Pilot-induced Oscillations Research Articles

Performance comparison of four pre-filters used to prevent pitch categoryIIPIO

Category Ⅱ PIO cases are mostly characterized by actuator saturation. FWB,DS,DASA,RLF are four nonlinear pre-filters that have already been used to compensate for the phase lag led by actuator saturation,thus to prevent pilot-induced oscillation( PIO). Different means were used to make comparison among four pre-filters. Dscribing function was used to analyze and compare the phase compensation in open loop analysis by linearizing the nonlinear components of the fourpre-filters. The study mainly focused on pitch attitude PIO,in a pilot-vehicle closed loop system with a neutrally stable pitch bare aircraft that could easily cause PIO,three typical pitch flight simulation tasks were used to make comparison in terms of actuator velocity saturation prevention,pitch output dynamic characters,degree of phase compensation and alleviation of pilot operation burden,measured by pilot input magnitude. DASA turns out to be a better design among the four pre-filters. The result can provide a useful reference for filter design and pitch attitude PIO prevention study.

Piloted Evaluation of a Control Allocation Technique to Recover from Pilot-Induced Oscillations

This paper describes the maturation of a control allocation technique designed to assist pilots in recovery from pilot-induced oscillations. The control allocation technique to recover from pilot-induced oscillations is designed to enable next-generation high-efficiency aircraft designs. Energy-efficient next-generation aircraft require feedback control strategies that will enable lowering the actuator rate limit requirements for optimal airframe design. A common issue on aircraft with actuator rate limitations is they are susceptible to pilot-induced oscillations caused by the phase lag between the pilot inputs and control surface response. The control allocation technique to recover from pilot-induced oscillations uses real-time optimization for control allocation to eliminate phase lag in the system caused by control surface rate limiting. System impacts of the control allocator were assessed through a piloted simulation evaluation of a nonlinear aircraft model in the NASA Ames Research Center’s Vertical Motion Simulator. Results indicate that the control allocation technique to recover from pilot-induced oscillations helps reduce oscillatory behavior introduced by control surface rate limiting, including the pilot-induced oscillation tendencies reported by pilots.

Multiloop Pilot Model for Boundary-Triggered Pilot-Induced Oscillation Investigations

This paper presents the development of a multiloop pilot model for use in boundary-triggered pilot-induced oscillation investigations. In doing so, the point-tracking and boundary-avoidance elements of the pilot control strategy are assumed to act simultaneously for a point-tracking-dominant task during a boundary-avoidance tracking event. The theoretical analysis indicates that the essence of the boundary-avoidance tracking phenomenon consists of an additional requirement for the pilot to provide lead equalization as the boundary is approached as the task transitions from a full-attention point-tracking task to both a point-tracking and boundary-avoidance task. This process leads to a narrower open-loop system bandwidth and a larger tracking error, or the triggering of a pilot-induced oscillation. It is also found that the severity of the boundary-avoidance influence can be reduced by including the effects of vestibular and proprioceptive cues into the model. The boundary-avoidance tracking pilot model exhibits pilot–vehicle responses that are consistent with those observed in a piloted-simulation study and can therefore be considered to be representative of pilot activity. Consideration of the results in the round indicates that the boundary-triggered pilot-induced oscillation can be categorized as Category III pilot-induced oscillation within the existing recognized taxonomy.

Application of the weight function method on a high incidence research aircraft model

AbstractThis paper assesses the application of a new method for system stability analysis, the weight functions method, to the longitudinal and lateral motions of a High Incidence Research Aircraft Model. The method consists of finding the number of weight functions that is equal to the number of differential equations required for system modelling. The aircraft’s stability is determined from the sign of the total weight function; which should be negative for a stable model. The Aero-Data Model In Research Environment (ADMIRE) simulation, developed by the Swedish Defence Research Agency, was used for the aerodynamic aircraft modelling, with the following configurations: Mach number = 0·25, altitude = 500m, angle-of-attack [–10 to 30]°, elevon deflection angle [–30 to 30]°, canard deflection [0° and 25°] and rudder deflection angles [–30° and 30°]. These flight configurations were selected because they are among the flight conditions for Cat. II Pilot Induced Oscillation (PIO) criteria validation, performed on the FOI aircraft model presented in the PIO Handbook by the Group for Aeronautical Research and Technology in Europe, Flight Mechanics/Action Group 12. This aircraft model has a known instability for longitudinal and lateral motions and so a control law was introduced to stabilise its flight.

Detection of Pilot-Induced Oscillation (PIO)

Since PIOs cannot be prevented, they will continue to occur, and methods are needed to mitigate the risk of catastrophic PIO. This paper discusses the challenges to detection of PIO in real-time. It describes the design of a method for the real-time detection of PIO that can be used to alert the flight crew in the early stage of a PIO, before the event has become catastrophic. With early warning, it is possible that loss of aircraft can be prevented through appropriate pilot response. Real Time Detection of a Pilot-Induced Oscillation (PIO) can potentially save both aircrew and aircraft. The method is effective at identifying any oscillatory behavior in real-time,Although limitations of the algorithm do exist, ROVER, if used correctly, will detect a severe PIO event and could allow a pilot to open the feedback control loop or adjust the control input strategy in order to suppress the aircraft oscillatory response.

A finite-state aeroelastic model for rotorcraft–pilot coupling analysis

Rotorcraft–pilot coupling (RPC) denotes interplay between pilot and helicopter (or tiltrotor) that may give rise to adverse phenomena. These are usually divided into two main classes: pilot-induced oscillations driven by flight dynamics and behavioural processes, and pilot-assisted oscillations (PAO) caused by unintentional actions of pilot on controls, owing to involuntary reaction to seat vibrations. The aim of this paper is the development of mathematical helicopter models suited for analysis of RPC phenomena. In addition to rigid-body dynamics, RPCs (especially PAO) are also related to fuselage structural dynamics and servoelasticity; however, a crucial role is played by main rotor aeroelasticity. In this work, the aeroelastic behaviour of the main rotor is simulated through a novel finite-state modeling that may conveniently be applied for rotorcraft stability and response analyses, as well as for control synthesis applications. Numerical results, first are focused on the validation of the proposed novel main rotor model, and then present applications of the developed comprehensive rotorcraft model for the RPC analysis of a Bo-105-type helicopter. Specifically, these deal with the stability of vertical bouncing, which is a PAO phenomenon caused by coupling of vertical pilot seat acceleration with collective control stick, driven by inadvertent pilot actions. Further, considering quasi-steady, airfoil theory with wake inflow correction and three-dimensional, potential-flow, boundary element method approaches, the sensitivity of PAO simulations to different aerodynamic load models applied within the main rotor aeroelastic operator is also investigated.

Robust Stability Analysis in the Presence of Time-Varying Uncertainties and Time Delay

The paper presents some robust stability conditions for systems affected by parametric uncertainties and time delay. The objective is to obtain a method allowing determining the admissible domain of the parametric uncertainties and the maximum value of the time delay that can be reached without destabilizing the system. The bounds of the parametric uncertain are obtained using the Small Gain Theorem. The norm-boundedness is studied using a version of the Bounded Real Lemma for time-delayed systems. Throughout the paper the theoretical developments are illustrated using a numerical example concerning the prediction of pilot induced oscillations (PIO).

Pilot-Induced Oscillations Prevention Using Anti-Windup Compensation in the Presence of Time-Delay

The paper presents a compensator design technique for systems with saturating actuators in order to recover as much as possible the performance of the unsaturated case. This anti-windup scheme is obtained using H-infinity optimization methodology. The proposed design approach allows to obtain a anti-windup controller that accounts for time-delays in the control system. Comparative result are presented for the two design methods and time simulation of the nonlinear system are used to analyze the performances of both designs. A reduced order anti-windup controller procedure is also investigated. Keywords: PIO, anti-windup, rate saturation, compensator.

Pilot modeling based on time-delay synthesis

A review of some human pilot models of the years 1970–1990 – the Kleinman–Baron–Levison optimal control model, the Davidson–Schmidt modified optimal control model, and the Hess optimal control model – has been presented from the perspective of a new model based on the optimal control synthesis of time-delay systems. In the first of the listed models, the ‘central nervous’ reaction of the pilot is naturally defined as a pure time delay in the measurement equation of the system. In the framework of the optimal control theory, the pilot’s behavior is modeled by linear quadratic regulator gain and Kalman–Bucy filter with a linear predictor. Starting from this optimal model of the 1970s, the other two models assumed the Padé approximation of the pure time delay, thus eliminating the linear predictor. In this article, the pure time delay of pilot reaction was reconsidered and divided, for convenience, into two equal parts: for the output measurement equation and for the input control. The pilot model problem has been first defined in the framework of rigorous time-delay synthesis and then solved by making reference to the control separation and duality principles. A closed-form expression of the solution is thereby obtained. The proposed model was then compared by numerical simulations with Kleinman and Hess consacrated models. The analysis of the results shows that this new pilot model is described by a simplified representation, instead denoting similar performance versus previous optimal models – which contains additional insertions as Kleinman–Baron predictor or Padé approximation, respectively. Finally, joint evaluation of the proposed model and Kleinman and Hess models with respect to the well-known Neal–Smith criterion confirms the consistency and viability of the employed strategy as a possible tool for pilot-induced oscillations phenomenon investigation.

The FWB Filter with Real-Time Detector to Suppress Category II Pilot Induced Oscillation

PIO (Pilot Induced Oscillation) is the key factor to damage the flight safety and decrease the quality of manned aircraft. Most of modern PIO records associating with rate limiting or position limiting from actuation systems are defined Category II PIO. The existing compensators, e.g. the Feedback-with-Bypass (FWB) filter, can reduced the oscillation potentiality of the pilot-in-the-loop aircraft system due to rate limiting of control surface, however a negative effect evidently arose which slow the response speed of whole system. This paper uses a Real-Time PIO detector based on Short-time Fourier Transform algorithm as a trigger to determine the switch-off time of the compensator working on the whole system so as to suppress the negative effect on responses of system by FWB. Simulation results show us the positive conclusion that this kind combined FWB filter with Real-Time PIO detector is effectively preventing the Category II PIO without performance decreasing.

Development of the Phase-Aggression Criterion for Rotorcraft—Pilot Coupling Detection

Significant effort has been expended to develop criteria to predict the susceptibility of an air vehicle to so-called pilot-induced oscillations . Much of this work has been carried out for fixed-wing aircraft and it is only recently that their applicability to rotorcraft has started to be assessed. Real time pilot-induced oscillation identification methods provide an alternative means to at least warn the pilot that a pilot-induced oscillation is in progress so that preventative action can be taken. Existing methods, however, have some limitations and have rarely been used for rotary-wing purposes. Specifically, the existing methods assessed in this paper do not provide an indication of the severity of the event and mask the underlying data that are being used to generate the warning. This paper proposes and presents a new method to identify pilot-induced oscillations, either in near real time or as a postprocessing aid for recorded flight-test data, that addresses both of these issues. The new method, entitled “phase-aggression criterion,” is compared with current methods. It is shown, for a specific set of test cases and a limited test pilot population, that not only can it provide more information about the pilot-induced oscillation but it can also provide an earlier warning of its onset.

Fuzzy-Logic Modeling of a Rolling Unmanned Vehicle in Antarctica Wind Shear

of wind shear are investigated using the recorded test data during a flight from the Pegasus white ice runway in Antarctica. The main purpose of this paper is to determine the reasons and characteristics of roll oscillations for improvement of autopilot design and ground operator training. The Meridian flight test is conducted with the necessaryrangeconstraint, which precludes steadylevel flight.The measured orestimated data,such as theangle of attack and sideslip angle, are filtered through a nonlinear compatibility analysis. The estimated aerodynamic coefficients are modeled as implicit functions of filtered flight variables in a concept of aerodynamic model identification, instead of parameter identification. A model identification method called fuzzy-logic modeling is used to set up the aerodynamic models without assuming the functional relations. The aircraft’s oscillatory stability and control derivatives are found to be significantly impacted by the environmental disturbances. The exhibited highly oscillatory motion in the manual-control mode is analyzed and identified as the pilot-induced oscillation with a new energy method. It is shown that the pilot-induced oscillation is identified as the motion when the motion energy is increasedbythecontrolinput.Intheautopilot flight,themotionisdeterminedtobewingrock,whichisexcitedbythe pilot-induced oscillation in the manual mode and sustained by the interaction between the wing wake and the V tail.

Absolute stability for the lateral-directional BWB model with rate limited actuator

In this paper the authors present a study regarding the interaction between the human pilot and the aircraft which may result in a dangerous phenomenon called Pilot Induced Oscillations (PIO), in the context of the lateral directional motion. The theoretical model of the airplane used is a Blended Wing Body (BWB) configuration and the human operator is expressed by the Synchronous Pilot Model (represented by a simple gain). The Popov criterion, in the case of the infinite parameter, is applied in order to investigate the absolute stability of the pilot-airplane linearized system in the presence of the rate saturation of the actuator.

Flight simulator study on the influence of vortex curvature on wake encounter hazard using LES wind fields

AbstractA flight simulator study has been carried out to evaluate the performance of modern flight control systems encountering curved wake vortices. During the decay process the shape of wake vortices alters significantly which has an influence on the encounter characteristics and thus on the encounter hazard. To analyse most realistic wake encounters, flow fields of matured vortices have been generated with large-eddy-simulations. These were used for the determination of induced force and moment histories during the encounter. The force and moment histories have been implemented into the equations of motions of a 6 DoF flight simulation. For the sake of comparison encounters have also been simulated with straight vortices as they were mostly used for many other investigations. The most important goal of the study was to analyse the difference between these ideally straight vortices and vortices with a more realistic shape regarding encounter acceptance. The simulator study was conducted in an A330 motion-based full-flight simulator with pilots-in-the-loop. The analysed scenario was a wake encounter during final approach. The encounter conditions corresponded to a heavy-behind-heavy situation for Instrument Flight Rules (IFR) operations. The aircraft was flown either manually (in normal law) or with autopilot engaged. Altogether 93 encounters have been simulated, 38 with straight and 55 encounters with curved vortices. For encounters under manual control the simulator study revealed a potential risk of pilot induced oscillations (PIO) during encounters with curved vortices. With autopilot engaged not even one encounter with curved vortices was classified to be unacceptable. Although significant aircraft response was experienced the autopilot never disengaged automatically in any encounter. Altogether about 12 percent of the encounters were not accepted by the pilots. This is indeed a significant number, especially as the analysed scenarios can be regarded to be realistic situations which can occur in reality even if the applicable separation distances are applied.

Tau Guidance in Boundary-Avoidance Tracking - New Perspectives on Pilot-Induced Oscillations

DOI: 10.2514/1.54065 Tau theory, introduced to the flight control discipline as a model for natural guidance, is shown to provide an approach to predicting a class of adverse aircraft-pilot couplings described as boundary-avoidance tracking events and pilot-induced oscillations. These have previously been modeled a posterior as discrete events using timedependent feedback gains. Drawing on the prospective nature of the time-to-contact variable optical tau � , a new method is proposed for modeling such phenomenon and also for determining the critical incipience for this class of aircraft-pilotcoupling.Inthepresentstudy,theapproachhasbeenappliedtotauguidanceinarotorcrafttrajectory tracking maneuver, to predict the conditions under which aircraft-pilot couplings may occur. In addition, a strong correlation between motion and control activity and the derivatives of tau adds substance to the hypothesis that the pilot’sperceptualsystemworksdirectlywithinvariantsintheoptical flowduringvisualguidance.Resultsfrom flight simulation tests conducted at the University of Liverpool and complementary flight tests carried out with the National Research Council (Canada) advanced systems research aircraft in-flight simulator support the tau control hypothesis. The theory suggests ways that pilots could be alerted to the impending threat of such adverse aircraftpilot couplings.

Pilot-Induced Oscillation Suppression by Using Adaptive Control

Despite significant technical advances, pilot-induced oscillation (PIO) continues to occur in both flight tests and operational aircrafts. Such a phenomenon has led to significant research activities that aim to alleviate this problem. In this paper, theL1adaptive controller has been introduced to suppress the PIO, which is caused by rate limiting and pure time delay. Due to its architecture, theL1adaptive controller will achieve a desired response with fast adaptation. The analysis of PIO and its suppression byL1adaptive controller are presented in detail in the paper. The simulation results indicate that theL1adaptive control is efficient in solving this kind of problem.

A Survey of Force Feedback in Flight Safety Enhancement

This survey presents the application of force feedback in aviation. In modern fly-by-wire control system, actuator rate limit is the primary source of PIO II (Pilot Induced Oscillation II). The combination of smart-cue and smart-gain method can alleviate PIO II effectively, and consequently enhance the flight safety. The recent research of applying force feedback in aviation demonstrates that it is worthy of theoretical exploration and extensive utilization.

Analysis of Interactions between Pilot-Operator and Advanced Flight Control System

This work discusses the application of techniques serving the purpose of prediction and detection of unfavorable man-machine interactions. Prediction criteria of Pilot Induced Oscillations (PIO) were applied during design process of experimental multi-modal fly-by-wire control system for small light aircraft. Estimated PIO susceptibility was further verified during real-time simulations and flight tests. The verification process included subjective pilot/operator expert opinions and the results obtained from the automatic detectors, used for the PIO identification in long sets of recorded data. The general idea of the detection algorithms is based on the Fast Fourier Transform (FFT). These are being presented in this paper in a form of detection applications along with the results of the flight experiments.

Application of parameter plane method to pilot-induced oscillations

The main purpose of this paper is to predict the phenomenon of pilot-induced oscillations (PIOs) by the use of parameter plane method. The position-limited and rate-limited are always existed in the practical pilot–vehicle systems. Based on the approximation of classical describing functions, the stability of linearized pilot–vehicle systems can be easily analyzed. In addition, the effects of varying controller or plant parameters on stability must be considered to achieve a robust design by parameter plane approach. In this work, a PID pilot–vehicle system with rate-limited is given for analyzing PIOs first. Although the control structure is simple, it still makes some complex situations. The effect of phase delay is also addressed. Finally, the stability analysis to a pilot–vehicle system with multiple nonlinearities is extended. Computer simulations demonstrate the estimation of PIOs by this method.

Stability Properties and Cross-Coupling Performance of the Control Allocation Scheme CAPIO

This paper presents a stability analysis and an application of a recently developed algorithm to recover from Pilot Induced Oscillations (CAPIO). When actuators are ratesaturated due to either an aggressive pilot command, high gain of the ight control system or some anomaly in the system, the eective delay in the control loop may increase. This eective delay increase manifests itself as a phase shift between the commanded and actual system signals and can instigate Pilot Induced Oscillations (PIO). CAPIO reduces the eective time delay by minimizing the phase shift between the commanded and the actual attitude accelerations. To establish theoretical results pertinent to CAPIO stability analysis, a scalar case is presented. In addition, we present simulation results for aircraft with cross-coupling which demonstrates the potential of CAPIO serving as an eective PIO handler in adverse conditions.