Electromechanical Brakes (EMB) work differently compared to traditional hydraulic or pneumatic braking systems. (In a previous post, we’ve examined the different Brake-by-Wire constellations.) EMB replaces the hydraulic actuators with electronic systems to control the brake action. Here’s a high-level overview of how EMBs function:
- Brake Application: An electric signal is sent from the brake system’s electronic control unit (ECU) to activate an electric motor.
- Mechanical Actuation: The electric motor drives a mechanical actuator, typically a ball screw mechanism, which applies pressure to the brake pads or shoes.
- Reversal Behaviour: For fail-safe operations, EMBs can be designed to reverse the mechanical actuation. When the motor stops or reverses, the actuator retracts, releasing the brake.
Key components and functionalities of EMB include:
– Motor Gear Unit (MGU): Used in the second generation of EMB for a reversible mechanical design. The MGU will normally include the following key components;
Electric Motor: This drives the actuator mechanism.
– Ball Screw Mechanism: Converts rotational motion from the motor into linear motion to apply the braking force.
– Gearbox: In some designs, a gearbox is used to increase the torque.
– Electronic Control Unit (ECU): Controls the motor and other electronic components to ensure proper operation.
– Latched Systems: In some designs, a latch can be used to hold the brake in position without continuous power.
Advantages of EMBs include precise control, integration with electronic stability control systems, and potential enhancements in vehicle automation and safety systems. Benefits such as increased vehicle efficiency (from energy supply changes and zero drag callipers) and increased stability control are offered at a vehicle level. In a production scenario, removing the fluid fill requirement from the vehicle factory can reduce investment. EMB systems are a form of brake by wire, and share many of the advantages of such a system, but crucially, there is no hydraulic element, so can be distinguished as a dry brake system.
Fig.1 – EMB actuator
Safety of new vs old
Conventional (4-wheel) brake actuation should be considered as utilising lever and hydraulic ratios to amplify driver effort to braking effort. (2-wheelers may have hydraulic or cable systems, and levers may be hand or foot operated). In this most basic system, a tandem Master Cylinder is used to compress fluid in the Master Cylinder, and thereby raise fluid pressure equally across the hydraulic circuit. This high pressure is also present in the callipers, and is used to clamp pads onto discs or shoes onto drums. (Hydraulic proportioning may occur to differentiate pressure from front to rear, where modulation systems aren’t fitted). Addtionally, a booster may be used to further multiply the driver muscular effort, and allow for better braking. Contemporary boosters may be vacuum, electro-mechanical or possible hydraulic.
While these mechanical and pneumatic actuation systems are relatively low cost, they are increasing being replaced by integrated actuation and modulation units (a so-called 1-Box brake system), which offer additional functionality and efficiency, but respect the same basic safety concepts.
In our basic Master Cylinder, two hydraulic circuits are pressurised with a single input rod movement. This is such that a single circuit leak will leave at least 50% (>0.5g) of the braking performance while alerting the driver to the fault condition. Seperation chambers in the reservoir mean even if a single circuit completely drains, the remaining circuit is operational. With a booster in the actuation system, we have to consider how faults in the boost function can affect performance. By design, a fully unboosted system must provide >0.25g, using driver muscular effort only. If a fully functional system provides >1.0g, this is a significant fault. However, in practice, the modulation unit can react to a boost failure, and provide some redundancy for the booster, meaning a greater level of braking is available even with a boost fail scenario. And if we consider a 1-Box system, where a single energy source can mean loss of boost and modulation, Master Cylinder layout can be such that >0.4g is available with driver muscular effort.
So when we consider how to implement electro-mechanical brakes, we need to achieve performance and redundancy that is at least competitive with today’s solutions.
Fig. 2: EMB X-split architecture
The future might be autonomous
EMB systems developed in the last decade were primarily considered as a direct replacement for hydraulic braking systems, and initially thought to be co-incident with a high level of vehicle autonomy. As such, a lot of the necessary architecture was assumed already on the way, and EMB systems could benefit from this. (If you need a quick refresher on Autonomous Driving and Braking, look here).
Full EMB systems require redundancy in the power network to ensure reliability and fault tolerance. The power supply must ensure energy and power availability for the braking system, even after a fault, and rule out common cause failures. That is to say, the power supply must meet a minimum safety level. Finally, a distributed and fault-isolated power network, that’s capable of isolating faults to prevent undervoltage scenarios that can affect the brake system. The design and implementation should adhere to relevant safety standards such as ISO26262 and VDA450 to ensure a fail-safe operation.
Such a power system is something that a L4 or L5 autonomous vehicle would need for most of it’s primary systems, and so for an EMB system, it made sense to develop braking solutions around a highly autonomous vehicle architecture.
In detail, the fault tolerance approach of a system was decisively influenced by the need to have a fault-operational system, where the level of available braking performance was still very high with a single fault (>0.7g as a working assumption). Without any human (muscular) intervention available, the EMB system needs to be able to support safe onward travel.
The near future might not be autonomous
But while we’re waiting for widespread adoption of autonomous fleets, what can we do with EMB in the meantime? If we still have a driver in the loop, and a relatively slow growth of ADAS above L2, then to deploy EMB in the short term, another approach can be profitable. EMB callipers on 1 axle (a hybrid approach) only can bring a lot of the benefits of tomorrow’s systems with many of the security of today’s systems. Importantly, many of the safety and redundancy approaches we know from hydraulic systems can be replicated for this hybrid approach.
The hybrid architecture that makes most sense is a hydraulic front axle together with an electromechanical rear axle. This means that the larger actuation forces we want for the front brakes can still benefit from hydraulic ratios, and the rear axle actuation can combine service and park brake into a single hardware solution.
Electronic Park Brake solutions are several decades in service, and have increased market share significantly in that time. If we consider the jump from a contemporary EPB calliper to an EMB calliper, we can see a lot of system reuse. We still want a sliding fist layout, we still want a motor, gearbox, electronics and diagnostics elements. We want to improve the pad control, the power and modulation capability, the endurance and the efficiency of these elements. But we can dispense with all hydraulic elements, such as pressure seals, bleed ports, pipe and hose arrangements.
Next Gen safety requirements
Implementing a safety concept here needs only a single energy source to each calliper, (which is consistent with contemporary hydraulic architectures) and so this is a much lower barrier to entry for EMBs. However, such an approach does have implications for both operational and fault tolerant behaviours. In any brake design, a failure which results in the calliper binding the pad to the disc must be precluded. With EMB, a loss of power during a high clamp brake event will mean a reduction of clamp force, but may not mean a complete release of the brake. Careful attention needs to be paid to both the gearing design and the back EMF characteristics of the actuation system, such that the brake can fully unlock from any fault injection scenario. These fundamental hardware requirements are distinct from Gen 1 EMB systems, due to the differences in safety approaches that support their architecture.
Outlook
Considering where we stand in 2024, it is clear that technical concepts are well advanced in EMB. The regulatory pathway and safety mechanisms are well understood. Therefore, the technology is primed for release on vehicles in the second half of this decade. There are clear advantages to the customer and the OEM, but also important strategic opportunities for the supply base with EMB. Recent developments suggest a divergence in approaches for HGV and passanger vehicles, however. These divergent approaches will mean some specific hardware solutions are required for the different sectors, and may point to differing priorities for those development teams.