S-AWC (Super All Wheel Control) is the brand name of an advanced full-time four-wheel drive system developed by Mitsubishi Motors. The technology, specifically developed for the new 2007 Lancer Evolution, is an advanced version of Mitsubishi's AWC system. Mitsubishi first exhibited S-AWC integration control technology in the Concept-X model at the 39th Tokyo Motor Show in 2005. According to Mitsubishi, "the ultimate embodiment of the company's AWC philosophy is the S-AWC system, a 4WD-based integrated vehicle dynamics control system".
It integrates management of its Active Center Differential (ACD), Active Yaw Control (AYC), Active Stability Control (ASC), and Sports ABS components, while adding braking force control to Mitsubishi Motors' own AYC system, allowing regulation of torque and braking force at each wheel. S-AWC employs yaw rate feedback control, a direct yaw moment control technology that affects left-right torque vectoring (this technology forms the core of S-AWC system) and controls cornering maneuvers as desired during acceleration, steady state driving, and deceleration. Mitsubishi claims the result is elevated drive power, cornering performance, and vehicle stability regardless of driving conditions.
Active Center Differential (ACD)
Active Center Differential incorporates an electronically-controlled hydraulic multi-plate clutch. The system optimizes clutch cover clamp load for different driving conditions, regulating the differential limiting action between free and locked states to optimize front/rear wheel torque split and thereby producing the best balance between traction and steering response.
Active Yaw Control (AYC)
Active Yaw Control uses a torque transfer mechanism in the rear differential to control rear wheel torque differential for different driving conditions and so limit the yaw moment that acts on the vehicle body and enhance cornering performance. AYC also acts like a limited slip differential by suppressing rear wheel slip to improve traction. In its latest form, AYC now features yaw rate feedback control using a yaw rate sensor and also gains braking force control. Accurately determining the cornering dynamics on a realtime basis, the system operates to control vehicle behavior through corners and realize vehicle behavior that more closely mirrors driver intent.
Active Stability Control (ASC)
Active Stability Control stabilizes vehicle attitude while maintaining optimum traction by regulating engine power and the braking force at each wheel. Taking a step beyond the previous generation Lancer Evolution, the fitting of a brake pressure sensor at each wheel allows more precise and positive control of braking force. ASC improves traction under acceleration by preventing the driving wheels from spinning on slippery surfaces. It also elevates vehicle stability by suppressing skidding in an emergency evasive maneuver or the result of other sudden steering inputs.
The Sports ABS system supports braking when entering into a corner by controlling power to all tires depending on handling characteristics. Braking can be controlled to obtain optimal damping at each tire based on information from four wheel-speed sensors and steering wheel angle sensor. The addition of yaw rate sensors and brake pressure sensors to the Sport ABS system has improved braking performance through corners compared to the Lancer Evolution IX.
Active Steering System
Active Steering System realizes handling with more linear response by adaptively controlling front wheel turn angle according to steering input and vehicle speed. At slower vehicle speeds the system improves response by shifting to a quicker steering gear ratio, while at higher speeds it substantially improves stability by moving to a slower gear ratio. For rapid steering inputs, S-AWC momentarily increases front wheel turn angle and Super AYC control to realize sharper response. In countersteer situations, S-AWC increases responsiveness further to assist the driver with steering precision.
Roll Control Suspension (RCS)
RCS effectively reduces body roll and pitching by hydraulically connecting all the shock absorbers together and regulating their damping pressures as necessary. Able to control both roll and pitching stiffness separately, RCS can operate in a variety of ways. It can, for example, reduce roll only when required during turn in or in other situations while being set up on the soft side to prioritize tire contact and ride comfort. Since the system controls roll stiffness hydraulically, it eliminates the need for stabilizer bars. In the integrated control of its component systems, S-AWC employs information from RCS's hydraulic system to estimate the tire load at each wheel.
The use of engine torque and brake pressure information in the regulation of the ACD and AYC components allows the S-AWC system to determine more quickly whether the vehicle is accelerating or decelerating. S-AWC also employs yaw rate feedback for the first time. The system helps the driver follow his chosen line more closely by comparing how the car is running, as determined from data from the yaw rate sensors, and how the driver wants it to behave, as determined from steering inputs, and operates accordingly to correct any divergence. The addition of braking force regulation to AYC's main role of transferring torque between the right and left wheels allows S-AWC to exert more control over vehicle behavior in on-the-limit driving situations. Increasing braking force on the inside wheel during understeer and on the outer wheel during oversteer situations, AYC's new braking force control feature works in conjunction with torque transfer regulation to realize higher levels of cornering performance and vehicle stability.
Using integrated management of the ASC and ABS systems allows S-AWC to effectively and seamlessly control vehicle dynamics when accelerating, decelerating or cornering under all driving conditions. S-AWC offers three operating modes:
When the driver selects the mode best suited to current road surface conditions S-AWC operates to control vehicle behavior accordingly and allow the driver to extract the maximum dynamic performance from his vehicle.
Two electronic control units (ECU) regulate vehicle motion. One is an ECU developed by Mitsubishi Electric to control ACD and AYC. The other is an ECU developed by Continental Automotive Systems of Germany that controls ASC and ABS. The two ECUs can communicate with other ECUs through a CAN, an in-vehicle LAN interface standard. In addition, the two ECUs are communicating with each other through a dedicated CAN, enabling vehicle motion to be controlled more quickly. The cable and communication standard for the dedicated CAN are the same as those for other CANs.
A longitudinal acceleration sensor, lateral acceleration sensor and yaw rate sensor are installed as one module near the gravity center of a vehicle, which is located between the driver's and passenger's seats. Other sensors, such as a wheel-speed sensor and steering-angle sensor, are installed in different places. However, no vertical acceleration sensor is used.
Also, when the vehicle is equipped with Mitsubishi's Twin Clutch SST transmission, S-AWC analyzes the behavior of the turning vehicle and if it judges that it is safer not to shift gears, it sends a signal to tell Twin Clutch SST that the gear must not be changed. However, S-AWC does not control vehicle motion by using control information from Twin Clutch SST. The co-operation is a one-way communication.
The control algorithms of vehicle motion were developed by Mitsubishi in-house, with MATLAB and Simulink: control system modeling tools. Mitsubishi adopted model-based method, which combines an algorithm and physical model of a vehicle to run a simulation. The physical model of a vehicle was constructed with CarSim, a simulation-package software developed by Mechanical Simulation Corporation of the United States. The algorithms were developed for each function such as ACD and AYC, not for each vehicle type. Therefore, the algorithms can be employed by various types of vehicles.
- "Mitsubishi Motors develops S-AWC vehicle dynamics control system", Mitsubishi Motors press release, July 10, 2007
- "All Wheel Control", Mitsubishi Motors website
- "Left-Right Torque Vectoring Technology as the Core of Super All Wheel Control", Kaoru Sawase, Yuichi Ushiroda, & Takami Miura, Mitsubishi Motors website
- "Mitsubishi Motors' S-AWC Integrally Controls Vehicle Behaviors with 2 ECUs", Naoshige Shimizu, Nikkei Electronics, July 11, 2007
- "Next-Generation Mitsubishi Lancer Evolution Introduces Super-All-Wheel Control (S-AWC) For Supercar Handling", Mitsubishi Motors North America press release, November 14, 2007
- "Mitsubishi Motors Tokyo Motor Show 2005 Press Pack"
- "S-AWC Super All Wheel Control - S-AWC Management", .pdf file, Mitsubishi Motors North America press release
- "S-AWC Super All Wheel Control - S-AWC Systematic", .pdf file, Mitsubishi Motors North America press release
- "S-AWC Super All Wheel Control - ACD (Active Center Differential)", .pdf file, Mitsubishi Motors North America press release
- "S-AWC Super All Wheel Control - AYC (Active Yaw Control)", .pdf file, Mitsubishi Motors North America press release
ACD/AYC programming information