automotive-powertrain-chassis

Automotive Powertrain Chassis

Abs Brake Systems

ABS/EBD Anti-lock Braking System Skill

Overview

Expert skill in Anti-lock Braking System (ABS) and Electronic Brake-force Distribution (EBD) development. Covers wheel slip control, pressure modulation, EBD algorithms, brake-by-wire, regenerative braking coordination, and brake fade compensation.

Core Competencies

1. Wheel Slip Control

• Target Slip Ratio: 10-20% for optimal braking (peak μ on most surfaces)
• Slip Calculation: λ = (vwheel - vvehicle) / vvehicle
• Control Modes: Build-up, Hold, Release phases
• Surface Adaptation: Adjust control aggressiveness for dry/wet/ice

2. Pressure Modulation

• Hydraulic Valve Control: Inlet/outlet solenoids, return pump
• PWM Frequency: 10-20 Hz typical ABS cycling
• Pressure Estimation: No direct sensor, estimate from valve timing
• Pedal Feel: Minimize pulsation feedback to driver

3. Electronic Brake-force Distribution (EBD)

• Dynamic Load Transfer: Front/rear brake bias based on deceleration
• Proportioning: Prevent rear wheel lockup (stability)
• Empty vs Loaded: Adjust distribution based on vehicle weight
• Trailer Detection: Modify EBD for towing conditions

4. Regenerative Braking Coordination

• Blending Control: Seamless transition hydraulic ↔ regenerative
• Regen Priority: Max energy recovery while meeting brake demand
• Pedal Feel: Maintain consistent deceleration feel
• Safety Backup: Full hydraulic if electric system fails

5. Brake-by-Wire (BBW)

• Electro-Mechanical Brakes (EMB): Electric caliper actuators
• Decoupled Pedal: Pedal feel simulator, no direct hydraulic link
• Redundancy: Dual ECUs, backup hydraulic circuit
• Latency: <100ms brake request to force application

Control Algorithms

ABS Slip Control (Bang-Bang with Hysteresis)

typedef enum {
    ABS_BUILD,      // Increase pressure
    ABS_HOLD,       // Maintain pressure
    ABS_RELEASE     // Decrease pressure
} ABS_Phase_t;

typedef struct {
    float wheel_speed_mps;
    float vehicle_speed_mps;
    float slip_ratio;
    ABS_Phase_t phase;
    uint16_t cycle_count;
} ABS_WheelController_t;

void ABS_WheelControl(ABS_WheelController_t *abs, float dt) {
    // Calculate slip ratio
    if (abs->vehicle_speed_mps > 0.1f) {
        abs->slip_ratio = (abs->vehicle_speed_mps - abs->wheel_speed_mps) / abs->vehicle_speed_mps;
    } else {
        abs->slip_ratio = 0.0f;
    }

    // State machine for pressure modulation
    switch (abs->phase) {
    case ABS_BUILD:
        // Normal braking: increase pressure
        Hydraulic_OpenInletValve();
        Hydraulic_CloseOutletValve();

        // Transition to HOLD if slip exceeds threshold
        if (abs->slip_ratio > SLIP_THRESHOLD_HIGH) {  // 20%
            abs->phase = ABS_HOLD;
        }
        break;

    case ABS_HOLD:
        // Hold current pressure
        Hydraulic_CloseInletValve();
        Hydraulic_CloseOutletValve();

        // Transition to RELEASE if slip still increasing
        if (abs->slip_ratio > SLIP_THRESHOLD_CRITICAL) {  // 25%
            abs->phase = ABS_RELEASE;
        } else if (abs->slip_ratio < SLIP_THRESHOLD_LOW) {  // 15%
            abs->phase = ABS_BUILD;
        }
        break;

    case ABS_RELEASE:
        // Decrease pressure to reduce slip
        Hydraulic_CloseInletValve();
        Hydraulic_OpenOutletValve();
        Hydraulic_ActivateReturnPump();

        abs->cycle_count++;

        // Transition to BUILD when slip decreases
        if (abs->slip_ratio < SLIP_THRESHOLD_LOW) {
            abs->phase = ABS_BUILD;
        }
        break;
    }
}