EPS System Description (2019 2020)
System Outline
This vehicle is equipped with electrical power steering (EPS). The driver's steering force is assisted by an electric motor at the steering gearbox. Compared to a hydraulic assist power steering system, EPS is more efficient because it does not need an engine driven oil pump to generate hydraulic pressure.
The EPS control unit monitors and controls the EPS motor's assisting force to match driving conditions.
- Low vehicle speeds: High power assist (for easy handling)
- High speed driving: Low power assist (for stable driving)
- Low speed to high speed driving: Change smoothly from high assist to low assist
Components
The EPS system mainly consists of the following parts:
- The EPS control unit: Control the EPS motor
- The EPS motor: Generate the assisting torque
- The torque sensor: Measure the steering force applied to the steering wheel
- The steering gearbox: Transmit the steering force
The EPS motor and the torque sensor are combined into the steering gearbox.
For locations of each component on the vehicle, refer to Component Location Index , and for functions of indicator, refer to How to Troubleshoot the EPS System .
EPS Control
Base Current
The base current is a basic current value to drive the motor and is calculated using the steering torque and the vehicle speed.
Inertia Compensation
The torque of the EPS motor tends to be lower as the vehicle begins to move and higher as it decreases in speed due to the inertia of the rotating body. To reduce the impact of the inertia, the inertia compensation increases the base current in acceleration and decreases in deceleration. The current of the inertia compensation is calculated by using the steering torque, vehicle speed, and motor speed.
Damping Compensation
The steering wheel receives vibration from the road surface during braking or cornering. The damping compensation reduces the vibration of the steering wheel by applying a damping effect through the motor current control. The damping compensation current is calculated by the steering torque, the vehicle speed, and the motor speed.
Target Current
The target current is a value necessary to perform feedback control of the motor and is determined by applying inertia and damping compensation to the base current and adding steering torque direction.
Current Feedback Control
Current feedback control monitors the motor current through sensors and reduces any deviation of motor current compared to the target current, thus accurately running the motor.
Unloader Control
The unloader control reduces motor current at the lock to lock (full right or left) of the steering wheel to protect the system. This value is determined by using the target current and the motor speed.
EPS Motor Control Circuit
The EPS motor control circuit is composed of a system control CPU, the FET (field-effect transistor) bridge drive circuit, the power relays, the fail-safe relays, the electric current detection circuit, and the EPS motor. With the signal from the input sensor, the CPU calculates and duty cycle outputs the appropriate three-phase current for the FET drive circuit. This operation is duty controlled.
Motor Output Limit Control
The motor output limit control reduces the motor current in the event of repeated turning of the steering wheel when the vehicle is not in motion. This control gradually reduces power assist force. The power assist force resumes gradually from the steering torque of 0 N.m (0 kgf-m, 0 lbf.ft) or from having the ignition switch in the off position and it may take up to 15 minutes to go back to normal assist force conditions. The activation of the motor limit control is based on the motor and the internal temperature of the control system.
Motion Adaptive-EPS Control
Overview
The motion adaptive-EPS control performs the steering torque correction control coordinated with VSA system for stabilizing the vehicle when the steering is unstable. The EPS system determines the state of the vehicle based on the data calculated by the VSA modulator-control unit, and the data from the yaw rate sensor and the acceleration sensor. When the system determines that the vehicle behavior is abnormal, the EPS control unit calculates the correction current based on the vehicle behavior and corrects the steering torque. The motion adaptive-EPS control is stopped in the event of failure in the VSA system.
Mitigates Oversteer Control
When oversteer occurs, it compensates for steering torque to steer in the countersteer direction, and enhances vehicle control.
Mitigates Understeer Control
When understeer occurs, it compensates for steering torque to control oversteer, then holds the tire grip to enhance vehicle control.
Stabilizes Braking on Road Surfaces With Different Friction Coefficients Control
When the vehicle behavior is abnormal while braking on an uneven road surface, it compensates for steering torque to enhance vehicle control.
Integrated Dynamics System
The EPS system has three control modes which correspond to the integrated dynamics system mode. The EPS control mode by the integrated dynamics system is shown below.
| EPS system | Integrated Dynamics System Mode | ||
|---|---|---|---|
| Comfort | Normal | Sport | |
| Control Mode | Mode 1 | Mode 2 | Mode 3 |
| Operation | Decreased steering effort | Balanced steering feel | Increased driver feedback |
Failsafe
The EPS control unit diagnoses and monitors the motor system, the torque sensor system, the power supply system, the communication system, and the CPU. When a failure is detected, the EPS indicator comes on and the unit takes one of the following failsafe actions:
- Stop assist
- Limit assist
- Start alternative assist (limited control)