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The Honda Insight uses a variable-assist rack and pinion electric power steering (EPS) system rather than a typical hydraulic power steering system.
A typical hydraulic power steering system is continually placing a small load on the engine, even when no steering assist is required. Because the EPS system only needs to draw electric power when steering assist is required, no extra energy is needed when cruising, improving fuel efficiency.
Electric power steering (EPS) is mechanically simpler than a hydraulic system, meaning that it should be more reliable. The EPS system is also designed to provide good road feel and responsiveness. The Insight's EPS system shares parts with the Honda S2000 steering system.
The system's compactness and simplicity offer more design freedom in terms of placement within the chassis. The steering rack, electric drive and forged-aluminum tie rods are all mounted high on the bulkhead, and steer the wheels via steering links on each front suspension strut. This location was chosen in order to achieve a more compact engine compartment, while improving safety.
The system is also smoother operating, more responsive to driver input, and has minimal steering kickback. The overall steering ratio is 16.4 to 1, and 3.32 turns lock-to-lock.
The operating principle of the EPS is basically the same as hydraulic power steering except for the following:
The rack is unusual in that it is mounted high on the rear engine bulkhead, and that the tie rods engage the rack in the center. The high mount location is used for crash safety, as it keeps these components out of the Insight's crumple zone.
The tie rods are aluminum, and they connect to an ackerman arm that is mounted to the struts just below the spring seat.
The EPS control unit is mounted inside the car on the right side bulkhead, underneath the dash. It receives input from the vehicle speed sensor and torque sensor mounted on the steering pinion shaft.
The torque sensor is identical in construction to the unit on the S2000. The pinion shaft engages the pinion gear via a torsion bar, which twists slightly when there is a high amount of steering resistance. The amount of twist is in proportion to both the amount of resistance to wheel turning, and to the steering force applied. A pin on the torsion bar engages a diagonal slot in the sensor core, which moves up or down depending on the amount of torsion bar twist, and the direction of rotation. Two coils surrounding the core detect both the amount, and the direction of movement.
Using this information, the EPS control unit determines both the amount of steering assist required, and the direction. It then supplies current to the motor for steering assist. The amount of assist is also modified in proportion to vehicle speed to maintain good steering feel.
The torque sensor is a device to detect steering turning direction and read resistance. The sensing section of the torque sensor consists of two coils and a core (slider). The steering input shaft and pinion gear are connected via a torsion bar. The slider is engaged with the pinion gear in a way that it turns together with the pinion gear but can move vertically. A guide pin is provided on the input shaft and the pin is in a slant groove on the slider.
When road resistance is low, the steering input shaft, pinion gear and slider turn together without the slider's vertical movement.
When road resistance is high, the torsion bar twists and causes a difference of steering angle between the input shaft and pinion gear. In other words, the turning angle of the guide pin and slider differ, and the guide pin forces the slider to move upward or downward.