EPS System Description
Overview
This vehicle is equipped with electrical power steering (EPS). The driver's steering force is assisted by an electric motor at the steering gearbox, instead of an engine-driven oil pump to generate oil pressure, so the EPS system improves engine efficiency.
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
EPS Control
The EPS control unit calculates the assistance torque based on the steering torque signal and the vehicle speed signal, and activates the motor to assist the steering force. To protect the system, the EPS control unit reduces motor current when repeated extreme steering force, such as turning the steering wheel continuously with the vehicle stopped or holding the steering at the lock position (full right or left). Also, the EPS control unit assists the steering force when the EPS control unit receives the steering assist request from the driving support system.
Steering Wheel Return Control
The steering wheel return control controls the return speed of the steering wheel based upon the steering angle and the vehicle speed. This control is responsible for the returning the steering wheel to the neutral position.
EPS Motor Control Circuit
The EPS motor control circuit is composed of a system control CPU, the FET (field-effect transistor) bridge, the FET drive circuit, the power relay, the fail-safe relays, the relay drive circuit, 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.
Steering Angle Detection
The EPS control unit calculates a steering angle from the motor angle sensor value.
The EPS control unit detects the motor rotational angle (A) with the motor angle sensor, and calculates the assist pinion shaft angle (B) by multiplying the motor rotational angle by the reduction ratio of the worm gear. The assist pinion shaft angle and the initial steering angle are converted by using the VGR (Variable Gear Ratio) conversion map, and the manual pinion shaft angle (C) is calculated. The EPS control unit also corrects the torsional angle of the torsion bar based on the torque sensor value for the manual pinion shaft angle, and calculates the steering wheel angle (D).
Steering Angle Neutral Position Learning
The EPS control unit learns the steering angle neutral position to detect an initial steering angle when turning the vehicle to the ON mode.
The steering angle neutral position learning is classified into initial learning and the regular learning.
Initial Learning
The initial learning is performed automatically at the time of the first driving after steering angle neutral position is cleared.
When the EPS control unit judges that the vehicle is going straight based on each sensor signals, the steering angle neutral position is learned.
For more details of the learning procedure, refer to Steering Angle Neutral Position Learning .
The steering angle neutral position is cleared whenever you do any of these actions:
- Replacing the EPS motor/control unit.
- Disconnecting the 12 volt battery cable from the 12 volt battery.
- Disconnecting the power from the EPS control unit.
- Decreasing the power supply voltage for the EPS control unit.
Regular Learning
The regular learning revises a gap of the neutral position that occurs due to normal wear and tear of the vehicle and a change of the wheel alignment during driving.
Learning using the HDS
The VSA sensor neutral position memorization for VSA system is performed using the HDS, the steering angle neutral position is cleared and overwritten.
Driver Attention Monitor Function
Overview
To help avoid a crash or collision due to drowsiness or fatigue, the driver attention monitor function can detect if the driver is tired or inattentive and prompt the driver to take a break.
System Description
The driver attention monitor function can detect driver drowsiness by analyzing the driving behavior based on the steering angle and the yaw rate when the vehicle speed is above a set point. If the system detects driver drowsiness or inattention, the driver attention monitor function warns the driver to take a break by showing a message on the multi-information display (MID) and/or vibrating the steering wheel.
Operation Conditions
The driver attention monitor function operates under these conditions:
- Vehicle speed is 25 - 112 mph (40 - 180 km/h).
- The EPS system, the SRS, and/or the gear system does not have a malfunction.
Function Prohibition
The driver attention monitor function is not activated under these conditions:
- Driving duration is less than 30 minutes.
- Driving on rough roads.
- Driving in a strong side wind.
- Active driving situation (changing the traffic lane or the vehicle speed).
Attention Level Reset
The driver attention monitor function resets the attention level displayed on the multi-information display (MID) if detects these conditions.
- The vehicle is turned to the OFF (LOCK) mode.
- The driver releases the seat belt and opens the driver's door with the vehicle in ON mode.
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 the 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.
Straight Driving Assist
Steering effort is detected and reduced by applying EPS motor assist when driving on a sloped road. This control will return to normal when no longer driving on a sloped road. This function only operates when the cruise control is on and the speed is set.
Assist image
Fail-Safe
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 fail-safe actions:
- Stop assist
- Limit assist
- Start alternative assist (limited control)