Power steering
The assisted steering is a mechanical device that helps the driver of a motor vehicle to control the direction of the vehicle when the physical effort required to turn the steering wheel increases, especially when stationary or when maneuvering at high speeds. low.
It consists of a hydraulic or electromechanical system that provides energy in a controlled manner to the steering mechanism, considerably reducing the physical effort required to operate the steering wheel of a vehicle that is stationary or moving slowly.
Automotive hydraulic power steering systems increase the force available to control steering through an actuator, a hydraulic cylinder that is part of a servo system mechanically connected directly between the steering wheel and the linkage that steers the wheels. This means that a failure in the power steering system still allows the vehicle to be steered using manual effort alone.
Electric power steering systems use electric motors to provide steering input. As with the hydraulic types, the power supplied by the actuator (an electric motor in this case) is controlled by the other elements of the device.
Other power steering systems (such as those on larger off-road construction vehicles) have no direct mechanical connection to the steering linkage; and they cannot function without an input of electrical energy. Systems of this type, without mechanical linkage, are sometimes referred to as "steer-by-wire", by analogy with "fly-by-wire" of aviation. In this context, "cable" refers to an electrical outlet, and not to thin steel cables of mechanical control.
Some construction vehicles have a split frame articulated by a central hinge, which allows the front and rear axles to steer independently. Opposing hydraulic cylinders move the frame halves relative to each other to steer the vehicle.
History
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The first steering motor was designed by John McFarlane Gray in 1868, this was a mechanical steam powered amplifier.
The first power steering system on a vehicle was apparently installed in 1876 by a man with the last name Fitts, but little else is known. on it. The following power steering system was installed on a 5-ton Columbia truck in 1903, where a separate electric motor was used to assist the driver in turning the front wheels.
Robert E. Twyford, a resident of Pittsburgh, Pennsylvania, included a mechanical power steering mechanism as part of his patent (US Patent 646,477) issued on April 3, 1900 for the first traction system on road four wheels.
Francis W. Davis, an engineer with the trucking division of the American company Pierce-Arrow, began to explore how to make steering easier, and in 1926 he invented and demonstrated the first practical power steering system. Davis went on to work at General Motors and refined the hydraulic power steering system, but the company calculated that it would be too expensive to produce. Davis later joined the Bendix Corporation, a manufacturer of parts for automobile manufacturers. Military needs during World War II to facilitate the driving of heavy vehicles increased the need for electrically assisted armored vehicles and tank recovery vehicles for the British and American armies.
Chrysler Corporation introduced the first commercially available passenger car power steering system in the 1951 Chrysler Imperial under the name 'Hydraguide'. Chrysler's system was based on some of the expired patents of Davis. General Motors introduced the 1952 Cadillac with a power steering system using work Davis had done for the company nearly twenty years earlier.
Charles F. Hammond of Detroit filed several patents for improving power steering with the Canadian Intellectual Property Office in 1958.
Starting in the mid-1950s, this technology was offered as standard or optional equipment by American manufacturers, while it became widely incorporated internationally in modern vehicles, due to trends toward adopting front-wheel drive, greater vehicle mass, lower assembly line production costs, and wider tires, which increase the steering effort required. Heavier vehicles, common in some countries, would be extremely difficult to maneuver at low speeds, while lighter weight vehicles might not need power steering at all.
A 1999 study on the perception of steering force found that ordinary real-world truck and car drivers expect an increase in steering wheel hardness as speed increases, and for this reason the first few Power steering forms, which lacked this effect, were met with disapproval.
Hydraulic systems
Hydraulic power steering systems work by using a hydraulic system to multiply the force applied from the steering wheel to the steered (usually the front) wheels of the vehicle. Hydraulic pressure usually comes from a rotary compressor or gerotor vane powered by vehicle engine. A double-acting hydraulic cylinder applies a force to the steering gear, which in turn steers the wheels on the road. The flywheel operates valves that control flow to the cylinder. The more torque the driver applies to the steering wheel and column, the more fluid the valves will let through to the cylinder, and the more force is applied to steer the wheels.
One design for measuring steering wheel torque has a torque sensor, a torsion bar located at the bottom end of the steering column. As the steering wheel is turned, so does the steering column, as well as the top end of the torsion bar. Since the torsion bar is relatively thin and flexible, and the lower end generally resists turning, 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, which allows fluid to flow into the cylinder, thus providing steering assistance. The greater the "twist" of the torsion bar, the greater the force applied.
Since hydraulic pumps are of the positive displacement type, the flow they deliver is directly proportional to the engine speed. This means that at high engine speeds, the steering would naturally work faster than at low engine speeds. Because this would be undesirable, a restriction orifice and flow control valve direct part of the pump output back to the hydraulic reservoir at high engine speeds. A pressure relief valve prevents a dangerous buildup of pressure when the hydraulic cylinder piston reaches the end of its stroke.
The steering servomotor is arranged so that if the servomotor fails, the steering will still work (although it will take much more to turn the steering wheel, which will feel heavier). Loss of power steering can significantly affect a vehicle's handling. The owner's manual for each vehicle provides instructions for checking the fluid levels and regular maintenance of the power steering system.
The working fluid, also called "oleo-hydraulic fluid" or "oil", is the medium by which pressure is transmitted. Common working fluids are based on a mineral oil.
Some modern systems also include an electronic control valve to reduce hydraulic supply pressure as vehicle speed increases; what is called a variable assist power steering.
DIRAVI variable power steering
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The DIRAVI system introduced an innovation that later became commonplace, speed-sensitive steering.
It is characterized in that the steering force that controls the wheels comes from the car's high-pressure hydraulic system and is always the same regardless of the speed at which it is driven. By turning the steering wheel, the wheels move simultaneously at a corresponding angle through a hydraulic cylinder. To give an artificial steering feel, there is an independent hydraulically operated system that tries to turn the steering wheel back to the center position. The amount of pressure applied is proportional to the speed at which you are driving, so at low speeds the steering is very light and at high speeds it is very difficult to move it more than a small amount off center. Furthermore, as long as there is pressure in the car's hydraulic system, there is no mechanical connection between the steering wheel and the wheels.
It was invented by Citroën of France, and was first introduced in the Citroën SM in 1970, being known as 'VariPower' in the UK and as 'SpeedFeel' in the U.S.
Electro-hydraulic systems
Electro-hydraulic power steering systems, sometimes abbreviated EHPS, and also called "hybrid" systems, use the same hydraulic power-assist technology as standard systems, but hydraulic pressure comes from a pump driven by an electric motor instead of being driven by a belt linked to the transmission in the engine.
In 1965, Ford experimented with a fleet of Mercury Park Lane equipped with "instant twist-of-the-wrist steering" which replaced the conventional large flywheel with two 5-inch (127 mm) rings, a fast 15:1 gear ratio, and an electric hydraulic pump in case the engine stalled.
In 1988, the Subaru XT6 was equipped with a unique Cybrid adaptive electro-hydraulic steering system that adjusted the level of assistance based on vehicle speed.
In 1990, Toyota introduced its second-generation MR2 with electro-hydraulic power steering, which prevented the hydraulic lines from the engine (which was located behind the driver in the MR2) from having to run to the steering rack.
In 1994 Volkswagen produced the Golf Mk3 Ecomatic, with an electric pump. This meant that the power steering would continue to work while the computer stopped the engine to save fuel. Electro-hydraulic systems can be found in some cars made by Ford, Volkswagen, Audi, Peugeot, Citroën, SEAT, Škoda, Suzuki, Opel, MINI, Toyota, Honda and Mazda.
Electrical systems
Electric Power Steering (EPS) or motor-driven power steering (MDPS) uses a system powered by an electric motor instead of a hydraulic system to help the driver of a vehicle actuate steering. Sensors detect position and applied torque on the steering column, and a computer module applies assist torque through the motor, which connects to the steering gear or steering column. This allows different amounts of assistance to be applied depending on driving conditions. Therefore, engineers can tailor the response of the steering gear to variable rate suspension and variable damping systems, optimizing the ride, handling and steering of each vehicle. In Fiat group cars, the amount of attendance can be regulated by means of a button called "CITY" which allows you to switch between two different assist curves, whereas most other EPS systems have variable assist, which increases as the vehicle slows down, and decreases at higher speeds.
With EPS, a mechanical connection is maintained between the steering wheel and the steering gear. In the event of component failure or power failure causing the servo mechanism to be inoperative, the mechanical linkage serves as a backup. If the EPS fails, the driver is in a situation where a lot of effort is required to drive. This heavy strain is similar to that of an inoperative power steering assist system. Depending on the driving situation and the driver's skill and strength, the loss of steering assistance may or may not lead to an accident. The difficulty of steering a vehicle with the power steering inoperative is compounded by the design of its gear ratios versus fully manual systems. NHTSA has worked with auto manufacturers to get them to recall fail-prone EPS systems.
Electric systems have an efficiency advantage because there is no belt-driven hydraulic pump that runs constantly regardless of whether assistance is required or not, and this is one of the main reasons for their introduction. Another major advantage is the elimination of a belt-driven element requiring multiple high-pressure hydraulic hoses between the engine-mounted hydraulic pump and frame-mounted steering gear. The adoption of an electrical system greatly simplifies manufacturing and maintenance. By incorporating electric power steering systems with stability control, levels of torque assistance can be instantly varied to assist the driver in corrective manoeuvres.
In 1986, NSK implemented the battery-connected EPS. In the following 8 years, the Japanese company Koyo Seiko (now JTEKT), developed a steering column system exclusively for minicars, sold only in Japan to Suzuki and Mitsubishi Motors. However, this simple design was not adopted, due to its unnatural performance at low and high speeds. The first electric power steering system appeared on the Suzuki Cervo in 1988, but it would not be until the late 1990s that a fully electronic direct control system of a power rack steering was put into practice on the Honda NSX in an automobile. Since then, the presence of alternating current motors (low-maintenance because they lack brushes) has become widespread in the automotive industry, and this power steering design has become the most widely used.
Electric power steering systems appeared on the Honda NSX in 1990; in the MG F, the FIAT Punto Mk2 and the Honda S2000 in 1999; in the Toyota Prius in 2000; in the BMW Z4 in 2002 and in the Mazda RX-8 in 2003. The system has been used by several car manufacturers, and has been applied more frequently in smaller and smaller cars as their manufacturing costs have decreased.
Electrically Variable Gear Ratio Systems
In 2000, the Honda S2000 Type V introduced the first electric power variable speed (VGS) drivetrain steering system. In 2002, Toyota introduced the "Variable Gear Ratio Steering" system.; (VGRS) in the Lexus LX 470 and Landcruiser Cygnus, and also incorporated the stability control system to alter steering gear ratios and steering assist levels. In 2003, BMW introduced the "active steering" in the BMW 5 Series.
This system should not be confused with variable-assist power steering, which varies the steering assist torque (and not the steering turn ratio), or systems where the gear ratio is only varies depending on the steering angle. These latter systems are more accurately called non-linear types (for example, the "Direct-Steer" offered by Mercedes-Benz), in which the graph of the position of the steering wheel against the angle of steering of the axle it curves progressively (and is symmetrical).
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