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Transmission control

Various types of automatic transmission

Those were the days, where (only) two parameters influenced the automatic gear-selection, the position of the gas pedal (pressure) and the revolutions of the driven wheel (speed). If the vehicle speeds up, when driving downhill with the gas pedal in a constant position, the gearbox shifts to a higher gear then down to a lower gear when driving uphill again. On the other hand, if the speed remains constant, the gearbox shifts down when accelerating and always shifts up when decelerating.

The old hydraulic control can also do all this and one small thing besides, e.g., the kick-down. If this is now done electronically, the additional possible functions are almost incalculable. Thus, buttons can be included for sporty- and also for economic driving. In the first case, the gear is held longer when accelerating, i.e. the gear is only shifted up at higher RPM. Recently these buttons can also be left out, because the gearbox- control can, through analysing the driving stlye, e.g., when accelerating, automatically recognize what type of driver is at work. When judging a situation for which there is no clear “no/yes” answer, the Fuzzy-logic plays an important role. The static gear selection has now become a dynamic selection.

If one would work only with a control device of this type, as a test, with the sensors connected and under certain driving conditions, one would discover that, with engines which are critical in this respect, certain driving conditions are not even possible. Instead of 2000 RPM in second gear, the control device simply selects 3500 RPM in first gear, perhaps because the engine, at these RPM, has a tendency to drone. Thus, for each gear there are two characteristic curves, which represent the limits for up- and down shifting. They are described as being dynamic if they can be varied during operation.

At the latest, one should now mention the networking, which should, about now, have begun between the engine and the automatic gearbox. The engine was perhaps too hot so that the gearbox shifts later, to increase the RPM, thus also increasing the performance of the coolant pump. The gearbox was too hot and the engine has limited it's performance or torque. So now, almost everything is linked with the car and the gearbox controller notices that the car is tilting strongly, it then chooses, taking other parameters into consideration as well, the setting for downhill driving, thus not allowing upshifting quite so easily.

Nowadays, a new software concept is possible almost every day. The previous button for 'winter' has become unnecessary, because the ABS announces slipping and the car possibly pulls off in a higher gear. If e.g., the engine control recognizes a fast, as a rule, very short deceleration, then the gearbox won't upshift, because it would have to downshift again straight away. Gear-shifting in a curve may also be suppressed, because it could interfere with the car's operational stability. Trailer-operation is recognised and the characteristic curves are all moved in the direction of later up- and earlier downshifting.

It really gets hairy for the electronic control, if the actuators are also faced with an uncertain situation. One good example here, is the setting element for the clutch when pulling off. To aggravate matters, we'll take reversing, best of all, between two other cars. Basically, the clutch should only be controlled by the position of the gas pedal. If e.g., it comes too quickly at increased idling RPM, the danger of reversing into the car behind is great. If it comes too hesitatingly, it looks very much like driving school and the clutch wears out more. This is a Herculean task for the controlling, which the receiving of added steering wheel-, tilting- and distance signals could alleviate.

Modern double-clutch and automatic gearboxes often control two clutches simultaneously, one for opening and one for closing. If the first one opens too early, the passengers may perceive a jerky traction interruption. If it opens too late, it causes tension and wear and tear, which although small, is repeated with each gear shifting operation. In this case, both actuators are given torque specifications, just when a certain action should take place. Thus, the aim of an action becomes the starting point of the controlling. One could also say, the controlling works directly to avoid jerking when pulling off or shifting gear. 01/12