Aleksander Hac

Estimation of vehicle side slip angle and yaw rate

An algorithm for estimation of vehicle yaw rate and side slip angle using steering wheel angle, wheel speed, and lateral acceleration sensors is proposed. It is intended for application in vehicle stability enhancement systems, which use controlled brakes or steering. The algorithm first generates two initial estimates of yaw rate from wheel speeds and from lateral acceleration. A new estimate is subsequently calculated as a weighted average of the two initial ones, with the weights proportional to confidence levels in each estimate. This preliminary estimate is fed into a closed loop nonlinear observer, which generates the final estimate of yaw rate along with estimates of lateral velocity and side slip angle. Parameters of the observer depend on the estimated surface coefficient of adhesion, thus providing adaptation to changes in road surface coefficient of adhesion. Performance of the algorithm is evaluated using vehicle test data involving representative maneuvers performed on various road surfaces.

Rollover Stability Index Including Effects of Suspension Design

With the growing popularity of vehicles with high centers of gravity, the evaluation of rollover propensity of these vehicles becomes an issue of increasing importance. This chapter on a rollover stability index is from a comprehensive textbook on occupant and vehicle responses in rollovers. The author proposes a simple model to predict vehicle propensity to rollover; the model includes the effects of suspension and tire compliance. The model uses only a few parameters, most of which are known at the design stage. The author compares the lateral accelerations at the rollover threshold that are predicted by the model to the results of simulations. In the simulations, vehicles with the same static stability factor, but with different suspension characteristics and payloads, are subjected to roll-inducing handling maneuvers. The results of simulations correlate well with the predictions based on the proposed model. An analytical expression for the optimal roll center height from the viewpoint of rollover resistance was developed.

Stability and Control Considerations of Vehicle-Trailer Combination

In this paper, dynamics and stability of an articulated vehicle in the yaw plane are examined through analysis, simulations, and vehicle testing. Control of a vehicletrailer combination using active braking of the towing vehicle is discussed. A linear analytical model describing lateral and yaw motions of a vehicle-trailer combination is used to study the effects of parameter variations of the trailer on the dynamic stability of the system and limitations of different control strategies. The results predicted by the analytical model are confirmed by testing using a vehicle with a trailer in several configurations. Design of the trailer makes it possible to vary several critical parameters of the trailer. The test data for vehicle with trailer in different configurations is used to validate the detailed non-linear simulation model of the vehicle-trailer combination. Analysis and the simulation model are used to examine the advantages and limitations of two active brake control methods: the uniform braking and the direct yaw moment (DYM) control of the towing vehicle.

Influence of Active Chassis Systems on Vehicle Propensity to Maneuver-Induced Rollovers

With the growing popularity of vehicles with high centers of gravity, the evaluation of rollover propensity of these vehicles becomes an issue of increasing importance. This chapter on the influence of active chassis systems on vehicle propensity to maneuver-induced rollovers is from a comprehensive textbook on occupant and vehicle responses in rollovers. The author considers the influence of three presently-available controlled chassis systems on vehicle rollover resistance. The systems are the active rear steer (ARS), the brake based vehicle stability enhancement system (VSE), and the active roll bar, referred to as dynamic body control (DBC) system. A 16 degree-of-freedom computer model of a full vehicle is used for the study. The maneuvers used in the simulation are the double lane change and the fishhook maneuvers, with increasing steering amplitudes. The results show that the uncontrolled vehicle rolls over in both maneuvers when the steering angle is sufficiently large. The author concluded that all three systems improved vehicle resistance to rollovers, but DBC system was the least effective (primarily due to insufficient speed of response, limited by the power of the hydraulic pump). The VSE system improved vehicle stability more than the ARS system did, and the vehicle with both of these control systems was the most stable of all.

Improvements in vehicle handling through integrated control of chassis systems

In this paper methods of improving vehicle stability and emergency handling using electronically controlled chassis systems are discussed. By analysing a simple nonlinear vehicle model in the yaw plane, it is shown that vehicles can become unstable during portions of handling manoeuvres performed at or close to the limit of adhesion. It is further demonstrated how small changes in the balance of tyre forces between front and rear axles may affect vehicle yaw moment and stability. The methods of effecting vehicle yaw dynamics using controllable brakes, steering, and suspension are discussed. Control authority of each chassis system, in terms of its ability to generate a corrective yaw moment, is evaluated and is shown to depend on the operating point of vehicle and tyres. Consequently, regions of effectiveness of each subsystem are defined, which is a prerequisite for development of integrated chassis control systems. Preliminary test results for a vehicle with integrated closed loop control of brakes and suspension, performing typical handling manoeuvres, are presented. They demonstrate the benefits of integrated control in terms of improved handling response, stability, and reduced driver steering effort.

Velocity Distributions of C Atoms in CO+ Dissociative Recombination: Implications for Photochemical Escape of C from Mars

We have carried out Monte Carlo calculations to determine the velocity distributions of C atoms produced by dissociative recombination of CO + using recent data for the branching ratios of various allowed channels and ion and electron temperatures appropriate to the Martian thermosphere. We find that the fractions of 12 C atoms with velocities greater than the escape velocity are ∼ 0.66 and ∼ 0.62, and those for 13 C are ∼ 0.47 and ∼ 0.48 at the plasma temperatures characteristic of the exobases at low and high solar activities, respectively. The ratio of the escape fractions of 13 C and 12 C, which is a measure of the isotope effect inherent in the CO + dissociative recombination mechanism, is thus in the range 0.72-0.77. Using model thermospheres and ionospheres for low solar activity from Viking, and for high solar activity from the Mars Thermospheric General Circulation Model, we have computed the global escape flux of C due to dissociative recombination of CO + as (3 - 5) x 10 5 cm -2 s -1 . This value is of the same order as current estimates for the escape flux of C in all forms due to sputtering by O + pickup ions at the current epoch but may be much less than that due to sputtering at earlier times in the history of the planet.

Elimination of Limit Cycles Due to Signal Estimation in Semi-Active Suspensions

Abstract Undesirable low frequency vibrations experienced on vehicles equipped with semi-active suspension are observed: when road inputs have significant repetitive component, when there are large control gains in the suspension control algorithm, and when the velocities of the body are estimated by filtering the measured suspension deflections. Stability analysis performed on simple vehicle models demonstrates that interaction between the dynamics of the vehicle and the estimation filter is the root cause of the problem. A vehicle with an active suspension becomes unstable while a vehicle with a semi-active suspension exhibits limit cycles when the above mentioned conditions are present. A solution is proposed in which the control gains are adaptively changed depending on the frequency content of the road spectrum.