Donald L. Margolis

ADAPTIVE SUSPENSION CONCEPTS FOR ROAD VEHICLES

SUMMARY Most vehicle suspensions are composed of passive spring and damper devices, although improved suspension performance is possible if an active system is used to control forces or relative velocities. The complexity, power requirements, and cost of fully active suspensions have restricted their use. Various partially active suspensions have been proposed and suspensions with slow load levelers and variable dampers are in widespread use. Here we analyze a class of basically passive suspensions the parameters of which can be varied actively in response to various measured signals on the vehicle. These suspensions can come close to optimal performance with simpler means than many of the active or semi-active schemes previously proposed.

Semi-Active Control of Wheel Hop in Ground Vehicles

ABSTRACT A two degree-of-freedom vehicle model is developed which incorporates passive, active, and semi-active secondary suspensions. The model is used to demonstrate the trade-offs which are inherent in attempting to provide desirable sprung weight isolation while at the same time controlling unsprung weight motions. A linear model is used first in order to compare passive and active suspensions in an analytically understandable configuration. The semi-active suspension is inherently nonlinear and is compared to the others through computer simulation. The passive suspension is, of course, the most restrictive in providing simultaneous isolation of sprung and unsprung weight; however, the active suspension is also compromised in providing both functions. The semi-active suspension does an excellent job of tracking its active counterpart.

Semi-Active Heave and Pitch Control for Ground Vehicles

SUMMARY A model is presented which includes both the heave and pitch motions of a vehicle traversing a roadway. Provision is made for testing totally passive, totally active, and semi-active secondary suspensions. Control strategies are developed for the totally active case and vehicle isolation is demonstrated. These active controllers are then modified to be semi-active, i.e., no power is provided from the controller to the vehicle. The semi-active isolation is shown to be comparable to the totally active system and much superior to the passive suspension.

A bond graph model incorporating sensors, actuators, and vehicle dynamics for developing controllers for vehicle safety

Abstract The process of developing controllers for vehicle dynamics requires reasonable models that expose the important dynamic effects without being excessively complicated. The controllers require sensors, signal processing, and actuation, and these parts must be incorporated and demonstrated using vehicle models. Bond graphs are a concise pictorial representation of all types of interacting energy domains, and are an excellent tool for representing vehicle dynamics with associated control hardware. This paper develops a four-wheel, nonlinear vehicle dynamic model with electrically controlled brakes and steering, as well as control at each suspension corner. Controllers are not developed here but are demonstrated through simulation.

Model-based road friction estimation

The tire/road friction coefficient, μ, has a significant role in vehicle longitudinal and lateral control, and there has been associated efforts to measure or estimate the road surface condition to provide additional information for stability augmentation systems of automobiles. In this paper, a model based road friction estimation algorithm is proposed from easily measured signals such as yaw rate and wheel speed. For the development of the estimator, a low order vehicle model incorporated with simple but effective tire model. Field tests of the estimator using actual vehicle measurements show promising results.

INTEGRATED CONTROLS OF LATERAL VEHICLE DYNAMICS

SUMMARY A study is presented dealing with the synergism coming from the integration of such controlled systems which are designed for improving the dynamic behaviour of road vehicles. A modified 1/4 car model is derived which allows suspension, torque and steering control to be applied to a low order vehicle model. A number of investigations, based either on complex vehicle models or on ground tests, show that important synergistic effects can be obtained from the integration of suspension, torque and steering controls. The straight running on irregular roads may take advantage from the co-operation of active suspension and four-wheel-steering systems. The braking under µ-split conditions can be improved with a proper control of ABS and rear wheel steering. The driveability of a car entering a curve with changing surface conditions could be substantially enhanced thanks to the combined exploitation of ABS and additional four wheel steering.

Reduction of models of large scale lumped structures using normal modes and bond graphs

Abstract Bond graphs are an extremely useful modeling procedure for representing the actual energy exchange mechanisms of interacting dynamic systems. Governing state equations are straightforwardly obtained from the bond graph; however, for large structures, a restrictively large number of equations can result. A procedure is developed whereby the original equations are reduced to a form suitable for modal decomposition. The resulting modes are reinterpreted in bond graph form with the resulting model being an extremely accurate system representation while requiring only a fraction of the original number of equations. The procedure is demonstrated through example.

A procedure for comparing passive, active, and semi-active approaches to vibration isolation

Abstract A procedure is developed which uses bond graph modeling and a digital computer to determine if semi-active control can provide a suitable performance in an application where totally active control is considered. The application areas principally addressed are those in which the disturbance inputs to the system are of zero mean, i.e. shock and vibration control. The procedure is developed through examples and then generalized to systems of high order and large complexity. The procedure consists basically of designing a control strategy suitable for totally active control and then enforcing a passivity constraint on the actuating device. Experience has shown that semi-active control approaches that of totally active control in most vibration isolation applications.

FINITE MODE BOND GRAPH REPRESENTATION OF VEHICLE-GUIDEWAY INTERACTION PROBLEMS

Abstract Classical modal analysis techniques for the prediction of vehicle-guideway dynamics are developed and interpreted with bond graph representations. Bond graphs are shown to allow easy generalization to multiple span guideways incorporating virtually any dynamic boundary conditions at the support locations. In addition, non-linear vehicle models can be used with any vehicle displacement-time history desired. Straightforward formulation procedures are also provided through the bond graph representation. The analysis procedure is demonstrated for a two-span Bernoulli-Euler Guideway with first-order dynamic boundary conditions. The results for a vehicle traveling at different speeds are shown to compare favorably with current literature.

Bond Graphs, Normal Modes and Vehicular Structures

SUMMARY Bond graph modeling techniques are used to represent the normal mode dynamic behavior of uniform Bernoulli-Euler beams. The independent beam models are then coupled together to form a distributed system structure of arbitrary complexity. The resulting overall system bond graph is shown to yield the governing state equations in a straightforward manner. Then, through proper ordering of the equations, the normal modes and frequencies of the coupled system are easily obtained This procedure is demonstrated for a vehicle A-frame structure. In addition, the bond graph model is modified to include primary and secondary suspension dynamics.