Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver can provide less effort to turn the steered wheels when driving at typical speeds, and reduce considerably the physical effort necessary to turn the wheels when a vehicle is stopped or moving slowly. Power steering can also be engineered to provide some artificial feedback of forces acting on the steered wheels.
Pritish Kumar Halder briefly explains a device that is known as Hydraulic steering in this blog post.
Hydraulic power steering systems for cars augment steering effort via an actuator, a hydraulic cylinder that is part of a servo system. These systems have a direct mechanical connection between the steering wheel and the linkage that steers the wheels. This means that power-steering system failure (to augment effort) still permits the vehicle to be steered using manual effort alone.
Electric power steering systems use electric motors to provide the assistance instead of hydraulic systems. As with hydraulic types, power to the actuator (motor, in this case) is controlled by the rest of the power steering system.
Other power steering systems (such as those in the largest off-road construction vehicles) have no direct mechanical connection to the steering linkage; they require electrical power. Systems of this kind, with no mechanical connection, are sometimes called “drive by wire” or “steer by wire”, by analogy with aviation’s “fly-by-wire”. In this context, “wire” refers to electrical cables that carry power and data, not thin wire rope mechanical control cables.
Some construction vehicles have a two-part frame with a rugged hinge in the middle; this hinge allows the front and rear axles to become non-parallel to steer the vehicle. Opposing hydraulic cylinders move the halves of the frame relative to each other to steer.
Hydraulic power steering systems work by using a hydraulic system to multiply force applied to the steering wheel inputs to the vehicle’s steered (usually front) road wheels. The hydraulic pressure typically comes from a gearmotor or rotary vane pump driven by the vehicle’s engine.
A double-acting hydraulic cylinder applies a force to the steering gear, which in turn steers the roadwheels. The steering wheel operates valves to control flow to the cylinder. The more torque the driver applies to the steering wheel and column, the more fluid the valves allow through to the cylinder, and so the more force is applied to steer the wheels.
One design for measuring the torque applied to the steering wheel has a torque sensor – a torsion bar at the lower end of the steering column. As the steering wheel rotates, so does the steering column, as well as the upper end of the torsion bar. Since the torsion bar is relatively thin and flexible, and the bottom end usually resists being rotated, the bar will twist by an amount proportional to the applied torque.
The difference in position between the opposite ends of the torsion bar controls a valve. The valve allows fluid to flow to the cylinder which provides steering assistance; the greater the “twist” of the torsion bar, the greater the force.
Electric power steering (EPS) or motor-driven power steering (MDPS) uses an electric motor instead of a hydraulic system to assist the driver of the vehicle. Sensors detect the position and torque exerted inside the steering column, and a computer module applies assistive torque via the motor, which connects either to the steering gear or steering column.
This allows varied amounts of assistance to be applied depending on driving conditions. Engineers can therefore tailor steering-gear response to variable-rate and variable-damping suspension systems, optimizing ride, handling, and steering for each vehicle.
A mechanical linkage between the steering wheel and the steering gear is retained in EPS. In the event of component failure or power failure that causes a failure to provide assistance, the mechanical linkage serves as a back-up. If EPS fails, the driver encounters a situation where heavy effort is required to steer. This heavy effort is similar to that of an inoperative hydraulic steering assist system.
Depending on the driving situation, driving skill and strength of the driver, steering assist loss may or may not lead to a crash. The difficulty of steering with inoperative power steering is compounded by the choice of steering ratios in assisted steering gears vs. fully manual. The NHTSA has assisted car manufacturers with recalling EPS systems prone to failure.
Electric systems have an advantage in fuel efficiency because there is no belt-driven hydraulic pump constantly running, whether assistance is required or not, and this is a major reason for their introduction.
Another major advantage is the elimination of a belt-driven engine accessory, and several high-pressure hydraulic hoses between the hydraulic pump, mounted on the engine, and the steering gear, mounted on the chassis. This greatly simplifies manufacturing and maintenance. By incorporating electronic stability control electric power steering systems can instantly vary torque assist levels to aid the driver in corrective maneuvers.
Electrically variable gear ratio systems
In 2000, the Honda S2000 Type V featured the first electric power variable gear ratio steering (VGS) system. In 2002, Toyota introduced the “Variable Gear Ratio Steering” (VGRS) system on the Lexus LX 470 and Landcruiser Cygnus, and also incorporated the electronic stability control system to alter steering gear ratios and steering assist levels. In 2003, BMW introduced “active steering” system on the 5 Series.
This system should not be confused with variable assist power steering, which varies steering assist torque, not steering ratios, nor with systems where the gear ratio is only varied as a function of steering angle. These last are more accurately called non-linear types a plot of steering-wheel position versus axle steering angle is progressively curved (and symmetrical).