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EPS System Description - 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. 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
EPS Motor Control Circuit
The EPS motor control circuit is composed of a system control CPU, the FET (Field-Effect Transistor) drive circuit, the FET bridge, the power relay, the fail-safe relays, the electric current detection circuit, relay drive circuit, and the EPS motor. With the signal from the input sensor, the CPU calculates the appropriate 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. |