EPS System Description - EPS Control
The base current is a basic current value to drive the motor. Base current is calculated using steering torque and vehicle speed.
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.
The steering wheel may receive a jiggling vibration from the road surface during braking or cornering. Damping compensation reduces the jiggling 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.
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.
To accurately run the motor, the current feedback control monitors the current through sensors and compares it to the target current. If there is a any deviation between the two current values, the current feedback control reduces the deviation.
To protect the system when the wheels are turned all the way to lock (full left or right), the unloader control reduces the motor current. This value is determined using target current and motor speed.
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 relay, the fail-safe relay, the electric current detection circuit, and the EPS motor. Using the signal from the input sensor, the CPU calculates and outputs the appropriate duty cycle of three-phase current for the FET drive circuit. This operation is duty controlled.
Motor Output Limit Control
In the event that the internal temperature of the system becomes too high, like the steering wheel is repeatedly turned when the vehicle is not moving, the motor output limit control activates to reduce the motor current. This control gradually reduces power assist force. Power assist force resumes gradually from steering torque of 0 N.m (0 kgf.m) or from having the ignition switch in the off position and it may take up to 10 minutes to go back to normal assist force conditions.