- Email: [email protected]
Recent and future trends in space and aeronautics – Special section on selected advanced control systems
Control Engineering Practice 60 (2017) 196–198
Contents lists available at ScienceDirect
Control Engineering Practice journal homepage: www.elsevier.com/locate/conengprac
Editorial
Recent and future trends in space and aeronautics – Special section on selected advanced control systems In our context there is a great deal of interest in present and future progress in the fields of both space and aeronautics; linked industries remain very attentive to new developments, applications or new technologies that may improve performance as well as reduce costs. This attention undeniably strengthens the motivation for engineers to cooperate with academia, and so innovative industry-relevant issues and goals are the focus of new researches. Indeed, looking at the recent past, rapid advancement of technologies has been achieved in space and aeronautics. This section focuses on some recent developments on spacecraft, aircraft and unmanned air vehicles (UAVs).
1. Some major trends in space In recent times there have been enormous improvements in control system technologies for spacecraft such as satellites, rockets, space exploration spacecraft, or planetary rovers; these improvements are a response to more and more sophisticated missions and a wider range of challenges. In the field of satellites, accuracy of attitude control (including determination, stability, and pointing precision) has been improved to meet higher levels of mission requirements; position control has seen similar improvements. For example, the joint project of the European Space Agency and NASA – “Lisa Pathfinder” – was launched in December 2015, and succeeded in positioning control of two onboard free-falling test masses with ultra-high accuracy, avoiding any disturbances; this development opened the way to space-borne gravitational wave observatories. For Earth observation satellites, ground resolution of onboard optical cameras has reached 31 cm – even with civil technology – which is realized in part thanks to advanced attitude stabilization and pointing technologies. As for rockets, experiments towards reusability have been performed by two companies, SpaceX and Blue Origin. They have succeeded in returning rockets from high altitude and making them stand vertically on the ground. Similar technological experimentation for reusable rockets is also being carried out in Europe and Japan, among other countries, where advanced control technologies to deal with unstable objects are being pursued. Space exploration has recently met success in a number of cutting-edge missions, including Japanese Hayabusa's sample-return from an asteroid, European Rosetta's orbiting a comet and Philae landing, and the USA's Dawn orbiting Vesta and Ceres and New Horizon's Pluto flyby, to name a few. As the probes go farther http://dx.doi.org/10.1016/j.conengprac.2017.01.015 0967-0661/& 2017 Elsevier Ltd. All rights reserved.
away from Earth, autonomous control becomes more critical. Planetary rovers are another example; after three successful Mars landings using parachutes and airbags, NASA JPL used the “sky crane” landing system as a new method for landing a very heavy (near 1 t) Mars rover. Improvements in autonomous navigation, path planning and obstacle avoidance technologies have been used by Mars rovers to cover more than 10 km distance on hazardous Martian terrain. In addition, the near future will see greater challenges for spacecraft, with missions requiring further advances in control technologies: for instance, debris catch and de-orbiting, sample return from various bodies, formation flying requiring precise relative position control, or constellations of thousands of satellites for communication. Another recent remarkable trend in space is the rapid improvement of technologies for smaller spacecraft. Small (100– 500 kg), micro (20–100 kg), nano (1–20 kg) and pico (1 kg or less) spacecraft, developed mainly by universities or venture companies, are beginning to be applied to practical missions such as space science and exploration, remote sensing, communication, and other novel applications, exploiting their low cost and quick development possibilities. Miniature components and attitude and orbit control system technologies have seen considerable advances to enable missions with requirements for high levels of attitude and orbit control. Space-exploration micro-spacecraft have also been developed, such as the Japanese PROCYON, and in 2018 a total of 13 6-kg (6U CubeSat) miniature explorers will be launched by NASA's SLS rocket. Small/micro/nano/pico spacecraft will see further use in the near future, not only by venture businesses but also even by governments to compensate for budget reductions. The realization of sufficient control-function requirements in advanced missions for this type of spacecraft – given their size, power, and budget constraints – has been and will be an important topic for research.
2. Some major trends in aeronautics 2.1. Aircraft There has been a spectacular evolution in aeronautics over the last decades. Both European and American aeronautical industries have designed, developed and manufactured new models of airtransport aircraft, both civil and military. In the subsonic and sonic ranges of air transport, much progress has been made in both the global conception and the design based on the breakthrough technologies.
Editorial / Control Engineering Practice 60 (2017) 196–198
Research has been focused particularly on increasing both safety and security, as well as improving the performance of the avionics and advancing the technology of composite materials. These developments have permitted higher performances, more optimized aerodynamic profiles, weight reduction, fuel savings, and reductions in environmental impact (emissions, noise) as well as operating costs. Most significant innovations have been derived from military requirements. Nowadays, aircraft are equipped with a new generation of sensors and networking capabilities that enable hard maneuvers and more efficient traffic-flow control, thanks to the modern and advanced guidance, navigation and control (GNC) systems that researchers and engineers have conceived, and also to the advancement of avionics systems to implement such functions requiring a large computational load. Many efforts have been directed towards improving the airport-approaching phase for a more efficient landing, especially in the case of adverse atmospheric conditions (such as high levels of wind shear, wind gusts, etc). Substantial efforts in research and development have been focused on increasing the maximum range, speed and ceiling of aircraft. The majestically designed supersonic aircraft Concorde, once the fastest civil aircraft in operation, showed a phenomenal performance, but its operation was stopped due to its environmental impact. The most recent exciting news in the civil domain is the redesigning of the aircraft line of Boeing and Airbus subsonic products; for instance, the so-called monster A380 has demonstrated the levels of performance that engineers and researchers are capable of bringing to society and its economy, from both the technical and practical points of view. Due to the projected future growth in transport of passengers and freight, efficient methods –including air traffic management and control – are under research to ensure fluid air traffic, to control delays to acceptable levels, and to prevent foreseeable bottleneck effects. Starting from some initial ideas that appeared at the early 1990s, a consortium of academics and industrial partners has already launched a large transnational program aiming to deal with the study and realization of fully automatic ground and inflight motions of passengers' aircraft. These kinds of program will need the implementation of efficient advanced methodologies to guarantee the reliability, safety and robustness of autopilots and a strong level of stabilization in uncertain environments to compensate for the absence of a human pilot. Nevertheless, it will probably take a long time before the general public has enough confidence to fly in unmanned aircraft. Some further innovative solutions have emerged to develop an electric aircraft; however, as seen from the Airbus prototype and the Solar Impulse model, the capability to transport a relatively heavy mass for a reasonable distance has not been achieved. That is due mainly to the capacity of the battery storage. In the near future, technical advances should improve the storage equipment and enable electric aircraft. 2.2. UAVs Meanwhile, as for satellites, aeronautics has already experienced decades of miniaturization in aircraft and other types of air vehicles. There are many different models depending on the mission they are designed for; they are typically classified according to their size and endurance: extremely small units, low altitude/long endurance, or high altitude/long endurance. The focus in this section is in particular on unmanned aircraft for which investments in research, development and innovation are crucial to reach higher levels of competitiveness. Starting from the 1990s, recent years have seen a considerable growth in the area of UAVs, popularly known as drones. Advances
197
in sensor and actuator technologies, as well as the exponential growth in onboard computational capacity, have enabled the development of a significant population of UAVs in both civil and military fields, with designs adapted to a rather large range of applications. UAVs have been demonstrated to be able to carry out a wide variety of missions at lower cost than traditional manned aircraft. In this technology, one of the key components in improving the autonomy and performance of the aircraft is the guidance, navigation and control (GNC) system, which must be adequately designed to guarantee that the aircraft is able to accomplish its mission safely and efficiently. Recent progress in control and estimation theory combined with improvements in technology nowadays provide the capability for designing more advanced GNC systems than ever. For instance, consider guidance systems. The objective of a guidance law is, in general, to produce the adequate command inputs to the attitude control system to follow a reference trajectory. Traditionally, aircraft guidance systems have been based on well-known missile guidance laws. While these laws have features such as simplicity (they are easily implemented on board) and robustness, they are reactive (not using future information), and may not provide precise tracking of a given trajectory. These limitations have motivated the development of guidance laws based on modern control techniques. One of the most promising control technique applicable to GNC in general and guidance in particular is model predictive control (MPC) which well exemplifies how complex control methods have become applicable thanks to advances in technology. MPC makes explicit use of a model of the process to obtain the control signal by minimizing an objective function over a finite receding horizon. In MPC the process model is used to predict the future plant outputs, based on past and current values and on the proposed optimal future control actions. Unfortunately, this procedure is oftentimes difficult to implement in practice, for two main reasons. First, the underlying optimization problem is frequently non-linear and therefore time-consuming; thus, real-time implementation becomes a challenge. This is sometimes referred to as the instantaneity problem. Second, there might be no guarantee of finding a feasible control solution (which could compromise the safety of the system). This is called the feasibility problem. As technology evolves, both the instantaneity and the feasibility problems have become solvable when using certain techniques or when the models are adequately simplified. Thus, a considerable number of examples of MPC designs for many aerospace problems can already been found in the literature. Many of these designs would have been considered impossible or impractical a few years ago. However, the unprecedented computational capabilities of todays' embedded electronic systems allow the implementation of advanced and complex algorithms (such as MPCbased designs) even in low-cost commercial off-the-shelf devices. As these capabilities keep increasing, we can expect to see more model predictive controllers (as well as other advanced control designs) implemented in real aerospace GNC systems. One may also find in the literature some designed controls that handle the strong non-linearities of the system and that have been shown to be quite efficient. However, questions concerning the verification and certification of these control systems remain open, representing a major obstacle to implementation in major aerospace industries such as commercial aviation. The advances that have already been achieved and those that will be achieved in the near future have unquestionably necessitated and will necessitate enormous efforts in areas such as mathematical modeling, estimation, and identification, as well new methodologies for control, guidance and navigation design.
198
Editorial / Control Engineering Practice 60 (2017) 196–198
3. Selected contributions related to space and aeronautics This special section contains a few contributions that have been accepted for publication after a thorough peer review of a series of papers that were pre-selected by technical sessions’ chairs from among the presentations from the recent IFAC Symposia on Automatic Control in Aerospace. 1) Pulse-Width Predictive Control for LTV Systems with Application to Spacecraft Rendezvous by Rafael Vazquez, Francisco Gavilan and Eduardo F Camacho. 2) Strategy for robust gust response alleviation of an aircraft model by Yuting Dai, Chao Yang and Chaolei Wang. 3) Robust Auto-landing of Fixed-wing UAVs using Neuro-Adaptive Design by Pradeep R. Ambati and Radhakant Padhi. The first paper presents a model predictive controller (MPC) that is able to handle linear time-varying (LTV) plants with pulsewidth modulated (PWM) control. Aerospace systems often need to be controlled by PWM actuators, i.e., actuators whose output level is fixed and can only be turned on and off, which poses a challenge because the system becomes non-linear in the switching times, even if the system is linear. However, most feedback design and motion planning methods ignore variable width pulses and approximate the control variables either by impulses (which produce instantaneous changes in some combination of the states) or pulse-amplitude modulated (PAM) control, which do not capture with precision the behavior of pulsed actuators such as spacecraft thrusters. The paper uses MPC, using a PAM or impulsive approximation as a hot-start, and then explicit linearization around successive PWM solutions for rapidly improving the solution by means of linear programming. As an example, the problem of rendezvous of spacecraft for eccentric target orbits (modeled by the LTV Tschauner-Hempel equations) is considered. The second paper describes the aeroelastic response to timedependent gusts or turbulence that should be considered in airplane design. A robust generalized predictive control law for gust response alleviation is designed and simulated on an aircraft model by using the real wind-tunnel response and approximated gust input. The GPC is thus applied to alleviate the wing tip acceleration under all test conditions, including varying flow velocities and varying gust frequencies. Finally, the alleviation effect of gust response under different test conditions is estimated on the basis of the comparison of simulated closed-loop acceleration with an experimental open-loop one. The comparison indicates that, after robust gust response alleviation control, the wing tip
acceleration response can be reduced by up to 70% under all the test conditions. Remarkably, the control law is robust to the parameter uncertainties and input uncertainties, which is applicable to the gust-alleviation wind-tunnel test. The third and last paper presents an innovative neuro-adaptive design philosophy embedding a Sobolev norm based Lyapunov function for directional learning of the unknown function, which is capable of learning both the unknown function in the system model as well as its Jacobian. This facilitates fast learning (model adaptation) without too many transient effects. The updated model is then used in the framework of dynamic inversion to design the guidance (outer) loop as well as the control (inner) loop. Using this philosophy a robust adaptive non-linear guidance and control design is presented for robust automatic landing. The performance of the proposed approach is successfully verified through numerous simulation studies using the six degrees of freedom (six-DOF) non-linear model of a prototype UAV. All possible disturbance effects that arises in practice have been considered in the simulation studies. As Associate and Guest Editors of this special issue, we would like to thank all the authors for their work and commitment and all reviewers for their efforts to evaluate these papers and make comments and remarks in order to ensure the scientific quality.
Associate Editor Houria Siguerdidjane L2S – Automatic Control Department, CentraleSupélec, Université Paris-Saclay, 3 rue Joliot Curie, 91192 Gif-sur-Yvette, France E-mail address: [email protected]
Guest Editor Shinichi Nakasuka Department of Aeronautics and Astronautics, School of Engineering, University of Tokyo, Hongo 7-3-1, Bunkoy-ku, Tokyo 113-8656, Japan E-mail address: [email protected]
Guest Editor Rafael Vazquez Aerospace Engineering Department University of Seville, 41092 Seville, Spain E-mail address: [email protected]
Contents lists available at ScienceDirect
Control Engineering Practice journal homepage: www.elsevier.com/locate/conengprac
Editorial
Recent and future trends in space and aeronautics – Special section on selected advanced control systems In our context there is a great deal of interest in present and future progress in the fields of both space and aeronautics; linked industries remain very attentive to new developments, applications or new technologies that may improve performance as well as reduce costs. This attention undeniably strengthens the motivation for engineers to cooperate with academia, and so innovative industry-relevant issues and goals are the focus of new researches. Indeed, looking at the recent past, rapid advancement of technologies has been achieved in space and aeronautics. This section focuses on some recent developments on spacecraft, aircraft and unmanned air vehicles (UAVs).
1. Some major trends in space In recent times there have been enormous improvements in control system technologies for spacecraft such as satellites, rockets, space exploration spacecraft, or planetary rovers; these improvements are a response to more and more sophisticated missions and a wider range of challenges. In the field of satellites, accuracy of attitude control (including determination, stability, and pointing precision) has been improved to meet higher levels of mission requirements; position control has seen similar improvements. For example, the joint project of the European Space Agency and NASA – “Lisa Pathfinder” – was launched in December 2015, and succeeded in positioning control of two onboard free-falling test masses with ultra-high accuracy, avoiding any disturbances; this development opened the way to space-borne gravitational wave observatories. For Earth observation satellites, ground resolution of onboard optical cameras has reached 31 cm – even with civil technology – which is realized in part thanks to advanced attitude stabilization and pointing technologies. As for rockets, experiments towards reusability have been performed by two companies, SpaceX and Blue Origin. They have succeeded in returning rockets from high altitude and making them stand vertically on the ground. Similar technological experimentation for reusable rockets is also being carried out in Europe and Japan, among other countries, where advanced control technologies to deal with unstable objects are being pursued. Space exploration has recently met success in a number of cutting-edge missions, including Japanese Hayabusa's sample-return from an asteroid, European Rosetta's orbiting a comet and Philae landing, and the USA's Dawn orbiting Vesta and Ceres and New Horizon's Pluto flyby, to name a few. As the probes go farther http://dx.doi.org/10.1016/j.conengprac.2017.01.015 0967-0661/& 2017 Elsevier Ltd. All rights reserved.
away from Earth, autonomous control becomes more critical. Planetary rovers are another example; after three successful Mars landings using parachutes and airbags, NASA JPL used the “sky crane” landing system as a new method for landing a very heavy (near 1 t) Mars rover. Improvements in autonomous navigation, path planning and obstacle avoidance technologies have been used by Mars rovers to cover more than 10 km distance on hazardous Martian terrain. In addition, the near future will see greater challenges for spacecraft, with missions requiring further advances in control technologies: for instance, debris catch and de-orbiting, sample return from various bodies, formation flying requiring precise relative position control, or constellations of thousands of satellites for communication. Another recent remarkable trend in space is the rapid improvement of technologies for smaller spacecraft. Small (100– 500 kg), micro (20–100 kg), nano (1–20 kg) and pico (1 kg or less) spacecraft, developed mainly by universities or venture companies, are beginning to be applied to practical missions such as space science and exploration, remote sensing, communication, and other novel applications, exploiting their low cost and quick development possibilities. Miniature components and attitude and orbit control system technologies have seen considerable advances to enable missions with requirements for high levels of attitude and orbit control. Space-exploration micro-spacecraft have also been developed, such as the Japanese PROCYON, and in 2018 a total of 13 6-kg (6U CubeSat) miniature explorers will be launched by NASA's SLS rocket. Small/micro/nano/pico spacecraft will see further use in the near future, not only by venture businesses but also even by governments to compensate for budget reductions. The realization of sufficient control-function requirements in advanced missions for this type of spacecraft – given their size, power, and budget constraints – has been and will be an important topic for research.
2. Some major trends in aeronautics 2.1. Aircraft There has been a spectacular evolution in aeronautics over the last decades. Both European and American aeronautical industries have designed, developed and manufactured new models of airtransport aircraft, both civil and military. In the subsonic and sonic ranges of air transport, much progress has been made in both the global conception and the design based on the breakthrough technologies.
Editorial / Control Engineering Practice 60 (2017) 196–198
Research has been focused particularly on increasing both safety and security, as well as improving the performance of the avionics and advancing the technology of composite materials. These developments have permitted higher performances, more optimized aerodynamic profiles, weight reduction, fuel savings, and reductions in environmental impact (emissions, noise) as well as operating costs. Most significant innovations have been derived from military requirements. Nowadays, aircraft are equipped with a new generation of sensors and networking capabilities that enable hard maneuvers and more efficient traffic-flow control, thanks to the modern and advanced guidance, navigation and control (GNC) systems that researchers and engineers have conceived, and also to the advancement of avionics systems to implement such functions requiring a large computational load. Many efforts have been directed towards improving the airport-approaching phase for a more efficient landing, especially in the case of adverse atmospheric conditions (such as high levels of wind shear, wind gusts, etc). Substantial efforts in research and development have been focused on increasing the maximum range, speed and ceiling of aircraft. The majestically designed supersonic aircraft Concorde, once the fastest civil aircraft in operation, showed a phenomenal performance, but its operation was stopped due to its environmental impact. The most recent exciting news in the civil domain is the redesigning of the aircraft line of Boeing and Airbus subsonic products; for instance, the so-called monster A380 has demonstrated the levels of performance that engineers and researchers are capable of bringing to society and its economy, from both the technical and practical points of view. Due to the projected future growth in transport of passengers and freight, efficient methods –including air traffic management and control – are under research to ensure fluid air traffic, to control delays to acceptable levels, and to prevent foreseeable bottleneck effects. Starting from some initial ideas that appeared at the early 1990s, a consortium of academics and industrial partners has already launched a large transnational program aiming to deal with the study and realization of fully automatic ground and inflight motions of passengers' aircraft. These kinds of program will need the implementation of efficient advanced methodologies to guarantee the reliability, safety and robustness of autopilots and a strong level of stabilization in uncertain environments to compensate for the absence of a human pilot. Nevertheless, it will probably take a long time before the general public has enough confidence to fly in unmanned aircraft. Some further innovative solutions have emerged to develop an electric aircraft; however, as seen from the Airbus prototype and the Solar Impulse model, the capability to transport a relatively heavy mass for a reasonable distance has not been achieved. That is due mainly to the capacity of the battery storage. In the near future, technical advances should improve the storage equipment and enable electric aircraft. 2.2. UAVs Meanwhile, as for satellites, aeronautics has already experienced decades of miniaturization in aircraft and other types of air vehicles. There are many different models depending on the mission they are designed for; they are typically classified according to their size and endurance: extremely small units, low altitude/long endurance, or high altitude/long endurance. The focus in this section is in particular on unmanned aircraft for which investments in research, development and innovation are crucial to reach higher levels of competitiveness. Starting from the 1990s, recent years have seen a considerable growth in the area of UAVs, popularly known as drones. Advances
197
in sensor and actuator technologies, as well as the exponential growth in onboard computational capacity, have enabled the development of a significant population of UAVs in both civil and military fields, with designs adapted to a rather large range of applications. UAVs have been demonstrated to be able to carry out a wide variety of missions at lower cost than traditional manned aircraft. In this technology, one of the key components in improving the autonomy and performance of the aircraft is the guidance, navigation and control (GNC) system, which must be adequately designed to guarantee that the aircraft is able to accomplish its mission safely and efficiently. Recent progress in control and estimation theory combined with improvements in technology nowadays provide the capability for designing more advanced GNC systems than ever. For instance, consider guidance systems. The objective of a guidance law is, in general, to produce the adequate command inputs to the attitude control system to follow a reference trajectory. Traditionally, aircraft guidance systems have been based on well-known missile guidance laws. While these laws have features such as simplicity (they are easily implemented on board) and robustness, they are reactive (not using future information), and may not provide precise tracking of a given trajectory. These limitations have motivated the development of guidance laws based on modern control techniques. One of the most promising control technique applicable to GNC in general and guidance in particular is model predictive control (MPC) which well exemplifies how complex control methods have become applicable thanks to advances in technology. MPC makes explicit use of a model of the process to obtain the control signal by minimizing an objective function over a finite receding horizon. In MPC the process model is used to predict the future plant outputs, based on past and current values and on the proposed optimal future control actions. Unfortunately, this procedure is oftentimes difficult to implement in practice, for two main reasons. First, the underlying optimization problem is frequently non-linear and therefore time-consuming; thus, real-time implementation becomes a challenge. This is sometimes referred to as the instantaneity problem. Second, there might be no guarantee of finding a feasible control solution (which could compromise the safety of the system). This is called the feasibility problem. As technology evolves, both the instantaneity and the feasibility problems have become solvable when using certain techniques or when the models are adequately simplified. Thus, a considerable number of examples of MPC designs for many aerospace problems can already been found in the literature. Many of these designs would have been considered impossible or impractical a few years ago. However, the unprecedented computational capabilities of todays' embedded electronic systems allow the implementation of advanced and complex algorithms (such as MPCbased designs) even in low-cost commercial off-the-shelf devices. As these capabilities keep increasing, we can expect to see more model predictive controllers (as well as other advanced control designs) implemented in real aerospace GNC systems. One may also find in the literature some designed controls that handle the strong non-linearities of the system and that have been shown to be quite efficient. However, questions concerning the verification and certification of these control systems remain open, representing a major obstacle to implementation in major aerospace industries such as commercial aviation. The advances that have already been achieved and those that will be achieved in the near future have unquestionably necessitated and will necessitate enormous efforts in areas such as mathematical modeling, estimation, and identification, as well new methodologies for control, guidance and navigation design.
198
Editorial / Control Engineering Practice 60 (2017) 196–198
3. Selected contributions related to space and aeronautics This special section contains a few contributions that have been accepted for publication after a thorough peer review of a series of papers that were pre-selected by technical sessions’ chairs from among the presentations from the recent IFAC Symposia on Automatic Control in Aerospace. 1) Pulse-Width Predictive Control for LTV Systems with Application to Spacecraft Rendezvous by Rafael Vazquez, Francisco Gavilan and Eduardo F Camacho. 2) Strategy for robust gust response alleviation of an aircraft model by Yuting Dai, Chao Yang and Chaolei Wang. 3) Robust Auto-landing of Fixed-wing UAVs using Neuro-Adaptive Design by Pradeep R. Ambati and Radhakant Padhi. The first paper presents a model predictive controller (MPC) that is able to handle linear time-varying (LTV) plants with pulsewidth modulated (PWM) control. Aerospace systems often need to be controlled by PWM actuators, i.e., actuators whose output level is fixed and can only be turned on and off, which poses a challenge because the system becomes non-linear in the switching times, even if the system is linear. However, most feedback design and motion planning methods ignore variable width pulses and approximate the control variables either by impulses (which produce instantaneous changes in some combination of the states) or pulse-amplitude modulated (PAM) control, which do not capture with precision the behavior of pulsed actuators such as spacecraft thrusters. The paper uses MPC, using a PAM or impulsive approximation as a hot-start, and then explicit linearization around successive PWM solutions for rapidly improving the solution by means of linear programming. As an example, the problem of rendezvous of spacecraft for eccentric target orbits (modeled by the LTV Tschauner-Hempel equations) is considered. The second paper describes the aeroelastic response to timedependent gusts or turbulence that should be considered in airplane design. A robust generalized predictive control law for gust response alleviation is designed and simulated on an aircraft model by using the real wind-tunnel response and approximated gust input. The GPC is thus applied to alleviate the wing tip acceleration under all test conditions, including varying flow velocities and varying gust frequencies. Finally, the alleviation effect of gust response under different test conditions is estimated on the basis of the comparison of simulated closed-loop acceleration with an experimental open-loop one. The comparison indicates that, after robust gust response alleviation control, the wing tip
acceleration response can be reduced by up to 70% under all the test conditions. Remarkably, the control law is robust to the parameter uncertainties and input uncertainties, which is applicable to the gust-alleviation wind-tunnel test. The third and last paper presents an innovative neuro-adaptive design philosophy embedding a Sobolev norm based Lyapunov function for directional learning of the unknown function, which is capable of learning both the unknown function in the system model as well as its Jacobian. This facilitates fast learning (model adaptation) without too many transient effects. The updated model is then used in the framework of dynamic inversion to design the guidance (outer) loop as well as the control (inner) loop. Using this philosophy a robust adaptive non-linear guidance and control design is presented for robust automatic landing. The performance of the proposed approach is successfully verified through numerous simulation studies using the six degrees of freedom (six-DOF) non-linear model of a prototype UAV. All possible disturbance effects that arises in practice have been considered in the simulation studies. As Associate and Guest Editors of this special issue, we would like to thank all the authors for their work and commitment and all reviewers for their efforts to evaluate these papers and make comments and remarks in order to ensure the scientific quality.
Associate Editor Houria Siguerdidjane L2S – Automatic Control Department, CentraleSupélec, Université Paris-Saclay, 3 rue Joliot Curie, 91192 Gif-sur-Yvette, France E-mail address: [email protected]
Guest Editor Shinichi Nakasuka Department of Aeronautics and Astronautics, School of Engineering, University of Tokyo, Hongo 7-3-1, Bunkoy-ku, Tokyo 113-8656, Japan E-mail address: [email protected]
Guest Editor Rafael Vazquez Aerospace Engineering Department University of Seville, 41092 Seville, Spain E-mail address: [email protected]