Permanent magnets in automotive electric power steering (EPS) systems

Permanent magnets in automotive electric power steering (EPS) systems

EPS stands for Electric Power Steering, which is the electronic power steering system in our cars. The system directly provides steering assistance by an electric power steering machine. As an important part of the vehicle, the steering system is one of the key systems that affect vehicle stability, comfort and driving safety. In the design of the steering system, in order to ease the contradiction between the steering convenience and steering sensitivity of the car, most commercial vehicles and 50% of the cars use power steering systems.

The electronic power steering system (EPS) uses an electronic control device to make the electric motor generate auxiliary power of corresponding size and direction according to the torque signal on the steering wheel and the driving speed signal of the car to assist the driver in steering operations. Although the structural components of different vehicles are different, the composition of the EPS system is generally the same. It usually consists 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:

1. Brushed motor

The brushed motor switches the current when the brush and commutator rotate, and can rotate when the power is turned on, and the cost is low. However, the winding of the brushed motor is arranged on the rotor side. As the output power increases, the inertia moment of the motor increases, and it is necessary to solve the problem of reduced steering sensitivity.

2. Brushless motor

The brushless motor itself does not have a rectifier function, so a built-in angle sensor is required to switch the current corresponding to the angle signal through the circuit, so the structure is complicated and the cost is high. However, the winding of the brushless motor is 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.

The EPS system has high requirements on the performance, weight and volume of the permanent magnet motor. Therefore, the permanent magnets in the EPS system are mainly high-performance NdFeB permanent magnets, and currently they are mainly sintered NdFeB magnets. Common grades are 45H, 48H, 38SH, 40SH, 42SH, 48SH, 35UH, etc.

In the future, hot pressed NdFeB magnets are expected to replace sintered NdFeB magnets in EPS systems, but currently, the technology is not mature and the cost is still high. Therefore, sintered NdFeB magnets are still the mainstream.