Jonathan Luke du Bois

On the Quantification of Eigenvalue Curve Veering: A Veering Index

Eigenvalue curve veering is a phenomenon that has found relevance and application in a variety of structural dynamic problems ranging from localization and stability studies to material property determination. Contemporary metrics for quantifying veering can be ambiguous and difficult to interpret. This manuscript derives three normalized indices in an effort to reconcile the deficit; two of these quantify the physical conditions which produce the behavior while the third provides a definitive measure of the overall intensity of the effect. Numerical examples are provided to illustrate the application of the methods, which are expected to form a basis for the development of advanced analytical tools.

A vision-based strategy for autonomous aerial refueling tasks

Autonomous aerial refueling is a key enabling technology for both manned and unmanned aircraft where extended flight duration or range are required. The results presented within this paper offer one potential vision-based sensing solution, together with a unique test environment. A hierarchical visual tracking algorithm based on direct methods is proposed and developed for the purposes of tracking a drogue during the capture stage of autonomous aerial refueling, and of estimating its 3D position. Intended to be applied in real time to a video stream from a single monocular camera mounted on the receiver aircraft, the algorithm is shown to be highly robust, and capable of tracking large, rapid drogue motions within the frame of reference. The proposed strategy has been tested using a complex robotic testbed and with actual flight hardware consisting of a full size probe and drogue. Results show that the vision tracking algorithm can detect and track the drogue at real-time frame rates of more than thirty frames per second, obtaining a robust position estimation even with strong motions and multiple occlusions of the drogue.

Control Methodologies for Relative Motion Reproduction in a Robotic Hybrid Test Simulation of Aerial Refuelling

In many applications it is advantageous to simulate the relative motion of two bodies in a laboratory environment. This permits the testing of sensors and systems critical to the safety of equipment and personnel with reduced risk, and facilitates stage-gate management of large projects to mitigate financial risks. The University of Bristol is collaborating with Cobham Mission Equipment to develop a large-scale facility for relative motion simulation, primarily for the purpose of testing automated air-to-air refuelling systems. The facility incorporates two 6DOF articulated robotic arms whose motion is dictated by real-time numerical simulations of the physical environment. Sensors on the robot-mounted equipment feed back into the numerical simulation to perform closed loop simulations with real hardware. This paper discusses the development of the facility and the different approaches considered for achieving real-time control of the robotic hardware. It then goes on to focus on aspects of the control topologies and motion optimisation which are used to maximise the performance of the facility. The current capabilities are demonstrated with respect to an aerial refuelling exercise and future challenges are explored.

Adaptive passive control of dynamic response through structural loading

This paper addresses the issue of the effect of structural load on structural dynamic behavior, and how an understanding of this behaviour can be used beneficially to reduce the amplitude of response. The processes discussed lead to a new method of adaptively retuning structures through the introduction of known loads, with particular reference to helicopter response suppression. In many applications it is desirable to reduce the response of a structure without the use of damping. This has been the subject of much research with regard to helicopter design. Passive methods exist but do not allow for multiple excitation frequencies, shifting frequencies or structural alterations arising from different flight configurations and loading. Active systems have the advantage that they can respond quickly and continuously to changes in system configuration but they require significant power and higher levels of maintenance than passive systems. Adaptive passive or semi-active systems are able to adapt to flight conditions while using far less power than fully active systems. They generally involve similar techniques to passive methods with some mechanism for tuning the parameters of the vibration absorber. In this paper, numerical and experimental techniques are used to investigate the effect of loading on structural response with a view to implementing a structural load based adaptive passive device. To set the scene for this proposal, a case study of structural loading on a helicopter tailboom is presented, illustrating the extent to which the dynamic response can be affected by loading. Finite element techniques are investigated and the results compared with experimental data. The difficulties in modelling the changes in dynamic performance are discussed with refence to a case study on joint parameterization. It is found that parameters updated for one load case may not apply across an entire loading regime. A preliminary study is then carried out to assess the feasibility of the proposal, and it is demonstrated that such a system can provide improved response with low power requirements across a range of narrow band excitation frequencies. This important result provides a starting point for further research into automated adaptive techniques for response suppression, and critical areas for new work to be focused on are identified.

Toward Robotic Pseudodynamic Testing for Hybrid Simulations of Air-to-Air Refueling

Hybrid simulation couples experimental tests of novel components to validated numerical models of the remainder of a system and provides high-confidence predictions of their coupled dynamic behavior. Air-to-air refueling (AAR) is an example of the type of system that can benefit from this development approach. The work in this paper concerns the on-ground validation and preflight verification of probe–drogue contact–impact scenarios in AAR maneuvers using off-the-shelf multiaxis industrial robots as part of a hybrid test to interface the refueling hardware with numerical models of the flight environment. While industrial manipulators present a cost-effective solution, bandwidth and power limitations inevitably cause practical problems for real-time hybrid testing. These deficiencies typically manifest themselves as significant tracking inaccuracies or instabilities when sharp nonlinearities or discontinuities are encountered as part of a contact phase. Here, the novel robotic pseudodynamic testing (RPsDT) method is employed to circumvent the contact-response speed limitations of industrial robots. This paper presents and discusses the application of RPsDT to contact–impact problems, outlines the challenges and limitations of the technique in an easily reproducible validation experiment, and details the first RPsDT hybrid simulation of an AAR maneuver using scaled refueling hardware. It is concluded that RPsDT provides a useful tool for the investigation of a particular subclass of multibody contact–impact problems including AAR, where the response of the contacting structures does not possess significant rate-of-loading effects. Future work will comprise tests with full-scale AAR hardware.

Weak signal detection based on two dimensional stochastic resonance

The analysis of vibrations from rotating machines gives information about their faults. From the signal processing perspective a significant problem is the detection of weak signals embedded in strong noise. Stochastic resonance (SR) is a mechanism where noise is not suppressed but exploited to trigger the synchronization of a non-linear system and in its one-dimensional form has been recently applied to vibration analysis. This paper focuses on the use of SR in a two-dimensional system of gradient type for detection of weak signals submerged in Gaussian noise. Comparing the traditional one-dimensional system and the two-dimensional used here, this paper shows that the latter can offer a more sensitive means of detection. An alternative metric is proposed to assess the output signal quality, requiring no a priori knowledge of the signal to be detected, and it is shown to offer similar results to the more conventional signal-to-noise ratio.

Robotic Pseudo-Dynamic Testing (RPsDT) of Contact-Impact Scenarios

This paper presents a hybrid test method that enables the investigation of contact-impact scenarios in complex systems using kinematically versatile, off-the-shelf industrial robots. Based on the pseudo-dynamic test method, the technique conducts tests on an enlarged time scale, thereby circumventing control rate and response time limitations of the transfer system. An initial exploratory study of a drop test demonstrates that non-rate dependant effects including non-linear stiffness and structural hysteresis can be captured accurately while limitations result from the neglect of rate- and time-dependant effects such as viscous damping and creep. Future work will apply the new method to contact scenarios in air-to-air refuelling.

A Strategy for Improving Performance in Real Time Hybrid Testing

Hybrid testing is an emergent technology encompassing a variety of contemporary methods such as hardware-in-the-loop, pseudo-dynamic, and real-time dynamic testing. A system to be tested is split into two or more subsystems, with some of the components represented by numerical models and the remainder being comprised of real, physical hardware. Forces and displacements are transmitted between subsystems via actuators and sensors. This paper is concerned with the challenges that emerge when trying to accommodate real-time simulations with highly nonlinear force characteristics in the physical substructure. A brief review of hybrid testing considerations is provided, including actuation hardware, controllers, and numerical time-integration schemes. An new approach is then proposed which unites novel methods in the numerical model, the integration scheme and the actuator control to achieve high performance levels independent of the physical component being tested, specifically for the case of highly nonlinear components and relatively coarse timesteps in the numerical substructure. Simulation results are provided to substantiate the projected benefits.

A Comparison of Analytical and Empirical Controllers for the SLIP Hopper

The Spring-Loaded Inverted Pendulum (SLIP) model poses a challenging control problem, important for the development of legged robots, due to the difficultly in solving the stance phase of the dynamics. Multiple attempts have been made to approximate these dynamics to allow for an analytical control method; here four of these methods have been compared for controlling agile hopping, where there are large changes in forward velocity across a single stance. In addition, a new, empirical, approach has been demonstrated. In this, a simple control law is formulated, based on some simple approximations, which allows the parameters to be selected empirically through simulation. This has led to a controller able to offer similar performance to the best analytical approximation but with a much simpler form. This empirical controller may present new opportunities for controlling more complex dynamics and the development of a self-tuning method in future work.

A Regenerative Approach to Energy Efficient Hydraulic Vibration Control

Active vibration control technologies are reaching maturity in many applications, in both periodic and transient operating regimes. Historically these systems have been designed without regard for the power they consume, which is not only inefficient and costly, but limits their adoption in applications where it is impractical to provide large power supplies. Strategies for reducing power consumption include semi-active and regenerative methods. The former limits the device action to dissipative forces, through adjustable spring and/or damping rates. The latter uses the dissipative portion of the cycle to store energy in a reservoir, which can then be used in the remainder of the cycle. This paper looks at the benefits of using hydraulic devices in this context instead of the prevalent electromechanical devices. A case study of regenerative hydraulic vibration control is presented using digital hydraulics concepts, analogous to the switching power supplies and amplifiers that have revolutionised the efficiency of modern electronic equipment. The limitations and trade-offs are examined and projections are made as to the performance that could be achieved as the limitations of contemporary hydraulic components are improved upon.