The safety of motor vehicles is of primary concern in the modern age as the death rate of road users are still at unacceptably high numbers and is the second largest cause for unnatural death worldwide. Consumers often expect unrealistic performance and comfort levels from their vehicles regardless of terrain or conditions, and the Sport Utility Vehicle class is often under the most pressure to meet these high expectations.
Literature reveals that the performance of Anti-lock Braking Systems (ABS) deteriorates on rough off-road terrains due to a number of factors such as axle oscillations, wheel speed fluctuations and deficiencies in the algorithms. This leads to complications such as loss of vertical contact between the tyres and the terrain and poor contact patch generation that eventually results in reduced longitudinal force generation.
In this study, an ABS modulator is retrofitted on a test vehicle to perform brake pressure control. The hydraulic modulator is controlled by an embedded computer, running the Linux operating system, onto which a slightly modified version of the Bosch ABS algorithm is coded in C-language. Brake tests are conducted with the vehicle on hard concrete terrains for both smooth roads and rough Belgian paving. The algorithm is also implemented in Matlab/Simulink using co-simulation with a validated non-linear full vehicle ADAMS model employing a validated FTire tyre model. The co-simulation model was validated with the test data on both flat and rough terrains and experimental results correlate well with simulation results when the recorded brake pressures from the test data are given as input to the simulation model.
Test data and simulation results indicate that wheel speed fluctuations can cause inaccuracies in the estimation of vehicle velocity and excessive noise on the derived rotational acceleration values. This leads to inaccurate longitudinal slip calculation and poor control decisions respectively. Although possible solutions to the identified problem are not explored in detail, the developed simulation model and test vehicle can be used to test improved ABS algorithms and suspension control strategies to solve the deterioration of ABS performance on rough terrain.
Dissertation (MEng)--University of Pretoria, 2016.