Abstract:
A hydrostatic power steering system including an electric motor is provided for a automotive vehicle. The hydrostatic power steering system utilizes a power steering piston responsive to movement of the rack element of the rack and pinion steering connection. The power steering piston is provided in a piston chamber divided by a valve land into first and second portions, each of which are connected to first and second hydraulic lines. A torque sensor is connected to the steering shaft for providing an output torque signal to a power steering controller. The electric motor of the system is responsive to a command signal generated by the power steering controller. An electrically operated valve is connected to the hydraulic lines and is adapted to receive a signal from the power steering controller for controlling flow of hydraulic fluid to the power steering chamber. The electrically operated valve arrangement is rendered inoperative within a predetermined zone defined by the command signal and the output torque signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a hydraulic and electric power steering control system for a motor vehicle and more particularly, for controlling the steering angle of the wheels to be steered of the vehicles in accordance with the state of movement of the motor vehicle to which the system is applied. 
     Various types of steering apparatuses for motor vehicles have been developed. Such examples include electric motor-driven pump-type power steering systems in which a conventional power steering system uses an electric motor as a power source to drive an oil pump so as to provide for hydrodynamic power steering. The flow rate of the oil delivered from the oil pump is controlled to allow a driver to operate a steering wheel with optimum steering force. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a new and improved steering apparatus which is capable of performing vehicle steering with high response and control accuracy while at the same time ensuring reliability of operation and precise control of the vehicle to which the power steering system is provided. 
     In accordance with the present invention, there is provided a hydrostatic power steering system including an electric motor and comprising a pinion fixedly connected to a steering shaft and meshingly connected with a rack of the steering mechanism. A hydraulic pump is provided and a power steering piston, responsive to movement of the rack, is positioned in a power steering chamber. The chamber is divided by a valve land into first and second portions, each of which are connected to first and second hydraulic lines. At least a pair of vehicle wheels are connected to the rack and piston by a linkage mechanism. A torque sensor arrangement is connected to the steering shaft for providing an output torque signal to a power steering controller and the electric motor is responsive to a command signal generated by the power steering controller. A fluid reservoir is provided for containing hydraulic fluid utilized in the hydraulic portion of the steering system. Electrically operated valves are provided in the steering system and connected to each of the first and second hydraulic lines and adapted to receive a signal from the power steering controller for controlling flow of hydraulic fluid to the power steering chamber. The electrically operated valves are inoperative within a predetermined zone defined by the command signal and the output torque signal. 
     A further object of the present invention is to provide the power steering controller with a command computing circuit and a motor control circuit. The output torque signal and a vehicle speed signal are transmitted to the command computing circuit and the motor control circuit outputs the command signal to control the electric motor. 
     Another object of the present invention is to provide a hydrostatic power steering system in which the predetermined zone of inoperation increases at increased vehicle speed when a ratio of the command signal current is divided by the output torque signal so as to define a line of minimum slope. The predetermined zone of inoperativeness will decrease at decreased vehicle speed when the ratio of the command signal current divided by the output torque signal defines a line having a maximum slope within the predetermined zone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and features of the present invention will become more readily apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is the first embodiment of the hydrostatic electric power steering system; 
     FIG. 1(A) is a second embodiment of the hydrostatic electric power steering system; 
     FIG. 2 is a block diagram of the first embodiment of the hydrostatic electric power steering system; 
     FIG. 3 is a power assist gain schedule of the first embodiment of the hydrostatic electric power steering; 
     FIG. 4 is a third embodiment of the hydrostatic electric power steering system; 
     FIG. 5 is a fourth embodiment of the hydrostatic electric power steering system; 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a configuration of the hydrostatic electric power steering system. Therein, a steering wheel 5 is connected via steering shaft 33 to a pinion gear 8 fixedly attached to the steering shaft. The pinion gear 8 is in meshing engagement with the rack 7 of the known type of rack and pinion steering. A hydraulic pump is indicated at reference numeral 2 and is of the bidirectional type so as to provide hydraulic fluid, under pressure, to through pump output conduits 2a. An electric motor 1 operates the pump 2. A reservoir 13 is provided for supplying the hydraulic fluid to the pump 2 and for receiving the working hydraulic fluid back into the reservoir from a drain system to be discussed later. The hydraulic fluid is pumped, under the action of the driving electric motor 1 and bidirectional oil pump 2 from the reservoir through an oil filter 12. Hydraulic conduit 9f provides the fluid to a pair of hydraulic lines 9c and 9d. In normal operation, the hydraulic fluid is pumped from one chamber to another. Fluid is supplied from the reservoir when the fluid level is low due to leakage or other flow problems. A check valve 10 is located in hydraulic line 9d as is a check valve 11 in hydraulic line 9c. The check valves 10 and 11 may be of any conventional structure which would permit flow in a first direction and block flow in the opposite direction. The hydraulic fluid in lines 9c and 9d is provided to first and second power steering chambers 9a and 9b, respectively. A valve land 9e separates the two chambers from one another. The hydraulic lines 9d and 9c are connected to the chambers 9a and 9b, respectively. The power steering system 9 is integral with the rack 7 and is connected, at either end, through a steering linkage 7b to the wheels 7a, to be steered. A pair of hydraulic return lines 13a and 13b are interconnected with hydraulic lines 9d  and 9c. The hydraulic return lines are normally closed by pressure relief valves 3 and 4, respectively. The pressure relief valves may be of any known type which would open under conditions of overpressure in lines 9c and 9d so as to permit the return of the hydraulic fluid from hydraulic lines 9c and 9d through the return lines 13a and 13b to the reservoir 13. Of course, the pressure relief valves would again shut-off flow of the hydraulic fluid through the hydraulic return lines upon the pressure in the system reaching equilibrium. 
     With reference to FIG. 1(A), the solenoid valve 29 of FIG. 1 has been omitted. The solenoid valve 29 functions to return the wheels of the vehicle to the straight ahead position when the driver has removed his hands from the steering wheel after a turning operation. However, valve 29 is only needed when the reduction ratio is very large or the oil pump does not allow reversible action. In the first case, the reaction force from the tires cannot move the rack since internal friction of the electric motor and pump are increased by the high reduction ratio. In the second case, the rack cannot move as the oil circuit is locked because the oil pump is not rotated by oil flow. 
     With reference to FIG. 2, a power steering controller 14 is provided for controlling the electric motor 1 which drives the hydraulic pump 2. The power steering controller comprises a command computing circuit 14a and a motor control circuit 14b. In order to provide control variables to the power steering controller 14, a vehicle speed sensor 32 is provided which transmits a signal indicative of the vehicle speed to the command computing circuit 14a. A torque sensor 6 connected with the steering shaft 33 also provides a signal which is indicative of the torque applied to the steering shaft to the command computing circuit 14a. The values received by the command computing circuit 14a are transmitted to the motor control circuit 14b. The motor control circuit is connected to the electric motor 1 so as to control operation thereof and, thereby, control operation of the bidirectional oil pump 2. 
     With reference to FIG. 3, a power assist gain schedule of the first embodiment is discussed. Therein, it can be seen that at a relatively high speed of the vehicle, the torque sensor output is greater than at low vehicle speed and the current command from the command computing circuit is greater at lower torque sensor output. Accordingly, at relatively high vehicle speed, a predetermined zone is provided in which a solenoid valve 29 (to be discussed later) is held in an open condition and the electric drive motor 1 is not actuated. The zone of inoperativeness of the solenoid valve 29 is relatively large and the slope of a line therethrough is relatively small. The slope is calculated by dividing the torque sensor output into the current command. Conversely, at relatively low vehicle speeds, the predetermined is small and the slope of a line, calculated in the foregoing manner, is larger. 
     With respect to the operation of the embodiment of FIG. 1, when the vehicle is moving in a straight line, power assist of the steering is unnecessary. In this condition, the solenoid valve 29 is open and the right chamber and left chamber 9b and 9a, respectively, are connected through the solenoid valve 29. The power steering controller provides an input signal to the solenoid valve 29 to control the opening or closing of the solenoid valve. When a driver of the vehicle turns the steering wheel 5 during movement of the vehicle and the torque sensor output is within the predetermined zone, the solenoid valve 29 remains open and the electric motor 1 is not driven. Once the torque sensor output exceeds the threshold of the predetermined zone, the solenoid valve 29 closes and the electric motor 1 is actuated. As discussed, the operation of the solenoid valve is dependent upon the output from the controller 14 based upon the torque sensor and vehicle speed sensor inputs thereto. 
     During normal power steering operation, when the driver of the vehicle turns the steering wheel 5, the torque applied to the steering shaft 33 builds up. The torque is detected by the torque sensor 6 and the torque signal is provided to the power steering controller 14. The command computing circuit 14a in the power steering controller 14 calculates the current command signal to the motor control circuit and transmits a signal to the solenoid valve 29 to open. The motor control circuit 14b provides the control signal to the electric motor. The oil pump 2, driven by the electric motor 1, will pump oil into one chamber 9a or 9b from the other chamber of the hydraulic cylinder. The oil flow will push against the power steering piston 9 and function as a hydraulic power assist in the steering of the vehicle. The control signal to the electric motor is based on a torque feedback control type in view of the torque sensor 6. 
     The oil pump 2 functions as a reduction gear on the hydrostatic electric power steering system. On the conventional electric power steering, as discussed above, the reduction ratio is usually around 10. The hydrostatic electric power steering allows a much greater reduction ratio using a large power steering piston diameter and a high speed small pump. In the conventional electric power steering system, the motor is operated at around 1000 rpm at maximum power so as to obtain the reduction ratio of 10. In contrast, the hydrostatic electric power steering motor of the present invention can be operated at about 3000 rpm or more at maximum power. This permits the hydrostatic electric power steering motor of the present invention be much smaller than the motors in the conventional power steering systems. 
     It is desirable for the steering wheel and steering system to return to the straight position when the driver removes his or her hands from the steering wheel or grips the steering wheel with a light relaxed grip. This phenomena is caused by the force applied to the rack which is generated by suspension geometry. In these situations, the torque applied to the steering shaft is within the predetermined zone and the solenoid valve is open. Thus, the force applied to the rack returns the steering to the straight-ahead position. 
     Under certain conditions, an electric power system can fail. Such conditions would be a lack of power for the electric motor, motor lock-up or pump lock-up. If such a failure of the electric power system of the present invention occurs, the driver of the vehicle can still safely operate the vehicle. If, for example, the driver wanted to steer to the right, oil would flow from the right to the left chamber, i.e., from chamber 9b to 9a, through the solenoid valve 29. Accordingly, the driver can steer the same as regular steering without power assist. Additionally, when the system experiences a pressure overload as previously discussed, the spring loaded pressure release valves open, and the steering system again functions in a non-power assist mode of operation. 
     In the second embodiment of the invention and in all subsequently discussed embodiments of the invention, like reference numerals will be utilized to identify like elements previously discussed. 
     The second embodiment of the invention is substantially similar to that discussed with regard to the preceding embodiment. A difference exists in the provision of a pair of normally open solenoid valves 29 and 30 which are arranged in the hydraulic circuit as follows. The pair of solenoid valves 29 and 30 replace the solenoid valve 29 of the previous embodiment. A difference in operation of the two embodiments is in the provision of the two solenoid valves which permit the hydraulic fluid to return to the reservoir 13 instead of to the other chamber of the cylinder in which the power steering piston 9 is located. Each of the solenoid valves 29 and 30 receive signals from the power steering controller 14 in the manner discussed with respect to the embodiment of FIG. 1. However, each of the solenoid valves 29 and 30 are located between the hydraulic lines leading to the chamber of the power assist steering arrangement and the return line to the reservoir. Accordingly, when the driver would want to steer to the left, the hydraulic fluid would flow from the right side of the chamber, i.e., side 9b through the hydraulic line 9c, the solenoid valve 30 and the return line 13b so as to drain back to the reservoir 13. 
     In the embodiment of FIG. 5, the bidirectional oil pump 2 of the first embodiments has been replaced by a unidirectional oil pump 36. In order to provide for the flow of hydraulic fluid to each side of the power steering chamber in which the power steering piston 9 is located, a three position solenoid valve 35 is provided. The solenoid valve receives its operational control from the power steering controller 14 in the manner of the previously discussed embodiments. The solenoid valve 29, for controlling the flow of the hydraulic fluid from one side of the chamber to the opposite side of the chamber remains the same as in the first embodiment and is likewise controlled by the power steering controller 14 in the same manner as in the first embodiment. 
     In the embodiment of FIG. 5, a pump outlet conduit 36a is provided which extends from the outlet of the pump to the three position solenoid valve 35. The three position solenoid valve 35 is designed so as to provide fluid flow to one side of the chamber 9a or 9b dependent upon steering conditions. As shown in the drawing, hydraulic fluid is discharged from the pump 36 through the outlet conduit 36a through the three position solenoid valve 35 and into the right hand side 9b of the power steering cylinder. The solenoid valve 29 would again function in the same manner as previously discussed so as to permit outflow of the fluid through the return line 36b leading to the reservoir 13 When moved to its lowermost position, the three position solenoid valve 35 provided fluid to the left side of the power steering piston 9 in the chamber section 9a and drains fluid from the right side 9b of the chamber back to the reservoir 13. In all other respects, operation of the embodiment of FIG. 5 is the same as the foregoing discussions with regard to FIGS. 1 and 4. 
     While the presently preferred embodiments of the present invention have been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.