JP2809715B2 - Lock-up clutch control device for automatic transmission - Google Patents

Description

The present invention relates to a torque provided with a lock-up clutch for engaging an input element such as a pump impeller and an output element such as a turbine runner in a relatively rotatable state. The present invention relates to a lock-up clutch control device for an automatic transmission that controls a slip state of a lock-up clutch with respect to an input element or an output element in an automatic transmission including a converter.

(Prior Art) A fluid coupling that constitutes an automatic transmission of a vehicle together with a transmission mechanism, a so-called torque converter, in addition to a pump impeller that constitutes an input element connected to an output shaft of an engine and a turbine runner that constitutes an output element. There is known a pump impeller provided with a lock-up clutch that frictionally engages with the pump impeller. In a vehicle equipped with such an automatic transmission having a torque converter with a lock-up clutch, when torque is transmitted from the engine to the wheel drive system via the automatic transmission in a predetermined traveling state. In order to achieve both a reduction in energy loss in the torque converter and a reduction in vehicle body vibration caused by transmission of engine torque fluctuations to the wheel drive system via the automatic transmission, It is known that slip control is performed in which the lock-up clutch is engaged to engage the pump impeller so as to generate a rotational speed difference between the pump impeller and the turbine runner, that is, a slip engagement state. Have been.

In the slip control of the lock-up clutch in such a torque converter, the engine load and the vehicle speed are usually set in a slip control area defined in a shift characteristic diagram which is preliminarily mapped and stored in a memory built in the control means. This is performed under the running condition of the vehicle that is located within the vehicle.For example, as disclosed in JP-A-57-33253, the target rotational speed difference between the pump impeller and the turbine runner in the torque converter is reduced. The feedback control is performed with a control amount that is determined and that is set according to the deviation of the actual rotational speed difference between the pump impeller and the turbine runner from the target rotational speed difference.

As described above, in a state where the slip control of the lock-up clutch in the torque converter is performed in a feedback control manner, when the speed-change operation is performed by the speed change mechanism that constitutes the automatic transmission, the lock-up clutch is moved to a specific position. Under the operating state, the shock generated in the vehicle during shifting is reduced.

However, as described above, the slip control of the lock-up clutch, which is performed when the engine load and the vehicle speed are within the slip control region, causes the rotational speed difference between the pump impeller and the turbine runner to be different. In the case where the slip control is performed in a portion where the engine load in the slip control region is relatively large, the engine load is increased under the form of the feedback control with the control amount based on the deviation from the target rotation speed difference. Is relatively large, the torque generated by the engine transmitted to the pump impeller is relatively large, so that the slip engagement state of the lock-up clutch is changed by the slip control, thereby causing an undesirable hunting phenomenon. May occur. When the slip control is performed in a portion where the engine load in the slip control region is relatively large, for example, the accelerator pedal is depressed with a relatively small amount so that the engine load does not deviate from the slip control region. In such a case, the engine speed is once increased by depressing the accelerator pedal, whereby the rotation speed on the pump impeller side in the torque converter is increased, and the rotation speed difference between the pump impeller and the turbine runner increases. So
By the slip control performed in the form of the feedback control, the engine speed is rapidly reduced to return the speed difference between the pump impeller and the turbine runner to the target speed difference, and therefore, the accelerator pedal is depressed. However, there is a problem that a sudden decrease in the engine speed occurs temporarily despite that the driver feels uncomfortable, and furthermore, the accelerator pedal moves the engine load out of the slip control region. If the vehicle is depressed with a relatively large amount, the smooth increase of the engine speed is hindered until the engine load falls outside the upper limit of the slip control region, and the driver is There is a disadvantage that it is not possible to obtain a great acceleration feeling.

In view of such a point, the present invention is also applicable to a case where the slip control for the lock-up clutch provided in the torque converter in the automatic transmission mounted on the vehicle is performed under a relatively large engine load. Undesirable hunting phenomenon does not occur, and furthermore, a situation in which the engine speed suddenly drops suddenly even though the accelerator pedal is depressed, or a smooth increase in the engine speed is hindered. It is an object of the present invention to provide a lock-up clutch control device for an automatic transmission which is designed to prevent a situation from occurring.

(Means and Actions for Solving the Problems) In order to achieve the above-mentioned object, a lock-up clutch control device for an automatic transmission according to the present invention has an input element and an output as shown in FIG. And an engine and an automatic transmission including a fluid coupling having a lock-up clutch capable of engaging a slip-up state in which the input element and the output element can be relatively rotatably engaged with each other. Load detecting means, rotational speed difference detecting means for detecting a rotational speed difference between the input element and the output element, and an operation for engaging the input element and the output element with the lock-up clutch; Hydraulic pressure supply means for supplying an operating hydraulic pressure for selectively causing the element to be disengaged, and hydraulic pressure control means. When the load detecting means detects that the load of the engine is smaller than a predetermined value in the state in which the engine speed is fastened, the hydraulic pressure supplying means responds to the rotational speed difference detected by the rotational speed difference detecting means. The operation of supplying the set hydraulic pressure to the lock-up clutch is performed, and the load detection means detects that the load of the engine is equal to or higher than a predetermined value under the condition that the lock-up clutch is in a slip-engaged state. When this is done, the hydraulic pressure supply means is configured to perform an operation of supplying a preset operating hydraulic pressure to the lock-up clutch.

(Embodiment) FIG. 2 shows an example of a lock-up clutch control device for an automatic transmission according to the present invention, together with a power plant of a vehicle to which the control device is applied.

The power plant shown in FIG.
And an automatic transmission 18, and the engine body 10
For example, it has four cylinders, and each cylinder has a throttle valve
The intake air from the intake passage 14 in which the 12 is disposed and the intake passage 14
Is supplied from the fuel injection valve disposed in the cylinder, and the mixture is burned by the operation of the ignition system in each cylinder and discharged to the exhaust passage. Then, torque from engine body 10 obtained by combustion of the air-fuel mixture is transmitted to the wheel drive system via a power transmission path including automatic transmission 18.

The automatic transmission 18 includes a torque converter 20, a multi-gear transmission mechanism 19, and a hydraulic circuit 40 that forms and sends out an operating oil pressure used for controlling the gear mechanism.

The torque converter 20 is provided with a hydraulic circuit 40 in FIG.
As shown in detail with a portion involved in the operation control of the torque converter 20 in FIG.
And a pump impeller 22 connected to the drive plate 21 and rotating therewith, a turbine runner 23 in which torque from the engine body 10 is transmitted via the pump impeller 22, and a pump impeller 22 and the turbine runner 23. A stator 24 and a one-way clutch 25 disposed between the stator 24 and a fixed portion of the torque converter 20.The turbine hub 26 is disposed between the drive plate 21 and the turbine runner 23. And a lock-up clutch 29 connected to the torsion damper 27 via a coil spring 27a.
The lock-up clutch 29 is entirely arranged to be close to and away from the drive plate 21 and is configured to rotate together with the turbine runner 23.

A lock-up clutch 29 is provided between the drive plate 21 and the turbine runner 23 in the torque converter 20.
The back pressure chamber 31 and the internal pressure chamber 32
The back pressure chamber 31 is supplied with an operating oil pressure that presses the lock-up clutch 29 in a direction away from the drive plate 21 from the hydraulic circuit section 40 through an oil passage 41, and is supplied to the internal pressure chamber 32. A hydraulic pressure is supplied from the oil passage circuit portion 40 through the oil passage 42 to press the lock-up clutch 29 in a direction to approach the drive plate 21.

When the value of the operating oil pressure in the internal pressure chamber 32 is higher than the value of the operating oil pressure in the back pressure chamber 31 by a predetermined value or more, the lock-up clutch 29 is pushed rightward in FIG. And the pump impeller 22 and the turbine runner 23 are brought into an engaged state, and the value of the operating oil pressure in the internal pressure chamber 32 is determined by a predetermined value from the value of the operating oil pressure in the back pressure chamber 31. 3, the frictional engagement with the drive plate 21 is released by being pushed to the left in FIG. 3, and the pump impeller 22 and the turbine runner 23 are brought into a disengaged state in which they are disengaged. It is assumed. Further, the lock-up clutch 29 can relatively rotate the pump impeller 22 and the turbine runner 23 when the differential pressure between the operating oil pressure in the internal pressure chamber 32 and the operating oil pressure in the back pressure chamber 31 is within a predetermined range. It will be engaged in a state. A slip engagement state with the drive plate 21 is assumed, and the slip ratio with respect to the drive plate 21 decreases as the differential pressure increases. The internal pressure chamber 32 is connected to an oil cooler 45 through an oil passage 44 provided with a check valve 43.

A lock-up shift valve 50 and a lock-up control solenoid valve 5 are provided in a portion of the hydraulic circuit section 40 that is involved in the operation control of the torque converter 20. The lock-up shift valve 50 includes a spool 51 provided with lands 51a, 51b and 51c, a spring 52 for urging the spool 51 rightward in FIG. 3, and ports a, b, c, d, e, f, g, and h. The ports a and f are connected to the back pressure chamber 31 via an oil passage 41, the port b is connected to an oil cooler 45 via an oil passage 46, and the port c is connected to the internal pressure chamber 32 via an oil passage 42. The oil passage 47 is connected to the oil passage 46 via an oil passage 42a branched from the oil passage 42, and the ports d and e are connected to an oil passage 47 provided with a pressure regulating valve 48.
The port g is connected to the oil pump OP via an oil passage 49 in which the lock-up control solenoid valve 5 is disposed, and the port h is connected to the oil pan T.

The speed change mechanism 19 is provided with various frictional engagement elements such as a planetary gear portion and a clutch or a brake for obtaining four forward steps and one reverse step. The release hydraulic pressure is
It is selectively supplied as appropriate via various control valves or shift valves provided therein. As a result, each frictional engagement element is set to the engaged state or the released state, and D that constitutes a P range (parking range), an R range (reverse range), an N range (neutral range), and a forward range. A range (drive range), an S range, and an L range, and first to fourth speeds in the forward range can be obtained.

Further, a control unit 70 is provided in addition to the above-described configuration. The control unit 70 includes, as shown in FIG.
A detection output signal St from a throttle opening sensor 61 for detecting the opening of the throttle valve 12 which is opened and closed in conjunction with the accelerator pedal, and a vehicle speed sensor 62 for detecting the vehicle speed
And a shift position sensor 63 that is connected to a manual valve provided in the hydraulic circuit unit 40 and detects an operation position of a shift lever operated by a driver to control the transmission mechanism 19. The detected output signal Ss obtained, the detected output signal Sn obtained from the engine speed sensor 64 for detecting the engine speed, and the detected output signal Sm obtained from the turbine speed sensor 65 for detecting the speed of the turbine runner 23 are While being supplied, other detection output signals Sx required for controlling the automatic transmission 18 are also supplied.

The control unit 70 supplies a drive signal Cf to various control valves or shift valves built in the hydraulic circuit unit 40 based on the above-described various detection output signals, and performs a shift control of the transmission mechanism 19, The drive pulse signal Ca is supplied to the lock-up control solenoid valve 5 built in the hydraulic circuit unit 40 to control the operation of the lock-up clutch 29 provided in the torque converter 20.

When the control unit 70 performs the shift control of the transmission mechanism 19 and the operation control of the lock-up clutch 29, for example, when the shift lever is detected to be in the D range position based on the detection output signal Ss. A shift characteristic line represented by the throttle opening TH on the vertical axis and the vehicle speed V on the horizontal axis, as shown in FIG. 4, which is mapped and stored in the internal memory of the control unit 70. In the figure, the shift lines Ua, Ub, Uc, Ud, Ue and Uf are compared with the opening degree of the throttle valve 12 represented by the detection output signal St and the vehicle speed represented by the detection output signal Sv,
It is determined whether a shift-up condition or a shift-down condition is satisfied, and a throttle valve representing lock-up operation lines La, Lb, and Lc, lock-up release lines Ld, Le, and Lf, and a detection output signal St. The opening degree of 12 and the vehicle speed represented by the detection output signal Sv are collated to determine whether a lock-up operation condition or a lock-up release condition is satisfied. Further, the slip control region Rs surrounded by the line r1 is compared with the opening of the throttle valve 12 represented by the detection output signal St and the vehicle speed represented by the detection signal Su, and the opening of the throttle valve 12 and the vehicle speed are compared. And
It is determined whether or not lock control clutch 29 is in a slip control region Rs, which is a region where slip control should be performed.

The shift lines Ua, Ub and Uc shown in FIG.
Are from 1st gear to 2nd gear, 2nd gear to 3rd gear, and 3rd gear, respectively.
Upshift from 4th to 4th, Ud, Ue and Uf
Are from 2nd gear to 1st gear, 3rd gear to 2nd gear, and 4
And the lock-up operation lines La, Lb and Lc correspond to the second gear and the third gear, respectively.
The lock-up release lines Ld, Le, and Lf relate to the lock-up operation at the second, third, and fourth speeds, respectively, for the lock-up operation at the first and fourth speeds. Further, the slip control region Rs sets an upper limit value of the throttle opening TH, for example,
1.5 / 8, throttle opening TH is less than a predetermined value, for example,
It is divided into a divided region Rs1 where the value is less than 0.5 / 8 and a divided region Rs2 where the throttle opening TH is not less than a predetermined value, for example, 0.5 / 8 or more.

When it is detected that the upshift condition or the downshift condition is satisfied, the control unit 70 sends a drive signal Cf to cause the transmission mechanism 19 to switch the gear position, and Is performed. When it is detected that neither the lock-up operation condition nor the slip control condition is satisfied, the control unit 70 does not supply the drive pulse signal Ca to the lock-up control solenoid valve 5. As a result, the lock-up control solenoid valve 5 is closed and the operating oil pressure from the oil pump OP is reduced through the oil passage 49 without causing a pressure drop.
Is supplied to the lock-up shift valve 50 through an oil passage 47.
Through its ports d and e. as a result,
In the lock-up shift valve 50, the spool 51 is moved in a direction opposing the biasing force of the spring 52,
It is assumed that the position is as shown in the figure, and ports b,
e and h are closed by the lands 51a, 51b and 51c of the spool 51, and the port c and the port d are in communication with each other. The pressure is supplied to the internal pressure chamber 32 of the torque converter 20 through the passage 42. At this time, the port f of the lock-up shift valve 50 connected to the back pressure chamber 31 of the torque converter 20 via the oil passage 41 is cut off from the port h, so that the pressure in the back pressure chamber 31 is reduced. No hydraulic oil is discharged,
When the operating oil pressure in the internal pressure chamber 31 exceeds a predetermined pressure, the internal pressure chamber 31
The working oil inside is discharged to an oil cooler 45 through an oil passage 44 provided with a check valve 43.

Therefore, in such a case, the lock-up clutch 29
However, the torque converter 20 is separated from the drive plate 21 and is in a released state, and the torque converter 20 assumes a converter state.

On the other hand, when it is detected that the lock-up operation condition is satisfied, the control unit 70 supplies the lock-up control solenoid valve 5 with the drive pulse signal Ca having the maximum pulse width. As a result, the lock-up control solenoid valve 5 is maintained in the open state, the operating oil pressure supplied to the lock-up shift valve 50 via its port g is reduced, and the spool 51 of the lock-up shift valve 50 By the operating oil pressure supplied from the pressure regulating valve 48 supplied through the ports d and e, the spring 52 is moved in a direction in accordance with the urging force of the spring 52 to assume a position to the right of the position shown in FIG. . As a result, the communication state between the port c and the port d of the lock-up shift valve 50 is maintained, and the port f and the port h are made into the communication state. While being supplied to the internal pressure chamber 32 of the converter 20, the hydraulic oil in the back pressure chamber 31 is discharged from the port h of the lock-up shift valve 50 to the oil pan T.

Therefore, in such a case, the lock-up clutch 29
Is pressed into engagement with the drive plate 21 by the operating oil pressure supplied into the internal pressure chamber 32, and is brought into a fastening state,
The torque converter 20 is in a lockup state.

Further, when the control unit 70 detects that the slip control condition is satisfied, the control unit 70 outputs a drive pulse signal having a pulse width that is changed according to the slip control state.
Ca is supplied to the lock-up control solenoid valve 5.

Accordingly, the period during which the lock-up control solenoid valve 5 is in the open state is changed according to the pulse width of the drive pulse signal Ca, and the operating oil pressure supplied from the oil pump OP to the port g of the lock-up shift valve 50 Decreases according to the period during which the lock-up control solenoid valve 5 is in the open state, and the amount of hydraulic oil discharged from the back pressure chamber 31 of the torque converter 20 through the port h of the lock-up shift valve 50 changes. Against me,
The operating oil pressure from the pressure regulating valve 48 is supplied to the internal pressure chamber 32 of the torque converter 20 as it is. As a result, the working oil pressure in the back pressure chamber 31 of the torque converter 20 and the internal pressure chamber 32
The differential pressure between the hydraulic pressure and the hydraulic pressure in the inside changes, and the lock-up clutch 29 causes the pump impeller 22 and the turbine runner 23 to engage with the drive plate 21 in a relatively rotatable state. In this state, the slip ratio at that time is changed according to the pulse width of the drive pulse signal Ca, and the slip control of the lock-up clutch 29 is performed.

In the slip control by the control unit 70, when the opening of the throttle valve 12 represented by the detection output signal St is compared with the slip control region Rs shown in the shift characteristic diagram shown in FIG. If the degree is less than 0.5 / 8 and is in the section area Rs1 of the slip control area Rs, the pump impeller 22 and the turbine runner
From among the target rotational speed differences between and, a target rotational speed difference corresponding to the shift speed that is taken at that time is determined, and
Based on the detection output signals Sn and Sm, the pump impeller 22
The actual rotational speed difference between the engine and the turbine runner 23 is calculated. Then, a correction duty value corresponding to a difference between the calculated actual rotation speed difference and the target rotation speed difference is obtained by being selected from a preset correction duty value. Is calculated by adding the correction duty value to the reference duty value corresponding to the target rotational speed difference, and the drive pulse signal Ca is formed as a signal having a pulse width corresponding to the calculated duty value, Slip control is sent to the up control solenoid valve 5, whereby slip control has a form of feedback control that functions to match the actual and target rotational speed difference between the pump impeller 22 and the turbine runner 23. Done.

On the other hand, when the opening degree of the throttle valve 12 takes a value within the range of 0.5 / 8 to 1.5 / 8 and is in the section area Rs2 of the slip control area Rs, the throttle valve 12 Feedback control such as is performed when the opening degree is in the segmented region Rs1 is not performed, and the drive pulse signal Ca is
It is formed as a signal having a pulse width corresponding to a duty value set in advance to a predetermined value, and is sent to the lock-up control solenoid valve 5, whereby slip control is performed in the form of feedforward control.

By doing so, the slip control is performed in a portion where the value of the throttle opening TH in the slip control region Rs is relatively small, and therefore, an engine acting on the pump impeller 22 is generated. Under a relatively small torque, feedback control is performed so that the relative rotation between the pump impeller 22 and the turbine runner 23 has a target rotation speed difference, and the rotation is properly performed. On the other hand, since the slip control is performed in a portion where the value of the throttle opening TH in the slip control region Rs is relatively large, the torque generated by the engine acting on the pump impeller 22 is relatively small. Under the condition that the rotation is large, the relative rotation between the pump impeller 22 and the turbine runner 23 is controlled with a constant control signal. Even when the accelerator pedal is depressed with a relatively small amount without causing the occurrence of hunting and causing the opening of the throttle valve 12 to deviate from the slip control region Rs, A sudden temporary decrease in engine speed is avoided, and the accelerator pedal is
When the opening of the throttle valve 12 is depressed with a relatively large amount that deviates from the slip control region Rs, the engine speed is smoothly increased following the step.

The control unit 70 that controls the operation of the lock-up clutch 29 as described above is configured using, for example, a microcomputer. In such a case, an example of a program for performing the slip control that is performed by the microcomputer Will be described with reference to the flowchart of FIG.

In the program shown by the flowchart of FIG. 5, after starting, in step 71, various detection output signals are fetched, and in step 72, it is determined whether or not the shift lever is in the D range position based on the detection output signal Ss. When it is determined that the shift lever is not in the D range position, the process returns to step 71, and when the shift lever is in the D range position, in step 73, the detection output signal St
It is determined whether or not the throttle valve 12 is in the fully closed state on the basis of. When it is determined that the throttle valve 12 is in the fully closed state, the operation returns to the original state. When it is determined that the throttle valve 12 is not in the fully closed state, the accelerator pedal is being depressed. At 74, the opening degree of the throttle valve 12 represented by the detection output signal St and the vehicle speed represented by the detection output signal Sv correspond to the slip control region.
The value within Rs is taken, and it is determined whether the slip control condition is satisfied.

As a result of the determination, when the slip control condition is satisfied, in step 75, the opening degree of the throttle valve 12 changes the feedback (F /
B) It is determined whether or not the value is within the segmented region Rs1 to be controlled. If the opening of the throttle valve 12 is a value within the segmented region Rs1, the detection output signal Sm is determined at step 76.
The actual engine speed difference ΔNo between the pump impeller 22 and the turbine runner 23 is calculated by subtracting the engine engine speed obtained based on the detected output signal Sn from the turbine engine speed obtained based on the engine speed. Then, in step 77, a correction duty value ΔDp corresponding to the rotation speed difference ΔNo is obtained. In step 78, the correction duty value ΔDP is added to the reference duty value Dp to calculate a duty value DF, and the process proceeds to step 79. . In step 79, a drive pulse signal Ca having a pulse width corresponding to the duty value DF calculated in step 78 is formed, supplied to the lock-up control solenoid valve 5, and the process returns to step 71.

Also, as a result of the determination in step 75, the throttle valve
When the opening degree of 12 is a value in the section area Rs2 in which the feedforward control is to be performed in the slip control area Rs, the process proceeds to step 80, where a duty value DF having a constant value DI is set, and the process proceeds to step 79.

Further, when it is determined in step 74 that the slip control condition is not satisfied, in step 81, the torque converter 20 determines the value of the operating oil pressure in the internal pressure chamber 32 based on the engine speed and the turbine speed. Is determined to be higher than the value of the operating oil pressure in the back pressure chamber 31 by a predetermined value or not. As a result of the determination, when the torque converter 20 is in the converter state, in step 82, the supply of the drive pulse signal Ca to the lock-up control solenoid valve 5 is stopped, and the operation returns to the original state. On the other hand, when it is determined in step 81 that the torque converter 20 is not in the converter state, the torque converter 20 is in the lock-up state.
After setting DF, go to step 79.

(Effect of the Invention) As is apparent from the above description, according to the lockup clutch control device for the automatic transmission according to the present invention, the slip of the lockup clutch provided in the torque converter in the automatic transmission mounted on the vehicle is provided. Control
In a state where the throttle opening is relatively small, that is, in a state where the torque generated by the engine acting on the pump impeller is relatively small, the pump impeller and the turbine runner have a target rotational speed difference. A state in which feedback control can be performed properly so that the throttle opening is relatively large, and thus a torque generated by the engine acting on the pump impeller is relatively large. Under the condition that the relative rotation between the pump impeller and the turbine runner is controlled in a feedforward manner by a constant control signal without causing hunting, and the accelerator pedal is controlled by the slip control of the throttle valve opening. Depressed with a relatively small amount that does not deviate from the area Also, when the accelerator pedal is depressed with a relatively large amount that causes the opening of the throttle valve to deviate from the slip control range without causing a sudden temporary decrease in the engine speed, Following this, the engine speed can be smoothly increased.

[Brief description of the drawings]

FIG. 1 is a basic configuration diagram showing a lock-up clutch control device for an automatic transmission according to the present invention according to the claims, and FIG. 2 is an example of a lock-up clutch control device for an automatic transmission according to the present invention. Is shown together with a power plant of a vehicle to which each is applied, FIG. 3 is a block diagram showing a main part of the example shown in FIG. 2, and FIG. 4 is provided for explaining the operation of the example shown in FIG. FIG. 5 is a flowchart showing an example of a program executed by the microcomputer when the microcomputer is used for the control unit in the example shown in FIG. In the figure, 5 is a solenoid valve for lock-up control, 10 is an engine body, 12 is a throttle valve, 18 is an automatic transmission, 20 is a torque converter, 21 is a drive plate, 22 is a pump impeller, 23 is a turbine runner, and 29 is A lock-up clutch, 31 is a back pressure chamber of the torque converter 20, 32 is an internal pressure chamber of the torque converter 20, 40 is a hydraulic circuit, 50 is a lock-up shift valve, 61 is a throttle opening sensor, 62 is a vehicle speed sensor, and 64 is a vehicle speed sensor. An engine speed sensor, 65 is a turbine speed sensor, and 70 is a control unit.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) F16H 61/14

Claims (1)

(57) [Claims] An automatic transmission includes an input element, an output element, and a fluid coupling having a lock-up clutch capable of engaging a slip engagement state in which the input element and the output element are engaged in a relatively rotatable state. Load detection means for detecting the load of the connected engine; rotation speed difference detection means for detecting a rotation speed difference between the input element and the output element; and the lock-up clutch, the input element and the output element. And an oil pressure supply means for supplying an operating oil pressure for selectively taking an operation of bringing the input element and the output element into the non-engagement state, and a step of bringing the lock-up clutch into the slip engagement state. When the load detecting means detects that the load of the engine is smaller than a predetermined value, the hydraulic pressure supplying means sends the engine speed difference detecting means to the hydraulic pressure supplying means. An operation of supplying an operating oil pressure set according to the detected rotational speed difference to the lock-up clutch is performed, and under the slip-up state of the lock-up clutch, the load detecting unit performs the operation. Hydraulic pressure control means for causing the hydraulic pressure supply means to perform an operation of supplying a preset operating hydraulic pressure to the lock-up clutch when it is detected that the load of the engine is equal to or greater than the predetermined value. Lock-up clutch control device for an automatic transmission.