Shorter braking distance with new research

Friday, December 19, 2014

Researchers at KTH have shown that the braking distance of an electrified vehicle can be shortened considerably by raising and lowering the chassis while braking. The research results were presented recently in a doctoral thesis by Johannes Edrén. The discovery represents a leap forward for the safety of future cars.

Johannes EdrénJohannes Edrén, PhD in vehicle dynamics, has had safety and energy efficiency as a basis for his research. He and his colleagues have made the discovery that the braking distance can be shortened considerably even at high speed, if the tire friction is increased by using a height-adjustable chassis. For a car driving at 100 km/h, the braking distance can be reduced by approximately one meter, shows Johannes Edrén in his doctoral thesis.

– When talking about shortening of braking distances, the improvements are often made in centimetres, so this really is a great step forward, says Johannes Edrén.

Height-adjustable chassis increases friction
The study was performed on electric cars with over-actuation, a technical solution where the vehicle has a larger number of actuators at each wheel than is needed to control a certain number of degrees of freedom. The technique is used for example for individual wheel motors and individual steering. Johannes Edrén and his colleagues have investigated the possibilities of raising and lowering the chassis when braking.

– During the time before the brakes fully engage, you lower the chassis. When the centre of gravity is lowered, the load on the front axle is reduced. Thus the load is distributed more evenly and the total friction is improved, explains Johannes Edrén. When the brakes are fully applied, the lowering of the chassis stops, which increases the load on the wheels even more so that you can brake extra hard. At the end of the deceleration, we use the car’s ability to raise the chassis again. At this point, the load on the wheels can be increased, so that the braking force is enhanced further.

A technique particularly suitable for electrified cars
Previous attempts to increase the friction force for braking have usually been made with ABS brakes. The potentials of active chassis and numerical optimisation now enabled the researchers to investigate whether it was possible to improve the braking performance further.

– With electrified powertrains you can have more compact actuators, and a greater number, for individual wheel steering and individual propulsion. Of course you can apply the same type of control on a standard vehicle with only a combustion engine as the power source. But the task becomes much more complicated. Over-actuation is a technical solution that is particularly well suited for electrified vehicles, says Johannes Edrén.

Several studies within the project
Within the scope of the research project, Johannes Edrén has also studied how over-actuation with individual propulsion and steering can be used to reduce the cornering resistance of electrified vehicles, and how much you can win in safety with active propulsion and steering. Studies on active camber control have also been included in the project. To investigate the effects of actuator limitations and response times, Johannes Edrén has constructed a scaled-down vehicle prototype.

The research results have recently been published in several scientific articles, as well as in Johannes Edréns doctoral thesi

More information
Johannes Edrén defended his thesis Motion modelling and control strategies of over-actuated vehicles at KTH on 3 December 2014.

Read the thesis >>

Johannes Edrén
Phone: +46 70-4474685

Participants in the project: Johannes Edrén, Annika Stensson Trigell (project leader), Jenny Jerrelind, Lars Drugge, Mats Jonasson

The research project “Generic vehicle motion modeling and control for enhanced driving dynamics and energy management” has been part of the Swedish Hybrid Vehicle Centre.

Text and photo: Emilia Lundgren