Steering: Description and Operation

EPS System Description

EPS Control Unit Inputs and Outputs for Connector A (2P) (Connector Disconnected)











EPS Control Unit Inputs and Outputs for Connector B (2P) (Connector Disconnected)











EPS Control Unit Inputs and Outputs for Connector C (2P) (Connector Disconnected)











EPS Control Unit Inputs and Outputs for Connector D (28P) (Connector Disconnected)











System Outline

The Acura TL is equipped with electric 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, the EPS is more efficient because it does not use engine power to drive an oil pump to generate hydraulic pressure.

The system uses an EPS control unit to monitor and control 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

System Operation

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. The torque sensor converts steering torque input and its direction into voltage signals that are sent to the EPS control unit. The EPS control unit uses these signals along with the engine and the vehicle speed signals from the ECM/PCM to calculate the direction and the amount of assist needed. The EPS control unit then sends out the appropriate command to the EPS motor. The EPS motor transmits its power through a ball screw mechanism to push the steering rack right or left.






Torque Sensor Operation

When the steering wheel is turned, inductance is changed by the movement of the core within the Torque Sensor. The amount this voltage changes varies with the amount of movement, and direction of the core, then is amplified by the interface circuitry of the sensor coil and output to the EPS control unit as a steering signal.






EPS Control Unit Operation

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

- Vehicle speed signal (for ECM/PCM)
- Engine speed signal (for ECM/PCM)
- 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.

If the EPS control unit detects a failure in the system, it stores a DTC, and sends a signal to the gauge control.






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. If 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 on and the power is provided to the EPS motor. If the CPU detects a failure in the system, the CPU turns the fail-safe relay off at the same time it turns the power relay off.

Current Sensor

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

EPS Motor Operation

The EPS uses an efficient brushless DC type motor. The EPS motor control circuit is composed of a system control CPU, FET drive circuit, H type FET bridge, power relay, fail-safe relay, electric current sensor, and the EPS motor. From the input sensor signals, the CPU calculates and outputs the appropriate three-phase current for the FET drive circuit.