It was at the Frankfort Motor Show in september 2002 that we saw the first Volvo S60 AWD with electronically controlled all-wheel drive. The system is also called the AOD (Active on demand) system and was developed by the Swedish company Haldex Group. The goal of the Haldex system or AOD is to evenly distribute power between front and rear wheels in an intelligent manner.
The beauty of the system like this is that it operates without any input from the driver. That's a nice option for people who don't want to be bothered with pressing button, locking out hubs etc, or forgetting to unlock hubs. It is also a great safety feature as it operates in all road conditions receiving feedback from all the cars various sensors. Modules that run the Haldex control the AOD in an instant giving the car superior road handling capabilities. Many folk with often compare with Suburu, stating "at least Suburu is real four wheel drive". Where the Suburu is bred from four wheel bush bashing, the Volvo is an highway car and the AOD gives it optimal road handling even on dirt roads.
So what makes this system so intelligent? The onboard differential electronic module (DEM) communicates with the engine control module and brake control module via the control area network (CAN) to determine loss of traction and to anticipate different driving situations.
Haldex
During normal driving conditions, the Volvo AWD operates mostly as a front-wheel-drive car for fuel-efficiency. The system senses loss of traction in the front wheels and selectively delivers power to the rear wheels. During FWD (front wheel drive) the car is operating 99:1 where 99% of power is delivered to the front wheels and when the DEM is activated power is delivered to the rear wheels by a mechanical pump and a wet multiplate clutch up to 20:80 torque power front to rear (note that rotational speeds of the rear wheels will never exceed the speed of the front wheels- i.e. the total maximum split is theoretically 50:50). This action results in reducing the speed difference between front and rear wheels during a time of lost traction.
The DEM is physically attached to the rear differential, which forms a unit with the stepper motor and the control valve. The DEM communicates with the engine control module and brake control module via the control area network to determine loss of traction and to anticipate different driving situations. The DEM uses sensor signals to control oil pressure on the wet clutch plates by adjusting the control valve. According to Volvo, the system can react to as little as a quarter turn difference between the input shaft and output shaft of the differential.
The electronically controlled AWD system is "intelligent"; by measuring front wheel spin, throttle position, and other data, the system can determine how quickly to distribute power and how much power to distribute. When accelerating on a difficult surface such as gravel, the rear wheels can be engaged quickly with maximum power transfer. Conversely, when cornering or parking at lower engine speeds, the system knows that the difference in speed between the wheels does not require engagement. As a result, the inertia experienced with other systems in similar situations is avoided, according to the company. In addition, it does not require special consideration in specific situations such as towing and tire changing, which can be carried out as normal.
AWD schematic
The AWD is connected to the vehicle's multiplex computer system. As a result, it is integrated with other systems in the car—including the ABS, the traction control system, and the optional Dynamic Stability Traction Control system—to optimize AWD during all driving situations.
