MOTOR-DRIVEN POWER STEERING SYSTEM FOR A VEHICLE

Abstract: A power steering system includes a motor (13) for power assisting the steering motion induced by an operator. The motor (13) is coupled to a steering rack (6) via a speed re­ duction gear (15, 16, 18) and first and second clutches (14, 17). The operation of the motor (13) and the clutches (14, 17) are controlled by signals supplied from a control unit (109). The system includes a torque sensor (3), a vehicle speed sensor (10), a deceleration sensor (119), an acceleration sensor (120), a power sensor (121), and a battery sensor (122). These sensors supply the control unit (109) with signals thereby enabling the system to control operation of the motor (13), and hence the power steering operation, in dependence upon the acceleration or deceleration of the vehicle, the steering torque applied to the wheel (1) by an operator, the vehicle speed, the battery power and the power supplied by an alternator. A vehicle height sensor (19) may also be included to enable control of the power steering in dependence upon the vehicle height. The control of the steering in dependence upon signals from these sensors enables the system to allow improved travel stability of the vehicle, particularly when the vehicle is travelling over uneven roads or when insufficient power is available from the battery (11) or alternator.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a motor-driven power steering system for a vehicle adapted to assist the operator-induced steering operation by means of the rotative force of a motor. 2. Description of the Prior Art In the past, a conventional power steering system of this type, as schematically illustrated in Fig. 7, has hitherto been known in which a steering whell 1 is operatively connected through a steering shaft 2 and a first rack and pinion gear T₁ including a first pinion 5 and first rack tooth portion 6a with a steering rack 6 which is connected at its opposite ends with a pair of steerable road wheels (not shown) through a pair of tie rods 8a, 8b so that when the steering wheel 1 is turned by an opeartor, the steerable road wheels (not shown) are appropriately steered in accordance with the steering motion of the steering wheel 1 imparted by the operator. On the other hand, the steering rack 6 is operatively connected with a motor 13 through a second rack and pinion gear including a second rack tooth 6b and a second pinion 18 and a speed-reduction gear R so that the driving force of the motor 13 is transmitted through the speed-reduction gear R and the second rack and pinion gear T₂ to the rack 6 so as to assist the steering operation of the steering wheel 1 imparted by the operator. The motor 13 is electrically connected with a battery 11 through a control unit 9 and a key or ignition switch 12 so that it is energized by the battery 11 under the control of the control unit 9. The control unit 9 is input with control signals from a steering-torque sensor 3 and a vehicle-speed sensor 10 so as to appropriately control the operation of the motor 13 on the basis of the steering torque and the vehicle speed measured. The power steering system as constructed above has the following problems. Specifically, when the vehicle is travelling on a bad road having many irregularities at a relatively low speed, the steering wheel 1 is sometimes forced to turn by the road surface irregularities through the power steering system against the opertor's will, thus impairing steerability. French patent specification FR-A-2 242 009 describes a power steering system which is controlled in dependence upon sensed steering torque in order to assist steering clarity, for example, parking manoeuvers. British patent specification GB -A-2 161 770 describes a system which controls steering in dependence upon steering torque and speed. Direct coupling or manual steering between the steering-wheel and the wheels is affected when the speed of the vehicle exceeds a predetermined value or in dependence upon the load on the vehicle, the load being determined by information from a vehicle level sensor or an in-tyre pressure sensor. SUMMARY OF THE INVENTION In view of the above, the present invention is intended to obviate the above-described problem of the prior art, and has for its object the provision of a novel and improved motor-driven power steering system for a vehicle of the kind described which is capable of ensuring the travelling stability of a vehicle during travel on a bad road by avoiding unintentional motion of the steering wheel which would otherwise be caused by road surface irregularities with power assist. According to the present invention, there is provided a motor-driven power steering system for a vehicle, the system comprising: a motor full power assisting the steering system; talk sensor means for providing a torque signal indicative of torque applied to a steering wheel of the vehicle by an operator; a speed sensor for providing a speed signal indicative of the vehicles's speed; clutch means for controlling the transmission of power assisting force from the motor to steerable road wheels of the vehicle; and control means for receiving the torque signal and the speed signal, for controlling the operations of the motor and/or the clutch means; characterized in that a vehicle height sensor is provided for generating a height signal indicative of the height of the vehicle, and the control means comprises a road-condition judging means for receiving the height signal and for judging that the vehicle is travelling on a bad road on the basis of the height signal, the motor and/or the clutch means being controllable in dependence upon the judgement of the road-condition judging means thereby to reduce or cut off power assistance when the vehicle is judged to be travelling on a bad road. Preferably, the clutch means comprises an electromagnetic sliding clutch adapted to continuously change the force transmitted from the motor to the steerabel road wheels, and/or an electromagnetic switching clutch disposed in series with the electromagnetic sliding clutch and adapted to switch the transmission of force from the motor toward the steerable road wheels on and off. The above and other objects, features and advantages of the present invention will become apparent from the following detailed description of several presently preferred embodiments of the invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 through 6 relate to a motor-driven power steering system in accordance with one embodiment of the present invention, in which Fig. 1 is a schematic view illustrating a general arrangement of the same: Fig. 2 is a block diagram showing a control unit and its related parts; Fig. 3 is a graphic representation showing current/torque characteristics of a first electromagnetic clutch; Fig. 4 is a characteristic view showing a steering-torque/motor-voltage relationship and a steering-torque/first-clutch-current relationship; Fig. 5 is a characteristic view showing a vehicle-speed/first-clutch-current relationship and a vehicle-speed/second-clutch-voltage relationship; Fig. 6 is a flow chart showing the control processes of the power steering system of Fig. 1 as controlled by the control unit illustrated in Fig. 2; and Fig. 7 is a view similar to Fig. 1, showing a general arrangement of a conventional motor-driven power steering system. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to several presently preferred embodiments thereof as illustrated in the accompanying drawings. In the following, the same parts or elements of the embodiments will be identified by the same reference numerals and reference characters as employed in Fig. 7. Referring to the drawings and to Figs. 1 through 6, there is shown a motor-driven power steering system for a vehicle constructed in accordance with a first embodiment of the present invention. Fig. 1 shows a general arrangement of the motor-driven power steering system which includes a steering wheel 1; a steering shaft 2 fixedly connected at its upper end with the steering wheel for integral rotation therewith; a torque sensor 3 mounted on the steering shaft 2 for detecting turning force imparted to the steering wheel 1 by an operator to generate an electrical signal representative of the detected steering torque, the steering shaft 2 including an upper section 2a and a lower section 2b joined with each other through a first universal joint 4a; a first pinion 5 operatively connected at its upper end with the the lower end of the steering shaft 2 through a second universal joint 4b; a rack 6 operatively connected at its opposite ends through ball joints 7a, 7b with tie rods 8a, 8b which are in turn connected with a pair of steerable road wheels (not shown), the rack 6 having a first tooth portion 6a in mesh with the first pinion 5 and a second tooth portion 6b axially spaced from or otherwise continuous with the first tooth portion 6a; a vehicle-speed sensor 10 for detecting the travelling speed of a vehicle to generate an output signal representative of the detected vehicle speed; a battery 11 adapted to be mounted on the vehicle; a key or ignition switch 12; a second pinion 18 in meshing engagement with the second rack tooth portion 6b on the rack 6; a motor 13 operatively connected through a speed-reduction gear R with the second pinion 18 for power assisting the steering motion of the rack 6 induced by the operator, the motor being preferably in the form of a direct current motor having a shunt winding field or a magnetic field and adapted to be driven to rotate by the battery 11; a first electromagnetic clutch 14 interposed between the motor 13 and the second pinion 18 for transmitting power therebetween substantially in proportion to the intensity of current supplied thereto, the first clutch 14 being preferably in the form of an electromagnetic sliding clutch such as, for example, a powder clutch, a hysteresis clutch or the like and directly coupled, in the illustrated embodiment, with an output shaft of the motor 13; a second clutch 17 in the form of an electromagnetic switching clutch interposed between the motor 13 and the second pinion 18 for selectively establishing or disconnecting an operative connection therebetween; a vehicle-height sensor 19 for detecting the height of the vehicle body from a ground surface to generate an output signal representative of the detected vehicle height; and a control unit 9 adapted to receive output signals from the torque sensor 3, the vehicle-speed sensor 10, and the vehicle-height sensor 19 for controlling the operations of the motor 13, the first and second clutches 14 and 17. In the illustrated embodiment, the speed-reduction gear R comprises a worm 15 operatively connected with an output shaft of the first sliding clutch 14, and a worm wheel 16 in mesh with the worm 15. The second electromagnetic switching clutch 17 serves to mechanically couple or disconnect the worm wheel 16 with the second pinion 18. As diagrammaticaly illustrated in Fig. 2, the control unit 9 comprises a steering-torque measuring means 9a adapted to receive an output signal from the torque sensor 3 for measuring the operator-induced steering torque, a vehicle-speed measuring means 9b adapted to receive an output signal from the vehicle-speed sensor 10 for measuring the vehicle speed, a vehicle-height measuring means 9c adapted to receive an output signal from the vehicle-height sensor 19 for measuring the vehicle height, a road-condition judging means 9d adapted to receive the output signals of the vehicle-speed measuring means 9b the vehicle-height measuring means 9c for judging that the vehicle is travelling on a bad road if the variation frequency and the variation magnitude of the output signal of the vehicle-height measuring means 9c are greater than respective predetermined levels (a reference frequency and a reference magnitude), and if the output signal of the vehicle-speed measuring means 9b is less than a predetermined level (a reference vehicle speed), and means C adapted to receive the output signal of the road-condition judging means 9d for controlling the operations of the motor 13 and the electric clutch means 14 and/or 17 in a manner such that the force transmitted from the motor 13 to the steerable road wheels (not shown) is decreased to reduce or dampen the response of the power steering system to inputs in the steering wheel 1 resulting from travelling over bad road if the road-condition judging means 9d judges that the vehicle is travelling on a bad road. In the embodiment illustrated in Fig. 2, the means C for controlling the operations of the motor 13 and the electric clutch means comprises a motor-voltage determining means 9e for determining an appropriate level of voltage imposed on the motor 13 in a manner such that the motor voltage is proportional to the steering torque value measured by the steering-torque measuring means 9a in the normal operating condition, whereas the motor voltage is decreased or zero if the road-condition judging means 9d judges that the vehicle is travelling on a bad road, a clutch-current determining means 9f for determining an appropriate level of clutch current in a manner such that the clutch current is proportional to the steering torque value measured by the steering-torque measuring means 9a in the normal operating condition, whereas the clutch current is decreased or zero if the road-condition judging means 9d judges that the vehicle is travelling on a bad road, a motor-voltage controlling means 9g for controlling the voltage to be imposed on the motor 13 on the basis of the output of the motor-voltage determining means 9e, a clutch-current controlling means 9h for controlling the current flowing through the first electromagnetic sliding clutch 14 on the basis of the output of the clutch-current determining means 9f, and a clutch controlling means 9i for controlling the second electromagnetic switching clutch 17 in a manner such that the electromagnetic switching clutch 17 is switched on in the normal operating condition, but switched off if the road-condition judging means 9d judges that the vehicle is travelling on a bad road. Now, the operation of the power steering system of this embodiment will be described with reference to Figs. 1 through 6. First, the case in which a vehicle is stationary or standing still will be considered. In this case, when the key switch 12 is first turned on to start the engine, the electromagnetic clutch 17 is automatically actuated to place the second pinion 18 into mechanical coupling with the worm wheel 16. In this state, when the steering wheel 1 is turned by an operator, the control unit 9 acts to control the operations of the motor 13 and the sliding clutch 14 in the manner as illustrated in Fig. 4 which shows a relationship between steering torque, motor voltage and clutch current. In this regard, it is to be noted that the steering torque as shown in Fig. 4 is an input torque periodically received when controlling the motor 13, or is an average torque when controlling the sliding clutch 14. More specifically, when steering torque increases in the righthand direction to point a in Fig. 4, the motor 13 is turned on and then imposed with 100% voltage at point b . As the steering torque further increases, current begins to flow through the electromagnetic clutch 14 at point c and the intensity of the current increases logarithmically with respect to an increase in the steering torque, and reaches 100% current at point d . On the other hand, as the steering torque decreases, current flowing through the electromagnetic sliding clutch 14 begins to decrease at point d and reaches 0% current at point c . In accordance with a further decrease in the steering torque, the motor 13 is turned off at point e so that the voltage imposed on the motor 13 becomes 0% at point f . Similar to this, the motor 13 and the electromagnetic sliding clutch 14 are controlled in the same manner when the steering torque increases or decreases in the lefthand direction. As shown in Fig. 3, the sliding clutch 14 is used in a range in which transmittting torque or sliding torque increases substantially in direct proportion to clutch current. Accordingly, as is clear from Fig. 4, the motor 13 is imposed with 100% voltage and energized to start rotating when steering torque increases to point a . As the steering torque further increases, current flowing through the sliding clutch 14 begins to gradually increase at point c so that the output torque transmitted from the sliding clutch 14 to the worm 15 increases gradually. As a result, auxiliary torque having an intensity corresponding to the turning force imparted to the steering wheel by an operator is transmitted from the motor 13 to the second tooth portion 6b on the rack 6 via the first sliding clutch 14, the worm wheel 16, the second switching clutch 17 and the second pinion 18, thereby lightening the steering operation for the operator. Now, the case in which the vehicle is travelling at a speed less than a predetermined level (for example, 40Km/H) is considered. In this case, if the variation frequency of the vehicle height is greater than a predetermined level (for example, 1Hz) and if the variation magnitude of the vehicle height is greater than a predetermined level (for example, + 2 cm), the road-condition judging means 9d judges that the vehicle is travelling on a bad road, as clearly shown in the flow chart of Fig. 6. As a result, the motor-voltage controlling means 9g, operated by the motor-voltage determining means 9e, acts to control the voltage imposed on the motor 13 to decrease or become zero, or the clutch-current controlling means 9h, operated by the clutch-current determining means 9f, acts to control the current supplied to the first sliding clutch 14 to decrease or become zero, or the clutch controlling means 9i operates to switch off the second electromagnetic switching clutch 17, so that the power-assist force is decreased or made zero, thereby preventing unintentional motion of the steering wheel 1 which would otherwise be caused by the road surface irregularities. This serves to improve safety in travelling as well as stability in maneuvering even during travel on a bad road. In this connection, it is to be noted that when the road-condition judging means 9d judges that the road is bad, at least two of the motor 13, and the first and second clutches 14 and 17 may be controlled simultaneously. Further, it is possible to omit the motor-voltage determining means 9e and the motor-voltage controlling means 9g, or the clutch-current determining means 9f and the clutch-current controlling means 9h, or the clutch controlling means 9i as necessary.

Claims: 1. A motor-driven power steering system for a vehicle, the system comprising: a motor (13) full power assisting the steering system; torque sensor means (3) for providing a torque signal indicative of torque applied to a steering wheel (1) of the vehicle by an operator; a speed sensor (10) for providing a speed signal indicative of the vehicle's speed; clutch means (14, 17) for controlling the transmission of power assisting force from the motor (13) to steerable road wheels of the vehicle; and control means (9) for receiving the torque signal and the speed signal, for controlling the operations of the motor (13) and/or the clutch means (14, 17); characterized in that a vehicle height sensor (19) is provided for generating a height signal indicative of the height of the vehicle, and the control means (9) comprises a road-condition judging means (9d) for receiving the height signal and for judging that the vehicle is travelling on a bad road on the basis of the height signal, the motor and/or the clutch means being controllable in dependence upon the judgement of the road-condition judging means (9d) thereby to reduce or cut off power assistance when the vehicle is judged to be travelling on a bad road. 2. A motor-driven power steering system according to claim 1, wherein the road-condition judging means (9d) is operative for judging that the vehicle is travelling on a bad road if the vehicle speed is less than a predetermined level. 3. A motor-driven power steering system according to claim 1 or claim 2, wherein the road-condition , judging means (9d) judges that the vehicle is travelling on a bad road if the variation frequency and the height variation magnitude of the height signal are greater than respective predetermined levels. 4. A motor-driven power steering system for a vehicle as set forth in claim 1, 2 or claim 3, wherein the clutch means (14, 17) comprises an electromagnetic sliding clutch (14) adapted for continuously changing the force transmitted from the motor (13) to the steerable road wheels in accordance with the torque signal. 5. A motor-driven power steering system for a vehicle as set forth in any one of the preceding claims, wherein the clutch means (14, 17) comprises an electromagnetic switching clutch (17) adapted for switching the transmission of force from the motor (13) toward the steerable road wheels on and off. 6. A motor-driven power steering system for a vehicle as set forth in any one of the preceding claims, wherein the control means (9) for controlling the operations of the motor (13) and/or the clutch means (14, 17) comprises a motor-voltage determining means (9e) for determining the voltage to be imposed on the motor (13) in a manner such that the intensity of motor voltage is proportional to the steering torque value represented by the torque signal in the normal operating condition, whereas the motor voltage is decreased if the road-condition judging means (9d) judges that the vehicle is travelling on a bad road; and a motor-voltage controlling means (9g) for controlling the voltage imposed on the motor (13) in accordance with the output of the motor-voltage determining means (9e). 7. A motor-driven power steering system for a vehicle as set forth in claim 4 or any claim dependent thereon, wherein the control means (9) for controlling the operations of motor (13) and/or the clutch means (14, 17) comprises; a clutch-current determining means (9f) for determining the current supplied to the electromagnetic sliding clutch (14) in a manner such that the intensity of clutch current is proportional to the steering torque value represented by the torque signal in the normal operating condition, whereas the clutch current is decreased if the road-condition judging means (9d) judges that the vehicle is travelling on a bad road; and a clutch-current controlling means (9h) for controlling the current supplied to the electromagnetic sliding clutch (14) in accordance with the output from the clutch-current determining means (9f). 8. A motor-driven power steering system for a vehicle as set forth in claim 5 or any claim dependent thereon, wherein the control means (9) for controlling the operation of the motor (13) and/or the clutch means (14, 17) comprises a clutch controlling means (9i) for controlling the electromagnetic switching clutch (17) in a manner such that the electromagnetic switching clutch is switched on in the normal operating condition, but switched off if the road-condition judging means (9d) judges that the vehicle is travelling on a bad road. 9. A motor-driven power steering system for a vehicle according to claims 4 and 5 and any claim dependent thereon comprising: a steering shaft (2) fixedly connected with the steering wheel (1) for integral rotation therewith; a first pinion (5) operatively connected with the steering shaft (2) and adapted to be driven by the steering wheel (1) through the intermediary of the steering shaft; a rack operatively connected through tie rods (8a. 8b) with the pair of steerable road wheels and having a first rack tooth portion (6a) and a second rack tooth portion (6b) formed thereon, the first rack tooth portion being in meshing engagement with the first pinion; and a second pinion (18) being in meshing engagement with the second rack tooth portion on the rack; wherein the motor (13) is adapted to be energized by a battery and operatively connected through a speed-reduction gear (R) with the second pinion for power assisting the steering motion of the rack induced by the operator through the intermediary of the steering wheel, the steering shaft, the first pinion and the first rack tooth portion; the electromagnetic sliding clutch (14) is interposed between the motor (13) and the second pinion (18) for transmitting power therebetween substantially in proportion to the intensity of current supplied thereto; and the electromagnetic switching clutch (17) is interposed between the motor (13) and the second pinion (18) for selectively establishing or disconnecting the operative connection between the motor (13) and the second pinion (18).

Inventors: KOHGE, SHINICHI C/O HIMEJI SEISAKUSHO, MORISHITA, MITSUHARU C/O HIMEJI SEISAKUSHO
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1987
Main classification: B62D5
IPC codes: B62D5, B60G17