Traditional hydraulic braking systems haven’t changed too much in the last century. Enhancements along the way have included a move from drum brakes to disc brakes and the addition of Anti-Lock Braking Systems (ABS) and Electronic Stability Programs (ESP). More recent additions include regenerative braking and Autonomous Emergency Braking.
The phasing out of mechanical control systems to ones that are electronically controlled was first seen in the aerospace industry as part of Fly-by-Wire technology. Control systems for steering, throttle and brakes became entirely electrical, removing the use of mechanical components such as cables and rods. The engineers found that removing these electrical components and using electronic systems instead greatly improved reliability and were also more compact. The automotive industry has only adopted this method of electronically controlled systems in throttle-by-wire tech.
Within the automotive industry, there are three types of brake-by-wire solutions: Electro-hydraulic, fully electric, and a combination of the two. The electro-hydraulic system removes the need for a brake booster. It uses sensors and actuators to measure the amount of pressure applied to the pedal, transferring this force to all brakes from the master cylinder using hydraulic fluid and slave cylinders. Using control units and powerful actuators, the pressure exerted to the brakes can be much higher than in a traditional hydraulic braking system (2,000 psi vs 800 psi). This was used on some models of Mercedes-Benz within their Sensotronic Braking Control (SBC) system. SBC was short-lived, manufactured for four years within their mainstream vehicles e.g. E-Class (211) 2002-06. This was due to issues with the fail-safe (increased stopping distance and pedal effort without the brake booster assistance) and the fact that the hydro-electric control unit was an expensive serviceable item (replaced after a predetermined number of brake actuations were carried out), costing close to £1,000 just for the component!
A fully electric braking system replaces all hydraulic and mechanical components with electric ones. Control units and electronic actuators are attached to the brake calipers at each wheel. Information is sent to them from a central control unit via communication bus wires. Extra sensors at each caliper include temperature, clamp force and actuator position. If there are one or more failures of the caliber control units, the central unit can still maintain system functionality. Currently, the systems reaction time is 90 ms vs 300 mss for a traditional hydraulic braking system, which can lend itself towards the pursuit for semiautonomous and autonomous vehicles. As the actuators on the calipers are fully electric, the pistons can be positively retracted when not in use, causing less drag for the vehicle, improving fuel economy and reducing emissions. Image on the left is a Brembo rear brake with ECU and actuator on the caliper.
The third system is a blend of electro-hydraulic and fully electric systems. It has electro-hydraulic and fully electric systems.It has electro-hydraulic on the front axle and fully electric components on the rear axle. This system is in use on vehicles with larger font brakes, as they cannot fit the fully electric components directly on the front brake calipers. Traditional hydraulic systems have their flaws, but drivers like the fact they have a direct link from the pedal to all four brakes the event of failure. Although brake-by-wire systems are advanced, a separate battery supply from the main battery may also be required for fully electric systems as one of many fail-safes. We may have to wait a bit longer before these systems can meet international ‘Road Vehicle-Functional Safety’ regulations.
Autodata’s online workshop application provides technical information to service and repair both traditional and modern braking systems.
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