Composition and working principle of electric power steering system EPS

The steering system is an important component of a car and an important connection tool between people and cars. It has been constantly updated with the overall development of the car and the emergence of new technologies, from the initial mechanical steering, mechanical hydraulic power steering HPS, electro-hydraulic power steering EHPS, electric power steering EPS, to the latest wire-controlled steering SBW.

Today we are going to talk about the electric power steering system EPS. In February 1988, Japan's Suzuki first equipped its Cervo car with electric power steering. After that, the electric power steering technology developed rapidly. Daihatsu, Mitsubishi, Honda, and Delphi Automotive Systems have successively developed electric power steering systems and installed them in their products. At present, electric power steering is very common in cars. The following will sort out the working principle of the electric power steering system, EPS control unit test, and EPS redundancy.

01.

System composition and working principle

The electric power steering system EPS consists of a mechanical steering system, a steering wheel torque sensor, a vehicle speed sensor, an electronic control unit (ECU), a motor and a reduction mechanism, as shown in the figure below.

▲ EPS system composition diagram

The steering angle sensor is used to detect the steering direction and position of the steering wheel in real time, and the torque sensor is used to detect the steering wheel torque in real time and transmit the signal to the EPS control unit. The accuracy of the steering angle and torque sensors determines the performance reliability of the electric power steering system.

The vehicle speed sensor is used to measure the vehicle speed. The vehicle speed sensor is generally an electromagnetic induction sensor installed on the gearbox. The sensor transmits the pulse signals of the main and auxiliary systems to the ECU according to the changes in vehicle speed.

The characteristics of the power-assisted motor directly affect the difficulty of EPS system control and the driver's feel. The basic requirements of the EPS system for the power-assisted motor are: it should have high reliability, high power, low noise and vibration, low friction torque, small size and weight; it should be able to output torque under stall; it should have good mechanical characteristics, and the torque fluctuation should be as small as possible during operation; the moment of inertia should be as small as possible; it should be able to reverse quickly.

At present, the power-assisting motors of EPS systems usually include permanent magnet DC motors, brushless DC motors and switched reluctance motors. Brushless DC motors have mature technology, simple controllers and low costs, but they have disadvantages such as easy wear of brushes, low power density, and electromagnetic interference caused by the sparks of the commutator. Brushless DC motors use electronic commutation, reduce sparks during commutation, do not require frequent maintenance, and have higher efficiency and power density, which attracts more and more attention.

The speed reduction mechanism is connected to the motor to reduce speed and increase torque. The speed reduction mechanism of the EPS system often adopts worm gear mechanism, recirculating ball screw nut, planetary gear mechanism, etc. In order to ensure that the EPS system only works within the preset vehicle speed range, some EPS systems are also equipped with a clutch. When the vehicle speed reaches a certain value, the clutch disengages, the motor stops working, and the steering system switches to manual steering. In addition, when the motor fails, the clutch will automatically disengage.

The function of ECU is to issue instructions to control the action of the power-assist motor after performing logical analysis and calculation based on the steering wheel angle, torque and vehicle speed signals. In addition, ECU also has safety protection and self-diagnosis functions. ECU determines whether the system is working normally by collecting signals such as the motor current, generator voltage, and engine operating conditions. Once the system is working abnormally, the power assistance will be automatically cancelled, and the ECU will perform fault diagnosis and analysis.

The working principle of the whole system is roughly as follows: the ECU determines the rotation direction and optimal power torque of the motor according to the signals of the torque sensor, the angle sensor and other information such as the vehicle speed sent by the communication bus, sends a control signal to the motor, controls the rotation of the motor through the power drive circuit, and the output of the motor is decelerated and torque-increased by the reduction mechanism, and then drives the gear rack mechanism to generate the corresponding steering assistance. Through precise control algorithms, the torque of the motor can be changed so that the transmission mechanism can obtain the required power assistance value.

Since the torque sensor is connected to the steering shaft, when the steering shaft rotates, the torque sensor starts to work, and the relative rotation angle displacement generated by the input shaft and the output shaft under the action of the torsion bar is converted into an electrical signal and transmitted to the ECU. The ECU determines the rotation direction of the motor and the magnitude of the power-assisting current according to the signals of the vehicle speed sensor and the torque sensor, thereby completing real-time control of the power-assisting steering. Therefore, it can easily provide different power-assisting effects of the motor at different vehicle speeds, ensuring that the car is light and flexible when turning at low speeds, and stable and reliable when turning at high speeds.

▲ EPS control schematic diagram (Source: Automotive Electronics and Software)

▲ EPS system control strategy diagram

02.

Design solutions of various chip manufacturers

Overall, the solutions are still provided by several major MCU manufacturers, including ST, Infineon, and Renesas.

First, let's look at ST's EPS solution, which consists of ST's automotive-grade 32-bit PowerPC architecture microprocessor SPC560P50, 6-channel MOS driver IC L9908 and automotive-grade MOS. The solution is suitable for 12V, 24V, and 48V systems, and also has corresponding diagnostic protection functions.

▲ ST system block diagram

SPC560P50 has FlexPWM suitable for motor development and ADC module adapted to CTU. It supports a core frequency of up to 64MHz to better meet the needs of application resources.

The driver IC L9908 supports three-way ADC sampling and six-way MOS drive output, and has an SPI interface that can read the full fault list, making it more suitable for the development of standard three-phase permanent magnet synchronous motor solutions.

The software of this solution is developed based on ST's SPC5Studio and MCTK (Motor Control Kit). The free IDE SPC5Studio provides various peripheral supports for MCU, and the mature MCTK platform also provides various interfaces for motor control, which can help customers reduce the underlying development cycle and be used for rapid system debugging.

Key features of the Motor Control Toolkit:

AEC-Q100 compliant

120MHz Flex PWM Module

Configure SPC5 FOC Lib via SPC5Studio plugin

Embedded software library for FOC control (SPC5-MCTK-LIB)

SPC560P + L9908 board

BLDC motor with sensor/sensorless

Below is Infineon's EPS solution, in which the main control chip is Tricore's TC33X or TC36X, the PMIC is the well-known TLF35584, the torque sensor is TLE499X, the three-phase drive chip is TLE9180, etc. The overall solution is shown in the figure below.

The solution provides unique fail-safe EPS chipsets that are all verified. Unlike standalone solutions, the calibrated chipsets can reduce development workload, reduce BOM material costs, and support scalability for different vehicle models and driving modes.

System Specific:

Powerful, scalable, and compact

Certified interoperability

EPS system supports fail-safe and fail-safe operation based on nx3 phases

Consistent safety concept focused on power supply and microcontroller

Software adjustable: suitable for various driving modes and vehicle types

Supports upgrades to meet future needs, such as steer-by-wire for autonomous driving

The solution provided by Renesas is mainly designed based on its own RH850/P1x and RAA270005 PMIC. The overall solution is shown in the figure below.

03.

Redundancy Solution

Redundancy means backup. First of all, it is the hardware level, including redundant sensors. The mainstream redundant sensors usually provide 4 torque signals and 2 angle signals. The ECU is the core part of the redundant EPS, which consists of the power-assisted motor and the drive and control unit. The power-assisted motor usually uses a six-phase or twelve-phase brushless DC motor. When three phases cannot be driven normally due to a single point failure, the remaining parts can still work normally. There is only one ECU in this solution.

Another solution is a fully redundant electric control solution based on dual three-phase brushless DC motors. The entire solution uses dual independent external power supplies, dual external CAN/CANFD communications, and redundant torque and angle sensor signals. The motor drive unit, motor position sensor, power management unit, and main control MCU also use a dual-backup redundant architecture. CANFD communication is used between the dual MCUs to achieve signal interaction, torque command transmission, fault diagnosis signal interaction, etc., which can play a role in mutual monitoring and support master-slave switching when necessary, improving the safety and reliability of the entire system.