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Anti-lock braking systems (ABS) were first developed for aircraft in 1929, by the French automobile and aircraft
pioneer, Gabriel Voisin, as threshold braking on aeroplanes is nearly impossible. An early system was Dunlop's
Maxaret system, introduced in the 1950s and still in use on some aircraft models. These systems used a flywheel
and valve attached to the hydraulic line that fed the brake cylinders. The flywheel was attached to a drum that
ran at the same speed as the wheel. In normal braking the drum and flywheel would spin at the same speed. If the
wheel slowed suddenly the drum would do the same, leaving the flywheel spinning at a faster rate. This caused
the valve to open, allowing a small amount of brake fluid to bypass the master cylinder into a local reservoir,
lowering the pressure on the cylinder and releasing the brakes. In testing, a 30% improvement in braking
performance was noted, because the pilots immediately applied full brakes instead of slowly increasing pressure
in order to find the skid point. An additional benefit was the elimination of burned or burst tires.
A fully mechanical system saw limited automobile use in the 1960s in the Ferguson P99 racing car, the Jensen FF and the experimental all wheel drive Ford Zodiac, but saw no further use; the system proved expensive and, in automobile use, somewhat unreliable.
Chrysler, together with the Bendix Corporation, introduced a true computerized three-channel all-wheel antilock brake system called "Sure Brake" on the 1971 Imperial. It was available for several years thereafter, functioned as intended, and proved reliable. General Motors introduced the "Trackmaster" rear-wheel (only) ABS as an option on their Rear-wheel drive Cadillac models in 1971.
In 1975, Robert Bosch took over a European company called Teldix (contraction of Telefunken and Bendix) and all patents registered by this joint-venture and used this acquisition to build the base of the ABS system introduced on the market some years later. The German firms Bosch and Daimler-Benz had been co-developing anti-lock braking technology since the early 1970s, and introduced the first completely electronic 4-wheel multi-channel ABS system in trucks and the Mercedes-Benz S-Class in 1978. The Honda NSX was the first mass produced automobile with the modern 4-channel ABS system sold in the United States and Japan. It applies individual brake pressure to each of the four wheels. ABS is offered, or comes standard, on most road vehicles produced today and is the foundation for ESC systems.
ABS has undoubtedly saved thousands of lives over the decades, but there is evidence in Europe that its fitment resulted in drivers using the system to increase vehicle performance and driving faster in adverse conditions. The net result was to increase rather than decrease accident rates in a small number of markets such as Germany.
As distinct from ABS, which manages the brake system pressure in response to slippage by one or more wheels, EBS/EBD systems enable the brake force to be optimally controlled at each wheel, according to the level of grip available. In this way, EBS/EBD can manage braking forces more effectively than ABS by making better use of any grip available to individual wheels. EBS/EBD builds on existing ABS and ASR technology and includes Brake Assist (BA).
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Brake Assist systems assist in emergency braking by increasing the brake pressure, independently of how hard the
brake pedal is pressed, when the pedal is applied suddenly. The system employs the ABS or EBS system and applies
the pressure required to achieve optimal braking performance under the prevailing road conditions.
Brake Assist systems typically outperform the capabilities of unassisted drivers and are effective in reducing frontal accidents, particularly at higher speeds, where serious injuries and fatalities are more likely.
Development work is being carried out with predictive Brake Assist systems that use sensors to detect the sudden approach of the driver's foot towards the brake pedal and prime the pressure in the braking system so that the brake pads are in touch with the rotors. This reduces the brake system reaction time, saving valuable milliseconds in emergency situations. Predictive systems can be utilised as a component of Active Cruise Control (ACC) to prime the braking system if the ACC system detects a possible emergency situation.
Autonomous emergency braking systems minimise the response time for a vehicle to brake in collision imminent situations with moving or stopped vehicles, because the system reacts without intervention from the driver, ensuring the most effective deceleration in bringing the vehicle to a complete stop.
This is achieved by integrating diverse information about road traffic from video and laser distance sensors, processing this complex information using advanced algorithms that significantly enhance object dimensioning and enable reliable anticipation of imminent danger.
A typical system of this type designed for driver assistance in commercial vehicles is Wabco's AEB technology, which detects, analyses and reacts to moving as well as stationary vehicles as the system simultaneously interfaces with braking and other systems in the truck such as engine, transmission and stability control to assist the driver.
Brake-by-wire separates the mechanical hydraulic connection between brake pedal and wheel brake. Sensors determine the driver's braking command and transmit this information to an electronic control unit. Using the corresponding actuators, the control unit creates the required brake force clamping effect at the wheels. Brake-by-wire technology is currently being applied successfully to hybrid powertrain vehicles, where the stop-start action of the engine means that the system has considerable operational advantages over vacuum driven brake boosting technology. A Bosch developed system was first launched on conventional vehicles with the Mercedes-Benz E-Class in the form of an electro-hydraulic system, but reliability problems lead to the OEM moving back to traditional systems while remaining issues were being resolved.
Brake-by-wire in theory brings many advantages including improved response times and shorter stopping distances, reduced weight, better packaging, improvements in pad wear and fuel economy, and better safety systems - because there is no longer the need for troublesome holes in the engine bulkhead. Beyond these improvements it is a key enabling technology for for active safety systems by providing control of each wheel independently.
In September 2005, Siemens VDO (now part of Continental AG) launched its Electronic Wedge Brake (EWB), which is based on innovative technology developed by eStop, a firm which was acquired by the company in early 2005. Its control-related foundations originate from German Aviation and Aerospace Centre applications. During the braking operation a brake a brake pad attached to a wedge is pressed between the brake calliper and the brake disk. As the wheel turns the wedge effect is automatically intensified. This allows any level of braking power with a minimum of intricacy.
The EWB eliminates the need for components such as hydraulic pipes, brake cylinders, brake boosters or antilock braking control units. This lowers overall weight and makes for greater reliability for improved safety, with reduced servicing requirements. By doing away with the hydraulic braking system, it also helps to reduce the vehicle's environmental impact.
Finally, the EWB is not only for braking while the vehicle is moving, but can also be used as an automatic parking brake. This allows vehicle designers to do away with the traditional hand-pulled brake, while helping prevent the vehicle of the future against rolling away when parked.
There are a number of proprietary vehicle stability systems, including Bosch's electronic stability program (ESP) or Delphi's TRAXXAR, that are variations on a theme - they all aim to be a back-up for the driver and to take control of some of the vehicle's dynamics before and during emergency manoeuvres.
A skidding vehicle can be stabilised by specific braking of individual wheels, by reducing the engine torque and making steering corrections. Techniques to influence the brake system automatically or to reduce torque automatically have been in use for some time, but systems with automatic steering intervention are still in the development phase.
Such systems typically comprise a hydraulic modulator and a control unit with sensors, which determine and evaluate the driving situation. The hydraulics quickly build brake pressure on a single wheel in critical situations to counteract the undesired skidding motion. This pressure increase occurs automatically on the wheel, without driver involvement. The brake impulse can thereby stabilise the vehicle at all times and return it to the proper course. The system can also reduce the engine torque, which provides an added stabilisation effect.
The benefits of ESP for the driver include improved steerability in critical situations, reduced danger of slipping or skidding, greater vehicle stability within physical limits and optimised stopping distances.
Torque vectoring (fully variable) systems are a new generation of devices that force small speed differences to arise within the driveline, which, in conjunction with the tire characteristics, transfer torque. Although providing the highest authority over torque distribution, the cost and potential complexity may limit this technology to premium vehicle classes.
Torque vectoring systems electronically monitor and manage torque delivery to each wheel, enabling an active AWD system to operate like an ESC system by transferring torque to stabilise the vehicle. It operates through all three of the AWD differentials and has the advantage of being able to pre-empt actual wheel slippage by transferring the appropriate amount of torque required. If any of the wheels do slip, the system will transfer torque to any wheels that still have traction.
Currently available systems can deliver variable torque in response to electronic signals provided by one or more of the vehicle's traction and/or stability control systems and can work in conjunction with other systems including ABS, ASR and ESC. Active AWD systems have proven to be reliable and durable; they are lighter than conventional passive AWD systems and are less costly to produce.
The Ricardo Torque Vectoring™ module, already well-proven in a number of automotive applications, has the potential to significantly improve the steering response and stability of military vehicles in slippery and difficult terrain, improving safety and enhancing dynamics for drivers of all skill levels.
Ricardo has developed a unique differential unit incorporating its Torque Vectoring™ module, which can be used as a replacement for the conventional differential. This unit can be used to direct drive torque between 100% left, centre or 100% right wheel bias. Coupled to a Ricardo vehicle control system, this fast-acting device can be used for example to enhance stability by countering the steering request if roll-over is imminent, so reducing the likelihood of roll-over.
