Automotive Steering Systems In a typical mechanical steering system the driver’s steering input is transmitted by a steering shaft through some type of gear reduction mechanism to generate steering motion at the front wheels. In the present day automobiles, power steering assist has become a standard feature. A hydraulic power steering uses hydraulic pressure supplied by an engine-driven pump. Power steering amplifies and supplements the driver-applied torque at the steering wheel so that steering effort is reduced. The recent introduction of electric power steering in production vehicles eliminates the need for the hydraulic pump.
Electric power steering is more efficient than conventional power steering, since the electric power steering motor only needs to provide assist when the steering wheel is turned, whereas the hydraulic pump must run constantly. The assist level is also easily tunable to the vehicle type, road speed, and even driver preference. An added benefit is the elimination of environmental hazard posed by leakage and disposal of hydraulic power steering fluid. Types of steering system 1. Electric systems Electric power steering (EPS or EPAS) is designed to use an electric motor to reduce effort by providing steering assist to the driver of a vehicle.
Sensors detect the motion and torque of the steering column, and a computer module applies assistive torque via an electric motor coupled directly to either the steering gear or steering column. This allows varying amounts of assistance to be applied depending on driving conditions. On Fiat group cars the amount of assistance can be regulated using a button named “CITY” that switches between two different assist curves, while most other EPS systems have variable assist, which allows for more assistance as the speed of a vehicle decreases and less assistance from the system during high-speed situations.
In the event of component failure, a mechanical linkage such as a rack and pinion serves as a back-up in a manner similar to that of hydraulic systems. Electric systems have a slight 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 evasive manoeuvres, Toyota has been doing so since 2002. 2. Electro-hydraulic systems Electro-hydraulic power steering systems, sometimes abbreviated EHPS, and also sometimes called “hybrid” systems, use the same hydraulic assist technology as standard systems, but the hydraulic pressure is provided by a pump driven by an electric motor instead of being belt-driven by the engine.
These systems can be found in some cars by Ford, Volkswagen, Audi, Peugeot, Citroen, SEAT, Skoda, Suzuki, Opel, MINI, Toyota, Honda, and Mazda. 3. Hydraulic systems Most power steering systems work by using a hydraulic system to turn the vehicle’s wheels. The hydraulic pressure is usually provided by a gerotor 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 applies a torque to the steering axis of the roadwheels.
The flow to the cylinder is controlled by valves operated by the steering wheel; the more torque the driver applies to the steering wheel and the shaft it is attached to, the more fluid the valves allow through to the cylinder, and so the more force is applied to steer the wheels in the appropriate direction. One design for measuring the torque applied to the steering wheel is to fix a torsion bar to the end of the steering shaft.
As the steering wheel rotates, so does the attached steering shaft, and so does the top end of the attached torsion bar. Since the torsion bar is relatively thin and flexible and the bottom end is not completely free to rotate, the bar will soak up some of the torque; the bottom end will not rotate as far as the top end. The difference in rotation between the top and bottom ends of the torsion bar can be used to control the valve that allows fluid to flow to the cylinder which provides steering assistance; the greater the “twist” of the orsion bar, the more steering assistance will be provided. Since the pumps employed are of the positive displacement type, the flow rate they deliver is directly proportional to the speed of the engine. This means that at high engine speeds the steering would naturally operate faster than at low engine speeds. Because this would be undesirable, a restricting orifice and flow control valve are used to direct some of the pump’s output back to the hydraulic reservoir at high engine speeds.
A pressure relief valve is also used to prevent a dangerous build-up of pressure when the hydraulic cylinder’s piston reaches the end of the cylinder. Some modern implementations also include an electronic pressure relief valve which can reduce the hydraulic pressure in the power steering lines as the vehicle’s speed increases (this is known as variable assist power steering). Comments/ Recommendation The next step in steering system evolution is steer-by-wire technology.
The aim of steer-by-wire technology is to completely do away with as many mechanical components (steering shaft, column, gear reduction mechanism, etc. ) as possible. Completely replacing conventional steering system with steer-by-wire holds several advantages, like: • The absence of steering column simplifies the car interior design. • The absence of steering shaft, column and gear reduction mechanism allows much better space utilization in the engine compartment. The steering mechanism can be designed and installed as a modular unit. • Without mechanical connection between the steering wheel and the road wheel, it is less likely that the impact of a frontal crash will force the steering wheel to intrude into the driver’s survival space. • Steering system characteristics can easily and infinitely be adjusted to optimize the steering response and feel.
