JP3204079B2 - Vehicle driving force control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely accelerate a driving wheel while restraining racing of it by computing the slip ratio of a fluid transmission means connecting a prime mover to a driving wheel, computing the lower limit value of driving force based on the value, and regulating the driving force to be the lower limit value or more when racing of the driving wheel is judged. SOLUTION: When the slip ratio of a driving wheel 101 connected to a prime mover through a fluid transmission means 100 is over a prescribed value, the driving wheel 101 is judged to be in racing by a slip judging means, and the driving force of the driving wheel 101 is reduced by a driving force restraining means 102. In a driving force control device preventing racing of the driving wheel 101 in this way, it is provided with a slip rate computing means 103 computing the slip ratio of the fluid transmission means 100, and a lower limit value setting means 104 computing the lower limit value of driving force based on the slip ratio. When racing of the driving wheel 101 is judged by the slip judging means, the driving force being reduced by the driving force restraining means 102 is regulated to the lower limit valve or more by a driving force lower limit value regulating means 105.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a driving force control device for preventing the idling of driving wheels and ensuring the stability and drivability of a vehicle.

[0002]

2. Description of the Related Art Conventionally, as a driving force control device (or traction control system) for preventing a driving wheel from spinning during acceleration or the like and deteriorating acceleration performance, a device for controlling an engine output and a braking force is conventionally used. For example, Japanese Patent Application Laid-Open No. 4-55156 is known.

[0003] In this method, when a driving wheel idles, a road surface friction coefficient μ is estimated based on a vehicle acceleration obtained each time from a driven wheel speed, and after a predetermined time elapses, a road surface friction coefficient μ is calculated according to an averaged vehicle acceleration. It is estimated to reduce the engine output and the like in accordance with the road surface friction coefficient μ.

[0004]

However, in such a conventional driving force control device, the road surface friction coefficient μ
When the driving wheels idle in a state where the running resistance (or road surface resistance) is large, such as a high slope road, the driving force suppression control is started based on the driven wheel speed, and the first control reduces the driving force. If it becomes excessive, not only is it not possible to obtain a sufficient acceleration force due to a large running resistance, but also it is not possible to quickly restore the driving force, and it takes time to reach a predetermined acceleration, and the acceleration performance becomes poor. In addition to the decrease, there is a problem that the driver feels strange.

Accordingly, the present invention has been made in view of the above problems, and suppresses an excessive decrease in the driving force and an idle rotation of the drive wheels at the same time even when the drive wheels idle in a case where the running resistance is large. It is an object of the present invention to provide a driving force control device capable of reliably accelerating while driving.

[0006]

According to a first aspect of the present invention, as shown in FIG. 11, a drive wheel 101 connected to a prime mover through a fluid transmission means 100 and a slip ratio of the drive wheel 101 to a road surface are determined. A slip determination unit 109 for determining idle of the drive wheel when the value exceeds the threshold value; and a drive force suppressing unit 102 for reducing the drive force of the drive wheel 101 when the slip determination unit 109 determines the idle of the drive wheel 101. A slip ratio calculating means 103 for calculating a slip ratio TRQSLP of the fluid transmission means 100, and a lower limit value for calculating a lower limit value TRQMN of the driving force based on the slip ratio TRQSLP. The driving force reduced by the driving force suppressing means 102 when the means 104 and the slip determining means 109 determine that the driving wheel is idling. And a driving force lower limit restricting means 105 for restricting the higher the lower limit TRQMN.

According to a second aspect of the present invention, as shown in FIG.
4 includes a road surface friction coefficient calculating unit 106 that calculates a road surface friction coefficient based on the slip ratio of the fluid transmission unit, and sets a lower limit value TRQMN of the driving force according to the road surface friction coefficient.

[0008] In a third aspect, as shown in FIG. 11, in the first aspect, the lower limit value setting means 10 is provided.
4 holds the lower limit value of the driving force when the idling of the driving wheel is determined for a predetermined time.

According to a fourth aspect of the present invention, as shown in FIG. 11, in the first aspect, the driving force lower limit value regulating means 105 determines a target driving force TRQE based on a longitudinal acceleration Xg generated in the vehicle. Target driving force calculating means 10 for calculating
7 and selecting means 108 for setting the larger of the target driving force and the lower limit of the driving force as the lower limit of the driving force.
And

[0010]

According to the first aspect of the present invention, the driving force suppressing means reduces the driving force transmitted to the driving wheels via the fluid transmission when the slip ratio of the driving wheels to the road surface exceeds a predetermined value, thereby reducing the driving force. Although the slip of the wheels is suppressed, the reduced driving force is regulated to a lower limit value or more determined based on the slip ratio of the fluid transmission means, so that the suppression of the driving force for preventing idling becomes excessive. By determining the lower limit of the driving force based on the slip ratio of the fluid transmission means, the lower limit of the driving force can be set in accordance with the magnitude of the running resistance. Is large, the vehicle can be quickly accelerated while preventing idle rotation of the drive wheels.

According to a second aspect of the present invention, the lower limit setting means includes:
The road friction coefficient can be reliably calculated from the slip ratio of the fluid transmission means, and the lower limit of the driving force is set according to the road friction coefficient. For example, even if the driving wheels idle on a high μ road or the like, the vehicle can be rapidly accelerated while suppressing the driving wheels from idling by preventing the reduction of the driving force from becoming excessive. it can.

In a third aspect of the present invention, the lower limit setting means includes:
Since the lower limit value of the driving force at the time when the idling of the driving wheel is determined is held for a predetermined time, the vehicle can be quickly accelerated while reliably suppressing the idling of the driving wheel.

According to a fourth aspect of the present invention, the driving force lower limit value restricting means includes a target driving force determined based on a longitudinal acceleration generated in the vehicle and a lower limit value of the driving force determined from a slip ratio of the fluid transmission means. Since the larger of the two is set as the lower limit of the driving force, the minimum driving force required for accelerating the vehicle can be secured, and the vehicle can be rapidly accelerated while suppressing the idling of the driving wheels.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows that the driving force control device comprises a TCS controller 1 composed of a microcomputer or the like and a second throttle 10 as driving force suppressing means controlled by the TCS controller 1 via an actuator 9. Show the case.

Reference numeral 4 denotes a transmission 6 via a torque converter 5.
The engine 4 is controlled by the engine controller 2 to control the fuel injection amount, ignition timing, and the like according to the operation state. The engine speed Ne of the engine 4 is transmitted from the engine controller 2 to the TCS controller 1. You.

The transmission 6 is of the FR type connected to the rear wheels RR, RL.
Are the driving wheels, and the left and right front wheels FL and FR are the driven wheels.

A first throttle 8 responsive to an accelerator pedal 7 and a second throttle 10 controlled by a TCS controller 1 via an actuator 9 are disposed in an intake passage of the engine 4. The opening sensor 11 that detects the opening outputs the opening TVO of the first throttle 8 to the TCS controller 1.

The transmission 6 connected to the engine 4 via the torque converter 5 is set by the transmission controller 3 to a gear ratio or a transmission ratio according to the operating state. The transmission controller 3 sets the gear ratio iGEA
R is sent to the TCS controller 1.

The TCS controller 1 has wheel speed sensors 12FR and 12FR for detecting the rotation speed of each wheel or axle.
FL, 12RR, and 12RL detection signals are input, and the TCS controller 1
R, VTFL, VTRR, drive wheel R based on VTRL
By detecting the idling of R and RL and the longitudinal acceleration Xg of the vehicle, when the driving wheels RR and RL are idling, the second throttle 10 is controlled via the actuator 9 to adjust the driving force of the engine 4. This is to suppress idling of the drive wheels.

An example of the control performed by the TCS controller 1 is shown in the flowcharts of FIGS. 2 to 4. Hereinafter, the driving force control will be described in detail with reference to these flowcharts. The control based on these flowcharts is executed every predetermined time.

First, in step S1, the output of each of the wheel speed sensors 12FR to 12RL is read, and in step S2
VTFR, VTFL, VTRR, VTR of each wheel
Calculate L.

Then, in step S3, the engine speed N
e, the gear ratio iGEAR is read.

In step S4, the average speed VF of the driven wheels
F is the wheel speed VTFR, VTFL of the left and right front wheels FR, FL
In step S5, the average speed VRR of the drive wheels is similarly calculated as the wheel speed VT of the left and right rear wheels RR and RL.
Determined from RR and VTRL.

Next, in step S6, a drive wheel speed threshold value VRRBS for detecting idling of the drive wheels is obtained by adding a predetermined constant β to the driven wheel average speed VFF corresponding to the current vehicle speed.

VRRBS = VFF + β In step S7, a target value VRRS of the driving wheel speed corresponding to the current vehicle speed is obtained by adding a predetermined constant α to the driven wheel average speed VFF.

VRRS = VFF + α where the predetermined value is set to α <β.

In step S8, the following driven wheel average speed VF
The current value VFF (n) of F and the value VF obtained in the previous process
The longitudinal acceleration Xg of the vehicle is obtained by multiplying the difference of F (n-1) by a predetermined conversion constant K.

Xg = {VFF (n) -VFF (n-1)}. Times.K In steps S1 to S8, the driving wheel average speed VRR, the driving wheel speed threshold value VRRBS, the driving wheel speed target value VRRS, and the longitudinal acceleration are calculated. After obtaining Xg, in step S9, idling of the driving wheel is detected by determining whether or not the average driving wheel speed VRR has exceeded the driving wheel speed threshold value VRRBS. Proceeding to the process of S10, a driving force control flag FTCS (hereinafter, referred to as a TCS control flag) for suppressing idling of the driving wheels RR and RL is set to 1.

Next, in step S11, based on the longitudinal acceleration Xg obtained in step S8, a target driving torque TRQE required for accelerating the vehicle is calculated by the following equation.

TRQE = Xg × INN where INN is the inertial mass of the vehicle and the drive train.

In step S12, the slip ratio TR, which is the ratio between the number of revolutions of the turbine of the tor converter 5 and that of the impeller,
QSLP calculation (hereinafter, torque converter slip rate TR
QSLP). This torque converter slip rate TRQSL
P becomes 1 when the output side turbine and the input side impeller are in the direct connection state in which the rotation speeds are equal, while in the idling stop state, the turbine rotation speed becomes 0, so the slip ratio T
RQSLP also becomes 0.

In the calculation of the torque converter slip rate TRQSLP, first, a known gear ratio iGEAR is obtained from the gear position GEAR of the transmission 6 read in step S3, and the torque converter slip rate TRQSLP is set to 1 based on the read engine speed Ne. The direct drive wheel speed VENG, which is the drive wheel rotational speed assumed as follows, is obtained from the following equation.

VENG = Ne / iGEAR Then, the ratio of the driving wheel average speed VRR, which is the actual driving wheel rotation speed obtained in step S5, to the directly connected driving wheel speed VENG, assuming that the torque converter 5 is in the directly connected state, is determined by the torque converter slip. The rate TRQSLP is expressed as follows.

TRQSLP = VRR / VENG If the torque converter 5 is provided with a turbine and an impeller, a rotation speed sensor, the ratio of the detected rotation speeds may be set to the torque converter slip ratio TRQSLP.

Next, in step S13, it is determined whether the current state of the drive wheels is in a steady state or not in steps S9 and S1.
If the TCS control flag FTCS set at 0 is 1 and the difference between the average driving wheel speed VRR and the target driving wheel speed value VRRS is less than the predetermined value DS1, the process proceeds to step S14, otherwise, the process proceeds to step S16. Proceed to processing.

In step S14, it is determined whether or not a predetermined time TS1 has elapsed. If the predetermined time TS1 has elapsed, the flow advances to step S15 to set a steady state determination flag FSRB to 1.

That is, even if the driving wheel average speed VRR exceeds the threshold value VRRBS and idles after the elapse of the predetermined time TS1, the driving wheel average speed VRR and the driving wheel speed target value VRRS are set.
If the difference is less than the predetermined value DS1, it is determined to be in a steady state.

The predetermined value DS1 indicates a threshold value of a difference between the average value VRR of the driving wheel speed and the target value VRRS. For example, as shown in FIG. 5, the predetermined value DS1 is larger than a predetermined value DS2 described later. Small, set to a small value to 0, and
For a predetermined time TS1, the TCS control flag FTCS changes from 0 to 1
Indicates a predetermined value of the elapsed time after the change to.

On the other hand, if the determination in steps S14 and S15 is NO,
In step S16, if the difference between the average value VRR of the drive wheel speed and the target value VRRS is equal to or greater than the predetermined value DS2 shown in FIG. 5, it is determined that the vehicle has deviated from the steady state. Proceed to S17 and set the steady-state determination flag FSTB to 0
To clear.

It should be noted that setting of the steady state determination flag FSTB,
The clear conditions are as shown in the following table.

[0042]

[Table 1]

The difference between the average value of the drive wheel speeds and the target value and the predetermined value D
By comparing S1 and S2, the steady-state determination flag FST
After setting B, the process proceeds to step S18, where a conditional branch is performed based on the state of the TCS control flag FTCS and the steady state determination flag FSTB.

This conditional branch is performed by the TCS control flag FTC.
Step S19 when S = 1 and the steady state flag = 0
To calculate the target drive torque lower limit value TRQMN, otherwise, to step S20, set the target drive torque lower limit value TRQMN to the TCS control flag FT.
The value at the time when CS = 1 is maintained.

In the calculation of step S19, the target drive torque lower limit value TRQMN is obtained from a preset map or function as shown in FIG. 6 according to the torque converter slip ratio TRQSLP obtained in step S12. In addition,
This map or function provides the torque converter slip rate TRQS
According to the increase in LP (increase in output turbine speed),
When the load is set to gradually decrease to a predetermined value and the input side impeller rotational speed is higher and the load is high, the target drive torque lower limit T
RQMN is set to a large value.

Next, at step S21, in order to limit the lower limit of the target drive torque, the larger of the target drive torque lower limit value TRQMN determined at step S20 and the target drive torque TRQE determined at step S11 is determined. Set as torque TRQE '.

In step S22, the target drive torque TRQE 'and the gear ratio iGE set for the transmission 6 are set.
From AR, the target engine torque TRQ is calculated as follows.

TRQ = TRQE '× iGEAR In step S23, a map (F) set in advance in accordance with the target engine torque TRQ and the engine speed Ne.
2), the opening degree THR of the second throttle 10 is obtained.

Thus, the torque converter slip rate TRQSL
The larger of the target drive torque lower limit value TRQMN determined according to P and the target drive torque TRQE determined according to the longitudinal acceleration Xg is set as the target drive torque TRQE ', and the second throttle opening degree is calculated from the target drive torque TRQE'. TH
R is required.

Then, in step S24, it is determined whether or not the second throttle opening THR is fully opened (8/8). If the second throttle opening THR is fully opened, the driving force suppressing process is terminated. If the TCS control flag FTCS and the steady-state determination flag FSTB are cleared to 0 and the second throttle opening THR is not fully open, step S2 is performed.
7, the second throttle opening TH obtained in step S3.
R is output, and the actuator 9 is driven to close the second throttle 10 by a predetermined amount.

By performing the above-described control at predetermined time intervals, it becomes possible to surely accelerate the vehicle while suppressing the idling of the drive wheels at the time of starting and accelerating. An example is shown below.

[0052] Now, as shown in FIGS. 7 to 9, starting with such a high uphill of the road surface friction coefficient mu, the description will be given of a case where subjected to an accelerated, depressed from the time t ₀ to the driver fully open the accelerator pedal 7 Go to the first throttle opening TV
O increases, the engine speed Ne also increases, and the average driving wheel speed VRR increases.
The FF remains at 0 and does not rise, and the drive wheels idle.

[0053] Then, the drive wheel average speed at time t ₁ VRR
Exceeds the threshold value VRRBS, and the above steps S9, 1
Since the TCS control flag FTCS is set to 1 at 0, the second throttle opening THR is set after step S24.
Is closed to a value corresponding to the target drive torque lower limit value TRQMN, and the target drive engine torque TRQ is suppressed.

At this time, as shown in FIG. 5, the average driving wheel speed VRR is a predetermined value DS2 from the target driving wheel speed value VRRS.
, The steady-state determination flag FSTB is cleared to 0 in steps S16 and S17, and the TCS control flag FT
Target drive torque lower limit value TRQM when CS becomes 1
N is maintained for a predetermined time TS1 as shown in FIG. 9 (steps S18 and S20).

The target drive torque lower limit value TRQMN when the TCS control flag FTCS is set to 1 is shown in FIG.
As shown in FIG. 8, the torque converter slip ratio TRQSL obtained from the ratio of the driving wheel average speed VRR to the directly connected driving wheel speed VENG
Set according to P, torque converter slip rate TR
As QSLP approaches 0, in other words, as the impeller rotational speed on the input side of the torque converter 5 becomes greater than the turbine rotational speed on the output side, the target drive torque lower limit TRQ
MN is set to a large value.

That is, as in the case immediately before the start at the time t ₀ , the load increases as the torque converter slip rate TRQSLP approaches 0, that is, the running resistance increases. On the other hand, after the vehicle starts after the time t ₂ , The load decreases as the torque converter slip ratio TRQSLP approaches 1, that is, the running resistance is small.

By the way, in the conventional example, when the racing of the driving wheels is started at time t _1, the second throttle opening THR
However, as shown by the dashed line in FIG. 7, the engine is almost fully closed, and the engine torque TRQ is excessively reduced as compared with the running resistance (FIG. 9). Therefore, although racing of the driving wheels is suppressed, as shown in FIG. 7, the drive wheel speed VRR 'and the driven wheel speed VFF' in the conventional example, even time t ₂ later hardly stretched, is possible to perform smooth acceleration Since it is not possible, the driver feels strange.

On the other hand, as in the present invention, the magnitude of the torque converter slip ratio TRQSLP is taken as the magnitude of the running resistance, and as shown in FIG.
By variably controlling the lower limit value TRQMN of the drive torque according to the RQSLP, the excessive lowering of the lower limit value TRQMN of the target drive torque as in the conventional example is regulated, and the running resistance such as the uphill road and the road surface friction coefficient μ is reduced. If it is large,
As described above, the second throttle opening THR (solid line) has a larger value than the conventional example (dashed line), the driven wheel speed VFF rises smoothly against the running resistance, and the drive wheel speed VRR increases. Slip is also suppressed, and the increase in vehicle speed (driven wheel speed) becomes larger than that of the VFF 'of the conventional example, so that it is possible to obtain an acceleration force according to the driver's driving intention, and the driving force control device Therefore, the drivability and acceleration performance of the vehicle provided with the above can be greatly improved as compared with the conventional example.

Here, the torque converter slip rate TRQSLP
Is captured as the magnitude of the running resistance, but the magnitude of the torque converter slip ratio TRQSLP can also be captured as the road surface friction coefficient μ. In this case, the target driving torque lower limit value TRQMN and the road surface friction coefficient μ for the torque converter slip ratio are The following table shows the relationship of running resistance.

[0060]

[Table 2]

Therefore, the torque converter slip rate TRQS
Based on the LP, the road surface friction coefficient μ can be accurately detected. As shown in FIG. 10, a road surface friction coefficient μ and a target driving torque lower limit TRQMN map or function are used in accordance with the road surface condition. Target drive torque lower limit T
RQMN is variably controlled, and reliable acceleration can be performed while suppressing idling of the drive wheels regardless of changes in the road surface condition, thereby improving the drivability and acceleration performance of the vehicle equipped with the driving force control device. You can do it.

In the above embodiment, the means for suppressing the driving force is replaced by the second means driven via the actuator 9.
Although not shown, the throttle 10 is used to control the engine such as fuel cut and timing retard, and to operate the brakes of the drive wheels RR and RL.

[0063]

As described above, according to the first aspect of the present invention, when the slip ratio of the driving wheel to the road surface exceeds a predetermined value, the driving force suppressing means controls the driving force transmitted to the driving wheel via the fluid transmission device. Although the driving force is reduced to suppress the slip of the driving wheels, the driving force to be reduced is restricted to a lower limit value or more determined based on the slip ratio of the fluid transmission means. It is possible to prevent the driving force from becoming excessive, and by determining the lower limit value of the driving force based on the slip ratio of the fluid transmission means, it is possible to set the lower limit value of the driving force according to the magnitude of the running resistance. Even if the driving wheels idle when the running resistance is large on a road or the like, the vehicle can be quickly accelerated while preventing the driving wheels from idling, as in the above-described conventional example. Driving force is excessively reduced by the second driving force suppression control Is prevented, and idling of the drive wheels can be suppressed while responding to the driver's intention to accelerate, and the drivability and acceleration performance of the vehicle including the driving force control device can be significantly improved. .

Further, according to the second invention, the road surface friction coefficient can be calculated based on the slip ratio of the fluid transmission means,
Since the lower limit value of the driving force is set according to the road surface friction coefficient, the lower limit value of the driving force can be set according to the road surface condition. For example, even if the driving wheels idle on a high μ road, the driving force is reduced. By preventing the vehicle from becoming excessively large, it becomes possible to accelerate the vehicle quickly while suppressing the idling of the driving wheels, and it is possible to suppress the idling of the driving wheels according to the driver's acceleration intention. Thus, the drivability and acceleration performance of the vehicle including the driving force control device can be significantly improved.

According to the third aspect of the present invention, the lower limit of the driving force when the idling of the driving wheel is determined is held for a predetermined time.
The vehicle can be rapidly accelerated while reliably suppressing the idling of the drive wheels, and the acceleration performance of the vehicle including the driving force control device can be improved.

According to a fourth aspect of the present invention, the driving force lower limit value restricting means is configured to determine a target driving force obtained based on the longitudinal acceleration generated in the vehicle and a lower limit value of the driving force obtained from the slip ratio of the fluid transmission means. Since the larger of the two is set as the lower limit of the driving force, the minimum driving force required for vehicle acceleration can be secured, and the vehicle can be quickly and reliably accelerated while suppressing idling of the drive wheels. By preventing the driving force from excessively decreasing as in the conventional example, it is possible to suppress idling of the driving wheels while accelerating the vehicle according to the driver's intention,
Drivability and acceleration performance of a vehicle including the driving force control device can be significantly improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a driving force control device according to an embodiment of the present invention.

FIG. 2 is a flowchart showing an example of control performed by the TCS controller, showing the first half.

FIG. 3 is also a flowchart showing an intermediate part.

FIG. 4 is a flowchart showing the latter half of the flowchart.

FIG. 5 is a map showing a target driving torque lower limit value TRQMN set according to the torque converter slip ratio TRQSLP.

FIG. 6 is a graph showing a relationship between each wheel speed and a target driving wheel speed, and a difference between a driving wheel speed average value and a target value and predetermined values DS1 and DS2.

FIG. 7 shows a state of driving force control at the time of starting, and includes a wheel speed,
4 is a graph showing a relationship between a throttle opening, an engine speed and time.

[FIG. 8] Similarly, the direct drive wheel speed, torque converter slip ratio,
5 is a graph showing a relationship between a target driving torque lower limit and time.

FIG. 9 is also a graph showing a relationship between a target drive torque, an engine torque and time.

FIG. 10 is a map showing a relationship between a target driving torque lower limit value TRQMN and a road surface friction coefficient μ according to another embodiment.

FIG. 11 is a claim correspondence diagram corresponding to any one of the first to fourth inventions.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 TCS controller 2 Engine controller 3 Transmission controller 4 Engine 5 Torque converter 6 Transmission 7 Accelerator 8 First throttle 9 Actuator 10 Second throttle 11 Throttle opening sensor 12FR, 12FL, 12RR, 12RL Wheel speed sensor 100 Fluid transmission means 101 Driving wheel 102 Driving force suppressing means 103 Slip ratio calculating means 104 Lower limit value setting means 105 Driving force lower limit value controlling means 106 Road surface friction coefficient calculating means 107 Target driving force calculating means 108 Selecting means 109 Slip determining means

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-285629 (JP, A) JP-A-3-202646 (JP, A) JP-A-6-66169 (JP, A) JP-A-1- 178036 (JP, A) JP-A-63-31862 (JP, A) JP-A-7-125561 (JP, A) JP-A-1-114523 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) F02D 29/00-29/06 F02D 41/00-45/00 395 B60T 8/00

Claims (4)

(57) [Claims] A drive wheel connected to a prime mover via a fluid transmission means; slip determination means for determining idle rotation of the drive wheel when a slip ratio of the drive wheel to a road surface exceeds a predetermined value; A driving force control device for reducing the driving force of the driving wheel when the judging device judges that the driving wheel is spinning, wherein a slip ratio calculation for calculating a slip ratio of the fluid transmission device is provided. Means, a lower limit value setting means for calculating a lower limit value of the driving force based on the slip ratio, and a driving force reduced by the driving force suppressing means when the slip judging means judges idling of the driving wheel. A driving force control device for a vehicle, comprising: a driving force lower limit value restricting means for restricting the driving force to a value not less than the lower limit value. 2. The lower limit value setting means includes a road surface friction coefficient calculating means for calculating a road surface friction coefficient based on a slip ratio of the fluid transmission means, and sets a lower limit value of the driving force according to the road surface friction coefficient. The vehicle driving force control device according to claim 1, wherein: 3. The driving force control device for a vehicle according to claim 1, wherein the lower limit value setting means holds a lower limit value of the driving force when the idling of the driving wheel is determined for a predetermined time. . 4. The driving force lower limit value restricting means includes a target driving force calculating means for calculating a target driving force based on a longitudinal acceleration generated in a vehicle, and a target driving force and a lower limit value of the driving force. 2. The driving force control device for a vehicle according to claim 1, further comprising: selection means for setting a larger one as a lower limit value of the driving force.