The Influence of Damper Properties on Vehicle Dynamic Behaviour

This paper details a non-linear hysteretic physical shock absorber model, and the processes utilised to identify the constituent parameters. In the current paper the model parameters are extracted from experimental data for the ‘sport’ setting of a prototype front shock absorber for a vehicle in the luxury class. The model is validated by comparing simulated results to experimental data for a test damper, for three discrete frequencies of sinusoidal excitation of 1,3 and 12 Hz. Finally the shock absorber model is included in a quarter car vehicle ride model and output characteristics are compared to those obtained with classical damper representations. INTRODUCTION The detailed dynamic properties of dampers are known to influence substantially some of the subtle, and yet nevertheless hugely important, refinement aspects of vehicle ride and handling. However, damper properties are typically characterised by quasi-steady properties for vehicle simulation purposes. The classic 14 speed test [1], for example, involves subjecting a damper to 14 differing frequency levels of fixed amplitude sinusoidal excitation, and then plotting the peak force values obtained versus the relevant test velocity. Such a representation of shock absorber behaviour is clearly deficient for the purpose of vehicle simulations as only a snap shot view of the damper’s behaviour is utilised and much information is discarded. As a direct consequence the process of damper valve tuning is still carried out to a great extent via ride work. This consists of ride engineers subjectively rating the performance of the prototype vehicle(s) over a series of test tracks/ride routes. The damper valve is then possibly adjusted to improve the vehicle’s ride/handling or to obtain characteristics expected for that class of vehicle. This process is clearly open to subjective evaluation which may vary driver to driver, and even from one day to the next. A more scientific approach to the issue of damper tuning, via vehicle simulations, would offer a number of significant benefits. This is simply due to the fact that the bulk of the damper selection process could be carried out prior to the manufacture of any vehicle prototypes, and this technique would be far less subjective in nature. Such an approach would necessitate an improved method of characterising the damper, such that the important dynamic features are represented, and comprehension of the links between more subtle features of the damper response. The current paper attempts to address the aforementioned requirement for improved damper characterisation in the context of the ‘sport’ setting of a triple rate prototype adaptive shock absorber. SHOCK ABSORBER MODELLING APPROACH In order to select the optimum damper modelling strategy for a ‘virtual damper tuning environment’, the suitability of the differing approaches found within published literature were determined with respect to the following criterion: Ability to capture damper non-linearity and dynamic behaviour. Flexibility to model different shock absorber types. Ease of model generation (Experiment/Parameter identification). Suitability for use in vehicle simulations. Usefulness as a predictive tool. Clearly black box methods such as the Restoring Force Mapping method [2-5] and neural networks [6], do not satisfy the need for a predictive tool as they are inherently non-tuneable. The same applies to elementary models constructed with spring and ideal viscous damping elements [7,8]. The required need for tuneable elements for this damper model application lends itself to an explicit physical model, where the damper control force is related to physical parameters that govern the dampers internal flows and pressures. The high detail model developed by Lang [9,10], for a twin tube shock absorber, was the first physical model that actually aimed to predict damper behaviour over a wide range of operating conditions. This 87 parameter model gave good correlation with experiment, but both simulation and parameter identification processes were highly iterative. It was also very specific to the shock 2002-01-0319 The Influence of Damper Properties on Vehicle Dynamic Behaviour Adrian Simms and David Crolla Copyright © 2002 Society of Automotive Engineers, Inc. The University of Leeds, School of Mechanical Engineering absorber under investigation. It did however identify the contribution of internal compliance’s to the hysteretic nature of dampers. More recently models by Duym [11-13], Lang and Sonnenburg [14] and Herr et al [16] attempted to generate more readily identifiable physical damper models. Each of these models are similar in overall structure in that they consist of a pressure model(s) and valve pressure/flow characterisations. The pressure models are a selection of first order non-linear differential equations, which are utilised to determine the various internal chamber pressures, and are derived from pressure dependent oil compressibility models. For the pressure/flow model Duym [11-13] identified valve parameters from several simple dynamometer tests and an additional ‘incompressible’ model (see definitions); these parameters were then used to analytically determine valve flows for given pressure drops in the main model. In contrast Lang and Sonnenburg [15] used experimentally obtained pressure/flow data, and Herr et al evaluated the pressure flow characteristics of the differing valve components using Computational Fluid Dynamics (CFD). In the absence of a large database of pressure/flow data for differing valve formulations, as was available to Lang and Sonnenburg, this non parametric form of pressure model does not satisfy the required criteria of tune-ability. The CFD approach is a time consuming process requiring detailed modelling even for varying damper valves of the same basic architecture. As a result of the aforementioned considerations the damper modelling approach to be taken is one of a physical model similar to that of Duym [11-13]. SHOCK ABSORBER ARCHITECTURE, AXES SYSTEMS AND SIGN CONVENTIONS The prototype shock absorber, in the ‘sport’ setting, falls into the mono-tube category; the only flow paths for the internal hydraulic fluid are thus through the piston’s valves (see Figure 1). The additional ‘softer’ damper settings, which are achieved by the solenoid activation of additional flow paths, will not be considered in this paper. The mono-tube shock absorber consists of a rebound chamber and a compression chamber which are both oil filled. A high pressure nitrogen gas volume, typically 20-30 Bar, is present at the end of the pressure tube, separated from the damper fluid by way of a floating piston. Expansion and compression of this gas volume compensates for the differing volumetric changes encountered in bump and rebound strokes as a result of the piston rod’s presence in the rebound chamber. Several valve assemblies are present on the piston, one for compression and one for extension strokes. The flow path architecture of these valves are outlined in Figure 1. For low damper velocities the preloaded blowoff valve remains closed and the net pressure/flow characteristic can be described by the series combination of the port channel and the fixed bleed. For higher velocities the blow-off valve progressively opens thus reducing the rate of increase of damper force with respect to velocity. Figure 1. Mono-tube layout / axis system (top), and flow path architecture (bottom). MONO-TUBE DAMPER MODEL The twin-tube damper model presented in [11-13] is a compressible model which is used solely for the purpose of simulations. To yield a readily identifiable damper model the parameters of the compressible model of [1113] were identified by fitting an additional incompressible model to experimental data from which the hysteresis had been removed as described in [16,17]. As a result of the differing model architectures the pressure/flow model, required for the parameter identification processes, was different to that of the full compressible model. In this paper a single pressure/flow model will be derived for a mono-tube shock absorber for the parameter identification process, and the valve pressure/flow characteristics will then be supplied to the compressible damper model in the form of a simple look-up table. This modelling approach should yield more rapid simulations and additional robustness without compromising model tune-ability. As will be discussed later this can also yield improvements in model accuracy and flexibility; for non-standard valve architectures it will now only be necessary to derive a single new pressure/flow model. Leak Restriction Port Restriction P1 P2 Rebound Chamber Preb Compression chamber, Pcom x , x Gas phase, Pcom,gas Piston valve assembly