What is a Steer-by-Wire system?
The Steer-by-Wire (SbW) system is an electric signal type intelligent steering system that transmits and controls the driver's steering will with electrical signals without mechanical connection between the driver's steering wheel and the vehicle wheels. It consists of an actuator (Road Wheel Actuator, RWA) that transmits the driver's steering intention to the vehicle wheels and moves the wheels, and an actuator (Steering Feedback Actuator, SFA) that provides the driver with the reaction force of the steering wheel.
Necessity of SbW system in terms of features and advantages In the midst of a huge flow of vehicles changing from means of transportation to mobility spaces, SbW is receiving a lot of attention as a future steering device. This is because it meets the demand for common parts with a free layout and can be applied to various vehicle platforms such as skateboards. The characteristics and advantages of SbW can be summarized as follows.
First, SbW directly controls the tire steering angle to correspond to the steering wheel input angle by the driver, enabling more direct and accurate steering control. In other words, since the compliance (degree of flexibility) of the steering system (steering wheel) is not affected, steering accuracy and responsiveness can be improved.
Second, the SbW has no axial linkage, so unwanted vibrations are essentially blocked. At this time, the necessary road surface input information is sensed by the RWA and transmitted to the SFA to provide load feedback corresponding to the road surface input to the driver.
Thirdly, SbW enables independent control of SFA and RWA to provide optimized performance that simultaneously satisfies steering feel and steering performance, and the steering gear ratio can be easily changed within a wide range according to vehicle speed without additional devices.
Fourth, SbW is a system more suitable for autonomous vehicles in that it fundamentally blocks the driver's unintended steering input during autonomous driving and enables a smooth and safe transition between driving modes.
Lastly, based on the SbW control technology, it is possible to implement steering systems with various concepts, such as a kingpin rotary steering system that steers around the kingpin rotation axis, a left/right independent steering system, and a steering system for in-wheel corner modules. In addition, it can be easily linked with various driver interface devices for high-level autonomous vehicles, such as a stowable column or joystick type, so it can be the basis for implementing innovative HMI functions.
SbW Mass Production Development Status
Since 2012, HL Mando has developed “True”, a fully detachable concept without steering clutch, by utilizing E/E redundancy technology developed for the purpose of preventing the risk due to rapid loss of steering assist power in case of EPS system failure. ” We reviewed the development of the SbW system, and since 2016, we have been developing SbW in earnest and are preparing for mass production. Every year, the Winter & Summer Ride Session event is held for global customers overseas to showcase various concepts of SbW systems and receive feedback to improve the technological perfection. Starting with the first mass production program of the SbW system in 2018, we registered the trademark under the name STEERITE® in 2019.
SbW Safety Architecture and Control Logic Design
The most important technology required for SbW system implementation is safety measures against system failure. EPS can be steered with the driver's power through mechanical connection even in the event of an electrical system failure, but SbW can cause a dangerous state in terms of vehicle safety in the same failure condition, so it is important to secure functional safety in various failure situations. There are mechanical backup methods and electrical backup methods to ensure safety against such system failures.
Since the mechanical backup method must maintain the same layout as the existing steering system, various advantages of SbW such as increased layout freedom cannot be utilized.
The electrical backup method is a concept that enables continuous fail-operation even in the event of a failure by immediately switching to the second system in case of a failure of the first system by redundant electrical/electronic systems such as ECU/sensor/communication/power. By adopting this electrical backup method, it is possible to implement a ‘True SbW’ system in which SFA and RWA are completely separated.
The system's redundancy concept and control strategy are established taking into account safety goals and safe states after failures. SFA places a redundant ECU on a single-winding motor so that it switches to the second ECU when the first ECU fails, allowing normal operation with 100% system output in case of system failure except for motor failure. The RWA implements a functional safety design that connects each ECU to a double-winding motor to generate 50% of the normal level of output even in the event of a system failure, including motor failure.
In addition to this, a response strategy at the vehicle level is being discussed in case of steering system failure through cooperative control with external systems such as Steer-by-Brake technology for safety.
In addition, the SbW system should implement advanced steering performance compared to the current EPS system, and all currently configured additional functions should be equally applicable.
For this, it is necessary to design the SbW control logic architecture. The SbW system can perfectly replace the existing steering system, and it is possible to implement a variable gear ratio free from mechanical connection. The variable gear ratio can be set in two dimensions according to the vehicle speed and steering angle, and individual settings are also possible according to the driving mode. The variable gear ratio provides a new user experience that has not been implemented in existing vehicles. In other words, it is possible to make a U-turn by turning the steering wheel half a turn, or to make a vehicle's response extremely fast when changing lanes while being comfortable while driving in a straight line. Also in autonomous vehicles, Silent Mode (steering wheel does not move, only lateral control works) or Sync. Mode (the steering wheel works with lateral control), etc. is free to implement.
In the SbW system, which has no physical connection, it is an important task to implement an appropriate steering feel, such as the reaction force generated from the tires or the vibration of the road surface. HL Mando has optimized performance by using a controller that is robust to disturbances and responds quickly to RWA, and SFA uses a controller of the same theory as RWA, but includes both a torque controller and an angle controller, depending on the driver mode and autonomous driving mode. made available selectively.
In order to implement an appropriate steering feel here, RWA controls the position of the rack and estimates the rack force acting on the wheels of the vehicle, and the estimated rack force is transmitted to the SFA through CAN communication. The SFA generates the driver's feedback torque from the estimated rack force delivered from the RWA. For the target feedback torque, functions such as EPS recovery control and damping control have been added using steering angle and steering angular velocity signals for optimal steering feel.
SbW performance
After starting SbW development, HL Mando established a new performance target to secure better performance than the existing EPS system, and secured performance that met the market's expectations through test drive feedback from major customers and benchmarking of premium vehicles.
HL Mando implemented vehicle behavior and load feedback similar to that of the EPS system through effective evaluation methods and development guidelines, enabling a new customer experience.
Since the SbW system is free from mechanical connection, unlike the EPS system, which only allows free tuning of the steering feel, in addition to tuning the steering feel, the gear ratio can be tuned to maximize vehicle performance according to the desired driving mode. This improved gear ratio (Variable Rack Stroke, VRS) tuning freedom has the advantage of increasing parking convenience by reducing the operating angle of the steering wheel when parking, and securing vehicle stability by lowering the gear ratio at high speeds.
In addition, since customers who value load feedback performance require that the input information of the road surface be properly delivered to the driver, the implementation of load feedback and securing robustness are the most important technologies in the SbW system. HL Mando's SbW can implement logic-based load feedback that reflects frequency response characteristics in various road conditions. HL Mando has been implementing and improving SbW performance based on feedback from customers who have participated in various test-drive events since 2016. In a test drive for a group of experts, it received a very positive evaluation of 87% of EPS performance equivalent or better.
SbW road driving real vehicle evaluation
On November 29, 2018, Tesla tweeted that its vehicle owners had driven 1 billion miles with Autopilot, and in 2020, Google's Waymo self-driving car recorded 20 million miles on the road in Fortune magazine. I have. In this way, it can be said that the mileage on the road is a measure that proves the safety level of a vehicle that can be easily accepted by ordinary consumers.
In the case of autonomous vehicles, standardization of the driving range and method is required in actual road driving and endurance driving tests in terms of safety verification and validation. There are active discussions in the process of standardization of ISO 21448 (SOTIF) standards and ISO 21448 (SOTIF) standards.
For example, Dr. Susanne Ebel, Bosch's functional safety expert, asserted that while finished vehicles and parts makers jointly conduct performance tests during the development stage, the finished vehicle bears responsibility during the quality test stage. In the end, discussions continue on who needs to perform the road driving test and how much to satisfy the functional safety and performance safety requirements, and the maturity level certification for road driving safety from parts companies is also required by automobile companies.
HL Mando is developing and operating a merit driving maturity certification process that is subdivided into 4 stages in mass production and development of the SbW system. It was the first to successfully conduct a road run.
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