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The Landrover Discovery has ABS as an option. ABS calibration for dirt surfaces & constant 4-wheel drive is quite complicated. Anti-lock braking systems: how they work Now commonplace on family cars, anti-lock braking systems require fancy electronic control circuitry to do their job. Here’s a rundown on how they work. By JULIAN EDGAR An anti-lock braking system (commonly known as ABS) prev­ ents a car’s wheels from locking during panic braking. This has two distinct advantages: (1) it gives shorter stopping distances; and (2) it allows the car to be steered during hard braking. A car with locked wheels cannot be controlled by steering input and will also take much longer to stop than one with the wheels still turning while it is being braked. 6  Silicon Chip Anti-lock braking systems have been used in automotive applications for around 25 years but have only recently found widespread use in mass-produced family cars. This has been made possible by a reduction in the cost of the electronic circuitry required and by increased public awareness of safety issues. Unlike airbags, which protect the car’s occupants after the car has hit something, ABS gives a car greater primary safety – meaning that it is less likely to be involved in an accident in the first place. In the vast majority of situations, an ABS-equipp­ ed car will have a braking advantage over a conventionally-braked car, although it should be noted that in some (rare) situations, an ABS will actually give longer stopping distances. The task An anti-lock braking system has an apparently simple role – to stop individual wheel lockup while still providing maximum braking efficiency. In stable situations where the frictional coefficient between the tyres and the road is constant, where vehicle mass is unchanging, and where the road surface is smooth, appropriate ABS behaviour is relatively easy to organise. However, in the real world, CONTROL ZONE BY ABS the driver pull the car fully back onto the bitumen, this lateral difference in braking effort could result in the car yawing rapidly. An optimal anti-lock braking system would thus give the following charac­ teristics during operation: (1). Driving stability maintained through the retention of suffi­cient lateral guiding forces at the rear wheels; (2). Steering ability retained through the provision of suffi­cient lateral guiding forces at the front wheels; (3). Reduced stopping distances; and (4). Rapid matching of the braking force to different adhesion coefficients. FRICTIONAL COEFFICIENT BETWE E N T YRE AN D ROAD S URF ACE,  ASPHALT ROAD ICE-SNO W ROAD 0 SLIP RATIO 100% Fig.1: the maximum braking effort is obtained when there is a certain amount of wheel slippage. While it depends on the road surface, best braking is usually obtained with a wheel slippage ratio of between 8% & 30% (Subaru). a large number of variables means that anti-lock braking systems need to be very sophisticated in the way they operate. An ABS control system must take into account: (1). Variations in the amount of adhesion between the tyres and road due to changes in the road surface and in wheel loads (especially during cornering); (2). Irregularities in the road surface which cause the wheels and suspension members to vibrate; (3). Out-of-round tyres and brake hysteresis characteristics; and (4). Different friction coefficients which might exist between the left and right-hand wheels, and a possible subsequent transi­tion to a homogeneous surface. Taking the last point as an example, if a car is heavily braked while the right-hand wheels are on dry bitumen and the left-hand wheels are on the dirt verge, then the ABS would obviously reduce the braking effort in the left-hand wheels. However, should Braking behaviour Obtaining the optimal braking force is more complicated than it first appears, with brake slippage actually necessary for best results. The brake slip ratio is defined as follows: Slip Ratio = (Vehicle speed - Wheel speed)/Vehicle speed x 100% When the slip ratio equals zero the wheel is travelling at the same speed as the car (ie, there is no slippage). Conversely, when the slip ratio is 100%, the wheel is locked and does not rotate at all. The relationship between the longitudinal fric­tional force of the wheel and the slip ratio depends on the road surface. Fig.1 shows this relationship for as- Fig.2: this diagram shows the main components of a typical anti-lock braking system, in this case for the Subaru Liberty (Subaru). November 1994  7 TOOTHED WHEEL +V FULL SPEED POLE PIECE 0 S N PERMANENT MAGNET -V SLOW SPEED Fig.3: inductive sensors are used to signal wheel speed to the electronic control unit & this then calculates the vehicle speed &the slippage for each wheel. phalt and ice-snow surfaces. It can be seen that in both cases the maximum frictional coefficient between the road and the tyre is achieved when in fact there is some slip. In other words, allowing the wheel to continue to rotate at the same forward speed as the car – that is, not skidding at all – will not give maximum retardation. The slip ratio at which the maximum friction exists is generally 8-30%, depending on the road surface. While an 8-30% slip ratio works well on dry and wet bitu­ men, ice and many other road surfaces, it does not hold true for fresh snow and gravel. In Australia, the latter road surface is especially important Operation Fig.4: the toothed wheel (tone wheel) is located on the inner hub of each wheel to excite the pick-up sensor. In some cars, the same sensors are also used for traction control (Subaru). Even cheap compact cars like this Holden Barina can now be supplied with ABS as an optional extra. 8  Silicon Chip and on gravel a slip ratio of 100% gives maximum retardation. In other words, locked wheels stop the car in a shorter distance on gravel than any other technique. This is because a small dam of gravel (or snow) builds up in front of the locked wheel and helps to slow the vehicle. The skidding wheel can also gouge its way down to a firmer surface beneath the gravel. Of course, while this is happening there will not be any steering control available! Some manufacturers provide a dash-mounted switch which allows the driver to switch off an anti-lock braking system while driving on surfaces for which it is not compatible. However, most manufac­turers avoid doing this, mostly because of potential driver confusion and the fact that the anti-lock braking system might be left deacti­vated just when it’s needed. An anti-lock braking system comprises a series of input sensors which read the wheel speeds, an electronic control unit (ECU), and a hydraulic control unit (HCU). Fig.2 shows the essential elements of a typical ABS. The wheel speed input sensors are typically inductive pick­ups and these use a permanent magnet and a coil. A toothed ring attached to the inner part of the wheel’s hub rotates past this sensor. As it does so, the teeth change the magnetic coupling into the coil and so the sensor generates an AC waveform whose frequency depends on the wheel speed. Fig.3 shows an example of a sensor and its output, while Fig.4 shows its location on the car. Note that in this Subaru system, the toothed ring is called a “tone wheel”. A typical ABS electronic control unit is shown in Fig.5. As well as the sensor amplification and shaping circuitry, it com­prises the ABS comparison and control circuitry, plus a number of output transistors which control the solenoids and pump within the HCU (hydraulic control unit). A self-diagnosis circuit is included to allow easy fault-finding and self-check circuits monitor the electrical condition of the input sensors and output actuators. If a fault is detected, a dash-mounted warning light is illuminated and the brakes then operate conventionally. In sophisticated 4-channel systems, Fig.5: block diagram of a Bosch ABS. Note the safety moni­toring & the self-diagnosis circuits (Subaru). the ECU uses the input signals from two diagonally opposite wheels to derive a vehicle reference speed. Using this speed and the individual wheel speeds, it then calculates the brake slip for each wheel. When a wheel’s deceleration exceeds a preset value, the ECU transmits a “hold” signal to the HCU. At the same time, the ECU computes a dummy vehicle speed, and – should the wheel speed drop below this – the ECU decreases the brake fluid pressure to prevent lockup. However, with the decrease in brake fluid pressure the wheel accelerates. When this acceleration passes a preset value, another “hold” signal is transmitted to the HCU; should wheel acceleration continue then the brake fluid pressure is increased. The frequency of this brake fluid pressure cycling varies from 4-10Hz. The HCU consists of solenoid valves, a hydraulic pump and accumulator chambers. Fig.6 shows an external view of an HCU. Depending upon the switching state, the brake cylinder is con­ nected to the corresponding circuit of the brake master cylinder, the return pump, or is isolated. When pressure is reduced, the return pump moves the fluid flowing out of the wheel brake cylin­ ders back to the master cylinder via the accumulators. The accu­mulators are pres­ent to absorb any temporary brake fluid surplus that may be produced when the pressure suddenly drops. Other systems Fig.6: external view of a Bosch hydraulic control unit (Subaru). Not all anti-lock braking systems use four input sensors and three or four control channels – indeed not all anti-lock braking systems are even electronic. Teves, Lucas Girling and individual vehicle manufac­turers use variations on the theme. Some, for example, set the hydraulic pressure applied to both rear wheels according to the wheel with the highest slippage (ie, the same pressure is applied to both wheels). Others may do the same for the front. Some anti-lock braking systems also use an acceleration sensor which measures the rate of vehicle slowing. For example, on the Subaru Liberty, a G-sensor is used when ABS is installed on manual constant 4-wheel SC drive cars. November 1994  9