Abstract:
Disclosed is a power steering apparatus using an electric motor as a drive source. The power steering apparatus includes a drive inhibiting circuit for inhibiting an electric motor from being driven when the temperature detected by a temperature sensor is not less than an upper-limit temperature, a judging circuit for judging, in a state where the electric motor is inhibited from being driven by the drive inhibiting circuit, whether or not the steering angle is included in a predetermined steering angle midpoint range when the temperature sensor detects a temperature which is not more than a lower-limit temperature, and a re-drive allowing circuit for allowing, when it is judged that the steering angle is within the steering angle midpoint range, the electric motor to be driven again.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to power steering apparatuses for applying a steering assist force to a steering mechanism by hydraulic pressure created by a pump driven by an electric motor. 
     2. Description of Related Art 
     Conventionally, power steering apparatuses for assisting in operating a steering wheel by supplying working oil from an oil pump to a power cylinder coupled to a steering mechanism have been employed. In such power steering apparatuses, the oil pump is driven by an electric motor. A steering assist force corresponding to the rotational speed of the electric motor is produced from the power cylinder. 
     Drive control of the electric motor is achieved by an electronic control unit carrying out on-off control of a motor driving element composed of an FET (Field Effect Transistor). In some circumstances, the steering wheel continues to be violently operated, so that the motor driving element is frequently turned on and off. There are also circumstances that the load on the electric motor is increased, resulting in a large current flow in the motor. In these situations, the motor driving element generates heat. Accordingly, the motor driving element may be destroyed. 
     The electronic control unit is constituted by a computer including a CPU, a RAM, and a ROM, for example, and has low resistance to heat applied from the exterior. When the motor driving element generates heat because the steering wheel continues to be violently operated, therefore, the electronic control unit may be destroyed by the heat generation from the motor driving element. 
     In the conventional power steering apparatus, therefore, there is provided a temperature sensor for detecting the internal temperature of the electronic control unit, for example. If the temperature detected by the temperature sensor is higher than a predetermined temperature, the electronic motor is forced to be stopped. After the temperature detected by the temperature sensor is lowered to not more than the predetermined temperature, the electronic motor which has been forced to be stopped is restarted. Consequently, the motor driving element and the electronic control unit can be prevented from being destroyed by the heat generation. 
     In the above-mentioned conventional control, however, after the electric motor is forced to be stropped, a driver may have a feeling of physical disorder in steering when the internal temperature of the electronic control unit is lowered to not more than the predetermined temperature while the steering wheel is being operated. That is, in a case where the driver is applying torque to the steering wheel, when the internal temperature of the electronic control unit is lowered to not more than the predetermined temperature, the electric motor is restarted in response thereto, thereby suddenly assisting in steering the steering wheel. Accordingly, the driver feels that the steering resistance is rapidly lowered. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a power steering apparatus capable of preventing a driver from having a feeling of physical disorder in steering. 
     A power steering apparatus according to the present invention uses an electric motor as a drive source, and produces a steering assist force for assisting in steering by oil pressure created by a pump driven by the electric motor, for example. The apparatus comprises a temperature detection section for detecting the temperature in a predetermined portion; a steering angle detection section for outputting steering angle data corresponding to a steering angle; a drive inhibiting circuit for inhibiting the electric motor from being driven when the temperature detected by the temperature detection section is not less than a predetermined upper-limit temperature; a judging circuit for judging, in a state where the electric motor is inhibited from being driven by the drive inhibiting circuit, whether or not the steering angle is included in a predetermined steering angle midpoint range on the basis of the steering angle data from the steering angle detection section when the temperature detected by the temperature detection section is lowered to not more than a predetermined lower-limit temperature; and a re-drive allowing circuit for allowing, when the judging circuit judges that the steering angle is within the steering angle midpoint range, the electric motor to be driven again. 
     According to the present invention, when the temperature detected by the temperature detection section is not less than the predetermined upper-limit temperature, the electric motor is inhibited from being driven by the drive inhibiting circuit. When the temperature detected by the temperature detection section is lowered to not more than the predetermined lower-limit temperature in a state where the motor is inhibited from being driven, it is judged whether or not the steering, angle is within the steering, angle midpoint range. The electric motor is allowed to be driven again by the re-drive allowing circuit, provided that the steering angle is within the steering angle midpoint range. 
     Consequently, the assistance in steering is prevented from being suddenly started while a driver is performing a steering operation, thereby making it possible to prevent the driver from having such a feeling of physical disorder in steering that the steering resistance is rapidly lowered. 
     The steering angle midpoint range is a predetermined range including a steering angle midpoint, which is a steering angle in a case where a vehicle goes straight on. 
     Furthermore, it is preferable that the predetermined portion is the inside of a control unit for controlling the drive of the electric motor. The predetermined portion may be a motor driving element such as an FET for controlling the supply of power to the electric motor. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a conceptual diagram showing the basic configuration of a power steering apparatus according to an embodiment of the present invention; 
     FIG. 2 is a flow chart for explaining operations performed by a CPU in relation to the drive of an electric motor; and 
     FIG. 3 is a graph showing the relationship between the internal temperature of an electronic control unit and the maximum motor rotational speed of the electric motor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a conceptual diagram showing the basic configuration of a power steering apparatus according to an embodiment of the present invention. The power steering apparatus is provided in relation to a steering mechanism  1  of a vehicle, and is for applying a steering assist force to the steering mechanism  1 . 
     The steering mechanism  1  comprises a steering wheel  2  operated by a driver, a steering shaft  3  connected to the steering wheel  2 , a pinion gear  4  provided at a front end of the steering shaft  3 , and a rack shaft  5 . The rack shaft  5  has a rack gear portion  5   a  extending along the width of the vehicle and engaged with the pinion gear  4 . Tie rods  6  are respectively coupled to both ends of the rack shaft  5 . The tie rods  6  are respectively coupled to knuckle arms  7  for supporting right and left front wheels FR and FL serving as steering wheels. The knuckle arm  7  is provided so as to be rotatable around a king pin  8 . 
     By this configuration, when the steering wheel  2  is operated so that the steering shaft  3  is rotated, the rotation is converted into a linear motion along the width of the vehicle by the pinion gear  4  and the rack shaft  5 . The linear motion is converted into rotation of the knuckle arms  7  around the king pins  8 , thereby achieving the steering of the right and left front wheels FR and FL. 
     In a halfway portion of the steering shaft  3 , there are interposed a torsion bar  9 , which is distorted depending on the direction and the magnitude of steering torque applied to the steering wheel  2 , and an hydraulic pressure control valve  23 , which changes its valve aperture depending on the direction and the magnitude of the distortion of the torsion bar  9 . The hydraulic pressure control valve  23  is connected to a power cylinder  20  for applying a steering assist force to the steering mechanism  1 . The power cylinder  20  has a piston  21  provided integrally with the rack shaft  5  and a pair of cylinder chambers  20   a  and  20   b , which are defined by the piston  21 . The cylinder chambers  20   a  and  20   b  are connected to the hydraulic pressure control valve  23 , respectively, through oil supply/return lines  22   a  and  22   b.    
     The hydraulic pressure control valve  23  is further interposed in a halfway portion of an oil circulation line  24  passing through a reservoir tank and an oil pump  26 . The oil pump  26  is driven by an electric motor  27 , to draw working oil stored in the reservoir tank  25  and supply the drawn working oil to the hydraulic pressure control valve  23 . The excess working oil is returned to the reservoir tank  25  from the hydraulic pressure control valve  23  through the oil circulation line  24 . 
     The hydraulic pressure control valve  23  supplies, when the torsion bar  9  is distorted in one direction, the working oil to one of the cylinder chambers  20   a  and  20   b  in the power cylinder  20  through one of the oil supply/return lines  22   a  and  22   b , while supplying, when the torsion bar  9  is distorted in the other direction, the working oil to the other cylinder chamber through the other oil supply/return line. When the torsion bar  9  is not virtually distorted, the hydraulic pressure control valve  23  enters a so-called equilibrium state. Accordingly, the working oil circulates in the oil circulation line  24  without being supplied to the power cylinder  20 . 
     When the working oil is supplied to either one of the cylinder chambers in the power cylinder  20 , the piston  21  moves along the width of the vehicle. 
     Consequently, a steering assist force is exerted on the rack shaft  5 . 
     Drive control of the motor  27  is achieved by a CPU  31  in an electronic control unit carrying out on-off control of a motor driving element  35  composed of an FET, for example. The electronic control unit  30  comprises a RAM  32 , a ROM  33  and a temperature sensor  34 , which are connected to the CPU  31  through a bus  37 . The RAM  32  provides a work area for the CPU  31 . The ROM  33  stores an operation program of the CPU  31 , and the like. The temperature sensor  34  is provided inside the electronic control unit  30 , to detect the internal temperature of the electronic control unit  30 . 
     Steering angle data outputted from a steering angle sensor  11  is fed to the CPU  31  through an I/O (Input/Output) port  36  connected to the bus  37 . The steering angle sensor  11  is provided in relation to the steering wheel  2 , and outputs steering angle data corresponding to a relative steering angle from an initial value, taking a steering angle of the steering wheel  2  when an ignition key switch of the vehicle is turned on to start the engine as the initial value “0”. 
     Furthermore, an output signal of a vehicle speed sensor  12  for detecting the speed of the vehicle is fed to the CPU  31  through the I/O port  36 . The vehicle speed sensor  12  may directly detect the speed of the vehicle, or may find the speed of the vehicle by calculation on the basis of pulses outputted from wheel speed sensors provided in relation to the wheels. 
     The CPU  31  finds a steering angle speed on the basis of the steering angle data fed from the steering angle sensor  11 . The drive of the motor  27  is controlled on the basis of the steering angle speed found from the steering angle data, the vehicle speed detected by the vehicle speed sensor  12 , and the temperature detected by the temperature sensor  34 . 
     FIG. 2 is a flow chart for explaining operations performed by the CPU  31  in relation to the drive of the electric motor  27 . FIG. 3 is a graph showing the relationship between the internal temperature T of the electric control unit  30 , which is detected by the temperature sensor  34 , and the maximum motor rotational speed N of the electric motor  27 . 
     The CPU  31  in the electronic control unit  30  first refers, when the ignition key switch of the vehicle is turned on, to an output signal of the temperature sensor  34 , to detect the internal temperature T of the electronic control unit  30  (step S 1 ). It is judged whether or not the detected internal temperature T is not less than a first threshold temperature T 1  previously determined (for example, 105° C.) (step S 2 ). If the internal temperature T is less than the first threshold temperature T 1  (NO at step S 2 ), the maximum motor rotational speed N of the electric motor  27  is set to a first rotational speed N 1  previously determined (step S 3 ). 
     When the internal temperature T is not less than the first threshold temperature T 1 , it is then judged whether or not the internal temperature T is not less than a second threshold temperature T 2  (for example, 110° C.) higher than the first threshold temperature T 2  (step S 4 ). When the internal temperature T is less than the second threshold temperature T 2 , that is, the internal temperature T is not less than the first threshold temperature T 1  and is less than the second threshold temperature T 2 , the maximum motor rotational speed N is set in accordance with a straight line P 1  (see FIG.  3 ), which changes almost linearly between the first rotational speed N 1  and the second rotational speed N 2  (N 1 &gt;N 2 ) with respect to the internal temperature T (step S 5 ). 
     When the internal temperature T is not less than the second threshold temperature T 2 , the CPU  31  judges whether or not the internal temperature T is not less than a third threshold temperature T 3  (for example, 12° C.) higher than the second threshold temperature T 2  (step S 6 ). When the internal temperature F is less than the third threshold temperature T 3 , that is, the internal temperature T is not less than the second threshold temperature T 2  and is less than the third threshold temperature T 3 , the maximum motor rotational speed N is set in accordance with a straight line P 2  (see FIG.  3 ), which changes almost linearly between the second rotational speed N 2  and the third rotational speed N 3  (N 2 &gt;N 3 ) with respect to the internal temperature T (step S 7 ). 
     When the maximum motor rotational speed N of the electric motor  27  corresponding to the internal temperature T of the electronic control unit  30  is thus determined, the CPU  31  reads out a motor control map corresponding to the vehicle speed at that time from the ROM  33  on the basis of the output signal from the vehicle speed sensor  12 . The motor control map is referred to in order for the CPU  31  to set a suitable target motor rotational speed corresponding to the steering angle speed, and is provided for each of a plurality of vehicle speed ranges previously determined (for example, a low speed range, an intermediate speed range, and a high speed-range) such that a good steering feeling can be realized. The CPU  31  refers to the motor control map read out of the ROM  33 , to control the drive of the electric motor  27  within the range of the maximum motor rotational speed N set at the steps S 3 , S 5 , and S 7  on the basis of the steering angle speed found from the steering angle data outputted by the steering angle sensor  11  (step S 12 ). 
     In the present embodiment, so-called idle-and-go control is carried out such that the electric motor  27  is rotated at a predetermined low rotational speed in a straight steering state where the steering wheel  2  is not turned, while the rotational speed of the electric motor  27  is increased to a rotational speed corresponding to the steering angle speed when the steering wheel  2  is turned. 
     On the other hand, when the internal temperature T is not less than the third threshold temperature T 3 , the CPU  31  sets the maximum motor rotational speed N to zero (step S 8 ). When the internal temperature T of the electronic control unit  30  is not less than the third threshold temperature T 3  while the electric motor  27  is being driven, for example, the electric motor  27  during the driving is forced to be stopped. In the present embodiment, the processing at the step S 8  corresponds to the function of a drive inhibiting circuit. 
     The CPU  31  sets the maximum motor rotational speed N to zero, and then always monitors an output of the temperature sensor  34 , to repeatedly judge whether or not the internal temperature T of the electronic control unit  30  is lowered to less than the first threshold temperature T 1  (step S 9 ). If the internal temperature T of the electronic control unit  30  becomes less than the first threshold temperature T 1 , it is judged whether or not the steering angle of the steering wheel  2  is within a steering angle midpoint range (for example, −5 to +5°) with reference to the steering angle data fed from the steering angle sensor  11  (step S 10 ). 
     The steering angle midpoint is a steering angle of the steering wheel  2  in a case where the vehicle goes straight on. For example, the electronic control unit  30  samples the steering angle data outputted from the steering angle sensor  11  after the ignition key switch of the vehicle is turned on, to prepare a histogram of steering angle data values. The electronic control unit  30  finds, after steering angle data corresponding to a predetermined number of times of sampling are collected, the most frequent steering angle data, takes the most frequent steering angle data as steering angle data at a steering angle midpoint, and sets a predetermined range including the data as a steering angle midpoint range. The steering angle midpoint range thus set is stored in the RAM  32  contained in the electronic control unit  30 . The electronic control unit  30  judges whether or not the steering angle data from the steering angle sensor  11  is data within the steering angle midpoint range that is held in the RAM  32 . 
     When the steering angle of the steering wheel  2  is not within the steering angle midpoint range it is repeatedly examined whether or not the internal temperature T of the electronic control unit  30  is less than the first threshold temperature, and the steering angle of the steering, wheel  2  is within the steering angle midpoint range until the steering angle of the steering wheel  2  is returned to the steering angle midpoint range. When the steering angle of the steering wheel  2  is returned to the steering angle midpoint range and it is judged that the steering angle of the steering wheel  2  is within the steering angle midpoint range (YES at step S 10 ), the maximum motor rotational speed N of the electric motor  27  is set to the first rotational speed N 1  (step S 11 ), and the drive control of the electric motor  27  is resumed (step S 12 ). Specifically, when the idle-and-go control is carried out as in the present embodiment, the electric motor  27 , which is being stopped, is restarted, to rotate the electric motor  27  at the predetermined low rotational speed. When the drive control of the electric motor  27  is started or resumed, the processing is returned. In the present embodiment the processing at the step S 10  corresponds to the function of the judging circuit, and the processing at the step S 11  corresponds to the function of the re-drive allowing circuit. 
     As described in the foregoing, according to the present embodiment, when the internal temperature T of the electronic control unit  30  is not less than the third threshold temperature T 3 , the drive of the electric motor  27  is stopped. Thereafter, when the internal temperature T of the electric control unit  30  is lowered to less than the first threshold temperature T 1 , it is judged whether or not the steering angle of the steering wheel  2  is within a predetermined steering angle midpoint range. The drive of the electric motor  27 , which is being stopped, is resumed, provided that the steering angle of the steering wheel  2  is within the steering angle midpoint range. Consequently, it is possible to prevent sudden assistance in steering the steering wheel  2  from being provided while the driver is operating the steering wheel  2 , thereby making it possible to prevent the driver from having such a feeling of physical disorder in steering that the steering wheel  2  is rapidly lightened. 
     Although description has been made of an embodiment of the present invention, the present invention is not limited to the above-mentioned embodiment. For example, although in the above-mentioned embodiment, so-called idle-and-go control is carried out such that the electric motor  27  is rotated at a predetermined low rotational speed even in a straight steering state where the steering wheel  2  is not turned, so-called stop-and-go control may be carried out such that the electric motor  27  is stopped in a straight steering state and is started, when the steering wheel  2  is steered at not less than a predetermined steering angle speed, in response thereto. When the stop-and-go control is carried out, the electric motor  27  may be restarted, when the steering wheel  2  is steered after the internal temperature T of the electronic control unit  30  is lowered to less than the first threshold temperature T 1 , and it is judged once that the steering angle of the steering wheel  2  is within the predetermined steering angle midpoint range, in response thereto. 
     Although in the above-mentioned embodiment, the temperature sensor  34  is provided inside the electronic control unit  30 , and the maximum motor rotational speed N of the electric motor  27  is determined on the basis of the internal temperature T of the electronic control unit  30 , the temperature sensor  34  may be provided in relation to the motor driving element  35 , for example, to determine the maximum motor rotational speed N of the electric motor  27  on the basis of the temperature of the motor driving element  35  which is detected by the temperature sensor  34 . Further, the temperature sensor  34  may be provided in relation to the electric motor  27 , to carry out an electric motor control similar to that in the above-mentioned embodiment. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims. 
     The present invention claims priority benefits under 35 § 119 of Japanese Patent Application No. 11-100408 filed with the Japanese Patent Office on Apr. 7, 1999, the disclosure of which is incorporated hereinto by reference.