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Electronic Brakes (Brake-By-Wire)

Copyright AA1Car

It seems that everything is wired these days. As technology advances, more and more mechanical systems have been replaced by electronics, and that includes the brakes. Electronics can provide significant improvements in many areas of vehicle control, including braking, but only if the electronics can provide trouble-free and reliable operation.

In August 2001, Mercedes introduced the world's first production brake-by-wire electronic brake system on the Mercedes-Benz SL500. Here's what Mercedes said about their new system:

"The company that invented ABS (Anti-Lock Brakes), traction control and ESP (Electronic Stability Control) is launching the world's first production car equipped with an electronic brake system, which provides faster, more sure-footed brake response, especially in emergencies.

"The brake pedal on the Mercedes-Benz SL500 works with a computer that tells four fast-acting valves exactly how hard to apply the brakes on each wheel. A backup hydraulic master cylinder comes into play only if there's a serious problem or electrical failure.

"With split-second accuracy, the system can change brake pressure on each wheel over uneven surfaces and can even increase brake pressure on just the outside wheels when braking in turns, taking advantage of the higher loading during cornering. Even ABS and ESP work more efficiently since they are more deeply integrated with the brakes instead of functioning as parallel systems.

"A special electrically-driven hydraulic pump and a high-pressure reservoir (or accumulator) provide "always-on" full brake pressure at each wheel. The computer-oriented modulator valves control how much pressure actually operates the brakes at each wheel. In the new system, the bulky vacuum brake booster is now obsolete.

"If the driver switches his foot quickly from accelerator to brake pedal, the electronic brake system recognizes the early signs of an emergency situation and reacts automatically. With the help of the high pressure reservoir, the system raises the pressure in the brake connectors and instantly moves the pads onto the brake disks, which can then spring into action with full force as soon as the brake pedal is depressed. At highway speeds, this pre-loading of the braking system reduces stopping distance by about three percent.

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"In addition, the system automatically senses when the road is wet and imperceptibly applies the brakes just enough to keep the disks dry, so that brake operation remains fast and consistent in the rain."

Sounds great, doesn't it? That was then. This is what happened after the car hit the road:

In 2004, Mercedes issued a huge recall of 680,000 Mercedes-Benz SL500 and E-Class vehicles equipped with the new "Sensotronic" electronic brake-by-wire system. The recall applied to SL-Class cars built since October 2001, and E-Class cars built since October 2002. It should be noted that only about 140,000 of the recalled vehicles were U.S. models, and that the recall was issued voluntarily by the auto maker as a "precaution" due to a few reported failures of the electronic brake system in unusually high mileage European taxi applications.

Details were sketchy about the cause of the failures. Some said there was a software glitch in the control module while others said the failures were caused by leaks or bubbles in the high pressure accumulator. Either way, the fault knocks the brake-by-wire system off-line. Fortunately, the backup hydraulic system is still able to stop the vehicle, but without the benefit of power assist.

Mercedes says the fix for the recall involved reprogramming the control module with new software. But in some cases hardware also had to be replaced.


Other Electronic Brake Systems

Bosch, Continental Teves, Delphi, TRW and other OEM supplier companies have all developed electronifc brake systems for potential use on future vehicles. These companies have invested millions of dollars in research and development of their new systems and have been working to find cusstomers for theier systems. But auto makers have been slow to accept the new technology over liability and reliability concerns. That's why most vehicles today still have conventional vacuum brake boosters and hydraulic brakes.

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Two Types of Electronic Braking Systems

Electronic brake systems fall into one of two categories: electro-hydraulic and electro-mechanical.

Electro-hydraulic brake-by-wire systems use conventional hydraulic brake calipers at each wheel, but pressure is applied by a computer-controlled high pressure pump and actuator solenoids. This is the approach used by Mercedes-Benz with their Bosch-built Sensotronic system. These kinds of brake-by-wire systems use inputs from a brake pedal position sensor (that works much like a throttle position sensor), wheel speed sensors, a steering angle sensor, yaw rate and lateral acceleration sensors to determine the optimum amount of brake pressure to apply at each wheel. The claimed benefits for this type of system are:

With electro-mechanical brake-by-wire systems, there are no hydraulics whatsoever. Braking force is generated at each wheel by a fully electronic caliper. Inside is a small, but powerful electric motor that pushes the pads against the rotor. Many of these systems work best with higher voltages (such as 42 volts) - which means electro-mechanical systems will probably remain on the shelf until auto makers decide whether or not changing to 42 volts is really necessary and worth the cost.

The benefits of electro-mechanical brake-by-wire are essentially the same as electro-hydraulic systems plus it eliminates brake fluid, hoses and lines, the need for a high pressure pump and accumulator, and provides improved braking safety by keeping three brakes operational should one caliper fail.

Being able to precisely control the amount of braking force at each wheel electronically also means a brake-by-wire system can shift more braking effort to the rear brakes during normal braking. This, in turn, can reduce front pad wear while reducing the forward weight shift and nose drive that normally occurs when the brakes are applied.


Electronic Brake Safety Concerns

The last thing any motorist wants to deal with is a finicky hi-tech electronic brake system that goes off-line while they are driving in heavy traffic. The brake system must be dependable and have back-up redundancy in case of a failure - otherwise there's no way to stop the vehicle. Since the late-1960s, all automotive brake systems have been split hydraulically into two separate circuits for safety. Should a caliper, wheel cylinder or hose fail in one circuit, the remaining circuit still has two brakes operational to stop the vehicle.

On most rear-wheel drive vehicles, the front brake circuit is separate from the rear brake circuit. If the front brakes fail, the vehicle still has the rear brakes to bring it to a halt. Plus there's a cable-operated mechanical parking brake ("emergency" brake) as a last resort should both hydraulic brake circuits fail.

On most front-wheel drive cars and minivans, the brake circuits are split diagonally with the left front and right rear brake sharing one circuit, and the right front and left rear brake sharing the other circuit. This is done so one front brake and one rear brake will remain operational if the other pair fail. On FWD vehicles, there is much more forward braking bias because the front wheels are also the drive wheels. Consequently, the rear brakes alone might not be sufficient to bring the vehicle to a halt if the front brakes shared the same circuit and happened to fail.

One of the primary goals of the engineers who are developing new electronic brake systems is to make electronic braking as safe and reliable, if not more so than conventional hydraulic braking systems. In this respect, an electro-mechanical system with redundant computer controls and a voltage supply may be safer than a conventional brake system.

Why? Because each wheel is on its own separate electrical circuit. If one caliper or circuit fails, it should not affect the other three wheels. This means the vehicle could still have three operational brakes (as opposed to two with today's hydraulic systems) in the event of a brake failure at one wheel.

The biggest gains in safety, though, are made possible when electronic brakes are integrated with ABS, traction control and stability control. As brake control strategies become more and more sophisticated, and use more and more sensor inputs, the brake system can better compensate for driver error and changing road conditions.

Brake-by-wire electronic brake systems can also be integrated with adaptive cruise control to provide automatic braking if a vehicle ahead suddenly stops and the driver fails to react quickly enough. It can also be combined with some type of crash avoidance system that is capable of detecting objects in the road ahead, or an oncoming vehicle,and apply the brakes before the driver can react.

All this will eventually come to pass. The question is how soon?

Brake technology Trends

Despite the Mercedes-Benz brake recall and slow growth of this technology, most experts predict electronic brake-by-wire systems will eventually become commonplace. As experience and confidence grows, so will the application of electronic brakes in new vehicle platforms.

Electronic Brake Assist

A growing number of vehicles today use electronic "brake assist" to decrease braking reaction times and reduce stopping distances in emergency situations. Most of these systems use boost pressure created by the ABS pump to precharge the brakes, making them ready to apply in less time when rapid braking is required. SOme of these systems precharge the brakes as soon as the driver lifts his foot off the throttle.

When some of these braking systems detect an abnormally fast application of the brake pedal, it applies full boost pressure to the brakes. This helps to make up for any hesitation the driver may of had in applying the brakes initially, and helps bring the vehicle to a stop much more quickly. Tests have shown that electronic brake assist can reduce average stopping distances in an emergency situation from 10 to 40 percent!

This same type of brake assist can also be provided by mechanical controls at less cost and complexity. A mechanical actuator inside the booster detects the speed at which the driver is applying the brakes. If it "feels" like a panic stop, the booster goes to full boost and slams on the brakes.

Electronic Parking Brakes

The same companies that have developed ABS and electronic brake systems have also developed electronic parking brake systems to replace cable-operated disc or drum brakes. Instead of pushing a pedal or yanking on a handle to set the parking brake (which few people seem to use anyway), the parking brake can be set by pressing a button. Or, it can be set automatically when the transmission is placed into park. More and more new vehicles today are being equipped with electronic parking brakes.

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The advantages of this approach are:

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