JPH06239159A - Differential limiting device for vehicle - Google Patents

Abstract

PURPOSE:To enhance control accuracy by transforming the signal of a current value at the time of switching-on into a periodic form around the center of a target current value corresponding to the target value of differential limiting force in the differential limiting device controlling its differential limiting force in response to the quantity of electricity turned on to each electoromagnetic clutch. CONSTITUTION:The engine 10 of an automobile has a center differential gear 12 located at the output side of a transmission 11 where a front propeller 13 and a rear propeller 14 are connected to the center differential gear 12, and the rotation of the respective propeller shafts 13 and 14 are transmitted to front wheels 16 and rear wheels 18 via a front differential gear 21 and a rear differential gear 22. Each differential gear 12, 19 and 20 contains an electromagnetic multi-plate clutch, and its differential limiting operation is continuously changed in a range from an unlocked condition to a fully locked condition with a switching-on condition changed. In this case, the transformation of a signal for the current value of supplied current into a periodic wave form around the center of a target current value corresponding to the target value of differential limiting force permits the hysteresis of each electromagnetic clutch to be relaxed, so that control accuracy for differential limiting force can thereby be enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention obtains a differential limiting force for a vehicle differential limiting device, in particular, a differential device interposed between left and right wheels or front and rear axles by engaging an electromagnetic clutch. , A differential limiting device configured to control the differential limiting force according to a current value of a current supplied to the electromagnetic clutch.

[0002]

2. Description of the Related Art As is well known, in a vehicle such as an automobile, when the output of the engine is transmitted to the wheel side, for example, when the vehicle turns, a difference in rotational speed occurs between wheels due to a difference in wheel locus. In order to prevent the occurrence of tire slip and ensure sufficient maneuverability of the vehicle as well, in the middle of the power transmission system from the engine to each wheel side, the rotational speed difference is mechanically affected by the differential action. Is provided with a differential device (differential device) capable of absorbing.
That is, in order to absorb the rotational speed difference between the left and right front wheels in a front-wheel drive vehicle and between the left and right rear wheels in a rear-wheel drive vehicle, respectively, a differential for the front wheel or the rear wheel is absorbed. A device (so-called front differential or rear differential) is provided. Further, in a four-wheel drive vehicle, in addition to the differential device between the left and right wheels described above, a differential is also provided between the front wheel side and the rear wheel side. A device (so-called center differential) is provided.

Further, as a differential limiting device for limiting the differential by the differential device in order to properly transmit the torque to each wheel, a differential limiting force is obtained by engaging an electromagnetic clutch and the electromagnetic limiting force is applied. It is known that the clutch engagement force, that is, the differential limiting force is controlled to a desired level by appropriately controlling the current supplied to the clutch (see, for example, Japanese Patent Application Laid-Open No. 64-12918).

As is well known, the electromagnetic clutch basically drives an actuator using an electromagnetic force generated by supplying a current to an electromagnetic solenoid, and the actuator presses a friction plate (clutch plate). Generally, a device that uses electromagnetic force to drive and controls the actuator in this way inevitably causes hysteresis in the reciprocating motion of the actuator. It is known that it becomes a big obstacle when improving the control accuracy of the electromagnetic clutch. In other words, regarding the stroke change of the actuator due to the increase or decrease of the current supplied to the electromagnetic solenoid, even if the current value of the supply current is the same on the forward path and the return path, there is a difference in the stroke value, and the stroke accuracy of the actuator is correspondingly increased. This lowers the control accuracy of the electromagnetic clutch.

It is to be noted that the electromagnetic clutch as described above is not intended to improve the control accuracy of the differential limiting clutch of the vehicle.
In Japanese Patent No. 137, a hydraulic clutch is used for limiting the differential,
When the temperature of the hydraulic oil is low, the pressure control valve is periodically opened and closed to cause the hydraulic oil to flow and raise the oil temperature only when the vehicle is stopped, which does not affect the running characteristics of the vehicle. It is disclosed that the operation delay of the hydraulic clutch due to the change in viscosity is prevented.

The above-mentioned hysteresis of the actuator of the electromagnetic clutch usually does not particularly occur in the maximum / minimum portion of the stroke, but it is remarkable in the intermediate region between them. Therefore, this hysteresis does not pose a particular problem in the ON / OFF type electromagnetic clutch, but it is an important problem in maintaining the control accuracy of the intermediate control type. Then, in the electromagnetic clutch of the type that performs this intermediate control, in order to ensure the control accuracy, it has been conventionally performed to change the target value of the supply current between the forward stroke and the backward stroke of the stroke to reduce the hysteresis. There is.

[0007]

However, in the above-mentioned conventional method, the setting and control of the target value of the supply current is
Actually, there was a practical problem that it became quite complicated and difficult. By the way, when controlling the drive of an actuator using electromagnetic force, so-called modulation control is performed on the current supplied to an electromagnet (for example, an electromagnetic solenoid) that generates this electromagnetic force, and the current value signal is centered around a predetermined target current value. It is known that the hysteresis generated with the increase and decrease of the supply current is alleviated by setting the periodic waveform as described above.

That is, for example, for an actuator having a stroke characteristic shown by a solid broken line in FIG. 12, the control current supplied to the electromagnetic solenoid is modulated and controlled, and a signal of the current value is predetermined as shown in FIG. 13, for example. When a periodic waveform centered on the target value is generated, a loop-shaped hysteresis occurs at the target current value, and the loop is, for example, as shown by a dashed line in FIG. In the case of a return trip, it occurs on the outward trip side. Since the actual stroke value for the current value is located at the center of each loop, the loop-shaped hysteresis causes a difference in the stroke of the actuator between the forward path and the return path for the same current value. Therefore, the hysteresis as a whole is relaxed.

Therefore, the present invention was made by paying attention to the fact that the hysteresis is alleviated by controlling the modulation of the current supplied to the electromagnet when the actuator is driven and controlled by utilizing the electromagnetic force. An object of the present invention is to provide a differential limiting device for a vehicle, which can alleviate the hysteresis of an electromagnetic clutch that applies a differential limiting force to a moving device and improve the control accuracy of the differential limiting force.

[0010]

Therefore, according to the first invention of the present application, the differential limiting force for the differential device provided between the left and right wheels or between the front and rear axles is obtained by engaging the electromagnetic clutch. In a vehicle differential limiting device configured to control the differential limiting force according to a current value of a current supplied to the electromagnetic clutch, a signal of the current value of the supply current is used as a target of the differential limiting force. This is a periodic waveform centered on the target current value corresponding to the value.

A second invention of the present application is the vehicle differential limiting device according to the first invention, wherein the electromagnetic clutch is in a half-locked state between a completely locked state and a completely unlocked state. Only, the signal of the current value of the supply current is the periodic waveform.

Further, a third invention of the present application is the differential limiting device for a vehicle according to the first invention, wherein the supply current of the supply current is changed only when the current value of the supply current is constant instead of changing. The current value signal is the periodic waveform.

[0013]

According to the first aspect of the present invention, since the signal of the current value of the current supplied to the electromagnetic clutch has the periodic waveform, at the target current value, the return path side is the forward path of the stroke. In addition, in the case of the return path, a loop-shaped hysteresis is generated on the outward path side, so that the hysteresis as a whole at the target current value is alleviated, and the control accuracy of the electromagnetic clutch can be improved.

Further, according to the second invention of the present application, basically, the same effect as that of the first invention can be obtained, and in particular, in the half-locked state in which the hysteresis remarkably appears, the above-mentioned effect is obtained. Since the signal of the current value of the supply current has a periodic waveform, it is possible to obtain good control characteristics with less influence of hysteresis over the entire region.

Further, according to the third invention of the present application, basically, the same effect as that of the first invention can be obtained. In particular, in this case, the signal of the current value of the current supplied to the electromagnetic clutch is set as a periodic waveform only when the current value of the supply current is not in a changed state but is constant, and when the current changes, the periodic waveform is not. Therefore, even when the engaging force of the electromagnetic clutch is suddenly changed, its responsiveness is not adversely affected.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described in detail below with reference to the accompanying drawings, in the case of being applied to a four-wheel drive type automobile, for example. FIG. 1 is an overall configuration diagram schematically showing an overall configuration of a driving force transmission system of an automobile according to the present embodiment. As shown in this diagram, in the automobile, the output side of an engine 10 is connected. Transmission 1
1, the transfer 12 is connected to the transfer 12
Front and rear propeller shafts 13 and 1
One end of each of 4 is connected.

The front end side of the front propeller shaft 13 is connected to a front axle 15 via a front differential 21 (hereinafter abbreviated as front differential), and the output of the engine 10 is output from the transmission 11 and the transfer 12. Front propeller shaft 13, front differential 21, and front axle 1
5 are sequentially transmitted to the left and right front wheels 16. The rear end side of the rear propeller shaft 14 is connected to a rear axle 17 via a rear differential 22 (hereinafter abbreviated as rear differential), and the output of the engine 10 is transmitted from the transmission 11 and the transfer 12 to the rear propeller. It is adapted to be transmitted to the left and right rear wheels 18 through the shaft 14, the rear differential 22, and the rear axle 17 in order.

The front differential 21 is provided on the left and right front wheels 1.
When there is a difference in rotation speed between 6 and 16, this is mechanically absorbed, and when the output of the engine 10 transmitted via the propeller shaft 13 is transmitted to the front wheels 16 and 16, the left and right wheels are The torque distribution to the front wheels 16 is controlled. Further, the rear differential 22 is provided on the left and right front wheels 1.
When there is a difference in rotational speed between 8 and 18, this is mechanically absorbed, and when the output of the engine 10 transmitted via the propeller shaft 14 is transmitted to the rear wheels 18 and 18, the left and right wheels are The torque distribution to each rear wheel 18 is controlled. In addition, the transfer 12 includes front wheels 1
A center differential 20 (hereinafter abbreviated as center differential) is provided to control the torque distribution between the 6 and 16 side and the rear wheels 18 and 18 side.

A wheel speed sensor 30 for detecting the wheel speed of each wheel is arranged near each of the wheels 16, 16, 18, and 18, and the detection signal (wheel speed signal) of each wheel speed sensor 30 is , Is input to the anti-skid brake device control unit 41 (hereinafter referred to as ABS control unit). In addition, the engine 10 has
A throttle sensor 32 for detecting the throttle opening of the engine 10 is attached, and a detection signal (throttle opening signal) of the throttle sensor 32 is input to the engine control unit 40. Further, a brake switch 31 for detecting ON / OFF of the brake is provided on the vehicle front side, and a detection signal (brake signal) of the brake switch 31 is input to the differential control unit 43. The differential control unit 43 is connected to a manual switch 44 and a battery 45 for selecting a diff lock mode, which will be described later.

In addition to the brake signal from the brake switch 31, the differential control unit 43 receives a throttle opening signal from the throttle sensor 32, and the ABS control unit 41 from the anti-skid brake device. The ABS signal indicating whether or not the wheel speed signal of each wheel 16, 16, 18, 18 is input, and the mode signal is further input from the manual switch 44. In the differential control unit 43, based on these input signals, each of the front, rear and center differentials 2
The control current values to be supplied to 1, 22 and 20 are calculated and supplied. The differential limiting force (locking force) of each of the differentials 21, 22 and 20 is controlled in accordance with each of the supplied control currents (front differential current, rear differential current and center differential current).
In addition, the differential control unit 43
From this, an ABS prohibition signal can be output to the ABS control unit 41.

The front, rear and center differentials 21, 22 and 20 are, for example, center differentials 2.
0 will be described as an example. An electromagnetic multi-plate clutch 50, an example of which is shown in FIG. 2, is provided, and the locking force of the center differential 20 is controlled by the engagement state of the multi-plate clutch 50. In the case of the center differential 20, the clutch 50 may be of any other type as long as it can limit the differential between the front propeller shaft 13 and the rear propeller shaft 14.

The multi-plate clutch 50 comprises a clutch plate 51 formed by combining a plurality of inner discs and outer discs, and an actuator 52 for exerting a pressing force on the clutch plate 51. Reference numeral 53 is a bearing, 54 is a transmission member that is transmission-connected to one propeller shaft, and 55 is a transmission member that is transmission-connected to the other propeller shaft. The actuator 52 is configured so that the armature 57 presses the clutch plate 51 by the magnetic force generated when the solenoid 56 is energized. In this electromagnetic multi-plate clutch 50, the pressing force for frictionally engaging the electric current flowing through the solenoid 56 and the clutch plate 51.
(That is, the torque generated by the electromagnetic multi-plate clutch 50) is in a proportional relationship, so that the differential rotation speed of the center differential 20 can be continuously changed by increasing or decreasing the current.
Incidentally, the front differential 21 and the rear differential 22 are also provided with an electromagnetic multi-plate clutch having the same configuration as described above.

Here, the front switch in each diff lock mode selected by the manual switch 44,
Center and rear differentials 21, 20 and 2
Table 1 shows an example of the control contents of No. 2. In the "Control content" column of Table 1, in the case of "unlock", the supply current to the electromagnetic multiple disc clutch of the differential device is 0 (zero), and in the case of "complete lock", the maximum Value current is supplied.

[0024]

[Table 1]

In each of the above modes, the manual switch 44
The driver can arbitrarily select by operating the button. For example, in the "A mode", the front diff 21 is in the unlocked state, so that the drivability is less affected and the operability is excellent. Suitable for on-road driving on ordinary roads such as urban areas. On the other hand, in the "F mode", all the differentials 21, 20, 22 are completely locked, so the operability is reduced but the driveability is excellent, which is suitable for off-road driving on rough roads. ing. Further, both the “C mode” and the “R mode” have characteristics between the above two modes, and are selected according to the driver's preference. In each of the above modes, each differential 2
Differential limiting force for 1,20,22 is controlled respectively,
Although various differential limiting patterns can be obtained, these differential limiting patterns are not limited to those obtained by the mode selection by the operation of the manual switch 44, but are also applicable to various conditions such as ABS signal and brake signal input. Different patterns can be obtained accordingly.

Next, the control of the current supplied to the electromagnetic multi-plate clutch 50 of each differential by the differential control unit 43 will be described.
When the control is started, first, based on the input signal from each wheel speed sensor 30, the left and right front wheels 16, 16, 18, 1.
Eight wheel speeds Nfl, Nfr, Nrl, Nrr are calculated. By comparing these calculated values with each other, it is possible to know whether or not any wheel is slipping. Also,
From the wheel speeds Nfl, Nfr, Nrl, Nrr, it is possible to calculate the vehicle body speed and the differential rotation speed of each differential 20, 21, 22. The vehicle speed Vsp and the differential speed ΔN of the center differential 20
Subroutines for calculating the differential rotation speed ΔNr of the c and the rear differential 22 are shown in FIGS. 3, 4 and 5, respectively.

As shown in the flowchart of FIG. 3, when calculating the vehicle body speed Vsp, the wheel speeds Nfl, Nfr, Nrl, Nrr are calculated.
(Step # 10), these wheel speeds Nfl, Nfr,
The minimum value of Nrl and Nrr is defined as the vehicle speed Vsp (step # 11). Further, as shown in the flowchart of FIG. 4, when calculating the differential rotation speed ΔNc of the center differential 20, each wheel speed Nfl, Nfr, Nrl, Nrr is input (step #
20) Based on this input value, the differential rotation speed ΔNc of the center differential 20, which is the rotation difference between the front wheel side and the rear wheel side, is calculated (step # 21). Further, as shown in the flowchart of FIG. 5, when calculating the differential rotation speed ΔNr of the rear differential 22, the left and right rear wheel speeds Nrl, Nrr are input (step #
30), based on this input value, the differential rotation speed ΔNr of the rear differential 22, which is the rotation difference between the left and right rear wheels, is calculated (step # 31). The differential rotation speed ΔNf of the front differential 21
Is the same as the rear differential 22, and N
The calculation can be performed by replacing rl and Nrr with Nfl and Nfr, respectively.

Next, each differential 2 obtained as described above
According to the differential rotation speed of 0, 21, 22 each differential 20, 21,
The control current value to be supplied to the solenoid 56 of the electromagnetic multi-plate clutch 50 of 22 is calculated. First, an example of calculating the control current value of the center differential 20 will be described. Figure 6
9 is a flowchart showing a center diff current setting routine during auto mode control. As shown in the figure, first, in step # 40, the center diff current Ic is set. The center differential current Ic is obtained from the center differential differential rotation speed ΔNc and the throttle opening TVO.
7 is a diagram showing the relationship between the current value I ₁ and the center differential differential rotation speed ΔNc, and FIG. 8 is the current value I ₂ and the throttle opening TV.
It is a diagram which shows the relationship with O. That is, when either the center differential differential rotation speed ΔNc or the throttle opening TVO reaches the maximum current value Imax, the center differential current Ic is
Set "Ic = Imax". Further, when both the center differential differential speed ΔNc and the throttle opening TVO are equal to or less than the maximum current value Imax, the current value I ₁ and the current value I at that time are obtained.
Based on ₂ , the center differential current Ic is calculated using a predetermined arithmetic expression.

Next, in step # 41, it is determined whether the center differential current Ic has the maximum current value Imax. If the center differential current Ic is different from the maximum current value Imax, that is, if it is smaller than the maximum current value Imax, the step is performed. In step # 42, "Ic = Ic" is set. At this time, the center differential 20 is in an intermediate locked state, and in the case of "Ic = 0", it is in an unlocked state. When the center differential current Ic is the maximum current value Imax, the timer is set in step # 43,
In step # 44, the center differential current Ic is changed to "Ic = Im
ax ”. At this time, the center differential 20 is completely locked. After that, the timer is counted up in step # 45, and it is determined in step # 46 whether or not a predetermined time has elapsed. That is, when the center differential differential rotation speed ΔNc suddenly increases due to slippage or the like, the center differential 20 is kept in a completely locked state for a predetermined time.

Next, an example of calculating the control current value of the rear differential 22 will be described. FIG. 9 is a flowchart showing a rear differential control current value setting routine during the automatic mode control. The rear differential control current value setting routine is basically the same as the above center differential control current value setting routine. That is, as shown in FIG. 9, first, step # 5
The rear differential current Ir is similarly set at 0, and it is determined at step # 51 whether the rear differential current Ir has the maximum current value Imax. If the rear differential current Ir is smaller than the maximum current value Imax, at step # 52, "Ir = Ir ". At this time, the rear differential 22 is in the intermediate lock state, and in the case of "Ic = 0", it is in the unlock state. When the rear differential current Ir has the maximum current value Imax, the timer is set in step # 53, and the rear differential current Ir is set to "Ic = Imax" in step # 54. At this time, the rear differential 22 is completely locked. next,
In step # 54, the timer counts up,
In step # 56, it is determined whether a predetermined time has passed. That is, when the rear differential differential rotation speed ΔNr suddenly increases due to slippage or the like, the rear differential 22 is kept in a completely locked state for a predetermined time.

In this embodiment, each differential 21,
For the purpose of alleviating the hysteresis of each electromagnetic clutch 50 that applies the differential limiting force to 20 and 22, respectively, and improving the control accuracy of the differential limiting force, under the predetermined conditions, the electromagnetic clutches The current supplied to 50 is so-called modulation-controlled, and the signal of the current value is set to have a periodic waveform centered on the target current value corresponding to the target value of the differential limiting force.

The current supply to the electromagnetic clutch 50 will be described below with reference to the flowchart of FIG. 10 and the graph of FIG. While the vehicle is running, the front, center and rear differentials 21, 2 are
0 and 22 actuation and these differentials 21,20
When the control of the differential limiting force with respect to 22 and 22 is started, the differential control unit 43 controls the supply current to each electromagnetic clutch 50. In the case of the center differential 20, for example, first, step # 60 and step # 6.
At 1, it is determined whether the differential limiting force with respect to the differential (center differential 20), that is, the lock value is 0 (zero), and whether the lock value is max. (Maximum),
If either one of the determination results is YES, the current supplied to the electromagnetic clutch 50 is not modulation-controlled but is normally controlled (step # 64). Lock value is 0
The reason why the modulation control of the supply current is not performed in the case of max. Is that, in this region, there is usually little hysteresis.

Steps # 60 and # 61 above
If the determination results in both are NO, step # 62.
Then, whether or not the lock value is in the changed state, that is, in FIG.
As shown in period T1 and period T3, it is determined whether or not the control current to the electromagnetic clutch 50 is being increased or decreased. The determination result in step # 62 is Y
In the case of ES, the supply current (control current) to the electromagnetic clutch 50 is also controlled as usual without modulation control (step # 64). When the lock value is in the changing state, the modulation control of the supply current is not performed. Normally, the responsiveness is particularly important when the lock value changes, but when the modulation control is performed, the current value signal is This is because there is a possibility that the control responsiveness of the electromagnetic clutch 50 may be adversely affected because of the periodic waveform of.

Then, the determination result in step # 62 is N
In the case of O, that is, as shown by the period T2 in FIG. 11, the control current to the electromagnetic clutch 50 is OFF and max.
When the electromagnetic clutch 50 is not changed and is constant in an intermediate region between the two, the electromagnetic clutch 50 is in a so-called half-locked state between the completely unlocked state and the completely locked state, and the lock value is not changed. In this case, the signal of the current value of the control current is modulated and controlled so as to have a periodic waveform with a predetermined amplitude centered on the target current value.

In this way, when the current value of the control current is increased by modulating the signal of the current value of the control current to the electromagnetic clutch 50 (outward path: T1 → T2 →
T3) and the case of decreasing (return path: T3 → T2 → T1), at the above target current value, a loop-like hysteresis is generated on the return path side in the case of the forward path, and on the forward path side in the case of the return path. The hysteresis as a whole at the target current value is alleviated, and the control accuracy of the electromagnetic clutch can be improved.

Further, in the present embodiment, in particular, when the electromagnetic clutch 50 is in the half-locked state in which a remarkable hysteresis appears, the supply current is modulated and controlled, and the signal of the current value is made into a periodic waveform. It is possible to obtain good control characteristics over which the influence of hysteresis is small.

Further, in the present embodiment, in particular, the signal of the current value of the supply current is modulated and controlled only when the current value is constant and not in a changing state, and when the current is changing, the modulation control is not performed. Even when the fastening force of is rapidly changed, its responsiveness is not adversely affected.

Although the above-mentioned embodiment is applied to the case of a 4WD vehicle provided with three differentials including a front differential 21, a center differential 20, and a rear differential 22, the present invention is not limited to this.
The invention is not limited to such a case, and the axle differential (center differential) and 1
It is needless to say that the present invention can be effectively applied to a vehicle provided with only two differentials such as an inter-wheel differential (for example, a rear differential) or a vehicle provided with only one of the differentials.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram schematically showing an overall configuration of a driving force transmission system of an automobile according to an embodiment of the present invention.

FIG. 2 is an explanatory sectional view showing an example of an electromagnetic clutch provided on the differential of the automobile.

FIG. 3 is a flowchart showing a subroutine for calculating a vehicle body speed of the automobile.

FIG. 4 is a flowchart showing a subroutine for calculating a differential rotation speed of the center differential of the automobile.

FIG. 5 is a flowchart showing a subroutine for calculating a differential rotation speed of the rear differential of the automobile.

FIG. 6 is a flowchart showing a center diff current setting routine during auto mode control.

FIG. 7 is a graph showing an example of a relationship between a current value and a center differential differential rotation speed.

FIG. 8 is a graph showing an example of a relationship between a current value and a throttle opening.

FIG. 9 is a flowchart showing a rear diff current setting routine during auto mode control.

FIG. 10 is a flowchart for explaining modulation control of a control current to the electromagnetic clutch.

FIG. 11 is a diagram showing an example of a supply pattern of a control current to the electromagnetic clutch.

FIG. 12 is a diagram showing an example of a stroke change with respect to a current value and its hysteresis.

FIG. 13 is a diagram showing an example of modulation control of a control current.

[Explanation of symbols]

 15 ... Front Axle 16 ... Front Wheel 17 ... Rear Axle 18 ... Rear Wheel 20 ... Center Differential (Center Differential) 21 ... Front Differential (Front Differential) 22 ... Rear Differential (Rear Differential) 43 ... Differential Control Unit 50 ... Electromagnetic Clutch

Claims (3)

[Claims] 1. A differential limiting force for a differential device provided between left and right wheels or between front and rear axles is obtained by engaging an electromagnetic clutch, and the differential limiting force is obtained according to a current value of a current supplied to the electromagnetic clutch. In the vehicle differential limiting device configured to control the differential limiting force, the current value signal of the supply current is a periodic waveform centered on a target current value corresponding to the target value of the differential limiting force. A differential limiting device for a vehicle characterized by the above. 2. The vehicle differential limiting device according to claim 1, wherein the current value of the supply current is provided only when the electromagnetic clutch is in a half-locked state between a completely locked state and a completely unlocked state. Signal of the above-mentioned periodic waveform, a differential limiting device for a vehicle. 3. The differential limiting device for a vehicle according to claim 1, wherein the signal of the current value of the supply current is applied to the periodic waveform only when the current value of the supply current is constant instead of changing. A differential limiting device for a vehicle characterized in that