Walter Fichter

• Dr.-Ing.
• Professor at University of Stuttgart

Guidance laws to steer missiles towards their targets are well established and have been a topic of research for decades. However, the problem of evading missiles is still a challenging task. This paper presents a novel approach to the problem of evasive maneuvers against missiles. The proposed deep reinforcement learning method leverages the Deep...

In manned aviation, the improvement of ride comfort becomes increasingly important. The phenomena that cause discomfort during flight are divided into two types. The first is the whole-body vibration mainly introduced through the high-frequency rotation of the rotor. The second type of discomfort is kinetosis. Passengers who experience kinetosis ca...

Motion sickness is a medical condition that presents with symptoms including nausea, disorientation, and vomiting. The condition is triggered by motion frequencies below 0.5 Hz, which affect the sensory organs of the passengers. The symptoms manifest over several minutes to hours and decrease when the motion exposure is terminated. In rotorcraft, l...

To reduce the computational footprint of model predictive control during online computation, a horizon-one scheme based on pre-computed inner-approximations of reach-able sets is proposed. The inner-approximated reachable set allows to virtually predict the future system behavior over the full-horizon instead of repeatedly solving potentially large...

The laser interferometer space antenna (LISA) senses gravitational waves by measuring distance fluctuations between three spacecraft (SC). These measurements rely on precise tracking of a beat note phase that is formed on a quadrant-photo-diode (QPD) at each SC by interference of a local laser with a laser sent from a distant SC. The crucial prereq...

This paper focuses on the application of the Covector Mapping Principle (CMP) to low-thrust trajectory optimization problems to analyze the performances of the algorithm when non-complete methods are employed. Non-complete methods are those numerical schemes that do not provide an analytical mapping between the dual variables of direct and indirect...

Knowledge about the future trajectory of the target is an essential part of the guidance and control of guided missiles. Most existing prediction models either implicitly or explicitly assume a deter-ministic behavior of the target. Using this prediction model, the guidance law computes the control commands for the missile. In reality, targets exhi...

The paper shows the design of a control augmentation system to transform helicopters into Personal Aerial Vehicles, accounting for handling qualities and actuator constraints. In this paper, PAVs are conceived as augmented rotorcraft that can be safely flown by minimal-trained pilots with a training comparable in length to that necessary to learn h...

In the context of the Laser Interferometer Space Antenna (LISA), the laser subsystems exhibit frequency fluctuations that introduce significant levels of noise into the measurements, surpassing the gravitational wave signal by several orders of magnitude. Mitigation is achieved by means of time-shifting individual measurements in a data processing...

Long-range missiles cannot lock onto their target at launch. Instead, they rely on estimates of the line-of-sight between the missile and the target to orient their seeker head toward the target when entering seeker range. Whereas inertial navigation, which works by integrating the angular rates and specific forces measured by the inertial measurem...

In the context of the Laser Interferometer Space Antenna (LISA), the laser subsystems exhibit frequency fluctuations that introduce significant levels of noise into the measurements, surpassing the gravitational wave signal by several orders of magnitude. Mitigation is achieved by means of time-shifting individual measurements in a data processing...

Future space observatories achieve detection of gravitational waves by interferometric measurements of a carrier phase, allowing to determine relative distance changes, in combination with an absolute distance measurement based on the transmission of pseudo-random noise chip sequences. In addition, usage of direct-sequence spread spectrum modulatio...

Expensive and time-consuming ŕight tests are required to evaluate different helicopters and ŕight control systems in terms of handling and ride qualities. If gusts occur during these ŕights, they can distort the ŕight crew's perception of the handling and ride qualities. Therefore, it is desired to quantify the inŕuence of gusts on these criteria....

Autonomous soaring is an appealing academic sample problem for investigating machine learning methods within the scope of aerospace guidance, navigation, and control. The stochastic nature of small-scale meteorological phenomena renders the task of localizing and exploiting thermal updrafts suited for applying a reinforcement learning approach. Wit...

The detection of gravitational waves requires a strain sensitivity at unprecedented precision. The planned space observatory LISA overcomes this extreme challenge by heterodyne laser interferometry at picometer-precision based on the exploitation of carrier phase measurements between spacecraft separated by millions of kilometers. In addition, data...

Current and future GNC systems for exploration missions to small celestial bodies, planets and moons require an increasing level of on-board autonomy. Drivers for this increase include the lack of direct ground control because of communication delays, the operation in partly unknown environments, and the enhancement of the scientific exploitation t...

Kinetosis, better known as motion sickness, is provoked by low-frequency vibrations below 0.5 Hz. The severity of kinetosis increases with increased vibrational magnitudes within the mentioned frequency range. In rotorcraft, these frequency range intersects with the bandwidth of common automatic flight control systems. The counterreactions of the a...

Ensuring passenger comfort is an essential task of manned aviation. The vibration environment of helicopters has changed in the last years through the use of passive and active anti-vibration measures. This also changes the effects that cause the discomfort. To describe the discomfort as a result of the entire vibration environment of a helicopter,...

This paper presents a flight simulator study that examines whether a display inside the cockpit can aid helicopter pilots with little to no experience in completing basic maneuvers. The study participants have no prior experience as helicopter pilots. The flight simulation uses a dynamic model of a coaxial ultralight helicopter horizontal motion th...

Machine Learning algorithms are used in this paper to synthesize models for an ultralight coaxial helicopter pilot controlling the roll- and pitch motion during stationary hover. Different topologies of neural networks modeling the pilot are evolved with a neuroevolutionary algorithm, and trained via a data-driven approach using datasets that are c...

The Laser Interferometer Space Antenna (LISA) mission is a space-borne observatory designed to detect and characterize gravitational wave sources inaccessible to ground-based detectors. The mission relies on laser interferometry to measure changes in space-time. In this context, non-avoidable noise sources within the LISA system, including tilt-to-...

Model predictive control allows solving complex control tasks with control and state constraints. However, an optimal control problem must be solved in real-time to predict the future system behavior, which is hardly possible on embedded hardware. To solve this problem, this paper proposes to compute a sequence of one-step optimizations aided by pr...

Future space observatories achieve detection of gravitational waves by interferometric measurements of a carrier phase, allowing to determine relative distance changes, in combination with an absolute distance measurement based on the transmission of pseudo-random noise chip sequences. In addition, usage of direct-sequence spread spectrum modulatio...

This work presents an approach to automatically track steep glidepaths during fixed wing aircraft landings by adopting the cross-controlled flight technique of forward slipping. The standard flight dynamics for fixed wing aircraft assuming coordinated flight conditions are extended in order to accurately describe the slipping motion with a delibera...

Coordinating groups of aircraft and unmanned aerial vehicles to reach target locations without collisions is crucial for deploying decentralized groups of vehicles in the real world. A key challenge is coordinating a variable number of agents without the need for agents to share information about their intended routes, reducing communication requir...

Stall prevention can be implemented in different ways, ranging from simple warning systems to automatic, active stall protection. The latter results in a nonlinear control problem. In addition to the nonlinear system behavior, the margin between avoiding a stall situation and interfering with the aircraft behavior intended by the pilot is small. Th...

Vorwort Sehr geehrte Damen und Herren, ob am Arbeitsplatz, im alltäglichen Verkehr oder in der Freizeit, mechanische Schwingungen wirken stetig auf den menschlichen Körper ein. Die Auswirkungen reichen von Komfort- und Leistungseinbußen bis hin zu gesundheitlichen Beeinträchtigungen. Neben einer fachspezifischen Betrachtung ist der interdisziplinär...

The present technology relates to a flight control system 110 that is adapted for controlling movements of a rotary wing aircraft 100 while considering passenger discomfort, to a rotary wing aircraft 100 with such a flight control system 110, and to a method 600 of operating a flight control system 110. The flight control system 110 includes sensor...

The illustrated preprint is an extended abstract of the proposed paper for the 79th annual forum of the vertical flight society.

Existing control allocation approaches usually minimize an L1 or L2 norm of the actuator commands that is loosely related to the thrust produced or the aerodynamic drag. This paper proposes a novel incremental control allocation method that directly minimizes the total power required for an over-actuated, propeller-driven transition aircraft. The m...

While single-gimbal control moment gyroscopes (CMGs) remain the actuators of choice for agile spacecraft, using over-redundant or overdesigned CMG arrays will cease to be a viable approach as agility requirements increase. The torque envelope of a CMG array under gimbal rate constraints is a zonotope, whose geometrical characteristics can be used t...

My paper manuscript for the 79th annual forum of the vertical flight society in West Palm Beach

Solving the decision-making problem between pursuing the objective of covering distance and exploiting thermal updrafts is the central challenge in cross-country soaring flight. The need for trading short-term rewarding actions against actions that pay off in the long term makes for a hard-to-solve problem. Policies resulting from reinforcement lea...

We bridged the sim-to-real gap by successfully flight-testing our novel end-to-end type autonomous soaring system. A deep artificial neural network featuring a Long Short-Term Memory implements the control policy for integrated updraft localization and exploitation. The recurrent structure facilitates observability and enables mapping the hard-to-...

In this practical approach to distributed propulsion, the solar-powered manned motorglider icaré 2 has been used as a testbed for wingtip propulsion. The large wingspan and slow cruising speed of icaré 2 are highly suitable to assess the influence and potential of propellers at the wingtips for yaw control. The paper describes the work of a multidi...

The Laser Interferometer Space Antenna (LISA) mission features a three-spacecraft long-arm constellation intended to detect gravitational wave sources in the low-frequency band up to 1 Hz via laser interferometry. The paper presents an open-loop control strategy for point-ahead angle (PAA) correction required to maintain the optical links of the mo...

The Laser Interferometer Space Antenna (LISA) mission aims to observe gravitational waves featuring a three-spacecraft interferometer constellation with an arm length of 2.5 Mio km. In support of maximizing LISA’s scientific return, the paper presents the design of coordinated spacecraft constellation maneuvers tailored for high-precision estimatio...

Coordinating groups of aircraft and unmanned aerial vehicles to reach target locations without collisions is crucial for deploying decentralized groups of vehicles in the real world. A key challenge is coordinating a variable number of agents without the need for agents to share information about their intended routes, reducing communication requir...

Disorientation, nausea, and vomiting of passengers that result from vehicle vibrations are characterized as kinetosis or motion sickness. It is mainly the low-frequency movements of these vehicles that contribute to the kinetosis of passengers. Frequencies that provoke kinetosis lie within the rigid-body flight characteristics of helicopters, which...

This paper presents the process of flight mechanical analysis and optimization of a helicopter tethering frame that is used in pilot training and research projects. An existing linear state space model of the CoAX 2D ultralight helicopter is fusioned with a parametrized analytical model of the tethering device. Using the eigenvalues of the free fly...

Autonomous soaring constitutes an appealing academic sample problem for investigating machine learning methods within the scope of aerospace guidance, navigation, and control. The stochastic nature of small-scale meteorological phenomena renders the task of localizing and exploiting thermal updrafts suited for applying a reinforcement learning appr...

A key requirement for orienting the seeker head of a guided missile towards the target is the knowledge about the missile's position and attitude. An inertial measurement unit determines the specific forces and the angular rates of the vehicle. This so-called inertial navigation, which integrates the angular rates and accelerations, is not sufficie...

This work addresses the implementation of a final approach guidance for fixed-wing aircraft using a nonlinear model predictive control concept with collision avoidance capability. This comprises an obstacle identification method using Euclidean clustering and least squares approximation. After the provision of initial trajectories with Dubins paths...

Glider pilots often struggle with the problem of stalling during the winch start, turning flight or final approach, which in most cases leads to serious or even fatal accidents. In general aviation, stall warning systems do exist, but they are not applicable and practical in all aircraft. On the one hand, the estimation of the angle of attack and t...

Time to get our autonomous soaring system, consisting of a multiple thermal updraft estimator and the cross-country soaring motion planning based on hierarchical reinforcement learning, ready for the upcoming flying season. The data displayed depict the localization and exploitation of a thermal updraft followed by the autonomous decision-making t...

Algorithms for attitude (and orbit) control usually provide a control command signal in spacecraft axes that corresponds to a torque (and force). However, actuators are usually not aligned with these axes and, moreover, may be redundant. Therefore, as part of the on-board processing chain, the controller outputs have to be properly distributed to t...

This chapter provides an introduction to attitude determintation and rate estimation. We focus on principles and general ideas rather than advanced algorithms. The latter can be found in many excellent references, some of them are listed in the literature section, e.g., publications by Markley or Crassidis. Attitude determination is usually based o...

Motion models are the basis for the design of spacecraft control systems and their verification in a simulation environment. Their understanding and proper interpretation is important for control system design. Three types of models are derived in this chapter, which are all composed of rigid bodies. The first model is a conventional spacecraft wit...

From a system engineering point of view, there are several fundamental concepts for three-axis attitude and momentum control, which are briefly discussed in the first section. It concludes with an overview of the most common control tasks in satellite attitude control. In the remaining part of the chapter, common control approaches for attitude and...

The design of a spacecraft control system is first of all a system engineering task. In the first chapter, we describe the most important system engineering aspects, such as mission objectives, interfaces and functional architecture of a control system, and important system design rules. In this context, we focus on attitude control.

The purpose of an accelerometer is to measure non-gravitational specific forces, whereas a drag-free system aims at reducing non-gravitational forces on a test mass. Both measurement and reduction of non-gravitational forces are important prerequisites for scientific satellite missions in fundamental physics and geodesy. In this chapter, the main p...

The proposed Laser Interferometer Space Antenna (LISA) mission features a three-satellite constellation realizing a long-arm interferometer designed to observe gravitational waves emitted by a multitude of stellar, galactic, and possibly cosmic sources not accessible to ground-based detectors. In support of maximizing LISA’s scientific return, this...

For a video of a test flight demonstrating the feasibility of the LSTM-based encoding approach in combination with a guidance scheme based on ANNs in a real-world environment, please see: https://schlichting.page.link/lstm_flight_test.

Reinforcement learning-based path planning for multi-agent systems of varying size constitutes a research topic with increasing significance as progress in domains such as urban air mobility and autonomous aerial vehicles continues. Reinforcement learning with continuous state and action spaces is used to train a policy network that accommodates de...

Reinforcement learning-based path planning for multi-agent systems constitutes a research topic with increasing significance. We apply reinforcement learning to train a policy network that accommodates desirable path planning behaviors and is applicable for time-critical applications. We propose to employ a Long Short-Term Memory module to encode a...

In 2021, we implemented both our "Multiple Thermal Updraft Estimator" and the latest version of the "Hierarchical Reinforcement Learning" cross-country soaring guidance on embedded hardware aboard an unpiloted glider aircraft. During a flight test in late summer (with the sole intention of adjusting the state-feedback controller), the glider starte...

Incremental nonlinear dynamic inversion has recently received increasing attention as a robust method for aircraft rate control with superior disturbance rejection properties. However, stability and performance of the incremental control law are strongly affected by the sampling rate, which is not accounted for in the standard controller formulatio...

View Video Presentation: https://doi.org/10.2514/6.2022-1595.vid In order to fulfill their tasks in the context of future air transportation, VTOL vehicles are intended to navigate through urban environments autonomously. Lacking a pilot on-board, an approved 3-dimensional reference flight path needs to be available that can be tracked and followed...

View Video Presentation: https://doi.org/10.2514/6.2022-1392.vid Low-bandwidth linear controllers are a commonly chosen solution for the attitude control of highly stable spacecraft; however, they typically exhibit poor transient behaviour due to their long time constants. This drawback can be mitigated through the use of the eigenmode initializati...

The exploration of asteroids and comets received increased attention over the last decades. Recently, a novel quadcopter-like lander with extended exploration capabilities for asteroids and comets has been proposed. In the exploration context, autonomous decision making plays a central role. Among others, decision variables such as energy and fuel...

Safe landing site detection is essential for crewed flight, autonomously operated aircraft, and space missions that involve landers. Lander missions on asteroids bring the additional problem that the direction of the gravity vector is not precisely known, mainly due to the unknown density distribution of the asteroid. This poses a significant chall...

Airborne Wind Energy has a wide range of applications and can help to cover the energy demand in the future. Using tethered kites is one possibility to harvest this green energy while having low material costs. The most important part of such a system is the control system, ensuring energy optimal trajectories and keeping the kite aloft, in the fir...

This work presents a control and guidance algorithm for automated tailwheel aircraft operation during takeoff and landing. The particular suitability of incremental control concepts for this application is shown to lie in the improved capability to deal with model uncertainties using measurements. Existing incremental controllers for fixed-wing air...

View Video Presentation: https://doi.org/10.2514/6.2022-1846.vid or https://www.youtube.com/watch?v=so1khOCFmm4. Most missile guidance laws are based on specific assumptions about the type and behavior of the target. However, this information is not necessarily available in time and with sufficient certainty. In fact, premature commitment to a spe...

View Video Presentation: https://doi.org/10.2514/6.2022-2216.vid This paper presents an approach to systematically reduce the error on the position estimate of thermal updrafts. The achievable accuracy of updraft position estimates is quantified using the Cramér-Rao lower bound for discrete-time filtering problems. An observability measure derived...

The paper proposes a method for generating and automatically executing smooth evasive maneuvers which allow to enforce a geofence for an optionally piloted fixed-wing aircraft. Feasible trajectories are planned online based on the current aircraft state and performance limitations, including finite roll rates. Collision checks are continuously carr...

This work presents the effect of wingtip-mounted propellers on the aircraft drag polar, identified from in-flight measurements. In previous wind tunnel experiments, significant reductions in drag and an increase in propulsive efficiency through interaction between induced flow by the wingtip-mounted propellers and the flow field of the wing itself...

View Video Presentation: https://doi.org/10.2514/6.2021-3192.vid This work presents an approach to efficient yaw control by differential thrust from distributed electric propulsion, avoiding an increase in aerodynamic drag associated with rudder deflection. Both a fully automated, scaled (1:3) version of the electric aircraft e-Genius, and the mann...

Recent lander concepts for small solar system bodies (SSSBs) under low-gravity environments close to the surface are limited to a relatively small exploration area. To overcome the limited range, a novel mobility concept for enhanced surface mobility is presented. The idea is to utilize a quadcopter-based spacecraft design to potentially hover in l...

Flight-data evaluation for the multi-updraft estimator: A particle filter is proposed for fast localization of multiple thermal updrafts.

The trained agent acts reasonably in the face of an unknown updraft distribution. The resultant trajectory comes close to the optimal solution (for the computation of which, the updraft distribution is explicitly evaluated!).

Working principle of the score function gradient estimator: The probability of achieving a superior trajectory is updated based on a batch of estimated gradients with respect to the policy's parameters. Adapted from: Karpathy, A., “Deep Reinforcement Learning: Pong from Pixels,” http://karpathy.github.io/2016/05/31/rl/, May 2016. Accessed 11/15/20...

The high-power density and good scaling properties of electric motors enable new propulsion arrangements and aircraft configurations. This results in distributed propulsion systems allowing to make use of aerodynamic interaction effects between individual propellers and the wing of the aircraft, improving flight performance and thus reducing in-fli...

Graph-based design languages have received increasing attention in the research community, because they offer a promising approach to address several major issues in engineering, e.g., the frequent manual data transfer between computer-aided design (CAD) and computer-aided engineering (CAE) systems. Currently, these issues prevent the realization o...

Maximizing the energy intake by thermalling updrafts while avoiding collisions between two gliders or gliders with obstacles exceeds existing guidance strategies. Existing thermalling algorithms cannot independently incorporate multiple other gliders with an arbitrary number of updrafts and obstacles. Therefore, they would need to incorporate heuri...

Glider pilots in soaring competitions are faced with the conflicting tasks of reaching a goal in minimal time and exploiting updrafts that are unknown a priori. They weigh short term benefits like gaining altitude through updrafts against their long term chances of reaching the goal. This problem can be formulated as a Markov Decision Process and p...

Autonomous soaring constitutes an appealing task for applying reinforcement learning methods within the scope of guidance, navigation, and control for aerospace applications. Cross-country soaring embraces a threefold decision-making dilemma between covering distance, exploiting updrafts, and mapping the environment. In addition to the stochastic n...

Existing control allocation approaches usually minimize an L1 or L2 norm of the input vector that is loosely related to the thrust produced or the aerodynamic drag. This paper proposes a novel incremental control allocation method that directly minimizes the power required. The cost functional is derived and the control allocation procedure present...

Reinforcement learning-based path planning for multi-agent systems of varying size constitutes a research topic with increasing significance as progress in domains such as urban air mobility and autonomous aerial vehicles continues. Reinforcement learning is used to train a policy network that accommodates desirable path planning behaviors and can...

The use of low-bandwidth attitude controllers is a standard solution for spacecraft facing tight frequency-domain requirements for attitude-stable payloads and platforms. However, such controllers have difficulty responding to transients both planned and unexpected. This paper introduces a novel method of controller re-initialization to counteract...

Especially in urban environments where space is closely confined, VTOL vehicles used for Urban Air Mobility will need to be controlled with respect to ground in the horizontal plane in order to comply with path restrictions set by fixed obstacles. This work presents a control scheme in the horizontal plane that allows a pilot to command ground-base...

The use of control moment gyroscopes (CMGs) will become increasingly common for Earth observation missions owing to higher agility requirements. With the gyroscopic nature of their torques and the constant rotation of individual angular momenta, CMGs pose a significant challenge to commanding algorithms, especially if actuator constraints are taken...

The automatic operation of fixed-wing drones plays an important role in research and commercial applications. Accurate planning and precise tracking of trajectories is a key aspect for flight in challenging environments with dense obstacles, e.g., urban areas. Within this paper, we propose a spline-based trajectory planner alongside an associated g...

The reduction of rotor-induced vibrations has been a major concern in the history of helicopter engineering. This has led to the development of numerous countermeasures, which cause the vibration levels to decrease considerably. Recent findings have shown that the reduced level of rotor vibrations highlights the importance of turbulence for the rid...

The serial introduction of passive and active anti-vibration means lead primarily to the reduction of the vibration levels at blade passage frequencies Nb/rev. Consequently, other - previously unnoticed - sources of vibration are perceived by rotorcraft occupants. Therefore, a comprehensive vibration assessment metric is required to characterize th...

This paper presents the synthesis of a flight mechanical model of a coaxial ultralight helicopter that features a teetering rotor system. The flight campaign, the identification process and the resulting model are presented. As a first step, the features that set the CoAX 2D apart from other helicopters are discussed briefly. A physical model that...

In this paper, the concept, design and implementation of a stability augmentation system for coaxial ultralight helicopters is presented. For an exemplary realization, the CoAX 2D ultralight helicopter is used. It is a 2-seated helicopter with two teetering mainrotors in a coaxial configuration with a maximum takeoff mass of 450 kg developed by edm...