Electrically driven power steering device

Description: TECHNICAL FIELD [Detailed description of the invention]

Industrial application field The present invention relates to an electric power steering device for automobiles and the like. Issues of prior art and invention FIG. 3 shows a conventional electric power steering device. This device consists of an electric motor (1) for assisting steering force, a motor drive circuit (3) composed of a bridge circuit consisting of four power transistors (2a) (2b) (2c) (2d), and a motor drive circuit (3). A key switch (5) and fail relay circuit (6) provided between the motor drive circuit (3) and the main power supply (battery) (4), and a torque sensor (7) for detecting the input torque of the steering shaft. ), The vehicle speed sensor (8) for detecting the vehicle speed, the detection torque of the torque sensor (7), the detection speed of the vehicle speed sensor (8), etc., to control the motor (1) and fail relay circuit (6). It is equipped with a control device (9). The motor (1) is PWM driven in order to save power. The control device (9) constantly monitors abnormalities in each part of the power steering device such as the torque sensor (7), the vehicle speed sensor (8), and the motor drive circuit (3). Then, the steering force assist amount is controlled to control the steering force assist amount of the motor (1) based on the detection torque of the torque sensor (7) and the detection speed of the vehicle speed sensor (8), and abnormalities of each part of the power steering device are detected. Faysafe control is performed to turn off the relay contact of the fail relay circuit (6) when it is detected. When a current is flowing through the motor (1) as shown by arrow A in Fig. 3, and a ground fault of the motor line (indicated by a broken line in the figure) occurs at point X, as shown by arrow B, A large current flows through the power transistor (2a), which may destroy the power transistor (2a) or cause a fire. Therefore, in order to prevent this, conventionally, the electric motor (1) is based on the voltage between the terminals of the current detection resistor (10) connected between the emitter of the power transistor (2c) (2b) and the ground. ) Is provided with a current detection circuit (11) that detects the current flowing through), and the ground fault of the motor line is detected by comparing this detected current with the current command value calculated by the control device (9). ing. However, the conventional abnormality detection method has a problem that it takes time to detect an abnormality in the motor drive circuit (3) such as a ground fault of the motor wire. That is, at the time of sudden steering, the rotation speed of the motor (1) increases, the counter electromotive force induced in the motor (1) also increases, and the current flowing through the motor (1) becomes smaller than the current command value. .. In the conventional abnormality detection method, it is necessary to take a long time for abnormality detection determination in order to prevent such a case from being determined as an abnormality. Further, when the power element is not energized, an abnormality cannot be detected. An object of the present invention is to solve the above-mentioned problems and to provide an electric power steering device capable of detecting an abnormality in a motor drive circuit in a short time regardless of whether the power element is energized or de-energized. Means to solve problems The electric power steering device according to the present invention includes an electric motor for assisting steering force, a motor drive circuit having two bridge-connected forward rotation power elements and two reverse rotation power elements, and an input torque of the steering shaft. Electric power steering equipped with a torque sensor that detects the voltage and a motor control means that controls the steering force assist amount by intermittently switching the motor and switching the polarity by the power element of the motor drive circuit based on the detection torque of the torque sensor. In the device, a first pull-up circuit that constantly applies a voltage to the first series connection point of one forward rotation power element and a reverse rotation power element, a first sample hold circuit that detects the voltage at the first series connection point, and the other A second pull-up circuit that constantly applies a voltage to the second series connection points of the forward rotation power element and a reverse rotation power element, a second sample hold circuit that detects the voltage at the second series connection point, and first and second sample holds. It is characterized by being provided with an abnormality detecting means for detecting an abnormality in a motor drive circuit based on the output of the circuit, and a means for prohibiting steering force assistance by an electric motor when an abnormality is detected by the abnormality detecting means. Is. Action Since the abnormality detecting means detects the abnormality of the motor drive circuit based on the voltage of the series connection point of the power element detected by the sample hold circuit, when the abnormality of the motor drive circuit such as the ground fault of the motor wire occurs. , That is immediately detected by the abnormality detecting means, and based on this abnormality detection, the steering force assist by the electric motor is prohibited. Further, since a voltage is constantly applied to the series connection point of the power element by the pull-up circuit, an abnormality is detected not only when the power element is energized but also when it is not energized. Example Hereinafter, examples of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 shows an overall schematic configuration of an electric power steering device. This device is a key switch (5) provided between an electric motor (1) for assisting steering force, a motor drive circuit (3), a motor drive circuit (3), and a main power supply (4). And the fail relay circuit (6), the torque sensor (7) that detects the input torque of the steering shaft, the vehicle speed sensor (8) that detects the vehicle speed, the detection torque of the torque sensor (7), and the vehicle speed sensor (8). It is equipped with a control device (9) that controls the motor (1) and the fail relay circuit (6) based on the detection speed and the like. Although not shown, the motor (1) is connected to an appropriate position such as an output shaft of a steering shaft or a steering gear via a speed reducer, a clutch, or the like. The torque sensor (7) comprises, for example, a potentiometer for detecting the twist of the torsion bar provided between the input shaft and the output shaft of the steering shaft. The motor drive circuit (3) is composed of a bridge circuit including two forward power transistors (2a) and (2b) and two reverse power transistors (2c) and (2d). The forward power transistor (2a) and the reverse power transistor (2d) are connected in series, and the reverse power transistor (2c) and the forward power transistor (2b) are connected in series. They are connected in parallel. Then, the connection point (first connection point) P1 between the forward rotation power transistor (2a) and the reverse rotation power transistor (2d), and the connection point between the reverse rotation power transistor (2c) and the forward rotation power transistor (2b) ( Second connection point) The electric motor (1) is connected to P2. The collector of the forward power transistor (2a) and the collector of the reverse power transistor (2c) are connected to the main power supply (4) via a fail relay circuit (6) and a key switch (5). The emitter of the forward power transistor (2b) and the emitter of the reverse power transistor (2d) are grounded via a current detection resistor (10). The bases of the power transistors (2a), (2b), (2c), and (2d) are connected to the control device (9), respectively. The current detection circuit (11) detects the current flowing through the motor (1) based on the terminal voltage of the current detection resistor (10). This detected current is sent to the control device (9). The first sample hold circuit (21) is connected to the first connection point P1, and the output e of the sample hold circuit (21) is sent to the control device (9). A second sample hold circuit (22) is connected to the second connection point P2, and the output f of this sample hold circuit (22) is sent to the control device (9). Each sample hold circuit (21) (22) is composed of, for example, a switch (23) controlled by a control device (9), a hold capacitor (24), and a voltage holer (25). Further, a first pull-up circuit (26) is connected to the first connection point P1, and a second pull-up circuit (27) is connected to the second connection point P2. The pull-up circuits (26) and (27) constantly apply a voltage to the connection points P1 and P2 via a resistor (28) and a diode (29) having a resistance value sufficiently larger than the armature resistance of the motor (1). It is a thing. The control device (9) controls the steering force assist amount for calculating the steering force assist amount of the motor (1) based on the detection torque of the torque sensor (7) and the detection speed of the vehicle speed sensor (8), and also controls the torque sensor. A microcomputer that performs fail-safe control that turns off the relay contact of the fail relay circuit (6) when an abnormality is detected in each part of the power steering device such as (7), vehicle speed sensor (8), and motor drive circuit (3). It includes a PWM modulation circuit (not shown) that obtains a PWM output according to the steering force assist amount obtained by the microcomputer (not shown). The PWM modulation circuit includes a triangular wave generation circuit and a comparison circuit that compares the output of the triangular wave generation circuit with the steering force auxiliary amount command signal and outputs a PWM signal for motor control. The motor control PWM signal output from the PWM modulation circuit is sent to the base of each forward rotation power transistor (2a) (2b) when the motor (1) is rotated forward. Therefore, in this case, each time the forward power transistors (2a) and (2b) are turned on in response to the PWM signal, the power supply (4), key switch (5), fail relay circuit (6), and forward power Current flows in the order of transistor (2a), motor (1), forward power transistor (2b), resistor (10), and power supply (4), and motor (1) is driven in forward rotation. When the motor (1) is reversed, the motor control PWM signal is sent to the base of each reverse power transistor (2c) (2d). Therefore, in this case, each time the reverse power transistors (2c) (2d) are turned on in response to the PWM signal, the power supply (4), key switch (5), fail relay circuit (6), and reverse power A current flows in the order of the transistor (2c), the motor (1), the reverse power transistor (2d), the resistor (10), and the power supply (4), and the motor (1) is driven in reverse. A sample instruction signal b is sent from the control circuit (9) to the switch (23) of each sample hold circuit (21) (22). As shown in FIG. 2, the sample instruction signal b is a pulse signal generated at each peak of the output a of the triangular wave generation circuit of the PWM modulation circuit and composed of pulses having a predetermined width centered on the peak time. The width of this pulse b is set to be equal to or less than the minimum pulse width of the PWM signal for motor control output from the PWM modulation circuit. The control device (9) detects an abnormality in the motor drive circuit (3) based on the outputs e and f of the sample hold circuits (21) and (22) that detect the voltages at the connection points P1 and P2. Further, a voltage is constantly applied to the connection points P1 and P2 by the pull-up circuits (26) and (27). Therefore, as described below, the abnormality of the motor drive circuit (3) can be detected in a short time regardless of whether the power transistors (2a), (2b), (2c), and (2d) are energized or de-energized. .. That is, when the power is off, as shown in FIG. 2A, voltages (signals c and d) are applied to the first connection point P1 and the second connection point P2 by the pull-up circuits (26) and (27), respectively. Therefore, the outputs e and f of the sample hold circuits (21) and (22) are both held at the H level. In this state, if a ground fault of the motor line occurs at the point X at the time point T1, the voltage c of the first connection point P1 becomes zero, so that the capacitor (24) of the first sample hold circuit (21) at the time of sampling The charge is discharged. Therefore, the output e of the first sample hold circuit (21) becomes the L level from the time point T1 to the time point T2 after a lapse of a minute time. When a ground fault occurs at point X at time point T1, a voltage corresponding to the voltage drop of the armature resistance of the motor (1) is generated at the second connection point P2, but the resistance (28) of the pull-up circuit (27) Since the resistance value is sufficiently larger than the resistance value of the armature resistance, the voltage d at the second connection point P2 also becomes almost zero, and the output f of the second sample hold circuit (22) also becomes zero at the time point T2. When the motor (1) is driven in the forward rotation, as shown in Fig. 2 (b), a pulsed voltage c corresponding to the PWM signal for motor control appears at the first connection point P1, and the first sample. The output e of the hold circuit (21) is held at the H level. On the other hand, the voltage d at the second connection point P2 becomes almost zero, and the output f of the second sample hold circuit (22) is held at the L level. In this state, when a ground fault of the motor line occurs at the point X at the time point T1, the voltage c of the first connection point P1 becomes zero, and the first sample is set to the time point T2 after a lapse of a minute time from the time point T1 as described above. The output e of the hold circuit (21) becomes L level. Further, the voltage d at the second connection point P2 remains almost zero, and the output f of the second sample hold circuit (22) also remains at the L level. When the motor (1) is driven in reverse, as shown in Fig. 2 (c), a pulsed voltage d corresponding to the PWM signal for motor control appears at the second connection point P2, and the second sample is held. The output f of the circuit (22) is held at the H level. On the other hand, the voltage c at the first connection point P1 becomes almost zero, and the output e of the first sample hold circuit (21) is held at the L level. In this state, if a ground fault of the motor wire occurs at the point Y at the time point T1, the voltage d at the second connection point P2 becomes zero, and similarly to the above, the second sample at the time point T2 after a minute time has passed from the time point T1. The output f of the hold circuit (21) becomes L level. Further, the voltage c at the first connection point P1 remains almost zero, and the output f of the first sample hold circuit (21) also remains at the L level. This can be summarized as follows. That is, in the non-energized state, the outputs e and f of the two sample hold circuits (21) and (22) are both H level under normal conditions, but both outputs e and f are at L level when an abnormality occurs. .. In the normal rotation drive state, the output e of the first sample hold circuit (21) is H level and the output f of the second sample hold circuit (22) is L level under normal conditions, but when an abnormality occurs, the outputs e and f Both become L level. In the reverse drive state, the output e of the first sample hold circuit (21) is at the L level and the output f of the second sample hold circuit (22) is at the H level under normal conditions. Both are at L level. From these, the logical sum (OR) of the outputs e and f of the two sample hold circuits (21) and (22) is 1 under normal conditions in any of the non-energized, forward rotation drive, and reverse rotation drive states, which is abnormal. When is generated, these logical sums become 0. Therefore, in the control device (9), only by detecting that the state in which the logical sum of the outputs e and f of the two sample hold circuits (21) and (22) becomes 0 lasts for a predetermined time, it is de-energized and energized. Abnormality can be detected regardless of. When a ground fault occurs at point Y in the normal rotation drive state, the voltage c at the first connection point P1 becomes zero because the motor (1) is located between the first connection point P1 and the ground fault occurrence point Y. Therefore, the logical sum of the outputs e and f of the two sample hold circuits (21) and (22) does not become 0. However, since the current does not flow through the current detection resistor (10) due to the ground fault, an abnormality is found by comparing the detected current of the current detection circuit (11) with the current command value calculated by the control device (9) as in the conventional case. Can be detected. In this case, since the current flows through the armature resistors of the power transistor (2a) and the motor (1), the current flowing through the power transistor (2a) does not become abnormally large, and even if it takes time to detect the abnormality. , The possibility of destruction of the power transistor (2a) due to overcurrent is small. Then, even if the same abnormality detection as in the conventional case is not performed due to the current not flowing through the current detection resistor (10), the next time the current is de-energized or the reverse drive state is entered, FIG. 2 (a) Anomalies can be detected as described in and (b). The same applies when a ground fault occurs at point X in the reverse drive state. Effect of the invention According to the electric power steering device of the present invention, as described above, it is possible to detect an abnormality in the motor drive circuit such as a ground fault of the motor line in a short time regardless of whether the power element is energized or de-energized. In response to such an abnormality, the steering force assist by the electric motor can be quickly prohibited. Therefore, even if an abnormality of the motor drive circuit such as a ground fault of the motor wire occurs, it is possible to prevent the motor drive element from being destroyed or a fire from occurring.

[Simple explanation of drawings]

FIG. 1 is a schematic electric block diagram of the entire electric power steering device showing an embodiment of the present invention, FIG. 2 is a time chart showing the operation of a sample hold circuit and a pull-up circuit, and FIG. 3 is electricity showing a conventional example. It is a block diagram. (1) ...... Electric motor, (3) ...... Motor drive circuit, (2a) (2b) (2c) (2d) ...... Power transistor, (6). ..... Fail relay circuit, (7) ...... Torque sensor, (9) ...... Control device, (21) (22) ...... Sample hold circuit, ( 26) (27) ...... Pull-up circuit.