MOTOR-DRIVEN POWER STEERING SYSTEM FOR A VEHICLE

Abstract: A motor-driven power steering system for a vehicle is disclosed which can be switched or transferred from power steering into manual steering or vice versa in an extremely smooth manner without causing any abrupt change in steering torque and hence unsure feelings on the part of the operator. The power steering system comprises: a torque sensor (3) for detect­ ing steering force imparted to the steering wheel by an operator; a vehicle-speed sensor (10) for detecting the vehicle speed; a motor (13) adapted to be energized by a source of electric power (11) for power assisting the steering wheel (1); an electromagne­ tic switching clutch (16) adapted to be switched off for interrupting the transmission of the power-assisting force from the motor (13) toward the steerable road wheels when the vehicle speed increases above a prescribed level, and switched on for estab­ lishing the transmission of the powersteering force when the vehicle speed decreases below the prescribed level; and a control unit (9) adapted to receive output signals from the torque sensor (3) and the vehicle-speed sensor (10) for controlling the operations of the motor and the clutch in a manner such that the power assisting force transmitted from the motor to the steerable road wheels is controlled in accordance with the travelling speed of the vehicle and the steering torque imparted to the steering wheel (1) by the operator. The current supplied from the source of electric power (11) to the motor (13) is gradually decreased to reduce the power-assisting force transmitted from the motor to the steerable road wheels as the vehicle speed increases before the clutch is switched off, and gradually increased to increase the power-assisting force as the vehicle speed decreases after the clutch (16) is switched on. The current for the motor may be controlled in dependence upon coefficients which are stored in memories (109c and 109d).

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 known to the Applicant. In the past, a conventional power steering system of this type, as schematically illustrated in Fig. 1, has hitherto been known in which a steering wheel 1 is operatively connected through a steering shaft 2 having an upper portion 2a and a lower portion 2b and a first rack and pinion gear T₁ including a first pinion 5 and a 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 operator, 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 T₂ including a second rack tooth portion 6b and a second pinion 17, a speed-reduction gear R, and a switching clutch 16 so that the driving force of the motor 13 is transmitted through the speed-reduction gear R, the switching clutch 16 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 operations of the motor 13 and the switching clutch 17 on the basis of the steering torque and the vehicle speed measured. However, the power steering system as constructed above has the following problems. Specifically, when the travelling speed of the vehicle increases above or decreases below a prescribed level, the control unit 9 operates to deenergize or energize the switching clutch 17 to interrupt or enable the transmission of the assisting force from the motor 13 to the rack 6 so as to switch the steering system into a manual steering mode without any power assist or into a power steering mode with power assist. Accordingly, whenever the vehicle speed changes across a prescribed speed level, the operating mode of the steering system abruptly changes from power steering into manual steering or vice versa, thus causing the operator to feel a certain unsmoothness, unsureness, or instability in the steering. European Patent Specification 0 174 137 describes an arrangement for disconnecting the clutch of a power steering system when the control circuits fail to operate properly. The system is arranged to vary the amplification of power assist in dependence upon applied steering torque only. European Patent Specification 0 174 203 describes a power steering system which varies the applied power assist in dependence upon the applied steering torque and measured rotational speed of the steering shaft. 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 can be switched or transferred from power steering into manual steering or vice versa in an extremely smooth manner without causing any abrupt change in steering torque and hence unsure feelings on the part of the operator, and which can be incorporated in an existing motor-driven power steering system without incurring any substantial increase in manufacturing costs. According to the present invention, there is provided a motor-driven power steering system for a vehicle, the system comprising sensor means for providing an output signal indicative of torque applied to the steering system by an operator, control means for providing a control signal in dependence upon the torque signal and the speed of the vehicle, a motor for power assisting the steering system, and clutch means for interrupting transmission of power from the motor to the steering system when the vehicle speed is above a predetermined level, characterised in that the motor and the clutch means are controlled in dependence upon the control signal such that the power-assisting force transmitted from the motor to the steering system is controlled in accordance with the vehicle speed and the torque applied to the steering system by the operator. In a preferred embodiment, the current supplied from the source of electric power to the motor is gradually decreased to reduce the power-assisting force transmitted from the motor to the steerable road wheels as the vehicle speed increases before the clutch means is switched off, and gradually increased to increase the power-assisting force as the vehicle speed decreases after the clutch means is switched on. In a preferred embodiment, the current supplied to the motor is increased or decreased in a stepwise fashion in accordance with the vehicle speed, and increased as the steering torque imparted to the steering wheel by the operator increases. The control unit comprises: a steering-torque measuring means adapted to receive the output signal from the torque sensor for measuring the operator-induced steering torque; a vehicle-speed measuring means adapted to receive the output signal from the vehicle-speed sensor for measuring the vehicle speed; a first memory means for storing motor-current multipliers which correspond to varying vehicle speeds; a second memory means for storing motor-current values which correspond to varying steering torque values; a motor-current determining means adapted to receive the output signals from the steering-torque measuring means, the vehicle-speed measuring means, and the first and second memory means and read out from the first and second memory means an appropriate motor-current multiplier corrresponding to the measured vehicle speed and an appropriate motor-current value corresponding to the measured steering torque to multiply the motor-current value by the motor-current multiplier so as to determine an appropriate motor-current value supplied to the motor; a motor-current controlling means for controlling the current supplied to the motor in accordance with the output from the motor-current determining means; and a clutch controlling means for controlling the operation of the clutch means in a manner such that the clutch means is switched off when the measured vehicle speed is above the prescribed level, and switched on when the measured vehicle speed is below the prescribed level. The motor-current determining means determines whether or not the measured vehicle speed is less than the prescribed speed level, and controls the operations of the motor and the clutch means in a manner such that the current, calculated by multiplying the read-out motor-current value by the read-out motor-current multiplier, is supplied to the motor and the clutch means is switched on, while on the other hand, if it is determined that the measured vehicle speed is equal to or greater than the prescribed level, the current supplied to the motor is made to be zero and the clutch means is switched off. The motor-current determining means further determines whether or not the current, calculated from the read-out motor current value and the read-out motor-current multiplier, is greater than a prescribed offset current level, and controls the operations of the motor and the clutch means in a manner such that if it is determined that the calculated motor current is greater than the prescribed offset current level and if the measured vehicle speed is less than the prescribed speed level, the calculated current is supplied to the motor and the clutch means is switched on, while on the other hand, if it is determined that the calculated motor current is equal to or less than the prescribed offset current level and if the measured vehicle speed is less than the prescribed speed level, the current supplied to the motor is made to be the offset current level and the clutch means is switched on. The control means further comprises a steering-interruption judging means adapted to determine whether or not steering operation is interrupted, and send out an output signal to the motor-current determining means when it is determined that steering operation is interrupted. The motor-current determining means further determines whether or not the absolute value of the measured steering torque is equal to or less than a prescribed reference value if it is determined that the measured vehicle speed is equal to or greater than the prescribed speed level, and controls the operations of the motor and the clutch means in a manner such that if it is determined that the absolute value of the measured steering torque is greater than the prescribed reference value, the current calculated from the read-out motor-current value and the read-out motor-current multiplier is supplied to the motor and the clutch means is switched on, while on the other hand, if it is determined that the absolute value of the measured steering torque is equal to or less than the prescribed reference value, the current supplied to the motor is made to be zero and the clutch means is switched off. The control unit further comprises a steering-torque change-rate calculating means adapted to calculate the rate of change per unit time of the steering torque on the basis of the presently measured steering torque value and the precedingly measured steering torque value, and sends out an output signal representative of the calculated steering-torque change rate to the motor-current determining means. The motor-current determining means further determines whether or not the calculated steering-torque change rate is equal to or less than a prescribed reference torque change rate if it is determined that the absolute value of the measured steering torque is equal to or less than the prescribed reference value, and controls the operations of the motor and the clutch means in a manner such that if it is determined that the calculated steering-torque change rate is greater than the prescribed reference torque change rate, it is then determined whether or not the clutch means is switched off and if it is so, the current supplied to the motor is made to be zero, but if not, the current calculated from the read-out motor-current value and the read-out motor-current multiplier is supplied to the motor, while on the other hand, if it is determined that the calculated steering-torque change rate is equal to or less than the prescribed reference torque change rate, the clutch means is switched off and the current supplied to the motor is made to be zero when the measured vehicle speed is equal to or greater than the prescribed speed level, but the clutch means is switched on and the current calculated from the read-out motor-current value and the read-out motor-current multiplier is supplied to the motor when the measured vehicle speed is less than the prescribed speed level. The above and other objects, features and advantages of the present invention will become apparent from the following detailed description of a few presently preferred embodiments of the invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view illustrating a general arrangement of a motor-driven power steering system for a vehicle having a conventional control unit; Figs. 2 through 6 relate to a motor-driven power steering system in accordance with one embodiment of the present invention, in which: Fig. 2 is a block diagram showing a control unit and its related parts; Fig. 3 is a characteristic view showing a relationship between the motor current and the steering torque; Fig. 4 is a characteristic view showing a vehicle-speed/motor-current relationship and a vehicle-speed/clutch-voltage relationship; Fig. 5 is a flow chart showing one control process of the power steering system of Fig. 1 as controlled by the control unit illustrated in Fig. 2; Fig. 6 is another flow chart showing another control process of the power steering system of Fig. 1 as controlled by the control unit illustrated in Fig. 2; Fig. 7 is a block diagram showing a control unit and its related parts in accordance with another embodiment of the present invention; Fig. 8 is a flow chart showing the control process of the power steering system of Fig. 1 as controlled by the control unit of Fig. 7; Fig. 9 is a block diagram showing a control unit and its related parts in accordance with a further embodiment of the present invention; and Fig. 10 is a flow chart showing the control process of the power steering system of Fig. 1 as controlled by the control unit of Fig. 9. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to a few 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 characters as employed in Fig. 1. Referring to the drawings and first to Fig. 2, there is shown a control unit 109 constructed in accordance with one embodiment of the present invention and adapted for controlling the operation of the motor-driven power steering system illustrated in Fig. 1. As diagrammatically illustrated in Fig. 2, the control unit 109 is adapted to receive the output signals from a torque sensor 3 and a vehicle-speed sensor 10 for controlling the operations of a motor 13 and a clutch means 16 in the form of an electromagnetic switching slutch in a manner such that the power-assisting force transmitted from the motor 13 to the steerable road wheels (not shown) is controlled in accordance with the travelling speed of a vehicle and the steering torque imparted to a steering wheel 1 by the operator. The control unit 109 controls the electromagnetic switching clutch 16 in a manner such that the clutch 16 is switched off when the vehicle speed increases above a prescribed level, and switched on for transmission of the power-steering force therebetween when the vehicle speed decreases below the prescribed level. The control unit 109 also controls the motor 13 in a manner such that the power-assisting force generated by the motor 13 is gradually decreased as the vehicle speed increases. To this end, the current supplied from a power source such as a battery (not shown) to the motor 13 is gradually decreased as the vehicle speed increases before the clutch 16 is switched off, and gradually increased as the vehicle speed decreases after the clutch 16 is switched on. Also, the current supplied to the motor 13 is increased as the steering torque imparted to the steering wheel 1 by the operator increases. As shown in Fig. 2, the control unit 109 comprises: a steering-torque measuring means 109a adapted to receive the output signal of the torque sensor 3 for measuring the operator-induced steering torque; a vehicle-speed measuring means 109b adapted to receive the output signal of the vehicle-speed sensor 10 for measuring the travelling speed of the vehicle; a first memory means 109c in the form of a motor-current multiplier memory means for storing motor-current multipliers which correspond to varying vehicle speeds; a second memory means 109d in the form of a motor-current memory means for storing motor-current values which correspond to varying steering torque values; a motor-current determining means 109e adapted to read out from the first and second memory means 109c and 109d an appropriate motor-current multiplier which corresponds to the measured vehicle speed and an appropriate motor-current value which corresponds to the measured steering torque, and multipy the read-out motor-current value by the read-out motor-current multipiler so as to determine an appropriate current value to be supplied to the motor 13; a motor-current controlling means 109f for controlling the current supplied to the electromagnetic switching clutch 16 in accordance with the output from the motor-current determining means 109e; and a clutch controlling means 109g for controlling the operation of the electromagnetic switching clutch 16 in a manner such that the clutch 16 is switched off when the measured vehicle speed is above a prescribed level, and switched on when the measured vehicle speed is below the prescribed level. Now, the operation of the above-described control unit 109 will be described in detail with reference to Figs. 3 through 6. First, the case in which a vehicle is stationary or standing still will be considered. In this case, when a key or ignition switch (not shown) is first turned on to start the engine, the electromagnetic switching clutch 16 is automatically actuated to place the second pinion 17 into mechanical coupling with the worm wheel 15. In this state, when the steering wheel 1 is turned by an operator, the control unit 109 acts to control the operation of the motor 13 in the form of a DC motor in the manner as illustrated in Fig. 3. More specifically, when steering torque increases in the righthand direction to point a in Fig. 3, the motor 13 is turned on and supplied with an offset current of a certain low level I OF (for example, about 2 to 10 amperes) in order to alleviate the inertia influence from the motor 13 and the mechanical portions. As the steering torque further increases, the current supplied to the motor 13 begins to increase, at point b , linearly in direct proportion to the intensity of steering torque, and reaches 100% current at point c . On the other hand, as the steering torque decreases, current flowing through the motor 13 begins to decrease at point c and becomes to be the offset level I OF at point b . When the steering torque further decreases and reaches point a , the current supply to the motor 13 is reduced to zero and the motor 13 is stopped. Similar to this, the motor 13 is controlled in the same manner when the steering torque increases or decreases is the lefthand direction. Is this connection, it is to be noted that the power-assisting torque generated by the motor 13 is in direct proportion to the current supplied thereto. Accordingly, as pictured in Fig. 3, as the steering torque increases, the motor 13 is energized or turned on at point a and supplied with the offset current I OF . As the steering torque further increases, the current supplied to the motor 13 is gradually increased at point b so that the output torque of the motor 13 to be transmitted to the worm 14 accordingly increases gradually. As a result, the assisting torque having the intensity corresponding to the operator's steering effort applied to the steering wheel 1 is transmitted to the second rack tooth portion 6b on the steering rack 6 through the intermediary of the worm wheel 15, the electromagnetic switching clutch 16 and the second pinion 17, thereby lightening the steering operation for the operator. Now, the case in which the vehicle is travelling will be considered. In this case, as seen from Fig. 4, the control unit 109 controls the operation of the motor 13 in a manner such that the current supplied to the motor 13 is controlled on the basis of the steering torque imparted to the steering wheel 1 by the operator and the travelling speed of the vehicle, that is the motor current is calculated by multiplying an appropriate motor-current value I M1 which corresponds to the measured steering torque T S by an appropriate motor-current multiplier K I which corresponds to the measured vehicle speed V. More specifically, as illustrated in the flow chart of Fig. 5, the control unit 109 is initialized after the key or ignition switch is turned on, and then, at step S1, the output of the vehicle-speed sensor 10 is input to the vehicle-speed measuring means 109b to measure the travelling speed V of the vehicle. At step S2, the steering-torque measuring means 9a measures the steering torque T S imparted to the steering wheel 1 by the operator, and the motor-valtage determining means 109e reads out from the motor-current multiplier memory means 109c an appropriate motor-current multiplier K I corresponding to the measured vehicle speed V at step S3, and also reads out from the motor-current memory means 109d an appropriate motor current value I M1 corresponding to the measured steering torque T S at step S4. Subsequently, at step S5, the motor-current determining means 109e calculates an appropriate motor current value I M to be supplied to the motor 13 by multiplying the read-out motor current value I M1 by the read-out motor-current multiplier K I . The above motor-current multiplier K I is set to be 100% until the vehicle speed increases above a certain level at point d in Fig. 4; decreased in a stepwise fashion as the vehicle speed further increases from point d ; and held constant in order to alleviate the inertia influence from the mechanical portions of the power steering system after the vehicle speed further increases from point e . If, at step S6, it is judged that the vehicle speed V is less than a predetermined level V₂, the current I M calculated in the above manner is supplied to the motor 13 under the control of the motor-current controlling means 109f at step S7, and at step 8, the clutch controlling means 109g acts to actuate or switch on the electromagnetic switching clutch 16 thereby to mechanically connect the motor 13 with the steering rack 6 via the speed-reduction gear R, the electromagnetic switching clutch 16, the pinion 17 and the second rack tooth portion 6b. Thereafter, when the vehicle speed V increases and reaches point f , as shown in Fig. 4, the current supplied to the motor 13 is made to be zero at step S9 in Fig. 5, and the electromagnetic switching clutch 16 is deenergized or disconnected at step S10 in Fig. 5 so that the mechanical connection between the worm wheel 15 and the second pinion 17 is released, thus making the steering system into a non-power assisted manual steering mode. Fig. 6 shows a modified form of control process carried out by the control unit 109 illustrated in Fig. 2. According to this modification, as shown in Fig. 6, after step 5, it is determined at step S11 whether or not the motor current I M is greater than the prescribed offset current I OF (for example, about 2 to 10 amperes) during travel of the vehicle, and if it is so, the control process proceeds from step S5 to step S6 through step S11. On the other hand, if it is determined that the motor current I M is equal to or less than the offset current I OF at step S11, the motor current I M is changed to and replaced by the offset current I OF at step S12, and then the control process proceeds to step S6. Thereafter the same process steps as those S7 through S10 in Fig. 5 are carried out. In this manner, when the vehicle speed is increasing above the prescribed level V₂, the current supplied to the motor 13 is gradually decreased to the prescribed offset level I OF to accordingly reduce the power-assisting force transmitted from the motor 13 to the steering rack 6 to a sufficiently small level before the electromagnetic switching clutch 16 is switched off or disconnected. On the other hand, when the vehicle speed V is decreasing below the prescribed level V₂, the clutch 16 is first switched on or connected with the motor deenergized, and then the motor current is gradually increased so that the power-assisting force transmitted to the steering rack 6 gradually increases. As a result, there will be no great shock generated due to disconnection or connection of the clutch 16, thus enabling smooth transfer of the steering system from a power steering mode to a manual steering mode or vice versa without causing any unsmooth, unsure or instable feeling on the part of the operator. Figs. 7 and 8 show a modified form of control unit in accordance with the present invention. This control unit 109′ is similar to the first mentioned embodiment as illustrated in Fig. 2 except for the provision of a steering-interruption judging means 109h which serves to determine whether or not the steering operation is interrupted or stopped. The steering-interruption judging means 109h generates an output signal if it is determined that the steering operation is interrupted or stopped. To this end, the steering torque T S measured by the steering-torque measuring means 109a is compared with a prescribed reference value T S1 , and if the absolute value of the measured steering torque IT S I is equal to or less than the prescribed reference value T S1 , the steering-interruption judging means 109h determines that the steering operation is interrupted. In this embodiment, the motor-current determining means 109e is adapted to receive the output signal from the steering-interruption judging means 109h as well as the output signals from the motor-current multiplier memory means 109c and the motor-current memory means 109d to control the current supplied to the motor 13 in a manner such that the motor current is calculated by multiplying an appropriate output read out from the motor-current memory means 109d by an appropriate output of the motor-current multiplier memory means 109c if the steering-interruption judging means 109h determines that the steering operation is not interrupted, but on the other hand, the motor current is made to be zero if the steering-interruption judging means 109h determines that the steering operation is interrupted. In operation of this embodiment, as shown in the flow chart of Fig. 8, the same operational processes as those in the flow chart of Fig. 6 are carried out from steps S1 through S5. After the motor current I M is calculated at step S5 by multiplying the motor current I M1 , which is read out from the motor-current memory means 109d and corresponds to the measured steering torque T S , by an appropriate multiplier K I , which is read out from the motor-current multiplier memory means 109c and corresponds to the measured vehicle speed V, it is determined at step S6 whether or not the measured vehicle speed V is less than the prescribed reference level V₂. If it is determined that the vehicle speed V is less than the prescribed reference level V₂, the motor current I M calculated at step S5 is output to the motor 13 by the motor-current controlling means 109f at step S7 and the electromagnetic switching clutch 16 is switched on or connected by the clutch controlling means 109g at step S8. On the other hand, if the steering-interruption judging means 109h determines that the vehicle speed V is equal to or greater than the prescribed reference level V₂, it is then determined at step S13 whether or not the absolute value of the measured steering torque IT S I is equal to or less than a prescribed steering-interruption reference level T S1 (about 0 to 0.2 Kg.m), and if it is so, the motor current output is made to be zero by the motor-current controlling means 109f at step S9, and at the same time the electromagnetic switching clutch 16 is switched off or disconnected by the clutch controlling means 109g at step S10. At step S13, if it is determined that the absolute value of the measured steering torque IT S I is greater than the steering-interruption reference torque T S1 , the motor current I M as calculated at step S5 is output to the motor 13 by the motor-current controlling means 109f and the electromagnetic switching clutch 16 is switched on or connected. According to this modification, even when the vehicle speed V is increasing the prescribed reference level V₂, the clutch 16 is held switched on or connected and the motor 13 is supplied with the current I M calculated at step S5 if the absolute value of the measured steering torque IT S I is greater than the steering-interruption judging torque T S1 , so that the power-assisting force is not interrupted but transmitted from the motor 13 to the steering rack 16 to maintain the power-steering operation of the steering system. In other words, where the vehicle speed V exceeds the prescribed level V₂, the power assist is interrupted only when the steering torque T S imparted to the steering wheel 1 by the operator is limited or negligible, thereby preventing the power assist from being interrupted during the steering operation. In this modification, however, if the change rate of the steering torque is great, there arises the problem that great shock may develop and be transmitted to the steering wheel 1 due to a time lag in operation upon switching off of the electromagnetic switching clutch 16 because the steering torque may become large at the time when the clutch 16 is actually switched off. Figs. 9 and 10 show a further modification of a control unit in accordance with the present invention which can solve the above-mentioned problem. This modified control unit 109" is similar to the above-mentioned modified embodiment as illustrated in Fig. 7 except for the provision of a steering-torque change-rate calculating means 109i which serves to calculate the rate of change of the steering torque K T by dividing the difference between the presently measured steering torque T S and the precedingly measured steering torque T SO by the time interval t therebetween. The change rate of the steering torque K T thus calculated is input to the steering-interruption judging means 109h wherein it is compared with a prescribed reference value K TS . The steering-interruption judging means 109h serves to determine that the steering operation is interrupted or not effected if the absolute value of the measured steering torque IT S I is equal to or less than the prescribed reference torque T S1 and if the change rate of the steering torque K T is equal to or less than the prescribed reference value K TS , and generates an output signal if it is determined that the steering operation is interrupted. In operation of this modification, as illustrated in the flow chart of Fig. 10, after initialization, it is determined whether or not a predetermined period of time t has elapsed at step S100, and if it is so, the same operational steps S101, S102 and S104 through S106 as those S1 through S5 of Fig. 8 are carried out but at step S103, the change rate of the steering torque K T is calculated by dividing the difference between the presently measured steering torque T S and the precedingly measured steering torque T SO by the time interval t therebetween. At step S107, the motor current I M , which is determined by the motor-current determining means 109 in the same manner as in the embodiment of Fig. 2 or Fig. 8, is output to the motor 13. In this modification, however, the operations of the motor 13 and the electromagnetic switching clutch 16 are further controlled in the following manner. Specifically, at step S108, it is determined whether or not the absolute value of the measured steering torque IT S I is equal to or less than the prescribed reference value T S1 (about 0 to 0.2 Kg.m), that is whether or not the steering operation is interrupted, and if it is so, it is then judged at step S109 whether or not the change rate of the steering torque K T as calculated at step S103 is equal to or less than the prescribed reference value K TS (about 0 to 2 Kg.m), that is whether or not the steering torque is changing gradually or slowly. If it is so, at step S110, it is determined whether or not the vehicle speed V as measured at step S101 is less than the prescribed reference level V₂. If it is so, there is no need to interrupt the power steering operation, and hence at step S111, the electromagnetic switching clutch 16 is switched on or connected by the clutch controlling means 109g, and the current calculated at step S106 is supplied to the motor 13. Thereafter, at step S112, the presently measured torque value T S is stored as the preceding torque value T SO by the motor-current memory means 109d for the next operation, and the control process returns from step S112 to step S100. On the other hand, at step S110, if it is determined that the measured vehicle speed V is equal to or greater than the prescribed reference level V₂, the electromagnetic switching clutch 16 is switched off or disconnected by the clutch controlling means 109g at step S115, and the motor-current output is made to be zero by the motor-current controlling means 109f at step S116. In this case, when the electromagnetic switching clutch 16 is switched off, there will be no great shock generated even if there is a time lag between the measurement of the steering torque and the switching off of the electromagnetic switching clutch 16 because both the steering torque and the change rate of the steering torque are limited so that the steering torque will not become great in a short time. Thereafter, the control process proceeds to step S112, and the above-described operational steps are repeated. Also, at step S108, if it is determined that the absolute value of the measured steering torque IT S I is greater than the prescribed reference value T S1 , or at step S109, if it is determined that the steering-torque change rate K T is greater than the prescribed reference value K TS , it is then determined whether or not the electromagnetic switching clutch 16 is switched off or disconnected, and if not, the control process returns to step S100 through step S112. In this case, the current calculated from the motor-current value I M1 and the motor-current multiplier K I at step S106 is supplied to the motor 13. At step S113, however, if it is determined that the electromagnetic switching clutch 16 is switched off or disconnected, the motor-current output is made to be zero by the motor-current controlling means 109f at step S114, and the control process proceeds to step S112.

Claims: 1. A motor-driven power steering system for a vehicle, the system comprising sensor means (3) for providing an output signal indicative of torque applied to the steering system by an operator, control means (9) for providing a control signal in dependence upon the torque signal and the speed of the vehicle, a motor (13) for power assisting the steering system, and clutch means (16) for interrupting transmission of power from the motor (13) to the steering system when the vehicle speed is above a predetermined level, characterised in that the motor (13) and the clutch means (16) are controlled in dependence upon the control signal such that the power-assisting force transmitted from the motor to the steering system is controlled in accordance with the vehicle speed and the torque applied to the steering system by the operator. 2. A motor-driven power steering system for a vehicle according to claim 1, wherein the control signal is produced in dependence upon preset coefficients which correspond to changes in the vehicle speed and/or the steering torque. 3. A motor-driven power steering system for a vehicle according to claim 2, wherein memory means (109c, 109d) is provided for storing the preset coefficients. 4. A motor-driven power steering system for a vehicle according to claim 1, the vehicle having a steering wheel (1) operatively connected with steerable road wheels so that the steering wheel is turned by an operator to appropriately steer said steerable road wheels, said motor-driven power steering system further comprising a vehicle-speed sensor (10) for detecting the vehicle speed and for generating an output signal representative of the detected vehicle speed; wherein: the motor (13) is adapted to be energized by a source (11) of electric power for power assisting the steering motion caused by the operator through said steering wheel (1); the clutch means (16) is adapted to be switched off for interrupting the transmission of the power-assisting force from said motor toward said steerable road wheels when the vehicle speed increases above said predetermined level, and switched on for establishing the transmission of the power-steering force when the vehicle speed decreases below the said predetermined level; and the control means (9) is adapted for receiving output signals from said torque sensor (3) and said vehicle-speed sensor (10) for controlling the operations of said motor (13) and said clutch means (16) in a manner such that the power-assisting force transmitted from said motor (13) to said steerable road wheels is controlled in accordance with the travelling speed of the vehicle and the steering torque imparted to the steering wheel by the operator. 5. A motor-driven power steering system for a vehicle as set forth in claim 4, wherein the current supplied from said source (11) of electric power to said motor (13) is gradually decreased to reduce the power-assisting force transmitted from said motor (13) to said steerable road wheels as the vehicle speed increases before said clutch means (16) is switched off, and gradually increased to increase the power-assisting force as the vehicle speed decreases after said clutch means (16) is switched on. 6. A motor-driven power steering system for a vehicle as set forth in claim 5, wherein the current supplied to said motor (13) is increased or decreased in a stepwise fashion. 7. A motor-driven power steering system for a vehicle as set forth in claim 6, wherein the current supplied to said motor (13) is increased as the steering torque imparted to said steering wheel (1) by the operator increases. 8. A motor-driven power steering system for a vehicle as set forth in claim 7, wherein the current supplied to said motor (13) is increased substantially in direct proportion to the steering torque. 9. A motor-driven power steering system for a vehicle as set forth in claim 4, wherein said control means (9) comprises: a steering-torque measuring means (109a) adapted for receiving the output signal from said torque sensor (3) for measuring the operator-induced steering torque; a vehicle-speed measuring means (109b) adapted for receiving the output signal from said vehicle-speed sensor (10) for measuring the vehicle speed; a first memory means (109d) for storing motor-current multipliers which correspond to varying vehicle speeds; a second memory means (109c) for storing motor-current values which correspond to varying steering torque values; a motor-current determining means (109e) adapted to receive the output signals from said steering-torque measuring means (109a), said vehicle-speed measuring means (109b), and said first and second memory means (109d, 109c) and read out from said first and second memory means an appropriate motor-current multiplier corresponding to the measured vehicle speed and an appropriate motor-current value corresponding to the measured steering torque to multiply the motor-current value by the motor-current multiplier so as to determine an appropriate motor-current value supplied to said motor; a motor-current controlling means (109f) for controlling the current supplied to said motor in accordance with the output from said motor-current determining means (109e); and a clutch controlling means (109g) for controlling the operation of said clutch means (16) in a manner such that said clutch means is switched off when the measured vehicle speed is above the prescribed level, and switched on when the measured vehicle speed is below the prescribed level. 10. A motor-driven power steering system for a vehicle as set forth in claim 9, wherein said motor-current determining means (109e) determines whether or not the measured vehicle speed is less than the prescribed speed level, and controls the operations of said motor (13) and said clutch means (16) in a manner such that the current, calculated by multiplying the read-out motor-current value by the read-out motor-current multiplier, is supplied to said motor (13) and said clutch means (16) is switched on, while on the other hand, if it is determined that the measured vehicle speed is equal to or greater than the prescribed level, the current supplied to said motor is made to be zero and said clutch means is switched off. 11. A motor-driven power steering system for a vehicle as set forth in claim 10, wherein said motor-current determining means (109e) further determines whether or not the current, calculated from the read-out motor current value and the read-out motor-current multiplier, is greater than a prescribed offset current level(I OF ) and controls the operations of said motor (13) and said clutch means (16) in a manner such that if it is determined that the calculated motor current is greater than the prescribed offset current level (I OF ) and if the measured vehicle speed is less than the prescribed speed level, the calculated current is supplied to said motor (13) and said clutch means (16) is switched on, while on the other hand, if it is determined that the calculated motor current is equal to or less than the prescribed offset current level (I OF ) if the measured vehicle speed is less than the prescribed speed level, the current supplied to said motor (13) is made to be the offset current level and said clutch means (16) is switched on. 12. A motor-driven power steering system for a vehicle as set forth in claim 10, wherein said control means (9) further comprises a steering-interruption judging means (109h) adapted for determining whether or not steering operation is interrupted, and send out an output signal to said motor-current determining means (109e) when it is determined that steering operation is interrupted. 13. A motor-driven power steering system for a vehicle as set forth in claim 12, wherein said motor-current determining means (109e) further determines whether or not the absolute value of the measured steering torque is equal to or less than a prescribed reference value if it is determined that the measured vehicle speed is equal to or greater than the prescribed speed level, and controls the operations of said motor (13) and said clutch means (16) in a manner such that if it is determined that the absolute value of the measured steering torque is greater than the prescribed reference value, the current calculated from the read-out motor-current value and the read-out motor-current multiplier is supplied to said motor (13) and said clutch means (16) is switched on, while on the other hand, if it is determined that the absolute value of the measured steering torque is equal to or less than the prescribed reference value, the current supplied to said motor is made to be zero and said clutch means (16) is switched off. 14. A motor-driven power steering system for a vehicle as set forth in claim 13, wherein said control means (9) further comprises a steering-torque change-rate calculating means (109i) adapted for calculating the rate of change per unit time of the steering torque on the basis of the presently measured steering torque value and the precedingly measured steering torque value, and sending out an output signal representative of the calculated steering-torque change rate to said motor-current determining means (109e). 15. A motor-driven power steering system for a vehicle as set forth in claim 14, wherein said motor-current determining means (109e) further determines whether or not the calculated steering-torque change rate is equal to or less than a prescribed reference torque change rate if it is determined that the absolute value of the measured steering torque is equal to or less than the prescribed reference value, and controls the operations of said motor (13) and said clutch means (16) in a manner such that if it is determined that the calculated steering-torque change rate is greater than the prescribed reference torque change rate, it is then determined whether or not said clutch means (16) is switched off and if it is so, the current supplied to said motor (13) is made to be zero, but if not, the current calculated from the read-out motor-current value and the read-out motor-current multiplier is supplied to said motor (13), while on the other hand, if it is determined that the calculated steering-torque change rate is equal to or less than the prescribed reference torque change rate, said clutch means (16) is switched off and the current supplied to said motor (13) is made to be zero when the measured vehicle speed is equal to or greater than the prescribed speed level, but said clutch means (16) is switched on and the current calculated from the read-out motor-current value and the read-out motor-current multiplier is supplied to said motor (13) when the measured vehicle speed is less than the prescribed speed level.

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