Automatic transmission control device

Abstract

To provide an automatic transmission control device capable of controlling an increase in an oil temperature in an automatic transmission.SOLUTION: When a shift lever is in either a D position or an R position with an oil temperature of a continuous transmission exceeding a first threshold (Yes in S2) and a torque converter is in an abnormally high temperature state (Yes in S3), a forward clutch and a backward clutch are released (S4). For example, when an accelerator pedal is continuously operated in a state where a vehicle is unable to move forward or backward with a tire spinning free in the mud, that is in a state where the vehicle gets stuck, the torque converter is determined to be in a state of abnormally high temperature.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a control device for an automatic transmission. [Background technology]

[0002] As transmissions mounted on vehicles, for example, continuously variable transmissions (CVTs) and stepped automatic transmissions (ATs) are widely known. Many commercial vehicles have been equipped with manual transmissions (MTs), but in recent years, the number of commercial vehicles equipped with automatic transmissions, including continuously variable transmissions, has been increasing.

[0003] However, because commercial vehicles are used more harshly than passenger cars, in commercial vehicles equipped with automatic transmissions, the oil temperature in the automatic transmission can rise significantly, raising concerns that the oil temperature may reach an oil temperature outside the warranty range, causing premature deterioration or damage. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 9-4708 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a control device for an automatic transmission that can suppress an increase in oil temperature in the automatic transmission. [Means for solving the problem]

[0006] In order to achieve the above-mentioned object, the automatic transmission control device of the present invention is an automatic transmission control device having a clutch on a transmission path of power input from a drive source via a torque converter, and includes: range determination means for determining whether the position of an operating member operated to indicate the shift range of the automatic transmission is a forward position indicating a forward range or a reverse position indicating a reverse range; temperature rise determination means for determining an abnormal temperature rise state of the torque converter; and clutch release means for releasing the clutch when the forward position or reverse position is determined by the range determination means and an abnormal temperature rise state is determined by the temperature rise determination means.

[0007] According to this configuration, when the operating member is in the forward or reverse position and the torque converter is in an abnormally heated state, the clutch on the power transmission path is released. By releasing the clutch, the power transmission path on the torque converter side of the clutch is separated from the power transmission path on the drive wheel side, so that the differential rotation between the input and output of the torque converter can be reduced. As a result, heat generation in the torque converter can be suppressed, and the temperature rise of the oil in the torque converter can be suppressed, and the temperature of the oil sprayed from the torque converter (sprayed oil temperature) can be reduced. Therefore, the rise in oil temperature of the automatic transmission can be suppressed, and early deterioration and damage caused by the oil temperature reaching an oil temperature outside the guaranteed range can be suppressed.

[0008] The temperature rise determination means may determine that an abnormal temperature rise has occurred when the oil temperature of the automatic transmission exceeds a predetermined threshold (temperature), or may determine that an abnormal temperature rise has occurred when the oil temperature rise rate (oil temperature rise gradient) is equal to or greater than a predetermined value even if the oil temperature does not exceed the threshold. The abnormal temperature rise may also be determined when the differential rotation between the input and output of the torque converter exceeds a predetermined threshold (rotations) and an accelerator operation (e.g., depressing the accelerator pedal) continues for a predetermined period of time or more. Furthermore, the abnormal temperature rise may be determined when the differential rotation between the input and output of the torque converter exceeds a threshold, an accelerator operation is being performed, and the wheel speed of at least one wheel is 0, and this state continues for a certain period of time or more.

[0009] The control device may further include clutch engagement means for engaging the clutch when the abnormally elevated temperature state is resolved or a predetermined time has elapsed after the clutch has been released by the clutch release means. Effect of the Invention

[0010] According to the present invention, an increase in oil temperature in an automatic transmission can be suppressed. [Brief description of the drawings]

[0011] [Figure 1] 1 is a skeleton diagram showing a configuration of a drive system of a vehicle equipped with a control device according to an embodiment of the present invention; [Diagram 2] 2 is a block diagram showing the configuration of a control system of the vehicle; FIG. [Diagram 3] 4 is a flowchart showing a flow of an oil temperature rise suppression process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0013] <Vehicle drive system> FIG. 1 is a skeleton diagram showing the configuration of a drive system of a vehicle 1.

[0014] The vehicle 1 is an automobile that uses an engine 2 as a driving source. The engine 2 may be a gasoline engine or a diesel engine.

[0015] The engine 2 is provided with an electronic throttle valve for adjusting the amount of intake air into the combustion chamber of the engine 2, an injector (fuel injection device) for injecting fuel into the intake air, and a spark plug for generating an electric discharge in the combustion chamber. The engine 2 is also provided with a starter for starting the engine 2. The power of the engine 2 is transmitted to a differential gear 5 via a torque converter 3 and a belt-type continuously variable transmission (CVT: Continuously Variable Transmission) 4, and is transmitted from the differential gear 5 to left and right drive wheels 7L, 7R via left and right drive shafts 6L, 6R, respectively.

[0016] The torque converter 3 is a torque converter with a lock-up mechanism, and includes a front cover 11, a pump impeller 12, a turbine runner 13, and a lock-up clutch (lock-up piston) 14. A crankshaft of the engine 2 is connected to the front cover 11, and the front cover 11 rotates integrally with the crankshaft. The pump impeller 12 is disposed on the opposite side of the front cover 11 to the engine side. The pump impeller 12 is provided so as to be rotatable integrally with the front cover 11. The turbine runner 13 is disposed between the front cover 11 and the pump impeller 12, and is provided so as to be rotatable about a common rotation axis with the front cover 11. The lock-up clutch 14 is disposed between the front cover 11 and the turbine runner 13.

[0017] The lockup clutch 14 is engaged / disengaged by the pressure difference between the hydraulic pressure in a release-side oil chamber 15 between the lockup clutch 14 and the front cover 11 and the hydraulic pressure in an engagement-side oil chamber 16 between the lockup clutch 14 and the pump impeller 12. That is, when the hydraulic pressure in the release-side oil chamber 15 is higher than the hydraulic pressure in the engagement-side oil chamber 16, the pressure difference separates the lockup clutch 14 from the front cover 11, and the lockup clutch 14 is in a released state (lockup off). When the hydraulic pressure in the engagement-side oil chamber 16 is higher than the hydraulic pressure in the release-side oil chamber 15, the pressure difference presses the lockup clutch 14 against the front cover 11, and the lockup clutch 14 is in an engaged state (lockup on).

[0018] In the lock-up off state, when the E / G output shaft rotates, the pump impeller 12 rotates. When the pump impeller 12 rotates, a flow of oil is generated from the pump impeller 12 toward the turbine runner 13. This oil flow is received by the turbine runner 13, causing it to rotate. At this time, the amplifying effect of the torque converter 3 occurs, and a torque larger than the torque of the E / G output shaft is generated in the turbine runner 13.

[0019] In the lock-up on state, when the E / G output shaft rotates, the E / G output shaft, the pump impeller 12 and the turbine runner 13 rotate together.

[0020] An oil pump 8 is provided between the torque converter 3 and the continuously variable transmission 4. The oil pump 8 is a mechanical oil pump, and its pump shaft is provided to rotate integrally with a pump impeller 12 of the torque converter 3. As a result, when the pump impeller 12 rotates due to the power of the engine 2, the pump shaft of the oil pump 8 rotates, and oil pressure is generated from the oil pump 8.

[0021] The continuously variable transmission 4 is housed together with the torque converter 3 in a unit case forming an outer shell, and transmits power input from the torque converter 3 to a differential gear 5. The continuously variable transmission 4 includes an input shaft 21, an output shaft 22, a belt transmission mechanism 23, and a forward / reverse switching mechanism 24.

[0022] The input shaft 21 is connected to the turbine runner 13 of the torque converter 3 and is provided so as to be rotatable integrally with the turbine runner 13 about the same rotational axis.

[0023] The output shaft 22 is disposed in parallel to the input shaft 21. An output gear 25 is supported on the output shaft 22 so as to be non-rotatable relative to the input shaft 21.

[0024] The belt transmission mechanism 23 includes a primary shaft 31 and a secondary shaft 32. The primary shaft 31 and the secondary shaft 32 are disposed coaxially with the input shaft 21 and the output shaft 22, respectively.

[0025] The belt transmission mechanism 23 has a configuration in which an endless belt 35 is wound around a primary pulley 33 supported on a primary shaft 31 and a secondary pulley 34 supported on a secondary shaft 32 .

[0026] The primary pulley 33 includes a fixed sheave 41 fixed to the primary shaft 31, and a movable sheave 42 disposed opposite the fixed sheave 41 with the belt 35 sandwiched therebetween and supported on the primary shaft 31 so as to be movable in the axial direction of the fixed sheave 41 but not rotatable relative thereto. A cylinder 43 fixed to the primary shaft 31 is provided on the opposite side of the movable sheave 42 to the fixed sheave 41, and an oil chamber 44 is formed between the movable sheave 42 and the cylinder 43 to which hydraulic pressure (primary sheave pressure) applied to the movable sheave 42 is supplied.

[0027] The secondary pulley 34 includes a fixed sheave 45 fixed to the secondary shaft 32, and a movable sheave 46 disposed opposite the fixed sheave 45 with the belt 35 sandwiched therebetween and supported on the secondary shaft 32 so as to be movable in the axial direction of the fixed sheave 45 but not rotatable relative thereto. A piston 47 fixed to the secondary shaft 32 is provided on the opposite side of the movable sheave 46 to the fixed sheave 45, and an oil chamber 48 is formed between the movable sheave 46 and the piston 47 to which hydraulic pressure (secondary sheave pressure) applied to the movable sheave 46 is supplied.

[0028] The groove width, which is the gap between the fixed sheave 41 and the movable sheave 42, changes continuously as the movable sheave 42 of the primary pulley 33 moves. The groove width, which is the gap between the fixed sheave 45 and the movable sheave 46 of the secondary pulley 34 changes continuously as the movable sheave 42 of the secondary pulley 34 moves. By continuously changing the groove widths of the primary pulley 33 and the secondary pulley 34, the winding diameter of the belt 35 around the primary pulley 33 and the secondary pulley 34 can be changed, and the gear ratio (pulley ratio) can be changed continuously and steplessly.

[0029] On the other hand, hydraulic pressure is supplied to the oil chamber 44 of the primary pulley 33 and the oil chamber 48 of the secondary pulley 34 so that a necessary and sufficient clamping pressure is applied to the belt 35 so as not to cause belt slippage.

[0030] Although not shown, a bias spring is interposed between the movable sheave 46 and the piston 47 to apply an initial clamping pressure (initial thrust) to the belt 35. The elastic force of the bias spring biases the movable sheave 46 and the piston 47 in directions away from each other.

[0031] The forward / reverse switching mechanism 24 is interposed between the input shaft 21 and the primary shaft 31 of the belt transmission mechanism 23. The forward / reverse switching mechanism 24 includes a planetary gear mechanism 51, a forward brake (forward clutch) B1, and a reverse clutch (reverse clutch) C1.

[0032] The planetary gear mechanism 51 includes a carrier 52, a sun gear 53 and a ring gear 54.

[0033] The carrier 52 is fitted onto the input shaft 21 so as to be capable of relative rotation. The carrier 52 rotatably supports a plurality of pinion gears 55. The plurality of pinion gears 55 are arranged circumferentially.

[0034] The sun gear 53 is supported by the input shaft 21 so as to be non-rotatable relative to the input shaft 21, and is disposed in a space surrounded by the plurality of pinion gears 55. The gear teeth of the sun gear 53 mesh with the gear teeth of each of the pinion gears 55.

[0035] The ring gear 54 is disposed so that its rotation axis coincides with the axis of the primary shaft 31. The primary shaft 31 of the belt transmission mechanism 23 is connected to the ring gear 54. The gear teeth of the ring gear 54 are formed so as to collectively surround the multiple pinion gears 55, and mesh with the gear teeth of each pinion gear 55.

[0036] The forward brake B1 is provided between the carrier 52 and a transmission case that houses the torque converter 3 and the continuously variable transmission 4, and is hydraulically switched between an engaged state (on) that brakes the carrier 52 and a released state (off) that allows the carrier 52 to rotate.

[0037] The reverse clutch C1 is hydraulically switched between an engaged state (ON) in which the carrier 52 and the sun gear 53 are directly connected (coupled so as to be rotatable together) and a released state (OFF) in which the direct connection is released.

[0038] A shift lever (select lever) is disposed at a position operable by the driver within the cabin of the vehicle 1. Within the movable range of the shift lever, for example, a P (parking) position, an R (reverse) position, an N (neutral) position, and a D (drive) position are provided in a line in this order.

[0039] When the shift lever is in the P position, both the reverse clutch C1 and the forward brake B1 are released and a parking lock gear (not shown) is locked, thereby forming a P range, which is one of the shift ranges of the continuously variable transmission 4. When the shift lever is in the N position, both the reverse clutch C1 and the forward brake B1 are released and the parking lock gear is not locked, thereby forming an N range, which is one of the shift ranges of the continuously variable transmission 4. When both the reverse clutch C1 and the forward brake B1 are released, the input shaft 21 and the sun gear 53 spin freely and the power of the engine 2 is not transmitted to the drive wheels 7L, 7R.

[0040] When the shift lever is in the D position, the forward brake B1 is engaged and the reverse clutch C1 is released, thereby forming a forward range, which is one of the shift ranges of the continuously variable transmission 4. In the forward range, when the power of the engine 2 is input to the input shaft 21, the sun gear 53 rotates integrally with the input shaft 21 while the carrier 52 is stationary. Therefore, the rotation of the sun gear 53 is transmitted to the ring gear 54 in the reverse direction and at a reduced speed. This causes the ring gear 54 to rotate, and the primary shaft 31 and the primary pulley 33 of the belt transmission mechanism 23 rotate integrally with the ring gear 54. The rotation of the primary pulley 33 is transmitted to the secondary pulley 34 via the belt 35, causing the secondary pulley 34 and the secondary shaft 32 to rotate. Then, the output shaft 22 and the output gear 25 rotate integrally with the secondary shaft 32. The output gear 25 meshes with the differential gear 5 (an input gear of the differential gear 5). When the output gear 25 rotates, the drive shafts 6L, 6R extending to the left and right from the differential gear 5 rotate, and the drive wheels 7L, 7R rotate, causing the vehicle 1 to move forward.

[0041] When the shift lever is in the R position, the forward brake B1 is released and the reverse clutch C1 is engaged, thereby forming the R range, which is one of the shift ranges of the continuously variable transmission 4. In the R range, when the power of the engine 2 is input to the input shaft 21, the carrier 52 and the sun gear 53 rotate integrally with the input shaft 21. Therefore, the rotation of the sun gear 53 is transmitted to the ring gear 54 without being reversed in the rotation direction. This causes the ring gear 54 to rotate, and the primary shaft 31 and the primary pulley 33 of the belt transmission mechanism 23 rotate integrally with the ring gear 54. The rotation of the primary pulley 33 is transmitted to the secondary pulley 34 via the belt 35, causing the secondary pulley 34 and the secondary shaft 32 to rotate. Then, the output shaft 22 and the output gear 25 rotate integrally with the secondary shaft 32. When the output gear 25 rotates, the drive shafts 6L, 6R extending to the left and right from the differential gear 5 rotate, and the drive wheels 7L, 7R rotate, causing the vehicle 1 to move backward.

[0042] <Vehicle control system> FIG. 2 is a block diagram showing the configuration of a control system of the vehicle 1.

[0043] Vehicle 1 is equipped with a plurality of ECUs (Electronic Control Units). Each ECU has a microcontroller unit (microcomputer), and the microcomputer has built-in, for example, a CPU, a non-volatile memory such as a flash memory, and a volatile memory such as a dynamic random access memory (DRAM). The plurality of ECUs are connected to enable two-way communication according to a controller area network (CAN) communication protocol. FIG. 2 shows one ECU 61 of the plurality of ECUs.

[0044] A hydraulic circuit 62 for supplying hydraulic pressure to each part is provided in the unit including the torque converter 3 and the continuously variable transmission 4. The ECU 61 controls various valves and the like included in the hydraulic circuit 62 for controlling the speed change of the continuously variable transmission 4 and the like.

[0045] Various sensors required for control are connected to the ECU 61, and detection signals of the connected sensors are input. For example, a shift position sensor 63 that outputs a detection signal according to the position of the shift lever, an engine rotation sensor 64 that outputs a detection signal in synchronization with the rotation of the crankshaft of the engine 2, a turbine rotation sensor 65 that outputs a detection signal in synchronization with the rotation of the turbine runner 13 of the torque converter 3, an oil temperature sensor 66 that outputs a detection signal in accordance with the oil temperature of the continuously variable transmission 4, and a wheel speed sensor 67 that outputs a detection signal in synchronization with the rotation of each wheel (drive wheels 7L, 7R and driven wheels) of the vehicle 1 are connected to the ECU 61. Based on the detection signals input from these sensors, the ECU 61 acquires the position of the shift lever, the engine speed which is the rotation speed of the engine 2, the turbine speed which is the rotation speed of the turbine runner 13, the oil temperature of the continuously variable transmission 4, and the wheel speed of each wheel.

[0046] Further, in addition to the detection signals input from the various sensors, the ECU 61 also receives information required for control from other ECUs.

[0047] <Oil temperature rise suppression treatment> FIG. 3 is a flowchart showing the flow of the oil temperature rise suppression process.

[0048] While the ignition switch of the vehicle 1 is turned on, the ECU 61 repeatedly performs an oil temperature rise suppression process for suppressing an increase in the oil temperature of the continuously variable transmission 4.

[0049] In the oil temperature rise suppression process, it is determined whether the shift lever is in the D position or the R position (step S1). If the shift lever is in a position other than the D position or the R position, the process does not proceed.

[0050] If the shift lever is in the D position or the R position (YES in step S1), it is determined whether the oil temperature of the continuously variable transmission 4 exceeds a first threshold value (step S2). The first threshold value is set to a temperature at which the oil temperature may cause fatal damage to the continuously variable transmission 4, for example, 135°C. If the oil temperature is equal to or lower than the first threshold value (NO in step S2), the process returns to the beginning, and it is determined whether the shift lever is in the D position or the R position (step S1).

[0051] When the position of the shift lever is the D position or the R position and the oil temperature of the continuously variable transmission 4 exceeds the first threshold value (YES in step S2), an abnormal temperature rise determination is performed (step S3). The abnormal temperature rise determination is a determination as to whether or not the torque converter 3 is in an abnormal temperature rise state. For example, when the accelerator operation is continued in a state where the wheels are spinning due to being stuck in mud, and the vehicle 1 is unable to move forward or backward, that is, the vehicle 1 is stuck, it is determined that the torque converter 3 is in an abnormal temperature rise state. Specifically, when the differential rotation (difference between the engine speed and the turbine speed) between the input and output of the torque converter 3 is 100 rpm or more, the wheel speeds of three or more wheels are 0, and the accelerator operation is continued for a predetermined time (for example, three seconds) or more, it is determined that the torque converter 3 is in an abnormal temperature rise state. When the torque converter 3 is not in an abnormal temperature rise state (NO in step S3), the process returns to the beginning and it is determined whether or not the shift lever is in the D position or the R position (step S1).

[0052] If it is determined that the torque converter 3 is in an abnormally high temperature state (YES in step S3), both the forward brake B1, which is a forward clutch, and the reverse clutch C1, which is a reverse clutch, are released, and the continuously variable transmission 4 is put into the N range state (step S4). Then, a warning is displayed by an indicator provided on the meter panel (step S5).

[0053] Thereafter, it is determined whether the oil temperature of the continuously variable transmission 4 has fallen to a second threshold value or lower (step S6). The second threshold value may be the same as or different from the first threshold value. In order to prevent the forward brake B1 and the reverse clutch C1 from being frequently engaged / disengaged, it is preferable that the second threshold value be set to a value lower than the first threshold value.

[0054] If the oil temperature has not dropped below the second threshold value (NO in step S6), it is determined whether a predetermined time (for example, 1 to 3 minutes) has elapsed since the forward brake B1 and the reverse clutch C1 were released (step S7).

[0055] When the oil temperature falls below the second threshold value (YES in step S6) or a predetermined time has elapsed since the forward brake B1 and reverse clutch C1 were released (YES in step S7), the warning display by the indicator is terminated (step S8), the forward brake B1 or reverse clutch C1 that was engaged before the release is engaged (step S9), and the series of processes is terminated.

[0056] <Action and effect> As described above, when the shift lever is in the D position or the R position, the oil temperature of the continuously variable transmission 4 exceeds the first threshold value, and the torque converter 3 is in an abnormally high temperature state, the forward brake B1 and the reverse clutch C1 are released. By releasing the forward brake B1 and the reverse clutch C1, the power transmission path on the torque converter 3 side and the power transmission path on the drive wheels 7L and 7R side are separated from each other by the forward / reverse switching mechanism 24, so that the differential rotation between the input and output of the torque converter 3 can be reduced. As a result, the generation of heat in the torque converter 3 can be suppressed, the temperature rise of the oil in the torque converter 3 can be suppressed, and the temperature of the oil sprayed from the torque converter 3 (sprayed oil temperature) can be reduced. Therefore, the rise in the oil temperature of the continuously variable transmission 4 can be suppressed, and early deterioration and damage caused by the oil temperature reaching an oil temperature outside the guaranteed range can be suppressed.

[0057] <Modification> Although one embodiment of the present invention has been described above, the present invention can be embodied in other forms.

[0058] For example, in the abnormal temperature rise determination, it may be determined that the torque converter 3 has already risen abnormally in temperature, or it may be determined that the torque converter 3 is approaching an abnormally high temperature state. Therefore, in the abnormal temperature rise determination, it may be determined that the torque converter 3 is in an abnormal temperature rise state when the oil temperature of the continuously variable transmission 4 exceeds a predetermined threshold, or it may be determined that the torque converter 3 is in an abnormal temperature rise state when the oil temperature rise rate (oil temperature rise gradient) is equal to or higher than a predetermined value even if the oil temperature does not exceed the threshold.

[0059] In the above embodiment, the continuously variable transmission 4 is taken up, but the control device according to the present invention can also be applied to a stepped automatic transmission. It can also be applied to a power split type continuously variable transmission. A power split type continuously variable transmission is, for example, a transmission that includes a belt transmission mechanism that changes the speed of the power steplessly by changing the gear ratio, and can transmit the power (torque) by dividing it between the input shaft and the output shaft through two paths. It can also be applied to a continuously variable transmission or a stepped automatic transmission that is mounted on a vehicle that employs a front-engine rear-wheel-drive (FR) layout and has a longitudinally mounted engine.

[0060] In addition, various design modifications can be made to the above-described configuration within the scope of the claims. [Explanation of symbols]

[0061] 2: Engine (power source) 3: Torque converter 4: Continuously variable transmission (automatic transmission) 7L, 7R: Drive wheels 61: ECU (control device, range determination means, temperature rise determination means, clutch release means) B1: Forward brake (clutch) C1: Reverse clutch (clutch)

Claims

1. A control device for an automatic transmission having a clutch on a transmission path of power input from a drive source via a torque converter, a range determination means for determining whether a position of an operating member operated to specify a shift range of the automatic transmission is a forward position specifying a forward range or a reverse position specifying a reverse range; a temperature rise determination means for determining an abnormal temperature rise state of the torque converter; a clutch release means for releasing the clutch when the range determination means determines that the vehicle is in the forward position or the reverse position and the temperature rise determination means determines that the vehicle is in the abnormal temperature rise state; and and clutch engagement means for engaging the clutch in response to a predetermined time having elapsed since the clutch was released by the clutch release means after the clutch was released by the clutch release means.

2. A control device for an automatic transmission having a clutch on a transmission path of power input from a drive source through a torque converter, a range determination means for determining whether a position of an operating member operated to specify a shift range of the automatic transmission is a forward position specifying a forward range or a reverse position specifying a reverse range; a temperature rise determination means for determining an abnormal temperature rise state of the torque converter; a clutch release means for releasing the clutch when the range determination means determines that the vehicle is in the forward position or the reverse position and the temperature rise determination means determines that the vehicle is in the abnormal temperature rise state; and and clutch engaging means for engaging the clutch in response to the elimination of the abnormal temperature rise state after the clutch is released by the clutch releasing means, A control device in which the temperature rise determination means determines that an abnormal temperature rise state exists, the control device includes a case in which the differential rotation between the input and output of the torque converter exceeds a threshold value, an accelerator operation of a vehicle equipped with the automatic transmission is being performed, and a wheel speed of at least one wheel of the vehicle is kept at 0 for a certain period of time or more.

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