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

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 the steering rack.

The EPS system mainly consists of the following parts:

• The EPS control unit: Controls the EPS motor.
• The EPS motor: Generates the assisting torque.
• The torque sensor: Measures the steering force applied to the steering wheel.
• The steering gearbox: Transmits the steering force.
• The system indicator: Indicates system status.

For locations of each component on the vehicle, refer to Components , and for indicator functions, refer to How to Troubleshoot the EPS .

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.

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.

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 the main CPU, the sub CPU, the field-effect transistor (FET) drive circuit, the FET, the relay drive circuits, the power relay, 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.

Courtesy of HONDA, U.S.A., INC.

Straight Driving Steering 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 

Courtesy of HONDA, U.S.A., INC.

Motion Adaptive-EPS Control 

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.

Courtesy of HONDA, U.S.A., INC.

Mitigates Understeer Control 

When understeer occurs, it compensates for steering torque to control the oversteer, then holds the tire grip to enhance vehicle control.

Courtesy of HONDA, U.S.A., INC.

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.

Courtesy of HONDA, U.S.A., INC.

Steering Wheel Vibration Control 

When the EPS control unit receives the steering wheel vibration request from the multipurpose camera unit via F-CAN, EPS control unit drives the motor and generates the vibration on steering wheel.

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 (vehicle speed fixed control)