Figure 4 – Double clutch transmission system from Volkswagen Besides just gear shifting, other systems—specifically all-wheel-drive and hybrid drives—have benefited from electronic control. In the latter, the wheels are driven either by electric motors or by the engine. It would be hard to imagine the manual management of a hybrid drive.
Basic Shifting Strategy Figure 5 shows a typical shifting characteristic curve. Note that this curve is plotted on a scale of accelerator pedal position vs. car speed. As is normal in a microprocessor-mediated system, the gas pedal is simply a wish-input device. So the accelerator pedal simply is telling the central microprocessor in the car how fast the driver wants to accelerate.
Figure 5 – Shifting characteristic curve Note that the characteristic consists of two curves, one for shifting up and one for shifting down. The “hysteresis” is built into the design of the shifting strategy to prevent constant shifting up and down if a driver happens to be driving near a shifting point. Rückschaltung means downshift and Hochschaltung means up-shift. The best way to understand these two curves is to consider simple examples first. So let’s see what happens from the AP, the Arbeitspunkt or just simply a typical operating point.
As a first case, consider the situation where the driver does not change the accelerator pedal position. So the shifting state stays on a horizontal line. If he/she encounters a hill, the speed of the car will drop off. Eventually point A will be reached on the Rückschaltung curve, so the car will downshift. If, on the other hand, the car starts to go downhill, point C will be reached and the transmission will shift into a higher gear.
Now consider a vertical movement on this shifting characteristic. This would be a sudden change in the driver’s wish without a change in speed. For example, if the driver wanted to pass a vehicle suddenly, he/she would step on the gas pedal and reach point B. The car would downshift to achieve quick acceleration. On the other hand, if the driver suddenly lets up on the gas pedal, point D will be reached, and the car will up-shift to reduce engine RPMs for less engine braking and more economical operation.
Of course there is such a pair of curves for each gear change. And since the shifting strategy is electronically controlled, these shifting characteristics can be postponed (pushed to the right on the speed axis) for sportier operation or shifted to the left for more economical/environmentally friendly operation.
The Big Picture As previously mentioned, in a fully mechatronic car the shifting control does not happen in isolation. The electronic control of the transmission needs to be aware of other electronically controlled systems and interact with them. At the same time, the shifting strategy of a manufacturer is unique and is separate from the actual hardware that constitutes the transmission. The strategy for managing this complexity is to adopt a layered approach. This scheme is illustrated in Figure 6.
