Exploiting human motion for the purpose of energy harvesting has been a popular idea for some time. Many
of the approaches proposed can be uncomfortable or they impose a significant burden on the person's gait. In
the current paper a hardware in-the-loop simulator of an energy harvesting backpack is employed in order to
investigate the effect of a suspended-load backpack on the human gait. The idea is based on the energy produced
by a suspended-load which moves vertically on a backpack while a person walks. The energy created from such
a linear system can be maximised when it resonates with the walking frequency of the person. However, such a
configuration can also cause great forces to be applied on the back of the user. The system which is presented here
consists of a mass attached on a rucksack, which is controlled by a motor in order to simulate the suspended-load
backpack. The advantage of this setup is the ability to test different settings, regarding the spring stiffness or
the damping coefficient, of the backpack harvester, and study their effect on the energy harvesting potential, as
well as on the human gait. The present contribution describes the preliminary results and analysis of the testing
of the system with the help of nine male volunteers who carried it on a treadmill.
Full suspension mountain bicycles exhibit unwanted suspension movement during pedalling. Damper manufacturers
frequently adopt what is known as platform damping to overcome this problem. Such dampers resist low frequency
pedalling inputs due to the presence of a threshold or 'platform' damping level. However, this platform compromises
shock absorption ability over rougher terrain.
In this paper, the authors describe a prototype rear shock absorber that utilises magnetorheological (MR) fluids to
implement semi-active platform damping. Results from recent field trials will be presented, and the current status of
commercialising the system will be discussed.
Smart fluid dampers can undergo large temperature changes due to the heating associated with energy dissipation. Such
heating will alter the fluid's properties and could degrade control system performance. For example, previous work by
the authors has shown that the stability of an MR damper under feedback control is dependent on the fluid's
compressibility and viscosity. In the present study, a temperature dependent model of a magnetorheological damper is
developed from experimental data, and it is shown that the fluid's yield stress, viscosity and compressibility parameters
vary significantly. An experimental and numerical control study is then performed to investigate the resulting effects of
temperature on the stability of two feedback controllers - a PID controller, and a proportional controller. Experimental
results indicate that both controllers can exhibit a reduction in stability with increasing temperature, particularly if the
controller gains are not suitably chosen. The temperature dependent MR damper model predicts this behaviour well, and
it is shown that the change in viscosity has the most significant effect on stability. Future work could focus on the
resulting effect on a complete vibration system, devices with different modes of operation, and alternative controllers.
Magnetorheological (MR) fluids provide a novel solution to adapt damping levels in aircraft landing gear, so that optimal performance can be achieved over a wide range of conditions. The present study helps to demonstrate the feasibility of this solution by sizing an MR valve within the constraints of an existing commercial (passive) oleopneumatic shock strut. Previous work on MR landing gear has tended to focus on potential control strategies rather than design and sizing issues. However these latter aspects are of great importance in aircraft systems, where space and weight are vital design constraints.
To aid the sizing analysis performed in this study, accurate quasi-steady and dynamic impact models of passive and MR oleopneumatic landing gears are developed. The model is validated against experimental data incorporating the passive device, which is then used as a benchmark for the MR designs and to assess fail safety. The dynamic model is particularly important as it incorporates fluid compressibility, which may be a significant contributor to the overall response of the device in an impact scenario. The present study also aims to give further insight into high velocity MR valve flow, which will be inevitable during impulsive loading. This area remains largely unexplored and particular importance is given to valve Reynolds number since turbulent values are known to reduce device performance. The feasibility of an MR landing gear will be largely dependant on these factors.
In recent years, much research has focused on the development of effective control strategies for smart fluid dampers. In particular, skyhook control principles are frequently shown to demonstrate significant performance improvements over conventional passive systems. However these investigations are often either model-based and assume that the controlled damper can accurately track a prescribed force, or they are based on on/off type control strategies where such accurate tracking is not required.
In this paper, the authors present an investigation of a magnetorheological (MR) skyhook controlled SDOF mass isolator subject to broadband input excitations. The semi-active element is an MR smart fluid damper. The study utilises feedback linearisation, which is demonstrated experimentally, to convert the non-linear damper into a linear controllable device. This approach can be effectively harnessed to implement skyhook control since it permits the accurate tracking of a desired force within the controllable limits of the MR damper.
Using a validated model of an MR damper, it is demonstrated that feedback linearisation can yield significant performance improvements over more simplistic on/off control strategies. The same strategy could be integrated within larger scale vibrating structures (such as vehicle suspensions or aircraft landing gear) to implement more complex control strategies, e.g. optimal control.
Regenerative chatter is a form of unstable, self-excited vibration that occurs in machining operations such as milling and turning. In high speed milling of many aircraft components, regenerative chatter is the fundamental factor that limits metal removal rate and consequently productivity. Regenerative chatter is essentially a feedback process: the cutting chip thickness produces a force between the tool and workpiece, and the dynamics of these components results in a change in the cutting chip thickness. An exciting method of avoiding chatter is to actively control the workpiece or tool during cutting. On thin walled workpieces, such as those for aerospace structural components, this can be achieved using piezoelectric devices. A wide range of control regimes could be applicable, such as feed-forward actuation, active constrained layer damping, or active damping, using either non-collocated or collocated sensors and actuators. This study focuses on active damping with collocated sensors and actuators. In this article, an experimental study is described whereby workpiece chatter during milling is reduced by using active vibration control with piezoelectric sensors and actuators.
There is much current interest in the development of smart fluid clutches for use in the design of high-speed machinery. This interest stems from the flexibility, controllability and fast response of such fluids. In this paper the authors outline the modifications to an Electro-rheological clutch mechanism for a robotics application. The clutch mechanism consists of twin ER clutches that are driven in opposite directions. By controlling the electric field applied to each clutch it is possible to cause a toothed belt to move in a desired manner in each direction. This belt motion can then be used to control the motion of a robot arm via a gear train. To improve the positional performance an ER brake is added to the robot arm mechanism. The extension to the dynamic model for the ER clutch mechanism to incorporate the robot arm and ER brake is outlined and is validated experimentally. The displacement response of the robot arm is then examined as a trend study using different motor driving speeds and load inertias. The positional accuracy of the robot arm and its repeatability is then demonstrated over a significant number of reciprocating tests.
Smart fluid dampers offer an attractive solution to vibration damping problems where there is a need for variable damping behaviour. In recent years there has been a great deal of research effort in developing these dampers, and implementing appropriate control strategies. Consequently a wide range of modelling techniques have been proposed, for both device design and controller design processes. In general, however, the development of modelling and control techniques has proceeded in parallel to the development of laboratory-based devices. Consequently the techniques become well suited to those particular devices, and their performance in a more generic device design process may not be guaranteed. A more thorough method of illustrating the performance of models would be to investigate their performance when based upon different devices. This would help to emphasise the strengths and potential weaknesses of the model when used in a generic device design process. In this paper, the authors will seek to assess the robustness of a modelling technique by assessing its performance when based upon an 'unrelated' damper design from a different research group.
Smart fluid devices are now seen as an attractive solution to vibration damping problems. They offer superior performance compared to passive devices, without involving the cost, weight and complexity of fully active damping strategies. However, the inherent non-linearity of smart fluid dampers makes it difficult to fully exploit their capabilities, due the problems in applying an effective control strategy. In the past much of the research focused on complex controllers involving techniques such as neural networks and fuzzy logic. In recent years, however, an alternative approach has been adopted, whereby classical control techniques are used to linearise the damper's response. As a result some applications for smart fluid damping now use combinations of proportional, integral, or derivative control methods. However, it appears that these controllers can become unstable in much the same way as for a truly linear system. In order to investigate this instability it is suggested that a sufficiently accurate model of the damper's response is required, so that the onset of instability can be reproduced numerically.
In this contribution, a model updating technique is described whereby an existing ER damper model is updated in line with experimental data. The paper begins with an overview of the experimental test facility and the modeling approach. The updating algorithm is then described, and it is shown how the updated model improves significantly on the accuracy of the model predictions.
The potential of smart fluids (both electrorheological, and magnetorheological) in damping devices is now well-known. Whilst both types of fluid can suffer from drawbacks such as sedimentation, fluid degradation, and problems with containment or sealing, these issues are not insurmountable and solutions have been engineered such that practical damping devices are now commercially available. However, one drawback is that the free-velocity characteristics of a smart fluid device are inherently non-linear, possessing the general form associated with a Bingham plate. This means that while practical devices have the potential to modify rapidly their behavior, it can be difficult automatically to adjust the device's response.
Ongoing research at the University of Sheffield is currently concerned with the design and construction of magneto- rheological (MR) squeeze-flow vibration damper. Previous work has demonstrated the feasibility of employing such a device as the key component in a controllable vibration isolator. The work also demonstrated the inadequacies of existing mathematical models which do not account for the observed behavior of MR fluids in squeeze flow. In parallel with investigations into the behavior of MR dampers, a collaborative programme between the Universities of Liverpool and Sheffield is also in progress. Here attention is focussed on ER fluids in squeeze-flow and a new test facility has been constructed for use in the development and validation of mathematical models. It is anticipated that this collaborative programme will assist in the development of both ER and MR squeeze-flow models. In this paper, the authors present a summary of progress to date.
It is now well known that smart fluids [electrorheological (ER) and magnetorheological (MR)] can form the basis of controllable vibration damping devices. With both types of fluid, however, the force/velocity characteristic of the resulting damper is significantly non-linear, possessing the general form associated with a Bingham plastic. In a previous paper the authors showed that by using a linear feedback control strategy is it possible to produce the equivalent of a viscous damper with a continuously variable damping coefficient. In the present paper the authors illustrate an extension of the technique, by showing how the shape of the force/velocity characteristic can be controlled through feedback control. This is achieved by using a polynomial function to generate a set point based upon the damper velocity. The response is investigated for polynomial functions of zero, 1st and 2nd order. It is shown how the damper can accurately track higher order polynomial shaping functions, while the zero order function is particularly useful in illustrating the dynamics of the closed-loop system.
It is now well established that magnetorheological (MR) fluids can provide the basis for constructing controllable vibration damping devices. Moreover, the characteristics of MR fluids are generally compatible with industrial requirements and there is enormous scope for commercial exploitation. In this paper the authors describe the design and construction of a vibration isolator which incorporates an MR damper. The damper is unusual in that it operates in the squeeze-flow mode. A quasi-steady model of the MR damper is summarized and then extended to include the vibration isolator dynamics. Model predictions are compared with experimental results. It is shown that by employing the MR damper a wide range of control can be exercised over the transmissibility of the vibration isolator. Numerical experiments are used to show that a feedback control strategy can provide even more control over transmissibility.
Magneto-rheological (MR) fluids are rapidly rising in prominence as a means of producing controllable damping devices for vibration control. MR fluids can be used in various modes of operation in order to provide damping forces. One of the least exploited of these modes is commonly known as squeeze-flow, where large, controllable forces can be generated over relatively small displacement ranges. In this paper the authors describe a recently constructed test facility in which an MR squeeze-flow device is incorporated as the damping element in a vibration isolator.
It is widely acknowledged that the inherent non-linearity of smart fluid dampers is inhibiting the development of effective control regimes, and mass-production devices. In an earlier publication, an innovative solution to this problem was presented -- using a simple feedback control strategy to linearize the response. The study used a quasi-steady model of a long-stroke Electrorheological damper, and showed how proportional feedback control could linearize the simulated response. However, this initial research did not consider the dynamics of the damper's behavior, and so the development of a more advanced model has been necessary. In this article, the authors present an extension to this earlier study, using a model of the damper's response that is capable of accurately predicting the dynamic response of the damper. To introduce the topic, the electrorheological long-stroke damper test rig is described, and an overview of the earlier study is given. The advanced model is then derived, and its predictions are compared to experimental data from the test rig. This model is then incorporated into the feedback control simulations, and it is shown how the control strategy is still able to linearize the response in simulations.