It’s a marriage made in vehicle-engineering heaven between Ford and its all-wheel drive (AWD) coupling supplier, JTEKT North America. An intelligent AWD torque coupling is combined with Ford software that controls it under a wide range of operating conditions, using up to 27 inputs from sensors and algorithms, working in various combinations.
The coupling debuts on the all-new 2013 Escape and will be phased into other Ford applications. It enables 100% of available torque to be transferred to the front or rear axles as conditions demand. The system operates in three primary areas: traction, handling, and steady-state operation.
There also are sub-control areas and overlapping modifier algorithms for specific conditions. The complex controls and rapid-fire changes the Ford software can command are possible, according to Joe Torres, Ford Senior Engineer, AWD, because the JTEKT coupling has virtually no hysteresis during torque split changes. A high level of precision is introduced with a “PID Controller”—Proportional-Integral-Derivative—a software controller that produces, when needed, closed-loop corrections, thus providing feedback for the open-loop numbers.
Traction control with launch boost control
Traction is covered primarily in two anticipatory control zones: low-speed initial launch for low-friction (such as icy) surfaces and an overall optimization of traction performance. Both zones also use an overlapping algorithm to determine wheel slip, which is based on readings from the four wheel speed sensors, steering angle, and a reference vehicle speed input.
As a basic example of Proportional in the PID Controller, in traction mode the software refers to a lookup table based on steering angle and average front wheel speed minus average rear wheel speed. The output is AWD torque needed to eliminate slip. In one of its uses, the Integral helps tighten control of coupling slip in traction mode.
The Derivative also may be used in traction to help prepare a pre-emptive response from the coupling in a particular traction control situation.
Normally, there is some torque transfer to the rear wheels, with the amount dependent on type of vehicle operation and road condition—less when it’s dry, and some more when the road is wet or there is snow. However, in dry road conditions, torque also may be split on hard acceleration to minimize torque steer.
The low-speed traction system includes a launch boost control that transfers torque to rear wheels based on throttle tip-in, calibrated to increase transfer as necessary to handle a worst case front-rear split in road friction, so there is no sense of slip. The overall traction algorithm uses as many as 10 inputs, primarily accelerator, brake pedal, steering angle, total powertrain torque at wheels, and driver-demanded torque request, plus braking system request and torque limit during high yaw and anti-lock brake system (ABS) operation.
AWD is not necessarily disabled immediately during electronic stability (ESC) operation, and in fact the two control systems may work well together in snow, Torres said.
AWD and handling
The system gradually moves out of traction mode starting at about 22 mph (35 km/h) and segues into handling control. The handling software, designed to mimic a true center differential, is written to provide balanced torque splitting both in turns and straight ahead.
The inputs are from powertrain-calculated torque to the wheels, braking request; reference values for road speed, yaw, lateral and longitudinal acceleration; plus linear and nonlinear target values for yaw based on an algorithm to determine driver intent. An algorithm that evaluates accelerator pedal action adjusts torque split, increasing torque to the rear wheels if the driver is “pushing it.”
The software also compares actual vs. target yaw rate as part of the closed loop feedback and adjusts torque split if necessary to reduce the difference.
Although a basic limited-slip algorithm prevents runaway slip and excessive heat, the handling algorithm has the primary job of reducing yaw rate error during cornering, which helps steer the vehicle along the driver’s intended path. Within the handling software, during cornering the PID controller can add or subtract to the torque split, up to about 65%, if understeer is detected.
If the electronic stability control (ESC) also is in use, as during cornering, some torque vectoring by brake (TVBB) from the ABS comes into play, and ESC and AWD software work together to steer the car in the intended direction. The side-to-side torque split with TVBB still is 50-50, but applying the brake to the slipping wheel means that torque will go from the wheel with grip to the road, while the brake on the wheel without traction absorbs the rest.
The steady-state architecture is basically an in-or-out of AWD. The software looks at the yaw, wheel slip, requested and actual torque production rates. An operating example cited by Ford was in a sudden decision to pass another vehicle with hard acceleration. The AWD re-engages during the pass, and if the software detects slip or yaw, it assumes a split-friction road surface front vs. rear. It also keeps the system in AWD for an extended period. The software additionally is capable of detecting high load, including trailer towing, and adjusts the split to ensure durability of heat-sensitive AWD parts is maintained.
Temperature, NVH monitors
The system is continuously monitored for operating temperatures to protect it from overheat, plus driveline oscillation or lugging that might produce objectionable NVH (noise, vibration, harshness). The algorithms that detect and modify operation for these conditions require many inputs, too.
Temperature monitoring of the AWD coupling is done without actual temperature sensors. Instead, there are four validated thermal models: oil sump, clutch pack, and electromagnetic coil in the AWD coupling and oil sump in the power transfer unit (PTU, the differential gearset on the transaxle). There are 11 inputs: torque converter (locked or open), ambient air, engine coolant and transmission oil temperatures, powertrain-calculated torque to wheels, transmission gear and selector position, and wheel speed sensors.
If the torque coupling itself gets hot, the software locks it up. If that doesn’t work, the coupling opens. If the PTU gets too hot, torque throughput is reduced, Torres explained.
The NVH modifiers make torque throughput adjustments based on 18 inputs, primarily the yaw rate, a pair of targeted yaw rates, lateral acceleration, braking, transmission gear and selector position; torque converter (locked or open), steering angle, engine rpm, wheel speed sensors; powertrain-calculated and driver-requested torque numbers, and a vehicle reference speed number. The adjustments were carefully calibrated to minimize any effect on AWD functions.
Oscillation-detection software predicts when the oscillation may occur and produces a validated adjustment for the specific condition, which may be more torque, reduced torque, or even a change in the torque application rates.
Although the coupling itself has virtually linear operation—the input current produces a virtually proportional torque transfer—Ford and JTEKT go a step further. JTEKT does an end-of-line (E-O-L) test to define the curve very precisely and attaches a bar-code label that reflects the result. Then Ford adds an algorithm that tightens the output curve. Torres said this is particularly important for maintaining very predictable torque transfer, particularly with units that are at the lower end of the nominal-performance curve.
The E-O-L procedure also eliminates engineering the coupling for torque overshoot, for which the driveline would have to be strengthened to compensate. With torque under precise control, the driveline doesn’t need additional strengthening, thus saving weight.
The JTEKT group companies include JTEKT Torsen North America, which makes the rear limited slip differential for the Boss 302 Mustang and the front LSD in the F-150 Raptor.