Cover Story E lectrificat ion of powertrains
E-Drive with electrically controlled differential Schaeffler is presenting an all-wheel drive electric vehicle named “Active eDrive”. The name is intended principally to convey innovation and the USP of the drive system: an electric differential with a torque vectoring function. The system combines the final drive with intelligent transverse torque distribution which, when used on axles, enables the distribution of torque over the longitudinal axis of the vehicle. The final drive can be integrated in both electric and hybrid vehicles with or without a range extender capability. The authors first explain the mechanical requirements and then describe the electrical systems that are intended to fulfill these.
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auth o r s
Dr. Tomas Smetana
is Head of Development Electric Drive Trains and Advanced Development Transmission Components at Schaeffler Technologies in Herzogenaurach (Germany).
Thorsten Biermann
is Head of Product Development Differential Systems at Schaeffler Technologies in Herzogenaurach (Germany).
Marco Rohe
is Project Manager in the Field of E -mobility at AFT in Werdohl (Germany).
Electric vehicle
The design concept of the electric drive system “Active eDrive” was perceived by Schaeffler as offering such significant prospects that a decision was taken at a very early stage not only to put the idea through transmission rig tests but also to assess it in an electrically driven vehicle under the most realistic conditions possible. The platform selected for the test vehicle was a Skoda Octavia Scout 1,8 TSI AWD, ❶. Since an all-wheel drive system was already in place, it was a relatively inexpensive matter to integrate the electric drive system in both axles, thus allowing maximum freedom in the testing of active torque distribution on the front and rear axles. This means the vehicle can be tested and compared in front-wheel drive, rear-wheel drive and all-wheel drive configurations. In addition, comparisons can be made with a conventional drive system. ❷ shows the components of the concept vehicle. The electric vehicle has a maximum power level of 220 kW and, despite the additional weight from about 400 kg compared to the standard vehicle, can accelerate from zero to 100 km/h within 8 s. It has an electronically regulated top speed of 150 km/h. Torque vectoring
Dr. Wolfgang Heinrich
is responsible for the Development of the Electric Motors for the Active eDrive at IDAM in Suhl (Germany).
In contrast to conventional differentials, so-called active differentials not only compensate for differences in speed but also enable torque to be individually distributed within a drive axle, a function known as
torque vectoring. Due to the different circumferential forces on the wheels, a yawing moment can be generated about the vertical axis of the vehicle that makes it possible to influence the driving dynamics and driving stability in a targeted manner. This function is similar to the control of tracklaying vehicles, which can travel a curved path by varying the speeds of the tracks relative to each other. In contrast to ESP systems, control intervention by means of torque vectoring does not have the effect of braking the vehicle but simply redistributes power within the axle. The relationship between wheel speed and drive torque is described in greater detail below. ❸ shows the differences in tyre slip when active torque distribution is used. In State A, it is initially assumed that the ve hicle is travelling straight ahead and both rear wheels have the same speed and drive torque. There is equal drive slip on both wheels. If the left wheel is then braked, the right wheel is simultaneously driven so hard that the total drive torque on the vehicle remains constant. State B shows the relationship between the braking torque of the left wheel and the required brake slip. In order to keep the drive force constant despite the braking force on the left rear wheel, the drive torque on the right wheel must be increased to State C. The diagram shows the required operating point on the slip curve of the right wheel. It can be deduced from the operating points of both slip curves that the distribution of drive torque on the driving axle requires a change in wheel speeds and vice versa. In order to generate the differential torque for
❶ Prototype vehicle with Active eDrive and approval for official roads 0 5 I 2 0 11
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Cover Story E lectrificat ion of powertrains
① Active differential ② Converter ③ Accumulator ④ Charger ⑤ Power socket ⑥ DC / DC converter ⑦ Control unit Protronic
❷ Schaeffler’s prototype vehicle with two electric driven differentials
the torque vectoring function, it is therefore necessary for one wheel to accelerate in relation to the other. Transmission
An essential component of the electric drive system is the planetary type differential that was developed by Schaeffler as an alternative to the bevel differential in conventional drive trains [2]. When the control system is inactive, this spur gear differential distributes the wheel torques evenly. The planets of the differential mesh with both “suns”, each of which is connected to one wheel. The three pairs of differential planets in principle fulfill the function of the differential bevel gears in the bevel differential. They roll against each other to compensate the speed difference between the wheels. In order to generate a differential speed between the wheels, relative motion between the planet pairs in the spur gear differential is required. This function is carried out in the transmission concept, ❹, by a superimposed transmission comprising a so-called coupled gear stage and a planetary gear. The coupled gear stage shares the crosspiece and one planet with the lightweight differential. When the sun of the coupled gear stage is rotated relative to the ring gear, this results in a relative speed between the differential planets of
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the differential. In turn, this generates a speed differential between the wheels. Both the coupled gear stage and the planetary gear allow a high transmission ratio between the servomotor and the wheel. The two planet stages of the same size in the planetary gear share a common crosspiece and the drive output is via two separate identical ring gears that are connected respectively to the sun and to the ring gear of the coupled gear stage. The sun of one planetary gear is fixed to the housing, the other sun is connected to the servomotor. If a torque is generated in the servomotor, the planetary gear rotates the ring gears of the planet stages of equal size in relation to each other. As a result,
a moment opposing the ring gear is applied to the sun of the coupled gear. When travelling straight ahead, there is no rolling of the differential, superimposed transmission or the rotor of the servomotor, thus minimising any losses. If the servomotor is not activated, no differential torque is applied. The wheel torques are identical and the servomotor does not support any torque. In the deactivated state, the active differential acts in the same way as a conventional differential with an increased lock-up value due to the mass inertia of the superimposed transmission and the servomotor. In order to enable transverse distribution of torque, such a system requires significantly less electrical system power than drive systems that have one electric motor per wheel, such as in electric motors close to the hubs or in-wheel motors. Furthermore, there is only one planetary gear stage between the primary motor and the differential. This design not only ensures a high degree of efficiency, but also has an extremely compact design and high power density due to the coaxial arrangement of the motors, the individual planetary gear stages and the differential, as shown in the cross-section of the prototype, ❺. In terms of its design, the prototype is already very close to implementation as a possible volume production version. Electric motors
The water-cooled synchronous motors for both the drive system and the active torque distribution system were developed by the company IDAM (INA Drives and Mechatronics). The main drive is designed
❸ Relationship between drive torque and wheel slip /FZG
❹ Technical principle of the electric transverse torque distribution: ① lightweight differential, ② linkage, ③ planetary drive, ④ servomotor, ⑤ primary motor
for speeds between 8000 and 14000 rpm. Particular attention was paid to the efficiency level. For this reason, a so-called HPMS (Hybrid Permanent Magnet Synchronous) motor was used. The permanent magnets recessed in the rotor plate not only give high energy density but also heavily restrict the active rotor heating that otherwise occurs due to eddy current losses in the magnets and the plate. The special rotor construction facilitates not only the normal magnetic torque but also the occurrence of an advantageous reluctance torque. This has a favourable influence on the speed-dependent moment curve in the field weakening range, but requires special measures in terms of control technology. The motor topology selected gives a high power density. The maximum efficiency level is 96 % in the moderate speed and dynamic range. The requirements placed on the servomotor are significantly different. High torque at low speeds is optimum in this case. A typical permanent magnet synchronous motor (torque motor) is used that develops a maximum power level of 10 kW at a speed of 1250 rpm. A suitable motor topology here is one with concentrated winding in the stator and integrated PM magnets and flow concentration in the rotor. Electrical safety system
The air-cooled lithium-ion battery has a total capacity of 17,8 kWh and comprises 0 5 I 2 0 11
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❺ Active electric differential – prototype assembly
110 cells each with a capacity of 45 Ah. In order to prevent “thermal runaway”, the battery is fitted with 22 temperature sensors connected to the “Battery Management System” (BMS). The BMS monitors the voltage of each cell and shuts down the high voltage (HV) battery if a defined temperature threshold is breached, in order to prevent a further increase in temperature. In a pure electric vehicle, however, shutting down the battery would lead to an abrupt and safety-critical loss of drive torque. In order to prevent this, the battery temperature is monitored by the vehicle control unit. If a temperature threshold is breached, the drive torque is restricted. If the temperature nevertheless continues to increase, the system switches to “emergency running” mode. In this mode, the maximum travel speed and maximum drive torque are reduced to the point at which all drive components can run without active cooling. It is only when the third threshold value is breached that the battery is shut down. Furthermore, the electrical safety system monitors the temperature of the electric motors and the inverter. In order to give the driver information about current system temperatures, the temperature of the most critical components is displayed in the instrument cluster. Since all the components are subject to different temperature thresholds, the individual temperatures are converted to give a percentage of the threshold values. The display then shows the maximum percentage.
In order to prevent direct contact with high voltage power, all the plug connectors in the HV circuit are incorporated in an interlock circuit. If any plug in the HV circuit is removed or loosened, the interlock circuit is broken and the high voltage via the main relay to the traction battery is shut down. The link capacitors are discharged in the inverters. Insulation of the HV circuit from the other vehicle components and chassis is monitored with the aid of an insulation monitoring device. If a defined acceleration of the vehicle is exceeded in the case of an accident, the airbag system sends the crash signal to the control units via the CAN-Bus. The vehicle is also fitted with longitudinal and transverse acceleration sensors. In response to the crash signal from the airbag system and if a defined acceleration is exceeded, the vehicle control unit implements an emergency stop and thus switches the entire HV network to a voltage-free state. The battery is also shut down and a special relay to the inverter is opened. Since this is a testbed, the emergency stop can also be manually initiated by the driver. Electronic control
In the same way as any control unit in conventional vehicles, the vehicle control unit in the eDrive electric vehicle performs control and regulating functions. Based on the pedal position and the current travel speed, the torque required by the driver is
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Cover Story E lectrificat ion of powertrains
to the control signals at the outputs. In the electric vehicle, Protronic also controls the accessory devices and, since signal conditioning and power output stages are already integrated, no other external modules are necessary. Summary
❻ Control unit Protronic from AFT
determined. Before this is distributed to the two drive motors on the front and rear axle, it is limited in accordance with system status and defect monitoring. The other functions of the vehicle control unit include monitoring of the connected components, actuators and accessory devices. A defect status is determined for each component and then evaluated in the defect manager system. This initiates an appropriate handling strategy if a defect is found. For monitoring of the cooling loop, which is configured separately for the drive on the front and rear axle, several temperature sensors and flow sensors are integrated. The vehicle control unit also controls the two coolant pumps and air coolers as well as the electric vacuum pump for the brake booster, the battery cooler, additional electric heating and the parking brake.
DOI: 10.1365/s38314-011-0045-2
Torque vectoring functionality
The vehicle control unit in the Active E-Drive offers the possibility of regulating the transverse torques generated within an axle by means of the torque vectoring system. Through its interfaces, the user can play out different control strategies in order to achieve optimum utilisation of the potential of the E-Differential system. This involves adjustment of the two inverters for the servomotors that then provide the corresponding torques. In a first development stage, simple torque maps as a function of vehicle speed and steering angle were used. This made it possible to investigate the basic functionality in simple travel dynamics tests, for example during stationary circular path
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travel. During further development, these were then replaced by a travel dynamics controller whose functionality is being continually expanded. Control Unit with degrees of freedom
Since the electric vehicle is a prototype and the use of torque vectoring functionality in this field is a novelty, an important requirement for the developers was to use a control unit that offers not only the necessary performance level but also degrees of freedom for the adaptation of input and output interfaces. A decision was made in favour of the development control unit “Protronic”, ❻, from AFT Atlas Fahrzeugtechnik GmbH. The software tool was helpful in developing the strategy on a model basis using Mathlab/ Simulink. This was an advantage especially in the higher abstraction levels of implementation. This means, for example, that single track models developed and validated in Simulink can be tested in the same development environment in the vehicle. Through automatic code generation and the completely automated build process, it was possible to achieve rapid development cycles that were necessary in particular for testing on the appropriate proving ground. As a prototyping control unit, Protronic offers the user the possibility of setting parameters (such as offset, thresholds or hysteresis) individually for each input. This is made possible by the use of programmable logic chips, socalled FPGAs (Field Programmable Gate Arrays). The same setting facility applies
With the electrically powered prototype Active eDrive, the Schaeffler Group is presenting a further concept vehicle for alternative drive systems that could offer the optimum basis for the control strategies of the future. On the basis of driver trials and tests, Active eDrive represents a milestone in terms of functionality. The system combines the final drive with intelligent transverse torque distribution which can be used for providing steering assistance, aiding traction or stabilising the vehicle when it comes close to its limits. In contrast to the solutions in current volume production, the system for transverse torque distribution is based on electric rather than hydraulic actuators. One significant advantage of electromechanical torque vectoring is the short response time. The full differential torque is available on the axle no more than 40 ms after the nominal torque is specified. Current measurements show that, particularly with rapid load changes, this is indispensable for controlling the vehicle in extreme driving situations and ensuring driving safety. The control unit Protronic from AFT offers flexible interfaces for the testing of different control strategies in order to further investigate and optimise the system potential of the Active eDrive. The Schaeffler Group vehicle shows that electric mobility, CO2 reduction and driving pleasure are development objectives that do not have to be contradictory but can in fact be realised together. References
[1] Biermann, T.; Smetana, T.;, Höhn, B.; Kurth, F.: Schaeffler Leichtbaudifferenzial, VDI-Kongress Getriebe in Fahrzeugen, Friedrichshafen, 2009 [2] Biermann, T.; Smetana, T.; Höhn, B.; Kurth, F; Wirth, C.: Schaeffler Active eDifferential for Future Drive Trains, Schaeffler Symposium, BadenBaden, 2010 [3] Biermann, T.; Smetana, T.: Schaeffler Lightweight Differentials and Electric Drive Modules, CTI Symposium Transmissions, Berlin, 2010 [4] Biermann, T.; Smetana, T.; Rohe, M.; Heirich, W.; Höhn, B.; Kurth, F.; Wirth, C.: Innovative Kom ponenten und Systeme für alternative Antriebe, VDI-Kongress Getriebe in Fahrzeugen, Friedrichs hafen, 2011
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