HOW deadly are heavy-vehicle rollover accidents? A National Highway Traffic Safety Administration report showed that rollover occurred in 52% of the accidents in which the driver was killed.
How environmentally dangerous are those accidents? A report showed that rollover of heavy vehicles was responsible for 95% of the bulk spillage of hazardous materials.
Moustafa El-Gindy, a researcher in the vehicle dynamics field for more than 24 years, believes that economic demands have put pressure on regulators to increase the size and weight limits of heavy trucks, and that the frequency of rollover accidents will grow unless measures are taken to compensate for the reduction in roll stability caused by those changes in regulations.
He thinks he might have a solution.
El-Gindy, director of the Vehicle Simulation Research Center at Penn State University's Transportation Institute, says a computer program they developed and have tested in simulation could automatically adjust the brake forces on the right and left sides of a heavy-truck cab to prevent rollover accidents during cornering maneuvers.
He believes the new Penn State controller for heavy trucks is the first of its type to offer a workable solution to the rollover problem, adding that a car manufacturer is currently trying to add a differential-braking concept to passenger cars.
“We've gotten very good results, so we're looking for some funding to develop the hardware,” said El-Gindy, who added that he has contacted some fleets that have experienced rollover problems. “The hardware would be a control system and sensors to measure the level of acceleration and the trailer's central gravity, which will give an indication of whether the vehicle will reach the rollover or not by knowing the rollover threshold — the maximum level of acceleration at which the vehicle would roll over or the wheels would leave the ground.
“Once that is sensed, the control system will send the command to apply a differential braking force on the sides — the inner wheels and outer wheels of the trailer axle, at a different magnitude. This will result in applying yaw moment on the tractor and continuously change its direction, which will eliminate the danger of rollover.
“It only takes a few seconds of differential braking to steady the vehicle. If the driver responds and gets the truck under control via other means, the controller will not deploy.”
Approach Applies to Any Configuration
A Scott Lewis, research associate at PSU's Applied Research Laboratory, says a computer simulation of a 75,000-lb tractor was used to demonstrate the effectiveness of the proposed active control system, but that the approach is applicable to any configuration of truck or tractor-trailer.
El-Gindy says the biggest obstacle was developing a computer program that could adjust to the continuous changes that take place in a tractor-trailer as it maneuvers around a corner. He says simulation tests have shown that the controller they developed can prevent rollover without significantly changing the direction of the vehicle.
El-Gindy has published more than 60 papers and technical reports related to heavy-vehicle dynamics and has extensive experience in computer simulation models of single and articulated heavy vehicles. He completed a temporary assignment with the Federal Highway Administration during which he participated in several research studies on truck size and weight, including those dealing with dynamic wheel loads of heavy trucks, and has been a senior research officer for the National Research Council of Canada.
El-Gindy presented his paper, “Nonlinear Active Rollover Prevention Control Strategies for a 5-axle Tractor/Semitrailer,” last November at the American Society of Mechanical Engineers' International Mechanical Engineering Congress and Exposition in New York City.
It presents new active control strategies to prevent heavy-vehicle rollover and focuses mainly on cases of maneuver-induced rollover, such as cornering and lane-change maneuvers.
Control Strategies
Two performance measures were used as control strategies: the lateral load-transfer ratio and the trailer lateral acceleration.
A 75,000-lb, five-axle tractor/semitrailer computer-simulation model was used to demonstrate the effectiveness of the proposed active-control system. A new sliding mode controller was designed and found to be effective in improving the dynamic performance and roll stability, regardless of parameter uncertainties such as tires or suspension characteristics. The controller torque requirement is limited by the differential dynamic braking forces that the tractor drive axles are able to produce as a function of the applied dynamic loads and road-surface condition. The results show that with this new controller, the vehicle lateral acceleration can be controlled to prevent rollover without significant change of the vehicle trajectory when active yaw torque is applied to the tractor drive axles. Simulation results also indicate that the vehicle rollover might be prevented using either the lateral load transfer ratio or the lateral acceleration at the trailer center of gravity as control strategies.
El-Gindy concluded that the control strategy based on limiting the lateral acceleration is more practical for implementation because “the lateral acceleration is much easier to measure than the load-transfer ratios. Also, the change in vehicle trajectory in the case of controlling the lateral acceleration is less than in case of controlling the load-transfer ratio.”
El-Gindy cites research done in 1986 by R D Ervin at the University of Michigan Transportation Research Institute. Ervin examined, via simulation, the sensitivity of a heavy truck's roll threshold from variations in size and weight parameters. His goal was to examine the effect on a truck's rollover threshold due to legislative changes in requirements of the size and weight variables — specifically the effect of changes in axle loading, gross vehicle weight, width, payload CG height, and lateral offset of the payload.
“The handling dynamics of an articulated vehicle, such as a tractor semi-trailer, differ from that of a non-articulated vehicle significantly,” El-Gindy says. “The ability for the trailer to articulate or pivot relative to the tractor contributes an additional mass with which the driver must be concerned. In addition, the response of the trailer to inputs from the tractor, such as steering maneuvers, is typically amplified and lags behind the response of the tractor, making it difficult to control. Many times this causes stability problems and a trailer may start to roll excessively or begin to jackknife (yaw instability) before the driver is aware of the problem and, therefore, may not have time to take corrective action.”