Friction dynamics in automotive braking operations and comparison of control techniques effectiveness

Stricter environmental regulations are driving vehicle development toward improved safety, comfort, and sustainability. Recent studies highlighted the significant contribution of brake and tire wear to overall vehicle emissions. To mitigate this, regenerative braking has been adopted in pure electric and hybrid electric vehicles, reducing fuel consumption, emissions, and brake wear. However, traditional brakes remain necessary for low-speed and emergency braking, introducing new challenges. This paper presents a comprehensive vehicle model featuring an advanced braking system, including: an electro-hydraulic actuator with fluid compressibility and thermal and rheological effects; a lumped capacitance model for disc temperature; a friction coefficient model dependent on contact temperature, pressure, and sliding speed. The drag torque effect, caused by the pads not completely separating from the disc at the end of braking phase is also considered. For the tribological analysis, detailed models were implemented. Several wheel slip controls were also compared to identify optimal strategies. Simulations show that using variable friction coefficient (CoF) yields more realistic, albeit harsher, braking conditions, increasing pressure, stress, and wear, yet preserving vehicle stability. Only with variable CoF the brake fading is detectable. First Order Sliding Mode Control (FOSM) and Second Order Sliding Mode Control (Second Order SMC) proved more efficient and robust than the Proportional-Integral controller.