Wade A. Smith

Hydraulically interconnected vehicle suspension: background and modelling

This paper presents a novel approach for the frequency domain analysis of a vehicle fitted with a general hydraulically interconnected suspension (HIS) system. Ideally, interconnected suspensions have the capability, unique among passive systems, to provide stiffness and damping characteristics dependent on the all-wheel suspension mode in operation. A basic, lumped-mass, four-degree-of-freedom half-car model is used to illustrate the proposed methodology. The mechanical–fluid boundary condition in the double-acting cylinders is modelled as an external force on the mechanical system and a moving boundary on the fluid system. The fluid system itself is modelled using the hydraulic impedance method, in which the relationships between the dynamic fluid states, i.e. pressures and flows, at the extremities of a single fluid circuit are determined by the transfer matrix method. A set of coupled, frequency-dependent equations, which govern the dynamics of the integrated half-car system, are then derived and the application of these equations to both free and forced vibration analysis is explained. The fluid system impedance matrix for the two general wheel-pair interconnection types—anti-synchronous and anti-oppositional—is also given. To further outline the application of the proposed methodology, the paper finishes with an example using a typical anti-roll HIS system. The integrated half-car systems free vibration solutions and frequency response functions are then obtained and discussed in some detail. The presented approach provides a scientific basis for investigating the dynamic characteristics of HIS-equipped vehicles, and the results offer further confirmation that interconnected suspension schemes can provide, at least to some extent, individual control of modal stiffness and damping characteristics.

Hydraulically interconnected vehicle suspension: theoretical and experimental ride analysis

In this paper, a previously derived model for the frequency-domain analysis of vehicles with hydraulically interconnected suspension (HIS) systems is applied to the ride analysis of a four-degrees of freedom roll-plane, half-car under a rough road input. The entire road surface is assumed to be a realisation of a two-dimensional Gaussian homogenous and isotropic random process. The frequency responses of the half-car, in terms of bounce and roll acceleration, suspension deflection and dynamic tyre forces, are obtained under the road input of a single profile represented by its power spectral density function. Simulation results obtained for the roll-plane half-car fitted with an HIS and those with conventional suspensions are compared in detail. In addition, sensitivity analysis of key parameters of the HIS to the ride performance is carried out through simulations. The paper also presents the experimental validation of the analytical results of the free and forced vibrations of the roll-plane half-car. The hydraulic and mechanical system layouts, data acquisition system and the external force actuation mechanism of the test set-up are described in detail. The methodology for free and forced vibration tests and the application of mathematical models to account for the effective damper valve pressure loss are explained. Results are provided for the free and forced vibration testing of the half-car with different mean operating pressures. Comparisons are also given between the test results and those obtained from the system model with estimated damper valve loss coefficients. Furthermore, discussions on the deficiencies and practical implications of the proposed model and suggestions for future investigation are provided. Finally, the key findings of the investigation on the ride performance of the roll-plane half-car are summarised.

Hydraulically interconnected vehicle suspension: handling performance

This paper extends recent research on vehicles with hydraulically interconnected suspension (HIS) systems. Such suspension schemes have received considerable attention in the research community over the last few years. This is due, in part, to their reported ability to provide stiffness and damping rates dependent on the suspension mode of operation (i.e. the bounce, roll, pitch or articulation of the unsprung masses relative to the sprung mass), rather than relying on the stiffness and damping characteristics of the single wheel stations. The paper uses a nine-degrees-of-freedom (DOF) vehicle model and simulations of a fishhook manoeuvre to assess the handling performance of a vehicle when it is fitted with: (a) a conventional independent suspension, and (b) an HIS. In the case of the latter, the fluid subsystem is modelled using a nonlinear finite-element approach, resulting in a set of coupled, first-order nonlinear differential equations, which describe the dynamics of the integrated mechanical-hydraulic vehicle system. The simulation results indicate that, in general, the HIS-equipped vehicle possesses superior handling, as measured by the sprung mass roll angle, roll rate, roll acceleration, lateral acceleration and the vehicles Rollover Critical Factor. The potential effects of the suspension set-up on ride performance are also considered by studying the transient response when one side of the vehicle traverses a half-sine bump. The obtained results are then discussed, and it is shown that they are consistent with previous findings, both by the authors and other researchers. The presented work outlines an alternative approach for studying the dynamics of HIS-equipped vehicles, particularly suited to analyses in the time domain.

Damage identification based on response-only measurements using cepstrum analysis and artificial neural networks

This article presents a response-only structural health monitoring technique that utilises cepstrum analysis and artificial neural networks for the identification of damage in civil engineering structures. The method begins by applying cepstrum-based operational modal analysis, which separates source and transmission path effects to determine the structure’s frequency response functions from response measurements only. Principal component analysis is applied to the obtained frequency response functions to reduce the data size, and structural damage is then detected using a two-stage ensemble of artificial neural networks. The proposed method is verified both experimentally and numerically using a laboratory two-storey framed structure and a finite element representation, both subjected to a single excitation. The laboratory structure is tested on a large-scale shake table generating ambient loading of Gaussian distribution. In the numerical investigation, the same input is applied to the finite model, but the obtained responses are polluted with different levels of white Gaussian noise to better replicate real-life conditions. The damage is simulated in the experimental and numerical investigations by changing the condition of individual joint elements from fixed to pinned. In total, four single joint changes are investigated. The results of the investigation show that the proposed method is effective in identifying joint damage in a multi-storey structure based on response-only measurements in the presence of a single input. Because the technique does not require a precise knowledge of the excitation, it has the potential for use in online structural health monitoring. Recommendations are given as to how the method could be applied to the more general multiple-input case.

Hydraulically Interconnected Vehicle Suspension: Optimization and Sensitivity Analysis

This paper extends recent research on vehicles with hydraulically interconnected suspension (HIS) systems. Such suspension schemes have received considerable attention in the research community over the last few years. This is due, in part, to their reported ability to provide stiffness and damping rates dependent on the suspension mode of operation (i.e. the bounce, roll, pitch, or articulation of the unsprung masses relative to the sprung mass), rather than relying on the stiffness and damping characteristics of the single wheel stations. In this paper, the optimization of such a system is considered. Use is made of a previously derived four-degree-of-freedom model of a roll-plane half-car fitted with a typical antiroll HIS system. Objective functions are then developed, based on the desire to improve ride comfort and to minimize suspension working space and tyre normal force fluctuations. With this formulation, a large number of optimal solutions are found and presented graphically, and the performance limitations and trade-offs between the desired objectives are illustrated. To contextualize these results, a similar optimization process is applied to a half-car with a conventional independent suspension. Four optimal parameter combinations are then selected as base points for further examination of the HIS vehicle. This is done by way of a basic sensitivity analysis, based on the local method, which involves single-parameter perturbations about a base point. The objective of the paper is to outline the dynamic performance, trade-offs, and limitations of an HIS-equipped vehicle, and to identify the systems most important parameters.

Robust Yaw Moment Control for Vehicle Handling and Stability

This paper presents a robust controller design method for improving vehicle lateral stability and handling performance. In particular, the practical load variation will be taken into account in the controller synthesis process such that the controller can keep the vehicle lateral stability and handling performance regardless of the load variation. Based on a two-degree-of-freedom (2-DOF) lateral dynamics model, a model-based TakagiSugeno fuzzy control strategy is applied to design such a controller and the sufficient conditions for designing such a controller are given in terms of linear matrix inequalities (LMIs) which can be solved efficiently using currently available numerical software. Numerical simulations are used to validate the effectiveness of the proposed control approach.

Experimental and Theoretical Investigation into the Dynamics of a Half-Car with an Interconnected Passive Suspension

In this paper, a previously derived theoretical model of an integrated hydraulically interconnected suspension (HIS) half-car system is experimentally validated. The paper outlines the development of the HIS fluid system model and its integration into a four degree-of-freedom, roll-plane half-car system. An experimental approach to validate the model is outlined, and the resulting purposebuilt half-car test facility is described in detail. Experimental results from both free and forced vibration testing are presented and compared with model-based simulations. In general, very good agreement is observed. Limitations of the testing approach and reasons for any discrepancies are discussed. Finally, the broader implications of the obtained results in terms of practical HIS system design are considered.

Hydraulically Interconnected Suspension Parameter Sensitivity in Half-Car Ride Performance

In this paper, the development of a hydraulically interconnected suspension (HIS) system model and the integration of this model into a four degree-of-freedom half-car system is briefly introduced. The appropriate frequency response functions are derived in order to simulate the system response to a stochastic road profile. The sprung mass vertical and roll accelerations, the dynamic normal tyre force, and the suspension deflection are considered in the frequency domain up to 20 Hz. Four key hydraulic system parameters are identified and investigated to gauge their effects on the system’s dynamic performance. The results indicate that HIS system performance can be greatly affected by these hydraulic parameters.

Vibration-Based Spall Size Tracking in Rolling Element Bearings

This paper extends recent research on the estimation of spall size in rolling element bearings (REBs) using laboratory measurements on a bearing test rig. The prognostics of REBs remains a formidable challenge, partly because many of the classic diagnostic indicators trend non-monotonically with bearing wear, as spalls that develop on the bearing races progress from localised to distributed faults. The direct estimation of spall size from the vibration signal has recently been proposed as a potential solution to this problem. Such estimation techniques typically rely on the identification and enhancement of the events in the signal corresponding to the rolling element entry into and exit from the spall. Identification of the entry event poses the most difficulty, and for this the paper proposes a novel method, which is then applied to signals obtained from a bearing with known spall dimensions. The resulting estimates correlate well with the actual spall size.

Transient Characteristics of a Hydraulically Interconnected Suspension System

This paper describes vehicle dynamic models that capture the large amplitude transient characteristics of a passive Hydraulically Interconnected Suspension (HIS) system. Accurate mathematical models are developed to represent pressure-flow characteristics, fluid properties, damper valves, accumulators and nonlinear coupling between mechanical and fluid systems. The vehicle is modeled as a lumped mass system with half- and fullcar configurations. The transient performance is demonstrated by numerical integration of the secondorder nonlinear differential equations. The stiffness and damping characteristics corresponding to vehicle bounce, roll and pitch motions are extracted from the transient simulation. Simulation results clearly demonstrate the superiority of the HIS system during vehicle handling and stability by providing additional roll stiffness and reduced articulation stiffness.