Bosch's electronic stability program
(ESP) was designed primarily to prevent passenger cars from skidding.
Loss of control can result from everyday driving events such as sudden
lane changes on the motorway, swerving to avoid animals running across
the road or simply misjudging the radius of a bend.
ESP is a complementary technology to anti-lock brakes (ABS) and traction control (TCS) in that it works to enable the driver of the vehicle to stay in control.
Now Bosch has evolved the ESP technology into a load-adaptive control (LAC) system for light commercial vehicles, which takes into account changes in payload and how the load is distributed. The system is fitted as standard to the new Mercedes-Benz Sprinter and Volkswagen Crafter.
|
LAC accounts for payload and how the load is distributed |
Kerb and gross vehicle weights in passenger cars may only differ by a few hundred kilos, and the distribution of the extra weight is predictable: passenger cell, boot and fuel tank. But commercial vehicles are quite the opposite. The load carried and how it is arranged in the cargo area can vary significantly.
The standard ESP system fitted to a commercial vehicle is configured for a specific mass but will operate with reduced effectiveness at any other load case because the control module has only an assigned value to work with. It would not be able to take account of the different vehicle dynamics associated with laden and unladen conditions.
Load-adaptive ESP does just that: it adapts the operation of ESP according to the load and its distribution. The system estimates the total mass of the vehicle and its centre of gravity by evaluating responses to braking and accelerating.
LAC also determines a parameter referred to as “characteristic speed”, an important variable for the function of ESP, related to the behaviour of the vehicle under steering. It is an estimated value computed using several inputs including actual vehicle speed, acceleration, engine and brake torques. Characteristic speed is also linked to LAC’s estimation of total vehicle mass and longitudinal centre of gravity.
Bosch says that the centre of gravity can change position by up to 25 per cent longitudinally, and 50 per cent vertically. The variances illustrate quite clearly how an ESP system for commercial vehicle applications benefits from being load-adaptive.
If the system believes that the behaviour of the vehicle is likely to result in a loss of control, the first action taken is a reduction in engine power. If this is not enough, braking can be applied to individual wheels to rectify the situation.
The ESP system also seeks to prevent the vehicle overturning when cornering. The risk of rollover through cornering too quickly or too tightly will differ according to the load carried. By acting to stabilise the vehicle early on, a rollover can be avoided.
The system can also control brake force distribution between front and rear axles, providing optimum braking on each (without locking) in accordance with the front/rear weight loadings. Bosch says this improves brake efficiency but also reduces front brake wear and fading by directing more braking effort to the rear brakes when the vehicle is laden.
|
|
