RESEARCH ANALYSIS: Review of electronic braking systems
Baffling terms such as ABS, ASR, BAS, EBD, EMD, ESP and TCS bound back and forth in brake engineering circles. But it's a foreign language to many. Do drivers really understand this braking technology, let alone know how to use it to best effect? Matthew Beecham releases just-auto's annual review of electronic braking technology.
While ABS prevents vehicle wheels from locking during braking, and traction control stops the wheels from spinning on acceleration, ESP reduces the risk of skidding in all traffic situations. Although ESP is already standard equipment on many luxury cars, an increasing number of mid-range and small-cars are being fitted with the unit as optional fitment. ABS provides a case study in how advanced electronic systems can rapidly filter down to mass-market vehicle segments. When Bosch produced the first ever ABS in 1978, it weighed in at just over 6 kilograms. Today, Bosch offers ABS units weighing 1.4 kilograms. The latest ABS technology also operates considerably faster and offers greater levels of safety.
The next frontier
Bosch has described vehicle safety as the 'next frontier for automotive electronics'. "Our premise is simple - vehicle safety is the next frontier for automotive electronics," said Jason Forcier, president North America, automotive electronics, Robert Bosch LLC. "Active safety technologies like forward collision warning, predictive braking and lane departure warning are helping to improve a driver's safety by completely avoiding or decreasing the severity of crashes." According to the US National Highway Traffic Safety Administration (NHTSA), vehicle-related fatalities in the US have remained constant for nearly a decade at approximately 40,000 deaths per year. Advancements in vehicle safety, designed to help reduce that alarming statistic, hinge on the research and development of advanced electronic technologies. "In addition to innovation, collaboration is essential for advancing safety," said Forcier. "The industry - automakers and suppliers - legislators, regulators and consumer advocacy groups must work in unison to achieve technology neutral legislation and a common understanding among consumers of the benefits of these safety technologies."
Yet are consumers really all that interested in braking technology? As long as their vehicle brakes in a manner they expect, surely they are not interested in how the technology actually works? "Consumers do appreciate the benefits of Electronic Stability Control (ESC) and other electronic brake technologies but are probably not very interested or knowledgeable in how this technology works," said Vince Austin, director, product planning for Global Braking Systems, TRW. "Consumers only appear to be interested in the benefits and reliability of these systems. As far as consumer preference for base braking goes, the traditional hydraulic system with booster/master cylinder has become so familiar in terms of brake feel that it is difficult to give consumers something that feels radically different. Thus the brake systems that are being developed that provide brake boost independent of engine vacuum cannot feel too artificial and require some sort of simulated brake feel that consumers are used to. The bottom line is they just want the brakes to stop the car effectively - and quickly if needed. But if the advantages of having electronic braking can be better understood - such as improved fuel economy in the case of regenerative systems - or the many advantages of being able to integrate with other chassis and driver assist systems for greater levels of vehicle comfort, control and safety, then a premium price might be possible."
Push button braking
A vehicle fitted with an electric parking brake puts an end to driving with the handbrake on or parked vehicles rolling away. In fact, electrically-operated parking brakes are becoming more common in new cars as manufacturers seek to save space and money by abandoning the traditional handbrake lever in the interior, plus the cables, linkages and mechanical adjusters beneath the car. Electric parking brakes (EPB) are typically operated by a button on the dashboard which operates an electric motor to actuate the brake cables.
There are two common types of EPB. Either a drive unit activates the cable, or an electric motor on each of the two rear-axle brake calipers applies the brake. In the second method, the cable and servo motor are no longer required. In addition to the hydraulic brake cable on the brake caliper, only an electric cable that transfers data and power is needed for connection to the separate control unit.
Five or six years ago, EPB systems were treated as independent technologies. Nowadays, it appears the theme is very much centred on integrated, combined electronic braking systems. "To comply with vehicle safety requirements, integrated systems are necessary," said Alan Botham, product development engineer, Dura Automotive. "Stand-alone systems used as back-up systems can be independent, but must share some integration to main vehicle networks. With DA [direct actuation] systems, braking management is more integrated, where the EPB function is only used for parking application and no dynamic application [normally managed through ABS system]. EPB systems can also be integrated with combined electronic braking systems."
So could light commercial vehicles use electric brakes? "Providing disc or drum-in-hat brakes are used, EPB system can be incorporated," added Botham. "Cable puller and DA systems would need to be specified to the obvious higher load requirements demanded of commercial vehicle GVW. Commercial delivery vehicles could benefit from EPB from a safety / roll-away aspect, although high stop-start cycles may be a limiting performance factor for some EPB's. Dura supplied EPB back-up system for Ford Transit for delivery vehicles."
Last March, TRW unveiled its integrated Electric Park Brake (EPBi) system, marking a further development of its EPB technology. EPBi removes the requirement for a separate electronic control unit due to its integration with the electronic stability control system. The company points out that its latest system is a fully integrated part of the braking system, which makes four wheel anti-lock braking functionality possible during emergency braking. EPB and EPBi includes additional functions such as Drive Away Assist, automatic hill hold and electronically controlled deceleration - features which are not available with conventional mechanical park brake systems. TRW launched its EPB in 2002 and now produces some 1.5 million units annually. TRW expects its EPBi system to enter production in 2011.
Braking by wire
Overall, the path towards brake-by-wire in the automotive industry has not been all that smooth. Presumably, brake-by-wire will only be successful if it can meet market needs, namely performance, quality and cost. Yet are OEM customers willing to pay extra given the current architecture? "First, it's important to clarify 'brake-by-wire' architectures as we know them today," said Austin. "Systems currently in the field are 'brake-by-wire' from a Brake Control perspective, but the wheel-end service brakes remain hydraulically actuated. TRW's first-to-market EPB is the closest product to an electric actuated wheel-end brake. There is no question there needs to be a value proposition for both the customer and the consumer if they are going to pay a premium for brake-by-wire. One area of potential interest to the customer and consumer would be reliability of the system. As brake NVH tends to be the number one reason consumers bring their cars to the dealership, if brake-by-wire could virtually eliminate these complaints, improve reliability and significantly cut warranty costs, this would be a possible advantage for customer and consumer alike."
Executives of Continental's Chassis & Safety division told us: "Vehicle manufacturers and system suppliers play an important role, with each contributing their own expertise. The vehicle manufacturer knows which requirements dominate the market and also knows the end consumer and where the demand lies. They are therefore willing to pay into new technologies. It is precisely now, when we expect incentives to buy new cars, for example driver assistance systems can add long-term value to new vehicles. Or, in plain terms: by installing driver assistance systems, a car manufacturer can show that it is different from its competitors."
In terms of tomorrow's electronic braking systems, Continental executives see further integration of functions and components as one of the key drivers in electronic brake systems. "Already state-of-the-art is the integration of several additional vehicle control functions such as our Full Speed Range ACC or the integration of inertial sensors like acceleration or yaw rate into the electronic brake system control unit. In the near future, the current separate electronics for the electric barking brake will be integrated. Safety functions in the scope of ContiGuard will lead to extensive functional integration into the future electronic brake system."
As far as emerging trends in electronic braking systems are concerned, Austin concludes that the increasing usage of direct injection, turbo-charging, diesel and hybrid engine architectures is resulting in a much lower availability of engine vacuum to assist with braking force. Therefore, the traditional booster/master cylinder configuration in many cases is no longer viable and a different solution must be presented. He said: "TRW already produces its Slip Control Boost stability control system that offers its own electro-hydraulic generated boost independent of engine vacuum - it is being used on General Motors full-size hybrids that offer the dual-mode hybrid system and works seamlessly with the regenerative braking system on these vehicles. In this case, it is a great blend of safety and fuel savings. We are also working on other solutions for the low vacuum challenge, such as using our premium ESC system with six piston pump that can build and deliver (via hydraulic boost) high pressure brake applies when demanded. This system can offer the brake pressure demand across a variety of vehicle architectures without the need to significantly redesign the brake system."