Operational diagram of ZF’s RAS-EC.
Especially well suited to long-wheelbase, urban, commercial vehicles with multiple rear axles, RAS-EC (Rear Axle Steering – Electronically Controlled), from ZF Lenksystme, Schwaebisch Gmuend, Germany, is the result of a joint venture between Bosch GmbH, Stuttgart, and ZF Friedrichshafen AG.
Based on our observations during a recent ride-and-drive in Kulmbach, Germany, RAS-EC looks like it has tremendous potential to increase vehicle maneuverability, especially during parking, in congested, city-delivery environments.
Due to the electronic control, says the company, the rear-axle steering angle can be altered as a function of whatever driving parameters are chosen. For example, opposite steering motions at the front and rear wheels, at low speed, can make for a much tighter turning circle, while same-direction front/rear steering at medium and high speeds promotes more exact straight-line driving and vehicle stabilization during lane changes.
The system works like this (see diagram): An ECU receives front steering-angle information, and calculates the needed degree and direction of rear-wheel steer, based on its desired-results programming.
The ECU then signals a control valve to supply the needed amount of hydraulic pressure to a rear-axle-mounted power steering cylinder. To complete the loop, a rear-axle steering angle sensor lets the ECU know that the desired direction and degree of steering angle has been achieved. If an error is detected, outputs are switched off, and the axle returns, mechanically, to a safe (straight) mode.
Special steering programs, says ZF, such as rear, swing-out minimization, as well as the ability to process additional signals (such as brake activation), will allow RAS-EC to be custom-tailored to many different vehicles and applications.