Motor control device and electric power steering device

Abstract

This motor control device is provided with an assist torque current calculation unit (25a) for calculating a current which should be supplied to a motor, and an overheat protection limiting current calculation unit (25b): for calculating, by using an overheat protection characteristic that indicates the relationship between an overheat protection coefficient and the magnitude of a current and that is specified by taking into account the influence on the atmospheric temperature exerted by heat generation of one component of a plurality of components disposed in a passage for passing current to be supplied to the motor and the influence on the atmospheric temperature exerted by heat generation of other components of the plurality of components, an overheat protection coefficient in a case where the current calculated by the assist torque current calculation unit flows; and for calculating an actual current which is to be supplied to the motor by using the calculated overheat coefficient.

Description

[Technical field]

[0001] The present disclosure relates to a motor control device and an electric power steering device. [Background technology]

[0002] An electric power steering device includes a motor that generates a steering assist torque for the steering wheel and a motor control device that controls the motor, and applies a steering assist force to the steering mechanism of a vehicle such as an automobile. Such an electric power steering device generates heat due to the current flowing through the motor control device and the motor, and therefore requires measures to deal with heat.

[0003] In conventional electric power steering devices, the current flowing through the motor control device is limited according to the detected value of a temperature detection means (e.g., a temperature thermistor) provided in the motor control device, thereby taking measures against heat in the motor control device. For details of such conventional electric power steering devices, please refer to, for example, the following Patent Documents 1 and 2. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4064600 [Patent Document 2] Patent No. 3605349 Summary of the Invention [Problem to be solved by the invention]

[0005] In the conventional electric power steering device, as described above, the detection value of the temperature detection means provided in the motor control device is treated as the ambient temperature to take measures against heat. However, since the temperature detection means provided in the motor control device is not necessarily located close to the part where the heat countermeasure is required, the detection value of the temperature detection means may differ from the actual ambient temperature.

[0006] For this reason, conventional motor control devices have tended to limit the current flowing through the motor too much, prioritizing safety. Specifically, conventional motor control devices limit the current early when a large current flows through the motor, and continue to limit the current after that, which leads to a reduction in the steering assist force.

[0007] The present disclosure has been made in consideration of the above circumstances, and aims to provide a motor control device and an electric power steering device that can implement more optimal heat countermeasures while preventing a reduction in steering assist force. [Means for solving the problem]

[0008] In order to solve the above problems, a motor control device according to one embodiment of the present disclosure includes a first calculation unit that calculates a current to be supplied to a motor, and a second calculation unit that calculates an overheat protection coefficient when the current calculated by the first calculation unit flows using an overheat protection characteristic that indicates the relationship between the magnitude of the current and an overheat protection coefficient, the overheat protection characteristic being defined by taking into consideration the effect on the ambient temperature of heat generated by one of a plurality of components arranged in a path through which the current supplied to the motor flows, and the effect on the ambient temperature of heat generated by other of the plurality of components, and calculates an overheat protection coefficient when the current calculated by the first calculation unit flows, and uses the calculated overheat protection coefficient to calculate a current actually supplied to the motor.

[0009] Moreover, an electric power steering device according to one aspect of the present disclosure includes a torque sensor that detects a steering torque of a steering wheel, the motor that generates a steering assist torque for the steering wheel, and the above-mentioned motor control device that controls the drive of the motor in response to the steering torque detected by the torque sensor. Effect of the Invention

[0010] According to the present disclosure, it is possible to implement more optimal heat countermeasures while preventing a reduction in steering assist force. [Brief description of the drawings]

[0011] [Figure 1] 1 is a block diagram showing the overall configuration of an electric power steering device according to a first embodiment. [Diagram 2] 3 is a block diagram showing a configuration related to overheat protection performed by a CPU in the first embodiment; FIG. [Figure 3A] FIG. 4 is a diagram showing an example of an overheat protection characteristic of a driving element (MOSFET) in the first embodiment. [Figure 3B] FIG. 4 is a diagram showing an example of an overheat protection characteristic of a capacitor in the first embodiment. [Figure 3C] 5 is a diagram showing an example of an overheat protection characteristic of the choke coil in the first embodiment. FIG. [Figure 4A] FIG. 11 is a diagram showing an example of an overheat protection characteristic of a driving element (MOSFET) in the second embodiment. [Figure 4B] FIG. 11 is a diagram showing an example of an overheat protection characteristic of a capacitor in the second embodiment. [Figure 4C] FIG. 11 is a diagram showing an example of an overheat protection characteristic of the choke coil in the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, a motor control device and an electric power steering device according to embodiments of the present disclosure will be described in detail with reference to the drawings.

[0013] [First embodiment] <Electric power steering device> Fig. 1 is a block diagram showing the overall configuration of an electric power steering device according to the first embodiment. As shown in Fig. 1, the electric power steering device 1 of this embodiment includes a torque sensor 11, a vehicle speed sensor 12, a motor 13, and a control unit 14 (motor control device), and applies a steering assist force to the steering mechanism of a vehicle such as an automobile.

[0014] The torque sensor 11 is disposed near the steering wheel of the vehicle and detects the steering torque of the driver. The vehicle speed sensor 12 detects the speed of the vehicle. The motor 13 is mounted on the steering column or rack shaft of the vehicle and generates a steering assist torque for the steering under the control of a control unit 14. Note that the motor 13 may be, for example, a three-phase brushless motor having a U-phase coil 13u, a V-phase coil 13v, and a W-phase coil 13w.

[0015] The control unit 14 controls the current supplied to the motor 13 according to the detection results of the torque sensor 11 and the vehicle speed sensor 12, thereby controlling the steering assist force applied to the steering mechanism of the vehicle. The source of the current supply to the motor 13 is the vehicle battery BT. The control unit 14 includes a choke coil 21, a power supply relay 22, a drive unit 23, a relay 24, a CPU (Central Processing Unit) 25, and a heat sink 26. In the following, the relay 24 will be referred to as a "motor relay 24" to distinguish it from the power supply relay 22.

[0016] The choke coil 21 is provided between the battery BT and the power supply relay 22 to remove noise. The power supply relay 22 is provided between the choke coil 21 and the drive unit 23, and is opened or closed under the control of the CPU 25. The power supply relay 22 is provided to cut off the current supplied from the battery BT to the motor 13, for example, when an abnormality occurs.

[0017] The drive unit 23 is provided between the power supply relay 22 and the motor 13, and under the control of the CPU 25, generates an AC current to be supplied to the motor 13 from a DC current supplied from the battery BT to drive the motor 13. The drive unit 23 includes positive drive elements Q11, Q12, and Q13, negative drive elements Q21, Q22, and Q23, smoothing capacitors C1, C2, and C3, and shunt resistors Ru, Rv, and Rw.

[0018] The positive electrode side driving elements Q11, Q12, and Q13 are connected to the positive electrode of the battery BT via a power supply relay 22 and a choke coil 21. The negative electrode side driving elements Q21, Q22, and Q23 are connected to the positive electrode side driving elements Q11, Q12, and Q13, respectively, and are connected to the negative electrode of the battery BT via shunt resistors Ru, Rv, and Rw, respectively.

[0019] Here, a U-phase series circuit is formed by the positive drive element Q11, the negative drive element Q21, and the shunt resistor Ru. In this U-phase series circuit, the connection point between the positive drive element Q11 and the negative drive element Q21 is connected to the connection point between the U-phase coil 13u and the W-phase coil 13w of the motor 13.

[0020] A V-phase series circuit is formed by the positive drive element Q12, the negative drive element Q22, and the shunt resistor Rv. In this V-phase series circuit, the connection point between the positive drive element Q12 and the negative drive element Q22 is connected to the connection point between the U-phase coil 13u and the V-phase coil 13v of the motor 13.

[0021] A W-phase series circuit is formed by the positive drive element Q13, the negative drive element Q23, and the shunt resistor Rw. In this W-phase series circuit, the connection point between the positive drive element Q13 and the negative drive element Q23 is connected to the connection point between the V-phase coil 13v and the W-phase coil 13w of the motor 13 via the motor relay 24.

[0022] The positive drive elements Q11, Q12, Q13 and the negative drive elements Q21, Q22, Q23 may be semiconductor switching elements such as MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors). A control signal CS output from the CPU 25 is supplied to the gate terminals of the positive drive elements Q11, Q12, Q13 and the gate terminals of the negative drive elements Q21, Q22, Q23. The positive drive elements Q11, Q12, Q13 and the negative drive elements Q21, Q22, Q23 are attached to a heat sink 26 to protect them from overheating.

[0023] The positive drive elements Q11, Q12, Q13 and the negative drive elements Q21, Q22, Q23 are turned on or off by a control signal CS output from the CPU 25. In this manner, the drive unit 23 generates an AC current to be supplied to the motor 13 from a DC current supplied from the battery BT.

[0024] The smoothing capacitors C1 to C3 are provided corresponding to the U-phase series circuit, the V-phase series circuit, and the W-phase series circuit, respectively. The smoothing capacitors C1 to C3 are connected in parallel to the U-phase series circuit, the V-phase series circuit, and the W-phase series circuit, respectively, and smooth the voltages across the U-phase series circuit, the V-phase series circuit, and the W-phase series circuit, respectively. Note that, although the example shown in FIG. 1 illustrates a configuration including three smoothing capacitors C1 to C3 corresponding to the U-phase series circuit, the V-phase series circuit, and the W-phase series circuit, a configuration may also be provided including only one smoothing capacitor common to the U-phase series circuit, the V-phase series circuit, and the W-phase series circuit.

[0025] The shunt resistor Ru detects a current Iu flowing through the U-phase coil 13u of the motor 13. The shunt resistor Rv detects a current Iv flowing through the V-phase coil 13v of the motor 13. The shunt resistor Rw detects a current Iw flowing through the W-phase coil 13w of the motor 13. The currents Iu, Iv, Iw detected by the shunt resistors Ru, Rv, Rw are input to the CPU 25 as a detection signal DS.

[0026] The motor relay 24 is provided between the connection point between the positive drive element Q13 and the negative drive element Q23 and the connection point between the V-phase coil 13v and the W-phase coil 13w of the motor 13, and is opened or closed under the control of the CPU 25. The motor relay 24 is provided to cut off the W-phase circuit when, for example, an abnormality occurs only in the W-phase. Note that, in this embodiment, for the sake of simplicity, an example is given in which the motor relay 24 cuts off the W-phase circuit, but the motor relay 24 may be provided in other phases (u-phase, W-phase).

[0027] The CPU 25 controls the current supplied to the motor 13 based on the steering torque detected by the torque sensor 11, the vehicle speed detected by the vehicle speed sensor 12, and the currents Iu, Iv, Iw (detection signal DS) detected by the shunt resistors Ru, Rv, Rw. Specifically, the CPU 25 determines the current to be supplied to the motor 13 to generate a required steering assist torque based on the steering torque detected by the torque sensor 11 and the vehicle speed detected by the vehicle speed sensor 12.

[0028] Meanwhile, the CPU 25 calculates a final current output from the deviation between the currents Iu, Iv, Iw (detection signal DS) detected by the shunt resistors Ru, Rv, Rw and the above-mentioned calculated current. Then, the CPU 25 converts the calculated final current output into a voltage and outputs a control signal CS for controlling the positive drive elements Q11, Q12, Q13 and the negative drive elements Q21, Q22, Q23 provided in the drive unit 23. In this way, the CPU 25 performs so-called feedback control by current.

[0029] Furthermore, when an abnormality occurs, the CPU 25 opens the power relay 22 to cut off the current supplied from the battery BT to the motor 13. Alternatively, when an abnormality occurs only in the W phase, the CPU 25 opens the motor relay 24 to cut off the W phase circuit.

[0030] In addition to the above controls, the CPU 25 performs overheat protection for components disposed in a path through which current supplied to the motor 13 flows, among the components provided in the motor 13 and the control unit 14. Here, components to be protected from overheating include, for example, the choke coil 21, power relay 22, positive drive elements Q11, Q12, Q13, negative drive elements Q21, Q22, Q23, smoothing capacitors C1, C2, C3, and motor relay 24 of the control unit 14, as well as the U-phase coil 13u, V-phase coil 13v, and W-phase coil 13w of the motor 13.

[0031] 1, components that should be protected from overheating include the permanent magnets built into the motor 13, wiring, and terminals that connect the wiring. Note that the shunt resistors Ru, Rv, and Rw of the control unit 14 are ignored here because they are more heat resistant than the above components due to their material and resistance value. Similarly, the wiring and connection terminals are ignored.

[0032] Fig. 2 is a block diagram showing the configuration related to overheat protection performed by the CPU in the first embodiment. In Fig. 2, the same components as those shown in Fig. 1 are denoted by the same reference numerals. As shown in Fig. 2, the CPU 25 includes, as components related to overheat protection, an assist torque current calculation unit 25a (first calculation unit), an overheat protection limit current calculation unit 25b (second calculation unit), and a motor control unit 25c.

[0033] The assist torque current calculation unit 25a calculates a current to be supplied to the motor 13 in order to generate a required steering assist torque, using the steering torque detected by the torque sensor 11. The overheat protection limit current calculation unit 25b calculates a current (limit current) actually supplied to the motor 13 in order to generate a steering assist torque while protecting components that should be protected from overheating, based on the current calculated by the assist torque current calculation unit 25a and the temperature detected by the temperature thermistor 27.

[0034] The temperature thermistor 27 is attached to the heat sink 26 and detects the temperature of the heat sink 26. This is because the components to be protected from overheating are close to or attached to the heat sink 26, and the temperature of the heat sink 26 has a large effect on the ambient temperature of the components to be protected from overheating. The thermal time constant of the heat sink 26 is longer than the thermal time constant of the components to be protected from overheating, and the heat capacity of the heat sink 26 is larger than the heat capacity of the components to be protected from overheating, so the temperature of the heat sink 26 has a large effect on the ambient temperature.

[0035] Specifically, the overheat protection limit current calculation unit 25b holds overheat protection characteristics prepared in advance for each component to be protected from overheating, and uses these overheat protection characteristics to determine the current (limit current) actually supplied to the motor 13. Here, the above-mentioned overheat protection characteristics indicate the relationship between the magnitude of the current and the overheat protection coefficient, which is determined by taking into consideration the influence on the ambient temperature of heat generated by one of the multiple components to be protected from overheating, and the influence on the ambient temperature of heat generated by the other components to be protected from overheating. Note that the overheat protection coefficient is synonymous with the current decrease rate [A / s].

[0036] The overheat protection limit current calculation unit 25b uses the overheat protection characteristics prepared for the components to be protected from overheating to calculate an overheat protection coefficient when the current calculated by the assist torque current calculation unit 25a flows. Then, the overheat protection limit current calculation unit 25b uses the calculated overheat protection coefficient to calculate the current actually supplied to the motor 13.

[0037] Here, when there are multiple components to be protected from overheating, the overheat protection limit current calculation unit 25b calculates an overheat protection coefficient for each of those components when the current calculated by the assist torque current calculation unit 25a flows. Then, the largest of the calculated overheat protection coefficients is used to calculate the current to be actually supplied to the motor 13. This is done to prevent the temperature of all the components to be protected from overheating from exceeding the rated (upper limit) temperature.

[0038] The motor control unit 25c generates a control signal CS for controlling the positive drive elements Q11, Q12, Q13 and the negative drive elements Q21, Q22, Q23 provided in the drive unit 23 based on the current calculated by the overheat protection limit current calculation unit 25b. Specifically, the motor control unit 25c calculates a final current output from the deviation between the current calculated by the overheat protection limit current calculation unit 25b and the currents Iu, Iv, Iw (detection signal DS) detected by the shunt resistors Ru, Rv, Rw. Then, the motor control unit 25c converts the calculated final current output into a voltage and outputs the control signal CS for controlling the positive drive elements Q11, Q12, Q13 and the negative drive elements Q21, Q22, Q23 provided in the drive unit 23.

[0039] <Overheat protection characteristics> Next, the overheat protection characteristics prepared for each of the above-mentioned components that need to be protected from overheating will be described. In general, when protecting a component from overheating, it is necessary to prevent the temperature of the component from exceeding the rated (upper limit) temperature of the component. Here, the characteristics for overheat protection against the current of the component depend on the temperature rise characteristics of the component when current is applied.

[0040] Generally, the temperature rise ΔT1 [℃] of a component that starts generating heat after time t is expressed by the following formula (1): In the following formula (1), RW1 is the thermal resistance of the component [℃ / W], W1 is the amount of heat generated by the component [W], and τ1 is the time constant [s].

[0041]

number

[0042] Here, the heat generation amount W1 of a component is expressed by the following formula (2) using the component resistance R1 [Ω] and the current I1 [A] flowing through the component.

[0043]

number

[0044] By substituting the above formula (2) into the above formula (1), the following formula (3) is obtained, where K1=RW1×R1.

[0045]

number

[0046] Here, if it is assumed that the ambient temperature (ambient temperature) of the component does not change from the initial ambient temperature Ti [° C.], the temperature T1 of the component is expressed by the following equation (4).

[0047]

number

[0048] However, in reality, the ambient temperature of a component also changes due to heat radiation from other components, so the actual temperature T1 of the component is expressed by the following formula (5).

number

[0049] In the above formula (5), I2 is the current [A] flowing through other components, K2 is the thermal resistance [℃ / W] around the component, and τ2 is the time constant [s] around the component. For example, if the component is surrounded by air, K2 is the thermal resistance of the air, and τ2 is the time constant of the air. In contrast, if a heat sink 26 is placed around the component, K2 is the thermal resistance of the heat sink, and τ2 is the time constant of the heat sink. In other words, the first term on the right side of the above formula (5) indicates the temperature rise due to the self-heating of the component, and the second term on the right side indicates the temperature rise of the component due to the rise in the ambient temperature caused by the heat generation of other components.

[0050] Here, to facilitate understanding, the temperature rise of the components will be described using a concrete example. Of the drive elements (drive elements Q11, Q12, Q13, Q21, Q22, Q23) of the drive unit 23 shown in FIG. 1, attention will be focused on the drive elements (drive elements Q11, Q21) that form a U-phase series circuit.

[0051] The driving elements Q11, Q12, Q13, Q21, Q22, and Q23 are attached to a heat sink 26. For this reason, the temperature (ambient temperature) of the heat sink 26 is affected not only by the driving elements (driving elements Q11 and Q21) that form the U-phase series circuit, but also by the driving elements (driving elements Q12, Q13, Q22, and Q23) that form the V-phase and W-phase series circuits.

[0052] Now, it is assumed that the drive elements Q11, Q12, Q13, Q21, Q22, and Q23 are all of the same specification, and that the thermal resistance when heat is dissipated from the drive elements Q11, Q12, Q13, Q21, Q22, and Q23 to the heat sink 26 is also the same. Also, it is assumed that the heat distribution of the heat sink 26 is uniform. Then, the temperature T fet_u can be expressed by the following equation (6).

[0053]

number

[0054] In the above formula (6), Iu is the current flowing through the drive elements (drive elements Q11, Q21) that form the U-phase series circuit, Iv is the current flowing through the drive elements (drive elements Q12, Q22) that form the V-phase series circuit, and Iw is the current flowing through the drive elements (drive elements Q13, Q23) that form the W-phase series circuit.

[0055] The first term on the right side of the above equation (6) represents the temperature rise due to self-heating of the drive elements (drive elements Q11, Q21) that form the U-phase series circuit. The second term on the right side represents the temperature rise of the heat sink 26 due to heat generation of the drive elements (drive elements Q11, Q21) that form the U-phase series circuit. The third term on the right side represents the temperature rise of the heat sink 26 due to heat generation of the drive elements (drive elements Q12, Q22) that form the V-phase series circuit. The fourth term on the right side represents the temperature rise of the heat sink 26 due to heat generation of the drive elements (drive elements Q13, Q23) that form the W-phase series circuit.

[0056] The second to fourth terms on the right side of the above formula (6) can be summed up to give the following formula (7).

[0057]

number

[0058] Furthermore, the above formula (7) can be expressed as the following formula (8) or (9).

[0059]

number

[0060]

number

[0061] In addition, Im in the above formula (8) is the motor current expressed by the following formula (10): Moreover, Km in the above formula (9) is a constant.

[0062]

number

[0063] The first term on the right side of the above equations (8) and (9) represents the temperature rise due to self-heating of the drive elements (drive elements Q11 and Q21) that form the U-phase series circuit. The second term on the right side represents the temperature rise of the heat sink 26 due to heat generation of the drive elements Q11, Q12, Q13, Q21, Q22, and Q23.

[0064] Here, the heat sink 26 has a larger heat capacity and a longer time constant than the drive elements Q11, Q12, Q13, Q21, Q22, and Q23, so τ1<τ2. The temperature rise of the drive elements (drive elements Q11 and Q21) that make up the U-phase series circuit is divided into the self-heating of the drive elements (drive elements Q11 and Q21) that make up the U-phase series circuit, and the temperature rise of the heat sink 26. The temperature rise of the heat sink 26 depends on the heat generated by all of the drive elements Q11, Q12, Q13, Q21, Q22, and Q23, so K1 <K2×Kmとなる。

[0065] From the above, when the current Iu flowing through the drive elements (drive elements Q11, Q21) forming the U-phase series circuit is large, the temperature T fet_u In the case of the drive element (drive element Q11, Q21), the temperature rise due to self-heating (the first term on the right side of equation (9)) tends to be dominant. Conversely, when the current Iu is small, the temperature T fet_u The atmospheric temperature (the temperature of the heat sink 26) (the second term on the right side of the equation (9)) tends to become dominant.

[0066] The following describes the overheat protection characteristics of each component that needs to be protected from overheating. Specifically, the following considers the overheat protection characteristics of the drive element (MOSFET), motor coils (U-phase coil 13u, V-phase coil 13v, and W-phase coil 13w), power relay 22, motor relay 24, capacitors (smoothing capacitors C1, C2, and C3), choke coil 21, and permanent magnets used in motor 13, in that order.

[0067] <Overheat protection characteristics of driving element> 3A is a diagram showing an example of the overheat protection characteristic of the driving element (MOSFET) in the first embodiment. In the graph shown in FIG. 3A, the horizontal axis represents the heat source current (current flowing through the driving element) [A], and the vertical axis represents the overheat protection coefficient L1 [A / s]. The overheat protection characteristic 31 of the driving element shown in FIG. 3A is calculated by using the above formula (9) to calculate the temperature T fet_uThe change in current is calculated and the overheat protection coefficient L1 for each heat source current is estimated so that the temperature of the drive element does not exceed the rated (upper limit) temperature.

[0068] The overheat protection characteristic 31 of the driving element shown in FIG. 3A is divided into four regions R11 to R14 according to the magnitude of the heat source current. Region R11 is a region (0 to A1) where the heat source current is extremely small. In this region R11, as the heat source current decreases, the overheat protection coefficient L1 increases linearly. For example, when the heat source current is 0, the overheat protection coefficient L1 is N1, and when the heat source current is A1, the overheat protection coefficient L1 is N0. Region R12 is a region (A1 to B1) where the heat source current is intermediate. In this region R12, the heat generated by the driving element and the heat dissipation by the heat sink 26, etc. are balanced, so the overheat protection coefficient L1 is constant (N0).

[0069] Region R13 is a region (B1 to C) where the heat source current is large. In this region R13, the overheat protection coefficient L1 gradually decreases gently in response to changes in the heat source current. Region R14 is a region (larger than C) where the heat source current is extremely large. In this region R14, the overheat protection coefficient L1 suddenly decreases in response to changes in the heat source current. Note that region R13 is a region where the ambient temperature (temperature of the heat sink 26) (second term on the right side of equation (9)) becomes dominant, and region R14 is a region where the temperature rise due to self-heating of the drive element (first term on the right side of equation (9)) becomes dominant. These regions R13 and R14 are regions where overheat protection of the drive element is required.

[0070] Here, when the current flowing through the drive element is equal to or less than B1, overheat protection of the drive element is not necessary. Therefore, when the current calculated by the assist torque current calculation unit 25a of the CPU 25 is equal to or less than B1, the overheat protection limit current calculation unit 25b sets the current calculated by the assist torque current calculation unit 25a as the current actually supplied to the motor 13.

[0071] On the other hand, when the current flowing through the drive element is larger than B1, the drive element needs to be protected from overheating. Therefore, when the current calculated by the assist torque current calculation unit 25a of the CPU 25 is larger than the above-mentioned B1, the overheat protection limit current calculation unit 25b calculates an overheat protection coefficient L1 according to the current calculated by the assist torque current calculation unit 25a, using the overheat protection characteristic 31 shown in Fig. 3A.

[0072] 3A, the overheat protection limit current calculation unit 25b calculates N11 as the overheat protection coefficient L1. Then, the overheat protection limit current calculation unit 25b performs current limiting by reducing the current calculated by the assist torque current calculation unit 25a by N11 amperes per second.

[0073] For example, consider a case where B1 in Fig. 3A is 20 [A], D is 50 [A], and N11 is 3 [A / s]. In this case, the overheat protection limiting current calculation unit 25b reduces the current (50 [A]) calculated by the assist torque current calculation unit 25a at a rate of 3 [A] per second. Then, after 10 seconds, the current reaches 20 [A] (B1 in Fig. 3A), so the overheat protection limiting current calculation unit 25b temporarily stops limiting the current.

[0074] The characteristic of the overheat protection characteristic 31 in the region R11 (the characteristic in which the overheat protection coefficient L1 increases linearly as the heat source current decreases) is provided to return the current limit value to the initial value in consideration of heat dissipation when the driver finishes steering. The initial value of the current limit value is set to, for example, 1.5 times the maximum value of the current amount required for assist (rated current).

[0075] <Motor coil overheat protection characteristics> Next, the overheat protection characteristics of the motor coils (U-phase coil 13u, V-phase coil 13v, W-phase coil 13w) will be considered. When a large current flows through the motor coil, the heat generated by the motor coil itself becomes dominant as to the temperature of the motor coil, and when a small current flows through the motor coil, the ambient temperature becomes dominant. Therefore, the overheat protection characteristics of the motor coil are similar to the overheat protection characteristics 31 of the drive element shown in FIG. 3A. However, the values ​​(A1, B1, C, N1) and the decay curve of the overheat protection characteristics of the motor coil are different from those of the drive element.

[0076] <Overheat protection characteristics of power relays and motor relays> Next, we consider the overheat protection characteristics of the power relay 22 and the motor relay 24. If these relays have a mechanical switch mechanism, the potential difference in the switch mechanism is almost nonexistent, so overheat protection can be ignored. However, if they are electronic relays using FETs, IGBTs (Insulated Gate Bipolar Transistors), etc., the same overheat protection characteristics 31 of the drive element shown in Figure 3A can be used.

[0077] <Capacitor overheat protection characteristics> Fig. 3B is a diagram showing an example of the overheat protection characteristic of a capacitor in the first embodiment. In addition, the graph shown in Fig. 3B has the heat source current (current flowing through the capacitor) [A] on the horizontal axis and the overheat protection coefficient L2 [A / s] on the vertical axis. The overheat protection characteristic 32 of the capacitor (smoothing capacitors C1, C2, C3) shown in Fig. 3B is divided into three regions R21 to R23 according to the magnitude of the heat source current. Region R21 is a region where the heat source current is extremely small (0 to A2), region R22 is a region where the heat source current is intermediate (A2 to B2), and region R23 is a region where the heat source current is large (larger than B2).

[0078] The smoothing capacitors C1, C2, and C3 are arranged near the drive elements Q11, Q12, Q13, Q21, Q22, and Q23 to remove ripples caused by switching of the drive elements Q11, Q12, Q13, Q21, Q22, and Q23. Therefore, the temperatures of the smoothing capacitors C1, C2, and C3 are dominated by the temperatures of the drive elements Q11, Q12, Q13, Q21, Q22, and Q23 and the temperature of the heat sink 26, and in particular become approximately the same as the temperature of the heat sink 26.

[0079] Therefore, regardless of the magnitude of the current flowing through smoothing capacitors C1, C2, C3, the temperature is dominated by the ambient temperature (the second term in equation (5)), and the overheat protection characteristics of smoothing capacitors C1, C2, C3 become like overheat protection characteristic 32 shown in FIG. 3B. Here, overheat protection characteristic 32 in region R23 is represented by a single quadratic curve. Note that the heat source current can be the current within the same phase (the current flowing through the drive element).

[0080] <Choke coil overheat protection characteristics> Fig. 3C is a diagram showing an example of the overheat protection characteristic of the choke coil in the first embodiment. In addition, the graph shown in Fig. 3C has the heat source current (current flowing through the choke coil) [A] on the horizontal axis and the overheat protection coefficient L3 [A / s] on the vertical axis. The overheat protection characteristic 33 of the choke coil 21 shown in Fig. 3C is divided into three regions R31 to R33 according to the magnitude of the heat source current. Region R31 is a region where the heat source current is extremely small (0 to A3), region R32 is a region where the heat source current is intermediate (A3 to B3), and region R33 is a region where the heat source current is large (larger than B3).

[0081] Although the temperature of the choke coil 21 is not as high as that of the smoothing capacitors C1, C2, and C3, the temperature of the heat sink 26 is dominant. Therefore, the overheat protection characteristic 33 of the choke coil 21 shown in FIG. 3C is similar to the overheat protection characteristic 32 shown in FIG. 3B. Here, if the choke coil 21 is disposed at a position away from the driving element and the motor coil, the temperature of the choke coil 21 depends not on the current flowing through the choke coil itself but on the total current flowing through each downstream component. Alternatively, it depends on the phase current controlled by the control signal CS output from the CPU 25.

[0082] <Overheat protection characteristics of permanent magnets used in motors> Next, the overheat protection characteristic of the permanent magnet (not shown) used in motor 13 will be considered. Since permanent magnets do not generate heat themselves, the temperature of the permanent magnet is determined by the ambient temperature. Also, the current flowing through motor 13 depends on the phase current, similar to choke coil 21. For this reason, the overheat protection characteristic of the permanent magnet used in motor 13 is similar to overheat protection characteristic 33 shown in FIG. 3C.

[0083] However, the values ​​(A3, B3, N2) and quadratic curve in the overheat protection characteristic of the permanent magnet used in the motor 13 may be different from those in the choke coil 21. Since the permanent magnet is disposed near the motor coil which generates a lot of heat, in some cases it may depend on each phase current. Also, if the temperature is distributed evenly throughout the motor 13, it may depend on the total current flowing through the motor 13. Therefore, the overheat protection characteristic of the permanent magnet used in the motor 13 may have two types of overheat protection characteristics, one that depends on each phase current and one that depends on the total current.

[0084] As described above, in this embodiment, the assist torque current calculation unit 25a calculates the current to be supplied to the motor 13 in order to generate the required steering assist torque. Then, the overheat protection limit current calculation unit 25b uses the overheat protection characteristics prepared in advance for each component to be overheat protected to calculate an overheat protection coefficient when the current calculated by the assist torque current calculation unit 25a flows, and limits the current flowing to the motor 13. Here, the above-mentioned overheat protection characteristics indicate the relationship between the magnitude of the current and the overheat protection coefficient, which is defined in consideration of the influence on the ambient temperature of heat generated by one of the multiple components to be overheat protected and the influence on the ambient temperature of heat generated by the other of the multiple components to be overheat protected.

[0085] In this manner, in this embodiment, the overheat protection coefficient is calculated using the overheat protection characteristic defined in consideration of the actual ambient temperature or an ambient temperature estimated to be close to the actual ambient temperature, and the current flowing through the motor 13 is limited. Therefore, in this embodiment, it is possible to implement more optimal heat countermeasures while preventing a reduction in the steering assist force.

[0086] In this embodiment, when there are multiple components to be protected from overheating, the overheat protection limit current calculation unit 25b calculates an overheat protection coefficient for each of the components when the current calculated by the assist torque current calculation unit 25a flows. Then, the largest of the calculated overheat protection coefficients is used to calculate the current to be actually supplied to the motor 13. This ensures continuity of control and effectively adds steering force assist.

[0087] Second Embodiment The configuration of the electric power steering device according to this embodiment is generally similar to the configuration of the electric power steering device 1 shown in Fig. 1. However, this embodiment differs from the first embodiment in that multiple types of overheat protection characteristics used in the overheat protection limiting current calculation unit 25b are prepared according to the rotation state of the motor 13.

[0088] Specifically, in this embodiment, the overheat protection limiting current calculation unit 25b holds an overheat protection characteristic (first overheat protection characteristic) when the motor 13 is stopped, and an overheat protection characteristic (second overheat protection characteristic) when the motor 13 is rotating. Then, the overheat protection limiting current calculation unit 25b uses the former overheat protection characteristic or the latter overheat protection characteristic depending on whether the motor 13 is stopped or not, to calculate an overheat protection coefficient when the current calculated by the assist torque current calculation unit 25a flows.

[0089] Equation (9) explained in the first embodiment is an equation when it is assumed that the motor 13 stops at a certain angle. When the motor 13 stops, the current flows concentratedly in the stopped phase. In contrast, when the motor 13 is rotating, the current does not flow concentratedly in one phase, but flows evenly in each phase.

[0090] For this reason, depending on whether the motor 13 is stopped or rotating, the constant Km in the second term on the right side of the above-mentioned equation (9) needs to be replaced with the following constants Km1 and Km2. When motor 13 is stopped Km1=(1 / √2) When motor 13 is rotating km2=(1 / √2)*(2 / π)

[0091] Fig. 4A is a diagram showing an example of the overheat protection characteristic of the driving element (MOSFET) in the second embodiment. The graph shown in Fig. 4A, like the graph shown in Fig. 3A, has the heat source current (current flowing through the driving element) [A] on the horizontal axis and the overheat protection coefficient L1 [A / s] on the vertical axis. The overheat protection characteristic 31a shown in Fig. 4A is the overheat protection characteristic of the driving element when the motor 13 is rotating, and the overheat protection characteristic 31 is the overheat protection characteristic of the driving element when the motor 13 is stopped. The overheat protection characteristic 31 shown in Fig. 4A is the same as the overheat protection characteristic 31 shown in Fig. 3A.

[0092] 4A, it can be seen that the overheat protection characteristic 31a of the drive element when the motor 13 is rotating changes more slowly than the overheat protection characteristic 31 of the drive element when the motor 13 is stopped. This is because the temperature rise of the drive element is smaller. In other words, when the motor 13 is rotating, compared to when the motor 13 is stopped, even if the temperature rise due to self-heating of the drive element (e.g., drive elements Q11, Q21) is almost the same, the current flowing through other drive elements (e.g., drive elements Q12, Q13, Q22, Q23) that contribute to the rise in the ambient temperature is smaller, so the temperature rise of the drive element (e.g., drive elements Q11, Q21) is smaller.

[0093] 4A, the upper limit of region R12 (the lower limit of region R13) changes from B1 to Ba, and the upper limit of region R13 (the lower limit of region R14) changes from C to Ca.

[0094] Fig. 4B is a diagram showing an example of the overheat protection characteristic of the capacitor in the second embodiment. Note that, like the graph shown in Fig. 3B, the graph shown in Fig. 4B has the heat source current (current flowing through the capacitor) [A] on the horizontal axis and the overheat protection coefficient L2 [A / s] on the vertical axis. Overheat protection characteristic 32a shown in Fig. 4B is the overheat protection characteristic of the capacitor when motor 13 is rotating, and overheat protection characteristic 32 is the overheat protection characteristic of the capacitor when motor 13 is stopped. Note that overheat protection characteristic 32 shown in Fig. 4B is the same as overheat protection characteristic 32 shown in Fig. 3B.

[0095] As shown in Fig. 4B, it can be seen that the capacitor overheat protection characteristic 32a when the motor 13 is rotating changes more slowly than the capacitor overheat protection characteristic 32 when the motor 13 is stopped. This is for the same reason as explained using Fig. 4A. In the example shown in Fig. 4B, the upper limit of region R22 (the lower limit of region R23) changes from B2 to Bb.

[0096] Fig. 4C is a diagram showing an example of the overheat protection characteristic of the choke coil in the second embodiment. The graph shown in Fig. 4C, like the graph shown in Fig. 3C, has the heat source current (current flowing through the choke coil) [A] on the horizontal axis and the overheat protection coefficient L3 [A / s] on the vertical axis. The overheat protection characteristic 33a shown in Fig. 4C is the overheat protection characteristic of the choke coil when the motor 13 is rotating, and the overheat protection characteristic 33 is the overheat protection characteristic of the choke coil when the motor 13 is stopped. The overheat protection characteristic 33 shown in Fig. 4C is the same as the overheat protection characteristic 33 shown in Fig. 3C.

[0097] As shown in Fig. 4C, it can be seen that the overheat protection characteristic 33a of the choke coil when the motor 13 is rotating changes more slowly than the overheat protection characteristic 33 of the choke coil when the motor 13 is stopped. This is for the same reason as explained using Fig. 4A. In the example shown in Fig. 4C, the upper limit of region R32 (the lower limit of region R33) changes from B3 to Bc.

[0098] Here, for components that generate little self-heat, such as the choke coil 21, the capacitors (smoothing capacitors C1 to C3), and the permanent magnets, if there is a thermal effect from components that generate a lot of self-heat, it is advisable to use the overheat protection characteristic 32a when the motor 13 is rotating, as shown in Fig. 4B. In contrast, if there is little thermal effect from components that generate a lot of self-heat, there is no need to distinguish between when the motor 13 is rotating and when the motor 13 is stopped, so the overheat protection characteristic may be determined according to the arrangement of the components in the device and the heat dispersion situation.

[0099] As described above, in this embodiment, the overheat protection limit current calculation unit 25b is provided with an overheat protection characteristic when the motor 13 is stopped and an overheat protection characteristic when the motor 13 is rotating. Then, depending on whether the motor 13 is stopped or not, the overheat protection coefficient when the current calculated by the assist torque current calculation unit 25a flows is calculated using the former overheat protection characteristic or the latter overheat protection characteristic. Therefore, it is possible to implement more detailed overheat protection measures than in the first embodiment, and it is possible to set the overheat protection more appropriately.

[0100] Although the embodiments have been described above, the present disclosure is not limited to the above-mentioned embodiments, and can be freely modified without departing from the spirit of the present disclosure. For example, in the above-mentioned embodiments, the overheat protection characteristics of the parts to be protected from overheating are expressed by mathematical expressions, but they may be stored in the CPU 25 as a map or table and used.

[0101] In the above embodiment, the motor 13 is a three-phase brushless motor, but the motor 13 is not limited to a three-phase brushless motor. For example, the motor 13 may be a brushed motor or a four or more phase motor.

[0102] In the above-described embodiment, the components that are arranged in the path of the current supplied to the motor 13 and that should be protected from overheating are, for example, the choke coil 21, power relay 22, positive drive elements Q11, Q12, Q13, negative drive elements Q21, Q22, Q23, smoothing capacitors C1, C2, C3, and motor relay 24 of the control unit 14, as well as the U-phase coil 13u, V-phase coil 13v, and W-phase coil 13w of the motor 13. However, the components that should be protected from overheating may be other components.

[0103] In the above-described embodiment, the temperature of the heat sink 26 to which the positive drive elements Q11, Q12, Q13 and the negative drive elements Q21, Q22, Q23 are attached is defined as the ambient temperature. However, a temperature other than the temperature of the heat sink 26 may be defined as the ambient temperature. For example, when the components to be protected from overheating are housed in a housing, the temperature of the housing may be defined as the ambient temperature. Alternatively, when the components to be protected from overheating are mounted on a board, the temperature of the board may be defined as the ambient temperature.

[0104] Also, in the above-described embodiment, an example has been described in which overheat protection is performed for the components provided in the electric power steering device 1. However, the present disclosure is not limited to overheat protection for the components provided in the electric power steering device 1, and can be applied to cases in which overheat protection is required for components arranged in a path through which current supplied to a motor flows.

[0105] Each of the components of the control unit 14 as the motor control device and the electric power steering device 1 has an internal computer system. A program for implementing the functions of each of the components of the control unit 14 and the electric power steering device 1 may be recorded in a computer-readable recording medium, and the program recorded in the recording medium may be read into a computer system and executed to perform processing in each of the components of the control unit 14 and the electric power steering device 1. Here, "reading a program recorded in a recording medium into a computer system and executing it" includes installing the program into the computer system. The "computer system" here includes hardware such as an OS and peripheral devices.

[0106] Furthermore, a "computer system" may include multiple computer devices connected via a network including the Internet or communication lines such as a WAN, LAN, or leased line. Furthermore, a "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, and storage devices such as hard disks built into a computer system. In this way, the recording medium storing the program may be a non-transitory recording medium such as a CD-ROM.

[0107] The recording medium also includes a recording medium installed inside or outside and accessible from a distribution server to distribute the program. The program may be divided into a plurality of parts, downloaded at different times, and then combined by the respective components of the control unit 14 and the electric power steering device 1, and each of the divided programs may be distributed by a different distribution server. Furthermore, the "computer-readable recording medium" includes a recording medium that holds a program for a certain period of time, such as a volatile memory (RAM) in a computer system that becomes a server or a client when a program is transmitted via a network. The above program may be a recording medium for realizing a part of the above-mentioned functions. Furthermore, the recording medium may be a so-called difference file (difference program) that can realize the above-mentioned functions in combination with a program already recorded in the computer system. [Explanation of symbols]

[0108] 1...electric power steering device, 11...torque sensor, 13...motor, 13u...U-phase coil, 13v...V-phase coil, 13w...W-phase coil, 14...control unit, 21...choke coil, 22...power relay, 24...motor relay, 25a...assist torque current calculation unit, 25b...overheat protection limit current calculation unit, 31 to 33...overheat protection characteristics, 31a to 33a...overheat protection characteristics, C1 to C3...smoothing capacitors, Q11 to Q13...drive elements, Q21 to Q23...drive elements

Claims

1. a first calculation unit that calculates a current to be supplied to the motor; a second calculation unit that uses an overheat protection characteristic that indicates a relationship between a magnitude of a current and an overheat protection coefficient, the relationship being defined in consideration of an increase in ambient temperature due to heat generation from one of a plurality of components arranged on a path through which a current supplied to the motor flows, and an increase in the ambient temperature due to heat generation from another of the plurality of components, to determine an overheat protection coefficient when the current determined by the first calculation unit flows, and determines a current actually supplied to the motor using the determined overheat protection coefficient; A motor control device comprising:

2. 2. The motor control device according to claim 1, wherein the second calculation unit holds the overheat protection characteristics for each of the plurality of components, calculates the overheat protection coefficient for each of the components when the current calculated by the first calculation unit flows, and calculates the current actually supplied to the motor using the largest of the calculated overheat protection coefficients.

3. At least one of the plurality of components is attached to a heat sink; The ambient temperature is the temperature of the heat sink. The motor control device according to claim 1 or 2.

4. The plurality of components are housed in a housing, The ambient temperature is the temperature of the housing. The motor control device according to claim 1 or 2.

5. The plurality of components are mounted on a substrate, The ambient temperature is the temperature of the substrate. The motor control device according to claim 1 or 2.

6. The second calculation unit has a first overheat protection characteristic which is the overheat protection characteristic when the motor is stopped; a second overheat protection characteristic which is the overheat protection characteristic when the motor is rotating; We hold determining the overheat protection coefficient when the current determined by the first calculation unit flows, using the first overheat protection characteristic or the second overheat protection characteristic depending on whether the motor is stopped or not; The motor control device according to any one of claims 1 to 5.

7. a drive unit that supplies the current calculated by the second calculation unit to the motor, One of the plurality of components is a drive element provided in the drive unit. The motor control device according to any one of claims 1 to 6.

8. A torque sensor for detecting a steering torque of a steering wheel; The motor generates a steering assist torque for the steering; The motor control device according to any one of claims 1 to 7, wherein the motor is controlled in response to the steering torque detected by the torque sensor. An electric power steering device comprising:

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