JPH10167086A - Control device for electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To escape from a steering difficult state by flowing an electric current as much as possible to a motor at the time when stationary steering is at low speed by changing a feedback system over to an open loop system at the time when a failure detection circuit detects a failure of an electric current detection circuit. SOLUTION: Judgement of a failure of a motor driving circuit 37 is carried out by a value of motor terminal voltage Vm at the time when all of an FET constituting an H bridge in the motor driving circuit 37 is put off. When the motor driving circuit 37 is normal, if all the FET is put off, the motor terminal voltage Vm becomes 0, and when it is abnormal, it does not become 0. Thereafter, when a motor electric current detection circuit 38 is judged as it fails by a failure detection circuit 310, a change-over signal SS is output, a contact point of a change-over circuit 320 is changed from a contact point (a) over to a contact point (b), a feedback system of electric current control is changed over to an open loop of a motor model 330, and a steering auxiliary command value from a steering auxiliary command value computing element 32 is input to the motor model 330.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control device for an electric power steering device which applies a steering assist force by a motor to a steering system of an automobile or a vehicle, and more particularly to a motor current detection circuit provided in a control unit. The present invention relates to a control device for an electric power steering device, which detects a failure of the power steering device and switches the control system.

[0002]

2. Description of the Related Art An electric power steering apparatus for energizing a steering apparatus of an automobile or a vehicle with an auxiliary load by the rotational force of a motor uses a transmission mechanism such as a gear or a belt through a speed reducer to transmit the driving force of the motor to a steering shaft or the like. An auxiliary load is applied to the rack shaft. The configuration of a general electric power steering device will be described with reference to FIG. The shaft 2 of the steering handle 1 is connected to a tie rod 6 of a steering wheel via a reduction gear 3, universal joints 4a and 4b, and a pinion rack mechanism 5. The shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1. A motor 20 for assisting the steering force of the steering wheel 1 is coupled to the shaft 2 via a clutch 21 and a reduction gear 3. Have been. Power is supplied from a battery 14 to a control unit 30 that controls the power steering device via an ignition key 11. The control unit 30 controls the steering torque T detected by the torque sensor 10 and the vehicle speed V detected by the vehicle speed sensor 12. The steering assist command value I of the assist command is calculated based on
The current supplied to the motor 20 is controlled based on the calculated steering assist command value I. The clutch 21 is ON / OFF controlled by the control unit 30 and is ON (coupled) in a normal operation state. The clutch 21 is turned off when the control unit 30 determines that the power steering device is out of order and when the power of the battery 14 is turned off by the ignition key 11.
(Disconnected).

The control unit 30 is mainly composed of a CP
FIG. 8 shows a general function executed by a program inside the CPU.
For example, the phase compensator 31 does not indicate a phase compensator as independent hardware, but indicates a phase compensation function executed by the CPU. Control unit 3
It is also possible to configure each functional element with independent hardware instead of configuring CPU 0 with a CPU.

[0004] The function and operation of the control unit 30 will be described. The steering torque T detected and input by the torque sensor 10 is phase-compensated by a phase compensator 31 to enhance the stability of the steering system. The steering torque TA is input to the steering assist command value calculator 32. The vehicle speed V detected by the vehicle speed sensor 12 is also input to the steering assist command value calculator 32. The steering assist command value calculator 32 determines whether the motor 20 has been turned on based on the input steering torque TA and vehicle speed V.
A steering assist command value I which is a control target value of a current supplied to the steering assist command value calculator 32 is provided with a memory 33. The memory 33 stores a steering assist command value I corresponding to the steering torque using the vehicle speed V as a parameter.
Used for the calculation of The steering assist command value I is subtracted by a subtractor 30A.
, And a feedforward differential compensator 34 for increasing the response speed,
The deviation (I-i) of A is input to a proportional calculator 35, and the proportional output is input to an adder 30B and also to an integral calculator 36 for improving the characteristics of the feedback system. The outputs of the differential compensator 34 and the integration compensator 36 are also added to the adder 30B, and the current control value E, which is the result of the addition in the adder 30B, is input to the motor drive circuit 37 as a motor drive signal. The motor current value i of the motor 20 is detected by the motor current detection circuit 38, and the motor current value i is input to the subtractor 30A and fed back.

An example of the configuration of the motor drive circuit 37 will be described with reference to FIG. 9. The motor drive circuit 37 uses a field effect transistor (FE) based on a current control value E from an adder 30B.
T) An FET gate drive circuit 371 for driving the gates of FET1 to FET4, an H bridge circuit composed of FET1 to FET4, a boost power supply 372 for driving the high side of FET1 and FET2, and the like. FET1 and FET2 are turned ON / OFF by a PWM (pulse width modulation) signal having a duty ratio D1 determined based on the current control value E.
It is turned off, and the magnitude of the current Ir actually flowing to the motor is controlled. FET3 and FET4 have a duty ratio D
In a small area of 1, the duty ratio D2 defined by a predetermined linear function expression (D2 = a.D1 + b where a and b are constants)
In the region where the duty ratio D1 is large, it is turned ON / OFF according to the rotation direction of the motor determined by the sign of the PWM signal. For example, when the FET 3 is in the conductive state, the current flows through the FET 1, the motor 20, the F
The current flows through the ET3 and the resistor R1, and a current flows in the motor 20 in the positive direction. When the FET 4 is in the conductive state, the current flows through the FET 2, the motor 20, the FET 4, and the resistor R2, and a negative current flows through the motor 20. Therefore, the current control value E from the adder 30B is also a PWM output.

The motor current detection circuit 38 detects the magnitude of the positive current based on the voltage drop across the resistor R1, and detects the magnitude of the negative current based on the voltage drop across the resistor R2. To detect. The motor current value i detected by the motor current detection circuit 38 is input to the subtractor 30A and fed back. The motor 20 is connected to a power supply Vig via a resistor R3 and a diode D1, and is grounded via a resistor R4. Resistance R3
R4 is much larger than the inter-terminal resistance Rm of the motor 20, and a motor terminal voltage Vm is obtained.

[0007]

In the control unit of the electric power steering apparatus as described above, when a failure of the motor drive system including the current detection circuit 38 is detected, the output of the motor 20 is ensured for safety. A fail-safe operation such as turning off or turning off the clutch 21 has been performed. However, when the electric power steering device is turned off, it is conceivable that the vehicle enters a manual steering state at a low speed such as stationary operation, the steering wheel becomes heavy, and steering becomes difficult. This is because the electric power steering device is uniformly turned off even when the motor 20 can be driven.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to reliably detect a failure in a current detection circuit of an electric power steering device and to control a control system when the failure is detected. Of the present invention is to provide a control device for an electric power steering device that switches from a feedback system to an open loop system. In other words, it is conceivable that the other parts, especially the motor drive circuit, are normal even if the current detection circuit is faulty. In this case, the motor can be driven in an open loop even if the motor current control cannot be performed. . Then, it is possible to escape from the steering difficult state by supplying a small amount of current to the motor at a low stationary speed.

[0009]

According to the present invention, there is provided a torque sensor for detecting a steering torque of a steering wheel, a motor for urging a steering shaft integrally provided with the steering wheel to apply an auxiliary load, and a magnitude of the steering torque. The present invention relates to a control device for an electric power steering device, comprising: a control unit that drives the motor by a feedback system in accordance with the control system, wherein the control unit includes: a current detection circuit for the motor; A failure detection circuit for detecting a failure in the detection circuit, and when the failure detection circuit detects a failure in the current detection circuit, the feedback system is switched to an open loop system. Further, the failure of the current detection circuit, the current control value for driving the motor,
The detection may be performed based on an output of the current detection circuit and a terminal voltage of the motor. Further, when switching to the open loop system, the duty ratio of the pulse width modulation for driving the motor is controlled by the vehicle speed and the battery voltage.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a failure of a current detection circuit of an electric power steering device is detected as follows.
That is, first, an abnormality of the motor drive system is detected based on the parameter variation, and when an abnormality is detected, the motor current output is set to zero, the diagnosis is stored, and the fail lamp is turned on. In the failure detection due to the parameter fluctuation, it is impossible to determine whether the failure is in the motor drive circuit or the current detection circuit. Therefore, the failure detection of the current detection circuit is performed next. Then, it is determined whether the motor drive circuit is normal or abnormal. If the motor drive circuit is normal, it is considered that the current detection circuit has failed. That is, when the current detection circuit is determined to be normal, the current cannot be supplied to the motor, so that the failure of the motor drive circuit is stored and the fail-safe is turned off to stop the electric power steering. If the current detection circuit is determined to be abnormal, the motor drive circuit is normal,
The fact that the current detection circuit has failed is stored, and the motor is driven in an open loop. In this case, the motor is driven with the current being suppressed as compared with the normal state while the fail lamp remains lit. Such control system switching according to the present invention can be easily performed only by changing the program of the CPU in the control unit.

An embodiment of the present invention will be described below with reference to the drawings.

In the present invention, in order to detect a failure in the motor current detection circuit 38 and switch the control system, the control unit is configured as shown in FIG. FIG. 1 corresponds to FIG. The motor detection current i from the motor current detection circuit 38 and the voltage control value E from the adder 30B
Is the fault detection circuit 31 together with the terminal voltage Vm of the motor 20.
The switching signal SS that is input to 0 and output from the failure detection circuit 310 is input to the switching unit 320. The contact a of the switching unit 320 is connected to the subtractor 30A, and the contact b is connected to the motor model 330 obtained by simulation. Further, the battery voltage Vb of the battery 14 and the ignition signal IG from the ignition key 11 are also input to the failure detection circuit 310. Motor model 330
Is represented by a transfer function using the internal resistance Rm and the inductance Lm of the motor 20.

First, the principle of detecting an abnormality in the motor drive system according to the present invention will be described. FIG. 2 is a diagram for explaining abnormality detection of the motor drive system. The motor 20 having the internal resistance Rm and the inductance Lm is connected to a motor drive circuit 37 configured by connecting the FETs 1 to 4 in an H bridge. The input side is supplied with the power of the voltage Vb from the battery 14, and one of the FETs is turned on according to the set operation state, as shown in FIG.
It operates at the tee ratio D, and the other FETs are set ON and OFF.

FIG. 2A shows a case where the motor 20 is in a normal state, and it is assumed that a current ia flows at this time. Hereinafter, the current ia is referred to as a theoretical current value.
Now, when the motor 20 is grounded (a state in which the motor winding is grounded by the resistor Rt), the ground fault current ie as shown in FIG.
Flows, and a current im flows through the current detection resistor Ra connected to the ground side of the motor drive circuit 37. Therefore, the theoretical current value ia when the motor 20 is in a normal state and the detected current i when the motor 20 is in an actual operating state are compared. If the detected current i = theoretical current value ia, it is determined that the motor 20 is normal, and the detection is performed. If the current i is smaller than the theoretical current value ia, it can be determined that the motor 20 is grounded.

An example of detecting an abnormality in the motor drive system such as a ground fault or a power short to the motor 20 will be described below. FIG. 3 shows a motor drive system for comparing and discriminating a mathematical model indicating that the motor 20 is in a normal state and an actual motor (when receiving a rightward rotation command) and detecting an abnormality (here, a ground fault) of the motor 20. Is a transfer function of a failure detection circuit 310 that detects an abnormality in the circuit.

In FIG. 3, reference numeral 311 denotes a mathematical model of the motor 20, and 312 denotes a state where the actual motor is grounded.
Reference numerals 313A and 313B denote adders (subtractors), which are _KT · ω and _KT * · ω with respect to the motor supply voltage Vb · D, respectively.
'Is added to the motor input side. 31
Reference numeral 4 also denotes an adder (subtractor), which indicates that the adder 314 outputs a difference e between a theoretical current value ia flowing through the mathematical model of the motor 20 and an actual current im flowing through the motor. Note that the mathematical model of the motor 20 is configured inside the CPU constituting the control circuit, and parameters indicating its electrical characteristics are stored in the memory 33. FIG.
Where L * is the motor inductance of the mathematical model,
R * is the motor internal resistance of the mathematical model, R is the actual motor internal resistance, L is the actual motor inductance, Vb is the supply voltage, D is the duty ratio for driving the FET, s is the Laplace operator, and Rf is the FET 1-4. , Rt represents the impedance at the ground fault location.

In the mathematical model of the motor 20, the theoretical current value ia flowing through the current detecting resistor Ra of the motor drive circuit 37 is expressed by the following equation (1).

[0018]

(Equation 1) When the motor 20 is grounded, for example, when the cold side of the motor 20 is grounded, the current im flowing through the current detection resistor Ra of the motor drive circuit 37 is expressed by the following equation (2).

[0019]

(Equation 2) When the motor 20 is grounded, the current flowing through the current detection resistor Ra is im and the current in the mathematical model of the motor 20 is ia.
When the absolute value e of the difference between the current ia and the current im is larger than a predetermined value α, it can be determined that the motor 20 has a ground fault.

[0020]

| Ia−im | = | e |> α In the detection based on the above equation, the motor angular velocity ω is detected because the motor back electromotive force _KT · ω is added to the input side of the motor drive circuit 37. It is necessary to provide an angular velocity sensor or an angular velocity detection circuit to calculate the motor back electromotive force _KT · ω. However, this is not preferable because it not only complicates the configuration but also increases the cost. Therefore, in the present invention, the ground fault of the motor 20 is detected by eliminating the influence of the motor back electromotive force _KT · ω by the method described below.

If the above equation 2 is simplified and rewritten, the current im flowing through the current detection resistor Ra when the motor 20 is grounded can be expressed by the following equation 4. Here, Le
Represents the inductance of the grounded motor, and Re represents the internal resistance of the grounded motor.

[0022]

[Number 4] im = Vb · D / (Le · s + Re) -K T ·
ω / (Le · s + Re) FIG. 4 is rewritten so that the determination circuit of FIG. 3 can be easily processed.
It is shown by a transfer function. In FIG. 4, reference numerals 315 and 316 denote high-pass filters, and the adder 314 outputs a difference e between the current flowing through the mathematical model of the motor 20 and the actual current flowing through the motor. im ′ is the value after passing the actual motor detection current im through the high-pass filter 316, and ia ′ is the value after passing the current ia of the mathematical model of the motor through the high-pass filter 315. Is the difference between the detected current im ′ of the mathematical model and the current ia ′ of the mathematical model, and the detected current e = (im′−ia ′) is expressed by the following Expression 5.

[0023]

(Equation 5) First, the case where the actual motor 20 is normal will be considered. Since the motor angular velocity ω is a physical quantity appearing in a frequency band synchronized with the steering wheel, the frequency band is at most 5 Hz. On the other hand, the dynamic characteristics of the motor drive system are shown in FIG.
As shown in (1), it has a characteristic of a combination of a high-pass filter and a gain, and its cutoff frequency is usually about several hundred Hz. Therefore, it has a cut-off frequency of 5 Hz or more, and the cut-off frequency of the motor drive system (several hundred H
If the time constant of the high-pass filter is set so as to have a lower cut-off frequency than (approximately z), the detection current e = (im′−ia ′) expressed by the above equation (5) is equal to the supply voltage Vb ·
Regardless of the frequency range of D, the motor back electromotive force _KT · ω appearing on the input side of the motor is removed by the high-pass filter, so that the actual electrical characteristics of the motor substantially match the mathematical model and are expressed by Equation 5. Detection current e = (ia'-im ')
Is close to zero.

Next, the case where the actual motor 20 is grounded will be discussed. If the actual motor 20 has a ground fault, its transfer characteristics are as shown by block 312 in FIG.
At this time, in order to detect a change in the characteristics of the motor drive system, the supply voltage Vb · D including a component equal to or higher than the cutoff frequency of the high-pass filter is required. This is because all components below the cutoff frequency are cut by the high-pass filter. When the ground of the motor 20 is grounded, the current feedback loop causes vibration when the impedance of the ground fault is large, and causes self-steering when the impedance of the ground fault is small. In the present invention, a ground fault location is detected before self-steering. When the impedance at the ground fault location is large, the current i actually flowing to the motor and the detected current im substantially match, so
There is no self-steering.

The current i flowing through the motor 20 oscillates inside the current feedback loop at a frequency equal to the time constant of the feedback loop due to a change in the dynamic characteristics of the motor drive system. FIG. 5 shows this state. The current ie and the detection current i which actually flow through the motor 20 at the time of a ground fault are shown in FIG.
It can be seen that m is vibrating. Since this vibration component is included in the supply voltage Vb · D as a feedback signal, it also appears in the detection signal e = (im′−ia ′) in FIG. 4. Therefore, by detecting the vibration component included in the detection signal e, the motor Twenty ground faults can be detected.

Although the abnormality of the motor drive system can be detected by the above-described method, it cannot be determined from this that the failure of the motor 20, the failure of the motor drive circuit 37, or the failure of the motor current detection circuit 38. Therefore, an operation for detecting a failure of the motor current detection circuit 38 will be described below. Even if the motor current detection circuit 38 fails, the motor 20 and the motor drive circuit 3
This is because when the number 7 is normal, the motor 20 can be driven in an open loop. According to the present invention, the failure of the motor current detection circuit 38 is not directly detected, but the normal / abnormal of the motor drive circuit 37 is determined after detecting the abnormality in the current loop due to the parameter variation.
If 7 is normal, it is considered that the current detection circuit 38 has failed.

The failure of the motor drive circuit 37 according to the present invention is determined by determining the motor terminal voltage V when all the FETs 1 to 4 constituting the H bridge in the motor drive circuit 37 are turned off.
This is performed according to the value of m. That is, if the motor drive circuit 37 is normal, the motor terminal voltage Vm becomes 0 if all of the FETs 1 to 4 are turned off, and does not become 0 if it is abnormal. When the failure detection circuit 310 determines that the motor current detection circuit 38 has a failure, the switching signal SS is output and the contact of the switching circuit 320 is switched from a to b. As a result, the feedback system of the current control is made an open loop of the motor model 330, and the steering assist command value I from the steering assist command value calculator 32 is input to the motor model 330.

FIG. 6 shows an operation example of the present invention. First, abnormality of the motor drive system is detected by parameter fluctuation (step S1), and if it is normal, normal operation (contact a)
If the abnormality is abnormal, the motor current output is set to 0 as described above, the diagnosis is stored, and the fail lamp is turned on (step S2). Then, all the FETs 1 to 4 in the motor drive circuit 37 are turned off (step S).
3), the motor terminal voltage Vm is read (step S4).
It is determined whether the motor drive circuit 37 is normal or abnormal based on the read motor terminal voltage Vm (step S10). If the motor drive circuit 37 is abnormal, the motor current detection circuit 38 is regarded as a failure, but the motor drive system is determined to be normal ( Step S11), the contact of the switching section 320 is switched from a to b by the switching signal SS, and the current control system is switched from the feedback system to the open loop system (step S12). Thereby, the motor 20 can be driven in an open loop system (step S13). If it is determined in step S10 that the motor drive circuit 37 is abnormal, the abnormality of the motor drive system is determined (step S14), and the fail relay is turned off.
F (step S515).

[0029]

As described above, according to the control apparatus of the electric power steering apparatus of the present invention, the failure of the current detection circuit of the control unit is reliably detected, and when the failure is detected, the control system is opened from the feedback system. Switching to a loop system allows a motor current to flow to perform fail-safe stable control, and has the merit of being realized at low cost. In other words, it is conceivable that the other parts, especially the motor drive circuit, are normal even if the current detection circuit has failed. In this case, the motor can be driven in an open loop even if the motor current control cannot be performed. . Then, it is possible to escape from the steering difficult state by supplying a small amount of current to the motor at a low stationary speed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration example of a control unit according to the present invention.

FIG. 2 is a circuit block diagram for explaining abnormality detection of a motor drive system.

FIG. 3 is a circuit block diagram illustrating abnormality detection of a motor drive system.

FIG. 4 is a block diagram in which a determination circuit is rewritten.

FIG. 5 is a diagram illustrating oscillation of a detected current in an abnormal state of the motor drive system.

FIG. 6 is a flowchart showing an operation example of the present invention.

FIG. 7 is a block diagram showing an example of a conventional electric power steering device.

FIG. 8 is a block diagram showing a general internal configuration of a control unit.

FIG. 9 is a connection diagram illustrating an example of a motor drive circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steering handle 5 Pinion rack mechanism 10 Torque sensor 12 Vehicle speed sensor 20 Motor 30 Control unit 31 Phase compensator 37 Motor drive circuit 38 Motor current detection circuit 310 Failure detection circuit 320 Switching circuit

Claims (3)

[Claims] 1. A torque sensor for detecting a steering torque of a steering wheel, a motor for urging an auxiliary load on a steering shaft provided integrally with the steering wheel, and a feedback system for controlling the motor according to the magnitude of the steering torque. A control unit for driving the electric power steering apparatus, the control unit comprising: a current detection circuit for the motor; and a failure detection circuit for detecting a failure in the current detection circuit. A control device for an electric power steering device, wherein the feedback system is switched to an open loop system when a failure detection circuit detects a failure in the current detection circuit. 2. The method according to claim 1, wherein the failure of the current detection circuit is performed by determining a current control value for driving the motor, an output of the current detection circuit,
The control device for an electric power steering device according to claim 1, wherein the control is performed based on a terminal voltage of the motor. 3. The control device for an electric power steering device according to claim 1, wherein when switching to the open loop system, a duty ratio of pulse width modulation for driving the motor is controlled by a vehicle speed and a battery voltage.