Software stabilizes electric vehicles on winterly roads
In principle, an electric all-wheel drive vehicle with several engines has a major advantage over vehicles with combustion engines: front and rear axles or all four wheels have their own electric machines, so that the driving force can be distributed variably. “It’s like having a separate accelerator pedal for each axle or wheel,” explains Ulf Hintze of Porsche Engineering. In conventional all-wheel drive vehicles, on the other hand, only one engine operates, the power of which is distributed to the axles via a central differential gear. The torque ratio is usually fixed. In the electric car, on the other hand, the torque is controlled purely electronically, which is much faster than via mechanical components. With reaction speeds in the single-digit millisecond range, software doses the forces so that the vehicle always behaves neutrally.
The software can be used for different constellations and engine arrangements. As a rule, the basic distribution is first developed, i.e. software that regulates how much force is transmitted to the front and rear axles. For example, a distribution of 50/50 would make sense when driving straight ahead and evenly distributed weight. When the driver accelerates, the software switches to complete rear-wheel drive; in a sharp curve, on the other hand, to pure front-wheel drive. Since the optimization is purely electronic, it is theoretically even possible to offer the driver various configurations: for example, one mode for sports car acceleration, another for smooth cruising.
The second task of the software is to adapt the torque to the wheel speed. The algorithms pursue a simple goal: all wheels should spin at the same speed. When driving on a snow-covered, winding mountain road, this task is not trivial: If the front wheels touch an icy surface, for example, they could spin without electronic intervention. But the torque control recognizes the non-optimal situation immediately and redirects the torque in fractions of a second to those wheels that rotate more slowly and still have grip. The software-controlled control reacts considerably faster than its mechanical counterpart, the speed-sensitive limited-slip differential.
The third and perhaps most important function of torque control is to control lateral dynamics, i.e. to defuse critical driving situations. For example, the control software immediately prevents understeer or oversteer.
A software module called Driving Condition Observer (“Observer”) is involved in all decisions to intervene. This constantly monitors a large number of factors such as steering wheel angle, position of the accelerator pedal or yaw torque. The data for this is provided by a yaw sensor, which is already present in today’s vehicles. The measured actual condition is compared with a dynamic model of the vehicle, which represents the target condition under normal conditions. If the “observer” detects deviations, the software intervenes.
Although a conventional driving stability program (ESP) does all this, the safety system can do more with an electrically powered all-wheel drive vehicle: while a classic ESP only brakes, individual wheels can accelerate additionally with an electric vehicle. In this way, the vehicle is “pulled” back into the right lane and does not lose speed. In addition, the intervention is smoother than with hydraulic ESP.
As the customer had specified that no additional sensors could be installed, the software developers had to ask the software developers’ creativity for this SUV project. “The observer must also use the important parameters from unusual data sources: For example, the torque control communicates with a sensor that detects the car’s angle of inclination and is usually used to automatically adjust the headlights.
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