Electric vehicle

Abstract

To suppress the vibration of a control system that may be generated when a current flowing into a reactor is small, in an electric vehicle comprising a bidirectional converter and an inverter.SOLUTION: An electric vehicle disclosed in the present specification comprises a motor for driving wheels, an inverter, a bidirectional converter and a controller. A low-voltage end of the bidirectional converter is connected to a battery, and a high-voltage end thereof is connected to a DC end of the inverter. The controller controls the bidirectional converter so that a voltage of the high-voltage end have a positive correlation with change in output of the motor. Further, the controller performs PWM control of the inverter when a voltage utilization rate in the inverter is lower than a utilization rate threshold, and performs rectangular-wave control of the inverter when the voltage utilization rate is higher than the utilization rate threshold. When an absolute value of a current flowing into a reactor is smaller than a predetermined current threshold, the controller controls the bidirectional converter so that the voltage of the high-voltage end increases until the voltage utilization rate falls below the utilization rate threshold regardless of output and rotation speed of the motor.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The technology disclosed in this specification relates to an electric vehicle equipped with a motor that drives the wheels and a battery that supplies power to the motor. In this specification, the term "electric vehicle" includes hybrid vehicles equipped with both a motor and an engine, and fuel cell vehicles equipped with a battery and a fuel cell as power sources. [Background technology]

[0002] In an electric vehicle, an inverter is connected between the battery and the motor. The inverter converts the DC power of the battery into AC power suitable for driving the motor. The inverter also converts AC power (regenerative AC power) generated by reverse driving of the motor during deceleration into DC power and supplies it to the battery. A bidirectional DC-DC converter is sometimes connected between the battery and the inverter (for example, Patent Document 1). The low-voltage end of the bidirectional DC-DC converter is connected to the battery, and the high-voltage end is connected to the DC end of the inverter.

[0003] The bidirectional DC-DC converter includes two switching elements and a reactor. The two switching elements are connected in series between the high-voltage terminal and ground. One end of the reactor is connected to the low-voltage terminal, and the other end is connected to the midpoint of the series connection of the two switching elements.

[0004] Known inverter control rules are rectangular control (rectangular wave control) and PWM control (pulse width modulation control). In this specification, "PWM control" includes various types of control that are based on pulse width modulation but differ in details, such as sinusoidal PWM control and overmodulation PWM control. Control that switches between PWM control and rectangular control depending on the situation is known (for example, Patent Document 2). Compared with PWM control, rectangular control has a higher power utilization efficiency. When efficiency is prioritized, rectangular control tends to be selected. Compared with rectangular control, PWM control has a higher control accuracy. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2018-191435 A [Patent Document 2] JP 2009-225633 A Summary of the Invention [Problem to be solved by the invention]

[0006] In general, a bidirectional DC-DC converter for electric vehicles is controlled so that the higher the motor output or rotation speed, the higher the voltage at the high-voltage end. In other words, the controller controls the DC-DC bidirectional converter so that the voltage at the high-voltage end (the high-voltage end of the bidirectional DC-DC converter) has a positive correlation with the motor output or rotation speed. In addition, the bidirectional DC-DC converter tends to have a lower control accuracy (response) when the absolute value of the current flowing through the reactor is small. In other words, the smaller the absolute value of the current flowing through the reactor, the lower the resonant frequency tends to be. Factors that lower the control accuracy include, for example, a delay in feedback control and the presence of a dead time in which both the upper switching element and the lower switching element are turned off. The bidirectional DC-DC converter and the inverter are connected in series, and each is feedback-controlled. In both PWM control and rectangular control of the inverter, a fluctuating component is included in the output. If the resonant frequency of the bidirectional DC-DC converter decreases, the resonant frequency may approach the frequency of the fluctuating component of the inverter. When the resonant frequency of the bidirectional DC-DC converter and the frequency of the inverter fluctuation component approach each other, the control of the bidirectional DC-DC converter and the control of the inverter may adversely affect each other, and the response of the entire control system of the bidirectional DC-DC converter and the inverter may become oscillatory. This specification provides a technology for reducing the possibility that the response of the entire control system of the bidirectional DC-DC converter and the inverter may become oscillatory. [Means for solving the problem]

[0007] The electric vehicle disclosed in this specification includes a motor for driving wheels, an inverter, a battery, a bidirectional DC-DC converter, and a controller. The AC end of the inverter is connected to the motor. The low-voltage end of the bidirectional DC-DC converter is connected to the battery, and the high-voltage end is connected to the DC end of the inverter. The bidirectional DC-DC converter includes two switching elements and a reactor. The two switching elements are connected in series between the high-voltage end and ground. One end of the reactor is connected to the low-voltage end, and the other end is connected to the midpoint of the series connection of the two switching elements. The controller controls the bidirectional DC-DC converter so that the voltage of the high-voltage end has a positive correlation with changes in the output or rotation speed of the motor. The controller also feedback-controls the inverter so that the actual output of the motor coincides with a target output.

[0008] The controller determines a target voltage at the high-voltage end of the bidirectional DC-DC converter so as to have a positive correlation with changes in the target output of the motor or the rotation speed of the motor. The controller feedback controls the bidirectional DC-DC converter so that the voltage at the high-voltage end coincides with the determined target voltage. The controller also performs PWM control on the inverter if the voltage utilization rate of the inverter is lower than a predetermined utilization rate threshold, and performs rectangular control on the inverter if the voltage utilization rate is higher than the utilization rate threshold.

[0009] There is a negative correlation between the voltage at the DC end of the inverter (i.e., the voltage at the high-voltage end of the bidirectional DC-DC converter) and the voltage utilization rate. Increasing the voltage at the DC end decreases the voltage utilization rate. When the absolute value of the current flowing through the reactor is smaller than a predetermined current threshold, the controller controls the bidirectional DC-DC converter to increase the voltage at the high-voltage end until the voltage utilization rate falls below the utilization rate threshold, regardless of the motor output and rotation speed. When the voltage utilization rate falls below the utilization rate threshold, PWM control is selected as the control law for the inverter.

[0010] In general, both PWM control and rectangular control include a fluctuating component in the inverter output, but it is known that the amplitude of the fluctuating component in PWM control is smaller than that in rectangular control. In the electric vehicle disclosed in this specification, when the absolute value of the current flowing through the reactor is smaller than a predetermined current threshold, PWM control is selected regardless of the motor output and rotation speed. The amplitude of the fluctuating component of the inverter output is smaller than that in rectangular control. Therefore, the possibility that the response of the entire control system of the bidirectional DC-DC converter and the inverter becomes oscillatory is reduced.

[0011] Details and further improvements of the technology disclosed in this specification are described in the following "Forms for Carrying Out the Invention". [Brief description of the drawings]

[0012] [Figure 1] 1 is a block diagram of a drive system of an electric vehicle according to an embodiment of the present invention; [Diagram 2] 4 is a flowchart of vibration suppression control executed by a controller. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] An electric vehicle 2 according to the embodiment will be described with reference to the drawings. Fig. 1 shows a block diagram of the drive system of the electric vehicle 2. The electric vehicle 2 according to the embodiment has a motor 31 for driving the wheels 44. An output shaft 41 of the motor 31 is connected to the wheels 44 via a differential gear 42 and an axle 43. The output torque of the motor 31 is transmitted to the wheels 44 and becomes a drive torque.

[0014] The electric vehicle 2 includes a battery 3, a bidirectional DC-DC converter 10, an inverter 20, and a controller 30. For ease of explanation, the "bidirectional DC-DC converter 10" will be simply abbreviated to "converter 10" below.

[0015] The AC terminal 20c of the inverter 20 is connected to the motor, and the DC terminals 20a, 20b are connected to the high voltage terminal 10b and the ground line 10c of the converter 10. The inverter 20 converts the DC power applied to the DC terminals 20a, 20b into AC power suitable for driving the motor 31. The converted AC power is supplied to the motor 31. The motor 31 may generate power by utilizing the inertial force of the vehicle, and the power generated by the motor 31 is called regenerative power. The inverter 20 also has a function of converting the AC regenerative power generated by the motor 31 into DC power and outputting it from the DC terminals 20a, 20b. The regenerative power converted into DC power is charged to the battery 3. The inverter 20 is well known, so a detailed description of the circuit will be omitted.

[0016] The converter 10 will now be described. The low-voltage terminal 10a of the converter 10 is connected to the positive electrode 3a of the battery 3, and the high-voltage terminal 10b of the bidirectional converter is connected to the DC terminal positive electrode 20a of the inverter 20. The ground line 10c of the converter 10 connects the negative electrode 3b of the battery 3 and the DC terminal negative electrode 20b of the inverter 20.

[0017] The converter 10 includes two switching elements (upper switching element 11a and lower switching element 11b), two diodes 12a and 12b, a reactor 13, two capacitors 14a and 14b, a current sensor 15, and voltage sensors 16a and 16b.

[0018] Two switching elements (upper switching element 11a and lower switching element 11b) are connected in series between high voltage terminal 10b and ground line 10c. Upper switching element 11a is connected to the high voltage terminal 10b side, and lower switching element 11b is connected to the ground line 10c side. Diode 12a is connected in anti-parallel to upper switching element 11a, and diode 12b is connected in anti-parallel to lower switching element 11b. Diodes 12a and 12b are connected in a direction that passes a current from ground line 10c to high voltage terminal 10b.

[0019] One end of the reactor 13 is connected to the low-voltage terminal 10a, and the other end is connected to the midpoint of the series connection of the two switching elements 11a and 11b.

[0020] Capacitor 14a is connected between low voltage end 10a and ground line 10c, and capacitor 14b is connected between high voltage end 10b and ground line 10c. Capacitor 14a suppresses voltage fluctuations at low voltage end 10a. Capacitor 14b suppresses voltage fluctuations at high voltage end 10b.

[0021] The current sensor 15 measures the current flowing through the reactor 13. The voltage sensor 16a measures the voltage between the low voltage end 10a and the ground line 10c, and the voltage sensor 16b measures the voltage between the high voltage end 10b and the ground line 10c. The measurement values ​​of the current sensor 15 and the voltage sensors 16a and 16b are sent to the controller 30.

[0022] When upper switching element 11a is appropriately turned on and off, the voltage at high voltage end 10b is stepped down and output from low voltage end 10a. When lower switching element 11b is appropriately turned on and off, the voltage at low voltage end 10a is stepped up and output from high voltage end 10b. When upper switching element 11a and lower switching element 11b are turned on and off with complementary PWM signals (or rectangular signals), a current flows between low voltage end 10a and high voltage end 10b (i.e., reactor 13) so that the voltage ratio between low voltage end 10a and high voltage end 10b is constant.

[0023] Controller 30 alternately turns on and off upper switching element 11a and lower switching element 11b so that the voltage of high-voltage terminal 10b coincides with a predetermined target voltage. Then, if the actual voltage of high-voltage terminal 10b is lower than the target voltage, a current flows from low-voltage terminal 10a to high-voltage terminal 10b. If the actual voltage of high-voltage terminal 10b is higher than the target voltage, a current flows from high-voltage terminal 10b to low-voltage terminal 10a. In this case, the power generated by motor 31 flows to battery 3, and battery 3 is charged.

[0024] The converter 10 is a voltage converter suitable for the electric vehicle 2 that frequently switches between power running and regeneration. Note that power running means that the motor 31 outputs positive torque using power from the battery 3, accelerating the electric vehicle 2. Regeneration means that the motor 31 is reverse-driven by the inertial force of the vehicle to generate electric power.

[0025] The controller 30 controls the inverter 20 and the converter 10. A vehicle speed sensor 37, an accelerator pedal sensor 38, and a brake pedal sensor 39 are connected to the controller 30. The controller 30 determines a target output of the motor 31 based on the vehicle speed obtained from the vehicle speed sensor 37, the accelerator opening obtained from the accelerator pedal sensor 38, and the brake pedal depression amount obtained from the brake pedal sensor 39, and controls the converter 10 and the inverter 20 so that the actual output of the motor 31 matches the target output.

[0026] The inverter 20 has three sets of two switching elements connected in series. The three sets of series connections are connected in parallel to the DC end of the inverter 20. When the switching elements are appropriately turned on and off, AC is output from the midpoint of each of the three sets of series connections. The circuit structure and operation of the inverter 20 are well known, so a detailed explanation will be omitted.

[0027] Controller 30 determines a target voltage at high voltage end 10b of converter 10 from the output of motor 31 and / or the rotation speed of motor 31. Controller 30 acquires an actual voltage value at high voltage end 10b from voltage sensor 16b. Based on the measurement value of voltage sensor 16b, controller 30 feedback controls converter 10 (upper switching element 11a and lower switching element 11b) so that the voltage at high voltage end 10b of converter 10 coincides with the target voltage.

[0028] The target voltage is set to a larger value as the output of motor 31 or the rotation speed of motor 31 increases. Conversely, the target voltage is set to a smaller value as the output of motor 31 or the rotation speed of motor 31 decreases. In other words, controller 30 determines the voltage (target voltage) at high-voltage end 10b so as to have a positive correlation with changes in the output of motor 31 or the rotation speed of motor 31. Then, controller 30 feedback-controls converter 10 so that the voltage at high-voltage end 10b coincides with the target voltage.

[0029] As described above, the controller 30 controls the inverter 20. The AC end 20c of the inverter 20 is provided with a current sensor 21, and the controller 30 obtains the actual output of the inverter 20 (i.e., the actual output of the motor 31) from the measurement value of the current sensor 21. The effective voltage of the AC power output by the inverter 20 is estimated from the voltage of the DC power input to the inverter 20. The voltage of the DC power input to the inverter 20 is output from the voltage sensor 16b. The controller 30 feedback-controls the inverter 20 based on the measurement values ​​of the above-mentioned sensors so that the output of the inverter 20 (i.e., the output of the motor 31) matches the target output. The output of the inverter 20 is AC power, and the controller 30 feedback-controls the inverter 20 so that the phase and frequency of the AC output of the inverter 20 match the targets.

[0030] As is well known, the control rules of the inverter 20 include PWM control and rectangular control. In this specification, "PWM control" includes various types of control that are based on pulse width modulation but differ in detail, such as sinusoidal PWM control and overmodulation PWM control. The control rules of PWM control and rectangular control are well known, so a detailed description will be omitted.

[0031] The controller 30 calculates the voltage utilization rate of the inverter 20, and selects either PWM control or rectangular control according to the voltage utilization rate. The controller 30 controls the inverter 20 using the selected control law. The voltage utilization rate of the inverter 20 can be calculated, for example, by the formula: [voltage utilization rate]=[coefficient]×[effective voltage at the AC end of the inverter] / [voltage at the DC end of the inverter]. As shown in this formula, the voltage utilization rate of the inverter has a negative correlation with the voltage at the DC end of the inverter.

[0032] The controller 30 selects PWM control if the calculated voltage utilization rate is lower than a predetermined utilization rate threshold, and selects rectangular control if the calculated voltage utilization rate is higher than the utilization rate threshold, and controls the inverter 20 using the selected control law.

[0033] As is well known, PWM control is characterized by its superior control accuracy (smoothness of change in motor rotation speed) but inferior efficiency to rectangular control. Conversely, rectangular control is characterized by its superior efficiency but inferior control accuracy to PWM control. When fine control is required, PWM control is selected, and when efficiency is more important than control accuracy, rectangular control is selected. The controller 30 switches between PWM control and rectangular control depending on the situation. In addition, both PWM control and rectangular control contain a fluctuation component in the inverter output. The frequency of the fluctuation component is the same, but the amplitude of the fluctuation component is larger in rectangular control than in PWM control.

[0034] The control accuracy (response) of the converter 10 tends to decrease when the absolute value of the current flowing through the reactor 13 is small. Causes of the decrease in control accuracy include, for example, a delay in feedback control and the presence of a dead time during which both the upper switching element 11a and the lower switching element 11b are turned off. A decrease in control accuracy is equivalent to a decrease in the resonant frequency.

[0035] On the other hand, as described above, in the control of the inverter 20, the amplitude of the output fluctuation component is larger in the rectangular control than in the PWM control. The converter 10 and the inverter 20 are connected in series, and each is feedback-controlled. When the resonant frequency of the converter 10 drops and approaches the frequency of the output fluctuation component of the inverter 20, the controls of the converter 10 and the inverter 20 may adversely affect each other, and the response of the entire control system composed of the converter 10 and the inverter 20 may become oscillatory. As described later, in the electric vehicle 2 of the embodiment, the PWM control is always selected when the current flowing through the reactor 13 falls below a predetermined current threshold. By doing so, the amplitude of the output fluctuation component of the inverter 20 becomes smaller than that in the rectangular control, and the possibility that the response of the entire control system becomes oscillatory can be reduced.

[0036] Fig. 2 shows a flowchart of vibration suppression control executed by the controller 30. The processing of the controller 30 will be described with reference to Fig. 2. The controller 30 repeats the processing of Fig. 2 at a constant control period. In Fig. 2, "reactor current" means a current flowing through the reactor 13. The reactor current is measured by the current sensor 15 and sent to the controller 30.

[0037] As shown in FIG. 2, if the absolute value of the reactor current is smaller than the "current threshold", the controller 30 sets the flag to "ON" (steps S2: YES, S3: YES, S4). If the absolute value of the reactor current is larger than the "current threshold + offset", the controller 30 sets the flag to "OFF" (steps S2: NO, S5). Here, the "flag" is a parameter defined in the program of the controller 30, and "ON" and "OFF" are constants defined in the program. The "offset" is a constant provided to prevent hunting, and is defined in advance in the program. Hunting here means that the value of the "flag" frequently switches between "ON" and "OFF".

[0038] If the absolute value of the reactor current is smaller than the "current threshold plus offset" (step S2: YES) and is larger than the "current threshold" (step S3: NO), the value of the flag is maintained at the previous value.

[0039] Next, the controller 30 checks the value of the flag (step S6). If the flag is "ON" (step S6: YES), the controller 30 controls the converter 10 to increase the voltage at the high-voltage end 10b until the voltage utilization rate of the inverter 20 falls below the utilization rate threshold (steps S7, S8: NO, S7). The voltage at the high-voltage end 10b is equal to the voltage applied to the DC end of the inverter 20. As described above, there is a negative correlation between the voltage at the DC end of the inverter 20 and the voltage utilization rate. When the voltage utilization rate of the inverter 20 falls below the utilization rate threshold, the PWM control law is selected. By selecting the PWM control law, a situation in which the reactor current is small and the rectangular control law is executed, which is likely to cause an oscillation phenomenon, is avoided.

[0040] On the other hand, if the flag is "OFF" (step S6: NO), controller 30 determines a target voltage for high voltage end 10b of converter 10 based on the vehicle speed, accelerator opening, and brake pedal depression amount, and controls converter 10 so that the actual voltage at high voltage end 10b coincides with the target voltage (step S9). In step S9, the target voltage is determined so as to have a positive correlation with the output or rotation speed of motor 31.

[0041] The processes in steps S2 and S3 when the determination is "NO" are processes for preventing hunting of the flag value when the reactor current fluctuates near the current threshold. That is, the essence of the process in FIG. 2 is as follows. When the absolute value of the current flowing through the reactor is greater than the current threshold, the controller 30 determines the target voltage of the high voltage end 10b of the converter 10 based on the vehicle speed, the accelerator opening, and the brake pedal depression amount, and feedback controls the converter 10 so that the target voltage is realized. On the other hand, when the absolute value of the current flowing through the reactor is smaller than the current threshold, the controller 30 controls the converter 10 so as to increase the voltage of the high voltage end 10b until the voltage utilization rate of the inverter 20 falls below the utilization rate threshold, regardless of the vehicle speed / accelerator opening / brake pedal depression amount. Such a process can reduce the possibility that the control system of the converter 10 and the inverter 20 becomes oscillatory when the reactor current is small.

[0042] Here are some points to note about the technology described in the embodiment. The frequency of the inverter output fluctuation component depends on the motor rotation speed. However, regardless of the motor rotation speed, if the resonant frequency of the converter decreases, it may approach the frequency of the inverter output fluctuation component.

[0043] The situation where the absolute value of the reactor current is small can occur regardless of the motor speed. For example, even when driving at high speed, if the accelerator pedal is only slightly depressed, the torque required of the motor becomes small, and as a result, the current flowing through the reactor becomes small.

[0044] In step S9 in FIG. 2, the controller 30 may determine the target voltage taking into consideration the temperatures of the inverter 20 and the motor 31 (or the temperature of the battery 3, etc.) in addition to the vehicle speed, the accelerator opening, and the brake pedal depression amount.

[0045] The controller 30 may determine the voltage of the high voltage end 10b so as to have a positive correlation with the change in the output of the motor 31, or may determine the voltage of the high voltage end 10b so as to have a positive correlation with the change in the rotation speed of the motor 31.

[0046] An upper limit is set for the voltage (output voltage) at the high voltage end of converter 10. In steps S7 and S8 in Fig. 2, a situation may occur in which the voltage utilization rate does not exceed the utilization rate threshold even if the output voltage of converter 10 reaches the upper limit. In such a case, controller 30 may stop the process in Fig. 2 and move to exception processing (not shown).

[0047] Although specific examples of the present invention have been described above in detail, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples exemplified above. The technical elements described in this specification or drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or drawings can achieve multiple objectives simultaneously, and achieving one of these objectives is itself technically useful. [Explanation of symbols]

[0048] 2: Electric vehicle 3: Battery 10: Bidirectional DC-DC converter (converter) 11a, 11b: Switching element 12a, 12b: Diode 13: Reactor 14a, 14b: Capacitor 15: Current sensor 16a, 16b: Voltage sensor 20: Inverter 21: Current sensor 30: Controller 31: Motor 37: Vehicle speed sensor 38: Accelerator pedal sensor 39: Brake pedal sensor 41: Output shaft 42: Differential gear 43: Axle 44: Wheel

Claims

[Claim 1] A motor that drives the wheels; an inverter having an AC end connected to the motor; A battery; a bidirectional DC-DC converter having a low voltage terminal connected to the battery and a high voltage terminal connected to a DC terminal of the inverter; a controller that controls the bidirectional DC-DC converter so that the voltage at the high voltage end has a positive correlation with a change in the output of the motor or a change in the rotation speed of the motor, and that PWM controls the inverter when a voltage utilization rate of the inverter is lower than a predetermined utilization rate threshold, and that rectangular-wave controls the inverter when the voltage utilization rate is higher than the utilization rate threshold; Equipped with The bidirectional DC-DC converter comprises: Two switching elements connected in series between the high voltage end and a ground line; a reactor having one end connected to the low-voltage end and the other end connected to a midpoint of the series connection of the two switching elements; Equipped with There is a negative correlation between the voltage at the high voltage end and the voltage utilization rate, When an absolute value of a current flowing through the reactor is smaller than a predetermined current threshold, the controller controls the bidirectional DC-DC converter to increase the voltage of the high voltage end until the voltage utilization rate falls below the utilization rate threshold, regardless of the output and the rotation speed. Electric car.

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