Definition: A steering system operated by a power source.
Subject: Mechanical Engineering_Automotive_Chassis_Steering
Related terms: Steering effort, Electric motor, Hydraulic power-assisted steering

A power-assisted steering system is a series of devices that convert part of the mechanical energy output by the engine into pressure energy or electrical energy and, under the driver’s control, applies an assisting force in different directions to a specific component within the steering train or steering gear, causing the steering wheels to turn and thereby enabling the vehicle to change direction. In vehicles equipped with this type of power steering system, under normal circumstances, only a small portion of the energy required for steering is provided by the driver’s physical effort, whilst the majority is supplied in the form of hydraulic, pneumatic or electrical energy by an engine-driven oil pump (power steering pump), air compressor or generator, thereby reducing the steering effort required of the driver.

The power steering system consists of a mechanical steering gear and a steering assist unit. Depending on the form of the assist energy, it can be classified into three types: pneumatic, hydraulic and electric. Pneumatic power steering systems are primarily used in certain lorries and coaches with a maximum front-axle load of 3 to 7 tonnes that are equipped with pneumatic braking systems. Pneumatic power steering systems are not suitable for lorries with exceptionally high payloads, as the operating pressure of pneumatic systems is relatively low (generally not exceeding 0.7 MPa); when used on such heavy-duty vehicles, the components would become excessively large. Hydraulic power-assisted steering systems can operate at pressures as high as 10 MPa, resulting in very compact components. Hydraulic systems operate silently, have minimal response delay and are capable of absorbing shocks from uneven road surfaces. Direct-assist electric power steering systems are a type of power steering system that relies directly on electric motor power to provide auxiliary torque; the motor can be controlled to deliver varying levels of auxiliary power depending on different operating conditions. Direct-assist electric power steering systems contain no hydraulic components and operate only during steering manoeuvres, which account for approximately 5 per cent of driving time; as a result, the vehicle’s fuel consumption increases by only around 0.5 per cent, leading to a significant reduction in energy consumption.

With the rapid development of the automotive industry and the growing demand for energy-efficient, intelligent and lightweight vehicles, electric power steering systems are now widely adopted in small passenger cars, whilst the proportion of hydraulic power steering systems is gradually decreasing.

Electric power steering systems primarily comprise a mechanical steering gear, a torque sensor, a reduction gear, a clutch, a motor, an electronic control unit (ECU) and a vehicle speed sensor. Depending on the location of the assist motor, direct-assist electric power steering systems can be categorised as steering-shaft-assist, gear-assist or rack-assist types. In steering-shaft-assisted electric power steering systems, the motor is mounted on the side of the steering shaft and directly drives the steering shaft to provide steering assistance. In gear-assisted electric power steering systems, the control unit is generally installed on the front passenger side, with the motor driving the gear shaft directly via a reduction gear to provide assistance. Rack-assisted systems utilise the motor and reduction gear to directly drive the rack to provide assistance.

Advantages of electric power steering: 1. High efficiency and low energy consumption; 2. Responsive with minimal lag; 3. Simple structure and low weight; 4. No need to fill with fluid, saving space; 5. No issues with fluid leakage, resulting in minimal environmental pollution. The disadvantages of electric power steering are: 1. The vehicle’s power supply voltage is relatively low, resulting in limited auxiliary power, which makes it difficult to use in large vehicles; 2. Friction and inertial forces from the reduction gear and motor components affect steering characteristics; 3. Components such as the motor and reduction gear increase operating costs.