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EPS System Description
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, 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.
Components
The EPS system is rack-assist type, and the steering gearbox is a dual pinion type, which includes two pinions that are engaged with steering rack. The EPS system mainly consists of the following parts:
The EPS control unit, the EPS motor, and the torque sensor are combined into the steering gearbox. For locations of each component on the vehicle, refer to Components, 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. Steering Speed Feedback Control
The steering speed feedback control improves stability in the high speed range by controlling the hysteresis of the steering torque. The steering speed feedback control current is calculated by using the deviation between the target and actual steering angle speed to adjust to the base current. 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 20 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. Steering Angle Detection
The EPS control unit calculates the 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:
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 method with 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 (For some models)
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:
Function Prohibition
The driver attention monitor function is not activated under these conditions:
Attention Level Reset
The driver attention monitor function resets the attention level displayed on the multi-information display (MID) if detects these conditions.
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:
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