EPS System Description - EPS Control
Base Current
The base current is a basic current value to drive the motor. Base current is calculated using steering torque and vehicle speed.
Inertia Compensation
EPS motor torque tends to be lower as the vehicle begins to move, and higher as it decreases in speed due to inertia of the rotating body. Inertia compensation increases the base current in acceleration and decreases it in deceleration to reduce the impact of the inertia. The inertia compensation current is calculated using steering torque, vehicle speed, and motor speed.
Damping Compensation
The steering wheel may receive a vibration from the road surface during braking or cornering. Damping compensation reduces the vibration on the steering wheel by applying damping effect through motor current control. Damping compensation current is calculated using steering torque, vehicle speed, and motor speed.
Target Current
The target current is a value necessary to perform a feedback control of the motor. Target current is determined by applying inertia and damping compensations 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
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 using target current and motor speed.
'17Steering Wheel Vibration Warning Control
During certain operations of the various driving support systems, the EPS control unit vibrates the steering wheel by the EPS motor, which receives signals from the multi-purpose camera via F-CAN.
'17Steering 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 the base current.
EPS Motor Control Circuit
The EPS motor control circuit is composed of a system control CPU, the field-effect transistor (FET) drive circuit, the H type FET bridge, the power relay, the fail-safe relay, the electric current sensor, and the EPS motor. With the signal from the input sensor, the CPU calculates 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. Power assist force resumes gradually from steering torque of 0 Nm (0 kgf-m) 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 motor output limit control activates based on the motor and the internal temperature of the control system.