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You will no longer have direct control over your car’s brakes The recently released Mercedes SL-Class is fitted with Sensotronic Brake Control as standard – the first car in the world with the system. (DaimlerChrysler). Electronic Braking Control in Cars Braking systems in cars are set for big changes with this new Bosch system which has just been introduced to Mercedes Benz cars. Incorporating the best of ABS, ESP and traction con­trol, the new braking system will even dry out the brakes after they have been wet and enable a smooth stop every time, without any dip in the car’s bonnet as you come to a complete stop. By JULIAN EDGAR 6  Silicon Chip www.siliconchip.com.au A PART FROM THE introduction of ABS (anti-skid braking sys­tems), braking systems in cars have changed little in over 50 years. As has been the case throughout this last half-century, all current systems use an hydraulic master cylinder which applies pressure in proportion to force on the brake pedal. The pressure in the brake lines causes movement of the pistons in the slave cylinders located in each wheel calliper, in turn applying the brake pads to the discs (or shoes to the drums, in older systems). A vacuum brake booster is also fitted to all cars to reduce the force required on the brake pedal. And ABS? – it allows the automatic modulation of fluid pressure in the slave cylinders of individual wheel brakes, to prevent wheel lock-up. Now, in the Bosch Sensotronic Brake Control (SBC) system, the control system becomes electronic. Further, the control logic is quite different to a traditional braking system, as SBC inte­grates a variety of other car control systems into the one archi­tecture. SBC represents a revolution in automotive braking – and it’s just been released in the SL-Class Mercedes production model. Integrating systems SBC needs to be considered not as a standalone braking system but in the context of a number of other car drive-line control systems. ABS (anti-skid braking) is now common on modern cars but ESP (electronic stability program) is much rarer. ESP uses the input of a vehicle yaw sensor, wheel speed sensors and a steering angle sensor to determine whether the vehicle is following the path requested by the driver. If the electronic system detects that, for example, the car is running wide in the corner (ie, it is under-steering), the ESP system will brake the inside rear wheel, pulling the nose of the car around. Similar one-wheel braking strategies can be used to control oversteer (the rear of the car running wide). It is important to note that ESP is not the same as trac­tion control. Traction control limits the spinning of the powered wheels, usually by reduc-ing engine torque (although also occa­sionally by braking the slipping www.siliconchip.com.au The components of a Sensotronic Brake Control system include wheel speed sensors, a steering angle sensor, accelera­tion sensor, electronic control unit and hydraulic control unit. (DaimlerChrysler). wheel). On the other hand, ESP can work very effectively when no throttle at all is being ap­plied. For example, a driver who enters a corner too fast, then realises his error and lifts the throttle sharply, can cause a power-off oversteer slide – the car attempting to spin. In this situation, ESP can brake the outside front wheel, preventing the incipient spin occurring. In the same situation, a traction control system is powerless. Another car system that is relevant to the braking system is Adaptive Cruise Control. This uses radar to maintain a preset distance to the car in front – an approach that needs to have control over the brakes in addition to being able to modulate the throttle. Finally, many luxury cars use Brake Assist technology, where if the system detects that the driver has braked very forcefully, the brakes are applied at maximum power – even if the driver reduces braking pressure a little. This approach was developed when testing of drivers in real on-road situations showed that when confronted with Fig.1: this graph shows the penetration of Electronic Stability Program systems in various automotive markets. Bosch has produced over three million ESP systems and expects Sensotronic braking to follow a similar growth pattern over the next five years. (Bosch). February 2002  7 1: Hydraulic unit with SBC control unit. 2: Fuses. 3: Actuation unit. 4: Speed sensor 5: Hydraulic line. Fig.2: the distribution of the Sensotronic Brake Control components in the SL Mercedes. (DaimlerChrysler) the need for an emergency stop, many simply didn’t push hard enough, and didn’t keep pushing hard enough, on the pedal. So anti-lock braking, brake assist, electronic stability program, traction control and adaptive cruise control are, to a greater or lesser extent, currently standalone systems that could be better integrated with the braking system. And as you may have now guessed, SBC does just that. SBC architecture SBC still uses hydraulic fluid, a master cylinder and wheel slave cylinders – electromagnetic operation of the brake pads does not occur. While technically, electromagnetic actuation of brake pads is feasible (and is currently used in electrically-braked trailers), the retention of an hydraulic system allows for fail-safe operation 8  Silicon Chip of the front brakes in the case of electronic failure. In SBC, fluid pressure is generated not by the driver’s foot movement and the brake booster operation but by a piston pump driven by an electric motor. This supplies brake fluid at a pressure of 140-160 Bar (2000 - 2350 psi) to a high-pressure gas diaphragm accumulator. This stored pressure is sufficient for several braking events, allowing full braking pressure to be applied a number of times, even when the engine is switched off. When the brakes are activated – either by the driver, Elec­tronic Stability Program, Traction Control or Intelligent Cruise Control – the electronic control unit calculates the desired target brake pressures on a wheel-bywheel basis. Remember, during ESP operation it’s likely that only single wheels will be braked – although of course in normal, straight-line, low 6: Box for control units with ESP. 7: Yaw angle sensor. decel­eration braking events, all four wheels are retarded. The interface with the driver is by means of the brake hydraulic actuation unit. This comprises a special tandem master cylinder which uses a simulator to provide normal pedal feedback. However, the important part of this mechanism is the pedal travel sensor, which determines both how fast and over what distance the brake pedal is being moved. During normal braking, the brake pedal is completely disconnected from the hydraulics of the system – it is simply an input into the electronic control unit to indicate the degree of braking requested by the driver. Only if a major fault or power failure occurs does the brake pedal actuate an hydraulic circuit. In addition to the brake pedal travel sensor, the electron­ic control unit also receives data on steering wheel angle, www.siliconchip.com.au Fig.3: the Sensotronic Brake Control system. An electric motor operates a piston pump that pressurises an accumulator. Fluid from the accumulator then operates individual brake cylinders as required. Except in emergency fail-safe operation, the brake pedal is not connected to the hydraulic system. (Bosch). individual wheel speeds, the selected gear and lateral (ie, sideways) acceleration. The latter is used to detect whether the car is cornering – if it is, then the braking bias is adjusted laterally, with the outside wheels braked more heavily than those on the inside of the corner. This reduces the likelihood of ESP needing to intervene during emergency braking manoeuvres. prevents the car rolling backward on a hill and simplifies the drive-away process. The function is activated by quickly and firmly pressing down the brake pedal when the car is stationary. The brake effect is automatically cancelled the next time pressure is applied to the accelerator pedal. • ‘ACC Stop & Go’ is an upgrade of the adaptive cruise control function that is applicable for stop-and-go traffic or city driving. If the vehicle ahead stops, SBC brakes the vehicle to a standstill. If the driver ahead accelerates, the car drives away automatically and follows. Safety features of SBC include: • ‘Dry Brake Function’, where on Advantages Bosch, the inventor of the system, believes that SBC will penetrate the luxury car market quickly, then filter down to cheaper cars in the way that ABS and more recently, ESP has. But what are the advantages to the driver of such a braking system? Says Gunther Plapp, Vice President of Bosch’s ABS and Brakes Division, “The crucial performance feature of the electro-hydraulic brake SBC is that it raises braking comfort”. A number of comfort and convenience features can be in­tegrated into the system. These include: • The ‘Soft-Stop-Function’, which provides for a soft and smooth stop during normal braking. • A ‘Traffic Jam Assistant’, which brakes the vehicle with pre-defined deceleration when the driver removes his or her foot from the accelerator pedal. On the Mercedes SL-Class, this mode is engaged by using the cruise control stalk when the car is sta­tionary. • The ‘Drive-Away Assistant’, which www.siliconchip.com.au The Sensotronic Brake Control master cylinder lacks a brake booster. In addition, the ‘hydraulic fluid displacement’ function of the brake pedal occurs only in an emergency, retarding just the front wheels. (Bosch). February 2002  9 Previous Model With Conventional Braking Technology New SL-Class With Sensotronic Brake Control Fig.4: Sensotronic Brake Control allows the braking force to be altered laterally, giving better stability when braking heavily while cornering. This reduces the likelihood of Stability Control needing to operate in these situations. (DaimlerChrysler). wet roads the SBC carries out regular short and weak brake applications to wipe the water film off the brake discs. Switching on the windscreen wipers activates this function. • Reduced stopping distances. This is possible because the system can be configured to pre-fill the brake circuits if a sudden lift of the accelerator pedal is detected. 10  Silicon Chip • The vibration of the brake pedal that normally occurs during ABS operation is not present with SBC. DaimlerChrysler driving simulator research shows that this absence is not just a comfort advantage but also has safety implications – almost two-thirds of tested people are startled when ABS pedal pulsation occurs. A proportion of these people do not maintain appro- priate brake pedal pressure when this happens and some even take their foot off the pedal! • Reduced requirement for Stability Control, because braking force can be varied laterally during cornering. The future SBC is currently available only on one very expensive car – the Mercedes SL. However by 2005, Bosch expects that lower cost SBC systems will be available for mass fitting to a wide SC range of vehicles. www.siliconchip.com.au