Steering: Description and Operation

EPS System Description

EPS Control Unit Inputs and Outputs for Connector A (11P)










EPS Control Unit Inputs and Outputs for Connector B (16P)










System Outline

The Fit is equipped with electrical power steering (EPS). The driver's steering force is assisted by an electric motor at the steering gearbox. Compared to a hydraulic assist power steering system, EPS is more efficient because it does not need an engine driven oil pump to generate hydraulic pressure.

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

Steering Gearbox

The steering force from the steering wheel is sent to the pinion shaft. The torque sensor measures the difference between the force applied to the pinion shaft and the resistance to turning the wheels due to road friction, and converts it to a voltage signal which is sent to the EPS control unit. Based on this signal, the EPS control unit controls the current to the EPS motor. The EPS motor shaft gear rotates the worm wheel gear which is part of the pinion shaft. This becomes the assist force in the steering system.






System Operation

The EPS control unit controls the EPS motor by these signals:

- Vehicle speed signal (from ECM/PCM)
- Engine speed signal (from ECM/PCM)
- EPS motor angle sensor signal
- Torque sensor signal

At idle or low vehicle speeds, the EPS control unit sends a signal to the ECM/PCM to increase the engine idle speed to prevent the engine from stalling.

When the EPS control unit detects a failure in the system, it stores a DTC and sends a signal to the gauge control module to turn the EPS indicator on.






EPS motor operation

The EPS uses an efficient brushless DC type motor. 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. From the input sensor signals, the CPU calculates and duty cycles outputs the appropriate three-phase current for the FET drive circuit. This operation is duty controlled.






Power relay (built into the EPS control unit)

When the system is operating normally, the CPU turns the power relay on, and the power is provided to the FET bridge. When the CPU detects a failure in the system which has the demand to shut down the system, the CPU turns the power relay off.

Fail-safe relay (built into the EPS control unit)

When the system is operating normally, the CPU turns the fail-safe relay to on, and the power is provided to the EPS motor. When the CPU detects a failure in the system which has the demand to shut down the system, the CPU turns the fail-safe relay off at the same time it turns the power relay off. This relay is a fail-safe in the event the power relay is faulty and does not turn off.

Electric current detection circuit

An electric current detection circuit monitors the current of each phase circuit to the motor, and sends a signal to the CPU.