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
This vehicle is equipped with electrical power steering (EPS). The driver's steering force is assisted by an electric motor at the steering gearbox, instead of an engine-driven oil pump to generate oil pressure, so the EPS system improves engine efficiency.
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
- 3-mode Drive System information 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 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 on, and the power is provided to the EPS motor. When the CPU detects a failure in the system that requires it 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.
3-Mode Drive System
The EPS control unit has two power assist control maps which correspond to the 3-mode Drive System information sent from the gauge control module.
When the 3-mode Drive System is in the Normal or ECON Mode, the EPS control map is set to the normal, and when it is in the Sport Mode, the EPS control map is set to sport.
When the control map is set to normal, power assist is mapped to provide light and easy steering effort at low speeds.
When the control map is set to sport, on the other hand, the system is set up to require more steering effort at low speed and it increases the self-alignment effort when turning compared with the normal control state.
When the control modes switches, the system changes gradually (normal to sport: about 0.5 second, sport to normal: about 1.5 second) to avoid shocks caused by a sudden change in power assistance.
