EPS (Electric Power Steering) generates the assist torque by the motor for steering operation and reduces the steering effort. Hydraulic power steering uses the power of the engine to generate the hydraulic pressure and obtain the assist torque. Since EPS uses a motor, it does not require the power of the engine and improves fuel economy.
2. Construction & Operation
(1) EPS ECU
The EPS ECU receives signals from various sensors, judges the current vehicle condition, and determines the assist current to be applied to the DC motor accordingly.
(2) Torque sensor
When the driver operates the steering wheel, the steering torque is applied to the torque sensor input shaft via the steering main shaft. Detection rings 1 and 2 are positioned on the input shaft (steering wheel side) and detection ring 3 is positioned on the output shaft (steering gear side). The input shaft and output shaft are connected via a torsion bar. Also, the detection rings have non-contact detection coils on their outer circumferences in order to form an excitation circuit. When the steering torque is generated, the torsion bar is twisted, generating a phase difference between detection rings 2 and 3. Based on this phase difference, a signal proportional to the input torque is output to the ECU. Based on this signal, the ECU calculates the motor assist torque for the vehicle speed and drives the motor.
(3) DC motor & Reduction mechanism
The DC motor consists of the rotor, stator, and motor shaft. The reduction mechanism consists of a worm gear and a wheel gear. The torque that is generated by the rotor is transmitted to the reduction mechanism. Then, this torque is transmitted to the steering shaft. The worm gear is supported by the bearings in order to reduce noise. Even if the DC motor breaks down, the rotation of the steering main shaft and the reduction mechanism is not fixed therefore the steering wheel can be steered. (4) ABS ECU Vehicle speed signal is outputted to EPS ECU. (5) Engine ECU Engine speed signal is outputted to EPS ECU. (6) Combination meter In case of a malfunction in the system, turns on the warning light. (7) Relay Supplies power to DC motor and EPS ECU.