Innolux's intelligent steer-by-wire solution makes cars smarter and safer

introduction:

汽车行业在“新四化”目标的牵引下快速发展，汽车电动化与智能化取得了显著的成果。汽车智能化的发展对汽车底盘提出了新的需求，传统汽车底盘在响应速度、执行精度、安全性等方面不再能满足智能汽车的要求，汽车底盘由传统底盘向线控底盘过渡。其中，线控转向（Steering-by-Wire，SBW）是线控底盘中控制横向运动的核心部件，是实现高阶自动驾驶的重要执行机构。国务院印发的《新能源汽车产业发展规划(2021-2035)》中将纯电动汽车底盘一体化、线控执行系统等列为重点技术攻关工程。国标《GB17675-2021 汽车转向系基本要求》中删除了不得装用全动力转向机构的要求(1999年的3.3)，法规层面已允许转向系统方向盘与转向器之间的物理解耦。国家战略的推动以及法规标准的落地对于线控转向（SBW）产品的大批量产业化应用具有直接的促进作用。本文将从转向技术的发展路径、技术方案、关键技术等方面对英创汇智的线控转向（SBW）解决方案进行详细介绍。

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Turning to the technology development path

The steering system is one of the key components of the automobile chassis and plays a vital role in the vehicle's handling stability, safety and comfort. With the deepening of electrification in the automobile industry, the steering system has gone through a development process from mechanical steering system (MS), mechanical hydraulic steering system (HPS) to electronic hydraulic power steering system (EHPS), electric power steering system (EPS) and then to steer-by-wire (SBW).

Figure 1-1 Development path of automobile steering system

Image source: Tianfeng Securities Research Report "Intelligent Electric Vehicle Track Depth II: Wire-controlled steering, the core component of high-level intelligent driving"

Compared with mechanical and hydraulic steering systems, the electric power steering system (EPS) has a simpler structure, faster power control response, lower energy consumption, and does not require maintenance. It is the mainstream product of the current automotive steering system. The biggest difference between steer-by-wire (SBW) and EPS is that there is no mechanical connection between the steering wheel and the actuator, so it has obvious advantages in cost control, design flexibility, rich functions, and space layout.

As a core component of the chassis, the steering system has very high technical barriers. my country's automobile industry started relatively late, so the current supplier giants in Germany, the United States, Japan, and South Korea still occupy most of the steering system market, especially high-end EPS and steer-by-wire (SBW) systems, and China is still in a state of catching up. However, as the domestic automobile industry chain matures, some companies have also begun to gradually master the core technology of the steering system, and are expected to rapidly increase their market share in the future.

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Innolux's T-SBW technology solution

2.1 System Solution of Steer-by-Wire

As shown in Figure 2-1, it is a schematic diagram of the SBW system solution of Innovative Intelligence, which is mainly composed of the steering wheel actuator (HWA) and the front wheel actuator (RWA). The biggest difference between SBW and EPS system is that there is no intermediate shaft, that is, the steering wheel actuator and the front wheel actuator are completely decoupled mechanically, so it has a more flexible and adjustable steering ratio and more comfortable road feedback, and also provides the possibility of silent steering wheel and folding steering wheel in autonomous driving scenarios.

Figure 2-1 Schematic diagram of the electrical architecture of the steer-by-wire (SBW) system

►Handwheel Actuator (HWA): It is mainly composed of a steering wheel, a steering column, a reducer, a TAS sensor, and a redundant electronic control unit. Its main function is to obtain the driver's intention and give the driver's desired steering wheel angle signal to the front wheel actuator (RWA). At the same time, it simulates the road feedback force of the vehicle based on the rack force fed back by the front wheel actuator to provide the driver with road feel feedback information.

►Front wheel actuator (Road Wheel Actuator, RWA): The front wheel actuator consists of a mechanical steering gear, a steering angle sensor, a redundant electronic control unit, etc. Its main function is to receive the desired steering angle command sent by the steering wheel actuator (HWA), and realize the lateral movement of the rack by controlling the motor, and finally realize the steering function.

►Redundant ECU (Fail-Operational Powerpack): Both the steering wheel actuator (HWA) and the front wheel actuator (RWA) require an ECU as an actuator to realize the functions of road sense feedback control and front wheel steering respectively. The use of redundant ECUs is mainly to support high-level autonomous driving conditions, that is, in the autonomous driving scenario, if the steering wheel actuator (HWA) or the front wheel actuator (RWA) of the steer-by-wire (SBW) system has any single point failure, the component must have a fail-operational function (Fail-Operational) to ensure that the road sense is not lost or the front wheel does not lose the ability to steer. The steering wheel actuator (HWA) and the front wheel actuator (RWA) ECUs of the InnoSmart steer-by-wire (SBW) system both use a fully redundant electronic control solution to drive the six-phase permanent magnet synchronous motor. After a single point failure, the system still has the ability to provide road sense feedback and perform steering control.

►Angle Sensor: The front wheel actuator (RWA) needs to accurately track the desired angle, so an angle sensor is required to measure the actual pinion input angle.

2.2 Electrical architecture of steer-by-wire

Figure 2-2 Schematic diagram of the electrical architecture of the steer-by-wire (SBW) system

As shown in Figure 2-2, it is a schematic diagram of the electrical architecture of the SBW system of Innolux. The vehicle has redundant power supplies and a public CAN communication network. The steering wheel actuator (HWA) and the front wheel actuator (RWA) both use fully redundant electronic control units, which are connected to different power supplies and CAN communication networks to achieve external electrical isolation of the two independent systems. The two ECUs of the fully redundant electronic control unit also use CAN communication to achieve signal interaction, so that signal interaction and collaborative control can be performed. The steering wheel actuator (HWA) and the front wheel actuator (RWA) communicate through private CAN to transmit the expected angle signal, rack force signal, etc. Each ECU of the steering wheel actuator (HWA) needs to collect dual-channel torque signals and a single-channel absolute angle signal (supporting functional safety ASIL D level), so it corresponds to the "4+2" TAS sensor; each ECU of the front wheel actuator (RWA) needs to collect a single-channel absolute angle signal (supporting functional safety ASIL D level), so it corresponds to the angle sensor of the 2-channel angle signal.

2.3 Algorithm architecture of steer-by-wire

As shown in Figure 2-3, it is a schematic diagram of the algorithm architecture of the Innovative Intelligent Steer-by-Wire (SBW) system. According to the function of the steering wheel actuator (HWA), the main algorithms implemented include reference hand force calculation, torque superposition control, hand force tracking control, angle superposition control, variable transmission ratio control, angle command calculation, etc. It can be seen that in the SBW system, the steering wheel actuator (HWA) implements the driver's hand feel control based on the torque closed-loop algorithm, which is completely different from the open-loop torque algorithm of the traditional EPS. The main algorithms of the front wheel actuator (RWA) include angle tracking control and rack force estimation algorithm. The angle tracking control measures the pinion angle as a feedback signal through the angle sensor to achieve closed-loop control of the angle, thereby achieving precise steering control. The rack force estimation algorithm observes the rack force of the steering gear and sends it to the steering wheel actuator (HWA) as a road feel feedback signal to achieve road feel feedback simulation calculation.

Figure 2-3 Schematic diagram of the algorithm architecture of the steer-by-wire (SBW) system

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Inno-Tech T-SBW key technologies

3.1 Rack force observation

As shown in Figure 3-1, this is the rack force estimation algorithm architecture of the steer-by-wire front wheel actuator (RWA), which uses a rack force estimation method based on a vehicle model (Vehicle Estimator) and an estimation method based on a steering system model (Steering Estimator).

Figure 3-1 Schematic diagram of rack force estimation algorithm

Image source: Kim, C., Son, D., Sabato, Z., and Lee, B., "Improvement of Steering Performance Using Steering Rack Force Control," SAE Technical Paper 2019-01-1234, 2019, https://doi.org/10.427/2019-01-1234.

Vehicle Estimator uses the vehicle's two-degree-of-freedom model to estimate the rack force based on the vehicle's lateral acceleration, yaw rate, and steering wheel speed; Steering Estimator uses a simplified steering system dynamics model to estimate the rack force through the pinion angular velocity, power-assisted motor speed, power-assisted motor output torque, etc. When the vehicle speed is low, the estimation error based on the vehicle's two-degree-of-freedom model is large, and the performance of Steering Estimator is better than Vehicle Estimator; when the vehicle speed is high, the rack force estimation based on the vehicle's two-degree-of-freedom model can achieve a higher accuracy, and the performance of Vehicle Estimator is better than Steering Estimator at this time. Therefore, in actual applications, the estimation results of the two algorithms are reconciled (Driving State Weighting) according to the vehicle speed signal, so as to obtain estimation results that are better in all working conditions.

3.2 Torque closed-loop algorithm

As shown in Figure 3-2, the torque closed-loop algorithm of the steering wheel actuator is shown. First, the road feel feedback control is performed according to the result of the rack force estimation. First, the rack force is properly filtered to filter out the high-frequency vibration signal, and then the initial target hand force is determined based on the MAP of the road feel feedback. Compared with the Boost Curve of the traditional EPS open-loop control, this part realizes the direct setting of the target hand force, which is more convenient for the debugging of the feel style. In order to further optimize the driving feel, the hand force component calculation of active return and damping control is added. The active return function simulates the return effect of the traditional EPS and improves the driving experience. Damping control increases the stability of the system and prevents the steering wheel from "shaking" during the return process. The torque closed-loop algorithm has completely compensated the friction of the system, so there is no need to compensate for the friction of the system. However, appropriately "reconstructing" the friction of the system can make the feel more realistic and the experience better, and it can also prevent oversteering. The hand force tracking control module is required to be able to accurately track the reference hand force, and have a faster response speed and stronger robustness. In addition, the reference hand force closed-loop tracking control algorithm must have good stability.