter define the tradeoff between performance and robustness of the closed-loop adaptive system.

To learn more about the theory of L1 Adaptive Control, please follow this link.

 

Applications

The features of L1 Adaptive Control described above have been verified –consistently with the theory– in a large number of flight tests and experiments in mid- to high-fidelity simulation environments. Brief overviews of the application of L1 Adaptive Control can be found below:

Flight Control

  • AirSTAR
    As part of the IRAC Project, an L1 flight control system was flight tested on the NASA’s AirSTAR Generic Transport Model (GTM) aircraft. The results of the flight tests demonstrated that, in the presence of aircraft component failure and significant changes in aircraft dynamics, the L1 flight control system is able to maintain aircraft safe operation and predictable performance with reduced pilot workload during both standard flight conditions and unusual flight regimes, like stall and post-stall. More >>
  • Learjet
    The first manned L1 flight control test was conducted by the U.S. Air Force Test Pilot School at Edwards Air Force Base, CA. This project consisted of flying and handling qualities assessment of Calspan’s variable-stability Learjet aircraft augmented with an L1 adaptive flight control law. The variable-stability capability of the aircraft was used to alter its apparent dynamics while in flight, allowing the validation of the L1 flight control law against a set of off-nominal aircraft configurations, some of them with aggressive tendencies to adverse pilot-aircraft interaction. All of these off-nominal configurations were opaque to the control law, and no fault detection and isolation methods were employed. The results demonstrated that the L1 flight control law was able to significantly restore the flying qualities of a baseline Learjet model, and also recover consistent and safe handling qualities. The evaluation also included a series of straight-in landings with two different aircraft configurations.
    More >>
  • F-16
    An L1 adaptive flight control system has been designed for the U.S. Air Force’s VISTA F-16 aircraft. This project will study the system’s performance in the more dynamically challenging environment provided by the VISTA F-16. With the VISTA F-16, additional failure configurations will be tested to demonstrate the ability of the L1 flight control law to compensate for off-nominal dynamics, actuator failures, and other types of uncertainties not included in the Learjet flight tests. More >>
  • SIMONA
    As a new effort in the development and technology transition of L1 adaptive control to general aviation aircraft, a simulation handling qualities assessment for a small business jet augmented with an L1 adaptive flight control system was conducted in collaboration with the Delft University of Technology (The Netherlands). The main objective of the study was to investigate the ability of an L1 adaptive FCS to provide enhanced handling qualities and maneuverability margins for safe landing in the presence of failures and in different atmospheric conditions. The experiments were conducted on the TU Delft’s 6DOF motion-based SIMONA Research Simulator. More >>

Energy Applications
L1 adaptive control has attracted StatOil and Schlumberger for possible applications in managed pressure drilling and rotary steerable systems. More >>

Hard Disk Control
In cooperation with Seagate Technology, ACRL has investigated applications of diverse adaptive schemes to increase overall tracking performance of hard disk drives under operational vibrations disturbances, including

  • dynamic control allocation for dual actuator vibration cancelation
  • design of adaptive actuator augmentation to mitigate drive to drive variation
  • active adaptive feedforward vibration cancellation methods
  • multi narrow-band adaptive disturbance observer to reduce tracking error

Optical Soliton Propagation
L1 adaptive control scheme has been developed for the problem of active dispersion management for propagation of solitons along uncertain fibers. More >>