GB2134055A

GB2134055A - Power steering system

Info

Publication number: GB2134055A
Application number: GB08333150A
Authority: GB
Inventors: Hajime Kozuka; Hiroshi Yoshida; Yoichiro Imamura
Original assignee: Mitsubishi Motors Corp
Current assignee: Mitsubishi Motors Corp
Priority date: 1982-12-20
Filing date: 1983-12-12
Publication date: 1984-08-08
Also published as: KR890000859B1; GB2134055B; FR2537936B1; DE3345896C2; KR840006943A; DE3345896A1; FR2537936A1; GB8333150D0

Abstract

A power steering system for a vehicle having a power cylinder (3) to which a hydraulic fluid delivered from a pump (1) driven by an engine is supplied through a fluid passage switching valve (2) includes a pressure control valve (11) provided in a branched fluid passage (8a) extending from a high pressure fluid passage (8) between the pump and the switching valve to a reaction piston (5) in the switching valve, and having a degree of opening controlled by a pilot pressure, and a pilot pressure control valve (12) provided in a low pressure fluid passage (8c) having one end connected by an orifice (7c) to the branched fluid passage downstream of the pressure control valve and another end leading to a fluid reservoir (4) via a further fluid passage (8d). <IMAGE>

Description

SPECIFICATION Power steering system This invention relates to power steering systems of the kind including a power cylinder to which a hydraulic fluid delivered from an oil pump driven by an engine is supplied through a fluidpassage changeover valve.

Known power steering systems have been unsatisfactory in various respects. They fail to satisfy a variety of requirements for performance.

They are unsatisfactory due to the failure to provide a light action during the travel of a vehicle at a low speed, the absence of an adequate reaction and the lack of stability for straight movement during the travel at a medium or high speed, the lack of a smooth return during the whole range of the vehicle speed, the excessive rotation of a steering handle in a case of emergency during the travel at a high speed, the insufficiency of reliability in operation, the lack of assurance for safety in a case of failure, and the lack of economy on energy consumption.

It is an object of this invention to provide an improved power steering system which overcomes the drawbacks of the known systems, and satisfies a variety of requirements for performance.

According to this invention a power steering system comprises an input shaft connected to a steering handle, a torsion bar provided for transmitting the rotation of the input shaft to an output shaft, a fluid passage changeover valve adapted to switch a fluid passage in accordance with a difference in angle of rotation between the input and output shafts, a power cylinder connected to the output shaft, a high pressure fluid passage by which a hydraulic fluid delivered from an oil pump is supplied to the power cylinder through the fluid passage changeover valve, a first low pressure fluid passage by which the hydraulic fluid is returned from the power cylinder to an oil reservoir through the fluid passage changeover valve, a reaction piston disposed between the input and output shafts for providing a force restricting the difference in angle of rotation between the input and output shafts, a branched fluid passage extending from the high pressure fluid passage to the reaction piston, a pressure control valve provided in the branched fluid passage for controlling the pressure of the hydraulic fluid so that it may not exceed a predetermined maximum level, a second low pressure fluid passage having one end connected by an orifice to the branched fluid passage downstream of the pressure control valve, while the other end thereof is connected to the oil reservoir, a pilot pressure control valve disposed in the second low pressure fluid passage for controlling a pilot pressure downstream of the orifice, and a pilot fluid passage transmitting the pilot pressure to the pressure control valve.

FIGURE 1 is a hydraulic circuit diagram showing a power steering system embodying this invention; FIGURE 2 is a longitudinal sectional view of a fluid passage changeover valve in the system of FIGURE 1; FIGURE 3 is a sectional view taken along the line Ill-Ill of FIGURE 2; FIGURE 4 is a longitudinal sectional view taken along the line IV--IV of FIGURE 5 and showing a switching valve; FIGURE 5 is a longitudinal sectional view taken along the line V-V of FIGURE 3 and showing the switching valve, a pressure control valve and a pilot pressure control valve; FIGURE 6 is a longitudinal sectional valve taken along the line VI--VI of FIGURE 5 and showing the pressure control valve; FIGURE 7 is a sectional view taken along the line VIl-VIl of FIGURE 2;; FIGURE 8 is a graph showing a relationship between the speed of a vehicle and a coefficient; and FIGURE 9 is a graph showing a relationship between the rotating speed of an engine and a coefficient.

A power steering system embodying this invention is diagrammatically shown in FIGURE 1.

It includes an oil pump 1 adapted to be driven by an engine not shown, a rotary fluid passage changeover valve 2, a power cylinder 3, an oil reservoir 4, a reaction piston 5 provided around the rotary valve 2, a chamber 6 defined behind the piston 5, a high pressure fluid passage 8 extending between the pump 1 and the valve 2, a low pressure fluid passage 9 extending between the rotary valve 2 and the reservoir 4, an orifice 7a provided in the high pressure fluid passage 8, a fluid passage 8a extending from the high pressure fluid passage 8 to the piston chamber 6, a parallel fluid passage 8b extending from the fluid passage 8a to the high pressure fluid passage 8 downstream of the orifice 7a, and a normally open switching valve 10 provided in the parallel fluid passage 8b.

A pressure control valve 11 is provided in the fluid passage 8a. A fluid passage 8c is connected by an orifice 7c to the fluid passage 8a downstream of the pressure control valve 11. A pilot pressure control valve 12 for the pressure control valve 11 is provided in the fluid passage 8c. A fluid passage 8d extends between the pilot pressure control valve 1 2 and the low pressure fluid passage 9. The fluid passage 8d is connected to the fluid passage 8c upstream of the pilot pressure control valve 12 by an orifice 7d and the fluid passage 8a downstream of the pressure control valve 11 by an orifice 7b.

The system also includes a vehicle speed sensor 1 3 having a speed meter, an engine speed sensor 14, an ignition coil 1 4a, a distributor 1 4b, and a control device 1 5. A control signal obtained by calculations is transmitted to a solenoid 1 2c in the pilot pressure control valve 12 to adjust the degree of opening of the valve 12.

The rotary fluid passage changeover valve 2 is shown by way of example in FIGURE 2. It comprises a valve housing 1 6 having a rack support 1 7 for a rack 1 8 engaged with a pinion 19, an input shaft 2a and a torsion bar 21 connecting the input shaft 2a and the pinion 1 9. A valve body 2b surrounds the input shaft 2a, and a valve sleeve 2c surrounds the valve body 2b. A hydraulic fluid leaving the oil pump 1 flows through the high pressure fluid passage 8, an annular groove 22a, a fluid passage 23a, a chamber 24, the low pressure fluid passage 9, the reservoir 4, and returns to the pump 1.If a steering handle not shown is turned to the right to rotate the input shaft 2a to the right, the high pressure fluid passage 8 is connected to the fluid passage 23b in the valve sleeve 2c through a fluid passage in the input shaft 2a, and the hydraulic fluid is supplied from the oil pump 1 to one of the chambers in the cylinder 3 through the high pressure fluid passage 8, the annular groove 22a and the fluid passage 23b to move the cylinder 3 to the right, while the hydraulic fluid in the other chamber of the cylinder 3 returns to the reservoir 4 through a fluid passage 23c, the chamber 24 and the low pressure fluid passage 9.If, on the contrary, the handle is turned to the left to rotate the input shaft 2a to the left, the high pressure fluid passage 8 is connected to the fluid passage 23c in the valve sleeve 2c through the fluid passage in the input shaft 2a, the hydraulic fluid is supplied from the oil pump 1 to the other chamber in the cylinder 3 through the high pressure fluid passage 8, the annular groove 22a and the fluid passage 23c to move the cylinder 3 to the left, while the hydraulic fluid in the one chamber of the cylinder 3 returns to the reservoir 4 through the fluid passage 23b, the chamber 24 and the low pressure fluid passage 9.

The switching valve 10 is shown by way of example in FIGURES 4 and 5. It comprises a valve body 1 Oa, a stop member 1 Ob secured to the valve housing 16, and a compression spring 10c disposed between the valve body 1 Oa and the stop member 1 Oc. The spring 1 Oc urges the valve body 1 0a to maintain the parallel fluid passage 8b normally open. The valve body 1 0a is movable over a stroke which is indicated at S.

The pressure control valve 11 is shown by way of example in FIGURES 5 and 6. It is disposed between a fluid passage 8a (Pi) (FIGURE 6), which is a passage on the Pi side in FIGURE 1, and a fluid passage 8a(Pc) (FIGURE 5), which is a passage on the Pc side in FIGURE 1, to control the degree of opening therebetween.The valve 11 comprises a valve body 11 a, a valve sleeve 11 b, a stop member 11 e secured to the valve housing 16, a compression spring 11 c disposed between the valve body 11 a and the stop member 11 e, and a compression spring 11 d disposed between the valve body 11 a and a valve sleeve 1 2b in the pilot pressure control valve 1 2. The valve sleeve 11 b is provided on its outer peripheral surface with a fluid passage 24a connected to the fluid passage 8a(Pi) as shown in FIGURES 6 and 7, a fluid passage 24b (FIGURES 5 and 7), which forms a part of a pilot fluid passage 8f which is shown in FIGURE 1, and a fluid passage 24c (FIGURES 5 and 7) connected to the fluid passage 8a(Pc).The valve body 11 a is provided on its outer periphery with a fluid passage 25a connected to the fluid passage 24a and a fluid passage 25b connected to the fluid passage 25a by the fluid passage 24c.

The fluid passage 25b is connected by the orifice 7c and the pilot fluid passage 8f (or fluid passage 24b) to that side of the valve 11 in which the compression spring 11 c is disposed, and that side of the switching valve 10 in which a counter compression spring 1 Od is disposed, as shown in FIGURES 1 and 5. The valve body 11 a is also provided with an outer peripheral fluid passage 26a and an axially extending fluid passage 26b having an upper end connected to the fluid passage 26a and a lower end opening toward the pilot pressure control valve 1 2. The fluid passage 26a is connected by the orifice 7d to the pilot fluid passage 8f (or fluid passage 24b), as shown in FIGURES 1 and 5. The fluid passage 25a on the outer periphery of the valve body 11 a is connected to the fluid passage 26b by the orifice 7b as shown in FIGURES 1 and 6.

The pilot pressure control valve 12 is shown by way of example in FIGURE 5. It comprises a valve body 12a, a valve sleeve 1 2b and a solenoid 1 2c adapted to move the valve body 12a. An inverted T-shaped fluid passage 27a is provided in the valve body 12a. A fluid passage 28a is provided in the valve sleeve 12b, and connected to the pilot fluid passage 8f (or 8c in FIGURE 1). The fluid passage 27a is connected to the low pressure fluid passage 9 (return fluid pressure PR) through the fluid passages 26b and 26a and a fluid passage 29 in FIGURE 6. FIGURE 5 also shows a nut 30.

Referring now to the operation of the power steering system as hereinabove described, the oil pump 1 driven by the engine has an output which varies in accordance with the rotating speed of the engine. The output of the pump shows a sharp increase during the start of the vehicle and its travel at a low speed and a sharp reduction during its travel at a medium speed, while it is maintained at a constant low level during the travel of the vehicle at a high speed. The oil pump is disclosed in detail in Japanese Utility Model Application No.

4939/1 982. When the output of the oil pump 1 shows a sharp increase, the fluid passage changeover valve 10 in the parallel fluid passage 8b is in its open position. Almost all the quantity of the hydraulic fluid delivered from the pump 1 is supplied to the power cylinder 3 through the rotary changeover valve 2, and substantially no hydraulic pressure develops in the branched fluid passage 8a. As no fluid pressure is fed to the reaction piston 5, no steering reaction is produced, but a light steering action is obtained. The control device 1 5 receives an output signal from the vehicle speed sensor 1 3 and an output signal from the engine speed sensor 14.It performs calculations of the following formula based principally on the output of the vehicle speed sensor, but also taking the output of the engine speed sensor into account: I = [1.0(a + b)] x 0.6 + c in which a and b make a total not exceeding 1.0, a is a coefficient shown in FIGURE 8 and representing the amount of a change in electric current, b is also a coefficient shown in FIGURE 9 and representing the amount of a change in electric current, and c represents an ineffective current, or an electric current which does not cause any displacement of the solenoid 12c. The control device 15, thus, generates a pilot pressure control signal 1A and transmits it to the solenoid 1 2c in the pilot pressure control valve 12.In accordance with an increase in the outputs of the vehicle speed sensor 13 and the engine speed sensor 14, the control device 1 5 controls the pilot pressure control valve 1 2 so as to close it, and a gradual increase appears in the pilot pressure P upstream of the valve 12. The control pressure Pc downstream of the pressure control valve 11 is fed to the fluid passage 25b, and the valve 11 cooperates with the compression spring 11 c to maintain the control pressure Pc at a level not exceeding a predetermined maximum value. If the pilot pressure P is fed to the compression spring 11 c, however, the valve 11 is opened to increase the control pressure P,, and a gradual increase in steering reaction is thereby obtained.When the vehicle travels at a high speed, the pilot pressure Pp increases to close the fluid passage switching valve 10 to increase the pressure P upstream of the pressure control valve 11. Accordingly, the pressure Pc downstream of the valve 11 increases, and acts on the reaction piston 5 to increase the steering reaction, even if the hydraulic fluid is supplied to the power cylinder 3 at substantially zero pressure when the steering handle is kept in the vicinity of its neutral position during the travel of the vehicle at a high speed.

The system as hereinabove described also has the advantage that the pressure control valve 11 and the pilot pressure control valve 12 can be assembled together accurately as shown in FIGURE 5. They can be assembled together as will hereinafter be described.

(I) The valve means 11 b of the pressure control valve 11 containing the valve body 11 a therein is inserted into the valve housing 1 6 until it rests on a shoulder 31 in the valve housing 16.

(II) The compression spring 11 d is inserted into the sleeve 11 b through the opposite end thereof.

(Ill) The valve sleeve 1 2b of the pilot pressure control valve 12 is inserted into the valve housing 1 6 through the opposite end thereof until it contacts the valve sleeve 11 b.

(IV) The solenoid 1 2c containing the valve body 1 2a is inserted until it contacts the sleeve 12b.

The two valves 11 and 12 can, thus, be accurately placed in position by insertion through the opposite ends of the valve housing 1 6 so that they may meet each other at the shoulder 31.

Although the invention has been described with reference to a preferred embodiment thereof, it is to be understood that variations or modifications may be easily made by anybody of ordinary skill in the art without departing from the scope of this invention which is defined by the appended claims. For example, it is possible to modify the control device 1 5 so that it may work in response to the output of a device for sensing the angular speed of the steering handle.

Claims

1. A power steering system comprising an input shaft connected to a steering handle, a torsion bar provided for transmitting the rotation of said input shaft to an output shaft, a fluid passage changeover valve adapted to switch fluid passages in accordance with a difference in angle of rotation between said input and output shafts, a power cylinder connected to said output shaft, a high pressure fluid passage by which a hydraulic fluid delivered from an oil pump is supplied to said power cylinder through said changeover valve, a first low pressure fluid passage by which said hydraulic fluid is returned from said power cylinder to an oil reservoir through said changeover valve, a reaction piston disposed between said input and output shafts for producing a force restricting said difference in angle of rotation between said shafts, a branched fluid passage extending from said high pressure fluid passage to said piston, a pressure control valve disposed in said branched fluid passage for maintaining a hydraulic pressure at a level not exceeding a predetermined maximum pressure, a second low pressure fluid passage having one end connected by an orifice to said branched fluid passage downstream of said pressure control valve, the other end of said second low pressure fluid passage being connected to said reservoir, a pilot pressure control valve disposed in said second low pressure fluid passage for controlling a pilot pressure downstream of said orifice, and a pilot fluid passage for transmitting said pilot pressure to said pressure control valve.

2. A system according to claim 1 , further including an orifice provided in said high pressure fluid passage, a parallel fluid passage bypassing the upstream and downstream portions of said orifice in said high pressure fluid passage, and a switching valve disposed in said parallel fluid passage operably by said pilot pressure.

3. A system according to claim 1 or 2, further including a sensor for detecting the operating condition of the vehicle in which said system is installed, and a control device which calculates a pilot pressure in accordance with the output of said sensor and transmits a pilot pressure control signal to said pilot pressure control valve.

4. A system according to claim 3, wherein a pluraiity of sensors are provided for detecting the operating condition of the vehicle simultaneously in a plurality of positions thereof, and the control device calculates a pilot pressure principally in accordance with the output of one of said sensors, but also takes the output of any other sensor into account, and transmits a pilot pressure control signal to said pilot pressure control valve.

5. A system according to claim 4, wherein said one sensor detects the speed of said vehicle, while said other sensor detects the rotating speed of an engine in said vehicle.

6. A system according to claim 4 or 5, wherein said pilot pressure control valve includes a solenoid which is displaceable to close said pilot pressure control valve, and said control device controls in accordance with the following formula an electric current I which is supplied to said solenoid to displace it so as to close said pilot pressure control valve: l=[1 -(a+b)] x0.6+c where a is a coefficient corresponding to the speed of said vehicle, and from 0 to 1.0, inclusive, b is a coefficient corresponding to the rotating speed of an engine in said vehicle, and from 0 to 0.6, inclusive, provided, however, that a and b make a total of O to 1.0, inclusive, and c represents an ineffective current which fails to cause any displacement of said solenoid.

7. A power steering system constructed, arranged and adapted to operate substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.