Alastair K. Cooke

Direct Method Based Control System for an Autonomous Quadrotor

This paper proposes a real time control algorithm for autonomous operation of a quadrotor unmanned air vehicle. The quadrotor is a small agile vehicle, which as well as being a excellent test bed for advanced control techniques could also be suitable for internal surveillance, search and rescue and remote inspection. The proposed control scheme incorporates two key aspects of autonomy; trajectory planning and trajectory following. Using the differentially-flat dynamics property of the system, the trajectory optimization is posed as a non-linear constrained optimization within the output space in the virtual domain, not explicitly related to the time domain. A suitable parameterization using a virtual argument as opposed to time is applied, which ensures initial and terminal constraint satisfaction. The speed profile is optimized independently, followed by the mapping to the time domain achieved using a speed factor. Trajectory following is achieved with a standard multi-variable control technique and a digital switch is used to re-optimize the reference trajectory in the event of infeasibility or mission change. The paper includes simulations using a full dynamic model of the quadrotor demonstrating the suitability of the proposed control scheme.

Simulation of wake vortex effects for UAVs in close formation flight

This paper addresses the development of multiple UAV deployment simulation models that include representative aerodynamic cross-coupling effects. Applications may include simulations of autonomous aerial refuelling and formation flying scenarios. A novel wake vortex model has been developed and successfully integrated within a Matlab/Simulink simulation environment. The wake vortex model is both sufficiently representative to support studies of aerodynamic interaction between multiple air vehicles, and straightforward enough to be used within real time or near real time air-to-air simulations. The model integration process is described, and computational results of a two-vehicle-formation flight are presented.

On-board trajectory generation for collision avoidance in unmanned aerial vehicles

This paper addresses the problem of collision avoidance with moving obstacles for unmanned aerial vehicles. It is assumed that obstacle detection and tracking can be achieved 60 seconds prior to collision. Such a time horizon allows on-board trajectory re-planning with updated constraints due to intruder and ownship dynamics. This trajectory generation problem is solved using a direct method, meaning the problem is transcripted to a nonlinear programming problem and solved with an optimization method. The main challenge in trajectory generation framework is to reliably provide a feasible (safe and flyable) trajectory within a deterministic time. In order to improve the methods reliability, a Monte Carlo analysis is used to investigate the convergence properties of the optimization process, the properties of the generated trajectories and their effectiveness in obstacle avoidance. The results show that the method is able to converge to a feasible and near-optimal trajectories within two seconds, except in very restrictive cases. Moreover, the dynamic feasibility of the generated trajectories is verified with nonlinear simulations, where the trajectory generation is integrated with the six degree-of-freedom nonlinear model of a fixed-wing research vehicle developed at Cranfield University. The results show that the generated trajectories can be tracked with a proposed two-degree-of-freedom control scheme. The improved convergence, fast computation and assured dynamic feasibility pave the way for on-board implementation and flight testing.

Detection of shaft-seal rubbing in large-scale power generation turbines with Acoustic Emissions; Case study

Abstract Rubbing between the central rotor and the surrounding stationary components of machinery such as large-scale turbine units can escalate into severe vibration, resulting in costly damage. Although conventional vibration analysis remains an important condition monitoring technique for diagnosing such rubbing phenomena, the non-destructive measurement of acoustic emission (AE) activity at the bearings on such plant is evolving into a viable complementary detection approach, especially adept at indicating the early stages of shaft-seal rubbing. This paper presents a case study on the application of high-frequency acoustic emissions as a means of detecting and verifying shaft-seal rubbing on a 217 MVA operational steam turbine unit. The generation of AE activity is attributed to the contact, deformation, adhesion and ploughing of surface asperities on the rubbing surfaces of the rotor and stator.

Wind Shear Energy Extraction using Dynamic Soaring Techniques

The paper describes results of a trajectory generation technique suitable for real-time implementation. The technique is used for energy extraction in wind shear. It is based on the direct method of Taranenko and is used for generating trajectories for a powered sailplane. The trajectories are generated to minimize the energy used per distance travelled in the lateral direction in the dynamic soaring cycle. The trajectories are also generated to maximise the speed. The trajectory following issues are addressed and an architecture for the control system is briefly described. Simulation is performed using a full model of a powered sailplane to verify the trajectory generation method. The results of fuel saving benefit quantification using linear wind shear models and also nonlinear windshear models are described.

Real-time optimal techniques for unmanned air vehicles fuel saving

Dynamic soaring can save fuel by the extraction of low-altitude wind shear energy. The problem of generating energy-saving trajectories in real time for an unmanned powered sailplane is considered. A method called ‘inverse dynamics in the virtual domain’ is investigated. The method results in rapid solution generation and is suitable for real-time applications. Three-dimensional unmanned aerial vehicle equations of motion and a non-linear wind gradientmodel are used. Trajectories are generated to minimize the total power used per horizontal distance travelled for cross-wind travelling and in the total power used per total time in air for loitering mode. Trajectory following issues are addressed and an architecture for the control system is briefly described. A method for the estimation of wind shear gradient parameters is proposed. Validation is performed using a full six-degree-of-freedom model of a powered sailplane to verify the trajectory generation method.

Review of pilot modelling techniques

As aircraft increase in size, unsteady aerodynamics and aeroelasticity pose new handling qualities challenges. These require a greater understanding of the pilot-vehicle system as a whole. The level of interaction between the flying control system and the pilot increases as structural mode frequencies enter into the manual control frequency range. Therefore, accurate physiological and control-theoretic models of the pilot are crucial. Interactions range from conscious changes in pilot gain and equalisation to unconscious biodynamic feedthrough of structural modes. So far, use of theoretical pilot models has been an effective way of gaining handling qualities insight. Pilot modelling can be split into three areas: human sensory modelling, biomechanical modelling and control-theoretic modelling. To date, a review that brings together these aspects and provides a holistic view of pilot modelling has not been presented. This paper aims to address this literature gap and presents a review of the state-of-the-art. he design of civil aircraft raises interesting issues involving weight and efficiency tradeoffs. The more passengers a civil aircraft carries the more efficient it becomes, but at the same time its size and weight increase. These have to be compensated for by the design of a lightweight airframe. The end product is a large lightweight airframe that is characterised by reduced rigidity and consequently increased levels of aeroelasticity. Aeroelasticity can affect aircraft stability and control in ways which are often not fully appreciated or understood. As structural modes enter into the rigid-body dynamic frequency range, the flight control systems (FCS) relying on feedback from the various sensors around the airframe are also affected. These sensors can no longer distinguish between aeroelastic and rigid body dynamics and this can lead to unexpected FCS behaviour. This inability to distinguish between rigid body dynamics and aeroelastic effects is also inherent to the human sensory dynamics. Now as airframe structural modes frequencies encroach into the frequency ranges of human senses, biodynamics and control, traditional assumptions in handling qualities analysis no longer remain valid. Pilot’s perception of aircraft states is corrupted just like the onboard FCS sensors. Therefore, pilot introduced gain and equalisation in the pilot-vehicle system may no longer be appropriate. During routine flying, pilots tend to command the aircraft with airframe structural limits in mind. However, now there are possibilities of certain scenarios where the pilot and FCS combination may load the aircraft beyond its limits. Such a scenario may be caused by various triggers and the pilot may either be aware or unaware of the scenario. Triggering events can range from changes in pilot’s strategy to extreme atmospheric conditions. Investigation of such scenarios require first an understanding of aircraft manual control. Current civil aircraft effectively have three modes of operation. Aircraft control can be achieved through complete manual control with objectives from the pilot’s mind or manual control with objectives from a flight director. The aircraft can also be controlled via the mode control panel which commands the various autopilot modes; the pilot plays a more supervisory role here. Figure 1 presents the key components involved in the manual control mode. The system is driven by an objective (derived consciously) that is subconsciously processed by higher brain functions to derive a control action; a function of pilot experience and skill. This control action is applied through the neuromuscular system that is in turn affected by the human body’s response to the environment. Then it goes through the flying control system and the aircraft responds accordingly.

Pilot-model-in-the-loop simulation environment to study large aircraft dynamics

Effects of aeroservoelasticity on the manual control of large aircraft are investigated through a pilot modelling approach based on the modified optimal control model. A synopsis of modelling techniques is presented followed by the description of the adopted method. A simulation environment suitable for investigating pilot–vehicle dynamics in the longitudinal axis has been developed which couples an aeroelastic model of a large transport aircraft with the modified optimal control pilot model. The pilot model parameter selection was based on limiting the bandwidth. A comparison between a conventionally designed height hold autopilot and the pilot model control in continuous turbulence is made to demonstrate this simulation capability.

Characterizing Stability and Control of Subscale Aircraft from Wind-Tunnel Dynamic Motion

This paper outlines a process of determining the stability and control characteristics of small-scale aircraft from wind-tunnel experiments which aim to emulate flight tests. Scaled aircraft models with representative control surfaces are flown in semifree flight on a test rig allowing motion in 4 degrees of freedom. The motion is measured and system identification and parameter estimation techniques are used to obtain a mathematical description of the dynamics of the model. Previous work has relied on the use of potentiometers to measure the models attitude about its pivot point with other motion variables being generated analytically using these measurements and well-known kinematic relationships. However, it is now possible to mount sensors within the model as advances in technology have produced microelectromechanical system packages of small size, lightweight, low cost, and low power consumption. The current work is investigating the effectiveness of using microelectromechanical system inertial sensors to measure motion of the model. An example of the system identification process carried out on a one-twelfth scale BAe Hawk model is also presented. Parameter estimation is performed in the frequency domain using the equation error and output error methods.

Modelling framework for flight dynamics of flexible aircraft

The flight dynamics and handling qualities of any flexible aircraft can be analysed within the Cranfield Aircraft Accelerated Loads Model (CA2LM) framework. The modelling techniques and methods used to develop the framework are presented. The aerodynamic surfaces were modelled using the Modified Strip Theory (MST) and a state-space representation to model unsteady aerodynamics. With a modal approach, the structural flexibility and each mode’s influence on the structure deflections are analysed. To supplement the general overview of the framework equations of motion, models of atmosphere, gravity, fuselage and engines are introduced. The AX-1 general transport aircraft model is analysed as an example of the CA2LM framework capabilities. The results showed that, according to the Gibson Dropback criterion, the aircraft with no control system lacks the stability and its longitudinal handling qualities are unsatisfactory. Finally, the steps for future developments of the CA2LM framework are listed within conclusions.