JP2018031467A - Travel control device for vehicle - Google Patents

Abstract

An object of the present invention is to accurately apply an engine brake when a vehicle travels on a downward slope even in a transmission having multiple gear stages. A gear stage selected from a map held by a cruise control gear stage setting means (16) is compared with a target gear stage for downhill set by a gear stage deriving means (17) at the time of a downward slope. However, when the gear stage selected in the map held by the cruise control gear stage setting means 16 is the high speed gear stage, the vehicle is driven at the downhill target gear stage set by the gear stage deriving means 17. to maintain the set vehicle speed. [Selection drawing] Fig. 1

Description

  The present invention relates to a vehicle travel control device for causing a vehicle to travel according to a set vehicle speed.

  A vehicle travel control device (cruise control device) that allows a vehicle to travel at a constant speed while maintaining a travel speed at a speed set by a driver has been put into practical use. The cruise control device is configured to maintain the traveling speed of the vehicle by controlling, for example, a gear position of the transmission (for example, Patent Document 1).

  When the vehicle travels on the cruise control device and the vehicle travels on a downward slope (when traveling on a downhill), the throttle is fully closed and the engine is operated. In this case, although the vehicle speed may increase, the traveling speed of the vehicle can be maintained by shifting down the gear position of the transmission to the low speed side.

  In recent years, with the aim of further improving fuel efficiency and improving ride comfort, there is a tendency that the gear stages of transmissions are multi-staged, and at present, 8th and 9th speed transmissions have appeared. is there. A vehicle equipped with a multi-stage gearbox is in a situation where the number of shift stages at which engine braking is difficult to operate is increasing. There was a risk that it would be difficult to maintain the speed.

JP 2002-29280 A

  The present invention has been made in view of the above situation, and provides a vehicular travel control device capable of maintaining the travel speed of a vehicle by accurately applying an engine brake even when the vehicle travels on a downward slope. The purpose is to provide.

  In order to achieve the above object, a vehicular travel control apparatus according to a first aspect of the present invention includes a cruise control speed setting means for setting a speed according to a driving state including a vehicle speed and an accelerator opening, and a downward slope. The shift speed deriving means for setting the corresponding target shift speed, the cruise control shift speed setting means, and the information of the shift speed deriving means are input to the travel control means so as to maintain the set vehicle speed of the vehicle. Control means for outputting travel control information including gear speed instructions, wherein the control means is such that the gear speed selected by the cruise control gear speed setting means is higher than the target gear speed. If there is, a target shift speed set by the shift speed deriving means is selected.

  In the present invention according to claim 1, when the shift stage selected by the cruise control shift stage setting means is on the high speed stage side with respect to the target shift stage set according to the down slope, the vehicle has a down slope. Since it is determined that the engine brake is insufficient due to traveling and the target shift stage (low speed stage side) is selected according to the downward gradient set by the shift stage deriving means, it is possible to secure a deceleration force by the engine brake operation. it can.

  In other words, at the time of the down slope, a shift speed (down speed selected by the cruise control speed setting means) is not applied but a travel speed for driving the vehicle at a preset set vehicle speed is not applied. The vehicle is driven at the target shift stage obtained from the information at the time of engine braking operation according to the gradient of the engine, that is, the target shift stage set by the shift stage deriving means, and the set vehicle speed is maintained. For this reason, even when the vehicle travels on a downward slope, it is possible to accurately apply the engine brake and maintain the traveling speed of the vehicle.

  According to a second aspect of the present invention, there is provided the vehicle travel control apparatus according to the first aspect, wherein the control means is configured to control the cruise control gear position setting means with respect to the target gear position. Selecting the gear set set by the cruise control gear setting means when the gears selected in the above are equal or when the target gear is a high speed gear. To do.

  According to the second aspect of the present invention, when the shift stage selected by the cruise control shift stage setting means is equal to the target shift stage corresponding to the downward gradient, or the target shift stage is a high speed side shift. It is assumed that the engine brake is in effect when the vehicle is in a gear position (the degree of downward slope, such as a downward slope that is looser than a predetermined slope, or a flat ground, is reduced), and the cruise control gear position setting means Since the gear stage to be set is selected, an appropriate gear stage can be selected even if the downward gradient changes.

  According to a third aspect of the present invention, there is provided the vehicular travel control apparatus according to the first or second aspect of the present invention, wherein the shift speed deriving means has a gradient determined by the gradient determination means. Accordingly, a tire driving force for maintaining the set vehicle speed of the vehicle is obtained, and a target shift stage in which the deceleration driving force by the engine braking operation becomes the tire driving force is set.

  In the present invention according to claim 3, when it is determined by the gradient determining means (for example, information detected by the inclination sensor) that the vehicle is traveling on a downward gradient, the deceleration driving force by the engine braking operation maintains the set vehicle speed. The gear speed that becomes the tire driving force for the purpose can be set as the target gear speed. For example, the gear ratio is determined according to the driving force and the vehicle speed determined based on the engine brake, the gear speed, and the differential ratio, and the gear speed is set.

  According to a fourth aspect of the present invention, there is provided the vehicle travel control apparatus according to the third aspect, wherein the gradient determination means includes the output of the vehicle, the air resistance of the vehicle, and A gradient resistance is derived in consideration of tire rolling resistance, a road surface inclination angle is calculated based on the gradient resistance, and a downward gradient is determined based on the calculated inclination angle.

  In this invention which concerns on Claim 4, it is judged that the vehicle is drive | working the down grade by calculating | requiring the gradient of a road surface by calculation.

  The vehicle travel control apparatus of the present invention can maintain the travel speed of the vehicle by accurately applying the engine brake even when the vehicle travels on a downward slope.

1 is a schematic block configuration diagram of a vehicle including a vehicle travel control device according to an embodiment of the present invention. It is a map for selection of the shift stage of auto-cruise control. It is a situation figure of the gear stage set by the gear stage deriving means. It is a flowchart showing an example of a process of the vehicle travel control apparatus which concerns on one Example of this invention. It is a time chart of the traveling control device for vehicles concerning one example of the present invention.

  FIG. 1 is a block diagram of a vehicle travel control apparatus according to an embodiment of the present invention, FIG. 2 is a map for selecting a shift stage for automatic cruise control (cruise control map), and FIG. The situation explanation for determining the gear position (target gear position) set in is shown.

  The vehicle travel control device according to the present embodiment is a cruise control device that causes the vehicle to travel while maintaining the travel speed of the vehicle at the speed set by the driver.

  As shown in FIG. 1, the driving force from the engine 3 is transmitted to the drive wheels 2 of the vehicle 1 via a transmission (a stepped automatic transmission: AT) 4. The engine 3 and the speed change device 4 are controlled in output and speed change operation based on a command from the control means 5 to control the travel of the vehicle 1.

  That is, the control means 5 is provided with an engine control unit 6 and an AT control unit 7, the output of the engine 3 is controlled based on a command from the engine control unit 6, and the transmission is controlled based on a command from the AT control unit 7. 4 shift operation is controlled. Based on the command from the control means 5, the operation of the brake, the output of the engine 3, and the gear position of the transmission 4 are controlled, and the running of the vehicle 1 is controlled while maintaining the set vehicle speed (cruise control).

  The control means 5 measures a vehicle speed sensor 11 that detects the speed of the vehicle 1, engine information 12 that is engine output information (accelerator opening information, gear speed information, etc.), and vehicle 1 tilt. Information on the inclination sensor 13 serving as a gradient determining means for determining the inclination of the travel path and the vehicle speed setting switch 14 for setting the vehicle speed set by the driver when performing cruise control are input. Then, in accordance with a command from the control means 5, the control of the engine output including the control of the operation of the brake and the switching operation of the shift stage of the transmission 4 is performed.

  On the other hand, the control means 5 is provided with a cruise control speed setting means 16 for setting the speed according to the traveling state including the vehicle speed and the accelerator opening. Further, the control means 5 is provided with a shift speed deriving means 17 for setting a target shift speed corresponding to the downward gradient.

  The cruise control gear position setting means 16 has a map for selecting a gear position for automatic cruise control for causing the vehicle 1 to travel at a preset vehicle speed. Based on the map for selecting the gear position, the gear position corresponding to the traveling state is selected so that the traveling speed of the vehicle 1 is maintained at the speed set by the driver. Information on the selected gear position is sent to the AT control unit 7.

  The gear stage deriving means 17 obtains a tire driving force for maintaining the set vehicle speed of the vehicle 1 according to the gradient determined by the inclination sensor 13 when traveling on a downward gradient, and a deceleration driving force by an engine brake operation. Sets the target shift speed at which the tire driving force becomes. The shift speed deriving means 17 sets a shift speed at which the deceleration driving force by the engine braking operation becomes a tire driving force for maintaining the set vehicle speed as the target shift speed. For example, the gear ratio is determined according to the driving force and the vehicle speed determined based on the engine brake, the gear speed, and the differential ratio, and the gear speed is set.

  As shown in FIG. 2, in the map that the cruise control gear stage setting means 16 has, a gear stage switching line is set according to the relationship between the accelerator opening and the vehicle speed. The switching line is shifted from the Y speed to the Y speed (for example, the seventh speed) and from the Y speed to the X speed (for example, the eighth speed) (shown by a solid line). It is set on the higher speed side than when shifting down from the X speed to the Y speed (shown by a dotted line). Thereby, the hunting operation at the time of the shift operation is prevented.

  In the shift speed deriving means 17, the target shift speed is calculated and set according to the relationship between the tire driving force (deceleration driving force by the engine braking operation) and the vehicle speed in accordance with the downward gradient, and is switched as shown in FIG. It is information (computation information) of the state partitioned by the line. The upper side of the boundary line between the Z speed and the Y speed indicated by the two-dot chain line in the figure is the range where the Z speed is set, and the upper side of the boundary line between the Y speed and the X speed indicated by the dashed line in the figure is In the range set to the Y speed, the upper side of the line indicated by the solid line in the figure is the range set to the X speed. The lower side of the boundary line between the Z speed and the Y speed indicated by a two-dot chain line in the figure is a range set to the (Zn) speed.

  For convenience of explanation, FIG. 3 shows the calculation result of the target shift stage as a switching line in the state of one map, but the shift stage deriving means 17 is for maintaining the set vehicle speed according to the downward gradient. Calculation of the tire driving force and the tire driving force by the engine braking operation that can be realized at each gear stage is performed, and thereby the target gear stage at which the set vehicle speed is maintained is determined (a switching line is set).

  Then, in the control means 5, when the shift speed selected on the map of the cruise control shift speed setting means 16 with respect to the target shift speed is the high speed speed side, the shift speed deriving means 17 sets it. A target gear position is selected. When the shift stage selected on the map of the cruise control shift stage setting means 16 is on the high speed side with respect to the target shift stage set according to the down slope, the vehicle travels on the down slope. Therefore, it is determined that the engine brake is insufficient, and the target shift stage (low speed stage side) corresponding to the downward gradient set by the shift stage deriving means 17 is selected and output to the AT control unit 7. For this reason, it is possible to operate at a shift speed (target shift speed) that secures a deceleration force by the engine braking operation.

  Therefore, when the vehicle is traveling on a downward slope, the cruise control shift is performed when the shift stage selected on the map of the cruise control shift stage setting means 16 is on the high speed side with respect to the target shift stage. Rather than traveling at the speed selected in the map of the speed setting means 16, the target speed calculated by the speed derivation means 17 (calculated by information on the speed during engine braking) is calculated. The set vehicle speed is maintained by running the vehicle at the target shift speed). For this reason, even when the vehicle travels on a downward slope, it is possible to accurately apply the engine brake and maintain the traveling speed of the vehicle.

  When the vehicle travels on a downward slope, the driver removes his / her foot from the accelerator pedal, and the accelerator opening is set to zero ((a) → (b) in FIG. 2). Since the vehicle speed increases, the map of the cruise control shift stage setting means 16 in FIG. 2 has the state (c) and may shift up to the X speed. . In the embodiment described above, when the vehicle travels on a downward slope, the target shift calculated by the shift speed deriving means 17 is used instead of the shift speed selected on the map of the cruise control shift speed setting means 16. Since the stage is applied, there is no state of shifting up to the X speed.

  Note that the inclination of the road is detected based on the information from the inclination sensor 13, and it is determined that the vehicle is traveling on a downward slope. However, the inclination of the road is obtained by calculation, and the vehicle travels on the downward slope. It is also possible to judge what is being done. That is, the slope resistance is calculated taking into account the vehicle output, air resistance, and tire rolling resistance, and the road surface slope angle is calculated based on the obtained slope resistance to obtain the slope angle. Thus, it may be determined that the vehicle is traveling on a downward slope.

  The vehicle travel control device having the above-described configuration is configured so that the vehicle output (accelerator opening degree) and the vehicle output (accelerator opening) are other than the downward gradient (from the predetermined downward gradient to the upward gradient side, that is, the downward gradient, the flat ground, and the upward gradient that are looser than the predetermined gradient). Based on the relationship with the vehicle speed, the vehicle is caused to travel at the gear stage selected on the map of the cruise control gear stage setting means 16. When it is determined that the vehicle is traveling on a downward slope and the shift stage selected on the map of the cruise control shift stage setting means 16 with respect to the target shift stage is on the high speed stage side, Instead of selecting the shift stage selected on the map of the control shift stage setting means 16, the vehicle is driven at the target shift stage calculated by the shift stage deriving means 17. When it is determined that the descending slope has been completed, the vehicle is caused to travel by returning to the gear stage selected on the map of the cruise control gear stage setting means 16.

  Based on FIG. 4, an example of an outline of a shift process when traveling on a downhill of the vehicular travel control apparatus having the above configuration will be described. FIG. 4 is a flowchart showing an example of processing of the vehicle travel control apparatus according to one embodiment of the present invention.

  As shown in FIG. 4, the set vehicle speed is determined in step S1 based on the operation information of the vehicle speed setting switch 14 (see FIG. 1), and in step S2 based on the detection information of the inclination sensor 13 (see FIG. 1). Road gradient is estimated. In step S3, it is determined whether or not the road gradient is a downward gradient. If it is determined in step S3 that the road gradient is not a downward gradient (step S3: NO), gear shift control other than the downward gradient is performed. In other words, since the shift control is performed by the shift stage selected by the map that the cruise control shift stage setting means 16 has, the process ends.

  If it is determined in step S3 that the road slope is a downward slope (step S3: YES), the tire driving force required to maintain the set vehicle speed is calculated in step S4, taking into account the situation of the downward slope, In step S5, the tire driving force by the engine braking operation that can be realized at each gear position is calculated. Then, in response to these results, in step S6, a target shift speed suitable for maintaining the set vehicle speed is determined.

  After the target shift stage is determined in step S6, the shift stage (crucon map shift stage) selected on the map of the cruise control shift stage setting means 16 is lower than the target shift stage in step S7. It is determined whether or not (equal) (target shift stage ≧ cru-con map shift stage).

  If it is determined in step S7 that the target shift speed is greater than or equal to the cruise control map shift speed (equal or high speed side) (step S7: YES), the gradient at which engine braking is obtained even if the target shift speed is on the high speed side. (The slope is lower than the predetermined slope, such as a downward slope, a flat ground, or the like, the slope of the downward slope is lowered), and the shift selected in the map that the cruise control speed setting means 16 has in step S8. The gear position is selected and the speed change process is executed, and the process ends.

  In step S7, the cruise control map shift speed is not the low speed side (equal) with respect to the target shift speed, that is, the cruise control shift speed setting means 16 is selected with respect to the target shift speed. When it is determined that the gear position is on the high speed side (target gear position <cruise control map gear position) (step S7: NO), the vehicle 1 (see FIG. 1) travels downhill and engine braking is insufficient. In step S8, a shift process using the target shift stage determined by the shift stage deriving means 17 (see FIG. 1) is executed, and the process ends.

  For this reason, when the cruise control map shift speed is on the high speed side with respect to the target shift speed set according to the downward gradient, the vehicle 1 travels on the downward gradient and the AT control unit 7 sets the target shift speed (low speed). Therefore, it is possible to secure a sufficient deceleration force to maintain the set vehicle speed by strengthening the engine brake. Therefore, even when the vehicle 1 (see FIG. 1) travels on a downward slope, it is possible to maintain the traveling speed of the vehicle by accurately applying the engine brake.

  When the cruise control map speed becomes equal to the target speed, or when the target speed becomes the high speed side, the engine brake is in effect (predetermined). It is assumed that the degree of the downward gradient, such as a downward gradient, flat ground, etc., which is gentler than the inclination), and the cruise control gear is selected, so that an appropriate gear can be selected even if the downward gradient changes.

  Based on FIG. 5, the flow of an example of the process of the vehicle travel control apparatus having the above configuration will be described in time series. FIG. 5 shows a specific situation of traveling from flat ground to uphill, downhill, and flat ground in the vehicle travel control apparatus according to one embodiment of the present invention, and FIG. FIG. 5 (b) shows the change over time in the gradient, and FIG. 5 (c) shows the change over time in the shift stage instructed by the travel control means.

  When the vehicle 1 (see FIG. 1) travels and the slope changes to the plus side (uphill side) at time t1, as shown in FIG. 5B, the vehicle speed decreases as shown in FIG. 5A. . When the vehicle speed decreases, at the time t2, the shift stage selected on the map (map of FIG. 2) possessed by the cruise control shift stage setting means 16 (see FIG. 1) changes from the X speed to the Y speed, for example. It will straddle the switching line at the time of downshift. As shown in FIG. 5 (c), at time t2, the Y-speed gear stage is selected and shifted down (kick down) to obtain the driving force at the set vehicle speed (1).

  As shown in FIG. 5 (a), the vehicle speed returns to the set vehicle speed at time t3, and at time t4, the shift stage selected in the map of FIG. 2 is switched at the time of shift-up from Y speed to X speed, for example. It will cross the line. As shown in FIG. 5C, at the time t4, the X-speed gear stage is selected and the driving force at the set vehicle speed is maintained.

  As shown in FIG. 5 (b), when the slope changes to a downhill at time t5 (when it is determined by the tilt sensor 13 that the vehicle 1 is traveling on the downhill), as shown in FIG. 5 (a). Then, the vehicle speed starts to rise. When the gear position is set (selected) because the slope has become a downhill, the target set by the gear speed deriving means 17 (see FIG. 1) from the control using the gear speed selected in the map of FIG. The control is switched to the control using the shift speed (control for comparing the target shift speed and the cruise control map shift speed).

  As shown in FIG. 5 (c), at the time t6, based on the situation diagram shown in FIG. 3, the Y speed is set as the target gear stage (marked with a circle in FIG. 3), and the tire is driven by the deceleration driving force by the engine braking operation. A driving force is obtained (2). If the downward gradient continues, the vehicle speed increases as shown in FIG. 5A, the deceleration driving force decreases (engine brake becomes insufficient), and at time t7 as shown in FIG. The Z speed on the low speed side is set as the target shift speed based on the situation diagram shown in FIG. 3 (□ in FIG. 3: (3)).

  The deceleration force by the engine brake is increased by the Z-speed gear position on the low speed side, and the vehicle speed is decreased as shown in FIG. As shown in FIG. 5 (b), at time t8, the gradient becomes gentle and the deceleration force by the engine brake is reduced, but the region of the downward gradient in which the engine brake becomes insufficient remains unchanged. As shown in FIG. 5C, at the time t8, based on the situation diagram shown in FIG. 3, the Z speed is maintained as the target gear position (Δ mark in FIG. 3: (4)).

  In the above-described embodiment, an example in which the target shift speed is selected in the control from (2) to (4) from time t6 to t9 has been described. However, the target shift speed and the cruise control map shift speed are compared. There is a case where the driving situation is such that the control for selecting the cruise control map gear is performed by the control.

  After time t8, as shown in FIG. 5 (b), the downward gradient continues to become gentle, and the vehicle speed is maintained at the set vehicle speed as shown in FIG. 5 (a). As shown in FIG. 5B, when the slope changes from a downhill to a plus side (uphill side) at time t9 (when the inclination sensor 13 does not determine that the vehicle 1 is traveling on the downhill). As shown in FIG. 5 (a), the vehicle speed decreases.

  When the shift stage is set (selected) because the gradient is on the uphill side, control using the target shift stage set by the shift stage deriving means 17 (see FIG. 1) (target shift stage and cruise control map shift stage) To the control using the shift stage selected on the map of the cruise control shift stage setting means 16 (see FIG. 1). As shown in FIG. 5 (c), at time t9, the accelerator opening is increased, and the operating state is, for example, the X speed region in the map of FIG. 2, and the X speed is selected (5).

  As described above, when the gradient other than the downward gradient is from zero to the plus side (upward gradient side) (until time t5), the cruise control shift speed is determined based on the relationship between the vehicle output (accelerator opening) and the vehicle speed. The vehicle is running at a gear selected on a map (map of FIG. 2) possessed by setting means 16 (see FIG. 1). When it is determined that the vehicle is traveling on a downward slope (from time t5 to time t9), the target set by the gear speed deriving means 17 is used instead of the gear speed selection by the map of FIG. The vehicle is driven at the shift speed, and when it is determined that the downward slope is finished (time t9), the vehicle is driven back to the shift speed selected in the map of FIG.

  As a result, even when the vehicle travels on a downward slope, the engine braking is accurately applied to change the vehicle traveling speed (solid line in FIG. 5A) to the set vehicle speed (middle dotted line in FIG. 5A). It becomes possible to maintain. In addition to the down slope, it is possible to include a down slope that is looser than a predetermined slope.

  In the above-described control, when the inclination sensor 13 determines that the vehicle is traveling on a downward slope (time t5), the slope has changed to a downhill. Instead of the determination by the inclination sensor 13, or In addition to the determination by the inclination sensor 13, the shift speed selected by using the map of FIG. 2 is compared with the target shift speed set by the shift speed deriving means 17 (see FIG. 1), and the gradient is reduced downhill. It is also possible to judge that it has changed.

  In the above-described control, when the vehicle is traveling on a downward slope, when the inclination sensor 13 determines that the vehicle is not traveling on the downward slope (time t9), the slope is changed to an upward slope. However, instead of the determination by the inclination sensor 13 or in addition to the determination by the inclination sensor 13, the shift speed selected using the map of FIG. 2 and the shift speed deriving means 17 (see FIG. 1) are set. It is also possible to determine that the gradient has changed from the downhill to the uphill side by comparing with the target shift stage.

  The above-described vehicle travel control device does not apply the gear position (the gear speed selected using the map of FIG. 2) for traveling the vehicle at a preset vehicle speed when the vehicle is descending. Rather, the shift speed selected using the map of FIG. 2 is compared with the target shift speed used for the downward gradient set by the shift speed deriving means 17, and the shift speed selected using the map of FIG. When the speed is on the high speed side, the vehicle is driven at the target speed set by the speed stage deriving means 17 to maintain the set vehicle speed. For this reason, even if the gear stage is multi-staged, for example, even if it is a transmission device of 8th speed and 9th speed, the engine brake can be applied accurately when the vehicle travels on a downward slope, It is possible to maintain the traveling speed of the vehicle.

  INDUSTRIAL APPLICABILITY The present invention can be used in the industrial field of a vehicle travel control device for traveling a vehicle according to a set situation.

DESCRIPTION OF SYMBOLS 1 Vehicle 2 Drive wheel 3 Engine 4 Transmission 5 Control means 6 Engine control part 7 AT control part 11 Vehicle speed sensor 12 Engine information 13 Inclination sensor 14 Vehicle speed setting switch 16 Cruise control gear position setting means 17 Gear speed deriving means

Claims (4)

Cruise control gear setting means for setting the gear according to the running state including the vehicle speed and the accelerator opening;
Shift speed deriving means for setting a target shift speed according to the downward slope;
Control for inputting information on the cruise control gear position setting means and the gear position deriving means and outputting travel control information including gear speed instructions to the travel control means so as to maintain the set vehicle speed of the vehicle. Means and
The control means includes
The target shift set by the shift speed deriving means when the shift speed selected by the cruise control shift speed setting means is higher than the target shift speed set by the shift speed deriving means. A vehicle travel control device, characterized by selecting a stage. The vehicle travel control apparatus according to claim 1,
The control means includes
When the shift speed selected by the cruise control shift speed setting means is equal to the target shift speed, or when the target shift speed is a high speed shift speed, the cruise control shift speed A vehicle travel control device, characterized in that a gear position set by a setting means is selected. In the vehicle travel control device according to claim 1 or 2,
The shift speed deriving means includes
The tire driving force for maintaining the set vehicle speed of the vehicle is determined according to the gradient determined by the gradient determining means, and the deceleration driving force due to the engine braking operation is set as a target shift stage that becomes the tire driving force. A vehicle travel control device. In the vehicle travel control device according to claim 3,
The gradient determining means includes
The slope resistance is derived by taking into account the vehicle output, the air resistance of the vehicle, and the rolling resistance of the tire, and the road surface slope angle is calculated based on the slope resistance. A vehicle travel control device characterized by determining a slope of the vehicle.

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