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2019-05
Analysis of Automotive Steering System Structure
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When we drive on a regular basis, simply holding the steering wheel steady is enough to make the car go where we want it to—yet we rarely stop to wonder how the steering wheel actually turns the wheels.
When we drive, simply holding the steering wheel steady is enough to steer the car in the direction we want—yet we rarely stop to wonder how the steering wheel actually turns the wheels. We also often hear terms like “hydraulic power steering,” “electric power steering,” and “active steering.” But how do these systems work, and what are the key differences among them?
● What is power steering?
Power steering refers to the use of an external force to enable drivers to steer with less effort. Initially introduced on larger vehicles to make steering easier and less physically demanding, it is now widely adopted across a broad range of vehicle types, enhancing driving comfort and responsiveness while also improving safety to a certain extent. Based on the source of power, power steering systems are classified into hydraulic power steering and electric power steering.
● Mechanical hydraulic power steering
The mechanical hydraulic power-assisted steering system primarily comprises two main components: a rack-and-pinion steering mechanism and a hydraulic system (including a hydraulic power pump, hydraulic cylinder, piston, and other elements). The operating principle is as follows: the hydraulic pump, driven by the engine belt, generates hydraulic pressure to actuate the piston, which in turn produces assistive force to push the steering tie rod, thereby aiding wheel steering.
So how exactly does it work? First, the mechanical valve body mounted on the steering gear—which rotates with the steering column—remains in its original position when the steering wheel is stationary, maintaining equal hydraulic pressure on both sides of the piston and thus a state of equilibrium. When the steering wheel is turned, the steering control valve opens or closes accordingly: on one side, the hydraulic fluid bypasses the hydraulic cylinder and flows directly back to the reservoir, while on the other side, fluid continues to be pumped into the cylinder. This creates a pressure differential across the piston, which drives it and, in turn, generates assistive force to act on the steering linkage, making steering much easier.
In hydraulic steering systems, when the wheels experience severe bouncing or encounter potholes that cause unintended steering movements, the hydraulic actuation of the piston effectively cushions and absorbs these vibrations, significantly reducing the amount of vibration transmitted to the steering wheel. Mechanical-hydraulic power-assist technology is mature, stable, highly reliable, and widely used; however, it has a relatively complex structure and higher maintenance costs. Moreover, in a purely mechanical-hydraulic system, the level of power assistance is fixed and cannot be adjusted, making it difficult to simultaneously meet the differing requirements for steering precision at low and high speeds.
● Electronic Hydraulic Power Steering
The structural principle of electronic hydraulic power steering is broadly the same as that of mechanical hydraulic power steering; the main difference lies in the method used to drive the hydraulic pump that generates oil pressure. In mechanical hydraulic power steering, the hydraulic pump is directly driven by the engine belt, whereas electronic hydraulic power steering employs an electrically driven electronic pump.
The electric pump in an electro-hydraulic power steering system does not draw power from the engine itself, and it is controlled by the vehicle’s electronic control system. When no steering input is required, the pump shuts off, further reducing energy consumption. The electronic control unit of the electro-hydraulic power steering system processes data from sensors such as the vehicle-speed sensor and the steering-angle sensor, and can adjust the flow rate of the electric pump to vary the level of steering assist.
● Electric Power Steering
Electric power assistance primarily consists of sensors, a control unit, and an electric assist motor. With the elimination of components such as the hydraulic pump, hydraulic lines, and steering column valve body found in hydraulic power steering systems, the overall architecture is remarkably simple.
The primary operating principle is as follows: when the steering wheel is turned, a torque sensor mounted on the steering column transmits the steering signal to the control unit. The control unit processes this signal and adjusts the voltage supplied to the electric motor accordingly, thereby driving the motor to rotate. The torque output by the electric motor is then amplified by a reduction mechanism and used to actuate the steering column or steering tie rods, providing power-assisted steering. Furthermore, the electric power-assisted steering system can vary the level of assistance based on vehicle speed, making the steering lighter at low speeds and more stable at high speeds.
Electric power steering can be implemented in two ways: one is to provide assistance directly to the steering column, in which the assist motor, after speed reduction and torque multiplication, is connected directly to the steering column, with the motor’s auxiliary torque applied directly to the column—effectively allowing the motor to help turn the steering wheel. The other approach is to provide assistance to the steering tie rod, where the assist motor is mounted on the tie rod and directly drives the rod to steer the wheels. The latter configuration is more compact and easier to arrange, and is currently more widely used.
● What is variable-speed power steering?
Speed-sensitive variable power steering means that the level of steering assist varies with vehicle speed. What are the benefits of this design? At low speeds, such as when parking or maneuvering in tight spaces, a light and effortless steering feel is indeed convenient. However, at high speeds, overly light steering can be hazardous, as it compromises the vehicle’s stability and handling.
Variable-ratio power steering makes this possible: at low speeds, it delivers substantial power assistance to ensure light and nimble steering; at higher speeds, the level of assistance is reduced to enhance driving safety and stability.
● What is a variable-ratio steering system (active steering system)?
So-called variable steering ratio can be simply understood as the ratio of the steering wheel’s angle of rotation to the corresponding angle of wheel rotation. In the speed-sensitive power-assisted steering system mentioned earlier, only the level of power assistance can be adjusted—meaning that the amount of assist during steering is the only parameter that can be varied. The steering ratio itself, however, remains fixed. By contrast, a variable steering-ratio system allows not only the adjustment of power-assist levels but also the variation of the steering-wheel-to-wheel-angle ratio under different driving conditions.
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