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reprinted from Automotive Engineering, September 1996

Mitsubishi combines active yaw control and active stability control

Mitsubishi chose to apply its active yaw control system (AYC) to a high-performance, all-wheel-drive model in the forthcoming Galant mid-size sedan replacement. The AYC is combined with the active stability control system (ASC) that employs braking force as a stabilizing factor.

The mechanical component of the AYC system is the torque-transfer differential located at the rear between the wheels incorporated with the final drive/differential unit, and is driven by the propeller shaft from the front-mounted transverse power plant. The torque-transfer differential comprises a planetary gear mechanism that generates two differing rotational speeds which are "fast (13%)" or "slow (13%)"–either relative to the differential case rotational speed; clutch units for left and right turns that transfer torque to either the left or the right wheel.

Five input sources to "read" the driver's intention are steering wheel angle, accelerator stroke, wheel (all four) speeds, longitudinal acceleration, and lateral acceleration. The ECU judges road surface frictional coefficient, actual vehicle behavior, the driver's intention, determines optimum transfer of driving torque between the rear wheels, and signals the hydraulic actuating unit.

Mitsubishi's system differs from Honda's in that it employs an electrically driven hydraulic pump and a magnetic hydraulic valve. Honda adopts a mechanically driven pump from the drive shaft. A senior engineer says that this system is much quicker, works from very low vehicle speeds, and its on-demand characteristics are energy efficient. The system functions in a circle as small as 14 m to, according to the Mitsubishi engineer's estimate, Honda's 30 m.

The AYC's benefits are, again, superior corner tractability, greatly improved cornering behavior, relief from a "held-back" feel during cornering, and traction control capability on split-µm surfaces. Applied to an all-wheel-drive car, the AYC improved accelerative performance on packed-snow, 30-m radius by as much as 30% over a front-wheel-drive model, claims Mitsubishi.

In the forthcoming Galant platform, the AYC will be combined with the active stability control that uses braking force as well as engine torque reduction to slow the vehicle and give a strong yaw moment to regain stability, should the vehicle exceed its cornering limit. The ASC is controlled by the same electronic control unit as the AYC. The combined systems add about 20 kg to the vehicle and a little over 100,000 yen (about $1000) to the price of the mid-size sedan. The AYC alone should be 50-60,000 yen, and there is already a prototype unit for front-wheel-drive application.

Jack Yamaguchi