ABS (Antiblockiersystem)

Anti-lock Braking System (ABS)
Engineers had dreamed about anti-lock brakes for at least 70 years and a patent for a brake force controller was granted to Karl Wessel in 1928 for a design which was never built. A few years later fundamental work was performed by Robert Bosch.

In 1961 a mechanical system acting on the center differential and derived from Dunlop's Maxaret system used in aircraft was installed in the Ferguson P66, and a complicated mechanical system similar to the Jensen also on the 1968 Ford Continental Mark III. Daimler-Benz had been monitoring mechanical systems since the late 1950s and concluded in their own tests that these systems were inadequate for road traffic, since they reduced brake pressure evenly at all wheels.

The name ABS is derived from the word Antiblockiersystem. Jürgen Paul at Mercedes-Benz and Heinz Leiber who worked on automobile ABS in 1964 at Teldix and later at Daimler-Benz are the inventors of the first working automotive ABS system. An ABS prototype vehicle was shown in 1970, and with ABS suitable for road vehicles, since it could reduce the brake pressure at each wheel individually. But it took many more years of development until the system was reliable enough, using digital integrated circuits instead of analog components. In 1981 Jürgen Paul at Mercedes-Benz and Heinz Leiber at Bosch were awarded the Prof. Ferdinand Porsche prize by Porsche and the TU Wien.

Bosch ABS 2 in 1981 Euro 500SEL

Mercedes-Benz started the first series production in 1978 with the ABS 2 system, as used in the legendary Euro '81 500SEL Mercedes-Benz. ABS was optional for 2599.00 marks, about 5% of the car's price and would be equivalent to a $5000 option today.

Bosch

The sideways force drops off faster than the brake force when the wheel slip increases. ABS keeps both close to optimum.

The idea behind ABS is simple: the braking-force coefficient and braking effectiveness are highest with the tire at the optimal brake slip. The controller modulates the brake pressure to keep the wheel in the optimal zone. A locked wheel's coefficient is about 10% lower than optimum, depending on the surface. Even more important is the lateral, or sideways, force coefficient, since it decreases to only about 10% of its full value when the wheel locks, as shown in the plot from Bosch above.

Continental shows the ABS control sequence plotting the speed and brake pressure versus time. The brake pressure increases, then the increase stops, then decreases, then increases again as the wheel speeds up again, and is thus modulated around the optimum to keep the wheel turning during braking which retains steerability. Cars cannot be steered with locked wheels.

ABS' major contribution is thus the lateral adhesion which allows steering during braking and, depending on the road surface, usually a reduced stopping distance as well.

Mercedes-Benz

Locked front wheels lost sideways force and the car without ABS pushes out of the curve.

ABS systems do not necessarily exploit all available traction. In 2-channel systems only the wheel speed at one front wheel is sensed which often locks up or under-brakes the other front wheel. In 3-channel and 4-channel systems, with rear brakes on different diagonal circuits, the rear wheels are low-value-tied where the modulation is based on the rear wheel with the lower adhesion. This is done for better stability, since the overriding goal is stable vehicle dynamics. Read on.

Mercedes Actros on mµ-split test surface

Tesla tested the 4 channel ABS of their new all electric roadster on the mµ-split surface at the proving ground in Arvidsjaur in Sweden. Movies are on their site. http://www.teslamotors.com/design/safety.php

Even basic ABS requires something called GMA (Gier Moment Anstiegsverzögerung; yaw moment increase delay) in vehicles with short wheelbases, so that these smaller vehicles won't spin if left and right wheels are braked hard on mµ-split surfaces, for example both left wheels on asphalt and both right wheels on a gravel shoulder. GMA delays the brake pressure increase on the wheels with higher traction by a fraction of a second, so that the driver can respond to a braking-induced turning/yawing motion - like in long wheelbase vehicles. GMA is turned off in curves when the lateral acceleration exceeds about 0.4 g, since it tends to cause over-steering. Although GMA is a passive system which is only used during ABS modulation, it may have inspired ESP and ABS Plus.

Mercedes-Benz

Large steering axle inclination results in zero or negative steering roll radius and high tracking stability. Shown is the front axle on top and the rear axle on the bottom, with a similar geometry. Codesigned with ZF and Bishop Pty.

Mechanical ESP

Even better would be an optimised suspension design, which helps the driver to steer when encountering uneven deceleration. Fritz Ostwald invented the zero or negative steering roll radius to accomplish this feat and patented it in 1958. The steering axle intersects the ground slightly outside of the wheel center in the ML, a slightly zero or negative steering roll radius. If one wheel is braked more than the other, it will turn slightly more towards the vehicle center, creating a very small steering correction away from the side with the harder braked wheel/s. This is a stabilizing moment, since the motion counters the vehicle yaw moment, caused by the higher deceleration on the side of the vehicle with higher traction. This geometry thus contributes to well behaved vehicle dynamics; long before electronic systems became available. And note that this stabilization works best on the steered axle. We have uncovered one reason why Mercedes prefers low-value-tied rear axle ABS.

The ML rear axle also uses a double wishbone suspension, just like the front, and the toe-in decreases under hard braking, which leads to great tracking stability on the rear as well, but it won't correct the vehicle direction as well as the front can, when decelerated unevenly, for example due to a failed brake, uneven tire traction or tire blowout. Double wishbone suspension first appeared on the Mercedes model 380 in 1933.

To test an ABS system , I brake hard so the pedal starts pulsating. If one can't feel the pulses, one can use a slightly higher initial speed. The pulsating frequency depends on the road surface and also the version of the ABS system. On slippery surfaces like wet asphalt the onset is earlier. One can also test the GMA delay when braking hard on split traction surfaces. I use loose sand or gravel which covers the right side of the road and can be found near windy beaches. One needs to correct the induced yaw with the steering wheel, less so in larger cars and less in cars with a zero or negative steering roll radius, which are autocorrecting. To feel a non-ABS brake in comparison, use an older car. Full braking without ABS leads to flat spots on the tires, so don't repeat it too often. If the tires flutter afterwards, have them re-balanced, which helps, although the flat spot remains.

I use a light colored asphalt road for full-force brake tests. The tires do scrub, but usually do not leave black tracks on the road when viewed directly from above, unless the wheel locked up completely. However when glancing at the test section from afar at a very low angle, one can see black tire tracks on a light colored road surface; even with correctly working ABS systems.

NASA Tire Hydroplaning movies
As an example, the tire adhesion coefficient on a wet road at 60 MPH is about 0.5 and drops to zero when encountering a puddle of water, just 3 mm deep.

Off-road ABS
Some off-roaders turn ABS off to improve braking, for example by removing the ABS fuse. Is this reasonable? Not on hard surfaces. But on surfaces with moveable material, e.g. snow, gravel, lose dirt, a locked wheel can build a wedge in front of it and thus increase grip and reduce stopping distances. A typical traction coefficient on a lose surface, with ABS modulated wheels, is 0.2 and it can rise up to 0.3 with complete lock.

How does the M-Class off-road ABS program work?
On road surfaces with good traction it works in the same manner as Mercedes passenger cars. But when off-road in low range, the ML can change into an off-road ABS mode if it detects rough road conditions by sensing uneven wheel rotation and the speed is below 30 km/h - 18.5 MPH. The brake apply phase is lengthened so the front wheels can "dig in". At higher speeds the off-road situation is also monitored, but it wont change into the off-road ABS mode until the speed drops below the limit.

Australian off-road ABS mode
Australian gravel roads turned out to be so smooth that the system would not reliably detect offroad conditions from jerky wheel motions. So in 2004 Bosch engineers in Australia developed a new optional algorithm especially for these conditions. Wheel slip is allowed to lengthen from the usual 10% to 40% on gravel, also depending on whether the car is in a curve.

Why didn't ABS work when I slowly slid down my plowed driveway?
ABS doesn't work under 5 MPH / 8 km/h and allows wheel lock, similar to the off-road ABS mode.

Does ABS work when driving backwards? Yes.

Does ABS reduce accidents?
According to a 1994 NHTSA study frontal multi vehicle impacts in 1990 to 1992 on wet, dry, snowy and icy surfaces are reduced 9% with ABS. Quite a bit of discussion ensued about fatal accidents: while fatalities on wet roads were reduced 24%, on icy, snowy roads 13% and fatalities of crashed into pedestrians, bicyclists, trains and animals 27%, fatal run-off-road crashes increased 28% when ABS was introduced, thus resulting in no significant overall change in fatalities. Further studies in 2000 to 2002 found the increase in run-off-road fatal crashes in the first 2 years after ABS was introduced to be temporary and they may have been due to driver inexperience. One study in 2002 could attribute the effect to drunken drivers who jerk the steering wheel violently in panic situations. To conclude although ABS may have no significant or only small effect on fatalities, its overall safety benefits of reducing accidents were confirmed.

ABS takes 26 years from innovation to standard
ABS becomes a standard option on all European cars starting in July 2004. The agreement was signed in 2001. Japanese and Korean automobile manufacturer associations signed similar agreements.

Bosch reaches 150 million brake controller milestone
Bosch made 150 million brake controllers since ABS was first introduced 30 years ago on the Mercedes-Benz S-Class. Currently the controllers are produced in six plants worldwide and a new plant in Brazil is to open in 2007.

         
Braking with and without ABS Plus.

ABS Plus
works just like ABS and uses the same hardware, but employs more advanced software algorithms. It's a relatively recent invention. The purpose is to assist the driver in extremely dynamic situations, by selectively reducing the brake pressure on one or more wheels. From the wheel speed sensors it senses the vehicle dynamics, for example if the vehicle is in a curve, is under- or oversteering, leaning etc and then optimises the braking pressure for each wheel accordingly. It can brake wheels individually. For example, some ESP systems, can brake the inner rear wheel in a curve differently than the outer rear wheel, helping the alignment and reducing the need for ESP intervention. Like ABS it only works during braking, in contrast to active systems like ASR, ESP or SBC and mainly improves the tracking stability and steerability, even during fast avoidance manoeuvers.

Electronic Brake Proportioning (EBP)
Electronic Brake Force Distribution
Elektronische Bremskraft Verteilung (EBV)
dynamically adjusts the hydraulic pressure supplied to the rear wheels during braking. Compared to more conventional methods, e.g. a fixed ratio valve, it thus enhances braking effectiveness by allowing the rear brakes to supply a greater proportion of the braking effort without a loss of vehicle stability. During braking, EBP senses if both rear wheels are tending to lock in equal amounts by comparing the sensed wheel speeds on the rear and front wheels, and limits or reduces the brake pressure to the rear wheels. This ensures proper brake proportioning for differing vehicle loads and maneuvering conditions. It also promotes increased brake pad life. The controller is a MK20e instead of the normal MK20 and also requires different, more sensitive DC wheel speed sensors.

Bosch plot

When braking, weight transfers from the rear to the front wheels. Applying equal braking pressures at all wheels would cause the rear wheels to lock first with high brake force, at high deceleration. Older systems use fixed values, based on pressure or load dependant ratio valves to reduce the pressures on the rear axles in this case. EBP works much more precisely and differentiated, automatically adjusting for total weight, transfered weight, center of gravity height, and even whether the vehicle is in a curve or straight line, thus exploiting all available traction for braking while at the same time ensuring vehicle stability.
Original press release announcing 1999 Mercedes M-Class with EBV

Regarding lifting of the vehicle. Continental put out a press release around 1999 showing why it is advantageous to have EBV in a lifted vehicle. Since it automatically adjusts to the higher center of gravity. In vehicles without EBV folks often tend to forget to adjust the fixed brake force proportioning valve when lifting it (according to ATE), so that the rears might lock up first under hard braking (unstable).
lifted M-Class

Continental shows a plot of the ideal brake force distribution between front and rear axle, for a vehicle with driver only, and a fully loaded vehicle. As the front brake force increases the rear increases faster, but then drops off to prevent the rear wheels from locking under heavy braking. In some cases this ideal brake force distribution may lead to more brake pad wear on the rear than the front axle, when, for example, a driver frequently brakes with 5 bar on the front which will apply 11 to 15 bar on the rears. Whereas a driver usually applying 15 or more bar of pressure to the fronts might wear the front pads faster.

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