Control method for electric vehicle and control device for electric vehicle

Abstract

This control method for an electric vehicle (1) is applied to the electric vehicle (1) having an electric motor (10) that drives a drive wheel (2). The control method includes determining a target rotational speed (Nt) in accordance with an accelerator opening degree (θa), and performing rotational speed limit control for outputting a torque command to the electric motor (10) so that the rotational speed (Na) of the drive wheel (2) reaches the target rotational speed (Nt).

Description

Control method for electric vehicle and control device for electric vehicle

[0001] The present invention relates to a control method for an electric vehicle and a control device for an electric vehicle.

[0002] Patent Document 1 discloses an electric vehicle drive system. This electric vehicle drive system includes a first traction motor and a second traction motor whose rotation axes extend parallel to but spaced apart from each other, a planetary gear mechanism, and a clutch. A first rotating element of the planetary gear mechanism is rotatably connected to the first traction motor, a second rotating element is rotatably connected to the second traction motor via a connecting portion, and a third rotating element is connected to an output shaft connected to the drive wheels. The clutch disconnectably connects any two of the first, second, and third rotating elements. This electric vehicle drive system can switch the driving characteristics of the electric vehicle by switching the clutch.

[0003] JP 2019-050705 A

[0004] However, the method disclosed in Patent Document 1 requires the insertion of a special mechanism, such as a gear, between the electric motor and the drive wheels in order to switch the driving characteristics, which increases the number of parts and requires space for installation, among other disadvantages.

[0005] An object of the present invention is to provide a control method and a control device for an electric vehicle that can obtain desired driving characteristics without adding a special mechanism.

[0006] A control method for an electric vehicle according to one aspect of the present invention is applied to an electric vehicle having an electric motor that drives drive wheels. The control method includes performing a rotation speed limit control that determines a target rotation speed in accordance with an accelerator opening degree and outputs a torque command to the electric motor so that the rotation speed of the drive wheels becomes the target rotation speed.

[0007] According to one aspect of the present invention, desired running characteristics can be obtained without adding a special mechanism.

[0008] Fig. 1 is a block diagram showing the configuration of an electric vehicle according to this embodiment. Fig. 2 is a block diagram showing the configuration of a controller. Fig. 3 is a diagram showing an opening characteristic map. Fig. 4 is a diagram showing a vehicle speed limit map. Fig. 5 is a timing chart showing a vehicle according to a comparative example. Fig. 6 is a timing chart showing an example of control according to this embodiment.

[0009] Hereinafter, a control method for an electric vehicle and a control device for an electric vehicle according to this embodiment will be described with reference to the drawings.

[0010] An electric vehicle according to this embodiment will be described with reference to Figure 1. The electric vehicle 1 is a vehicle that uses an electric motor as a drive source. Vehicles that use an electric motor as part of their drive source are also included in the electric vehicle category. In other words, electric vehicles include not only electric vehicles but also hybrid vehicles and the like.

[0011] The electric vehicle 1 (hereinafter simply referred to as “vehicle 1 ”) includes an electric motor 10 , a battery 12 , an inverter 14 , and a controller 20 .

[0012] The electric motor 10 is, for example, a three-phase AC synchronous motor, and generates a driving torque using AC power supplied via an inverter 14. The driving torque of the electric motor 10 is transmitted to the drive wheels 2 via a speed reducer 16 and a drive shaft, generating a driving force on the drive wheels 2. When the electric motor 10 rotates in accordance with the rotation of the drive wheels 2, it converts the kinetic energy of the electric vehicle 1 into electrical energy, thereby generating a regenerative braking force.

[0013] The inverter 14 supplies the electric power supplied from the battery 12 to the electric motor 10 by turning on or off a switching element in response to a current command input from the controller 20. The inverter 14 also supplies the electric power generated by the electric motor 10 using regenerative braking force to the battery 12.

[0014] 1, a set of wheels (left and right wheels) is depicted as drive wheels 2, but these drive wheels 2 are representatively depicted as left and right front wheels and left and right rear wheels. Similarly, the electric motor 10 and inverter 14 are representatively depicted as being one that drives the front wheels and one that drives the rear wheels. However, one electric motor 10 and inverter 14 may also be configured to drive all four wheels, including the left and right front wheels and the left and right rear wheels.

[0015] The battery 12 is connected to the electric motor 10 via an inverter 14. The battery 12 supplies electric power to the electric motor 10. The battery 12 is charged with regenerative electric power supplied from the electric motor 10.

[0016] The controller 20 is configured by a computer having a hardware processor such as a CPU (Central Processing Unit), memory, and various interfaces. The CPU reads various computer programs stored in the memory and executes various commands contained in the programs. By executing the programs, the controller 20 functions as multiple information processing circuits, which will be described later. Note that the controller 20 realizes the functions of each information processing circuit through software, but it is also possible to realize those functions through dedicated hardware.

[0017] The controller 20 controls the electric motor 10. Specifically, the controller 20 calculates a torque command and outputs the torque command to the electric motor 10. That is, the controller 20 calculates a current command to be supplied to the electric motor 10 based on the torque command. The controller 20 causes the inverter to perform a switching operation in accordance with the current command, and supplies a drive current from the inverter to the electric motor 10.

[0018] The controller 20 receives signals from various sensors and the like.

[0019] The accelerator opening sensor 30 detects the accelerator opening, which varies depending on the amount of depression of the accelerator pedal by the driver.

[0020] The rotation speed sensor 31 detects the rotation speed of the drive wheels 2. The rotation speed sensor 31 is provided on each of the plurality of drive wheels 2.

[0021] The operation switch 34 is a switch for selecting a driving mode and is operated by the occupant. The driving mode is a mode for setting the driving characteristics of the vehicle and includes a normal mode and a 4L mode. When the 4L mode is selected, the driving characteristics of the vehicle 1 shift to a lower speed and higher torque side compared to the normal mode (low speed, high torque mode). The 4L mode is a driving mode suitable for road surfaces with bumps, road surfaces with high loads such as going over steps such as rocks, or slippery road surfaces such as ice, sand, and mud. By operating the operation switch 34, the occupant can select one of the normal mode and the 4L mode as the driving mode.

[0022] Furthermore, when the 4L mode is selected, the degree of action of the 4L mode can be selected using the operation switch 34. In this specification, a high level of the 4L mode means that the 4L mode is strongly active. Conversely, a low level of the 4L mode means that the 4L mode is weakly active.

[0023] Next, a control method for the vehicle 1 according to this embodiment will be described. The control method for the vehicle 1 according to this embodiment is a method suitable for when the 4L mode is selected as the driving mode. The control method for the vehicle 1 is executed by the controller 20.

[0024] As shown in FIG. 2, the controller 20 includes a target torque calculation unit 21, a target rotation speed calculation unit 22, a feedback control unit 23, and a torque command determination unit 24 as a plurality of information processing circuits.

[0025] The target torque calculation unit 21 determines the first target torque Tt1 based on the accelerator opening θa and the 4L mode level Lev. Specifically, as shown in Fig. 3, the target torque calculation unit 21 has an opening characteristic map that defines the relationship between the accelerator opening θa and the target torque (first target torque Tt1).

[0026] In Figure 3, the opening characteristic map Lts used when the normal mode is selected is shown by a dashed line, and the opening characteristic map Lta used when the 4L mode is selected is shown by a solid line. The opening characteristic map Lta when the 4L mode is selected is set to provide a larger target torque than the opening characteristic map Lts when the normal mode is selected. In other words, when compared at the same accelerator opening θa, the target torque in the 4L mode is larger than the target torque in the normal mode. Therefore, according to the opening characteristic map Lta, a larger target torque (driving torque) can be obtained when the 4L mode is selected than when the normal mode is selected, even if the accelerator opening θa is the same.

[0027] The opening characteristic map Lta has a tendency for the target torque to increase as the accelerator opening θa increases. In this case, the rate of increase in the target torque (i.e., the slope of the opening characteristic map Lta) changes in stages. Specifically, the slope of the opening characteristic map Lta is greatest in the region where the accelerator opening θa is small. In the region where the accelerator opening θa is intermediate, the slope of the opening characteristic map Lta is smaller than the slope in the region where the accelerator opening θa is small. In the region where the accelerator opening θa is large, the slope of the opening characteristic map Lta is also smaller than the slope in the region where the accelerator opening θa is intermediate. Therefore, according to this opening characteristic map Lta, a large target torque (driving torque) can be obtained even in the region where the accelerator opening θa is small.

[0028] Furthermore, this opening characteristic map Lta changes depending on the 4L mode level Lev. The opening characteristic map Lta shown in FIG. 3 is a map used when the 4L mode level Lev is the highest. As the 4L mode level Lev selected by the occupant increases, the opening characteristic map Lta approaches the profile of the opening characteristic map Lta shown in FIG. 3. Conversely, as the 4L mode level Lev selected by the occupant decreases, the opening characteristic map Lta approaches the profile of the opening characteristic map Lts shown in FIG. 3. In this way, the opening characteristic map Lta continuously changes between the opening characteristic map Lts and the opening characteristic map Lta (the opening characteristic map Lta with the 4L mode being the highest level Lev) depending on the 4L mode level Lev selected by the occupant. As can be seen from FIG. 3, the higher the 4L mode level Lev, the greater the target torque when the accelerator pedal is depressed.

[0029] Since the electric motor 10 has performance limitations, the target torque is limited by an upper limit value that corresponds to the characteristics of the electric motor 10. For this reason, the opening characteristic map Lta has a smaller gradient on the high accelerator opening θa side as the 4L level Lev increases. In particular, the opening characteristic map Lta shown in FIG. 3 (the opening characteristic map Lta with the highest level Lev in the 4L mode) has a gradient of zero on the high accelerator opening θa side. In other words, according to this opening characteristic map Lta, the target torque becomes a constant value when the accelerator opening θa is equal to or greater than a predetermined value. This constant value is determined based on the characteristics of the electric motor 10, as described above.

[0030] The target rotation speed calculation unit 22 determines the target rotation speed Nt based on the accelerator opening θa and the 4L mode level Lev. The target rotation speed Nt is an upper limit value for limiting the rotation speed (vehicle speed) of the drive wheels 2. Specifically, as shown in Fig. 4, the target rotation speed calculation unit 22 has a vehicle speed limit map that defines the relationship between the accelerator opening θa and the target rotation speed Nt.

[0031] In FIG. 4 , the vehicle speed limit map Lns used when the normal mode is selected is indicated by a dashed line, and the vehicle speed limit map Lna used when the 4L mode is selected is indicated by a solid line. The vehicle speed limit map Lna when the 4L mode is selected is set so that the target rotation speed is smaller than that of the vehicle speed limit map Lns when the normal mode is selected. That is, when compared at the same accelerator opening θa, the target rotation speed in the 4L mode is smaller than the target rotation speed in the normal mode. Therefore, according to this vehicle speed limit map Lna, even if the accelerator opening θa is the same, the target rotation speed when the 4L mode is selected is smaller than that when the normal mode is selected. Therefore, even if the accelerator opening θa is the same, the rotation speed of the drive wheels 2 is more greatly limited when the 4L mode is selected than when the normal mode is selected.

[0032] The vehicle speed limit map has a tendency that the target rotation speed Nt increases as the accelerator opening degree θa increases. In the example shown in Figure 4, the rate of increase of the target rotation speed Nt (i.e., the slope of the vehicle speed limit map) is constant.

[0033] The vehicle speed limit map changes depending on the 4L mode level Lev. The vehicle speed limit map Lna shown in FIG. 4 is an example of a map used when the 4L mode level Lev is the highest. As the 4L mode level Lev selected by the occupant increases, the vehicle speed limit map Lna approaches the profile of the vehicle speed limit map Lna shown in FIG. 4. Conversely, as the 4L mode level Lev selected by the occupant decreases, the vehicle speed limit map Lna approaches the profile of the opening characteristic map Lts shown in FIG. 4. In this way, the vehicle speed limit map Lna continuously changes between the opening characteristic map Lts and the vehicle speed limit map Lna (the opening characteristic map Lta with the 4L mode being the highest level Lev) depending on the 4L mode level Lev selected by the occupant. As can be seen from FIG. 4, the higher the 4L mode level Lev, the greater the proportion of the rotational speed of the drive wheels 2 that is limited when the accelerator pedal is depressed.

[0034] The feedback control unit 23 determines the second target torque Tt2 based on the rotation speed (actual rotation speed) Na of the drive wheels 2 obtained by the rotation speed sensor 31 and the target rotation speed Nt determined by the target rotation speed calculation unit 22.

[0035] The torque command determiner 24 outputs a torque command to the electric motor 10 based on the first target torque Tt1 and the second target torque Tt2. When the rotation speed of the drive wheels 2 is lower than the target rotation speed Nt, the torque command determiner 24 outputs a torque command to the electric motor 10 based on the first target torque Tt1. On the other hand, when the rotation speed of the drive wheels 2 reaches the target rotation speed Nt, the torque command determiner 24 outputs a torque command to the electric motor 10 based on the second target torque Tt2. In other words, the torque command determiner 24 switches between acceleration control and rotation speed control, which will be described later, based on the rotation speed of the drive wheels 2 and the target rotation speed Nt.

[0036] The following describes in detail the control method for the vehicle 1. The control method for the vehicle 1 according to this embodiment is executed by the controller 20 on the condition that the 4L mode is selected as the driving mode.

[0037] When the 4L mode is selected as the operation mode, the controller 20 first performs acceleration control. In this acceleration control, the torque command determination unit 24 outputs a torque command to the electric motor 10 in accordance with the first target torque Tt1 determined by the target torque calculation unit 21.

[0038] Specifically, the target torque calculation unit 21 determines an opening characteristic map Lta corresponding to the level Lev of the 4L mode. Then, the target torque calculation unit 21 determines a target torque corresponding to the accelerator opening θa based on the determined opening characteristic map Lta. The target torque calculation unit 21 outputs the determined target torque as a first target torque Tt1 to the torque command determination unit 24. The torque command determination unit 24 outputs a torque command to the electric motor 10 in accordance with the first target torque Tt1.

[0039] In this way, in acceleration control, a torque command is output to the electric motor 10 based on the first target torque Tt1. This acceleration control makes it possible to obtain a larger drive torque than when the normal mode is selected. A large drive torque can be obtained even in a range where the accelerator pedal opening θa is small. In addition, once the drive torque increases to a specified torque, the drive torque is controlled to a constant value. This allows the vehicle 1 to start and travel with a small accelerator pedal opening θa even on high-stress road surfaces, such as those with bumps or when going over steps such as rocks. Furthermore, even when towing a stuck vehicle, a high drive torque can be continuously output with a small accelerator pedal opening θa, making it easier to assist the vehicle in escaping from the stuck state.

[0040] When acceleration control is performed, the rotation speed Na of the drive wheels 2 reaches the target rotation speed Nt. That is, the vehicle speed (speed of the vehicle 1) reaches a target vehicle speed (e.g., 20 km / h) determined from the target rotation speed Nt. When the vehicle speed reaches the target vehicle speed, the controller 20 switches from acceleration control to rotation speed limit control. In this rotation speed limit control, the torque command determiner 24 outputs a torque command to the electric motor 10 in accordance with the second target torque Tt2 determined by the target rotation speed calculator 22 and the feedback controller 23.

[0041] Specifically, the target rotation speed calculation unit 22 determines a vehicle speed limit map Lna corresponding to the level Lev of the 4L mode. Then, the target rotation speed calculation unit 22 determines a target rotation speed Nt corresponding to the accelerator opening θa based on the determined vehicle speed limit map Lna. The target torque calculation unit 21 outputs the determined target rotation speed Nt to the feedback control unit 23.

[0042] The feedback control unit 23 calculates the difference (error) between the rotation speed Na of the drive wheels 2 obtained from the rotation speed sensor 31 and the target rotation speed Nt. The feedback control unit 23 determines a second target torque Tt2 using, for example, PID control, so that the rotation speed Na of the drive wheels 2 reaches the target rotation speed Nt. The feedback control unit 23 outputs the determined second target torque Tt2 to the torque command determiner 24. The torque command determiner 24 outputs a torque command to the electric motor 10 in accordance with the second target torque Tt2. At this time, the feedback control unit 23 repeatedly adjusts the second target torque Tt2 until the rotation speed Na of the drive wheels 2 converges to the target rotation speed Nt.

[0043] As described above, the rotation speed limit control uses servo control of the electric motor 10, and outputs a torque command to the electric motor 10 so that the rotation speed of the drive wheels 2 becomes the target rotation speed Nt. This rotation speed limit control limits the rotation speed of the drive wheels 2 to the target rotation speed Nt. Therefore, even if the occupant depresses the accelerator pedal, the rotation speed Na of the drive wheels 2 is limited to the target rotation speed Nt. Furthermore, even if the occupant fully depresses the accelerator pedal, the rotation speed Na of the drive wheels 2 is maintained constant at a low rotation speed. This limits the rotation speed Na of the drive wheels 2, and therefore the vehicle speed, compared to when the normal mode is selected. By limiting the rotation speed of the drive wheels 2, the occupant can control the angular acceleration of the drive wheels 2 by the speed at which the accelerator pedal is depressed. That is, the occupant can suppress the angular acceleration of the drive wheels 2 by controlling the accelerator pedal. This makes it easier to control the drive wheels 2 with the accelerator pedal, even on slippery road surfaces, and suppresses changes in the behavior of the vehicle 1 due to slippage. Furthermore, the slip ratio can be appropriately controlled immediately before or after the drive wheels 2 become stuck, thereby suppressing the vehicle 1 from sinking and improving the possibility of escaping from being stuck.

[0044] FIG. 5 shows a comparative example of a timing chart for a vehicle that has a transfer case between the drive source and the drive wheels, which is a mechanism for switching between two-wheel drive and four-wheel drive, and that has switched to four-wheel drive using this transfer case. In this comparative example, period Ta is the time period during which the grip of the drive wheels is at its limit. During this period Ta, part of the energy is converted into spinning of the drive wheels (region E1). The spinning of the drive wheels causes a decrease in vehicle speed (region R1). At this time, the driver reduces the accelerator pedal depression to maintain vehicle speed (region R2). When the driver subsequently depresses the accelerator pedal, the drive wheels spin again at a certain timing, and this cycle is repeated. In the example shown in FIG. 5, the wheel speed (speed of drive wheels 2) and vehicle speed fluctuate.

[0045] In contrast to this, the rotation speed limit control according to this embodiment limits the rotation speed of the drive wheels 2, allowing the occupant to control the angular acceleration of the drive wheels 2 by the speed at which the accelerator pedal is depressed. This allows the occupant to maintain a constant vehicle speed, as shown in FIG.

[0046] As described above, the control method for the vehicle 1 according to this embodiment includes determining the target rotation speed Nt in accordance with the accelerator opening θa and performing rotation speed limit control based on the target rotation speed Nt. According to this method, the rotation speed limit control can achieve desired driving characteristics. Because the rotation speed limit control simply uses servo control of the electric motor, the desired driving characteristics can be achieved without adding any special mechanism.

[0047] In the control method for the vehicle 1 according to this embodiment, when the rotation speed Na of the drive wheels 2 is lower than the target rotation speed Nt, a target torque (first target torque Tt1) is determined according to the accelerator opening θa, and acceleration control is performed to output a torque command to the electric motor 10 based on the target torque. On the other hand, when the rotation speed Na of the drive wheels 2 reaches the target rotation speed Nt, rotation speed limit control is performed. According to this method, a desired drive torque can be obtained even in a range where the accelerator opening θa is small.

[0048] In the control method for the vehicle 1 according to this embodiment, when compared at the same accelerator pedal depression θa, the target torque in the low-speed, high-torque mode is greater than the target torque in the normal mode. That is, as shown in FIG. 3 , the opening characteristic map Lta when the 4L mode is selected projects higher than the opening characteristic map Lts when the normal mode is selected. This method allows for a large drive torque to be obtained even in a region where the accelerator pedal depression θa is small. This allows for the desired driving characteristics to be obtained.

[0049] If the profile of the opening characteristic map Lta when the 4L mode is selected does not project upward, i.e., if the profile of the opening characteristic map Lts when the normal mode is selected is used, the vehicle 1 itself will not move unless the accelerator pedal is depressed before a driving force sufficient for acceleration is reached. Furthermore, even if the vehicle starts to move, acceleration control will remain and the vehicle will not transition to engine speed limit control. In this regard, according to the control method for the vehicle 1 according to this embodiment, the opening characteristic map Lta when the 4L mode is selected projects upward, so that a large driving torque can be obtained even in a region where the accelerator pedal opening θa is small. This allows the target engine speed to be reached quickly, allowing the vehicle 1 to appropriately switch to engine speed limit control.

[0050] In the control method for the vehicle 1 according to this embodiment, during acceleration control, once the drive torque of the electric motor 10 increases to a specified torque, the drive torque is maintained constant. This specified torque is determined based on the characteristics of the electric motor 10. With this configuration, the drive torque can be maximized within the range of the characteristics of the electric motor 10. Furthermore, since the electric motor 10 can be prevented from outputting a torque exceeding its performance, damage to the electric motor 10 can be prevented.

[0051] In the control method for the vehicle 1 according to this embodiment, the target torque is set to be constant when the accelerator pedal depression amount θa is equal to or greater than a predetermined depression amount. With this configuration, the drive torque of the electric motor 10 can be maintained at a specified torque when the accelerator pedal depression amount θa is large.

[0052] The control method for the vehicle 1 according to this embodiment changes the characteristics of the target rotation speed relative to the accelerator opening θa in response to the driver's operation, thereby enabling the vehicle to achieve driving characteristics that meet the user's preferences.

[0053] The control method for the vehicle 1 according to this embodiment changes the target torque characteristic relative to the accelerator opening θa in response to the driver's operation, thereby enabling driving characteristics to be obtained according to the user's preferences.

[0054] Note that a control device for the vehicle 1 including a controller 20 that controls the electric motor 10 that drives the drive wheels 2 is also included as part of this embodiment. This controller 20 executes the above-described control method for the vehicle 1. This control device for the vehicle 1 simply uses servo control of the electric motor, so it is possible to obtain desired driving characteristics without adding any special mechanism.

[0055] Although the embodiments of the present invention have been described above, the descriptions and drawings that form part of this disclosure should not be understood to limit the present invention. Various alternative embodiments, examples, and operating techniques will become apparent to those skilled in the art from this disclosure.

[0056] REFERENCE SIGNS LIST 1 vehicle (electric vehicle) 2 drive wheel 10 electric motor 12 battery 14 inverter 16 reducer 20 controller 21 target torque calculation unit 22 target rotation speed calculation unit 23 feedback control unit 24 torque command determination unit

Claims

1. A control method for an electric vehicle having an electric motor that drives drive wheels, comprising: determining a target rotation speed for limiting the rotation speed of the drive wheels in accordance with an accelerator pedal position; and performing rotation speed limit control by using servo control of the electric motor to output a torque command to the electric motor so that the rotation speed of the drive wheels becomes the target rotation speed.

2. A control method for an electric vehicle according to claim 1, wherein, when the rotation speed of the drive wheels is lower than the target rotation speed, a target torque is determined in accordance with the accelerator opening, and acceleration control is performed to output a torque command to the electric motor based on the target torque, and when the rotation speed of the drive wheels reaches the target rotation speed, the rotation speed limit control is performed.

3. The control method for an electric vehicle according to claim 2, wherein the low-speed, high-torque mode is selected as a vehicle driving mode including a normal mode and a low-speed, high-torque mode, and when compared at the same accelerator opening, the target torque in the low-speed, high-torque mode is greater than the target torque in the normal mode.

4. The control method for an electric vehicle according to claim 3, wherein, in the acceleration control, once the drive torque of the electric motor has increased to a specified torque, the drive torque is maintained constant.

5. The method for controlling an electric vehicle according to claim 4, wherein the specified torque is determined based on the characteristics of the electric motor.

6. The method for controlling an electric vehicle according to claim 4, wherein the target torque is set to be constant when the accelerator opening is equal to or greater than a predetermined opening.

7. The method for controlling an electric vehicle according to claim 1, wherein the characteristic of the target rotation speed relative to the accelerator opening is changed in response to an operation by a driver.

8. The method for controlling an electric vehicle according to claim 2, wherein the characteristic of the target torque relative to the accelerator opening is changed in response to an operation by a driver.

9. A control device for an electric vehicle, comprising: a controller that controls an electric motor that drives drive wheels, wherein the controller determines a target rotation speed for limiting the rotation speed of the drive wheels in accordance with an accelerator opening, and performs rotation speed limit control that uses servo control of the electric motor to output a torque command to the electric motor so that the rotation speed of the drive wheels becomes the target rotation speed.

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