SAAB ESP TAKES THE NERVES OUT OF SWERVES
NORCROSS, Ga. - The new Saab 9-5 range offers an Electronic Stability Program (ESP) for the first time, standard on Arc and Aero models, and available on Linear as part of the optional Premium Package.
Extra sensors have been added to the traction-control system and anti-lock braking systems to provide a highly sophisticated stability function. Coupled with the Saab 9-5's many chassis improvements, the addition of ESP gives an even sharper edge to the latest Saab's sports performance dynamics, while providing the driver with a greater degree of car control.
Saab engineers have tested the system extensively - in typical Saab fashion - by taking the proprietary Bosch hardware and then re-engineering the software to accommodate Saab's own specific preferences. The system was benchmarked against rival carmakers' systems. The result is the most sophisticated and "intelligent" ESP system on the market, one that's able to "read" differences in surface grip.
However, ESP Saab-style isn't intended to work in extremis, or to smooth over chassis faults to dampen wayward handling behavior. Saab's goal was to enhance the driving experience, Lars-Goran Warmark, manager of braking systems, explained. "Our target was to make the system 'scared' before the driver was, instead of having a system that reacts vigorously once the levels of adhesion have been exceeded. It makes the driving experience more comfortable."
It took a good deal of testing by Warmark and his team to achieve the desired goal. During this intensive period of development, though, the ABS system was also modified to ensure stability and control during avoidance maneuvers without lengthening stopping distances.
Six months were spent testing in northern Sweden on low-traction surfaces, before a test session on Germany's Hockenheimring for high-performance driving. "Hockenheim was particularly appealing," commented Peter Stavered, who was one of the development engineers. "It was close to our supplier Bosch, so feedback, consultation and recalibration were speeded up, while the circuit is a high-speed course with good grip characteristics. So we could also test the system at really fast speeds."
The result is one of the best ESP systems on the market, one that's been thoroughly engineered to meet Saab's driving-experience parameters and benchmarked against Saab's premium-segment rivals to provide superior performance.
Bosch hardware fine-tuned by Saab
The nuts and bolts of the system consist of a proprietary Bosch yaw sensor. The yaw sensor works in conjunction with a steering-angle sensor mounted on the steering shaft, a pressure sensor on the anti-lock (ABS) unit, a hydraulic control unit (HCU) and the ECU electronic control unit.
Saab engineers have tuned the system by changing twenty of the pre-set Bosch parameters to meet Saab driving requirements. However, the testing procedure meant that more than 100 parameters were changed, tested, assessed and then rejected or accepted for further development - that's over 10 percent of the pre-set Bosch parameters, a substantial workload.
Saab's ESP works like this: it assists the driver in the direction of his steering efforts. If he goes into a corner on which there is less grip than he had anticipated, resulting in an oversteering slide as the tail starts to lose control, the ESP system applies brake force to the outer wheels to nullify the yaw rate of the car and gently bring the car back into line. In that way it complements the driver at the wheel, rather than aggressively offering help at the last available moment.
The new Saab ESP system also works when a slippery road causes the car to understeer - when the nose of the car starts to push wide instead of following its intended course. ESP offers just enough brake-force control to help bring the car back into line, because too much assistance might suddenly switch the car from an understeer to an oversteer situation.
"In most cases," Warmark said, "the driver will have backed off by then, or the Traction-Control System will have done its work."
Latest ABS with TCS and ESP
The new 9-5's ABS system incorporates electronic brake-force distribution (EBD) and an electronic traction-control system (TCS). The new ESP system works with TCS, which requires a modified ABS system tuned to the parameters of ESP.
ABS prevents the wheels from locking while the brakes are on by the action of a solenoid-operated ABS valve in the central hydraulic modulator. As soon as the onset of skidding is detected by any one of four sensors, located at each wheel, the system releases the pressure to the locked brake and diverts pressurized fluid to a supplementary low-pressure hydraulic reservoir. When the wheel accelerates back to the vehicle speed, pressure to the affected brake is restored by feeding high-pressure fluid from the ABS pump. The system can cycle in this way up to 12 times a second to allow the wheel to keep rotating at the point of locking, allowing the driver to maintain steering control while stopping in the shortest possible distance.
This ABS system also incorporates improved electronic brake-force distribution (EBD) that works during hard braking before ABS activation. It correctly balances the friction available at each axle with the braking forces being applied. The system is controlled by the integrated ABS controller, which compares the rear-wheel slip with the front as a reference point. As soon as significant differences are detected, pressure to the rear hydraulic system is modulated to provide the most balanced effect without operating the electric ABS pump, so the driver is unaware of the adjustments taking place.
Traction control integrated with ABS
TCS works in conjunction with the engine-management system to prevent the driving wheels from spinning if tire grip is lost. The dual-mode system operates on the front brakes individually, to transfer torque to the wheel with the most available friction. Should it be necessary, it also signals the engine-management system to reduce engine torque by over-riding the electronically activated throttle. A warning lamp on the instrument panel lights up when the traction control is operating, alerting the driver to the hazardous nature of the roads.
The TCS system is fully automatic in operation, but can be turned off for special circumstances, such as when snow chains are used. It can only be de-activated at low speeds (below 40 mph).
The system uses the ABS signals from the front wheel sensors to continuously evaluate the degree of wheel slip and acceleration on the driven wheels, comparing these data with similar information from the rear wheels. If excessive wheel slip is detected at only one front wheel (usually at low speed or when moving off on a slippery surface or a steep hill), its brake is applied to transfer engine torque to the other side of the differential, where more grip is available. If both wheels start to spin together, the engine-management unit reduces engine torque until grip is regained.
ABS+EBD+TCS +ESP= more chassis control
Saab's ESP system uses all these inputs to calculate the car's behavior. But it also has the advantage of the extra yaw and steering wheel sensors, allowing smoother TCS and ESP control because the engine-management system is given more information to "understand" the car's behavior and then react accordingly.
"This is particularly crucial for us," said Warmark. "It means we can tune the parameters in a more sophisticated way, balancing the use of brake and torque management to provide a smoother system that works early.
"But it only works if you have a good chassis to start with. You can't make ESP work in the sophisticated manner we intended if it's just compensating for chassis faults," he added.
This is especially true when making quick lane change maneuvers. ESP on a good chassis helps neutralize the weight build-up at the rear of the car as it changes direction, causing an oversteer effect, by gently "dragging" the car's mass back into line by judicious and unfelt use of the brakes.
"We like to think of ESP as a yaw damper," commented Peter Stavered, development engineer. "You can steer any car to avoid something. But to come back into a lane after a quick change of direction is the toughest problem. You need to be a rally driver or to have ESP. And ESP is better than the rally driver because a rally driver can't brake two wheels - he can only brake four. ESP can brake the two outer wheels. It makes the car's behavior more neutral, but it acts in such a subtle way that you don't realize it's working. To give you another analogy connected to rallying, it's like driving in Sweden along a constant snow bank to softly correct the direction of your travel!"