Overview
Steer-by-wire (SBW) decouples the relationship between a steering command input—such as from an autonomous driving system—and the wheel steering angle, enabling control of the steering mechanism independent of a mechanical linkage. SBW directly governs precise path and heading control for autonomous driving.
1. Development history
Since the first passenger car steering wheel-equipped steering system appeared in 1894, steering systems have evolved through roughly five stages:
- Early purely mechanical steering systems;
- Hydraulic power steering introduced by Ford;
- Electro-hydraulic power steering introduced by Toyota;
- Next-generation electric power steering systems;
- Steer-by-wire systems that eliminate mechanical connections and front-wheel active steering systems with active steering functions.
1.1 Electro-hydraulic power steering (EHPS)
The driver applies torque and angle at the steering wheel. The steering wheel turns the steering column, which via a gear-and-rack mechanism converts the column rotation into lateral linear motion of the rack. A torque sensor detects the driver input. Based on the driver torque and vehicle speed, the ECU controls an electric motor to drive a hydraulic pump that generates high-pressure fluid. Hydraulic fluid is delivered to the hydraulic power steering unit, where it acts on a double-acting hydraulic cylinder piston to assist the rack's lateral movement. Tie rods at both ends of the steering gear push or pull the steering knuckles to change wheel direction, generating lateral tire forces and producing vehicle turning.
1.2 Electric power steering (EPS)
There are three common EPS configurations. Column-EPS (C-EPS) assists the steering column torque. Pinion-EPS (P-EPS) assists the pinion in the column-to-rack gear set. Rack-EPS (R-EPS) assists the rack's linear motion. R-EPS can be further classified by power transmission: direct R-EPS, DP-EPS (dual-pinion EPS), and BD-EPS (belt-drive EPS).
1.3 Steer-by-wire (SBW)
Narrowly defined, SBW refers to steering systems without mechanical connections. Functionally, any system that decouples driver input from front-wheel angle can be regarded as SBW. A typical SBW architecture includes multiple possible motor installation locations and an electromagnetic clutch. The electromagnetic clutch provides mechanical redundancy and enables mechanical decoupling between the steering wheel and the wheels. Based on the presence of that clutch, SBW systems are commonly classified as SBW with a retained mechanical soft link and SBW without mechanical connection. This has led to research into dual-motor redundant SBW systems. The architecture typically includes a steering input mechanism, steering actuator(s), electronic wire-control network, power system, and supporting structures. The system allows switching between traditional mechanical steering and wire-controlled active steering via the electromagnetic clutch; mechanical steering can serve as a backup to enhance safety through fault detection.
2. Main research areas
Current SBW research focuses on three main areas: road-feel feedback control strategies, steering execution control strategies, and fault diagnosis with fault-tolerant control.
2.1 Road-feel feedback control strategies
Because SBW removes the mechanical link between the steering wheel and the road-contact wheels, road feel cannot be transmitted directly to the driver. To address this safety issue, the steering-wheel assembly typically includes a haptic motor that generates a resisting torque on the wheel to simulate road feel. Road feel is commonly defined as the steering resisting torque perceived by the driver and mainly comprises return torque and friction torque. Return torque, which tends to restore the wheels to straight-ahead, depends directly on front-wheel loading, which in turn depends on vehicle state and road adhesion. In practice, total return torque divided by the transmission ratio from wheel to steering wheel is treated as steering-wheel hand torque, i.e., road feel.
Two main design approaches are used. Empirical-design methods define road feel as a nonlinear function of steering-wheel angle, vehicle speed, yaw rate, and other parameters, offering simple and effective solutions but with limited adaptability and accuracy. Model-based methods derive road feel from vehicle dynamics principles: using vehicle dynamic response and driver inputs to compute tire forces and friction torques and then generate the desired steering torque. Once the target torque is obtained, a control algorithm—often PID—is used to command the haptic motor to produce the required torque.
2.2 Steering execution control
Steering execution control is usually divided into a higher-level strategy and a lower-level strategy. The higher-level controller computes the desired front-wheel angle from vehicle state and driver or autonomous inputs while satisfying control objectives and constraints. The lower-level controller commands the steering actuator motor to reach that angle accurately and quickly. Control algorithms divide broadly into empirical-design approaches and model-based dynamic approaches. Empirical methods tune steering responsiveness and return characteristics based on maneuvering stability requirements across different speed regimes: at low speeds, the goal is light but not excessive steering effort with good returnability; at higher speeds, the focus is on yaw-frequency response, straight-line stability, returnability, and sufficient steering feel. Model-based approaches aim to improve vehicle stability by specifying control objectives from current vehicle state and environment, computing a reference front-wheel angle, and adjusting lateral tire forces to compensate yaw moments.
2.3 Fault diagnosis and fault-tolerant control
In SBW, motors provide both the haptic feedback to the driver and the steering actuation. Motor and controller reliability are therefore critical. Real-time monitoring and hardware redundancy are common measures to ensure stable operation and enable fault-tolerant control. Research addresses fault detection and the compensatory control strategies needed when a motor or controller fails, with the aim of maximizing SBW system reliability.
3. Application and market status
Global suppliers such as Bosch, ZF, JTEKT, NSK, and Nexteer have mature SBW products and technologies, but full commercialization remains constrained by several practical and regulatory challenges.
From 2020 onward, the production deployment of Level 3 autonomous driving is expected to drive commercialization of SBW products. Foreign companies that first deploy in the Chinese market may obtain a first-mover advantage. In the Chinese market, relatively few domestic companies have made significant progress on SBW technology, and those that have are relatively small.
