Late Braking News

by Steve Sturgess

You might be tempted to shake your head at Transport Canada’s proposed new rules requiring anti-lock brakes on new trucks starting this December and call the agency’s efforts another attempt to play catch-up with the United States. The U.S. now requires all new commercial vehicles over 10,000 pounds GVW, including buses and even heavy recreational vehicle chassis, to have ABS with whatever braking system they employ.

Sure, it’s true that Canada has drafted its rules to mirror those in force south of the border. But the real race for regulators on both sides of the fence is with technology. Newer, more advanced brake systems are steaming full-speed ahead.

Already, there are demonstration Freightliners and Internationals running around North America with the next generation of braking technology: electronic braking systems, or EBS.

Europe is further ahead. Across the Atlantic, there are something like 70,000 trucks running today with full electronically controlled brakes. A standout is the Mercedes-Benz Actros, which has had EBS as standard since its launch two years ago. This high-tech cabover is made by the truck division of DaimlerChrysler, Freightliner Corp.’s parent corporation.

But EBS is just the first step in a brave new world of electronically enhanced braking and dynamic stability for heavy vehicles. Just around the corner is a new technology that radically enhances a combination vehicle’s cornering ability, especially on slick surfaces. The upcoming electronic stability control (ESC) is based on the fast-acting and wheel-by-wheel braking made possible by the sophisticated electronic braking.

An excellent account of these upcoming technologies, along with a history of air-brake systems, was delivered by Meritor WABCO president and general manager Len Buckman at last year’s Society of Automotive Engineers Truck and Bus Meeting. He delivered the Buckendale Lecture on air brakes and ABS. (A 170-page transcript is available as SAE paper SP-1405.)

As Buckman pointed out, the electronically controlled stability technology is still emerging, so the acronym for stability control isn’t yet set in stone. It may be ESC, or it may be known as vehicle stability control (VSC), vehicle dynamic control (VDC) or electronic stability program (ESP). Whatever the name, it promises a major safety improvement for drivers of heavy trucks.

To understand how ESC and EBS functions, it’s worth a quick recap of how ABS and conventional air braking systems work. Air brakes go back before the dawn of the truck 100 years ago to George Westinghouse’s braking system for railroad rolling stock, for which he received a patent in 1869. It was the first air-brake system.

The distinguishing feature of air-brake systems is the use of compressed air to generate the large forces needed to clamp the brake shoe to the braking surface. All air-brake systems use this force-generation method. Early systems pushed all the compressed air through the foot or hand valves. Today, the systems have separate lines for the system air and much smaller lines for signaling air.

Pressure on the foot treadle valve causes proportional air pressure to enter the system’s signal line. From the relay valves, the working air-again proportional to the signal pressure-is directed to the brake chambers at the wheel-ends for the amount of braking called for by the pressure exerted by the driver’s foot on the treadle valve.

Today’s ABS uses microprocessor technology to sense when the wheels are about to lock up under braking and then limit the brake pressure to prevent the wheels from locking up. Wheel speed is measured constantly with a sensor and a notched or toothed ring mounted at the wheel end or inside the axle. As the ring (often called an exciter or tone ring) rotates, the sensor counts the teeth or notches to measure speed.

Speed information is sent to an Electronic Control Unit (ECU) which constantly analyzes the situation. Before the wheel locks up, the ECU instructs the modulator or relay valve at the wheel to reduce braking force.

Modern digital electronics allow all of this monitoring and braking to take place in a split second, pumping the brakes far faster than any human could.

ABS is particularly helpful on split-coefficient surfaces, where the wheels on one side of the vehicle are on a slippery surface and those on the other are on dry pavement.

If you braked on this surface in a non-ABS vehicle, the wheels on the slippery surface would almost certainly lock up because the foundation brake system is designed to apply pressure evenly to all wheels.

On most ABS-equipped vehicles, pressure is modulated to achieve optimum braking performance on both sides without lockup (some ABS-equipped vehicles may not perform this way if they have different modulator valve setups).

Some systems are also fitted with Automatic Traction Control (ATC), which uses the same principles in reverse. If a wheel is spinning on a slick surface, the ATC momentarily brakes the wheel and/or reduces engine power to improve traction.

Today’s ABS also does a good job of telling you if something’s wrong. Systems monitor circuitry constantly and shut themselves down if they sense a problem. However, an ABS malfunction does not mean brake failure. An ABS “fail-safe” feature allows the brakes to function normally without ABS. If the system fails, the driver is warned by a light on the dash and/or a light on the front of the trailer.

Mechanics get help in finding the problem by using blink codes from a light on the dash, on the ECU, or from a hand-held diagnostic tool. If there is a problem, one of the more likely causes is a loose connector or a wiring failure. Or sometimes it’s a simple timing issue: if your front and rear tire sizes are more than 5% different, the ECU will be confused as it tries to sense wheel speed and it may shut the ABS down. If you’re planning different tire sizes, talk to the OEM or ABS manufacturer first. Some offer systems capable of automatically adjusting to greater differences in tire size.

With true electronic braking systems, the braking force is still applied in the traditional way, using compressed air on the brake chambers. But the signal is carried electrically from the foot-valve to the relay valve. So the signal arrives instantaneously at the relay valve instead of having to rely on the conventional system’s pressure rise in the signal air line.

Similarly, when the driver releases the brake pedal, air can dump much faster from the brake chambers for a much faster release.

In engineering terms, this faster application and faster release is termed a reduction in the hysteresis of the system. As might be expected, faster application has been shown to achieve significant improvements in stopping distances, as well as improving the feel at the brake pedal.

But there’s another factor that enhances the EBS feel and performance: movement at the brake pedal does not dictate a certain value of air pressure to apply the shoes to the drum. Instead, the EBS signal sets a level of deceleration. The wheel and speed sensors and the electronic control unit work together to apply the brakes for the desired braking performance, and the braking is independent of the loaded condition of the truck.

EBS enables other safety and performance features. For instance, the application of so-called smart electronics allows for better brake balance between tractor and trailer, less brake drag, a fade pre-warning, and enhanced brake diagnostics.

Add a few more sensors on the brake components and EBS can read brake temperatures, torque, and brake stroke, among others. This allows for overcoming lining friction variations, misadjustment of brakes, brake pulls, and so on.

In fact, several years ago, I drove a prototype Freightliner Century Class fitted with WABCO EBS. The feel of the brakes was far more like a well-braked car than a heavy truck. The improvement in feel and control was marked, even though we never came close to exploring the braking limits in a panic stop. Drivers of Navistar vehicles with Bendix EBS also report improved “brake feel.”

With EBS able to sense and vary the braking in accordance with what is happening at each wheel position, the field is wide open for the application of a control system that allows for the braking of individual wheels. By understanding how individual wheel braking affects the overall stability of the vehicle, it becomes possible to affect the dynamic cornering characteristics of a vehicle as it starts to lose grip in a corner.

You may already have experienced this sort of control yourself when driving a tractor-trailer in slippery conditions. Drivers with very sensitive hands and a lot of experience can sometimes detect an impending jackknife. With just the right amount of trailer hand valve, they can straighten out the tractor-trailer and avoid a potential accident.

With EBS, the system responds faster, automatically and more thoroughly, applying braking to wheel positions all around the vehicle to optimize the control and prevent an out-of-control slide out of a corner.

If you need any convincing, you just need to view the video clips presented as part of Buckman’s lecture.

Produced by Meritor WABCO and Mercedes-Benz, they show how such an experimental system fitted to a German cabover Actros and trailer enables it to stay in its lane through a number of different manoeuvers. AlliedSignal’s Bendix division has demonstrated the same type of system in North America on a Navistar vehicle. Included is footage of a fast lane change and a slippery cobble-stoned corner. Braking to the trailer’s tri-axle and to its outside front wheel in each manoeuver was able to transform a potential accident into a completely controlled avoidance situation.

With EBS on board, additional sensors are required for the application of ESC. The braking control computer gets added input on steering wheel angle, lateral acceleration of the truck, and yaw rate. Lateral acceleration is that centrifugal feeling you get in a corner, which suddenly drops off as a vehicle starts to slide. Yaw is the rotational motion of the tractor or trailer that determines if either component of the vehicle is starting to spin.

It’s easy enough to explain how the system works. Getting it to do what’s needed is a lot tougher, because the computers have to be programmed to know what’s happening in a manoeuver and what is supposed to happen, and then do whatever is necessary to bring the vehicle back into control.

The exciting thing is that development systems are doing exactly that, and doing it quickly and unobtrusively so that a driver is protected if he or she gets in trouble on an unexpectedly slippery surface.

Europe is moving forward with factory-fitted EBS on Mercedes-Benz and Scania trucks. Other nameplates follow this year.

North America, though, is lagging behind because braking regulations don’t allow for a cost-effective application of EBS. As they are, the rules say an air-braked vehicle must have a fully functional, two-line air system.

This is a safety measure, but the addition of electronic control means adding yet another expensive system on top of what is available today for three-level, doubly redundant brakes.

Unfortunately, until the rules are rewritten, that’s what we must work with. Fortunately research is still going forward, and some brave pioneers will be spec’ing and buying EBS Freightliners this year, albeit at a premium price.

Experience with these trucks will be used as evidence in efforts to convince the U.S. National Highway Traffic Safety Administration-and perhaps eventually Transport Canada-that EBS is a safe system in and of itself. In fact, it is a safer system than conventional air brakes.

Once that hurdle has been overcome, be prepared for electronic stability control. It’s just around the corner.

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