JP2007295753A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering device that is capable of detecting a DC motor current while including even its direction without increasing a circuit scale while reducing a power loss of an H-bridge circuit. <P>SOLUTION: The electric power steering device drives a pair of vertical transistors of at right or left arms of the H-bridge circuit 31 which drives a DC motor 2 assisting a steering force of an operator in a complementary PWM manner, and also, drives so that the High state of either of the right and left arms and the Low state of the other of the right and left arms have a relation reverse to each other. A shunt resistor 331 is inserted into a GND line of the H-bridge circuit 31 so as to detect a current. A motor current is calculated by providing a subtraction circuit for subtracting a current detection value in the Low state of the arm from a current detection value in the High state of the arm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electric power steering device that assists a steering force of a driver of an automobile with a DC motor.

2. Description of the Related Art In an automobile, there is an electric power steering device that assists a driver's steering force on a steering wheel with a motor. Some motors use DC motors. Hereinafter, the electric power steering apparatus referred to in the present invention refers to the one using this DC motor.
An electric power steering apparatus for an automobile using a DC motor disclosed in Patent Document 1 PWM-drives one of the upper FETs of four FETs constituting an H bridge circuit and supplies power to the H bridge circuit A method of detecting a motor current by inserting a shunt resistor in a line has been proposed.
In addition, the electric power steering apparatus for an automobile using a DC motor disclosed in Patent Document 2 PWM-drives one of the upper FETs of the four FETs constituting the H bridge circuit and uses the lower two A method has been proposed in which a shunt resistor is inserted between the sources of two FETs and GND, and the motor current is obtained from the voltage drop.
That is, in the prior art, a DC motor is driven by an H bridge circuit, one of the four FETs constituting the H bridge circuit is PWM driven, and a shunt is provided at a predetermined location of the H bridge circuit. A resistor is inserted to detect the current flowing through the motor from the value of the voltage drop.

  As can be seen from FIG. 2 of Patent Document 1, since the motor current passes through the parasitic diode of the other FET paired with the FET while the PWM driven FET is OFF, this diode A large power loss is caused by the voltage drop due to. In addition, the motor current is obtained from a single shunt resistor inserted in the power supply line of the H-bridge circuit. In this configuration, the absolute value of the motor current can be detected, but the direction of the motor current cannot be detected. There is.

As is clear from FIG. 3 of Patent Document 2, the direction of the motor current can be detected by inserting shunt resistors R1 and R2 between the sources of the two lower FETs and GND, as is clear from FIG. However, this configuration has a problem that the circuit scale increases because two shunt resistors and a current detection circuit associated therewith are required.
JP 2002-359919 A JP-A-8-150946

  In the conventional electric power steering apparatus, since a motor current passes through a parasitic diode, a large power loss is caused by a voltage drop caused by the diode. Further, although the absolute value of the motor current can be detected, the direction of the motor current cannot be detected. In addition, there is a problem that the circuit scale increases because two shunt resistors and a current detection circuit associated therewith are required.

  The present invention solves the problems of such a conventional DC motor type electric power steering device, and reduces the power loss of the H-bridge circuit, and the direction of the current of the DC motor without increasing the circuit scale. An object of the present invention is to provide an electric power steering device that can be detected.

An electric power steering apparatus according to the present invention is mounted on an automobile and includes a first arm and a second arm, each of which is formed by connecting two switching elements in series. Are connected to each other in an H-bridge shape, both ends are connected to a battery power source mounted on the vehicle, a connection point between the two switching elements of the first arm, and a second arm A motor drive circuit configured by connecting a DC motor for assisting the steering force of the driver of the automobile between the connection points of the two switching elements;
A switching element driving circuit for driving the two switching elements connected in series to each other in a complementary PWM manner, and driving the first arm transistor and the second arm transistor connected to the same electrode of the battery in a complementary manner to each other. ,
A shunt resistor inserted in series with the battery power supply,
A motor current detection circuit for detecting a voltage across the shunt resistor;
A motor current calculation circuit that controls the polarity of the detection voltage in synchronization with the operation timing of the switching element driving circuit is provided.

The pair of upper and lower transistors of the left and right arms of the H bridge circuit that drives the DC motor that assists the steering force is driven in a complementary PWM manner, and the High and Low states of the left and right arms are driven in opposite relations. By doing so, current is prevented from flowing through the parasitic diode of the transistor. Thereby, the power loss of the H bridge circuit can be reduced.
Further, a current is detected by inserting a shunt resistor in the GND line of the H bridge circuit, and the motor current is calculated by subtracting the current detection value in the low state from the current detection value in the high state of the arm. Therefore, the current of the DC motor including the direction can be detected with one shunt resistor.

  FIG. 1 is a diagram showing a configuration of an electric power steering apparatus according to Embodiment 1 of the present invention. A current torque signal is detected from a torque sensor 1 that is attached to a steering wheel shaft of an automobile (not shown) and detects a driver's steering force, and is input to the control device 3. A DC motor 2 that assists the driver's steering force is attached to a handle shaft (not shown). The control device 3 includes an H bridge circuit 31 that receives power from the vehicle battery 4 and drives the DC motor 2, and four FETs (FET 1, FET 2, FET 3, FET 4) that constitute the H bridge circuit 31. An FET driver 32 to be driven, a current detection circuit 33 for detecting the source current of the FET of the H bridge circuit 31 (total current of four FETs), and a CPU 34 for controlling the current of the DC motor 2 are provided. The H bridge circuit 31 is configured by connecting a first arm and a second arm, each of which is formed by connecting two switching elements in series, with the same polarity sides of both ends of each arm connected to each other. It is. Both ends are connected to a battery power source mounted on the vehicle, and the steering is performed between the connection point of the two switching elements of the first arm and the connection point of the two switching elements of the second arm. A direct current motor that performs force assist is connected. The H bridge circuit is also called a motor drive circuit.

Next, the operation of the complementary PWM drive of the H bridge circuit 31 will be described. In order to help understanding, a comparative explanation of the operation of a conventional H-bridge circuit (of Patent Document 1) will be described later in the present embodiment.
The PWM drive operation of the H bridge circuit 31 of FIG. 1 will be described first with reference to FIG.
FIG. 2 is a timing chart showing ON and OFF operations of each FET within one cycle of PWM driving. The PWM cycle is operated when one cycle (TPWM) is around 20 μs. In the H-bridge circuit 31 of the present invention, each FET performs so-called complementary PWM driving. That is, in the complementary PWM drive, the ON and OFF states of the upper and lower FETs constituting the left and right arms of the H bridge circuit are opposite to each other. That is, the lower FET is driven to be OFF when the upper FET is ON, and the lower FET is driven to be ON when the upper FET is OFF.
In addition, the transistors of the first arm and the transistors of the second arm connected to the same electrode of the battery are also driven in a complementary manner.
Here, the ON / OFF timing of the PWM drive pulse is changed as shown in FIG. 2 by a timer value 101 constituted by an up / down counter, and a duty ratio setting value 102 for setting the duty ratio (which varies depending on the control state). ) To obtain.

  FIG. 2 shows a case where the duty ratio setting value 102 is set so that the duty ratio of the PWM pulse is just 50%. The timer value 101 and the set value 102 are compared with each other to obtain ON / OFF timing of the pulse, and the FET1, FET2, FET3, and FET4 are driven as shown in FIG. That is, the upper FET 1 of the left arm of the H-bridge circuit 31 is turned on when the timer value 101 is larger than the duty ratio set value 102 and turned off when the timer value 101 is less than the set value. The upper FET 2 that is paired with the FET 1 is driven in a state in which ON and OFF are opposite to those of the FET 1. Contrary to the left arm, the right arm is turned off when the timer value is greater than or equal to the set value and turned on when the timer value is less than or equal to the set value. The lower FET 4 that is paired with the FET 4 is driven so that ON and OFF are opposite to the FET 2. In this way, in FIG. 2, the left and right arms are both driven at a duty ratio of 50%, and the same average voltage is applied to the M + terminal and M-terminal of the motor. There will be no voltage difference. Since the time constant of the motor circuit has a sufficiently long time constant (L / R is large) as compared with the PWM drive cycle, no current flows to the motor as a result.

  Next, FIG. 3 shows a case where the duty ratio of the PWM pulse is set to be smaller than 50%. In FIG. 3, the FET 1 ON period (shown as section M <b> 1) shows a case where α is reduced by less than 50% as shown in FIG. 3. The relationship between ON and OFF of FET1, FET2, FET3, and FET4 at this time is the same as in FIG. With this set value, the time ratio at which the left arm of the H-bridge circuit is High is α lower than 50%, and the time ratio at which the right arm is High is α higher than 50%. Therefore, the average voltage applied to the M + terminal of the motor is lower than the average voltage applied to the M− terminal, and current flows from the M− terminal to the M + terminal.

4 and 5 show current paths through which current flows in the state of FIG. In the figure, for the sake of understanding, instead of FET circuit symbols, ON and OFF operations are schematically represented using switch contact symbols. A diode inserted in parallel with the contact of the switch represents a parasitic diode of the FET.
FIG. 4 shows a current path in the section where FET1 and FET4 indicated by “M0” in FIG. 3 are OFF and FET2 and FET3 are ON. The motor current is output from the battery plus FET2, motor, It flows through the FET 3 through the shunt resistor 331 downstream and returns to the negative battery. Now, since the time ratio is longer in the section M0 than in the section M1 and the time constant of the motor circuit is long as described above, the current in the same direction as that in the section M0 remains in the motor portion even when the section M1 is reached. Keep flowing. FIG. 5 shows the current path of the section in which FET1 and FET4 indicated by “M1” in FIG. 3 are ON, and the motor current flows from the negative of the battery and flows upward through the shunt resistance, It flows through a path that returns to the battery plus through the motor and FET1.

  FIG. 6 shows a case where the duty ratio of the PWM pulse is set to be higher than 50% contrary to FIG. In this setting, the ON period of FET1 shows a case where α is increased by more than 50% as shown in FIG. The relationship between ON and OFF of FET1, FET2, FET3, and FET4 at this time is the same as in FIG. With this set value, the time ratio at which the left arm of the H-bridge circuit is High is α higher than 50%, and the time ratio at which the right arm is High is α lower than 50%. Accordingly, the average voltage applied to the M + terminal of the motor is higher than the average voltage applied to the M− terminal, and current flows through the motor from the M + terminal to the M− terminal on average.

The current path in the H-bridge circuit when PWM driving is performed as described above will be described with reference to FIGS. In the figure, instead of the circuit symbol of the FET, the ON / OFF operation of the FET is schematically represented by using the contact symbol of the switch.
7 and 8 are diagrams showing in-circuit current paths when the H-bridge circuit 31 is driven at the duty ratio shown in FIG. FIG. 8 shows a current path in the section where FET1 and FET4 indicated by “M1” in FIG. 6 are ON, and the motor current goes out from the battery positive and goes through the FET1, motor and FET4 to the battery negative. And return path.
Now, since the time ratio in the section M1 is longer than that in the section M0 and the time constant of the motor circuit is long as described above, the current in the same direction as that in the section M1 remains in the motor portion even when the section M0 is reached. Keep flowing. That is, FIG. 7 shows the current path of the section in which FET2 and FET3 indicated by “M0” in FIG. 6 are ON, and the motor current flows from the battery minus and flows to the upstream of the shunt resistor. It flows through a path that returns to the battery plus via FET3, motor, and FET2.

  As described above, in this PWM driving method, since any pair of two pairs of FETs on a diagonal line is always in an ON state, the motor current does not flow through the parasitic diode of the FET. There is no power loss due to the voltage drop of the parasitic diode. Needless to say, the voltage drop in the FET is very small compared to the diode. In this PWM driving method, the motor current always flows through the shunt resistor 331, and the flowing direction depends on both the direction of the motor current and which FET set is operating. That is, even if the direction of the motor current is the same, the directions are opposite to each other depending on the ON and OFF states of the FET. Therefore, the voltage difference between the both ends of the shunt resistor is detected by the differential amplifier, and the output is processed by the CPU 34 as described below, whereby the motor current including the direction can be obtained. In FIG. 1, the shunt resistor 331 is inserted at a position connected to the ground, but may be a position where the shunt resistor 331 is inserted in series with the battery 4 if necessary considerations such as using an insulation type differential amplifier.

Hereinafter, a method for obtaining a motor current including a direction will be described with reference to FIG. 1, a signal chart explanatory diagram, and FIG. Note that FIG. 9 shows a state of current detection in a state where the M1 section is longer, corresponding to FIGS. 6 to 8 as an example. The current detection circuit 33 includes a shunt resistor 331 and a differential amplifier 332. As shown in FIG. 1, the differential amplifier 332 amplifies the voltage obtained by subtracting the lower end potential from the upper end potential of the shunt resistor 331, that is, the voltage difference between both ends of the shunt resistor. The output of the differential amplifier 332 becomes positive when flowing toward the downstream of 331 (downstream toward the drawing, ie, the direction of flowing to the negative terminal of the battery), and flows upstream (upstream toward the drawing, ie, from the negative terminal of the battery). Negative) when flowing in the direction). That is, 200 in FIG. 9 indicates the drive timing of the element for explanation, and 201 indicates the motor current (gradual increase state as an example). Reference numeral 202 denotes a voltage between both ends of the shunt resistor 331 and an output of the differential amplifier 332, and indicates that the polarity is inverted like 202a and 202b depending on the drive timing of the element. The CPU 34 includes a timer 343 configured by an up / down counter, a sample hold circuit 346 that operates in synchronization with the timer, and an A / D converter 347. The CPU 34 has a maximum output waveform peak. The output of the differential amplifier 332 is sampled and held at the timing of the value (center of the interval M1, indicated as X in FIG. 9) and the timing of the minimum value at the valley (center of the interval M0, indicated as Z in FIG. 9). To do. The sampled and held value is indicated by 203 in FIG. The CPU further performs A / D conversion on the sampled and held value, and subtracts the A / D conversion result (Ip) obtained at the peak timing from the A / D conversion result (Ib) obtained at the valley timing (ie, Calculate the motor current (IM) by adding the same polarity. The calculated result is shown at 204 in FIG.
Here, the peak timing of the timer 343 is the timing at the center of the ON section of the FET1 and FET4 indicated by “M1” in FIGS. 2, 3, and 6, and the valley timing is “ This is the timing at the center of the OFF section of FET1 (M0) (the end of the figure in each figure).

First, the case where the duty set value shown in FIG. 3 is smaller than 50% will be described. In this case, the motor current flows from the M− terminal to the M + terminal of the motor as shown in FIGS. 4 and 5. Now, let the absolute value of the motor current be Im. As shown in FIG. 4, current flows downstream of the shunt resistor 331 at the valley timing in FIG. 3, so that a + Im signal is obtained at the output of the differential amplifier 332. As shown in FIG. 5, since a current flows upstream of the shunt resistor 331, a signal of −Im is obtained at the output of the differential amplifier 332. Therefore, the detection current value (Im) of Im − (− Im) = 2Im is obtained by subtracting −Im obtained at the peak timing from + Im obtained at the valley timing. Next, the case where the duty set value shown in FIG. 6 is larger than 50% will be described. In this case, the motor current flows from the M + terminal to the M− terminal of the motor as shown in FIGS. At the valley timing in FIG. 6, current flows upstream of the shunt resistor 331 as shown in FIG. 7, so that a signal of −Im is obtained at the output of the differential amplifier 332, and at the peak timing in FIG. As shown in FIG. 4, since a current flows downstream of the shunt resistor 331, a signal of + Im is obtained at the output of the differential amplifier 332. Accordingly, the detection current value (IM) of −Im − (+ Im) = − 2Im is obtained by subtracting + Im obtained at the timing of the peak from −Im obtained at the timing of the valley.
As described above, according to this method, a positive current value is obtained when the motor current flows from the M− terminal to the M + terminal of the motor, and the motor current flows from the M + terminal to the M− terminal. Get a negative current value. That is, the motor current value including the direction can be obtained.
The configuration of the portion where the CPU 34 performs the above calculation on the output signal of the differential amplifier is referred to as a motor current calculation circuit in the present invention.

  Hereinafter, control of the motor current will be described with reference to FIGS. 1, 10, and 11. FIG. FIG. 10 is a characteristic diagram of the torque sensor 1. As shown in FIG. 10, the torque sensor 1 outputs a signal proportional to the steering torque of the vehicle driver. When the driver steers the steering wheel clockwise, a positive signal is output, and when the driver steers counterclockwise, a negative signal is output. In response to the output of the torque sensor 1, the target current setting block 341 of the CPU 34 sets a target current value (IT) having the characteristics shown in FIG. 11, that is, a target value of current to be supplied to the DC motor 2. When the output of the torque sensor 1 is positive, a positive target current value (IT) is set. When the output of the torque sensor 1 is negative, a negative target current value (IT) is set. Next, the difference between the target current value (IT) and the detected current value (Im) is obtained, and the duty ratio calculation block 342 makes an H bridge circuit in a direction in which the difference becomes zero, for example, by the method of proportional integral control. The duty ratio of PWM to drive 31 is calculated. As described above, the comparator 344 at the next stage compares the value of the timer 343 composed of an up / down counter with the duty ratio setting value calculated by the duty ratio calculation block 342 and compares the pulse ON / OFF timing. appear. The next-stage pulse generator 345 generates drive pulses for the four FETs constituting the H-bridge circuit 31 in response to the ON and OFF timings. The H bridge circuit 31 is driven by this drive pulse via the FET driver 32. As described above, the DC motor 2 is supplied with a predetermined current according to the steering force of the driver, and the auxiliary force is controlled.

  In order to help understand how the operation of the H-bridge circuit of the present application described above differs from the conventional one, a conventional device, for example, the load driving means 8 shown in FIG. For the sake of convenience, the operation of the H-bridge circuit will be described with an explanation method similar to that of the present application. 12, 13, 14, 15, and 16 show the PWM drive operation of the H-bridge circuit of the conventional device, and FIG. 12 shows the ON and OFF timings in one cycle of the PWM drive. . In many cases, this type of PWM drive is chopped at a frequency of 20 Khz, and therefore one cycle (TPWM) is 20 μs, and the FET is turned on in a predetermined period (T1), and the remaining predetermined period. Turned off at (T2). 13 and 14 show the case where the upper FET 1 of the left arm is PWM-driven, and FIGS. 15 and 16 show the case where the upper FET 2 of the right arm is PWM-driven. In FIG. 13, FIG. 14, FIG. 15, and FIG. 16, the ON / OFF operation of the FET is schematically shown using the contact diagrams of the switch. A diode inserted in parallel with the contact of the switch represents a parasitic diode of the FET.

  First, the operation when the upper FET 1 of the left arm is PWM driven will be described with reference to FIGS. In this case, FET2 and FET3 are always in an OFF state, and FET4 is always in an ON state. 13 shows a current path in the ON period (T1) shown in FIG. 11, and FIG. 14 shows a current path in the OFF period (T2). During the period when the FET 1 is ON, the motor current flows from the M + terminal to the M− terminal of the motor according to the path indicated by the broken line and the arrow in FIG. Next, even if FET1 is turned off during the period T2, the motor circuit time constant is sufficiently longer than the PWM cycle (TPWM), so that the current in the same direction continues to flow through the motor. That is, the motor current flows through a path indicated by a broken line and an arrow in FIG. As shown in the figure, the motor current circulates through the parasitic diode of the FET 3 in the interval T2. Next, the operation when the upper FET 2 in the right arm is PWM driven will be described with reference to FIGS. In this case, FET1 and FET4 are always in an OFF state, and FET3 is always in an ON state. 15 shows a current path in the ON period (T1) shown in FIG. 11, and FIG. 16 shows a current path in the OFF period (T2). During the period when the FET 2 is ON, the motor current flows from the M− terminal to the M + terminal of the motor according to the path indicated by the broken line and the arrow in FIG. Next, even if the FET 2 is turned off in the period T2, the motor circuit time constant is sufficiently longer than the PWM cycle (Tpwm), so that current in the same direction continues to flow through the motor. That is, the motor current flows through a path indicated by a broken line and an arrow in FIG. As shown in the figure, the motor current flows through the parasitic diode of the FET 4 and circulates in the section T2.

  As described above, in the conventional system, the motor current circulates through the FET parasitic diode during the PWM drive OFF period (T2). The voltage drop of the parasitic diode is usually about 0.7V. Therefore, a large power loss is caused by the motor current flowing. Further, as indicated by broken lines and arrows in FIGS. 13, 14, 15, and 16, in this driving method, the current flowing through the power supply line or GND line of the H-bridge circuit is always independent of the direction of the motor current. Since it flows in a certain direction, it is not possible to detect the direction of the motor current by inserting shunt resistors in these lines.

Embodiment 2. FIG.
Hereinafter, the electric power steering apparatus according to the second embodiment provided with a detection circuit for an abnormal state of the apparatus and a treatment circuit for an abnormality will be described with reference to the configuration diagram of FIG.
As described in the first embodiment, while the device is operating normally, the positive / negative polarity of the output of the torque sensor 1 and the positive / negative polarity of the output of the subtracting circuit are the same except for the control transient. Should be. The case where the polarities do not match, for example, indicates that the motor is generating assist force in the opposite left direction even though the driver is steering in the right direction. It is extremely dangerous to continue for a long time beyond the extremely short time.
The monitoring circuit 35 compares the output of the torque sensor 1 with the output of the subtraction circuit and determines that the device is abnormal when the positive and negative polarities of both signals continue for a predetermined time, and the DC motor Shut off the power to 2. FIG. 17 shows a configuration in the case where the energization path to the DC motor 2 is interrupted by a mechanical relay. As this interruption method, for example, a method of interrupting the FET driver or a power source of the H bridge circuit 31 is used. A blocking method or the like may be used.

Embodiment 3 FIG.
The first embodiment has been described using an example in which sample hold is performed by a digital circuit as a current calculation method. Of course, this type of circuit can also be constructed in analog. Hereinafter, the method for obtaining the motor current including the direction will be described with reference to the signal chart FIG. 9 described in the first embodiment. Note that FIG. 9 shows a state of current detection in a state where the M1 section is longer, corresponding to FIGS. 6 to 8 as an example. The current detection circuit 33 includes a shunt resistor 331 and a differential amplifier 332. Since the differential amplifier 332 amplifies the voltage obtained by subtracting the lower end potential from the upper end potential of the shunt resistor 331, that is, the voltage difference between both ends of the shunt resistor, the motor current is downstream of the shunt resistor 331 (toward the drawing). When the output of the differential amplifier 332 becomes positive when flowing toward the downstream, that is, the direction flowing to the negative terminal of the battery, and when flowing toward the upstream (upstream toward the figure, that is, the direction flowing out from the negative terminal of the battery). Negative. That is, 200 in FIG. 9 indicates the drive timing of the element for explanation, and 201 indicates the motor current (gradual increase state as an example). Reference numeral 202 denotes the voltage across the shunt resistor 331 and the output of the differential amplifier 332, and the polarity is inverted depending on the drive timing of the element.
Since the CPU 34 grasps the section M0 and the section M1, the output of the differential amplifier is directly output to the input terminal of the duty ratio calculation 342 as the detection signal IM during the section M0 and is differential during the section M1. The polarity of the output of the amplifier 332 is inverted through an inverting amplifier (not shown), and then output to the input terminal of the duty ratio calculation unit 342 as the detection signal IM.

  The electric power steering apparatus according to the present invention is not limited to steering control of an automobile, and can be used for a general servo motor system such as a machine tool or a robot system.

1 is a block diagram showing an electric power steering apparatus according to Embodiment 1 of the present invention. 2 is a timing chart showing an operation with a duty ratio of 50% of the electric power steering apparatus of FIG. 1. It is a timing chart which shows operation | movement of the electric power steering apparatus of FIG. FIG. 4 is a diagram illustrating a current path in the timing chart of FIG. 3. FIG. 4 is a diagram illustrating a current path in the timing chart of FIG. 3. It is a timing chart which shows operation | movement of the electric power steering apparatus of FIG. It is a figure which shows the path | route of the electric current in the timing chart of FIG. It is a figure which shows the path | route of the electric current in the timing chart of FIG. It is a signal time chart explaining operation | movement of a current detection circuit. It is a figure which shows the characteristic of a torque sensor. It is a figure which shows the characteristic of a target electric current setting block. It is a figure which shows the timing chart of the conventional H2 bridge circuit for comparative explanation. It is a figure which shows the electric current path in the state of FIG. 11 of the conventional H bridge circuit. It is a figure which shows the electric current path in the state of FIG. 11 of the conventional H bridge circuit. It is a figure which shows the electric current path in the state of FIG. 11 of the conventional H bridge circuit. It is a figure which shows the electric current path in the state of FIG. 11 of the conventional H bridge circuit. It is a block diagram which shows the electric power steering apparatus by Embodiment 2 of this invention.

Explanation of symbols

1 torque sensor, 2 DC motor, 3 control device,
31 H bridge circuit, 32 FET driver,
33 motor current detection circuit, 34 CPU, 35 monitoring circuit,
201 Motor current value, 202 Voltage across shunt resistor,
203 sampled and held signal, 204 subtractor output signal,
331 shunt resistor, 332 differential amplifier,
341 target current setting block, 342 duty ratio calculation block,
343 timer, 344 comparator,
345 pulse generator, 346 sample hold circuit,
347 A / D converter.

Claims (5)

A first arm and a second arm, each of which is mounted on an automobile and configured by connecting two switching elements in series, are connected to each other in an H-bridge shape that connects the opposite poles of each arm to each other. The both ends are connected to a battery power source mounted on the vehicle, the connection point of the two switching elements of the first arm, and the connection point of the two switching elements of the second arm A motor drive circuit constructed by connecting a direct current motor that assists the steering force of the driver of the car between
A switching element driving circuit for driving the two switching elements connected in series to each other in a complementary PWM manner, and driving the first arm transistor and the second arm transistor connected to the same electrode of the battery in a complementary manner to each other. ,
A shunt resistor inserted in series with the battery power supply,
Motor current detection circuit that detects the voltage across this shunt resistor,
An electric power steering apparatus comprising a motor current calculation circuit that controls the polarity of the detected voltage in synchronization with the operation timing of the switching element drive circuit.   The motor current detection circuit includes a differential amplifier that amplifies the voltage across the shunt resistor, and the motor current calculation circuit outputs the output of the differential amplifier according to the output timing of the switching element drive circuit. A sample hold circuit that samples and holds every half cycle of the output of the switching element drive circuit, and a subtracter circuit that subtracts the other from one of the two sample values obtained every half cycle. The electric power steering apparatus according to claim 1.   The motor current detection circuit includes a differential amplifier that amplifies the voltage across the shunt resistor, and the motor current calculation circuit switches the output of the differential amplifier according to the output timing of the switching element drive circuit. The electric power steering apparatus according to claim 1, further comprising a polarity inversion circuit that inverts the polarity every half cycle of the output of the element driving circuit. A torque sensor for detecting a steering force of the driver;
When the polarity of the output of the torque sensor and the polarity of the output of the motor current calculation circuit are compared, and the state where they do not match each other continues for a predetermined time, the electric power steering device is in an abnormal state A monitoring circuit that determines and outputs a signal,
The electric power steering apparatus according to any one of claims 1 to 3, further comprising a cutoff circuit that cuts off the energization to the DC motor based on the signal of the monitoring circuit. The electric power steering apparatus according to any one of claims 1 to 4, wherein the switching element is an FET element.

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