JP3296218B2 - Anti-creep device for automatic transmission - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent erroneous learning and drops of the operating pressure for a frictional element in a creep prevention system in creep prevention where the operating pressure command value for the frictional element is suddenly lowered first to an initial set pressure and is thereafter gradually lowered, by bringing the gradually lowering gradient coincident with the actual lowering velocity of the operating pressure. SOLUTION: The command value α of the fluid pressure Pc is suddenly lowered at a control start time t1 to an initial set pressure Ps and is thereafter gradually lowered to thereby lower the fluid pressure Pc as indicated by the continuous line βand thus bring the forward clutch slipping for creep prevention. If the fluid temperature remains lower, the fluid pressure Pc drops with a timelag by the command as indicated by the dotted line γ, with the results that the command pressure α deviates from the actual pressure γ at a slip start time t3 as indicated by the alternate long and short dash line to adversely cause erroneous learning at the time t3 and drops of the operating pressure Pc in Δt1 . In order to counter that, the lowering gradient k of the command pressure is made gradual as indicated by the continuous line δwhile the fluid temperature lies lower so as to bring the actual pressure γ coincident with the command pressure δat the slip start time t3 .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a creep preventing device for an automatic transmission for preventing a creep phenomenon of an automatic transmission caused by drag transmission of a fluid transmission device.

[0002]

2. Description of the Related Art In an automatic transmission, since the engine rotation is input via a fluid transmission, the drag torque of the fluid transmission is stopped during a stop in a state where the power can be transmitted with the automatic transmission in a travel range. Is running at very low speed in the vehicle, so-called creep phenomenon cannot be avoided. The driver operates the brakes to prevent this very slow running, but during this time,
Fuel economy is likely to deteriorate and vibration problems are likely to occur.

[0003] Therefore, by lowering the operating oil pressure of the hydraulically operated friction element to bring the automatic transmission into a state in which power can be transmitted in the traveling range, the operating friction element is brought into a slip state. There has been proposed a creep preventing device that reduces transmission of creep torque by a fluid transmission device.

[0004] By the way, when preventing the creep, the operating oil pressure of the friction element is reduced at a stretch and the friction element is suddenly slipped, so that the creep torque is reduced too rapidly and a shock occurs.

In order to solve this problem, conventionally, as disclosed in Japanese Patent Application Laid-Open No. 5-79562, for example, the operating oil pressure of a friction element is first rapidly reduced to an initial set pressure and then gradually reduced at a predetermined gradient. There has been proposed a creep preventing device for an automatic transmission in which the friction element is gradually brought into a slip state.

Further, another document, Japanese Patent Application Laid-Open No. 5-87234, further teaches that the mode of lowering the operating oil pressure of the frictional element is controlled by learning so that the transition time to the creep prevention state is constant, so that various variations and It has also been proposed that a reliable creep-preventing action can be obtained while always suppressing shock regardless of aging.

[0007]

The above-mentioned creep prevention control will be described below with reference to FIG. 7. The command value of the friction element operating oil pressure (forward clutch operating oil pressure P _c ) is indicated by a solid line α as shown in FIG. Instant t to start
_{At 1} , the pressure is rapidly decreased to the initial set pressure P _S , and then gradually decreased at a predetermined gradient. After the moment t ₂ at which the frictional element starts to slip and the turbine speed N _t rises rapidly, the engine speed N
the slip amount ΔN for the hydraulic power transmission device, represented by the difference between _e and the turbine speed N _t to a feedback control value such as to maintain a predetermined value.

[0008] In response to the command pressure α, the actual frictional element operating oil pressure (forward clutch operating oil pressure _Pc is approximately equal to the two-dot chain line β if the transmission operating oil temperature is a specified temperature during operation). Control is performed so as to follow the command value α, and the desired creep prevention action can be obtained.

However, at low temperatures, the actual friction element operating oil pressure (forward clutch operating oil pressure P _c ) changes to a broken line γ.
As shown by, the command value α ′ indicated by the one-dot chain line is inevitably reduced with a large time delay due to the increase in viscosity, and the slip of the friction element (forward clutch) is not reached until the instant t _3. Is started.

In this case, the actual operating oil pressure γ still remains at the instant t _{3 at} which the friction element (forward clutch) starts slipping.
And the command pressure α ′ may not match. By the way when the learning, based on the command pressure α'at the slip starting instant t ₃ of the frictional element (the forward clutch) is the routine to perform learning, command pressure α'at the instant t ₃ the friction element There is a concern that the above learning may be erroneous because it does not correspond to the hydraulic pressure at which (forward clutch) slip is started.

[0011] As shown in FIG. 7 on the other hand, although the instant t ₃ subsequent little change for command pressure α'is feedback control value of the, the instant t ₃ initially is the actual hydraulic oil pressure γ thereto Since the values do not match, the control for bringing the actual working oil pressure γ to the command pressure α 'as shown by the broken line is continued, so that the actual operating oil pressure γ causes the friction element to start slipping. Equivalent pressure (instantaneous t ₃
Hydraulic pressure γ is lowered beyond the value) of the engine from the instant t ₃ of the engine speed N _e shown by a broken line is clear from time-series change is empty indulge in.

[0012] Due to such an excessive decrease in the actual working oil pressure γ (improper filling), the instantaneous working oil pressure γ starts to operate at the instant t _3.
Δt until the moment when is equal to the return spring equivalent pressure
_{During one} period, when the friction element (forward clutch) is to be re-engaged in order to end the creep prevention control in response to the depression of the accelerator pedal, this re-engagement has a response delay due to the in-built defect. become.

[0013] The response delay causes the engine to run at a higher speed in response to the depression of the accelerator pedal, and when the friction element (forward clutch) is re-engaged, the engine uses the energy corresponding to the engine speed. This causes a large fastening shock, that is, a large starting shock.

It is an object of the present invention to propose a creep preventing device for an automatic transmission in which all of the above problems can be solved by changing the gradual decrease gradient of the operating oil pressure command value.

[0015]

According to a first aspect of the present invention, the rotation of an engine is input via a fluid transmission and selected by a selective hydraulic operation of a plurality of friction elements. In the automatic transmission that changes the engine speed and outputs the gear at the selected gear, the hydraulic pressure of the operating friction element is first initialized to reduce the transmission of creep torque by the fluid transmission device. In the creep preventing device of the automatic transmission in which the friction element is brought into the slip state, the operation oil pressure control command for the friction element is suddenly reduced to the pressure and thereafter gradually decreased at a predetermined gradient over a predetermined time. It is characterized in that the gradual decrease gradient with respect to the value is set to be gentler as the oil temperature is lower, according to the transmission operating oil temperature.

[0016] The second invention described in claim 2 is:
In the first invention, the gradual decrease gradient of the working oil pressure control command value is determined so that the working oil pressure control command value and the actual working oil pressure coincide with each other when the operating friction element starts slipping. It is characterized by having comprised.

[0017]

According to the first aspect of the present invention, the automatic transmission operates at a shift speed selected by selective hydraulic operation of a plurality of friction elements.
The rotation of the engine from the fluid transmission is shifted and output.

On the other hand, in order to reduce the transmission of creep torque by the fluid transmission device, the anti-creep device first reduces the operating oil pressure of the friction element during operation to an initial set pressure, and then takes a predetermined time for a predetermined time. The friction element is brought into a slip state by an operation oil pressure control command that is gradually reduced by a gradient.

By the way, in the first invention, in particular, the gradual decrease gradient relating to the operation oil pressure control command value of the friction element is made gentler as the oil temperature becomes lower in accordance with the transmission oil temperature. can get.

In other words, even when the actual hydraulic pressure of the friction element decreases at a low temperature with a large time delay due to an increase in viscosity, the gradually decreasing gradient of the command value becomes gentler at a lower temperature. However, the decrease of the actual friction element working oil pressure with respect to the command value does not have much delay, and therefore, when the friction element starts slipping due to the decrease of the working oil pressure, the actual operating oil pressure and the command pressure are not Can be reduced.

Therefore, it is used for various learning controls.
The command pressure at the moment when the friction element starts slipping substantially corresponds to the oil pressure at which the friction element starts slipping, and it is possible to eliminate the concern that learning will be erroneous.

Further, since the difference between the actual operating oil pressure and the command pressure when the friction element starts slipping is small, the actual operating oil pressure is reduced to the return spring equivalent pressure of the friction element immediately after the instant when the slip starts. And the frictional element in response to the depression of the accelerator pedal has a response delay corresponding to the indentation failure, causing the engine to blow. Therefore, there is no problem that the re-fastening of the friction element is accompanied by a large fastening shock with the energy of the engine blowing.

According to the second aspect of the present invention, the operating oil pressure control command value and the actual operating oil pressure just match the gradually decreasing gradient of the operating oil pressure control command value when the friction element in operation starts slipping. With such a configuration, the deviation between the actual operating oil pressure and the command pressure when the friction element starts slipping due to a decrease in the operating oil pressure can be completely eliminated. The effect of the present invention can be completely achieved.

[0024]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a system diagram showing a creep preventing device for an automatic transmission according to an embodiment of the present invention. The automatic transmission creep prevention device 10 according to the present embodiment includes a forward clutch (start clutch) 11, which is a starting friction element to be engaged in the first forward speed of the automatic transmission, in a semi-engaged state (slip state). ) To prevent creep.

In FIG. 1, reference numeral 12 denotes a torque converter (fluid transmission), which comprises a pump impeller 12p, a turbine runner 12t, and a stator 12s.
The torque converter 12 is provided with a pump impeller 12p.
To the engine crankshaft 13, the turbine runner 12 t to the input shaft 14 of the automatic transmission, and the stator 12 s to the one-way clutch 1 on the transmission case 15.
It is a normal one that is put into practical use by putting it through 2o.

Here, the torque converter 12 transmits the engine rotation (rotation speed _Ne ) from the crankshaft 13 to the input shaft 14 of the automatic transmission while increasing the torque via the internal working fluid. The input rotation speed (torque converter output rotation speed) to the machine is denoted by N _t (turbine rotation speed).

The automatic transmission includes a plurality of friction elements (not shown) including a forward clutch 11 and selectively operates (engages) the plurality of friction elements including the forward clutch 11 in a corresponding gear. Is to select.

In this embodiment, the forward clutch 1 is engaged while the automatic transmission is in the travel range and the vehicle is stopped to set the automatic transmission in the first forward speed selection state.
To a 1 to perform anti-creep in the semi-engaged state (slip state), the oil pressure modulator 16 to control the hydraulic pressure P _C of the forward clutch 11
Is provided.

The oil pressure modulator (hereinafter, referred to as OPM) 16 is provided with a line pressure P of the automatic transmission.
_L is supplied from the oil passage 17 to the input port 18, the output port 19 is connected to the operating hydraulic chamber 11 a of the forward clutch 11 via the oil passage 20, and has a drain port 21. ) 23 an electromagnetic force to act in the figure left by the output pressure of the port 19 via the port 22a to the chamber 24 facing the other end of the spool 22, i.e., is fed back to hydraulic pressure P _C of the forward clutch 11.

[0030] Such OPM16 as source pressure line pressure P _L to the input port 18, and functions as a linear solenoid valve which produces the hydraulic pressure P _C which corresponds to the electromagnetic force corresponding to the current value i _X to the solenoid 23, the the oil passage of the hydraulic pressure P _C 20
To the operating hydraulic chamber 11a of the forward clutch 11 as the operating pressure of the forward clutch. Here, the forward clutch 11 can generate the fastening force proportional to hydraulic pressure P _C.

The current value i _X to the solenoid 23 is controlled by the control unit 30, and a signal from an input sensor group 31 described later is supplied to the control unit 30.

The control unit 30 is a vehicle-mounted type using a microcomputer, and has an input circuit 151, a RAM 152, a ROM 153, a CPU 154,
It is assumed that the circuit includes a clock circuit 155 and an output circuit 156.

The input circuit 151 is a circuit that converts an input signal from each sensor of the sensor group 31 into a digital signal that can be processed by the CPU 154 and inputs the digital signal. RAM1
Reference numeral 52 denotes a readable and writable memory for writing an input signal from each sensor, writing information being calculated by the CPU 154, and reading the written information.
Further, the ROM 153 is a read-only memory,
Information and the like necessary for the arithmetic processing in U154 are stored in advance, and the information is read out by a command from the CPU 154 as necessary.

The CPU 154 is a device for performing arithmetic processing on various types of input information in accordance with predetermined processing conditions. The CPU 154 performs input information processing in creep prevention control, forward clutch operation control, and the like.
This is a circuit for setting the calculation processing time in the CPU 154. The output circuit 156 is a circuit for outputting a control current signal i _X to the solenoid 23 as an actuator on the basis of the calculation result signal from the CPU 154.

In the present embodiment, as sensors constituting the input sensor group 31, a select position switch 161,
Idle switch 162, oil temperature sensor 163, output shaft speed sensor 164, engine speed sensor 165, turbine speed sensor 166, and brake switch 167
Shall be used.

The select position switch 161 is a switch for outputting a signal corresponding to the selected range (selected position) of the automatic transmission. In the present embodiment, the switch is turned ON when the selected range is in the neutral range (N range). ,
The selected range is the driving range (drive range; D range)
It is assumed that the switch is turned off and the switch signal _PSW is output only when.

Note that the switch signal _PSW output from the select position switch 161 indicates that the range has been switched from the N range to the D range (N → D select operation has been performed). It is used as a signal to be performed.

The idle switch 162 is a switch for detecting whether or not the throttle valve of the engine is in a fully closed state (idle state). The switch is turned off when the throttle valve is in an open state, and is turned on only when the throttle valve is in a fully closed state. It outputs a switch signal Id. Note that a throttle sensor may be used instead of the idle switch 162.

The oil temperature sensor 163 detects the transmission operating oil temperature (A
TF temperature) _Tm is a sensor that detects the oil temperature signal T
_Atf is output, and the output shaft speed sensor 164 is a sensor for detecting the speed No of the transmission output shaft. A signal output from the output shaft speed sensor 164 is used as a signal representing the vehicle speed.

The engine speed sensor 165, the rotational speed of the crankshaft 13 is a sensor for detecting the (engine speed) N _e, the turbine speed sensor 166, the rotation speed of the transmission input shaft 14 (turbine speed) a sensor for detecting the N _t.

The brake switch 167 is disposed on a brake pedal or the like, and is a sensor for detecting the operation of a foot brake or a side brake, and outputs a brake operation signal B when the brake is operated.

The CPU 154 executes the main routine of FIG. 2 and FIG. 3 based on the input information from the input sensor group 31 to execute the creep prevention control aimed at by the present invention and the start control from creep prevention. Perform as follows.

First, in step 51 of FIG. 2, the signals P _SW , Id, and N are output from the corresponding sensors of the input sensor group 31, respectively.
_o and B are read. In the next step 52, it is determined whether or not the vehicle is in the travel range (D range) based on the presence or absence of the signal _PSW. If it is the travel range, the control proceeds to step 53; Normal running control is performed.

In step 53, it is determined whether or not the idle switch is in the idle state based on the presence or absence of the signal Id.
If it is in the idle state, the control proceeds to step 54, and if it is not in the idle state, the control proceeds to step 57 to perform normal traveling control.

In step 54, it is determined whether or not the brake is ON based on the presence or absence of the signal B. If the brake is ON, the control proceeds to step 55, and if the brake is OFF, the control proceeds to step 57 to perform the normal traveling control. Do.

[0046] At step 55, the vehicle speed to determine whether the vehicle is stopped in (transmission output shaft speed) N _o is approximately 0, and starts the operation of the anti-creep control at step 56 if N _o approximately 0 , carried out during the normal running control proceed to N _o approximately 0 step 57 the control unless.

Eventually, the operation of the creep prevention control described later in step 56 is started only when all of the four conditions of “running range, idle state, brake ON, and vehicle speed are almost 0” are all satisfied. Yes, otherwise, normal traveling control is performed. After executing step 56, the control proceeds to step 61 of the start control program in FIG.

[0048] At step 61 in FIG. 3, the signal from the idle switch 162 Id, signal N _o from the output shaft speed sensor 164, reads a signal B from a brake switch 167, the next step 62, the presence or absence of the signal Id Determine whether the idle switch is in the idle state of ON,
If it is in the idle state (accelerator OFF), the control proceeds to step 63, where the accelerator switch of the idle switch OFF is turned ON.
If so, the control proceeds to step 65, where predetermined forward clutch engagement control (start control) when the accelerator is ON is performed.

In step 63, it is determined whether or not the brake is ON based on the presence or absence of the signal B. If the brake is ON, the control proceeds to step 64, and if the brake is OFF, the control proceeds to step 66, and the control proceeds to step 66 to determine whether the brake is ON. Is performed.

[0050] At step 64, it is determined whether or not the vehicle speed N _o is approximately 0, the control if N _o approximately 0 back after the step 61, the control When larger than N _o approximately 0 Step Proceeding to 66, the forward clutch engagement control at the time of accelerator OFF is performed.

After all, it is the case that “the creep prevention control is in operation and the idle state”, and the “brake OF
F or N _o > 0 ”, step 6
In step 6, the forward clutch engagement control is performed when the accelerator is off. In this forward clutch engagement control, the forward clutch 11 is slowly engaged.

This forward clutch engagement control is repeated until it is determined in the next step 67 that the engagement of the forward clutch has been completed. The determination of the forward clutch engagement completion, for example turbine speed and vehicle speed rotational speed difference with time became (Nt-N _o) is approximately 0, so as to determine the engagement completion.

At step 68, which is performed when the forward clutch engagement is completed, in which the determination at step 67 is YES, and after step 65, at which the forward clutch engagement control is performed when the accelerator is turned on, the creep prevention control is activated. End (OFF).

Next, the creep prevention control performed in step 56 of FIG. 2 according to the gist of the present invention will be described. This control is shown in detail in FIG. As described above, this anti-creep control is performed when the automatic transmission is set to the driving range, the engine is idle, and the brake is activated (O
N) to which, four conditions that it is the vehicle speed N _o is substantially 0 is everything, are intended to be started at instant t ₁ in FIG.

[0055] Then, the control start instant t ₁ to to sharply forward clutch hydraulic pressure P _C to the initial set pressure P _S, a subsequent forward clutch hydraulic pressure P _C, is gradually decreased at a predetermined gradient over the predetermined operational response time The main point is to prevent the creep by putting the forward clutch 11 in the slip state, but the combination of the rapid decrease and the gradual decrease allows the anti-creep control to be started quickly but without any shock. To do.

[0056] First, the map representing the relationship between reading a transmission working oil temperature T _m at step 71 in FIG. 4, then in step 72, the forward clutch hydraulic pressure P _c and OPM current i _X as illustrated in FIG. 5 based on the initial setting of the forward clutch hydraulic pressure P _C pressure command value P _S
Find the OPM current i _X = i _S for causing the sharply up,
This is commanded to the solenoid 23 of FIG.

[0057] In a next step 73, it retrieves the decreasing slope k of the forward clutch hydraulic pressure P _c corresponding to the transmission working oil temperature T _m based on the map exemplified in FIG. Here decreasing gradient k of the forward clutch hydraulic pressure P _c is a forward clutch hydraulic pressure P _C at a reduced slope when gradually decreasing after rapid decrease to the initial set pressure P _S, as preferably exemplified by δ in Figure 7 , the actual even at low temperatures when delayed, as shown by the decrease of the forward clutch hydraulic pressure P _c is gamma, just instantaneously t ₃ when the forward clutch 11 begins to slip, the forward clutch hydraulic pressure P _c command value δ is the forward clutch a gradient to match the actual value γ of the working oil pressure P _c, and the previously obtained by an experiment or the like in advance for each transmission working oil temperature T _m.

The gradually decreasing gradient k of the forward clutch operating oil pressure _Pc thus determined is α (at high temperature) in FIG.
And δ As is apparent from comparison between (low temperature), the transmission operating oil temperature T _m is the lower the lower the gradient, delayed as shown by the decrease in the actual forward clutch hydraulic pressure P _c as during low temperature γ Agree with the facts.

In step 74, the above-mentioned gradually decreasing gradient is used.
k and the instant t at which creep prevention control starts. ₁Elapsed time t
From the forward clutch operating oil pressure P_cCommand pressure of P
_c= P_S−k · t, and in step 75
The command pressure P_cOPM current i corresponding to_XTo
The OPM current i is searched from the map corresponding to FIG._X
Is output to the solenoid 23.

Next, at step 76, the turbine speed _Nt is read, and at step 77, it is determined whether or not the turbine speed _Nt has occurred, that is, whether or not the forward clutch 11 has started to slip. until begins to slip, by repeating the above control steps 75, 75, gradually decreasing the hydraulic pressure P _C of the forward clutch 11 with a gradient k.

If it is determined in step 77 that the forward clutch 11 has begun to slip, step 7
8, hold the OPM current i _X of the instant, the OPM current i _X in the slip start instantly at the same time, the clutch 11 is set as the slip start current value i _min to become a slip state, which in various learning control To contribute.

Then, in step 79, after the instant of the start of the slip, the input / output rotational speed difference of the torque converter 12, that is, the torque converter slip amount ΔN is calculated by ΔN = N _e −N
together determined by _t, as the torque converter slip amount ΔN is retained on preventing creep suitable predetermined value, determined by the feedback control of the command pressure of the forward clutch hydraulic pressure P _C. Then, as the lower limit a slip start current value i _min of the, seeking OPM current i _X corresponding to the forward clutch command pressure above and outputs it to the solenoid 23.

According to the creep preventing apparatus according to the embodiment of the present invention described above, when the operating oil pressure control command value of the forward clutch 11 is abruptly reduced to the initial set pressure P _S , the value gradually decreases. depending on the transmission working oil temperature T _m as illustrating the gradient k in FIG. 6, the oil temperature is in the shelving as low as ultimately show a decreasing gradient k in FIG. 7, the forward clutch 11 slips just when started, since it has the structure set forth in gradient as real and hydraulic pressure γ and the operating oil pressure control command value δ match, at t ₃ when the forward clutch starts to slip due to a decrease in hydraulic pressure, the actual divergence between actuation pressure γ and command pressure δ of, can be completely eliminated even under any transmission operating oil temperature T _m.

Therefore, the command pressure δ (slip start current value i) at the slip start instant t ₃ used in various learning controls
_min) is also under any transmission operating oil temperature T _m, will correspond exactly to the oil pressure (return spring equivalent pressure) for causing rise to slip start of the forward clutch, it becomes learning erroneous learning The concern can be dispelled.

Further, since there is no divergence between the actual operating oil pressure γ and the command pressure δ at the time t ₃ when the forward clutch starts slipping, the actual operating oil pressure γ is shifted to the forward clutch after the slip start instant. The return spring does not drop below the equivalent pressure of the return spring (into the state of poor packing), and the re-engagement of the forward clutch in response to the depression of the accelerator pedal has a response delay corresponding to the poor packing. In addition, there is no problem that the engine blows and the re-fastening of the friction element is accompanied by a large fastening shock due to the energy of the engine blowing.

[Brief description of the drawings]

FIG. 1 is a system configuration diagram showing a creep preventing device for an automatic transmission according to an embodiment of the present invention.

FIG. 2 is a flowchart showing a first half of a shift control program of the automatic transmission including a creep prevention control, which is executed by a control unit in the embodiment.

FIG. 3 is a flowchart showing a latter half of the transmission control program.

FIG. 4 is a flowchart specifically showing creep prevention control in the speed change control program.

FIG. 5 is a relationship diagram between a forward clutch operating hydraulic pressure and an OPM current for driving a solenoid.

FIG. 6 is a relationship diagram between a gradually decreasing gradient of a forward clutch operating oil pressure command value and a transmission operating oil temperature.

FIG. 7 is a time chart showing a comparison between a creep prevention control operation according to the present invention and a conventional creep prevention control operation.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 Creep prevention device of automatic transmission 11 Forward clutch (friction element during operation) 12 Torque converter (fluid power transmission) 13 Crankshaft 14 Transmission input shaft 16 Oil pressure modulator 23 Solenoid 30 Control unit 31 Input sensor group 12 _p pump Impeller 12 _t Turbine runner 12 _S Stator 161 Select position switch 162 Idle switch 163 Oil temperature sensor 164 Output shaft speed sensor 165 Engine speed sensor 166 Turbine speed sensor 167 Brake switch

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-87235 (JP, A) JP-A-7-139620 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) F16H 61/00-61/24

Claims (2)

(57) [Claims] 1. An automatic transmission which receives the rotation of an engine via a fluid transmission and shifts and outputs the engine rotation at a speed selected by selective hydraulic operation of a plurality of friction elements. In order to reduce the transmission of creep torque by the fluid transmission device, the operating oil pressure of the friction element during the operation is first rapidly reduced to an initial set pressure, and thereafter, by an operating oil pressure control command that gradually decreases at a predetermined gradient over a predetermined time. A creep preventing device for an automatic transmission in which the friction element is set in a slip state, wherein a gradually decreasing gradient relating to an operation oil pressure control command value of the friction element is changed in accordance with a transmission operating oil temperature. A creep preventing device for an automatic transmission, wherein the creep preventing device is configured to: 2. The gradual decrease gradient of the operating oil pressure control command value according to claim 1, wherein the operating oil pressure control command value matches the actual operating oil pressure just when the operating friction element starts slipping. An anti-creep device for an automatic transmission, characterized in that the device is configured to have a constant gradient.