Steering system

Abstract

A steering system installed on a vehicle, including: a steering mechanism including a steering member and configured to enable a wheel to be steered in accordance with a movement of the steering member; an operation assist device configured to apply a torque to a steering shaft for assisting an operation of a steering operation member; a steering assist device configured to assist the movement of the steering member utilizing a working fluid; and a controller configured to control a torque applied to the steering shaft by the operation assist device and control a supply flow rate of the working fluid, wherein the steering assist device assists the movement of the steering member with an assist force that depends on the supply flow rate, and the controller increases the supply flow rate when an operation speed of the steering operation member becomes equal to or higher than a set threshold speed.

Claims

1. A steering system installed on a vehicle, comprising: a steering operation member operated by a driver; a steering shaft coupled to the steering operation member and configured to rotate in accordance with an operation of the steering operation member; a steering mechanism including a steering member coupled to a wheel and configured to cause the steering member to move in accordance with rotation of the steering shaft so as to enable the wheel to be steered in accordance with a movement of the steering member; an operation assist device configured to apply a torque to the steering shaft so as to assist the operation of the steering operation member; a steering assist device including a working-fluid supply source for supplying a working fluid and a flow-rate adjusting mechanism configured to adjust a supply flow rate that is a flow rate of the working fluid supplied from the working-fluid supply source, the steering assist device being configured to assist the movement of the steering member of the steering mechanism utilizing the working fluid supplied from the working-fluid supply source; and a controller configured to control the torque applied to the steering shaft by the operation assist device and configured to control the flow-rate adjusting mechanism of the steering assist device so as to control the supply flow rate of the working fluid, wherein the steering assist device is configured to assist the movement of the steering member with an assist force that depends on the supply flow rate of the working fluid, and wherein the controller is configured to increase the supply flow rate of the working fluid such that the working fluid increases from moving at a first flow rate to moving at a second flow rate when an operation speed of the steering operation member becomes equal to or higher than a set threshold speed.

2. The steering system according to claim 1, wherein the steering mechanism includes an input shaft to which the steering shaft is coupled and which is rotated by the steering shaft, and wherein the steering assist device is configured to assist the movement of the steering member with the assist force corresponding to a steering torque that is a torque applied to the input shaft by the steering shaft.

3. The steering system according to claim 1, wherein the steering assist device includes, as the working-fluid supply source, an engine pump configured to be rotated by an engine of the vehicle.

4. The steering system according to claim 1, wherein the operation assist device includes an electric motor as a source of generation of the torque applied to the steering shaft.

5. The steering system according to claim 1, further comprising an operation torque sensor for detecting an operation torque that is a torque applied to the steering shaft by the operation of the steering operation member, wherein the controller is configured to control, in accordance with the detected operation torque, the torque applied to the steering shaft by the operation assist device.

6. The steering system according to claim 1, wherein the controller is configured to control, based on the operation speed of the steering operation member, the torque applied to the steering shaft by the operation assist device.

7. The steering system according to claim 1, wherein the controller is configured to control, based on an operation amount of the steering operation member, the torque applied to the steering shaft by the operation assist device.

8. The steering system according to claim 1, wherein the controller is configured to control, based on a running speed of the vehicle, the torque applied to the steering shaft by the operation assist device.

9. The steering system according to claim 1, wherein the first flow rate is a flow rate greater than zero.

10. The steering system according to claim 1, wherein before the operation speed of the steering operation member becomes equal to or higher than the set threshold speed, the working fluid is circulated between the working-fluid supply source and a working-fluid reservoir.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The objects, features, advantages, and technical and industrial significance of the present disclosure will be better understood by reading the following detailed description of an embodiment, when considered in connection with the accompanying drawings, in which:

(2) FIG. 1 is a view schematically illustrating an overall structure of a steering system according to one embodiment;

(3) FIG. 2A is a graph indicating a gain for determining an operation-torque-dependent component as one of three components of an assist torque applied by an operation assist device of the steering system according to the embodiment;

(4) FIG. 2B is a graph indicating a gain for determining an operation-speed-dependent component as another one of the three components of the assist torque applied by the operation assist device of the steering system according to the embodiment;

(5) FIG. 2C is a graph indicating a gain for determining an operation-angle-dependent component as still another one of the three components of the assist torque applied by the operation assist device of the steering system according to the embodiment; and

(6) FIG. 3 is a flowchart of a control program executed in the steering system of the embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENT

(7) Referring to the drawings, there will be explained in detail a steering system according to one embodiment of the present disclosure. It is to be understood that the present disclosure is not limited to the details of the following embodiment but may be embodied based on the forms described in Various Forms and may be changed and modified based on the knowledge of those skilled in the art.

(8) A. Hardware Configuration of Steering System

(9) As illustrated in FIG. 1, a vehicle 10 on which is installed a steering system according to one embodiment includes two front wheels 12 each as a steerable wheel and as a drive wheel. The vehicle 10 includes an engine 14 as a drive source. Rotation of the engine 14 is transmitted to each of the front wheels 12 held by respective steering knuckles 22a via a torque converter/transmission 16, a differential 18, and respective drive shafts 20R, 20L. The vehicle 10 runs by the rotation of the front wheels 12.

(10) The steering system according to the present embodiment includes (a) a steering wheel 30, as a steering operation member, operated by a driver, (b) a steering shaft 34 which is held by a steering column 32 and one end of which is coupled to the steering wheel 30 so as to rotate in accordance with an operation of the steering wheel 30, and (c) a steering mechanism 38 including a steering rod 36, as a steering member, opposite ends of which are respectively coupled to the front wheels 12 and which extends in a width direction of the vehicle, the steering mechanism 38 being configured to move the steering rod 36 in accordance with rotation of the steering shaft 34 so as to enable the front wheels 12 to be steered in accordance with a movement of the steering rod 36.

(11) The steering mechanism 38 includes an input shaft 42 to which the other end of the steering shaft 34 is coupled and which is held by a gear box 40. A pinion is formed on a portion of the input shaft 42, which portion is located within the gear box 40. The steering rod 36 is also held by the gear box 40, and a rack 44 meshing with the pinion is formed on the steering rod 36. That is, the steering mechanism 38 includes a rack and pinion motion converting mechanism. The rotation of the steering shaft 34 causes the input shaft 42 to rotate, so that the steering rod 36 moves in the vehicle width direction. The steering rod 36 is coupled at its opposite ends respectively to knuckle arms 48 of the respective steering knuckles 22 that respectively hold the front right and left wheels 12, via respective link rods 46. The front wheels 12 are steered by the movement of the steering rod 36.

(12) The one end of the steering shaft 34 is coupled to the steering wheel 30 via a first torsion bar 60. The steering column 32 is provided with an operation torque sensor 62 for detecting a twisted amount of the first torsion bar 60 and detecting a torque applied to the steering shaft 34 by the operation of the steering wheel 30. This torque will be hereinafter referred to as “operation torque” where appropriate. In this respect, the operation torque can be regarded as a torque that the driver applies to the steering wheel 30 by the steering operation. The other end of the steering shaft 34 is coupled to the input shaft 42 of the steering mechanism 38 via a second torsion bar 64. A twisted amount of the second torsion bar 64 corresponds to a torque applied to the input shaft 42 by the steering shaft 34. This torque will be hereinafter referred to as “steering torque” where appropriate.

(13) The steering system includes a steering assist device 70 configured to assist the movement of the steering rod 36 of the steering mechanism 38 utilizing a working fluid (working oil). The steering assist device 70 includes a hydraulic actuator 78 including a piston 72 fixed to the steering rod 36 and a housing 76 whose interior is partitioned into two fluid chambers 74R, 74L by the piston 72. The steering assist device 70 further includes an engine pump 80 as a working-fluid supply source driven by the engine 14. The engine pump 80 pumps up the working fluid out of a reservoir 82 storing the working fluid via a pump-up passage 84 and supplies the pumped working fluid to a supply-flow controlling mechanism 88 via a supply passage 86.

(14) The supply-flow controlling mechanism 88 has a known ordinary structure as illustrated in FIG. 2 of Japanese Patent Application Publication No. 6-8840, for instance. Specifically, the supply-flow controlling mechanism 88 has a function of controlling the flow rate of the working fluid to be supplied to the actuator 78 and a function of determining to which one of the two fluid chambers 74R, 74L the working fluid is to be supplied, based on the twisted amount of the second torsion bar 64 and a direction of the twisting, that is, based on the steering torque and a direction of the steering. The supply-flow controlling mechanism 88 is configured to receive the working fluid supplied from the engine pump 80. When the steering torque is not acting on the input shaft 42, the supply-flow controlling mechanism 88 permits the received working fluid to return to the reservoir 82 via a return passage 90. That is, the working fluid is circulated. When the steering torque is acting on the input shaft 42, on the other hand, the supply-flow controlling mechanism 88 supplies at least a part of the received working fluid to one of the two fluid chambers 74R, 74L of the actuator 78 that corresponds to the direction of the steering, at a flow rate corresponding to the steering torque and receives, from the other of the two fluid chambers 74R, 74L, the working fluid at the same flow rate as the supplied working fluid. The supply-flow controlling mechanism 88 permits the working fluid received from the other of the two fluid chambers 74R, 74L also to return to the reservoir 82. In the actuator 78, a force corresponding to the pressure of the working fluid supplied from the supply-flow controlling mechanism 88 to the one of the two fluid chambers 74R, 74L acts on the piston 72, so that the force (hereinafter referred to as “assist force” where appropriate) assists a steering force that is a force by which the steering rod 36 steers the front wheels 12. In other words, the steering assist device 70 is configured to assist the movement of the steering rod 36 with the assist force corresponding to the steering torque.

(15) The engine pump 80 is configured to eject the working fluid at a flow rate corresponding to a rotational speed of the engine 14. The steering assist device 70 is provided with a flow-rate restricting mechanism 92 disposed on an ejection side of the engine pump 80 for restricting the flow rate of the working fluid ejected from the engine pump 80. The flow-rate restricting mechanism 92 has a known ordinary structure including a valve, as described in Japanese Patent No. 3218788, Japanese Patent Application Publication No. 8-301132, or Japanese Patent Application Publication No. 6-8840, for instance. Specifically, the flow-rate restricting mechanism 92 has a function of restricting, to a set flow rate, the flow rate of the working fluid to be supplied to the supply-flow controlling mechanism 88 after passing therethrough, in the case where the rotational speed of the engine 14 becomes high to a certain extent. Further, the steering assist device 70 is provided with a flow-rate controlling mechanism 94 disposed on an ejection side of the flow-rate restricting mechanism 92 so as to be in series therewith and configured to control the flow rate of the working fluid sent from the flow-rate restricting mechanism 92 to the supply-flow controlling mechanism 88. The flow-rate controlling mechanism 94 is a known ordinary electromagnetic valve mechanism as illustrated in FIG. 2 of Japanese Patent Application Publication No. 2014-19290, for instance. The flow-rate controlling mechanism 94 has a function of allowing passage of the working fluid at a flow rate corresponding to an electric current supplied to a solenoid. The steering assist device 70 includes a mechanism constituted by the flow-rate restricting mechanism 92 and the flow-rate controlling mechanism 94, namely, a flow-rate adjusting mechanism 96 for adjusting the flow rate of the working fluid supplied from the engine pump 80 to the supply-flow controlling mechanism 88. This flow rate will be hereinafter referred to as “supply flow rate” where appropriate.

(16) As understood from the explanation above, the steering assist device 70 is configured to assist the movement of the steering rod 36 with the assist force that depends on the supply flow rate. In general, the assist force increases with an increase in the supply flow rate and decreases with a decrease in the supply flow rate.

(17) The steering system further includes an operation assist device 100 configured to apply a torque to the steering shaft 34 so as to assist the operation of the steering wheel 30. The operation assist device 100 includes an electric motor 102 as a source of generation of the torque applied to the steering shaft 34. This torque will be hereinafter referred to as “assist torque” where appropriate. The torque generated by the electric motor 102 is applied, as the assist torque, to the steering shaft 34 via a speed reducer 104. The electric motor 102 is a three-phase brushless DC motor. The magnitude of the assist torque corresponds to an amount of an electric current supplied to the electric motor 102. This electric current will be hereinafter referred to as “supply current” where appropriate. The assist torque increases with an increase in the supply current and decreases with a decrease in the supply current.

(18) The control of the steering assist device 70 and the operation assist device 100 in the present steering system, specifically, the control of the flow-rate controlling mechanism 94 of the flow-rate adjusting mechanism 96 and the electric motor 102, is executed by an electronic control unit (ECU) 110 as a controller. The ECU 110 is constituted by: a computer, as a main constituent element, including a CPU, a ROM, a RAM, etc.; a drive circuit of the solenoid of the flow-rate controlling mechanism 94; and an inverter that is a drive circuit of the electric motor 102. The drive circuit of the solenoid of the flow-rate controlling mechanism 94 and the inverter are activated by a command from the computer. The ECU 110 controls the supply flow rate of the working fluid and the assist torque. The steering system is equipped with an operation angle sensor 112 for detecting the operation angle of the steering wheel 30 as the operation amount of the steering operation member and four wheel speed sensors 114 (only one of which is illustrated in FIG. 1) each for detecting a rotational speed of a corresponding one of the four wheels including the two front wheels 12. The operation angle sensor 112, the wheel speed sensors 114, and the operation torque sensor 62 described above are connected to the ECU 110.

(19) B. Control of Steering System

(20) i) Control of Supply Flow Rate of Working Fluid in Steering Assist Device

(21) As understood from the explanation above, the steering mechanism 38 is for enabling the front wheels 12 to be steered in accordance with the operation angle θ of the steering wheel 30, and the steering assist device 70 is for assisting the movement of the steering rod 36 in order to achieve appropriate steering of the front wheels 12. In the present steering system, the ECU 110 controls the operation angle sensor 112 to detect the operation angle θ of the steering wheel 30 and identifies an operation speed dθ/dt of the steering wheel 30 based on the detected operation angle.

(22) In the case where the operation speed dθ/dt of the steering wheel 30 is relatively high, namely, in the case where the driver performs a relatively quick steering operation, the piston 72 of the actuator 78 moves quickly and therefore the actuator 78 needs to receive the working fluid at a relatively high flow rate. In view of this, the present steering system is configured such that, when the operation speed dθ/dt becomes equal to or higher than a set threshold speed (dθ/dt).sub.0, the supply flow rate Q of the working fluid is made equal to a set high flow rate Q.sub.H that is set as a relatively high flow rate, so as to prevent the flow rate of the working fluid to be received by the actuator 78 from becoming low.

(23) On the other hand, in the case where the operation speed dθ/dt of the steering wheel 30 is relatively low, namely, in the case where the driver performs a relatively slow steering operation, the piston 72 of the actuator 78 moves slowly and therefore the flow rate of the working fluid to be received by the actuator 78 need not be high. In view of this, the present steering system is configured such that, when the operation speed dθ/dt is lower than the set threshold speed (dθ/dt).sub.0, the supply flow rate Q of the working fluid is made equal to a set low flow rate Q.sub.L that is set as a relatively low flow rate, so as to reduce the load of the engine pump 80. The reduction of the load of the engine pump 80 leads to a reduction of the load of the engine 14, thus enhancing fuel economy of the vehicle 10.

(24) As explained before, the supply flow rate Q is changed by changing the electric current supplied to the solenoid of the flow-rate controlling mechanism 94.

(25) ii) Control of Assist Torque Applied by Operation Assist Device

(26) The present steering system is provided with the operation assist device 100 mainly for giving, to the driver, a better operational feeling of the steering wheel 30 felt by the driver in the steering operation. An assist torque T.sub.A applied to the steering shaft 34 by the operation assist device 100 is determined in the following manner.

(27) In the present steering system, three components, i.e., an operation-torque-dependent component T.sub.A-TO, an operation-speed-dependent component T.sub.A-R, and an operation-angle-dependent component T.sub.A-θ, are set as components constituting the assist torque T.sub.A. These three components, i.e., the operation-torque-dependent component T.sub.A-TO, the operation-speed-dependent component T.sub.A-R, and the operation-angle-dependent component T.sub.A-θ, are added up according to the following equation so as to determine the assist torque T.sub.A to be applied to the steering shaft 34:
T.sub.A=T.sub.A-TO+T.sub.A-R+T.sub.A-θ

(28) The operation-torque-dependent component T.sub.A-TO is set as a function based on an operation torque T.sub.O and a running speed v of the vehicle as parameters, the operation-speed-dependent component T.sub.A-R is set as a function based on the operation speed dθ/dt of the steering wheel 30 and the running speed v of the vehicle as parameters, and the operation-angle-dependent component T.sub.A-θ is set as a function based on the operation angle θ of the steering wheel 30 and the running speed v of the vehicle as parameters, as indicated below. The operation-torque-dependent component T.sub.A-TO, the operation-speed-dependent component T.sub.A-R, and the operation-angle-dependent component T.sub.A-θ are determined according to the respective functions.
T.sub.A-TO=f.sub.TO(T.sub.O,v)
T.sub.A-R=f.sub.R(dθ/dt,v)
T.sub.A-θ=θ(θ,v)

(29) According to the respective functions, the operation-torque-dependent component T.sub.A-TO, the operation-speed-dependent component T.sub.A-R, and the operation-angle-dependent component T.sub.A-θ change with respect to changes of the corresponding parameters, as indicated in respective graphs of FIGS. 2A, 2B, and 2C. The three components will be specifically explained. The operation-torque-dependent component T.sub.A-TO is a basic component, namely, one component of the assist torque T.sub.A for promoting the steering operation in order to reduce the load of the driver for the steering operation. As shown in FIG. 2A, the operation-torque-dependent component T.sub.A-TO is determined such that the assist torque T.sub.A increases with an increase in the operation torque T.sub.O and decreases with an increase in the running speed v of the vehicle. The operation-speed-dependent component T.sub.A-R is a component that may be regarded as a damping force with respect to the steering operation. The operation-speed-dependent component T.sub.A-R is a component for applying the assist torque T.sub.A in a direction reverse to a direction of the operation-torque-dependent component T.sub.A-TO. This assist torque T.sub.A in the reverse direction will be referred to as a counter torque T.sub.A with respect to the steering operation. As illustrated in FIG. 2B, the operation-speed-dependent component T.sub.A-R is determined such that the counter torque T.sub.A increases with an increase in the operation speed dθ/dt of the steering wheel 30 and increases with an increase in the running speed v the vehicle. The operation-angle-dependent component T.sub.A-θ is a component that may be considered as a force for returning the steering wheel 30 to a neutral position. This force is what is called spring force. Like the operation-speed-dependent component T.sub.A-R, the operation-angle-dependent component T.sub.A-θ is a component for applying the counter torque T.sub.A in the direction reverse to the direction of the operation-torque-dependent component T.sub.A-TO. As illustrated in FIG. 2C, the operation-angle-dependent component T.sub.A-θ is determined such that the counter torque T.sub.A increases with an increase in the operation angle θ of the steering wheel 30 and decreases with an increase in the running speed v of the vehicle. As for the sign (±) of the operation-torque-dependent component T.sub.A-TO, the operation-speed-dependent component T.sub.A-R, and the operation-angle-dependent component T.sub.A-θ, the sign of the operation-torque-dependent component T.sub.A-TO generally differs from the sign of the operation-speed-dependent component T.sub.A-R and the sign of the operation-angle-dependent component T.sub.A-θ for one steering operation, as apparent from the graphs of FIGS. 2A-2C.

(30) There is supplied, to the electric motor 102 of the operation assist device 100, an electric current based on the thus determined assist torque T.sub.A so as to apply the assist torque T.sub.A to the steering shaft 34. It is noted that the operation torque T.sub.O is obtained based on detection by the operation torque sensor 62, and the operation angle θ of the steering wheel 30 is obtained based on detection by the operation angle sensor 112. The operation speed dθ/dt of the steering wheel 30 is identified based on the obtained operation angle θ of the steering wheel 30, and the vehicle running speed v is identified based on wheel rotation speeds v.sub.W detected by the corresponding wheel speed sensors 114 provided for the respective four wheels.

(31) iii) Control Flow

(32) The computer of the ECU 110 repeatedly executes a supply flow rate/assist torque control program indicated by a flowchart of FIG. 3 at a short time pitch, e.g., about several to several tens of milliseconds (msec), so that the control of the supply flow rate Q of the working fluid in the steering assist device 70 and the control of the assist torque T.sub.A applied by the operation assist device 100 are executed. Referring to the flowchart, there will be briefly explained a process of the two controls.

(33) The process according to the program starts with Step 1 at which the ECU 110 obtains the operation angle θ of the steering wheel 30, the wheel rotation speeds v.sub.W, and the operation torque T.sub.O applied to the steering shaft 34 from the steering wheel 30 respectively based on the detection by the operation angle sensor 112, the detection by the wheel speed sensors 114, and the detection by the operation torque sensor 62. Step 1 is abbreviated as “S1” and other steps are similarly abbreviated. At S2, the ECU 110 identifies the operation speed dθ/dt of the steering wheel 30 and the vehicle running speed v respectively based on the operation angle θ and the wheel rotation speeds v.sub.W obtained as described above.

(34) At S3, the ECU 110 determines whether the identified operation speed dθ/dt is not lower than the set threshold speed (dθ/dt).sub.0. When the operation speed dθ/dt is not lower than the set threshold speed (dθ/dt).sub.0, the ECU 110 determines, at S4, the supply flow rate Q of the working fluid that should be attained to be equal to the set high flow rate Q.sub.H, for increasing the assist force applied by the steering assist device 70. On the other hand, when the operation speed dθ/dt is lower than the set threshold speed (dθ/dt).sub.0, the ECU 110 determines, at S5, the supply flow rate Q of the working fluid that should be attained to be equal to the set low flow rate Q.sub.L, for keeping the assist force applied by the steering assist device 70 small. At S6, the ECU 110 supplies an electric current based on the determined supply flow rate Q to the solenoid of the flow-rate adjusting mechanism 96.

(35) At S7, the ECU 110 determines the operation-torque-dependent component T.sub.A-TO, the operation-speed-dependent component T.sub.A-R, and the operation-angle-dependent component T.sub.A-θ based on the obtained operation torque T.sub.O, the obtained operation angle θ of the steering wheel 30, the identified operation speed dθ/dt of the steering wheel 30, and the identified vehicle running speed v according to the respective three functions indicated above. At S8, the ECU 110 adds up the operation-torque-dependent component T.sub.A-TO, the operation-speed-dependent component T.sub.A-R, and the operation-angle-dependent component T.sub.A-θ so as to determine the assist torque T.sub.A to be applied by the operation assist device 100.

(36) At S9, the ECU 110 supplies an electric current based on the assist torque T.sub.A determined as described above to the electric motor 102 of the operation assist device 100. One cycle of the process according to the control program is ended by execution of the process described above.