Rollover probability through fish-hook maneuver
One of the biggest challenges related to chassis engineering today is developing the systems that will, in effect, maintain the chassis of a given car, truck or SUV on the road (or in the case of the last-named, on the road when it is intended to be there). The reason why is simple: research shows that when a vehicle is involved in a rollover, 90% of the time it has left the road. And in cases where vehicles rollover—especially if those vehicles are vans, pickups, or SUVs—there is a considerable level of fatalities that occur. Staying on the road is essential for safer driving.
One company that is taking a holistic approach to the issue of keeping vehicles where they ought to be is TRW Automotive. It is not necessarily alone in this task, but, as Douglas P. Campbell, vice president, Engineering, observes, the big players in airbags don’t do much, if anything, in the areas of chassis and braking, and the big players in chassis and braking don’t play a role in airbags. TRW is a player in all three areas. Consequently, they’re looking at preventative measures as well as post-event contingencies with the objective of developing systems that are sufficiently integrated—both in terms of hardware and software—so that the packages can have a comparatively low cost: “This is an integration story.” Or, as his colleague, Dr. Aly Badawy, vice president, Engineering, Steering, Linkage and Suspension, puts it: “Our philosophy here is that there is not a single silver bullet that would resolve all of the issues for rollover. It is a set of technologies to cover the whole scenario of driving to give the driver maximum safety.”
To learn about what happens, we sat down with TRW Automotive’s Campbell, Badawy, and Danny R. Milot, chief engineer, New Products, N.A., Advanced Control Systems. They took us through the four steps—or, in Badawy’s simplification: “normal driving, about to get yourself in trouble, almost in trouble, then in big trouble”—on the road (or off the road) to an accident and the developments that have been made to help mitigate the situations.
1.) NORMAL DRIVING
GOAL: Stay in the lane.
- Lane Guard. System is based on sensors. Said to be the first implementation of vision for chassis. System determines where the lane markings are and by working with the electric power steering system creates “virtual camber” so that the wheels are maintained on the center of the lane. In addition to which, an audio system creates a “virtual rumble strip” that’s activated should there be departure from the center of the lane. System also helps prevent moving out of a lane and into one when there is a vehicle in the driver’s blind spot. High effort is put into the electric steering system so that the driver can move the car, but has a difficult time doing so.
- Active Roll Control (ARC). This system controls the stiffness of vehicle suspension as a function of the vehicle’s dynamic behavior. A suspension link on each sway bar is replaced with a hydraulic actuator. Based on inputs from such things as the steering angle and the readings from a lateral accelerometer, the actuators are adjusted. The steering pump is used to initiate the actuation. The hydraulic reservoir for the ARC system is based on the same valve block that TRW produces for its ABS systems (thereby achieving manufacturing cost savings).
2.) ABOUT TO GET IN TROUBLE
GOAL: Bring the vehicle back.
- Active Dynamic Control. A variation of Active Roll Control as sway bars are also used for adjustment. If there is a single-channel system (front), the system provides lateral dynamic control and improves steering response. If there is a two-channel system, then the sway bars are actuated independently, which also provides roll compensation and roll damping.
- Rear Wheel Steering (RWS). While the RWS system can provide benefits such as reducing the turning circle and increasing towing stability, in this context the issue is dramatically influencing the vehicle’s yaw response. An actuator assembly is integrated into the rear suspension. This is a by-wire system. There are a wheel position sensor, a vehicle speed sensor, and an inertial sensor deployed, along with an electronic control unit. An example of where this would come into play is when braking on a split mu surface. Without the system, there is a need to “fight” the vehicle by aggressively moving the steering wheel back and forth. This system automatically adjusts the angle of the rear wheels so that the vehicle yaw rate is minimized, as is the driver input (i.e., no wheel fighting).
3.) ALMOST IN TROUBLE
GOAL: Bring the vehicle back from being closer to the rollover edge.
- Vehicle Stability Control (VSC). Sensors measure steering wheel angle (to determine what the driver wants to do) and the vehicle yaw rate and lateral acceleration (to determine what the vehicle is doing). These measures are compared. So appropriate braking adjustments are made to modulate the vehicle yaw moment, reducing rollover potential. Through the additional integration of the electric power steering unit, there can be an 8% improvement in stopping distance.
- Roll Stability Control (RSC). Essentially an enhancement of the VSC system with additional sensors and algorithms. In the first generation RSC system there is a roll model that can predict roll over conditions. The system controller can then make the necessary adjustments (e.g., induce understeer in cases where there could be wheel lift) to minimize the states that could lead to roll over. In the second-generation system, additional sensors and would be deployed to accommodate control under varying conditions, such as off-camber turns.
- Active Control Retractor (ACR). Unlike the other systems, this one is inside the vehicle. In fact, an ACR is deployed in the Mercedes S-Class. ACR employs a retractor with an electric motor that is activated based on measures for various inputs, ranging from ABS to VSC to short-range radar (as can be used for automatic cruise control). Essentially, the retractor removes the slack from the seat belt and helps bring the seat occupant into the proper position prior to an event. This is not a replacement for the traditional pyrotechnic pretensioners that are used in the event of a crash. (The ACR could also be used in concert with a sensor that would determine whether the driver was beginning to nod off. . .and then jerk him back to a less-restful state.)
4.) IN BIG TROUBLE (THE ROLLOVER)
GOAL: Minimize injury.
- Inflatable curtain airbag. The sensor array must determine whether a vehicle is about to roll or has rolled (i.e., discriminate between a rollover and a side or front collision). The curtain airbag is housed in the roof rail. Unlike the airbags for the front or side, the curtain uses a cold-gas inflator because the bag must stay inflated for a longer period of time (say, seven seconds): rollovers are comparatively slow events. The objective is to provide head protection and to help keep passengers in the vehicle.