Electronic Transmission Control by Frank Owen (last revised 2 August 2008) Introduction
Figure 6 - Overview of transmission control
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- Figure 7 - Gear selection strategy
- Figure 8 - Servo valve controlling pressure in an automated transmission
Figure 6 - Overview of transmission control Driving Strategy – The most abstract function is the driving strategy. This incorporates the shifting characteristic curve previously discussed. Note that there are 2(n-1) such curves, n being the number of gears, since one cannot up-shift from the highest gear nor downshift from first. Thus the normal five-speed transmission has eight such curves. These curves can be modified for Eco-friendly driving – early up-shifting, later downshifting Sport – the opposite of eco-friendly, late up-shift, early downshift Normal – between 1 and 2 Loaded shifting – going uphill or carrying a trailer, late up-shifting, late downshifting (to avoid flip-flopping gears) Unloaded shifting – going downhill, late up-shift, early downshift (to use the engine as a brake) Special – winter (cold engine, heat catalytic converter) Note that since these driving strategies are programmed, they are selectable. I.e. the same car can behave differently simply by choosing a different driving strategy from a menu. So, in general the scheme for selecting the gear can be characterized as shown in Figure 7 
Figure 7 - Gear selection strategy But this simplified scheme is not optimal for all driving situations. For example, if the drive wheels slip, it looks to the sensor as if the car is going faster. You wouldn’t want to up-shift in this case. Also in a curve it may not be wise to up-shift or downshift. Also there is the “fast-off” response, where a driver quickly moves his/her foot off the gas pedal because of a perceived obstacle ahead. A delayed shifting response to such an immediate return of the gas pedal to the null position can be implemented. Driving strategies vary from manufacturer to manufacturer. Each manufacturer does lots of tests under all kinds of conditions and situations and then incorporates the developed strategies into their shifting control. In fact this can be considered proprietary information, part of what makes a BMW and BMW. The developed software embodies a driving strategy worked out painstakingly and with the outlay of a great deal of money and effort by a manufacturer. It is worth noting also that Fuzzy Logic control is now being implemented for assessing driving situations and producing responses thereto. It is also worth noting that since driving strategies eventually wind up expresses as software, it is theoretically possible that 1) third-party developers could come up with their own strategies for improving driving strategies and then implement them on various manufacturers’ automobiles and 2) improved driving strategies could be developed which could be up-loaded into a car’s control system after its date of manufacturer. So as an owner, you could be faced with being able to actually improve the performance of your car with as little as a software upgrade. The mechatronic car is becoming more and more like a computer. Driving Functions – One big purpose of this layer between Driving Strategy and Transmission Control is to separate them, so that a manufacturer’s driving strategy can be implemented on more than one type of car and transmission. For example, a manufacturer could have both an automatic transmission and an automated manual transmission, yet the driving strategy is the same for this manufacturer, regardless of the type of transmission. At this second level, there are subfunctions that are important. For example, starting, stopping, how to implement shifting, dealing with the slip of couplings, gathering the overall state of the transmission (not low-level stuff but a layer above this), how to deal with the engine torque during shifting, evaluating gas and brake pedal positions, implementing characteristic engine performance. Also at this stage one finds the management of the lower level, a function call set to interface with all the servo-valves and solenoid actuators in the transmission. With the double-clutch transmission, it is at this level that the transfer of torque steady from one output shaft to the other takes place. Lower Level Functions – This level interfaces with the actual actuators and sensors in the transmission. For example, to transfer into a certain gear, it may mean applying an increasing and controlled pressure to a coupling. This pressure is supplied by a hydraulic servo valve. The pressure would be governed by a current to a valve. So the control loop would appear as shown in Fugure 8. 
Figure 8 - Servo valve controlling pressure in an automated transmission There may be an adaptation to this because of hysteresis in the servo valve, wear in the system, aging, change in operation of the system as fluid warms up, change due to altering fluid properties over time (fluid gets dirty, burnt), etc. Directory: ~fowen -> AutoMech AutoMech -> Gasoline (Otto) Engine Control Hochschule München International Summer School Download 0.82 Mb. Share with your friends:
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