Drivetrain control technology originally developed for high-performance rally cars competing in the World Rally Championship (WRC) has been adapted for heavy-duty, off-highway, all-wheel-drive systems, with the objective of improving safety and driveability while reducing maintenance costs. U.K.-based engineering and motorsport consultancy Prodrive saw the potential to adapt its proprietary differential control algorithm. Branded Active Torque Dynamics (ATD), it can be applied to any heavy-duty all-wheel drivetrain as original equipment and in many cases as a retrofit.
Since the system was developed to provide rapid activation and deactivation of differential locks, adapting it for heavy vehicles involved scaling it up to include more axles. The development work was carried out on a 6x6 drivetrain with a differential lock for each axle and a fourth differential to split torque between the front steer/drive axle and the rear drive axles. Hydraulic pressure is used to engage the differential locks.
Heavy vehicles working on low-friction surfaces in mine or quarry sites face distinct vehicle control issues. Normally, control of the differential locks is carried out by the driver switching the differential locks in and out, according to the conditions. Locking the differentials when descending a slippery gradient provides better braking performance but limits steering control since outer and inner wheels on an axle are rotating at the same speed. The same applies to ascents, with the driver engaging and disengaging the differential locks to balance maintaining traction and steering control. In practice, it would be difficult for a driver to respond quickly enough to engage each differential lock individually, so all are engaged or disengaged simultaneously.
ATD needs input data for wheel speed, steering wheel angle, and yaw rate, as Matt Taylor, head of vehicle dynamics at Prodrive explained. “Despite the fact that these differentials locked, we were able to adapt them to make them proportionally controlled,” he said. This means that although the differential locks were originally set up to be either engaged or disengaged, it was possible to lock them progressively.
“So we took control of all four differentials in a proportional sense, adding accelerometers around the vehicle to measure understeer directly,” Taylor continued. “We took some command input from the driver, from the point of view of where he steered, then set about improving the performance of the vehicle, that was our task.
“Basically, we extract enough information from the vehicle to understand what it’s doing—how fast it’s turning, how fast the wheels are going. We take input from the driver—does he want to go or not and which way does he want to go? The system is controlled by our in-house microprocessor unit. It’s basically got a linear map of the vehicle in it, so for a given speed or steer angle, it knows what the vehicle ought to be doing. From this information we can make demands of the differentials.
“What appears to be the control strategy development in the field is typically done by one of the attributes engineers, not by one of the control engineers,” Taylor noted. “The control engineers have made that possible to use, prior to us going out into the field, so the efficiency benefits of that are enormous.”
The end result is improved vehicle control for the driver, without the need to manually engage and disengage the differential locks. Since it was designed to improve the turning circle for rally cars on low-friction surfaces, it also produces the same effect for the heavy vehicle as well as providing better traction and stopping ability.
The sensor data is used to build a mathematical model to compare the actual dynamic behavior of the vehicle with the behavior requested by the driver. From that, adjustments can be made to each differential to ensure that the optimal engine or braking torque is apportioned to wheels individually. Other benefits include reduced tire wear and driveline shock, which could reduce repair and maintenance requirements.