JP7779161B2 - Electric vehicle drive unit - Google Patents

Description

The present invention relates to a drive system for an electric vehicle, and in particular to a system equipped with two electric motors.

Patent Documents 1 and 2 listed below show a drive unit for an electric vehicle that has two electric motors and combines the output of these two motors using a planetary gear mechanism to output the combined power. By using the planetary gear mechanism, the speed of each electric motor can be changed even when the vehicle speed is the same. Patent Document 1 listed below describes that when a vehicle is driven by two electric motors, each electric motor is operated at its most efficient rotational speed (see paragraph 0027). Patent Document 2 listed below describes that by determining the planetary gear ratio based on the maximum power or maximum torque of the two electric motors, even when the two electric motors are controlled so that the planetary gear mechanism does not cause differential rotation, torque characteristics roughly equivalent to when differential rotation is allowed over a wide operating range of the drive unit can be obtained (see paragraph 0027).

Japanese Patent Application Laid-Open No. 2018-100709 Japanese Patent Application Laid-Open No. 2018-117409

In a drive system for an electric vehicle equipped with two electric motors and a planetary gear mechanism connecting them, the rotational speeds of the two electric motors can be selected relatively freely, although there are restrictions on the planetary gear ratio. However, if the output of the drive system is set to maximum torque or close to it, the rotational speeds of the two electric motors may be limited.

When a sudden acceleration request is made to this drive unit, the rotational speed of the two electric motors at that time is not necessarily sufficient to generate a large torque commensurate with the sudden acceleration request. Therefore, in order to generate a large torque commensurate with the sudden acceleration request, it may be necessary to change the rotational speed of the two rotating electric machines, which may result in a delay before the large torque is actually generated.

The purpose of this invention is to reduce the delay until the appropriate torque is generated when sudden acceleration is required.

The drive device for an electric vehicle of the present invention includes a first electric motor and a second electric motor, and further includes a planetary gear mechanism having a first input element connected to the first electric motor, a second input element connected to the second electric motor, and an output element connected to a drive wheel, and the rotational speeds of the first input element and the second input element are set to rotational speeds that can generate a maximum torque of the output element at each vehicle speed within a speed range used by the electric vehicle .

By setting the rotational speed as described above, when a torque increase is required, the output torque of the drive unit at that vehicle speed can be maximized without changing the rotational speeds of the first and second motors.

In addition, the rotational speeds of the first input element and the second input element can be set so that the absolute value of the input speed difference, which is the difference between the rotational speeds of the first input element and the second input element, is minimized. By minimizing the input speed difference, it is possible to reduce differential losses in the planetary gear mechanism.

Furthermore, when at least one of the first and second electric motors operates in the upper limit constant torque region, the input speed difference, which is the difference in rotational speed between the first input element and the second input element, can be set to have a mathematical relationship similar to the mathematical relationship between the speed of the output element and the input speed difference when both the first and second electric motors operate in the upper limit constant power region and at less than their maximum rotational speed. By setting the input speed difference as described above, the method for calculating the speeds of the first and second electric motors when they operate below their maximum rotational speeds is unified.

By being able to set the output torque of the drive unit to its maximum value at that time without changing the rotational speed of the first and second electric motors, it is possible to reduce response delays when a request for increased torque is made.

1 is a schematic diagram illustrating a schematic configuration of a drive device according to an embodiment. FIG. 3 is an explanatory diagram showing the output characteristics of an electric motor. FIG. 10 is a diagram showing upper torque limits of the first and second electric motors when the vehicle speed is 20 km/h. FIG. 10 is a diagram showing the speed difference between the first sun gear and the second sun gear when the vehicle speed is 20 km/h. FIG. 10 is a diagram showing upper torque limits of the first and second electric motors when the vehicle speed is 37 km/h. FIG. 10 is a diagram showing the speed difference between the first sun gear and the second sun gear when the vehicle speed is 37 km/h. FIG. 10 is a diagram showing upper torque limits of the first and second electric motors when the vehicle speed is 50 km/h. FIG. 10 is a diagram showing the speed difference between the first sun gear and the second sun gear when the vehicle speed is 50 km/h. FIG. 10 is a diagram illustrating an example of the relationship between the vehicle speed and a set speed difference.

Embodiments of the present invention will now be described with reference to the drawings. Figure 1 is a schematic diagram showing the configuration of a drive unit 10 for an electric vehicle according to this embodiment. The drive unit 10 includes a first electric motor M1 and a second electric motor M2, each connected to a separate input element of a planetary gear mechanism 12. The output element of the planetary gear mechanism 12 is connected to left and right drive wheels 16 via a final reduction gear 14, which includes a differential.

The planetary gear mechanism 12 has two input elements: a first sun gear 18 to which a first electric motor M1 is connected, and a second sun gear 20 to which a second electric motor M2 is connected. The second sun gear 20 meshes with a plurality of outer planetary pinions 24 (hereinafter referred to as outer pinions 24) rotatably supported on a planetary carrier 22 (hereinafter referred to as carrier 22). The first sun gear 18 meshes with a plurality of inner planetary pinions 26 (hereinafter referred to as inner pinions 26) rotatably supported on the carrier 22. Each inner pinion 26 also meshes with one outer pinion 24. The first sun gear 18, the second sun gear 20, and the carrier 22 are rotatable about a common axis. The carrier 22 is the output element of the planetary gear mechanism 12, and is equipped with an output gear 28 integral with the carrier 22. This output gear 28, together with a driven gear 30 that rotates integrally with the differential, forms a final reduction gear pair 32.

The planetary gear mechanism 12 is a compound planetary gear mechanism including a first planetary gear train 34 consisting of a first sun gear 18, an outer pinion 24, and an inner pinion 26, and a second planetary gear train 36 consisting of a second sun gear 20 and an outer pinion 24. The first planetary gear train 34 is a double-pinion planetary gear train, and the second planetary gear train 36 is a single-pinion planetary gear train.

The first sun gear 18 is fixed to the first input shaft 38, which is connected to the first motor shaft 42, the output shaft of the first electric motor M1, via the first input gear pair 40. The first motor shaft 42 itself may also serve as the first input shaft 38. The second sun gear 20 is fixed to the second input shaft 44, which is connected to the second motor shaft 48, the output shaft of the second electric motor M2, via the second input gear pair 46. The second motor shaft 48 itself may also serve as the second input shaft 44. The first and second input gear pairs 40, 46 represent a transmission mechanism between the first and second motor shafts 42, 48 and the first and second input shafts 38, 44. Other configurations, such as a gear train consisting of three or more gears, may also be used. The first and second input gear pairs 40, 46 may also be replaced with other speed conversion mechanisms, such as a transmission mechanism including a chain and sprockets.

This planetary gear mechanism 12 is a three-element, two-degree-of-freedom mechanism; once the rotational speeds of two of the three elements are determined, the rotational speed of the remaining element is uniquely determined. For example, once the rotational speeds of the first sun gear 18 and second sun gear 20 are determined, the rotational speed of the carrier 22 is determined accordingly.

A clutch element is provided in the transmission system from the second electric motor M2 to the second sun gear 20. This clutch element allows rotation of the second sun gear 20 in the rotational direction when the vehicle moves forward, but prevents rotation in the reverse direction. The clutch element is, for example, a one-way clutch 50 provided on the second input shaft 44.

The drive unit 10 further includes a control unit 52 that controls the first electric motor M1 and the second electric motor M2. The control unit 52 controls the output torque and rotation speed of the first electric motor M1 and the second electric motor M2.

The drive unit 10 can run in two modes: a first mode in which the vehicle is driven solely by the output of the first electric motor M1, and a second mode in which the vehicle is driven by the output of both the first electric motor M1 and the second electric motor M2. The first mode is used under low-speed, low-load conditions, while the second mode is used under high-speed, high-load conditions.

Figure 2 shows the output characteristics of a typical electric motor. The output characteristics of an electric motor are such that the upper torque limit is constant relative to the rotational speed below the base rotational speed, and the upper power limit is constant relative to the rotational speed above the base rotational speed. A constant upper power limit means that the upper torque limit decreases in inverse proportion to the rotational speed. Hereinafter, the region where the upper torque limit is constant will be referred to as the upper torque limit constant region, and the region where the upper power limit is constant will be referred to as the upper power limit constant region. The first electric motor M1 and second electric motor M2 described above also have similar output characteristics.

As in drive unit 10, when two electric motors are connected to two of the three elements of a planetary gear mechanism and the remaining element is an output element, there are multiple selectable rotational speeds for the two electric motors for a given rotational speed of the output element, provided there is a margin from the upper torque limit. If both electric motors are operating in a region with sufficient margin from the maximum rotational speed in the upper torque limit range and at a torque lower than the upper torque limit, the torque of the output element can be maximized for that rotational speed by maintaining both motors' current rotational speeds and increasing the torque to the upper torque limit, regardless of the rotational speed of the two motors. On the other hand, when two electric motors are operating in the upper power limit range, increasing the torque of both motors to the upper torque limit for that rotational speed while maintaining the current rotational speed of the two motors does not necessarily maximize the torque of the output element for that rotational speed. In order to maximize the torque of the output element, it may be necessary to change the rotational speed of the two electric motors. In this case, it takes time to change the rotational speed of the electric motors, and therefore time to reach the upper torque limit. As a result, when the vehicle driver requests sudden acceleration, there may be a time delay before the output torque increases accordingly. Furthermore, even if the torque required by the driver is close to the upper torque limit, a similar time delay may occur.

Even if the required torque of the output element is low, if the two electric motors are operated at a rotational speed that can generate the maximum torque of the output element at that rotational speed, there is no need to change the rotational speed of the electric motors when sudden acceleration is requested, and the time delay until the torque corresponding to the requested sudden acceleration is reached can be reduced. Below, we will explain how to set the rotational speed of the two electric motors using specific examples.

The parameters of the drive unit 10 and their specific numerical values are defined as follows:

The planetary gear ratio ρ is defined as the absolute value of the ratio of the relative speeds ΔN _sd (= N _sd - N _c ) of the first sun gear 18 (first input element) to the carrier 22 (output element) and ΔN _ss (= N _ss - _{N c} ) of the second sun gear 20 (second input element) when these speeds have opposite signs (ρ = |ΔN _sd /ΔN _ss |). In the planetary gear mechanism 12 shown in the drive unit 10, the planetary gear ratio ρ is the ratio of the number of teeth Z _sd of the first sun gear 18 to the number of teeth Z _ss of the second sun gear 20 (ρ = Z _sd /Z _ss ).

The torque T _C of the carrier 22 is balanced with the torque T _sd of the first sun gear 18 and the torque T _ss of the second sun gear 20 (T _C = T _sd + T _ss ). The rotational speeds of the elements are constrained by the relationship shown in the following equation (1).
ρN _sd +N _ss = (1+ρ)N _c ...(1)

By transforming equation (1) using the difference ΔN _s between the rotational speed N _sd of the first sun gear 18 and the rotational speed N _ss of the second sun gear 20 (ΔN _s = N _sd - _{N ss} ), the rotational speeds N _sd and N _ss of the first and second sun gears 18, 20 can be expressed as the following equations (2) and (3).
N _sd = N _c +ΔN _s / (1+ρ) ... (2)
N _ss =N _c -ρΔN _s /(1+ρ) ...(3)
Furthermore, the torque _Tsd of the first sun gear 18 and the torque _Tss of the second sun gear 20 have the relationship expressed by the following equation (4).
T _sd = ρT _ss (4)

Fig. 3 is a diagram showing the torque upper limits T M1_u and T M2_u of the first and second electric motors M1 and _M2 when both the first and second electric motors M1 and M2 are operating in the upper limit constant torque region and in a region away from the maximum rotation speed of the upper limit constant torque region. In particular, Fig. 3 illustrates the torques of the first and second electric motors _M1 and M2 when the vehicle speed is 20 km/h and the parameters of the drive device 10 are the specific values shown in Table 1. The portion of the torque upper limit value T _{M2_u} of the second electric motor M2 represented by a solid line corresponds to the range in which the second electric motor M2 can generate the maximum torque T _{M2_max} , and the portion of the torque upper limit value T _{M1_u} of the first electric motor M1 corresponds to the range in which the second electric motor M2 can generate the maximum torque T _{M2_max} . In this drive system 10, due to the limitation of equation (4), when the vehicle is driven by the outputs of both the first electric motor M1 and the second electric motor M2, the first electric motor M1 does not generate the maximum torque T _{M1_max} . The torque T _M1 of the first electric motor M1 is approximately 70% of the torque T _M2 of the second electric motor M2 due to the relationship between the reduction ratios γ _M1 and γ _M2 of the first and second input gear pairs 40, 46 and equation (4). The thin lines indicate the torque characteristics of the first and second electric motors M1 and M2 individually.

As can be seen from equation (1), the rotation speed N _M1 of the first electric motor M1 and the rotation speed N _M2 of the second electric motor M2 have a relationship in which when one is high, the other is low. The dashed portion on the right side of the figure of the torque upper limit value T _{M1_u} of the first electric motor M1 slopes downward to the right in accordance with the output characteristics of the electric motor alone, shown by the thin line. Correspondingly, the torque upper limit value T _{M2_u} of the second electric motor M2 slopes downward to the left, as shown by the dashed line, at a rotation speed higher than the downward-sloping portion of the output characteristics of the electric motor alone, so as to satisfy equation (4). The downward-sloping portion of the torque upper limit value T _{M1_u} of the first electric motor M1 also corresponds to the downward-sloping portion of the torque upper limit value T _{M2_u} of the second electric motor M2. The drive device 10 maximizes output torque when the first and second electric motors M1 and M2 are operating within the portions of the torque upper limit values T _{M1_u} and T _{M2_u} shown by the solid lines. Therefore, if the first and second electric motors M1, M2 are operating at rotational speeds within the range indicated by the solid lines, when a request is made to increase the output torque, the drive unit 10 can set the output torque to the maximum value that can be generated at that time without changing the rotational speeds of the first and second electric motors M1, M2.

In a specific example, when a vehicle is traveling at 20 km/h, the rotational speed of the drive wheels 16 is 126.01 rpm, and the carrier rotational speed _Nc is 541.86 rpm. At this time, when the first electric motor M1 operates at 153.67 to 4055.64 rpm and the second electric motor M2 operates at 2850.54 to 108.01 rpm, the output torque of the drive unit 10 can be maximized. Therefore, if the rotational speeds of the first and second electric motors M1, M2 are kept within the above ranges while the vehicle is traveling at 20 km/h, when a request to increase the output torque is made, the output torque can be increased to the maximum value at that time (vehicle speed 20 km/h) while maintaining the rotational speeds of the two electric motors.

Fig. 4 is a diagram showing the relationship between the speed difference _ΔNs between the first sun gear 18 and the second sun gear 20 and the torque of the drive wheels 16 under the same conditions as in Fig. 3 . The portion shown by the solid line corresponds to the range of the maximum value of the output torque of the drive device 10 at this vehicle speed, that is, the range of the torque upper limits T _{M1_u} and T _{M2_u} shown by the solid lines in Fig. 3 . Fig. 4 also shows the minimum speed difference ΔN _{s_min} at which the speed difference _ΔNs is minimum within the range in which the output torque of the drive device 10 is at its maximum value. In this case, the minimum speed difference ΔN _{s_min} is 0. Fig. 3 shows the operating points D _M1 (ΔN _{s_min} ) and D _M2 (ΔN _{s_min} ) of the first and second electric motors M1 and M2 that achieve this minimum speed difference ΔN _{s_min} . In a specific example, when ΔN _{s — min} =0, the rotation speed of the first electric motor M1 is 1896.50 rpm, and the rotation speed of the second electric motor M2 is 1625.57 rpm.

If the speed difference _ΔNs is 0, then the differential of the planetary gear mechanism 12, that is, the loss due to the speed difference between the two input elements, becomes zero.

5 is a diagram showing the torque upper limit values T M1_u and T _{M2_u} of the first and second electric motors M1 and _M2 when both the first and second electric motors M1 and M2 are operating near the maximum rotational speed in the upper limit torque constant region. In particular, FIG. 5 illustrates the torque upper limit values of the first and second electric motors M1 and M2 when the vehicle speed is 37 km/h and the parameters of the drive device 10 are the specific values shown in Table 1. As in FIG. 3 , the solid lines of the torque upper limit values T _{M1_u} and T _{M2_u} indicate the range in which the second electric motor M2 can generate the maximum torque T _{M2_max} . The thin lines indicate the torque characteristics of the first and second electric motors M1 and M2 alone.

As can be seen from FIG. 5 , the range in which the second electric motor M2 can generate maximum torque is very narrow. In this specific example, the rotational speed of the drive wheels 16 is 233.12 rpm, and the carrier rotational speed _Nc is 1002.44 rpm. In this case, when the first electric motor M1 operates at 3731.58 to 4055.64 rpm and the second electric motor M2 operates at 2850.54 to 2622.77 rpm, the output torque of the drive unit 10 can be maximized. Therefore, if the rotational speeds of the first and second electric motors M1 and M2 are within the above ranges, when a request to increase the output torque is made, the output torque can be maximized without changing the rotational speeds of the first and second electric motors M1 and M2. Conversely, if the rotational speeds of the first and second electric motors M1, M2 are outside the above range, maintaining the rotational speeds will only result in the output torque indicated by the dashed line, and the output torque of the drive unit 10 will not be the maximum value obtainable at that speed. Therefore, in order to obtain the maximum output torque, it is necessary to change the rotational speeds of the first and second electric motors M1, M2.

FIG. 6 is a diagram showing the relationship between the speed difference _ΔNs between the first sun gear 18 and the second sun gear 20 and the torque of the drive wheels 16 under the same conditions as in FIG. 5 . The portion shown by the solid line corresponds to the range of maximum values of the output torque of the drive device 10 at this vehicle speed. FIG. 6 also shows the minimum speed difference _{ΔNs_min} , at which the speed difference _ΔNs is minimum within the range in which the output torque of the drive device 10 is maximum. This minimum speed difference _{ΔNs_min} is 115.99 rpm, which is not 0. FIG. 5 also shows the operating points D _M1 ( _{ΔNs_min} ) and D _M2 ( _{ΔNs_min} ) of the first and second electric _{motors M1 and M2} when the speed difference _ΔNs = 0. 5, when the drive unit 10 is operating with the speed difference _ΔNs being 0 and a request for an increase in torque is made, the rotation speeds of the first and second electric motors M1, M2 must be changed in order to obtain the maximum value of the output torque at that time (vehicle speed 37 km/h). Therefore, when the vehicle is traveling at a speed of 37 km/h, it is preferable to control the first and second electric motors M1, M2 so that the speed difference _ΔNs is 115.99 rpm in preparation for a sudden request for an increase in torque and to suppress loss due to differential rotation of the planetary gear mechanism 12.

7 is a diagram showing the torque upper limit values T _{M1_u} and T _{M2_u} of the first and second electric motors M1 and M2 when both the first and second electric motors M1 and M2 are operating in the upper limit constant power region and at speeds lower than the maximum rotational speeds N _{M1_max} and N _{M2_max} . In particular, FIG. 7 illustrates the torque of the first and second electric motors M1 and M2 when the vehicle speed is 50 km/h and the parameters of the drive system 10 are the specific values shown in Table 1. There is only one set of operating points of the first and second electric motors M1 and M2 that provides the maximum output torque of the drive system 10 at this time. The rotational speed of the drive wheels 16 and the carrier rotational speed _Nc at this operating point are 315.03 rpm and 1354.64 rpm in this specific example. At this time, the rotation speed of the first electric motor M1 is 5261.62 rpm, and the rotation speed of the second electric motor M2 is 3698.16 rpm.

8 is a diagram showing the relationship between the speed difference _ΔNs between the first sun gear 18 and the second sun gear 20 and the torque of the drive wheels 16 under the same conditions as in FIG. 7. The speed difference _ΔNs that gives the maximum value of the output torque of the drive unit 10 at this time is uniquely determined. When the rotation speeds of the first and second electric motors M1, M2 are both less than the maximum rotation speed, the speed difference _ΔNs is calculated from equation (4) as follows:
P _sd /N _sd = ρ(P _ss /N _ss )...(5)
Substituting equations (2) and (3) into this and rearranging it to find the speed difference _ΔNs , we obtain the following equation (6).

The rotational speeds N M1 and N _M2 of the first and second electric motors M1 and M2 are determined so that the rotational speed N _c of the carrier 22 at that time, i.e., the speed difference ΔN _s in equation (6 ₎ is minimized depending on the vehicle speed, and by operating them at these rotational speeds, when there is a sudden request to increase the output torque, the output torque can be made the maximum value at that vehicle speed without changing the rotational speed.

When one of the first and second electric motors M1, M2 reaches its maximum rotation speed and the vehicle speed is increased further, one of the electric motors cannot output maximum power because the torque of the three elements must be balanced, and therefore, equation (6) cannot be applied. In this case, the speed difference _ΔNs is determined based on the above-mentioned equation (1).

In the drive device 10, the first electric motor M1 reaches the maximum rotational speed first, so the rotational speed N _{sd_max} of the first sun gear 18 corresponding to the maximum rotational speed of the first electric motor M1 is substituted into equation (1) to determine the rotational speed N _ss of the second sun gear 20.
N _ss = (1+ρ)N _c -ρN _{sd_max}
The speed difference ΔN _s is expressed by the following formula.
ΔN _s = (1 + ρ) (N _{sd_max} - N _c ) ... (7)

FIG. 9 is a diagram showing the relationship between vehicle speed and the speed difference _ΔNs between the first sun gear and the second sun gear when the first and second electric motors M1, M2 are operating at less than the maximum rotational speed. The solid lines indicate the minimum speed difference _{ΔNs_min} among the speed differences _ΔNs that can maximize the output torque of the drive unit 10 at each speed. When the vehicle speed exceeds approximately 39 km/h, the speed difference _ΔNs that maximizes the output torque is uniquely determined as shown in FIGS. 7 and 8 . The speed difference _ΔNs at this time is expressed by Equation (6). When the vehicle speed is less than approximately 39 km/h, the speed difference _ΔNs that maximizes the output torque is not uniquely determined but can be selected within a certain range. The solid line in FIG. 9 indicates the minimum speed difference _{ΔNs_min} at this time. In particular, the speed difference _ΔNs can be set to zero at speeds less than approximately 35 km/h. By controlling the rotational speeds of the first and second electric motors M1, M2 so as to achieve this minimum speed difference _{ΔNs_min} , it is possible to reduce the loss due to the differential of the planetary gear mechanism 12 and also to reduce the response delay when there is a sudden request to increase the output torque.

The dashed-dotted line in Figure 9 indicates the case where Equation (6), which expresses the speed difference _ΔNs when the vehicle speed exceeds approximately 39 km/h, is applied to the range where the vehicle speed is approximately 39 km/h or less. In other words, the dashed-dotted line indicates the case where the speed difference _ΔNs , which maximizes the output torque when the first and second electric motors M1, M2 are operating in the upper limit constant power range, is applied when the two electric motors are not operating in the upper limit constant power range. In this case, when the first and second electric motors M1, M2 are operating below their maximum rotational speeds, the speed difference _ΔNs can be determined by a single equation, simplifying the control of the first and second electric motors M1, M2. Furthermore, because the speed difference _ΔNs is not zero, the meshing positions of the gears of the planetary gear mechanism 12 move, preventing gear wear from concentrating in one location.

The control unit 52 determines the rotation speeds of the first and second electric motors M1, M2 based on the minimum speed difference _{ΔNs_min} shown by the solid line in FIG. 9 from the vehicle speed. In the drive device 10, since the vehicle speed and the carrier rotation speed _Nc have a one-to-one relationship, the vehicle speed can be detected and the minimum speed difference _{ΔNs_min} can be calculated based on the detected vehicle speed. For example, the relationship between the vehicle speed and the minimum speed difference _{ΔNs_min} can be correlated in advance, and the detected vehicle speed can be applied to this correlation to calculate the minimum speed difference _{ΔNs_min} at that time. By applying this minimum speed difference _{ΔNs_min} to equations (2) and (3), respectively, the rotation speed _Nsd of the first sun gear and the rotation speed _Nss of the second sun gear can be calculated. Since the rotational speeds _Nsd and _Nss of the first and second sun gears have a one-to-one relationship with the rotational speeds _Nm1 and _Nm2 of the first and second electric motors, respectively, the rotational speeds _Nm1 and _Nm2 of the first and second electric motors can be calculated from the rotational speeds _Nsd and _Nss of the first and second sun gears.

Alternatively, the control unit 52 may calculate the rotational speeds of the first and second electric motors M1, M2 based on equation (6). The carrier rotational speed _Nc calculated from the vehicle speed is applied to equation (6) to calculate the speed difference _ΔNs , and this is then applied to equations (2) and (3), respectively, to calculate the rotational speeds _Nsd , _Nss of the first and second sun gears. Then, the rotational speeds _Nm1 , _Nm2 of the first and second electric motors can be calculated from the rotational speeds _Nsd , _Nss of the first and second sun gears.

The planetary gear mechanism 12 of the drive unit 10 shown in FIG. 1 has two sun gears 18, 20 as input elements and a carrier 22 as output element, but planetary gear mechanisms with other structures can be used. For example, the most common planetary gear mechanism having a sun gear, a ring gear, and a planetary carrier supporting planetary pinions meshing with the sun gear and ring gear can be used. In this case, electric motors are connected to the sun gear and ring gear, respectively, and drive wheels are connected to the planetary carrier. The planetary gear ratio ρ is ρ = _Zs / _Zr , where _Zs is the number of teeth on the sun gear and _Zr is the number of teeth on the ring gear. This allows the rotational speeds of the two electric motors to be determined in the same way as for the drive unit 10 shown in FIG. 1.

Another type of planetary gear mechanism is a so-called double-pinion planetary gear mechanism, which has a sun gear, a ring gear, an inner planetary pinion that meshes with the sun gear, and a planetary carrier that supports an outer planetary pinion that meshes with the ring gear and the inner planetary pinion. In this case, electric motors are connected to the sun gear and the carrier, respectively, and the ring gear is connected to the drive wheels. The planetary gear ratio ρ is ρ = _Zs / ( _Zr - _Zs ), where _Zs is the number of teeth on the sun gear and _Zr is the number of teeth on the ring gear, and the rotational speeds of the two electric motors can be determined in the same way as with the drive unit 10 shown in FIG. 1.

Other aspects of the invention are set forth below.
[1]
A first electric motor;
A second electric motor;
a planetary gear mechanism including a first input element connected to the first electric motor, a second input element connected to the second electric motor, and an output element connected to a drive wheel;
a control device that controls the first electric motor and the second electric motor so that the rotational speeds of the first input element and the second input element are rotational speeds that can generate a maximum value of torque of the output element at each vehicle speed;
A drive device for an electric vehicle comprising:
[2]
The drive device for the electric vehicle according to [1] above,
the control device controls the first electric motor and the second electric motor so that an absolute value of an input speed difference, which is a difference in rotation speed between the first input element and the second input element, is minimized.
Drive unit for electric vehicles.
[3]
The drive device for the electric vehicle according to [1] above,
the control device controls the first electric motor and the second electric motor such that, when at least one of the first electric motor and the second electric motor operates in an upper limit constant torque region, an input speed difference, which is a difference in rotation speed between the first input element and the second input element, has a mathematical relationship similar to a mathematical relationship between the speed of the output element and the input speed difference when both the first electric motor and the second electric motor operate in an upper limit constant power region and at a speed less than a maximum rotation speed.
Electric vehicle drive unit

10 Drive device, 12 Planetary gear mechanism, 14 Final reduction gear, 16 Drive wheel, 18 First sun gear, 20 Second sun gear, 22 Planetary carrier, 24 Outer planetary pinion, 26 Inner planetary pinion, 28 Output gear, 30 Driven gear, 32 Final reduction gear pair, 34 First planetary gear train, 36 Second planetary gear train, 38 First input shaft, 40 First input gear pair, 42 First electric motor shaft, 44 Second input shaft, 46 Second input gear pair, 48 Second electric motor shaft, 50 One-way clutch, 52 Control unit, M1 First electric motor, M2 Second electric motor, _Nsd First sun gear rotational speed, _Nss Second sun gear rotational speed, _Nc Carrier rotational speed, _ΔNs Rotational speed difference between the first sun gear and the second sun gear, ΔN _{s_min:} the minimum rotational speed difference that allows the output torque at that time to be maximized, _Tsd: first sun gear torque, _Tss: second sun gear torque, _Tc: carrier torque, _{Tm1_u} : upper torque limit value of the first motor, _{Tm2_u:} upper torque limit value of the second motor.

Claims (3)

A first electric motor;
A second electric motor;
a planetary gear mechanism including a first input element connected to the first electric motor, a second input element connected to the second electric motor, and an output element connected to a drive wheel;
A drive device for an electric vehicle comprising :
the rotational speeds of the first input element and the second input element are set to rotational speeds that can generate a maximum value of torque of the output element at each vehicle speed within a speed range used by the electric vehicle ;
Drive unit for electric vehicles. The drive device for an electric vehicle according to claim 1,
the rotational speeds of the first input element and the second input element are set so that an absolute value of an input speed difference, which is a difference between the rotational speeds of the first input element and the second input element, is minimized;
Drive unit for electric vehicles. The drive device for an electric vehicle according to claim 1,
When at least one of the first electric motor and the second electric motor operates in an upper limit constant torque region, an input speed difference, which is a difference in rotation speed between the first input element and the second input element, is set to have a mathematical relationship similar to a mathematical relationship between the speed of the output element and the input speed difference when both the first electric motor and the second electric motor operate in an upper limit constant power region and at a speed less than a maximum rotation speed.
Drive unit for electric vehicles.