Controlling the wheels - a potted history

As the automobile has evolved over the last century, one problem has plagued the development for 75 years before it was resolved – a locked front wheel cannot steer so it is essential that the front, steering wheels do not lock in emergency braking maneuvers. From the moment this was realized, anti-lock brakes became the dream of all automotive engineers endeavoring to improve handling safety substantially and, above all, to retain the vehicle’s steerability during emergency braking.

The automotive industry was just entering the era of mass production in 1920 when work began on anti-lock brakes but the road to success proved long and difficult. It was not before 1978 that ABS became available in large-scale production cars when Mercedes Benz developed the first-generation anti-lock braking system.

Perhaps even more significant has been the rapid development of additional systems based on the knowledge gained in developing ABS. Building upon ABS, acceleration skid control (ASR, start of large-scale production in 1981) was developed to control the interplay of the longitudinal forces between tires and road surface not only under braking but, for the first time, also under acceleration by acting upon both the brakes and the engine torque. This was followed by the automatic locking differential (1985) and the innovative permanent four-wheel drive 4MATIC (1985).

What all these systems have in common is the recording and limiting of wheel slip by means of advanced micro-electronics and hydraulics with the aim of improving the so-called longitudinal dynamics of a motor vehicle. The ABS signals are equally used by the Brake Assist (BAS, 1996), the revolutionary Electronic Stability Program (ESP, 1995) and the electrohydraulic Sensotronic Brake Control system (SBC, 2001). It’s an interesting read to see how the forces of human engenuity overcame the obstacles along the way.

As early as 1920, the French automotive and aircraft pioneer Gabriel Voisin had been using a hydraulically operating system to prevent wheel lock. In 1928, German Karl Wessel was granted a patent on a brake power regulator for motor vehicles but did not get beyond the drawing-board stage. In 1936, Bosch filed a patent for a “device for preventing the locking of a motor vehicle’s wheels under braking,” but this design was not taken any further than the test stage. In 1941, an anti-lock regulator was tested with “modest success,” as the “Automotive Engineering Manual” stated succinctly.

However, these first practical tests pointed the way: an anti-lock braking system must have sensors to measure the rotational speed at each front wheel. A control unit registers and compares the measured values – and corrects excessive deviations by controlling the brake pressure at each wheel individually until immediately before wheel lock.

Transfer to the road proved to be much more difficult than expected, however. While appropriate sensors already worked satisfactorily as early as 1952 in so-called anti-skid systems on aircraft and from 1954 in “Knorr wheel slide control” systems for railways, a great deal more was demanded of mechanical friction sensors in cars. They had to be capable of registering the deceleration and acceleration of the wheels, of working reliably while cornering and on rough ground, and of operating faultlessly even when heavily soiled and at high temperatures. In 1961, a mechanical system acting on the central differential and derived from the Maxaret system used in aircraft production was installed in the Ferguson P 66; a complex mechanical system operated in the Ford Continental Mk III of 1968 as well as in the Jensen. The engineers at Daimler-Benz had been monitoring the use of mechanical wheel-lock preventing systems since the late 1950s but had to recognize in their own test series that these systems were inadequate for road traffic as they reduced the brake pressure evenly at all wheels.

Induction rather than mechanics

The problem occupied the engineers not only at Daimler-Benz but also at Teldix GmbH, an affiliate of German company Telefunken and American company Bendix headquartered in Heidelberg, Germany. Their aim was to control each wheel individually to make full use of the existing tire-to-road friction in each case. The basic idea was to calculate imminent wheel lock from wheel speed and wheel acceleration and then to control the brake fluid pressure at each individual wheel. The first proposals concerning the measurement of wheel speed and acceleration involved very large mechanical sensors which were difficult to accommodate at the wheels or wheel hubs, and which were pressed against the brake discs like bicycle dynamos. These sensors were complemented by a simple electronic control unit and a hydraulic control block with fast-acting solenoid valves.

...continued