Electronically controlled power steering system

DETAILED DESCRIPTION The invention relates generally to power steering systems and, in particular, to methods and apparatuses for electronically controlled power steering systems.
Power steering systems are used in motor vehicle applications to augment the steering effort provided by the driver of a vehicle.
Many conventional systems employ a four-way open-center "rotary" control valve having "follow along" position feedback in which road feel is artificially induced by deflection of a torsion bar.
Other power steering systems typically employ a four-way open-center "reaction" control valve also having "follow along" position feedback wherein road feel is induced directly via hydraulic reaction forces.
Conventionally, the control valves of the power steering systems are mechanically coupled to the vehicle's steering shaft and are typically located just ahead of the vehicles's steering gear assembly.
Unfortunately, this placement often results in significantly compromising the static and dynamic performance of the host power steering systems.
For example, such power steering systems may generate poor response during lower steering wheel torque inputs and/or generate excessive response to small changes in the steering wheel torque input at higher levels thereof.
Dynamically, some conventional power steering systems are also subject to sluggish behavior which produces a response that is delayed beyond normally anticipated human perception times.
In addition, some power steering systems are subject to undesirably rapid steering wheel motion without a concomitant increase in required steering wheel torque.
In other words, the steering force output of such systems cannot be said to be rate stable.
Further, power steering systems have been recently modified to include "speed sensitive" steering.
In general, most "speed sensitive" systems increase steering forces at high vehicular speeds by reducing hydraulic fluid flow through the control valves in order to reduce valve gain.
An alternative power steering system, known as a "Variable-Assist" Power Steering System is described in SAE Technical Paper No