Audi’s production electric e-tron quattro may set a new standard for vehicle handling; advanced vehicle dynamics control
1 December 2015
The production version of Audi’s battery electric e-tron quattro SUV (earlier post), due out in 2018, is an important vehicle for the brand, and especially for Audi of America, which is driving many of the requirements for the C-segment electric SUV. At the LA Auto Show, Audi of America President Scott Keogh said that the brand is targeting at least 25% of its US sales to be e-tron—i.e., plug-in hybrid or full electric—models by 2025. (Earlier post.)
He also observed, in a conversation on the eve of the LAAS, that if an automaker can launch a defining product, the game changes. Audi intends for the e-tron quattro to become such a defining product—an Audi electric SUV in the high-volume C-segment that is reasonably priced (in the luxury sector) and delivers longer range along with all the “known” benefits of electric drive: speedy acceleration, quiet, lower cost of fueling, and zero tailpipe emissions (as well as the high-value HOV sticker for some markets). But to make this a benchmark product, Audi intends to go further, leveraging the three-motor electric quattro powertrain through very advanced software controls to deliver a ride and handling experience not possible even in an all-wheel drive electric vehicle equipped with a torque vectoring differential mechanism (e.g., a Tesla).
|Audi’s e-tron quattro concept shows its grille at the LA Auto Show. Click to enlarge.|
As Telsa’s Yifan Tang notes in the company’s recently published patent on a control system for an all-wheel drive electric vehicle (Nº 9,162,586), Tesla couples its axle motors to a differential assembly. “…the differentials used with the present invention can be configured as open, locked or limited slip, although preferably an open or limited slip differential is used.” By contrast, the two motors on the rear axle of the Audi e-tron quattro concept provide discrete control, one linked to each rear wheel, without the use of a differential.
Current vehicle dynamics control (VDC) systems preserve vehicle stability under challenging conditions, but do not routinely enhance vehicle handling.
In the filing for a patent awarded in 2014 on the design of a system for vehicle dynamics control in electric vehicles, Ian Wright, currently the founder and CEO of Wrightspeed (earlier post) and a co-founder of Tesla Motors, and his co-inventor Ryan Cash explain that existing control systems for vehicle dynamics—e.g., Electronic Stability Control, Traction Control, and Anti-Lock Braking—exercise control over engine power, and may automatically apply and release brakes on each wheel independently. Some systems allow “torque vectoring”—directing drive torque from the engine more to one output shaft of a differential than to the other.
These existing systems use a form of exception-based closed-loop control—i.e., the traction control, ABS, and stability control programs do not intervene until an anomaly is detected, such as a sudden change in wheel speed or yaw rate. Then, the control system intervenes, applying or releasing individual wheel brakes, reducing engine power, and in some cases, vectoring drive torque towards specific wheels, until the anomaly disappears.
Audi wants to change that—and the best vehicle for embodying that change is an electric one. Instead of being applied only on an exception basis, such an advanced electric vehicle dynamics control system could deliver exceptional safety and handling at lower speeds as well as at high speeds or when correcting an anomaly, suggested Siegfried Pint, Audi’s new Head of Electric Powertrain.
Although, in Pint’s phrase, the e-tron quattro SUV can “turn like a hunting dog after a rabbit”, the C-BEV (as Audi calls it internally) should be capable of consistently removing some of the impacts of dynamic forces on the driver in much more mundane driving events. The steady, smooth quality of the dynamic driving experience could prove to be unparalleled, at least in current production vehicles. If Audi can deliver on that vision, it would not only go a long way toward meeting that 25% target, but it could add a significant and distinctive new value point for electric drive vehicles in general, thereby helping to accelerate market adoption.
Pint, who has been at Audi for less than one year, came from BMW where he worked on the electric powertrains for the i3 and i8. However, he started his career as a Formula 1 engineer and is a motorcycle racer, and thus embodies both an analytical and empirical appreciation of the realities of vehicle dynamics, as well as the potential inherent in certain types of electric drive.
Vehicle dynamics. The movements of vehicles on a road surface are the subject of the field of vehicle dynamics. Specific movements of interest are acceleration and braking; ride; and turning or cornering.
In his well-known 1992 book Fundamentals of Vehicle Dynamics, Dr. Thomas Gillespie noted that:
In the early decades of the 1900s, most of the engineering energy of the automotive industry went into invention and design that would yield faster, more comfortable, and more reliable vehicles. The speed capability of motor vehicles climbed quickly in the embryonic industry as illustrated by the top speeds of some typical production cars ... With higher speeds the dynamics of the vehicles, particularly turning and braking, issued greater importance as an engineering concern.
The status of automotive engineering during this period was characterized in the reminiscences of Maurice Olley as follows: “There had been sporadic attempts to make the vehicle ride decently, but little had been done. The rear passengers still functioned as ballast, stuck out behind the rear wheels. Steering was frequently unstable and the front axle with front brakes made shimmy almost inevitable. The engineers had made all the parts function excellently, but when put together the whole was seldom satisfactory.”
Since those days, the industry has developed a large body of knowledge about the mechanistic explanations of the behavior of automobiles in response to the forces of movement. However, Gillespie adds, it was only with the advent of accessible computational power that major shortcomings of analytical methods could be overcome, enabling the creation of comprehensive models that can simulate and evaluate vehicle behavior before being rendered in hardware.
SAE first published a standard (J670) defining vehicle dynamics terminology in 1952; the standard is now in its seventh edition, having been revised most recently in 2008 to keep pace with the emergence of new technologies such as a four-wheel drive and active control.
Electric vehicle dynamics. The notion of using electric traction motors for the ongoing active control of a vehicle to deliver a much superior ride and handling experience is by no means unique to Audi. There has emerged a significant body of academic work exploring different approaches to delivering on the concept.
For example, the European (E-VECTOORC) (Electric-VEhicle Control of individual wheel Torque for On- and Off-Road Conditions) project, which ran from 2011 to 2014, involved 11 partners—including Jaguar Land-Rover and TRW—to address the individual control of the electric motor torques of fully electric vehicles to enhance safety, comfort and fun-to-drive in both on- and off-road driving conditions.
Key objectives of that project were:
Development and demonstration of yaw rate and sideslip angle control algorithms based on the combination of front/rear and left/right torque vectoring to improve overall vehicle dynamic performance.
Development and demonstration of novel strategies for the modulation of the torque output of the individual electric motors to enhance brake energy recuperation, Anti-lock Brake function and Traction Control function.
In addition to improving ride and handling, these strategies also reduce: I) vehicle energy consumption, II) stopping distance, and III) acceleration times.
In a paper presented about E-VECTOORC at the 2014 SAE World Congress, Andrew Pennycott of the University of Surrey noted that:
The handling and fun-to-drive elements of fully electric vehicles can be enhanced using continuously-acting control systems. For instance, the understeer characteristic which governs the steady-state lateral acceleration response of the car to steering wheel inputs can be modified. Electric motor actuators that are individually controllable offer greater improvements in vehicle performance over alternative technologies such as torque vectoring differentials which typically have limitations in torque transfer and bandwidth.
Control systems produce reference levels for the overall longitudinal force to satisfy acceleration and deceleration requirements and a corrective yaw moment for cornering maneuvers. For electric vehicles equipped with four electric motors, these two targets can be realized via an infinite number of combinations in the individual wheel torques; the assignment of the four wheel torques to meet the high level targets is referred to as the control allocation problem. Although each wheel torque combination will produce the same net traction and yaw moment, the combinations will differ with regard to other aspects such as power utilization and tire slip.—Pennycott et al. (2014)
More specifically, the Surrey team noted in the paper, the main objective of the control allocation is to alter the performance of the vehicle dynamics through continuously producing a corrective yaw moment. In a passive, uncontrolled vehicle, the understeer characteristic changes with the longitudinal acceleration. The project’s goal was to reduce the impact of the longitudinal acceleration on the yaw behavior; to extend the linear region of the understeer characteristic; and to alter the gradient of the relationship in this linear region.
More broadly, members of the Surrey team noted in a paper published earlier this year in IEEE Transactions on Vehicular Technology:
Fully electric vehicles with individually controlled electric motor drives allow significantly improved vehicle dynamics over vehicles with equal torque on the left and right wheels and constant torque distribution between the front and rear axles. Torque-vectoring—the controlled distribution of the traction and braking torques among the wheels, also called direct yaw moment control—enables the design of the steady-state and transient cornering responses of the vehicle. This has a potential impact on vehicle behavior with benefits regarding safety and the handling performance, more so than through the traditional approach of fine tuning hardware parameters such as mass distribution and suspension elasto-kinematics. From a terminology viewpoint, since the first torque-vectoring systems for internal-combustion-engine-driven vehicles were based on torque-vectoring differentials with very limited effect in braking, the concept of torque-vectoring was mainly associated with traction conditions. In the context of an electric vehicle with significant regeneration capability, torque-vectoring can refer to individual wheel torque control in both traction and braking.
The precise controllability of the torque generated by the electric motor drives together with their fast dynamics allows a continuous and seamless actuation of torque-vectoring, which is a significant improvement over conventional stability control systems found on current production vehicles. With the conventional systems, the interventions are based on the reduction of the engine torque and the generation of friction braking torques on specific wheels. As a consequence, the activation of the stability control system reduces vehicle velocity, making the intervention practical for enhancing vehicle safety in critical transient conditions, identified when the offset between the reference value of vehicle yaw rate and the actual value of vehicle yaw rate exceeds assigned thresholds. Alternatively, the activation of the stability control system can be triggered by an excess of estimated sideslip angle, corresponding to a potential vehicle oversteer situation in comparison with the nominal conditions. These safety functionalities can be enhanced by means of torque-vectoring through the electric motors...a yaw moment can be continuously generated without variation of the net traction force.
Further expansion on the concept comes from Wright and Cash, in the patent noted above. (The patent is assigned to Wright’s current company, Wrightspeed, which is currently focused on range-extended advanced electric driver powertrains for trucks.)
The vehicle dynamic control system includes a non-transitory computer readable medium having computer executable program code embodied thereon, the computer executable program code configured to receive sensor inputs, calculate a required wheel speed, and send appropriate signals to the electric drive motor to command the required wheel speed.
In some embodiments, the program code may be further configured to use the received sensor inputs to calculate friction coefficients, slip rates and angles, velocity vectors and resultant wheel motor speed and torque commands. In some configurations, the electric drive motor delivers torque to the wheel with precise vehicle dynamics control including stability, regenerative braking, and traction control. The gear reduction component is configured to reduce the speed of the motor by a predetermined factor to a lower speed suitable for driving the wheel, while the gear reduction component is configured to increase the torque of the wheel by the same predetermined factor.
... According to the above-described system, the vehicle dynamic control system receives inputs from the driver controls and a velocity vector sensor, which measures the speed and direction of the vehicle over the roads. In some implementations, the velocity vector sensor comprises a module including a CCD camera chip and lens, MEMS accelerometers, and a DSP for image processing. The driver controls may include a steering angle sensor for measuring steering position, a brake pressure sensor or brake pedal switch for measuring brake pressure, and an accelerator including a sensor for measuring accelerator position. The vehicle dynamic control system uses the inputs received from the velocity vector sensor and the driver controls to calculate a required speed of the wheel and send corresponding signals to the electric drive motor to command the calculated speed. The electric drive motor and drive electronics component operate to drive the wheel to the speed commanded by the vehicle dynamic control system. In some embodiments, the vehicle dynamic control system is configured to calculate effective tire circumference, tire pressure, tire slip rate, and tire slip angle.—US Patent Nº 8,718,897
Audi quattro. A key factor to keep in mind is Audi’s deep experience with mechanical four-wheel drive and torque vectoring. Audi first introduced the mechanical quattro permanent all-wheel drive (the Ur-Quattro) in 1980 at the Geneva Motor Show. (That vehicle was developed under the guidance of Dr. Ferdinand Piëch, who was the development director at the time.)
The Ur-quattro’s all-wheel-drive system featured a hollow shaft running through the transmission, obviating not only a heavy transfer case but also an auxiliary shaft to the front axle. A bevel-gear differential distributed power equally to the front axle and rear axle. On slippery surfaces, the driver could manually lock it as well as the rear-axle differential.
Quattro technology made its way into all model series. In 1986, Audi introduced a new self-locking center differential—called Torsen (“torque sensing”)—into series production. Specially toothed worm gears inside the differential could rapidly redistribute the engine’s power, sending as much as 7% to whichever axle had better traction. The Torsen differential’s flexibility made the anti-lock brake system was invariably effective.
Since 1998, even Audi compact models with transverse-mounted engines have been available with four driven wheels. These use an electronically controlled multi-plate clutch mounted on the end of the propeller shaft; the clutch is supplied with oil from an electric pump. When the oil pressure forces the plates together, they transfer more torque—up to 100 percent under extreme circumstances—in a continuously variable manner from the front axle to the rear axle.
In 2005, Audi introduced a new self-locking center differential with asymmetric/dynamic torque distribution. During normal driving, it distributes power at a ratio of 40:60 between the front and rear axles. Should it become necessary, this differential can send as much as 60% of torque to the front and as much as 8% to the rear. The self-locking center differential is configured as a planetary gear and works strictly mechanically.
Audi’s S launched in 2008 featuring a sport differential that actively distributed engine torque between the rear wheels in accordance with a given driving situation. This torque vectoring was accomplished by two superposition gears at the differential, which were operated by an actuator via multi-plate clutches. When the driver steered or accelerated in a curve, much of the torque flowed to the outside wheel—re-orienting the car. The system thus nipped in the bud tendencies toward understeer or oversteer.
In 2010, Audi introduced the RS 5 Coupé featuring the crown-gear center differential and torque vectoring. Inside the new center differential were two rotating crown gears. The rear crown gear drove the propeller shaft to the rear-axle differential while the front crown gear drove the output shaft to the front-axle differential. The crown gears meshed with four rotatable pinion gears. These were arranged at right angles to each other and were driven via the differential’s housing by the transmission output shaft.
|Crown-gear differential. Click to enlarge.|
Under normal driving conditions, the two crown gears rotated at the same speed as the housing. If the torques change because one axle loses grip, then different rotational speeds arose inside the differential; axial forces causes the adjacent plate packages to press against one another. The resulting self-locking effect subsequently diverted the majority of the torque to the axle achieving better traction; up to 80% could flow to the back. Conversely, if the rear axle has less grip, the opposite happened; up to 7% of the torque is correspondingly diverted to the front axle.
Audi combined the crown-gear differential with a torque vectoring software solution. An advancement of the ESP with electronic axle-differential lock standard on many front-wheel-drive models, it could act on each of the four wheels.
During cornering at high speeds, the software used the driver’s steering input, the lateral acceleration, and the desired degree of acceleration to calculate the optimal distribution of drive forces to all four wheels. If the software detected that the wheels on the inside of the curve—which are under a reduced load—would otherwise soon lose their grip, then it slightly braked these wheels. Various drive torques generated an additional steering torque, which stops any wheel slippage the moment it starts. ESP intervenes later and more gently if any intervention at all is necessary.
As a complement to the quattro drivetrain, Audi made an optional sport differential available, which actively distributes torque between the rear wheels. When turning into or accelerating in a curve, the majority of the torque flows to the outside wheel and pushes the four-seat convertible into the curve, counteracting the tendency to oversteer or understeer early.
|RS5 quattro with self-locking crown-gear center differential and torque vectoring. Click to enlarge.|
With the sport differential, a superposition gear is added to each side of the classic rear differential; it comprises two sun gears and an internal gear which turns ten percent faster than the drive shaft. A multi-plate clutch in an oil bath and operated by an electrohydraulic actuator makes the power connection between the shaft and the superposition gear.
When the clutch engages, it steplessly imposes the higher speed of the superposition stage on the outside wheel. The additional torque required is obtained from the inside wheel via the differential. In this process, nearly all of the torque can be directed to one wheel. The maximum difference between the wheels is 1,800 N·m (1,327.61 lb-ft). (Keep this figure in mind.)
The sport differential works in overrun as it does under load and when coasting; it is electronically controlled with response times of just a few hundredths of a second. The controller calculates the ideal distribution of the forces for each driving situation as a function of the steering angle, yaw angle, lateral acceleration, speed and other information.
In July 2014, the six-millionth Audi with quattro drive rolled off the production line at the Ingolstadt plant—about one and a half years after the five-millionth quattro.
Audi’s first e-tron quattro: 2011. In March 2011, Audi unveiled its first e-tron quattro concept: a plug-in parallel hybrid designed at Audi’s ePerformance project house. An additional electric motor drove the rear axle; its power was divided between both wheels. At the time, Audi said that the concept demonstrated the potential of an innovative, partly electric quattro drive system.
An intelligently and gently operating dry clutch connected the front electric motor with both the TFSI engine and the automatic four-speed transmission to drive the front wheels of the technology demonstrator.
|Driveline layout of the 2011 e-tron quattro PHEV demonstrator. Click to enlarge.|
Under normal driving conditions, the concept PHEV used the front hybrid drive; the electric motor at the rear axle cut in as a supporting measure when the driver accelerated. The direct connection was via a friction-optimized, single-stage planetary gear. If an axle lost its grip on a slippery road, or if optimal transverse dynamic performance so required, the drive torque was transmitted to the corresponding axle within fractions of a second.
The recuperation potential of the technology demonstrator was independent of the engaged gear at the front axle transmission. During braking on a curve the torque vectoring could freely distribute the recuperation torque between the two rear wheels. The maximum possible recuperation could then be further increased and driving safety in critical situations noticeably improved.
The two electric motors also allowed ultra-sensitive actuation via the traction control system on slippery roads.
The PSM motor in the rear generated more than 60 kW with torque of 300 N·m (221.27 lb-ft); forming a structural unit with the differential, it integrated a single-stage gear. On both sides of the differential, which is located in the rotor of the electric motor, were electrohydraulically controlled superposition gears. Very much like the sport differential described above, these allowed a dynamic distribution of the drive forces—i.e., torque vectoring.
When the dynamic mode was selected, the superposition gears actively distributed the torque from the electric motor. When the vehicle was steered into or accelerated in a curve, for example, the power flowed predominantly to the outside wheel, virtually pressing the e-tron quattro into the curve. All tendencies towards understeer or oversteer were neutralized, so that the technology demonstrator took corners as if on rails, with maximum lateral acceleration.
Audi R8 e-tron, Gen 1 and Gen 2. Audi introduced the R8 e-tron, intended to be a very small series electric sports car, in 2010. (Earlier post) The R8 e-tron featured four asynchronous motors—two each on the front and rear axles. Power was transmitted to the wheels via the single-staged transmission and short input shafts.
The distribution of the electric drive torque in the R8 e-tron distinctly favored the rear axle. Like the standard Audi R8, about 70% of the power acted at the rear in regular operation, with the remaining 30% going to the front wheels. Should slip occur at an axle, this balance changes in a fraction of a second.
The R8 e-tron also managed lateral dynamics through its four motors. This allowed torque vectoring, the selective acceleration of individual wheels and therefore active distribution of torque. Understeering and oversteering can be compensated by small power boosts and brake interventions. The electrically driven high-powered sports car retained neutrality even at maximum lateral acceleration.
In 2015, Audi introduced the Gen 2 R8 e-tron, and shifted to a rear-wheel design with two rear electric motors. (Earlier post.) The two electric motors on the rear axle each output 170 kW and 460 N·m (339.3 lb-ft) of torque—i.e., combined torque is 920 N·m (678.6 lb-ft).
|Torque vectoring in the Gen 2 R8 e-tron. Click to enlarge.|
Targeted Torque Vectoring—a need-based distribution of drive power between the rear wheels—gives the car maximum stability and dynamism.
The Audi e-tron quattro. This wealth of IP and experience with quattro mechanical four-wheel drive, torque vectoring and electric drive systems is the basis for the emergence of the fully-electric quattro drive as represented in the e-tron quattro concept. This concept, according to Audi, is very close to the the promised production version of the electric SUV coming to market in 2018.
The e-tron quattro drive uses an axial motor for the front and two co-axial, Audi-designed motors in the rear; each rear motor drives a rear wheel. Audi engineers are drawing heavily on the control experience gained with the RS8 e-tron for these rear motors. At low load, the motor on the front axle is solely responsible for propulsion.
Due to their short axial length and high relative output, axial flux motors offer size, weight, performance, and efficiency advantages when compared to traditional radial flux electric motors.
The new two-motor rear axle system leverages the work Audi has already done with the two-motor rear-wheel drive on the R8 e-tron sports car. The two powerful motors (individual output is not yet specified) enable superb lateral dynamics with huge potential for torque vectoring, Pint noted. The rear motors offer a good balance between maximum and continuous output.
The design provides direct control of the wheels; there is no differential mechanism as in some other electric vehicle designs—a key point, Pint emphasized, for a number of factors including weight, response, and span of control.
The key is clearly the software control of the interplay of the torque of the two motors—not just for performance, but also for safety. Regulators want to ensure that the car will drive straight, Pint said. “Mis-torque vectoring” must be precluded. A massive differential torque of 5,000 N·m is possible, Pint added. (Recall the 1,800 N·m differential torque for the RS5 quattro above.)
The Audi team is very far along with the high level Torque Control Manager, which works together with the Electronic Stabilization Control (ESC) to distribute the power between the rear wheels, and on the regen controls, Pint said. There is still work to be done on control and coordination with the front motor, and with controls for the other dynamic forces involved with vehicle movement.
Furthermore, as an academic research team earlier remarked (Crolla and Cao, 2012):
...despite the significant volume of theoretical studies of torque-vectoring on vehicle handling control, there is no widely accepted design methodology of how to exploit it to improve vehicle handling and stability significantly.
In other words, this is new territory, requiring a tremendous software development effort.
Nevertheless, Pint and his team are driving mules equipped with the powertrain and the advanced dynamics control system (Pint says he reserves the most extreme testing for himself). The team is pushing hard through multiple iterations of the development cycle of model in the loop; software in the loop; hardware in the loop; and prototype to hone the capabilities of the C-BEV.
The vehicle is still about two years away from production, and a number of factors—both internal and external—can still affect the ultimate feature set of the battery-electric SUV. Publicly available details about the scope of the dynamic control system are clearly absent, and numerous other critical elements need to come into play for the vehicle to be realized as envisioned. In the worst case, the vehicle could be canceled.
But if Pint and his team can deliver on the promise of such continuous dynamic control of the electric drive vehicle, they will contribute to setting a new benchmark. (Too, this all-electric quattro system would be only the first generation; the approach to active and continuous control of vehicle dynamics can expand to all four directly driven wheels, Pint suggested, as well as other elements of the vehicle, such as the suspension.)
That, in turn, could give Scott Keogh the kind of reaction he said he’d like to hear from buyers for his game-changing vehicle: “Damn! That thing’s cool! I want it!”
Tommaso Goggia, Aldo Sorniotti, Leonardo De Novellis, Antonella Ferrara, Patrick Gruber, Johan Theunissen, Dirk Steenbeke, Bernhard Knauder, and Josef Zehetner (2015) “Integral Sliding Mode for the Torque-Vectoring Control of Fully Electric Vehicles: Theoretical Design and Experimental Assessment” IEEE Transactions on Vehicular Technology, Vol. 64, No. 5 doi: 10.1109/TVT.2014.2339401
- Pennycott, A., De Novellis, L., Sorniotti, A., and Gruber, P., (2014) “The Application of Control and Wheel Torque Allocation Techniques to Driving Modes for Fully Electric Vehicles,” SAE Int. J. Passeng. Cars - Mech. Syst. 7(2):488-496 doi: 10.4271/2014-01-0085. Erratum published in SAE Int. J. Passeng. Cars - Mech. Syst. 7(4):1446, 2014, doi: 10.4271/2014-01-0085.01
US Patent Nº 8,718,897: Vehicle Dynamics Control in Electric Drive Vehicles
David A. Crolla and Dongpu Cao (2012) “The impact of hybrid and electric powertrains on vehicle dynamics, control systems and energy regeneration” Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility Volume 50, Supplement 1 doi: 10.1080/00423114.2012.676651
Gillespie, T. D. Fundamentals of Vehicle Dynamics. Warrendale, PA: Society of Automotive Engineers, 1992. Print.
US Patent Nº 9,162,586, Control system for an all-wheel drive electric vehicle (Tesla Motors, published Oct 2015)