Brake-by-wire technology in the automotive industry is the ability to control brakes through electronic means, without a mechanical connection that transfers force to the physical braking system from a driver input apparatus such as a pedal or lever.
The three main types of brake-by-wire systems are: electronic parking brakes which have, since the turn of the 21st century, become more common; electro-hydraulic brakes (EHB) which can be implemented alongside legacy hydraulic brakes and as of 2020 have found small-scale usage in the automotive industry; and electro-mechanical brakes (EMB) that use no hydraulic fluid, which as of 2020 have yet to be successfully introduced in production vehicles.
Electro-hydraulic braking systems control or boost the pressure applied to the hydraulic pumps through the brake pedal. Safety requires that the system remains fail-operational in the event of a power failure or an electronic software or hardware fault. Traditionally this has been achieved by means of a mechanical linkage between the brake pedal and the brake master cylinder. With a mechanical linkage, the braking system still operates hydraulically via the pedal, whether or not electrical control is present. EHBs can be implemented by-wire, without legacy hydraulic systems and mechanical connections. In such a case, fail-operational redundancy is implemented, allowing the vehicle to brake even if some of the brake systems fail.
Electro-mechanical brakes offer the advantage of reduced braking system volume and weight, less maintenance, easier compatibility with active safety control systems, and absence of toxic braking fluid. Their novel actuation methods such as wedge brakes have kept them, as of 2020, from successfully being introduced in production vehicles.
Since by-wire systems have no mechanical linkages that would provide manual control over the brakes, they require fail-operational redundancy as specified by the ISO 26262 standard level D. Redundant power supplies, sensors, and communication networks are required.
Implementations
Prevalence
Some x-by-wire technologies have been already installed on commercial vehicles such as steer-by-wire, and throttle-by-wire. Brake-by-wire technology has been widely commercialized with the introduction of Battery Electric Vehicles and Hybrid Vehicles. The most widely used application by Toyota in the high volume Prius was preceded by the GM EV1, the Rav4 EV, and other EVs where the technology is required for regenerative braking. Ford, General Motors, and most other manufacturers use the same general design, with the exception of Honda, who designed a notably different design.
Brake-by-wire is used in most common hybrid and electric vehicles produced since 1998 including all Toyota, Ford, and General Motors Electric and hybrid models. The Toyota Synergy Drive and the Rav4 EV use a system where a modified ABS (antilock brake system) actuator is coupled with a special hydraulic brake master cylinder to create a hydraulic system, coupled with the brake control unit (computer). Ford's system is almost identical to the Toyota system, and the General Motors system uses different nomenclature for components while the operation is virtually identical.
Electro-hydraulic brakes
The hydraulic force generated by pressing the brake pedal is used only as a sensor input to the computer unless a catastrophic failure occurs including a loss of 12-volt electrical power. The brake actuator has an electric pump that provides the hydraulic pressure for the system, and valves to pressurize each wheel caliper to apply the friction brake when required by the system.
The system includes all of the complexity of a vehicle stability control system (VSC), antilock brake system (ABS), and the requirement to use the Regeneration Braking as the primary mode of slowing the vehicle unless the Traction Battery (high-voltage battery) state of charge is too high to accept the additional energy or a panic stop or ABS situation is detected by the system.
The sensors monitored as inputs for the brake system include the wheel speed sensors, traction battery state of charge, yaw sensor, brake pedal, stroke sensor, steering wheel angle, hydraulic actuator pressure, hydraulic pressures of each wheel caliper circuit, and accelerator position. Other information and inputs are also monitored.