System Description
VEHICLE SPEED SENSING, ELECTRONICALLY CONTROLLED, HYDRAULIC REACTION TYPEPurpose
A hydraulic reaction mechanism is installed between the pinion shaft and input shaft to vary the oil pressure according to the vehicle speed. This permits the driver to use less steering effort during low-speed driving and to have a firm, rigid response during high-speed driving.
Feature
1. Less steering effort is required even when the steering wheel is fully turned or when driving at reduced speeds. The same light steering feeling as with a conventional power steering system can be enjoyed.
2. The steering power is controlled according to the vehicle speed responding to the road conditions.
3. A built-in fail-safe function enables the system to maintain the same steering characteristic as a conventional steering system even when a problem occurs in such electrical units as ECU, vehicle speed signal, engine speed signal, solenoid valve, etc.
Operation
Hydraulic
GEARBOX ASSEMBLY
The gearbox components are: Rotary type control valve composed of sleeve and pinion shaft; torsion bar and reaction chamber; reaction piston and solenoid direct drive control valve. The control valve is made up of a direct drive solenoid valve, solenoid plunger, reaction valve and a sleeve (that forms a passage). It drives the solenoid by current from the ECU to control reaction pressure. The power cylinder is the same as that used in the engine speed sensing type power steering.
REACTION MECHANISM
Construction
The input shaft has 3 reaction arms, each with a "V" groove cut lengthwise to convert thrust from the reaction piston into turning torque. The reaction piston moves back and forth in the axial direction to provide thrust to the V groove via a ball. The pinion shaft has holes on its flange to hold steel balls. The ball corresponding with the V groove moves freely in the axial direction.
Operation
1. When driving straight ahead: The ball is pressed onto the innermost position of the V groove by means of reaction spring and preset hydraulic pressure.
2. On turns: When steering wheel is turned, a relative displacement occurs between input shaft and pinion shaft in the corresponding direction because of the tire load on the pinion shaft. This causes the ball to push the reaction piston via the reclining angle of the V groove. Since the reaction piston is under reaction hydraulic pressure, the ball receives thrust F from the reaction piston. Thrust F creates a vector (Ft = F tan 0), which is converted into a reaction torque (T = Ft x l) by the reaction arm.
OIL PUMP & RESERVOIR TANK
The construction is basically the same as the engine speed sensing type, but flow control valve design is different because of difference in the flow control characteristic.
SYSTEM OPERATION
The ECU, receiving signals input from vehicle speed and engine speed sensors, sends current (corresponding with vehicle speed) to the control valve to control reaction pressure via reaction control valve.
1. When steering wheel is fully turned or during low speed driving: As a large current flow is supplied to the solenoid, the solenoid plunger moves right while the spool valve, unitized with the plunger, is pushed right. The power steering reaction chamber and oil reservoir tank are connected by a path, making the reaction pressure O kPa (0 psi). In this condition, the reaction pressure does not work on the reaction piston, nor does the reaction piston operate. So, light, effortless steering as with conventional power steering, can be enjoyed.
2. When driving at a medium speed: From low to high or from straight ahead to turns, the reaction pressure of the power steering changes continuously.
3. When driving at a high speed: As the current being supplied to the plunger becomes the minimum value, and the solenoid plunger moves left to move the spool valve in the same direction, the pump pressure divided at the valve spot is led into the power steering reaction chamber to act on the reaction piston. As a result, a rigid steering response is felt by the driver during high-speed, straight ahead driving.