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FOUR WHEEL STEERING SYSTEM
This paper deals with the details of four wheel steering (4WS) system. With the help of this system all the four wheels can be turned to any direction using the steering. Thus the vehicle can be controlled more effectively especially during cornering and parking. Also the speed of the vehicle can be increased or decreased.
`There are three types of production of four-wheel Ã‚Â¬steering systems:
1. Mechanical 4WS
2. Hydraulic 4WS
3. Electro/hydraulic 4WS
The mechanical 4WS uses two separate steering gears to control the front and rear wheels. The hydraulic 4WS uses a two-way hydraulic cylinder to turn both the wheels in the same direction. It is not possible to turn them in the opposite direction. The electro/hydraulic 4WS combine computer electronic controls with hydraulics to make the system sensitive to both steering angle and road speeds.
This system finds application in off-highway vehicles such as fork lifts, agricultural and construction equipment and mining machinery. It is also useful in passenger cars, mainly SUVs.
Four-wheel steering, 4WS, also called rear-wheel steering or all-wheel steering, provides a means to actively steer the rear wheels during turning maneuvers. It should not be confused with four-wheel drive in which all four wheels of a vehicle are powered. It improves handling and help the vehicle make tighter turns.
Production-built cars tend to understeer or, in few instances, oversteer. If a car could automatically compensate for an understeer/oversteer problem, the driver would enjoy nearly neutral steering under varying conditions. 4WS is a serious effort on the part of automotive design engineers to provide near-neutral steering.
The front wheels do most of the steering. Rear wheel turning is generally limited to 50-60 during an opposite direction turn. During a same direction turn, rear wheel steering is limited to about 10-1.50.
When both the front and rear wheels steer toward the same direction, they are said to be in-phase and this produces a kind of sideways movement of the car at low speeds. When the front and rear wheels are steered in opposite direction, this is called anti-phase, counter-phase or opposite-phase and it produces a sharper, tighter turn.
2. WHY FOUR-WHEEL STEERING SYSTEM
To understand the advantages of four-wheel steering, it is wise to review the dynamics of typical steering maneuvers with a conventional front -steered vehicle. The tires are subject to the forces of grip, momentum, and steering input when making a movement other than straight-ahead driving. These forces compete with each other during steering maneuvers. With a front-steered vehicle, the rear end is always trying to catch up to the diÃ‚Â¬rectional changes of the front wheels. This causes the vehicle to sway. As a normal part of operating a vehicle, the driver learns to adjust to these forces without thinkÃ‚Â¬ing about them.
When turning, the driver is putting into motion a comÃ‚Â¬plex series of forces. Each of these must be balanced against the others. The tires are subjected to road grip and slip angle. Grip holds the car's wheels to the road, and moÃ‚Â¬mentum moves the car straight ahead. Steering input causes the front wheels to turn. The car momentarily resists the turning motion, causing a tire slip angle to form. Once the vehicle begins to respond to the steering input, cornering forces are generated. The vehicle sways as the rear wheels attempt to keep up with the cornering forces already generated by the front tires. This is referred to as rear-end lag, because there is a time delay between steering input and vehicle reaction. When the front wheels are turned back to a straight -ahead position, the vehicle must again try to adjust by reversing the same forces developed by the turn. As the steering is turned, the vehicle body sways as the rear wheels again try to keep up with the cornering forces generated by the front wheels.
The idea behind four-wheel steering is that a vehicle requires less driver input for any steering maneuver if all four wheels are steering the vehicle. As with two-wheel steer vehicles, tire grip holds the four wheels on the road. However, when the driver turns the wheel slightly, all four wheels react to the steering input, causing slip angles to form at all four wheels. The entire vehicle moves in one direction rather than the rear half attempting to catch up to the front. There is also less sway when the wheels are turned back to a straight-ahead position. The vehicle responds more quickly to steering input because rear wheel lag is eliminated.
3. TYPES OF 4WS
There are three types of production of four-wheelÃ‚Â¬ steering systems:
3.1 Mechanical 4WS
3.2 Hydraulic 4WS
3.3 Electro-hydraulic 4WS
3.1 Mechanical 4WS
Figure 1. Mechanical 4WS
In a straight-mechanical type of 4WS, two steering gears are used-one for the front and the other for the rear wheels. A steel shaft connects the two steering gearboxÃ‚Â¬es and terminates at an eccentric shaft that is fitted with an offset pin. This pin engages a second offset pin that fits into a planetary gear.
The planetary gear meshes with the matching teeth of an internal gear that is secured in a fixed position to the gearbox housing. This means that the planetary gear can rotate but the internal gear cannot. The eccentric pin of the planetary gear fits into a hole in a slider for the steerÃ‚Â¬ing gear.
A 120-degree turn of the steering wheel rotates the planetary gear to move the slider in the same direction that the front wheels are headed. Proportionately, the rear wheels turn the steering wheel about 1.5 to 10 degrees. Further rotation of the steering wheel, past the 120Ã‚Â¬degree point, causes the rear wheels to start straightenÃ‚Â¬ing out due to the double-crank action (two eccentric pins) and rotation of the planetary gear. Turning the steerÃ‚Â¬ing wheel to a greater angle, about 230 degrees, finds the rear wheels in a neutral position regarding the front wheels. Further rotation of the steering wheel results in the rear wheels going counter phase with regard to the front wheels. About 5.3 degrees maximum counter phase rear steering is possible.
Mechanical 4WS is steering angle sensitive. It is not sensitive to vehicle road speed.
3.2 Hydraulic 4WS
Figure 2. Hydraulic 4WS
The hydraulically operated four-wheel-steering system is a simple design, both in comÃ‚Â¬ponents and operation. The rear wheels turn only in the same direction as the front wheels. They also turn no more than 11/2 degrees. The system only activates at speeds above 30 mph (50 km/h) and does not operate when the vehicle moves in reverse.
A two-way hydraulic cylinder mounted on the rear stub frame turn the wheels. Fluid for this cylinder is supÃ‚Â¬plied by a rear steering pump that is driven by the differÃ‚Â¬ential. The pump only operates when the front wheels are turning. A tank in the engine compartment supplies the rear steering pump with fluid.
When the steering wheel is turned, the front steering pump sends fluid under pressure to the rotary valve in the front rack and pinion unit. This forces fluid into the front power cylinder, and the front wheels turn in the direction steered. The fluid pressure varies with the turning of the steering wheel. The faster and farther the steering wheel is turned, the greater the fluid pressure.
The fluid is also fed under the same pressure to the control valve where it opens a spool valve in the control valve housing. As the spool valve moves, it allows fluid from the rear steering pump to move through and operate the rear power cylinder. The higher the pressure on the spool, the farther it moves. The farther it moves, the more fluid it allows through to move the rear wheels. As mentioned earlier, this system limits rear wheel movement to 11/2 degrees in either the left or right direction.
3.3 Electro-hydraulic 4WS
Figure 3. Electro-hydraulic 4WS
Several 4WS systems combine computer electronic controls with hydraulics to make the system sensitive to both steering angle and road speeds. In this design, a speed sensor and steering wheel angle sensor feed information to the electronic control unit (ECU). By processing the information received, the ECU commands the hydraulic sysÃ‚Â¬tem steer the rear wheels. At low road speed, the rear wheels of this system are not considered a dynamic facÃ‚Â¬tor in the steering process.
At moderate road speeds, the rear wheels are steered momentarily counter phase, through neutral, then in phase with the front wheels. At high road speeds, the rear wheels turns only in phase with the front wheels. The ECU must know not only road speed, but also how much and quickly the steering wheel is turned. These three factors - road speed, amount of steering wheel turn, and the quickness of the steering wheel turn - are interpreted by the ECU to maintain continuous and desired steer angle of the rear wheels.
The basic working elements of the design of an electro-hydraulic 4WS are control unit, a stepper motor, a swing arm, a set of beveled gears, a control rod, and a control valve with an output rod. Two electronic sensors tell the ECU how fast the car is going.
The yoke is a major mechanical component of this electro-hydraulic design. The position of the control yoke varies with vehicle road speed. For example, at speeds below 33 mph (53 km/h), the yoke is in its downward position, which results in the rear wheels steering in the counter phase (opposite front wheels) direction. As road speeds approach and exceed 33 mph (53 km/h), the conÃ‚Â¬trol yoke swings up through a neutral (horizontal) posiÃ‚Â¬tion to an up position. In the neutral position, the rear wheels steer in phase with the front wheels.
The stepper motor moves the control yoke. A swing arm is attached to the control yoke. The position of the yoke determines the arc of the swing rod. The arc of the swing arm is transmitted through a control arm that passes through a large bevel gear. Stepper motor action eventually causes a push-or-pull movement of its output shaft to steer the rear wheels up to a maximum of 5 degrees in either direction.
The electronically controlled, 4WS system regulates the angle and direction of the rear wheels in response to speed and driver's steering. This speed-sensing system optimizes the vehicle's dynamic characteristics at any speed, thereby producing enhanced stability and, within certain parameters, agility.
4. ACTUAL 4WS
The actual 4WS system consists of a rack and pinion front steering that is hydraulically powered by a main twin-tandem pump. The system also has a rear-steering mechanism, hydraulically powered by the main pump. The rear-steering shaft extends from the rack bar of the front-steering assembly to the rear-steering-phase conÃ‚Â¬trol unit.
The rear steering is comprised of the input end of the rear-steering shaft, vehicle speed sensors, and steering-phase control unit (deciding direction and degree), a power cylinder, and an output rod. A cenÃ‚Â¬tering lock spring is incorporated that locks the rear system in a neutral (straight-ahead) position in the event of hydraulic failure. Additionally, a solenoid valve that disengages the hydraulic boost (thereby activating the centering lock spring in case of an electrical failure) is included.
5. FAIL-SAFE MEASURES
All 4WS systems have fail-safe measures. For examÃ‚Â¬ple, with the electro-hydraulic setup, the system autoÃ‚Â¬matically counteracts possible causes of failure: both electronic and hydraulic, and converts the entire steering system to a conventional two-wheel steering type. Specif-ically, if a hydraulic defect should reduce pressure level (by a movement malfunction or a broken driving belt), the rear-wheel-steering mechanism is automatically locked in a neutral position, activating a low-level warn-ing light.
In the event of an electrical failure, it would be deÃ‚Â¬tected by a self-diagnostic circuit integrated in the four Ã‚Â¬wheel-steering control unit. The control unit stimulates a solenoid valve, which neutralizes hydraulic pressure, thereby alternating the system to two-wheel steering. The failure would be indicated by the system's warning light in the main instrument display.
On any 4WS system, there must be near-perfect com-pliance between the position of the steering wheel, the position of the front wheels, and the position of the rear wheels. It is usually recommended that the car be driven about 20 feet (6 meters) in a dead-straight line. Then, the position of the front/rear wheels is checked with respect to steering wheel position. The base reference point is a strip of masking tape on the steering wheel hub and the steering column. When the wheel is positioned dead cenÃ‚Â¬ter, draw a line down the tape. Run the car a short disÃ‚Â¬tance straight ahead to see if the reference line holds. If not, corrections are needed, such as repositioning the steering wheel.
Even severe imbalance of a rear wheel on a speed sensitive 4WS system can cause problems and make basic troubleshooting a bit frustrating.
6. ADVANTAGES OF 4WS
The vehicle's cornering behavior becomes more staÃ‚Â¬ble and controllable at high speeds as well as on wet or slippery road surfaces.
The vehicle's response to steering input becomes quicker and more precise throughout the vehicle's entire speed range.
The vehicle's straight-line stability at high speeds is improved. Negative effects of road irregularities and crosswinds on the vehicle's stability are minimized.
Stability in lane changing at high speeds is improved. The vehicle is less likely to go into a spin even in situÃ‚Â¬ations in which the driver must make a sudden and relatively large change of direction.
By steering the rear wheels in the direction opposite the front wheels at low speeds, the vehicle's turning circle is greatly reduced. Therefore, vehicle maneuÃ‚Â¬vering on narrow roads and during parking becomes easier.
7. APPLICATIONS OF 4WS
Some of the vehicles in which the 4WS is applied are:
7.1 Chevrolet Suburban 2500:
The purely electronic system works so that, at low speed, the rear wheels turn the opposite direction of the front wheels, thus shortening the turning circle. At higher speeds all four wheels turn in the same direction for better stability in lane change maneuvers. The system works spectacularly well with the Suburban and the turning circle diameter drops down from 44.5 feet to 35.2 feet. There is a switch to turn the system off and the Suburban drives like a regular two-wheel steering machine and, in contrast, it feels quite ponderous.
Unfortunately the four-wheel steering system also pushes the width of the Sub out past 80 inches. But the very worst thing about the four-wheel steering system is its $4495 option cost. Hopefully as the four-wheel steering system becomes more ubiquitous across the GM range of products the cost of the system will drop.
7.2 GM Concept Truck:
QUADRASTEERTM (four-wheel steering system) by Delphi is featured on General Motor Corp.'s GMC Terradyne concept vehicle. QUADRASTEERTM by Delphi is an electronic four-wheel steering system that enables vehicles to significantly improve handling and maneuverability in full-size vehicles. Based on tests with full-size SUVs and pickup trucks, QUADRASTEER by Delphi reduces the minimum turning circle diameter by an average of 19 percent. In fact, one full-size pickup's turning radius was reduced from 46.2 feet to 37.4 feet, making it comparable to a Nissan Ultima at 37.4 feet and a Saturn Coupe at 37.1 feet.
QUADRASTEERTM by Delphi combines conventional front-wheel steering with an electrically powered rear-wheel steering system. The system has four main components - a front-wheel position sensor, steerable solid hypoid rear axle, electric motor-driven actuator, and control unit. Hand wheel position and vehicle speed sensors continuously report data to the control unit, which in turn determines the appropriate angle of the rear wheels. Algorithms are then used to determine the correct phase of operation. The QUADRASTEERTM by Delphi Systems also provides a controlled return to regular two-wheel steering if the four-wheel steering system is damaged.
7.3 Jeep Hurricane:
The Jeep Hurricane, a radical off-road machine with two 5.7 litre V8 engines features a turn radius of absolutely zero, using skid steer capability and toe steer: the ability to turn both front and rear tires inward. In addition, the vehicle features two modes of automated four-wheel steering.
The first is traditional with the rear tires turning in the opposite direction of the front to reduce the turning circle. The second mode is an innovation targeted to off-road drivers: the vehicle can turn all four wheels in the same direction for nimble crab steering. This allows the vehicle to move sideways without changing the direction the vehicle is pointing. The multi-mode four-wheel steering system offers killer performance and maneuverability.
Figure 4. Jeep Hurricane
Figure 5. Ford Suburban 2500
Thus the four-wheel steering system has got cornering capability, steering response, straight-line stability, lane changing and low-speed maneuverability. Even though it is advantageous over the conventional two-wheel steering system, 4WS is complex and expensive. Currently the cost of a vehicle with four wheel steering is more than that for a vehicle with the conventional two wheel steering. Four wheel steering is growing in popularity and it is likely to come in more and more new vehicles. As the systems become more commonplace the cost of four wheel steering will drop.
1. Automotive Technology-A Systems Approach, Jack Erjavec.
2. Automotive Suspension and Steering Systems, Thomas W Birch.
3. Automotive Service-Inspection, Maintenance, Repair, Tim Gilles.