Torque-Vectoring mit Koppelgetrieben und reduzierten Koppelgetrieben in elektrischen Achsantrieben Torque Vectoring in electrical drives with coupled plane- tary gears and complex compound planetary gears

Additional Authors: M.Sc. Michael Wein and Dipl. -Ing. David Reitze (Audi AG, Ingolstadt) Electrically driven axles in cars equipped with torque vectoring functionality provide enhanced driving dynamics – in addition to the provision of a hybrid vehicle with AWD-system. Electrical drives with torque vectoring function can be displayed through two electric machines, each driving one wheel. Also the systems, which are currently used in series production, and in which a multi-disk clutch or a multi-disk brake generates differential speed between the wheels through direct slippage, and in which generally a superimposing unit is switched between the wheels or between one wheel and the differential carrier, can be used for the redistribution of driving torques in electrical gearboxes. Thus, one e-machine is sufficient for electric traction. If an electric actor – instead of clutch elements is interconnected with the superimposing unit, then the losses inherent to the system, caused by clutch traction during control, cease to exist. The “ZG Zahnräder und Getriebe GmbH” has proposed a particularly suitable gearbox structure for an electrically actuated torque vectoring gearbox which permits the occurrence of high levels of difference torque between the wheels by means of a quite small (second) superimposing electric machine; this takes place independently from the driving torque of the axle, which is represented via the primary traction machine. For a high ratio to the superimposing electric machine, in particular in coaxial gearbox layouts, it is recommended to execute the differential of the vehicle in the form of a complex compounded planetary gear. This multiple use of the differential’s components results in a high level of performance in combination with a high level of transfer ratio. With the help of an assembled prototype, the presentation will explain the layout of a differential which is comprised on a complex compounded planetary gear. The prototype was developed during a research project, funded by “Bayern Innovativ” with the participating project partners Audi, Hör Technologie, FZG (“Gear Research Centre”) and of the ZG GmbH. Audi has successfully tested the electric drive at the test stand and during road tests. What’s more the presentation also shows the potential of single-gear axle drives which have the entire differential unit, including the coupled gear, arranged in the power-flow between the e-machine and the axle drive ratio. This makes integration into the rotor of the traction machine particularly efficient with regard to weight and construction space. By means of a concept design the principles are explained. 1. Basic Information 1.1 Torque Vectoring Torque Vectoring gearboxes enable individual torque distribution to the wheels of a drive axle. Through different peripheral forces at the wheels it is possible to generate yaw moment around the vehicle vertical axis, which enables direct influencing of driving dynamics and driving stability. Unlike the ESP system, the vehicle will not be slowed down by control intervention. In contrast to current ESP, Torque Vectoring gearboxes are capable to effectively prevent understeer and thus to enhance safety and dynamics of the vehicle. Systems currently used in series production create the required differential speed at the wheels through a superimposing unit and by enforcing fixed speed ratio on the closing of a multi-disk clutch or a multi-disk brake. Differential speeds at the wheels deviating from the fixed transmission ratio in the superimposing unit will be generated through slippage in the control element. This type of clutch-based systems shows drawbacks regarding efficiency, responding behaviour and controllability. From [2], [3] and [4] systems have been presented, in which both control elements have been replaced by an electric machine. Generally, the wheels’ differential speed onto the wheels is transmitted by means of a superimposing unit. In case of inactive system and straight-line drive (i.e. wheels turn with equal speed), the electric machine stands still so that no significant losses will occur. In the following, the basic principle of this gearbox structure will be explained. For analysis and synthesis of such gearboxes, a procedure according to Helfer [1] has turned out adequate. The procedure will be presented in brief in the following. 1.2 Planetary Gears – Analogue Model according to Helfer In [1] Helfer has published an analogue procedure basing on the similarity of a planetary gear and a bar. It can be demonstrated in a generally accepted way that torque moments and speeds at the shafts of a planetary gear are in the same ratio toward each other like forces and speeds in the nodal points of a bar. For this, the distances between the nodal points of the bar must have a specific ratio which is exclusively characterised by the transmission in the planetary gear. Fehler! Verweisquelle konnte nicht gefunden werden. shows the analogue procedure by means of an example: If the sun shaft is determined in the planetary gear according to Figure 1, a fixed transmission ratio between carrier shaft and ring gear shaft will occur. The same applies to the bar: If the displayed bar is deviated around node 1, peripheral speeds will occur at the nodes; these speeds match the speeds in the planetary gear as long as distances a and b between the nodes are set correspondingly. Figure 1: Engine Speed/ Speed Analogy according to Helfer [1] Also the torque forces developing at the shafts and the forces developing at the nodal points match proportionally if sections a and b have been set correctly. An example of this analogy is displayed in Figure 2. Figure 2: Torque/ Forces Analogy according to Helfer [1] This analogy cannot only be applied to individual elementary planetary gears, but also to gearbox systems consisting of several elementary gearboxes, which are connected arbitrarily. 1 s 2 ns n2 1 s 2 vs v2 b a : 0 1  n ) 1 ( 12 12 2 i n i n s      12 12 2 1 i i n n s   