An electric power-assisted steering (EPS) ECU senses driver input and vehicle state, calculates the assistance needed, and controls a motor through a three-phase inverter. Building one safely means designing the sensors, power electronics, software, communications, diagnostics and validation as a single safety-critical system—not simply choosing an MCU and motor driver.
What an EPS ECU does
The ECU turns steering intent into controlled motor assistance. A torque sensor measures the driver’s input; steering-angle and motor-position feedback help establish system state. The controller computes an assist request, then drives a brushless DC (BLDC) motor through a three-phase power stage. Depending on the design, the motor applies torque to the steering column or force directly to the rack.
Infineon’s functional-safety documentation describes the ECU as directly controlling the BLDC motor to apply additional torque or force to the steering column or steering rack. The central hazard is unwanted steering: unintended or incorrectly directed motor assistance can affect vehicle control. Infineon says this hazard must be detected within a fault-tolerant time interval on the order of milliseconds, so fast fault detection and a defined response are core design requirements.
EPS removes the hydraulic pump and can vary assistance with vehicle speed and driving mode. Infineon’s 2021 automotive application guide reports an approximate 3 percent fuel-efficiency improvement for EPS; treat that as a reported approximate benefit, not a guaranteed result for every vehicle or operating condition.
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What hardware belongs in the ECU
A production-oriented design connects sensing, computation, actuation, power and vehicle communications, with diagnostics and supervision across those functions.
| Block | Typical role in an EPS ECU | Design considerations |
|---|---|---|
| Safety MCU | Runs motor-control and application software, reads sensors and supervises system behavior. | Choose motor-control peripherals and safety mechanisms such as lockstep or equivalent mechanisms; establish deterministic timing for critical control paths. |
| Power management and supervision | Provides regulated supplies and monitors voltage, reset and watchdog conditions. | Define behavior for brownout, reset and loss of a monitored supply. |
| Three-phase power stage | Gate or pre-driver and MOSFET inverter switch motor phases in response to PWM commands. | Size for the motor and vehicle supply, and provide over-current protection and fault detection appropriate to the architecture. |
| Sensors and current measurement | Torque, steering-angle and rotor-position sensors inform the assist calculation; current sensing supports control and diagnostics. | Check sensor plausibility and detect conditions such as bias, open or short faults. |
| Vehicle-network interfaces | CAN or CAN FD carries vehicle data, diagnostics and coordination messages; LIN may connect lower-speed peripherals when the vehicle architecture allows it. | Define supervision, timeout responses and message-integrity measures required by the system. |
| Input protection and thermal monitoring | Protects and monitors the ECU’s 12 V or 48 V supply and temperature-sensitive components. | Account for reverse polarity, load dump, thermal limits and derating behavior. |
| Diagnostics and support interfaces | Support fault reporting, nonvolatile data, calibration and service. | Specify diagnostic trouble-code meanings and control access to calibration and service functions. |
Infineon’s EPS portfolio spans safety MCUs, power management, gate drivers, MOSFETs, torque and angle sensors, and wired connectivity; it also presents a fail-operational dual-lane architecture. NXP’s EPS application material identifies automotive MCUs, integrated power supplies, CAN/LIN connectivity and MOSFET pre-drivers as control-unit elements. These portfolios illustrate the range of required blocks, but do not by themselves determine the parts or safety architecture for a particular vehicle.
Choose the system architecture before selecting parts
Parts selection follows requirements: motor and supply capability, safety goals, operating modes, network interfaces, thermal limits and desired availability. Three recurring decisions shape the design.
| Decision | Option A | Option B | What to resolve |
|---|---|---|---|
| Fault response | Fail-safe: detect a fault and move to a defined safe behavior, which may include reducing or disabling assistance. | Fail-operational: redundant lanes may preserve reduced assistance after a lane failure. | Vehicle-level safety analysis determines whether continued assistance is required and what independence the lanes need. A redundant block diagram alone does not establish safety. |
| Supply architecture | 12 V input. | 48 V input. | Choose against motor and power needs, protection requirements and thermal headroom. The available evidence does not prescribe a universal voltage choice or provide a component-level sizing rule. |
| Software integration | Bare-metal or otherwise tightly controlled deterministic implementation. | AUTOSAR Classic partitioning for application, communication and platform services. | Keep time-critical motor control and safety mechanisms on deterministic paths, regardless of the broader software architecture. |
Do not select a microcontroller or inverter from a generic EPS block diagram alone. Confirm that the MCU’s motor-control peripherals, safety mechanisms and timing fit the control concept, and that the power stage matches the motor and supply. Infineon and NXP describe relevant component categories; the exact selection remains specific to the vehicle program.
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Build the safety case around vehicle hazards
EPS is a safety-critical steering function, so ISO 26262 work must cover the system, hardware and software rather than only the ECU schematic. SAE’s EPAS paper emphasizes the importance of motor and ECU reliability because electric power-assisted steering affects vehicle stability and dynamics. The ASIL target is vehicle-specific: derive it from the hazard analysis, including exposure and controllability, rather than assuming one universal ASIL for EPS.
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- 【Standalone Control with Steering Assist】 Connects compatible with Toyota Prius/Yaris EPS steering columns to a standalone 12V power source, enabling electric power steering operation without the factory ECU or CAN bus. The adjustment knob lets you fine-tune steering assist—turn clockwise for lighter steering or counterclockwise for firmer steering. Ideal for steering conversions, custom builds, restorations, off-road vehicles, and motorsport applications.
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- Define the item. Document operating modes, interfaces, boundaries, dependencies and relevant vehicle functions.
- Perform hazard analysis and risk assessment (HARA). Analyze hazards including unintended torque, loss of assistance and erroneous assist direction, then derive safety goals.
- Allocate the technical safety concept. Assign safety requirements to sensing, computation, actuation, power and communications, including fault detection and response.
- Determine the ASIL target. Use the vehicle-specific exposure and controllability analysis; do not infer an ASIL solely from the fact that the system is EPS.
- Design diagnostics and safe behavior. Include independent diagnostics where required, plausibility checks, watchdog supervision, fault handling and a defined response to detected faults.
- Analyze hardware faults. Use hardware FMEA/FTA, FMEDA or equivalent quantitative analysis, and address latent faults and dependent failures.
- Verify and validate. Verify software timing, motor-control limits, diagnostics and communication behavior; validate the integrated steering function against its safety requirements.
JTEKT reports EPS ECU hardware work conforming to ISO 26262 with quantitative electronic-component fault analysis. SAE’s later safety-architecture paper notes that increased steering forces and ADAS functions raise the consequences of losing assistance and affect ASIL computation. This reinforces why safety targets and architecture need to be established in the context of the vehicle and its intended functions.
Design the control loop and fault response together
A typical control cycle samples torque, steering angle, rotor position and phase current; computes a current or torque command; applies limits and diagnostics; and updates inverter PWM. The implementation needs deterministic timing, including ADC sampling aligned with motor-control execution.
Plan for over-current shutdown, phase-loss detection, sensor plausibility checks and a controlled torque ramp-down. Specify what the ECU does when a measurement is missing or inconsistent, when a watchdog activates, or when voltage or temperature crosses a limit. The response must meet the system safety concept; a generic “turn the motor off” rule is not a substitute for defining safe behavior for each fault and operating mode.
For higher availability, a dual-lane design can use independent power and motor-control paths so that one lane’s failure may leave reduced assistance available. The independence, fault detection and permitted degraded behavior have to be established by the safety architecture. Redundancy adds hardware, software and validation work; it is not automatically safer merely because two lanes are present.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Partition the software without compromising timing
AUTOSAR Classic defines three high-level software layers: application, runtime environment (RTE) and basic software (BSW). BSW includes services, ECU abstraction and microcontroller abstraction. AUTOSAR describes a top-down method that starts with a vehicle system description and allocates functions to ECUs and the network communication matrix.
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For EPS, keep fast motor-control and safety mechanisms on deterministic paths. Use the RTE to exchange vehicle signals where appropriate, and BSW services for communication, diagnostics, memory, watchdog and security. The architecture should make the boundaries between safety-critical control, platform services and vehicle-level signals clear enough to analyze and verify.
Define network behavior and diagnostics explicitly
CAN or CAN FD can carry vehicle-speed data, torque-assist requests, diagnostics and coordination with ADAS or chassis controllers. LIN may serve lower-speed peripherals where the vehicle architecture permits it. NXP’s EPS application architecture specifically lists CAN and LIN connectivity alongside MOSFET pre-drivers.
For each relevant message, specify expected timing and behavior if data is absent, stale or invalid. Define message counters, alive supervision, timeout handling and CRC or end-to-end protection where required by the system. Give diagnostic trouble codes clear semantics so that detected faults can be identified and service behavior is predictable.
Bring up and validate on a representative bench
A useful EPS bench records the signals needed to connect the driver input, ECU decision and motor response. Yokogawa’s EPS application note identifies monitoring and recording sensor, motor, battery, ECU and CAN signals—including assist torque calculated by the ECU—as part of design and evaluation.
- Steering torque and angle, plus motor position.
- Three-phase current and phase or DC-link voltage.
- Battery power, ECU inputs and outputs, PWM behavior and temperatures.
- CAN traffic and the assist torque calculated by the ECU.
Exercise normal operation as well as failure and recovery cases. The test plan should include sensor bias, open and short faults; inverter faults; brownout and reset; communication loss; watchdog activation; thermal derating; and single-lane failure and recovery where the architecture is redundant. Capture both the triggering condition and the ECU’s response so that timing and behavior can be checked against the safety requirements.
Use prototypes to learn, not to claim production readiness
For early bench integration, “CAN bus development board” is a practical search phrase for finding a way to exercise and observe CAN traffic. An automotive motor-control development board and an oscilloscope or data-acquisition instrument can support control and signal measurements. These are prototyping aids, not production EPS hardware. Production use requires automotive qualification, environmental testing, cybersecurity controls and a vehicle-program safety case.
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