|Electronic Transmission Control
by Frank Owen
(last revised 2 August 2008)
Gear shifting is yet another automotive subsystem that has been affected heavily by automotive mechatronics. Prior to the explosion of micro-processor-controlled systems in cars, there was, of course, the automatic transmission. As we shall see, both the manual transmission and the automatic transmission are still in use, as well as yet another transmission that allows the car to change gears without removing the load.
A manual transmission consists of two shafts: an input shaft from the engine and an output shaft that delivers power to the drive wheels (see Figure 1). These two shafts can be coupled together in a number of different ways by gear pairs. Each gear pair represents a different speed reduction ratio. Usually a car has five forward speeds and one reverse speed. When shifting gears, the load on the gear pair must first be removed, so that the transmission is not transmitting any load while the gear change is taking place. This is done with the manual clutch. (See http://auto.howstuffworks.com/transmission.htm for a more detailed explanation of how a manual transmission works.) From a stop the driver applies gas. When the first gear has “wound out”, he or she pushes in the clutch while removing gas, then shifts into second gear. Thus the power transmission is interrupted while the gear changing is taking place.
Figure 1 – Simplified model of a manual transmission
An automatic transmission, by contrast, is a set of planetary gears or multiple sets really. By keeping some of these gears stationary and allow others to move, various speed reductions corresponding to the various forward gears can be achieved. There is also a torque converter in the transmission that represents a hydraulic coupling between the engine and the drive wheels. A hydraulic linkage decouples the engine and the drive wheels, so that shifting can take place without damaging mechanical parts. (For a more complete description of the function of a hydraulic automatic transmission, see http://auto.howstuffworks.com/automatic-transmission.htm.) To a driver the gear changing is automatic, that is takes place without his or her direct intervention. But the driver can perceive the discreet gears by feeling the transition and watching the drop in engine speed on the tachometer with each specific gear speed.
Both manual and automatic transmissions are now controlled by automatic shifting systems. With a manual transmission, the action of pushing in a clutch and then moving the gearshift lever are performed by electro-mechanical actuators that make these motions for the driver. Thus the distinction between a manual transmission and an automatic transmission has been blurred, as far as the driver is concerned. He or she just steps on the gas. With electronic control, the direct link between the input (driver) and the output (gear) has been severed. The microprocessor mediates the input and decides what the output should be. The gear shift lever becomes merely an input device for the driver’s wishes. The microprocessor senses the car’s environment and makes an intelligent decision on what the appropriate gear should be.
In the simplest case, consider a straightforward acceleration from a stop with an automated transmission. By “automated transmission” I mean a transmission under control by a central processor. The microprocessor senses the speed of the car and the speed of the engine. When, for a certain speed, the engine RPM reaches a certain value, the microprocessor sends out commands to actuators to shift the transmission into the next gear. If the car has an automated manual transmission (AMT), i.e. a transmission with an automated clutch and gear shift, the microprocessor sends out commands to actuators to
actuate the clutch
reduce the throttle to idle
let out the clutch
Of course the commands would be slightly different with an automatic transmission because the transmission itself is different. But the shifting strategy, at what engine RPMs to shift gears, would be the same because a given engine has a given performance curve.
Regarding the AMT, the shifting sequence can be optimized so that the unloading, shifting, reloading is made as smooth as possible. Thus programming and the control by an automated system would allow someone who doesn’t know how to drive a stick shift to drive one like an expert. A slightly more complicated shifting strategy allows the shifting points to change. For example, when you are driving a manual transmission and you want to accelerate quickly, you usually stay in any given gear longer before shifting up to the next highest gear. A microprocessor can be programmed to sense this wish to accelerate quickly and then to delay the normal shifting points until the car reaches respectively higher speeds. A wish for a leisurely acceleration could also be sensed and early up-shifting implemented.
Besides these operation advantages, the mediation of the microprocessor also allows for the correction of driver errors, for example accidentally shifting into reverse or shifting the transmission into an inappropriately low gear when the car has too high a speed. Thus, again, the insertion of a microprocessor into the system allows for convenience, flexibility, and safety not found in a non-automated transmission.
In a fully automated vehicle, the transmission control can be integrated with the electronic engine control and indeed with the overall electronic management of a fully mechatronic automobile. Transmissions must work closely, for example, with the ABS system and the ESP (electronic stability control) systems in modern mechatronic cars.
To bring home the point that the transmission has become a programmable unit, consider Figure 2. This shows a gearshift. What is obviously missing is the transmission. This is possible because the gearshift lever is simply a wish input device in today’s mechatronic auto. Note that activating the gearshift lever simply generates electronic signals that are then interpreted by the car’s central control system.