Electronically variable orifice (EVO) steering varies the amount of power steering assist based on electrical inputs to provide the driver with improved road feel at higher speeds, and at the same time providing power assist at lower speeds and during parking maneuvers.
EVO systems utilize information such as vehicle speed, steering wheel position, and steering-wheel turn rate to calculate and deliver optimum assist to match road conditions.
The EVO system uses a solenoid valve attached directly to the PS system pump’s output fitting or the rack and pinion housing. It is electrically controlled by a digital control signal using pulse width modulation (PWM). By varying the amount of "on-time," or duty cycle of the control signal to the solenoid, variable levels of fluid flow and resulting effort levels can be obtained.
Depending on the year and model, the solenoid could be directed by various control modules. A "dedicated" EVO module was used in the earliest versions of this system. Since that time, however, the electronic-brake and traction-control module (EBTCM) has taken over the responsibilities of VES in most GM products.
Vehicle weight, steering design and suspension geometry vary greatly from platform to platform. Seemingly identical cars, with different tire or handling/suspension options, may require slightly different VES behavior. The EVO software is altered to provide ideal assist levels at each speed for this wide variety of vehicles, tailoring the vehicle’s handling "feel" to the car and its intended customer’s preferences. Four or five EVO-module part numbers for the exact same year and model of car are not uncommon.
An enhancement to EVO-VES systems that began to appear in 1992 included a steering-wheel position sensor (SWPS) or handwheel speed sensor (HWSS), depending on the vehicle platform. This sensor has the ability to input steering-wheel position, direction and turn-velocity data into the EVO controller. Using this data, the EVO control module can quickly react to emergency evasive maneuvers. If the driver were to suddenly swerve or change lanes to steer around an object on the highway, the EVO module provides an immediate boost in assist levels, maximizing steering control.
One of the renditions of GM VES systems is known as magnetic speed variable assist (MSVA), or MagnaSteer. MSVA was designed specifically to minimize one of the few negative aspects of EVO type steering systems. EVO systems, at highway speeds, restrict the flow from the power steering pump. This restriction raises internal pump pressures as well as the pump’s belt load on the engine. This additional engine load will have a negative impact on fuel consumption, and will increase exhaust emissions as well.
The first GM MVSA system, dubbed MagnaSteer, was initially used in 1995 on the G-body platform, the Olds Aurora and Buick Riviera. Cadillac also enveloped this MVSA system into their Integrated Chassis Control System in 1996. MVSA became even more popular with its release in GM’s 1997/98 versions of the Pontiac Grand Prix, Olds Intrigue, and Buick Regal W-body platform.
While the EVO-VES systems were a "bolt-on" addition to an existing power-steering system, MSVA requires a specially designed rack and pinion assembly. An electromagnetic coil within the steering rack’s spool valve has the ability to alter the assist/effort levels by varying the strength and direction of a magnetic field.
To simplify the process, the MVSA module directs the current flow to the electromagnet coil in one direction to increase the amount of assist. Reversing the direction of current flow has the opposite effect.
When no current is flowing through the electromagnetic poles, there is a fixed moderate amount of power steering assist, which is the default should trouble arise. This default (zero coil current) will approximately equate to the amount of power assist at 30-40 km-h.