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Author Daniel E. Williams, an industry professional with more than 30 years of experience in chassis control systems from concept to launch, brings this experience and his unique approach to readers of Generalized Vehicle Dynamics. This book makes use of nomenclature and conventions not used in other texts. This combination allows the derivation of complex vehicles that roll with multiple axles, any of which can be steered, to be directly predicted by manipulation of a generalized model. Similarly the ride characteristics of such a generalized vehicle are derived. This means the vehicle dynamic behavior of these vehicles can be directly written from the results derived in this work, and there is no need to start from Newton's Second Law to create such insight. Using new and non-standard conventions allows wider applicability to complex vehicles, including autonomous vehicles. Generalized Vehicle Dynamics is divided into two main sections-ride and handling-with roll considered in both.

A large percentage of commercial vehicles transport freight on our interstate highway system. These vehicles spend the vast majority of their duty cycle at high speed maintaining a lane. As steering is integrated into ADAS, objective performance measures of this most common mode of commercial vehicle operation will be required. Unfortunately in the past this predominant portion of the commercial vehicle duty cycle was overlooked in evaluating vehicle handling. This lanekeeping mode of operation is also an important, although less significant portion of the light vehicle duty cycle. Historically on-center handling was compromised to achieve acceptable low speed efforts. With the advent of advanced active steering systems, this compromise can be relaxed. Objective measures of lanekeeping are developed and performance of various advanced steering systems is quantified in this important operating mode.

Today's hydraulic steering systems suffer from poor efficiency due to their use of throttling valves to build pressure inside the steering gear. In this work, we propose a novel way to build and control pressure by controlling the flow from the pump and without throttling. As a result, such a system will be more energy efficient. Moreover, the ability to control pressure inside a steering gear, and thus assistance torque, allows the steering system to become an active closed-loop system rather than a passive open-loop system. Specifically, by controlling pressure, one can closely control the hand wheel torque the operator feels. Consequently, the new pressure control concept has the potential to not only significantly improve the efficiency of steering systems, but also offers the numerous benefits of torque overlay without the use of an electric motor.

With the expectation that means of redundant steering will be necessary for highly autonomous vehicles, different methods of providing redundant steering can be considered. One potential for redundancy is to steer the rear axle for directional control of the vehicle in the event of a failure in the primary steered front axle. This paper will characterize the dynamics of directional control of a three-axle vehicle when steered at the rear, and compare it to a conventionally steered three-axle vehicle. Several compensators are suggested that allow similar vehicle dynamic behavior when steering the rear axle as a driver would expect when steering the front, giving hope that a steerable rear axle can provide acceptable redundancy for a failed primary steering system on the front axle.

The classic two-degree-of-freedom yaw-plane or “bicycle” vehicle model is augmented with two additional states to describe lane-keeping behavior, and further augmented with an additional control input to steer the rear axle. A simple driver model is hypothesized where the driver closes a loop on a projected lateral lane position. A rear axle steer control law is found to be a function of front axle steering input and vehicle speed that exhibits high speed stability and improved low speed maneuverability. The theoretically derived control law bears similarity to practical embodiments allowing a deeper understanding of the functional value of steering a rear axle.

Self-steered or caster steered axles are commonly used to support load on multi-axle commercial vehicles. Such axles can allow more payload to be hauled in some vehicle configurations under the existing bridge formulas. These self-steered axles cannot generate a side load, and serve to unload surrounding fixed axles that do generate lateral forces to turn the vehicle with payload. Since the tire's ability to generate a side load is dependent upon its load, the use of caster-steered auxiliary axles can upset the balance (or the understeering) properties of the vehicle. This work will define the effect of adding a caster steered auxiliary axle and compare it with a steerable axle that positively controls the steer angle and thereby generates a lateral force. This work assumes the reader has a basic knowledge of the well publicized “bicycle” model, and particularly its extension to multi-axle vehicles.

A novel speed and position dependent Lane Keeping Assistance (LKA) control strategy for heavy vehicles is proposed. This LKA system can be implemented with any torque overlay system capable of accepting external position or torque commands. The proposed algorithm tackles the problem of lane keeping in two ways from a heavy vehicle's perspective. First, it stabilizes the vehicle's lateral position by bringing it to the center of the lane and giving it the correct heading to stay there. This is done using a speed and position dependent control strategy that becomes less aggressive as the vehicle's speed increases and as it gets closer to the center of the lane. Such speed and position dependency is especially critical in heavy vehicles where unnecessary aggressive control can lead to oscillations about the lane's centerline when cruising at high speeds.

Commercial vehicles must transport an increasing volume of freight on a relatively fixed infrastructure. Some of these vehicles are highly specialized and customized to perform particular tasks. One way to increase freight hauling efficiency is to allow longer vehicles with more axles. These vehicles will have different handling properties and must be driven on existing infrastructure. Longer term, autonomous-like vehicles could be used to increase vehicle utilization. In both cases characterizations of multi-axle vehicle dynamics are required. A two-dimensional yaw plane model is used in practice to analyze handling performance of two-axle passenger cars. Commonly known as the "bicycle" model because it combines all tire forces associated with a given axle to act on the centerline of the vehicle, the yaw plane model allows lateral velocity and yaw rate degrees of freedom.

The conventional rear tandem axle of a three-axle vehicle produces a yaw resisting moment that adversely impacts vehicle performance. This work examines the effect of steering the rear axle on tire wear. Using actual vehicle test data, a tire wear model is developed. This tire wear model is then used to predict tire wear savings over an actual commercial vehicle duty cycle when the rear axle is steered. The result of this projection is shown to be consistent with reported third party field experience.

Electrically Powered Hydraulic Steering (EPHS) has provided value in passenger car applications by reducing power consumption at engine idle, providing only the required power during high speed lane-keeping, and allowing engine-off operation of vehicles with alternative power sources. This work discusses the design modifications made to use EPHS for medium duty commercial vehicle applications. Configuration options along with communication and diagnostic interface are discussed. Bench tests show the steady-state performance of the system. Experiments are done on a medium duty truck with the EPHS as the sole source of steering power to determine the speed of steer at various vehicle speeds. Finally, the power consumption for the EPHS system is compared to a conventional engine driven pump.

The conventional rear tandem axle of a three-axle vehicle produces a yaw resisting moment that adversely impacts vehicle performance. This work examines the steady-state handling effect of steering the rear axle of a three-axle vehicle in terms of equivalent wheelbase and understeer. It is found that a very simple rear axle steer control strategy improves both the equivalent wheelbase and understeer. The equivalent wheelbase of the rear axle steered vehicle calculated from vehicle performance data equals the intuitively expected kinematic result. Finally, improvement in tire wear by steering the third axle is reported.

Hydraulic power steering systems traditionally are sized in a straightforward manner with easily verifiable results. The source of power in conventional systems is an engine driven pump that is effectively a source of hydraulic flow. As energy consumption of auxiliary functions becomes significant, on-demand power sources are considered. Best typified by hydraulic pumps driven by electric motors, these on-demand sources are often power limited, and established sizing practices should be re-visited.

Synthetic torque feedback (artificial feel) is valued in the market for enhancing steering performance as perceived by the driver. The possibility of using this same hardware, with minimal control modifications, to aid the driver in responding to tire failures, is discussed.

A computer controlled steering system providing an artificial feel or synthetic torque feedback to the driver has recently been launched into production in the commercial vehicle market. This work compares the artificial feel control strategy with prior electric power steering control strategies and hydraulic power steering. Suitability for integration with other vehicle control systems such as lane sensing and electronic stability enhancement is explored.

The effect of an artificial feel steering device providing a synthetic torque feedback to the driver is studied for a transit bus operating in an urban drive cycle. Driver workload is greatly reduced and the results are shown to be statistically valid for a wide range of transit bus routes.

This paper is concerned with power consumption analysis for conventional steering system, the importance of duty cycle before choose appropriate pump drive system, and the energy saving potential of the proposed systems. After reviewing the recent efforts in developing energy-efficient steering system, two new on-demand pump drive systems are proposed to provide varying flow according to vehicle/engine speed: one is the combination of a sized pump without flow control valve and an Electrical Power Hydraulic Steering (EPHS) unit; the other is the combination of multiple EPHS units. The energy saving advantage of the combinations will be emphasized for different duty cycles.

This paper reviews activities relating to understanding, and improving the on-center performance of heavy truck steering systems. Initially, the on-center steering performance characteristics for commercial vehicles were quantified. Steering wheel torque and angular position were the prime measurables. Graphical analyses of the on-center handling data were performed. To better understand the data, and to insure statistical significance, an algebraic model of the analyzed data was developed, with confidence intervals determined. The calculated system stiffness, as determined from the steering wheel data, was found to be a key discriminator between steering gears. System stiffness is a function of several component values, which were measured in the laboratory. Finally, to test the above findings, a correlation study of subjective driver impressions with measured steering gear characteristics and objective vehicle measures was performed.

A prototype active cab suspension has been built and evaluated. As the system uses low bandwidth valves and is powered by the existing power steering system, it is perhaps a good example of expected performance from a production active control system. System architecture is described. The control system is presented, and system performance is reported.