Permanent Magnetic Materials in Automotive EPS

EPS is an electronic power steering system, which uses the power generated by the electric motor to assist the driver in power steering. As an important part of the vehicle, the steering system is one of the key systems that affects the vehicle's handling stability, comfort and driving safety.

In the design of the steering system, in order to ease the contradiction between the lightness of automobile steering and steering sensitivity, most commercial vehicles and 50% of cars use power steering systems. Since the 1950s, the power steering system has gone through three development stages: conventional hydraulic power steering system (HPS), electro-hydraulic power steering system (EHPS), and electric power steering system (EPS), and there is a trend of continuing to develop towards electronic and intelligent.

The composition of EPS system

The EPS electronic power steering system is based on the traditional mechanical steering system. According to the torque signal on the steering wheel and the vehicle speed signal, the electronic control device makes the motor generate auxiliary power of corresponding size and direction to assist the driver in steering operations. It can easily provide different power assistance effects to the motor at different vehicle speeds, ensuring that the car is light and flexible when turning at low speeds, has sufficient road feel when turning at high speeds, and is stable and reliable. Although different vehicles have different structural components, the structure of the EPS system is generally the same. It is generally composed of a torque (steering) sensor, an electronic control unit, a motor, a reducer, a mechanical steering gear, and a battery power supply.

The motors used in EPS are divided into brushed motors and brushless motors

Brushed motors switch current while the brushes and commutator rotate, and can rotate when the power is turned on. They are low-cost, but the windings of brushed motors are arranged on the rotor side. As the output power increases, the inertia moment of the motor increases, and the problem of poor steering sensitivity needs to be solved.

The brushless motor itself does not have a rectifier function, so a built-in angle sensor is required. The current structure of the corresponding angle signal is complex and costly to switch through the circuit. However, the windings of the brushless motor are arranged on the stator side, and the rotor is a permanent magnet. Even if the output power increases, the inertia moment problem can be suppressed.

Permanent magnet materials in EPS motors

EPS has very high requirements for the performance, weight and volume of permanent magnet motors, so the permanent magnet materials used are mainly high-performance NdFeB magnets, which are currently mainly sintered NdFeB. Common grades are 45H, 48H, 38SH, 40SH, 42SH, 48SH, 35UH, etc. In the future, hot-pressed NdFeB is expected to replace sintered NdFeB in EPS, but the technology is not yet mature and the cost is still high, so sintered NdFeB is still the mainstream.