Reducing Braking Distance by Control of Semi-Active Suspension

Dissertation / Doktorarbeit von Tobias Niemz

Abstract:

This thesis presents a control algorithm for semi-active suspensions to reduce the braking distance of passenger cars. Active shock absorbers are controlled and used to influence the vertical dynamics during ABS-controlled full braking. In today’s series cars the active shock absorbers are switched to a passive damping - usually hard damping - during ABS-braking. Several approaches to reduce oscillations of vertical dynamic tire forces are known, implemented and some of them tested in non-braking situations (refer to Yi, Valàšek, and Nouillant).

The approach presented in this paper goes a step further by connecting the vertical with the longitudinal dynamics. To influence the vertical dynamics a switching control logic, called MiniMax-controller, is used. It is named after the fact that it changes only from soft to hard damping and vice versa. A control quantity was identified that connects the vertical dynamics with the longitudinal dynamics: the integral of dynamic wheel load. The control algorithm is implemented in a compact class passenger car. Simulations with a quarter-car model have been undertaken as well as tests on a 4-post-test rig, driving tests with defined excitations (like defined obstacles), and test drives on a real road, using a braking machine for reproducibility reasons.

It could be shown that it is possible to reduce the braking distance by affecting on the vertical dynamics of a passenger car in general. The amount of reduction depends on the elevation profile of the chosen testing track and on the initial velocity. On a road with an unevenness comparable to the one that is found on a typical German Autobahn, a reduction of typically 1-2%, compared to the best passive damping, was achieved.

Table of Contents:

Text Sample:

Chapter 7, Summery:

When it comes to the design of a suspension system, a classic conflict needs to be solved: One fixed set of suspension parameters may lead to the best result with respect to handling performance for one given driving maneuver. Changing the type of maneuver, however, changes the demand on the suspension and therefore leads to a different solution for the optimal set of suspension parameters. Such a change may take place even during the course of a driving maneuver. To treat this conflict in a better manner, semi-active suspension systems can be used.

This thesis presents a control algorithm for a semi-active suspension system with the objective to reduce the braking distance of passenger cars. Active shock absorbers are controlled and used to influence the vertical dynamics during ABS-controlled full braking. This thesis’ objective is to determine if it is possible to affect the longitudinal dynamics in an aimed way under close to reality conditions by means of controlling the vertical dynamics. Results of previous research work by Reichel already suggested that this could be possible. He showed for a braking maneuver with constant velocity (front axle braking, rear axle powering) and for a specific obstacle that the longitudinal dynamics can in fact be influenced positively.

For a standard tire in standard conditions (dry or wet asphalt) the maximum braking force is applied to the ground at a braking slip level that lies between zero and one. For slip equal to zero no braking force is applied at all, for slip equal to one the braking force generally is smaller than at its maximum. Furthermore a today’s ABS-controller is not able to keep the braking slip at the optimal level during the whole braking process, because the braking slip is not only controlled by the braking torque but also by the actual wheel load—a quantity that cannot be influenced by the ABS-controller. The ABS-controller can only react on whatever happens in the wheel’s vertical direction. Due to those facts there is still a gap between optimal braking distance and realized braking distance. This is why it is possible to increase the average braking force and to decrease the braking distance at all.

The approach presented in this thesis makes use of a switching control logic, called MiniMax-controller. It is named after the fact that it changes the active shock absorbers’ setting only from soft to hard damping and vice versa. The damper settings between those extrema are no selectable states for the controller. In test rig trials on a 4-post test rig it is shown that by switching the shock absorbers it is possible to purposefully increase or decrease the wheel load in a time and magnitude frame that is utilizable for braking procedures. The same holds true for the braking force and the braking slip. Increasing and decreasing in this context always refers to the course of the respective quantity that would have been established if the shock absorber had not been switched. Hence, the active shock absorber can be treated as a quasi-active element. The energy of those elements mostly comes from the potential energy of the body.

The effect that is caused by switching the active shock absorber takes place after a certain amount of time. The wheel load does not change its value immediately, but it rather takes approximately 25ms for the wheel load to increase or decrease. After this period of time the wheel load changes its former value continuously. It is shown that the wheel load’s integral with respect to time is connected to the braking slip. A short drop of wheel load does not affect the braking slip, because it takes time for the wheel to change its angular velocity due to its mass moment of inertia. If the drop of wheel load holds on for a longer time, which will also lead to a drop of the integral of wheel load, the braking slip will increase. At the same time a rising integral of wheel load lowers the braking slip. This is shown in braked test drives. Hence, a connection between the vertical and the longitudinal dynamics of a vehicle is established.

With this knowledge gained it is possible to control the active shock absorbers in a way that the wheel load is increased - and with it the integral of wheel load - at the right time. The right time is measured in terms of an integral of wheel load that is too negative. By switching the shock absorber, this integral and with it the wheel velocity is increased. Thus, the braking slip is lowered and kept at a level where the tire can transmit the maximum braking force.

Applying the switching logic to a real car and defining a setting for test drives that allows to determine the braking distance in a high accuracy, it could be shown that, generally speaking, it is possible to reduce the braking distance by affecting on the vertical dynamics of a passenger car. On a road with an unevenness like a typical German autobahn and for an initial velocity of 70 km/h it is possible to reduce the braking distance by an average of 1.3% compared to the best passive damping. The reduction is statistically significant. Furthermore, the integral of the square of the longitudinal velocity with respect to the traveled distance, which is a measure for the probability of a high damage in case of a crash, could be reduced as well, and at the same time as the braking distance was reduced.

Uncertainties in the determination of the proper switching time in a real car cause high deviations of those results. Not for every single braking process is it possible to obtain a shorter braking distance by controlled shock absorbers than with a constant setting. But the mean value for controlled damping is smaller and the standard deviation could also be decreased. The same control algorithm as it is used in this case could be applied to other situations, as for example braking in curves or enhancing the performance of the ESP-controller.

All the results of this thesis were gained by using the longitudinal and the vertical controller - namely the ABS- and the active shock absorber-controller - at the same time but independently from each other. Since the positive effect of the switching of the active shock absorbers is always followed by a negative effect, the full effectiveness of a controller of the vertical dynamics in terms of shorter braking distance can only be gained if the controller interacts with the ABS-controller. It is therefore essential to combine both strategies, to let the ABS know about what the active shock absorber will do in the next time step and vice versa. The results of this thesis not only let expect an improvement of braking performance but also an improvement of any other kind of controller of horizontal tire forces by a combination of vertical and horizontal control strategies - which is yet another step on the way to a global chassis control that includes every chassis control functionality.