MOTOR-DRIVEN POWER ASSISTED STEERING CONTROL APPARATUS

Abstract: A motor-driven power assisted steering control apparatus comprises a steering torque sensor (1), a motor (14) connected to a steering shaft via a clutch to thereby exert an auxiliary force for steering to the steering shaft and a control means (3,10) for controlling the motor in correspondence to an output from the steering torque sensing means, wherein a torque neutral point detector (23) detects the neutral point of steering torque, a first level detecting means (23) detects that a motor current exceeds a first level, and a second level detecting means (27) detects that the motor current exceeds a second level which is greater than the first level, whereby when the motor current exceeds the first level in a case that a steering torque value is at other than the neutral point, a current flowing in the clutch is reduced, and when the motor current exceeds the second level in a case that the steering torque value is at other than the neutral point, the current flowing in the clutch is interrupted to thereby stop the motor (14).

Description: The present invention relates to a motor-driven power assisted steering control apparatus for use e.g. in an automobile according to the first part of claim 1 (EP-A-0 249 902). A conventional motor-driven power assisted steering control apparatus which is similar to that of the EP-A-0 245 902, is so constructed that a torque sensor detects a torque of steering; an output from the torque sensor is supplied to a control means so that an auxiliary torque in proportion to the output of the torque sensor is generated by means of a motor, and the auxiliary torque is given to a steering shaft to thereby assist a power for operating the steering wheel. Figure 2 is a diagram of a control circuit for such a conventional motor-driven power assisted steering control apparatus wherein a reference numeral 1 designates a torque sensor, a numeral 16 a motor control circuit, numerals 15A, 15B input terminals and a numeral 2 an interface circuit for a steering torque detection signal, through which the steering torque detection signal is supplied to a microcomputer 3 (hereinbelow, referred to as CPU). A numeral 4 designates an interface circuit for supplying a signal to turn a motor in the right direction, through which a signal for driving the motor in the right direction is input from the CPU 3 to a motor driving circuit 10. A numeral 5 designates an interface circuit for supplying a signal to turn the motor in the left direction, through which a signal for driving the motor in the left direction is inputted from the CPU 3 to the motor driving circuit 10. Numerals 13A, 13B respectively designate output terminals through which terminals of the motor 14 are connected. A numeral 6 designates a D/A conversion circuit. An error amplifier 7 for controlling the motor 14 has an input terminal which receives a torque detection signal from the CPU 3, which is inputted as an analogue signal after having been subjected to D/A conversion, and another input terminal which receives an output from a detection circuit 11 for detecting a motor current. A numeral 9 designates a pulse width modulation signal osillator which produces a signal for pulse width modulation (PWM) control, and the output of the oscillator 9 is inputted to a PWM modulator 8 along with an output from the error amplifier 7. The PWM modulator 8 supplies a PWM signal to the motor driving circuit 10. A numeral 12 designates a resistor for detecting a motor current. Figure 3 is a diagram showing the output characteristics of the torque sensor 1 wherein the abscissa represents a steering torque in the right or left direction and the ordinate represents a torque output. A symbol T₀ indicates a neutral point of steering torques (the zero point of torque). The right region of T₀ represents a steering torque for a steering wheel turned in the right direction and the left region represents a steering torque for the steering wheel turned in the left direction. A symbol T₂ represents a steering torque in which a control in the right direction is started and a symbol T₁ represents a steering torque in which a control in the left direction is started. Simbols V₀-V₂ respectively designate detection outputs corresponding to T₀-T₂. The output characteristic is substantially linear in a range of control. As a result, the region between T₁ and T₂ constitutes a non-sensitive zone of control. Figure 4 is a diagram showing the output characteristics of the motor 14 wherein the symbols V₀-V₂ are the same as defined above, V₃ desigantes a saturated torque detection output in the right direction and a simbol V₄ designates a saturated torque detection output in the left direction. The motor output changes in a substantially linear fashion between V₂ and V₃, and it changes in a substantially linear fashion V₁ and V₄ so as to be symmetric therewith. The motor output becomes zero between V₁ and V₂, and the motor output exhibits a constant value Pmax in the region greater than V₃ or smaller then V₄. The operation of the conventional device as shown in Figure 2 will be described. When an operator handles a steering wheel, a torque detection signal in proportion to a steering torque of the steering wheel is outputted from the torque sensor 1, and the signal is inputted to the CPU 3 through the interface circuit 2. The CPU 3 outputs a torque detection signal as a digital signal. The torque detection signal is converted into an analogue signal by means of the D/A conversion circuit 6. The level of the torque detection signal is determined in the CPU 3. As a result, a driving signal for driving the motor in the right direction is supplied to the interface circuit 4 or a driving signal for driving the motor in the left direction is supplied to the interface circuit 5. The motor driving circuit 10 receives a driving signal from the interface circuit 4 or 5 to drive the steering wheel in the right or left direction; thus an instruction of a turning direction of the motor 14 is provided. The output of the D/A conversion circuit 6 is inputted to the error amplifier 7, and the ouptut of the error amplifier 7 is inputted to the PWM modulator 8. The output of the PWM modulation oscillator 9 is inputted to the PWM modulator 8. As a result, a control signal having a pulse width in proportion to the torque detection signal is supplied from the PWM modulator 8 to the motor driving circuit 10 so that the torque of the motor 14 is controlled on the basis of the output of the motor driving circuit 10. A current flowing in the motor 14 is detected by the motor current detection circuit 11, whereby the motor current is limited or is interrupted depending on a detected level of the current. However, when a locking phenomenon or the like takes place in the motor 14 in the conventional apparatus, the steering wheel enters a state of locking or its rotation requires an extremely high handling force to be afforded by the driver, whereby it is very difficult to handle the steering wheel, which causes a dangerous situation. It is a problem of the present invention to provide a motor-driven power assisted steering control apparatus ,which prevents a steering wheel from falling into a state of locking or difficult handling due to a locking state of the motor to thereby assure safe steering permanently. This problem is accomplished by the subject matter of the claim. In the drawings: Figure 1 is a block diagram of an embodiment of the control circuit for a motor-driven power assisted steering control apparatus according to the present invention; Figure 2 is a block diagram of a conventional control circuit for a power assisted steering control apparatus; Figure 3 is a diagram showing the output characteristics of a torque sensor; and Figure 4 is a diagram of the output characteristics of a motor. A preferred embodiment of the motor-driven power assisted steering control apparatus of the present invention will be described with reference to the drawings. Figure 1 is a diagram showing a control circuit for the power assisted steering control apparatus of the present invention, wherein a reference numeral 20 designates a differentiation circuit, a numeral 21 designates a clutch driver for driving a clutch, a numeral 23 designates a torque steady state detecting circuit torque neutral point detection circuit) for detecting a neutral point of torque, a numeral 24 designates a self-sustaining circuit, a numeral 25 designates a fail-relay interrupting circuit, a numeral 26 designates a fail relay which comprises stationary contacts 41a, 41b, a movable contact 42 and an exciting coil 43, a numeral 27 designate a first level detector, numerals 28 and 32 designate AND-gates, a numeral 29 designates a second level detector, a numeral 30 designates a clutch current control circuit, a numeral 31 designates a clutch coil, and a numeral 36 designates a power source. The motor driving circuit 10 is constituted by a bridge circuit comprising four power MOS FETs 37-40. Numerals 44, 45 designate output terminals and a numeral 46 designates a key switch. The operation of the control apparatus of the present invention will be described. On activating the key switch 46, the fail relay 26 is activated and the movable contact 41 comes to contact with stationary contacts 41a, 41b, whereby a current is fed to the motor driving circuit 10 and the clutch driver 21 so that the motor 14 is connected to the steering shaft by means of the clutch. A steering torque produced in the steering wheel is detected by the torque sensor 1, and the output of the torque sensor is inputted to the CPU 3 and the torque steady state detecting circuit 23 through the differentiation circuit 20. The CPU 3 processes the torque detection signal to detect the direction and the magnitude of the torque produced in the steering wheel, and outputs a signal to the motor driving circuit 10. The motor driving circuit 10 controls the magnitude and the direction of a current flowing in the motor 14 through the output terminals 44, 45, so that the motor 14 is driven. The driving force of the motor 14 is supplied to the steering shaft to thereby assist an operational force for the steering wheel. The motor current detection circuit 11 detects the current flowing in the motor through the resistor 12. The torque steady state detecting circuit 23 detects the neutral point of torque T₀, i.e. a neutral state of the steering wheel. The output of the torque steady state detecting circuit 23 is inputted to the AND gate 28 along with the output of the second level detector 29. The output of the AND gate 28 is supplied to the fail relay 26 through the self-sustaining circuit 24 and the fail-relay interrupting circuit 25. The output of the torque steady state detecting circuit 23 is inputted to the AND-gate 32 along with the output of the first level determinig device 27. The output of the AND-gate 32 is supplied to the clutch current control circuit 30. When the torque steady state detecting circuit 23 detects the fact that the torque to the steering wheel is out of the neutral point region and when a current flowing in the motor 14 is beyond the first level, a control signal is supplied to the clutch current control circuit 30 to reduce the clutch current, whereby a slipage occurs in the clutch. Accordingly, when the motor 14 falls in a state of locking or nearly locking while the motor current increases, the clutch current is reduced to reduce the driving connection between the motor and the steering shaft to thereby bring the steering wheel into a state of manual operation. On the other hand, when the motor 14 falls into a state of locking and the motor current is further increased, the AND-gate 28 is turned on to open the fail relay 26, whereby the current supplied to the motor driving circuit 10 and the clutch driver 21 is interrupted. Accordingly, the steering wheel is brought to a state of complete manual operation. As described above, in accordance with the present invention, when the motor falls in a state of locking or nearly locking, the steering wheel is more or less isolated from the clutch or the motor so that the steering wheel is brought into a state of manual operation or nearly manual operation. Accordinly, there is no possibility that the operator feels the operation of the steering wheel to be abnormally heavy, whereby an automobile can be safely driven.

Claims: A motor-driven power assisted steering control apparatus comprising a steering torque sensing means (1) for detecting a steering torque of a steering shaft, a motor (14) connected to a steering shaft via a controllable clutch to thereby exert an auxiliary steering torque to the steering shaft to assist an operational torque applied to the steering wheel, a control means (3,10) for controlling said motor in correspondence to an output from said steering torque sensing means (1), a clutch driving circuit (21), a clutch current control circuit (30), and a fail relay interrupting circuit (25), characterized by the clutch driving circuit (21) being arranged to reduce the current flowing in said clutch to thereby reduce the moment to be transmitted by the clutch; a torque steady state detecting circuit (23) detecting the steady state of torque applied to the steering wheel; a first level detector (27) detecting that a motor current exceeds a first level and a second level detector (29) detecting that the motor current exceeds a second level which is greater than the first level, the clutch current control circuit (30) being connected via an AND-gate (32) to the torque steady state detecting circuit and the first level detector and the fail relay interrupting circuit (25) being connected to the torque steady state detecting circuit (23) and the second level detector via a second AND-gate, the clutch current control circuit (30) reducing the current flowing in said clutch when the motor current exceeds the first level with the torque applied to the steering wheel being not at a steady state, whereby the fail relay interrupting circuit (25) interrupts the current flowing in said clutch and said motor (14) if the motor current exceeds the second level with the torque applied to the steering wheel being not at a steady state.

Inventors: MORISHITA, MITSUHARU C/O MITSUBISHI DENKI K.K., UOTA, KOSAKU C/O MITSUBISHI DENKI K.K., YASUKAWA, TAKESHI C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1989
Main classification: B62D5
IPC codes: B62D5