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The Mazda Speed Sensing Computerised 4-Wheel Steering System.
Three and a half decades ago, two young Mazda designers arrived at a far-sighted and well-calculated conclusion that was quite revolutionary for the time. In their technical presentation at the October 26, 1962 Japanese Automotive Engineers' Society Technical Conference, Dr Tadashi Okada and engineer Toshiaki summarised their arduous research concerning vehicle dynamics as follows.
1. The basic difference in the characteristics of oversteer and understeer lies in the magnitude of time delay and response.
2. a vehicle that is stable under high speed must possess understeer characteristics
3. the rear wheel tyre reflects heavily on the stability and
4. a major improvement on control and stability may be anticipated by means of the automatic rear wheel steering system.
The conclusions and formulations presented by these two engineers established the foundation for Mazda's present-day reputed suspension technology. Over years of dedicated research and development expertise, their original discoveries and theories have contributed to some of the most significant achievements within the recent history of automotive chassis engineering, incorporated by Mazda within its series production products. These developments include the twin trapezoidal link rear suspension, first employed in the original front-wheel drive Mazda 323 (1980) and the Mazda 626 (1982), and then perfected within the updated Mazda 626; the award winning Dynamic Tracking Suspension System of the second generation Mazda RX-7 (1985); and the elaborate E-link rear suspension of the new Mazda 929 (1987).
While various external forces and loads are exerted to the rear wheels of a vehicle as it combats the elements of the law of motion as defined by Sir Isaac Newton, these new suspension systems convert those forces into "4WS effects" which positively aid in vehicle stability and agility.
The Mazda designers' and engineers' ultimate goal was still a positive measure to generate forces for positive controls; a Four-Wheel Steering system.
In 1983, Mazda astonished the automotive world with the introduction of an engineering concept car, the MX-02, exhibited at the Tokyo Motor Show. This four-door Sedan, with generous passenger accommodation on an unusually long wheelbase, incorporated among its numerous advanced features a true 4WS system that aided high-speed stability as well as its low-speed manoeuvring. The degree of rear wheel steering was determined by the measurement of both front wheel steering angle and vehicle speed, by means of a central computer unit.
The MX-02 was followed by another exciting concept car; the MX-03, first exhibited at the Frankfurt Motor Show in September 1985. This sleek four seat futuristic coupe of the 1990s combined a refined electronically-controlled 4WS system with a continually varying torque-split, four-wheel drive system and a powerful three-rotary engine.
Mazda Electronically -Controlled Four-Wheel Steering System:
A Beneficial Technology
Mazda's electronically-controlled, vehicle-speed-sensing Four-Wheel Steering System (4WS) steers the rear wheels in a direction and to a degree most suited to a corresponding vehicle speed range. The system is mechanically and hydraulically actuated, producing greatly enhanced stability, and within certain parameters, agility.
The driver of a Mazda 4WS-equipped car derives five strategic benefits, over and above the conventional vehicle chassis.
1. Superior cornering stability
2. Improved steering responsiveness and precision
3. High-speed straightline stability
4. Notable improvement in rapid lane-changing manoeuvres
5. Smaller turning radius and tight-space manoeuvrability at low vehicle speed range
The most outstanding advantage of the Mazda 4WS is that it contributes to a notable reduction in driver fatigue over high-speed and extended travelling. This is achieved by optimally:
1. reducing the response delay to steering input and action and
2. eliminating the vehicle's excessive reaction to steering input
In essence, by providing the optimum solution to the phenomena researched by the two young Mazda engineers in the early sixties - by the method advocated by them - the 4WS system has emerged as a fully beneficial technology.
Strategic Construction
The Mazda 4WS consists of a rack-and-pinion front steering system that is hydraulically assisted by a twin-tandem pump main power source, with an overall steering ratio of 14.2:1. The rear wheel steering mechanism is also hydraulically assisted by the main pump and electronically controlled - according to the front steering angle and vehicle speed. The rear steering shaft extends from the rack bar of the front steering gear assembly to the rear steering-phase control unit.
The rear steering system is comprised of the input end of the rear steering shaft, vehicle speed sensors, a steering-phase control unit (determining direction and degree), a power cylinder and an output rod. A centering lock spring is incorporated, which locks the rear system in a neutral (straightforward) position in the event of hydraulic failure. Additionally, a solenoid valve that disengages hydraulic assist (thereby activating the centering lock spring) in case of an electrical failure is included.
The 4WS system varies the phase and ratio of the rear-wheel steering to the front wheels, according to the vehicle speed. It steers the rear wheels toward the opposite phase (direction) of the front wheel during speeds less than 35km/h (22mph) for a tighter turn and "neutralizes" them (to a straightforward direction, as in a conventional two-wheel steering principle) at 35km/h (22mph). Above that speed, the system steers toward the same phase-direction as the front wheels, thereby generating an increased cornering force for stability. The maximun steering angle of the rear wheels extends 5 degrees to either left or right, a measurement that Mazda has determined to be optimally effective and natural to human sensitivity.
Primary Components
1. Vehicle speed sensors Interpret speedometer shelf revolutions and send signal to the electronic computer unit. two sensors, one within the speedometer and the other at the transmission output, are used to crosscheck the other for accuracy and failsafe measures.
2. Steering phase control unit* Conveys to the power steering cylinder booster valve the direction and stroke of rear wheel steering by the combined movement of the control yoke angle and bevel gear revolutions.
3. Electric stepper motor Performs altering of the yoke angle and bevel gear phasing
4. Rear steering shaft Transmits front wheel steering angle by turning the small bevel gear in the steering phase control unit, which rotates the main bevel gear in the assembly.
5. Control valve Feeds hydraulic pressure to the steering actuator, according to the phase and stroke required for appropriate rear wheel steering.
6. Hydraulic power cylinder Operates the output rod by hydraulic pressure and steers the rear wheels. It locks the rear wheels in a "neutral" (straightforward) position with the centering lock spring, which is activated by a solenoid valve in case of failure to ensure a normal 2WS function for the vehicle.
7. Hydraulic pump. Provides hydraulic pressure to both the front and rear steering systems.
Details of Steering Phase Control Unit
The steering phase control unit alters the direction and degree of rear wheel steering. It consists of a stepper motor that controls the rear steering ratio, a control yoke, a swing arm, a main bevel gear engaged to the rear steering shaft via a small bevel gear, and a control rod connected to the control valve. It operates:
a. Opposite phase (direction) steering under 35km/h (22mph)
1. Control Yoke is at an angle activated by the stepper motor
2. Front wheels are steered to the right. The small bevel gear is rotated in direction X by the rotation of the rear steering shaft. The small bevel gear, in turn, rotates the main bevel gear.
3. Rotation of the main bevel gear causes movement of the control rod toward the control valve.
4. Input rod of the control valve is pushed to the right, according to the degree of the control rod's movement (determined by the disposition of the swing arm), which is positioned to move in an upward direction, to the right. The rear wheels are thus steered to the left, in an opposite direction to the front wheels.
5. As the angle of the control yoke is increased in direction A as vehicle speed decreases, the rear-to-front steering ratio proportionately increases and the vehicle's steering lock tightens.
b. Same phase (direction) over 35km/h (22mph)
The operation of this phase is the reverse of the opposite phase one, because the control yoke is angled toward "positive" in this vehicle speed range, as illustrated. The phasing of the swing arm, yoke rod and bevel gear steers the rear wheels toward the right-the same direction as the front wheels.
c. Neutral phase, at 35km/h (22mph) The control yoke's angle is horizontal (neutral). Thus, the input rod is not affected, even if the control rod is moved with the rotation of the bevel gear unit. As a result, the rear wheels are not steered in this mode.
Power Cylinder
The movement of the input rod of the control valve unit is transmitted to the power cylinder's spool. The spool's displacement to the sleeve causes a pressure difference between the right and left side chambers in the hydraulic power cylinder. The pressure difference overcomes the output shaft load and initiates sleeve movement. The sleeve-power rod assembly is moved in the direction of the input rod by a proportionate degree. The output rod transmits steering action to the tie rod on either end of the rear wheel steering control-mechanism unit, thereby steering the rear wheels.
Fail-Safe Measures
The system automatically counteracts possible causes of failure, both electronic and hydraulic. In either case, the centering lock spring housed in the steering system unit returns the output rods in the "neutral" straightforward position, essentially alternating the entire steering system to a conventional 2WS principle.
Specifically, if a hydraulic defect should render a reduction in 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 warning light.
In the event of an electrical failure, such would be detected by a self-diagnostic circuit integrated within the 4WS control unit, which stimulates a solenoid valve and then neutralizes hydraulic pressure and return lines, thereby alternating the system again to that of a 2WS principle. Henceforth, the warning light referencing the 4WS system within the main instrument display is activated, indicating a system failure.