With the rapid advancement of electrification and intelligence in the automotive industry, electric cars are undergoing significant transformations, particularly in braking systems. These systems are evolving from partial to full brake-by-wire configurations, playing a pivotal role in the intelligent chassis control of electric cars. As a foundation for autonomous vehicle braking control, brake-by-wire systems utilize electronic hydraulic, electronic pneumatic, or fully electric methods to apply intelligent braking torque to wheels, enabling precise deceleration, stopping, and parking. The growing demand for vehicle intelligence has heightened requirements for braking systems, making redundant backup functions increasingly critical. This article explores the current state and future prospects of redundancy in electric car brake-by-wire systems, focusing on Electronic Hydraulic Brake (EHB) and Electronic Mechanical Brake (EMB) systems. The rise of China’s EV market underscores the importance of these technologies, as electric cars become more integrated into global automotive landscapes.
Electronic Hydraulic Brake (EHB) systems build upon traditional hydraulic braking by replacing mechanical components with electronic elements, using brake fluid as the power transmission medium. These systems feature centralized control units and execution mechanisms, retaining hydraulic backup for enhanced safety. EHB systems offer advantages such as short braking response times, rapid pressure buildup, and precise braking force control, making them suitable for modern electric cars. They can be classified based on coupling degree between the brake pedal and wheel cylinder into non-decoupled, semi-decoupled, and fully decoupled types. Additionally, they are categorized by hydraulic pressure buildup methods into push-fluid direct pressure and boost-assisted pressure, and by integration level into “Two-box” and “One-box” forms. The following table summarizes the key characteristics of different EHB types, highlighting their relevance to electric cars and the evolving China EV sector.
| Type | Structural Features | Coupling Degree | Advantages | Disadvantages |
|---|---|---|---|---|
| Two-box | Electronic Booster + ESC | Non-decoupled/Semi-decoupled | Natural redundant backup | Larger volume |
| One-box | Integrated EHB System | Fully decoupled | Small volume, light weight, high integration | No redundant backup in single unit |
In Two-box EHB systems, the electronic power assistance system works alongside a vehicle ESC or ABS dual control unit, replacing traditional vacuum assistance. For instance, Bosch’s iBooster + ESP®HEV solution exemplifies this approach, where the iBooster electronic booster and ESP®HEV collaborate to provide active pressure buildup and mutual backup. In manual driving scenarios, ESP®HEV serves as a backup to iBooster, while in autonomous modes, they function as redundant counterparts for brake-by-wire operations. Similarly, Asia-Pacific Mechanical’s IBS system employs a semi-decoupled design with a build-up unit and ESC acting as mutual backups, ensuring reliability for electric cars. The braking force in such hydraulic systems can be described by the formula: $$ F_b = P \cdot A $$ where $F_b$ is the braking force, $P$ is the hydraulic pressure, and $A$ is the effective area of the brake caliper. This principle is fundamental to EHB systems in electric cars, contributing to their efficiency in China’s EV applications. The table below compares various Two-box solutions, emphasizing their redundancy features for electric car safety.
| Supplier/Product | Structural Features | Product Characteristics |
|---|---|---|
| Bosch iBooster+ESP®HEV | iBooster+ESP®HEV | Mutual backup, active pressure buildup, meets L2 and above autonomous driving requirements for electric cars |
| Asia-Pacific Mechanical IBS | Execution Unit + Build-up Unit | Semi-decoupled form, build-up unit and ESC mutual backup, suitable for China EV markets |
One-box EHB systems integrate the vehicle stability system and electronic brake assistance into a single controller, offering higher integration and compactness. Prominent examples include Bosch’s IPB system, Continental’s MK-C1+HBE, and other integrated solutions. Bosch’s IPB eliminates traditional components like the vacuum pump and ESP, combining the electronic brake booster and ESP®HEV into one unit with full decoupling from the brake pedal. In failure scenarios, One-box systems rely on pure mechanical braking or EPB backups, but for L3 and above autonomous electric cars, additional redundancy like a Redundant Brake Unit (RBU) is essential. Bosch’s IPB+RBU solution employs a two-in, two-out circuit design, providing independent redundant braking, while Continental’s MK-C1+RBU positions the RBU between the wheel end and MK-C1 for active pressure control post-failure. The pressure in these systems can be modeled as: $$ P = \frac{F}{A} $$ where $P$ is the pressure, $F$ is the applied force, and $A$ is the area, which is crucial for understanding braking dynamics in electric cars. The following table outlines One-box redundancy schemes, reflecting innovations in China’s EV industry.
| Supplier/Product | Structural Features | Product Characteristics |
|---|---|---|
| Bosch IPB+RBU | Two-in two-out, two circuits | Fully decoupled, independent redundant backup, meets L3 and above levels for electric cars |
| Continental MK-C1+RBU | RBU located between wheel end and MK-C1 | Active pressure buildup and pressure control after failure, with low-select ABS function, ideal for China EV safety |
| Other EHB+EMB, four-wheel independent braking | Four-wheel independent braking mutual redundancy, drive motor regenerative braking | Mutual redundancy between four wheels, regenerative braking by drive motor, enhancing electric car efficiency |
Electronic Mechanical Brake (EMB) systems represent a fully electric approach by completely eliminating hydraulic components such as the brake assistance mechanism, master cylinder, ESC, and pipelines. With simpler structures and independent electric motor brakes for each wheel, EMB systems allow for mutual redundancy among the four brakes. Each EMB actuator consists of a power unit and brake caliper block, controlled by an EMB controller that processes signals from sensors like pedal displacement and wheel speed. Compared to hydraulic braking, EMB offers faster response times, more precise braking force control, and reduced maintenance costs due to the absence of brake fluid, making it a promising technology for electric cars. The braking force in EMB systems is derived from motor torque: $$ F_b = \frac{T}{r} $$ where $F_b$ is the braking force, $T$ is the motor torque, and $r$ is the effective radius of the brake disc. This formula highlights the direct control achievable in EMB systems, which is beneficial for the precision required in China’s EV advancements.
Redundancy in EMB systems leverages the independence of each wheel’s braking mechanism. For example, Brembo’s brake-by-wire system includes a pedal simulator, brake control unit, hydraulic actuator, and electronic mechanical calipers, with front wheels using hydraulic calipers (similar to EHB) and rear wheels using EMB calipers. If the front brake control unit fails, the hydraulic circuit activates for backup braking on the front wheels, while the rear wheels remain operational. In cases of complete failure, mechanical braking via the pedal ensures emergency compliance. Great Wall Motors’ Jingong Feige EMB takes integration further by canceling traditional systems like ESP and EPB, incorporating four-wheel motor calipers with triple redundancy in power, sensors, controllers, and actuators. Even if all redundancies fail, kinetic energy recovery provides deceleration, showcasing innovative safety for electric cars. The table below compares EMB redundancy approaches, emphasizing their role in the evolving China EV sector.
| Supplier/Product | Structural Features | Product Characteristics |
|---|---|---|
| Brembo EMB | Front wheel hydraulic calipers + Rear wheel EMB calipers | Fully decoupled, front wheel hydraulic redundant backup, suitable for electric car applications |
| Great Wall Jingong Feige EMB | Four-wheel independent EMB calipers | Four-wheel independent braking mutual redundancy, triple redundancy system, aligns with China EV safety standards |
In conclusion, the diversity in brake-by-wire systems leads to varied failure handling and control strategies. EHB systems are widely adopted in electric cars due to their inherent redundant backups and high safety, whereas EMB systems, though capable of redundancy, face challenges in widespread implementation due to technical limitations. Mastering the software, hardware, and redundant backup technologies is essential for developing efficient, safe, and reliable brake-by-wire systems. For L3 and above autonomous electric cars, which minimize driver intervention, enhanced braking redundancy is crucial, involving active pressure devices like ESP, iBooster, IBC, and MKC1. Future research should focus on fast fault detection, redundant mode control, longitudinal stability redundancy, steerable anti-lock redundancy, and regenerative braking redundancy. These advancements will drive the evolution of brake-by-wire systems, particularly in the context of China’s EV market, where electric cars are at the forefront of automotive innovation. Multi-level redundancy and control will undoubtedly be a focal point, ensuring that electric cars meet the highest safety and performance standards globally.