A power steering is a mechanical device equipped on a
motor vehicle that helps drivers
steer Steer, Steers or Steering may refer to:
Animals
* Steer or bullock, castrated male cattle
* Ox, a steer used as a draft animal
People
* Steer (surname)
* Steers (surname)
Places
* Steer Creek (West Virginia), a tributary of the Little ...
the vehicle by reducing steering effort needed to turn the
steering wheel, making it easier for the vehicle to turn or maneuver at lower speeds.
Hydraulic
Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counte ...
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.
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
Drive by wire, DbW, by-wire, steer-by-wire, fly-by-wire or x-by-wire technology in the automotive or aviation industry is the use of electrical or electro-mechanical systems for performing vehicle functions traditionally achieved by mechanical link ...
" or "steer by wire", by analogy with aviation's "
fly-by-wire
Fly-by-wire (FBW) is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires, and flight control ...
". In this context, "wire" refers to
electrical cable
An electrical cable is an assembly of one or more wires running side by side or bundled, which is used to carry electric current.
One or more electrical cables and their corresponding connectors may be formed into a ''cable assembly'', whic ...
s that carry power and data, not thin
wire rope
Steel wire rope (right hand lang lay)
Wire rope is several strands of metal wire twisted into a helix forming a composite
''rope'', in a pattern known as ''laid rope''. Larger diameter wire rope consists of multiple strands of such laid rope in a ...
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.
History
The first power steering system on a vehicle was apparently installed in 1876 by a man with the surname of Fitts, but little else is known about him.
The next power steering system was put on a Columbia 5-ton truck in 1903 where a separate
electric motor
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate for ...
was used to assist the driver in turning the front wheels.
Robert E. Twyford, a resident of
Pittsburgh
Pittsburgh ( ) is a city in the Commonwealth of Pennsylvania, United States, and the county seat of Allegheny County. It is the most populous city in both Allegheny County and Western Pennsylvania, the second-most populous city in Pennsylva ...
,
Pennsylvania
Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...
, included a mechanical power steering mechanism as part of his patent (U.S. Patent 646,477) issued on April 3, 1900 for the first four-wheel drive system.
Francis W. Davis, an engineer of the truck division of
Pierce-Arrow
The Pierce-Arrow Motor Car Company was an American motor vehicle manufacturer based in Buffalo, New York, which was active from 1901 to 1938. Although best known for its expensive luxury cars, Pierce-Arrow also manufactured commercial trucks ...
, began exploring how steering could be made easier, and in 1926 invented and demonstrated the first practical power steering system.
Davis moved to
General Motors and refined the hydraulic-assisted power steering system, but the automaker calculated it would be too expensive to produce.
Davis then signed up with
Bendix, a parts manufacturer for automakers. Military needs during
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
for easier steering on heavy vehicles boosted the need for power assistance on
armored cars and tank-recovery vehicles for the British and American armies.
Chrysler Corporation introduced the first commercially available passenger car power steering system on the 1951
Chrysler Imperial under the name "Hydraguide". The Chrysler system was based on some of Davis' expired patents. General Motors introduced the 1952
Cadillac with a power steering system using the work Davis had done for the company almost twenty years earlier.
Charles F. Hammond from
Detroit
Detroit ( , ; , ) is the largest city in the U.S. state of Michigan. It is also the largest U.S. city on the United States–Canada border, and the seat of government of Wayne County. The City of Detroit had a population of 639,111 at t ...
filed several patents for improvements of power steering with the
Canadian Intellectual Property Office in 1958.
Starting in the mid-1950s American manufacturers offered the technology as optional or standard equipment while it is widely offered internationally on modern vehicles, owing to the trends toward
front-wheel drive
Front-wheel drive (FWD) is a form of engine and transmission layout used in motor vehicles, where the engine drives the front wheels only. Most modern front-wheel drive vehicles feature a transverse engine, rather than the conventional longitu ...
, greater vehicle mass, reduced assembly line production costs, and wider
tire
A tire (American English) or tyre (British English) is a ring-shaped component that surrounds a wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide traction on the surface over which t ...
s, which all increase the required steering effort. Heavier vehicles, as are common in some countries, would be extremely difficult to maneuver at low speeds, while vehicles of lighter weight may not need power assisted steering at all.
A study in 1999 on the perceptual fidelity of steering force feedback, found that ordinary real-world truck and car drivers naturally expect an increase in feedback torque as speed increases, and for this reason early forms of power steering, which lacked such effect, were met with disapproval.
Hydraulic systems
![Jeep 2](https://upload.wikimedia.org/wikipedia/commons/f/ff/Jeep_2.5_liter_4-cylinder_engine_chromed_h.jpg)
Hydraulic power steering systems work by using a
hydraulic system
Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid coun ...
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
gerotor
A gerotor is a positive displacement pump. The name ''gerotor'' is derived from "generated rotor". A gerotor unit consists of an inner and outer rotor. The inner rotor has ''n'' teeth, while the outer rotor has ''n''+1 teeth; with ''n'' defined a ...
or
rotary vane pump driven by the vehicle's engine. A double-acting
hydraulic cylinder
A hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in construction equipment ( engineering vehicles ...
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
A torsion bar suspension, also known as a torsion spring suspension, is any vehicle suspension that uses a torsion bar as its main weight-bearing spring. One end of a long metal bar is attached firmly to the vehicle chassis; the opposite end termi ...
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.
Since the hydraulic pumps are 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 direct some of the pump's output back to the hydraulic reservoir at high engine speeds. A pressure relief valve prevents a dangerous build-up of pressure when the hydraulic cylinder's piston reaches the end of its stroke.
The steering booster is arranged so that should the booster fail, the steering will continue to work (although the wheel will feel heavier). Loss of power steering can significantly affect the handling of a vehicle. Each vehicle owner's manual gives instructions for inspection of fluid levels and regular maintenance of the power steering system.
The working liquid, also called "
hydraulic fluid" or "oil", is the medium by which
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
is transmitted. Common working liquids are based on
mineral oil
Mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum, as distinct from usually edible vegetable oils.
The name 'mineral oil' by itself is imprecise, ...
.
Some modern systems also include an electronic control valve to reduce the hydraulic supply pressure as the vehicle's speed increases; this is variable-assist power steering.
DIRAVI variable-assist power steering
DIRAVI
DIRAVI is the name given by Citroën to its proprietary power steering system, first seen in 1970.
DIRAVI is an acronym for "''Direction à rappel asservi''" literally meaning "steering with controlled return" more accurately described in English ...
innovated the now common benefit of
speed sensitive steering.
In this power steering system, the force steering the wheels comes from the car's high pressure hydraulic system and is always the same no matter what the road speed is. Turning the steering wheel moves the wheels simultaneously to a corresponding angle via a hydraulic cylinder. In order to give some artificial steering feel, there is a separate hydraulically operated system that tries to turn the steering wheel back to centre position. The amount of pressure applied is proportional to road speed, so that at low speeds the steering is very light, and at high speeds it is very difficult to move more than a small amount off centre.
It was invented by
Citroën of France.
As long as there is pressure in the car's hydraulic system, there is no mechanical connection between the steering wheel and the roadwheels. This system was first introduced in the
Citroën SM
The Citroën SM is a high-performance coupé produced by the French manufacturer Citroën from 1970 to 1975. The SM placed third in the 1971 European Car of the Year contest, trailing its stablemate Citroën GS, and won the 1972 ''Motor Trend' ...
in 1970, and was known as 'VariPower' in the UK and 'SpeedFeel' in the U.S.
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 comes from a pump driven by an
electric motor
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate for ...
instead of a drive belt at the engine.
In 1965, Ford experimented with a fleet of "wrist-twist instant steering" equipped
Mercury Park Lane
The Mercury Park Lane is a full-sized automobile that was produced by the Mercury division of Ford Motor Company. While not officially introduced as the replacement of the Mercury Turnpike Cruiser, the Park Lane became the flagship of the Mercur ...
s that replaced the conventional large steering wheel with two rings, a fast 15:1 gear ratio, and an electric hydraulic pump in case the engine stalled.
In 1988, the
Subaru XT6
The Subaru XT was a four-passenger, 2-door coupé manufactured and marketed by Subaru for model years 1985-1991, with a facelift in 1987. Assembly took place at Subaru's Yajima Plant in Ota, Japan and during its single generation, production ...
was fitted with a unique Cybrid adaptive electro-hydraulic steering system that changed the level of assistance based on the vehicle's speed.
In 1990, Toyota introduced its second-generation
MR2 with electro-hydraulic power steering. This avoided running hydraulic lines from the engine (which was behind the driver in the MR2) up to the steering rack.
In 1994 Volkswagen produced the
Golf Mk3 Ecomatic, with an electric pump. This meant that the power steering would still operate while the engine was stopped by the computer to save fuel. Electro-hydraulic systems can be found in some cars by
Ford
Ford commonly refers to:
* Ford Motor Company, an automobile manufacturer founded by Henry Ford
* Ford (crossing), a shallow crossing on a river
Ford may also refer to:
Ford Motor Company
* Henry Ford, founder of the Ford Motor Company
* Ford F ...
,
Volkswagen
Volkswagen (),English: , . abbreviated as VW (), is a German motor vehicle manufacturer headquartered in Wolfsburg, Lower Saxony, Germany. Founded in 1937 by the German Labour Front under the Nazi Party and revived into a global brand post-W ...
,
Audi
Audi AG () is a German automotive manufacturer of luxury vehicles headquartered in Ingolstadt, Bavaria, Germany. As a subsidiary of its parent company, the Volkswagen Group, Audi produces vehicles in nine production facilities worldwide.
Th ...
,
Peugeot
Peugeot (, , ) is a French brand of automobiles owned by Stellantis.
The family business that preceded the current Peugeot companies was founded in 1810, with a steel foundry that soon started making hand tools and kitchen equipment, and the ...
,
Citroën,
SEAT,
Škoda
Škoda means ''pity'' in the Czech and Slovak languages. It may also refer to:
Czech brands and enterprises
* Škoda Auto, automobile and previously bicycle manufacturer in Mladá Boleslav
** Škoda Motorsport, the division of Škoda Auto respons ...
,
Suzuki,
Opel,
MINI
The Mini is a small, two-door, four-seat car, developed as ADO15, and produced by the British Motor Corporation (BMC) and its successors, from 1959 through 2000. Minus a brief hiatus, original Minis were built for four decades and sold during ...
,
Toyota
is a Japanese multinational automotive manufacturer headquartered in Toyota City, Aichi, Japan. It was founded by Kiichiro Toyoda and incorporated on . Toyota is one of the largest automobile manufacturers in the world, producing about 10 ...
,
Honda
is a Japanese public multinational conglomerate manufacturer of automobiles, motorcycles, and power equipment, headquartered in Minato, Tokyo, Japan.
Honda has been the world's largest motorcycle manufacturer since 1959, reaching a producti ...
, and
Mazda
, commonly referred to as simply Mazda, is a Japanese multinational automotive manufacturer headquartered in Fuchū, Hiroshima, Japan.
In 2015, Mazda produced 1.5 million vehicles for global sales, the majority of which (nearly one m ...
.
Electric systems
![EPAS power steering column](https://upload.wikimedia.org/wikipedia/commons/2/27/EPAS_power_steering_column.jpg)
Electric power steering (EPS) or motor-driven power steering (MDPS) uses an
electric motor
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate for ...
instead of a hydraulic system to assist the
driver of the
vehicle
A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), ...
. Sensors detect the position and
torque
In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
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. This new technological feature also gave engineers the ability to add new driver assistance features. This include features such as lane assist, wind drift correction, etc. 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. These give more assistance as the vehicle slows down, and less at faster speeds.
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 ratio Steering ratio refers to the ratio between the turn of the steering wheel (in degrees) or handlebars and the turn of the wheels (in degrees).
The steering ratio is the ratio of the number of degrees of turn of the steering wheel to the number of de ...
s in assisted steering gears vs. fully manual. The
NHTSA
The National Highway Traffic Safety Administration (NHTSA ) is an agency of the U.S. federal government, part of the Department of Transportation. It describes its mission as "Save lives, prevent injuries, reduce vehicle-related crashes" rel ...
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.
In 1986, NSK put the world’s first electric power steering system for battery forklifts into practical use. In 1988, Koyo Seiko (currently JTEKT) and NSK co-developed a column system exclusively for minicars sold only in the domestic market of Japan. The first-ever electric power steering system for mass-produced passenger cars appeared on the Suzuki Cervo in 1988. However, this simple method was not widely adopted by other automakers in the initial years due to the unnatural steering feel of the motor caused by the inertia at the time of rapid steering for danger avoidance in slower speed driving, as well as at the time of faster speed driving in which the electromagnetic clutch makes the steering force smaller, returning to the manual steering mode. In the year 1990, a direct full control system of a rack assist without a clutch was put into practical use in the Honda NSX (initially installed in automatics only). Since then, there has been a transition of trend from brush-attached motors to brushless motors in the rack type for ordinary vehicles and this method has become the mainstream.
Other electric power steering systems (including 4WS) later appeared on the Honda NSX after 1990, the Honda Prelude and the Subaru SVX in 1991, the Nissan 300ZX (Z32; after the Version 3 onwards), Silvia, Skyline, and the Laurel in 1993, the MG F, the FIAT Punto Mk2 in 1999, the Honda S2000 in 1999, Toyota Prius in 2000, the BMW Z4 in 2002, and the Mazda RX-8 in 2003.
The system has been used by various automobile manufacturers, and most commonly applied for smaller cars to reduce fuel consumption and manufacturing costs.
Electrically variable gear ratio systems
In 2000, the
Honda S2000
The Honda S2000 is an open top sports car that was manufactured by Japanese automobile manufacturer Honda, from 1999 until 2009. First shown as a concept car called the SSM at the Tokyo Motor Show in 1995, the production version was launched on ...
Type V featured the first electric power variable
gear ratio
A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage.
Gear teeth are designed to ensure the Pitch circle diameter (gears), pitch circles of engaging gears roll on each other without slipping, pr ...
steering (VGS) system. In 2002,
Toyota
is a Japanese multinational automotive manufacturer headquartered in Toyota City, Aichi, Japan. It was founded by Kiichiro Toyoda and incorporated on . Toyota is one of the largest automobile manufacturers in the world, producing about 10 ...
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 (e.g. Direct-Steer offered by
Mercedes-Benz
Mercedes-Benz (), commonly referred to as Mercedes and sometimes as Benz, is a German luxury and commercial vehicle automotive brand established in 1926. Mercedes-Benz AG (a Mercedes-Benz Group subsidiary established in 2019) is headquartere ...
); a plot of steering-wheel position versus axle steering angle is progressively curved (and symmetrical).
See also
*
Servomechanism
*
Steering engine
A steering engine is a power steering device for ships.
History
The first steering engine with feedback was installed on Isambard Kingdom Brunel's ''Great Eastern'' in 1866.
Designed by Scottish engineer John McFarlane Gray and built by George ...
*
Direct-drive sim racing wheel
References
External links
{{DEFAULTSORT:Power Steering
Auto parts
Automotive accessories
Automotive steering technologies
Vehicle safety technologies
American inventions
19th-century inventions