CREEP TORQUE CONTROL SYSTEM FOR A VEHICLE

Abstract

Aspects of the present invention relate to a control system for a vehicle. The control system comprises one or more controllers, and is configured to select a relationship between torque and speed based, at least in part, on a determined terrain mode. The control system is further configured to control a drive torque of the vehicle in accordance with the selected relationship between torque and speed when the vehicle is operating in a creep control mode. The vehicle may be a hybrid or electric vehicle and the terrain mode may be determined from a Terrain Response™ switch input or automatically determined.

Claims

1-15. (canceled)

16. A creep control system for a vehicle, the control system comprising at least one controller, the control system being configured to: determine a terrain mode of the vehicle; determine that the vehicle is operating in a creep control mode; select a relationship between creep torque and speed torque based, at least in part, on the terrain mode; and control a drive torque of the vehicle in accordance with the selected relationship between creep torque and speed torque.

17. The control system according to claim 16, wherein the speed of the vehicle is controlled to a target speed while maintaining drive torque at or below a level corresponding to the selected relationship between creep torque and speed torque.

18. The control system according to claim 17, wherein the speed of the vehicle is controlled by accelerating the vehicle from stationary to the target speed.

19. The control system according to claim 17, further configured to maintain the target speed once said target speed is reached.

20. The control system according to claim 17, further configured to determine the target speed based on the selected relationship between creep torque and speed torque.

21. The control system according to claim 16, wherein the terrain mode is dependent on a determined vehicle terrain based on a manual selection or an automatic selection.

22. The control system according to claim 16, wherein the selected relationship between creep torque and speed torque is dependent on an intended direction of travel of the vehicle and/or a current direction of travel of the vehicle.

23. The control system according to claim 16, wherein the selected relationship between creep torque and speed torque is dependent on a terrain gradient.

24. The control system according to claim 16, further configured to receive a torque demand from a user and provide control of drive torque to the user, such that the vehicle is not operating in the creep control mode.

25. The control system according to claim 24, further configured to reinstate the creep control mode when the vehicle speed passes below the target speed and the driver demand falls below the selected relationship between creep torque and speed torque.

26. The control system according to claim 16, further configured to provide control of drive torque to a user when a torque demand from the user exceeds the selected relationship between creep torque and speed torque.

27. The control system according to claim 16, further configured to control a torque applied to each wheel or each axle of the vehicle based on the selected relationship between creep torque and speed torque.

28. The control system according to claim 16, further configured to implement the selected relationship between creep torque and speed torque by applying a braking torque opposing drive torque from an internal combustion engine and/or a traction motor.

29. The control system according to claim 28, wherein the braking torque is provided by the traction motor opposing drive torque from the internal combustion engine.

30. The control system according to claim 16, further configured to receive a creep control indication from a user for operating in the creep control mode.

31. A vehicle comprising the control system according to claim 16.

32. A method comprising: determining a terrain mode of a vehicle while the vehicle is operating in a creep control mode; selecting a relationship between torque and a speed of the vehicle based, at least in part, on the terrain mode; and controlling a drive torque of the vehicle in accordance with the selected relationship.

33. A non-transitory computer readable medium comprising computer readable instructions that, when executed by at least one processor, cause the at least one processor to perform the method of claim 32.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0042] In an embodiment of the invention;

[0043] FIG. 1 shows a schematic illustration of a vehicle suitable for use with an embodiment of the invention in a side view;

[0044] FIG. 2 shows a powertrain arrangement suitable for use with an embodiment of the invention;

[0045] FIG. 3 shows a control system suitable for use with an embodiment of the invention;

[0046] FIG. 4 shows a graph of road speed vs. torque according to an embodiment of the invention;

[0047] FIG. 5 shows a graph of road speed vs. torque for several terrain modes according to an embodiment of the invention;

[0048] FIG. 6 shows an example state diagram for controlling the control system according to an embodiment such as that shown in FIG. 3.

DETAILED DESCRIPTION

[0049] A control system for a vehicle in accordance with an embodiment of the present invention is described herein with reference to the accompanying figures.

[0050] FIG. 1 shows a vehicle 100 according to an embodiment of the present invention. The vehicle 100 may be two-wheel drive or four-wheel drive and includes an automatic transmission. The vehicle may be a hybrid vehicle including an internal combustion engine (ICE) and an electric traction motor. Alternatively, the vehicle may be an electric vehicle, or it may include an ICE with no electric traction motor.

[0051] FIG. 2 shows a powertrain 200 suitable for use with an embodiment of the present invention. The powertrain 200 comprises a prime mover, in the form of an internal combustion engine 210, a torque converter 220, a traction motor 230, a transmission 240, a final drive 250 and driving wheels 256 and 258. The engine 210 provides torque to drive the torque converter 220 by means of flex plate. Drive passes from the flex plate to the torque converter impeller and the torque converter provides hydrodynamic drive to the turbine, drive then leaves the torque converter through the transmission input shaft into the transmission 240. The traction motor 230 has a stator and a rotor and the rotor provides torque to the transmission input shaft either to provide drive or braking torque through the transmission. The transmission 240 provides a range of selectable gear ratios and the output from the transmission passes to the prop shaft. The prop shaft drives the final drive 250 which may contain a differential and drives side shafts which themselves drive wheels 256 and 258.

[0052] FIG. 3 shows a control system 300 suitable for use with an embodiment of the present invention. The control system comprises a plurality of control modules, input signals and controlled outputs. While an embodiment is shown, the functions of individual modules may be combined or distributed across controllers on a network. Similarly, the embodiment shows an arrangement of sensors and controlled outputs, these may be substituted by equivalent signals without departing from the invention. Signals between controllers are also shown in the embodiment and these may be carried by a network such as a controller area network (CAN) linking a plurality of controllers.

[0053] The control system 300 comprises an engine control module (ECM) 310 which accepts signals from an accelerator pedal 314 to indicate a driver demand for torque and an engine speed sensor 312. These are used by the controller to determine fuel injection and, in a gasoline engine, the ignition timing. The engine speed and pedal position signals are sent to the vehicle supervisory controller (VSC) 320.

[0054] The vehicle supervisory controller (VSC) 320 receives signals from other controllers and from a terrain mode switch 322 which allows the driver to indicate the terrain over which the vehicle is travelling. This switch may have an automatic mode which allows the controller to select an appropriate mode depending on sensor inputs. A creep mode switch 323 also provides an input to the VSC for the driver to select or de-select creep mode, this may be a physical switch or may be provided by a multipurpose input device such as a touch screen. The VSC provides an output to a traction motor controller 324 and the traction motor controller provides control signals to an inverter 326 which controls power flow between a battery 327 and the traction motor 328. The inverter can control the motor to provide drive torque by supplying battery power to the motor or to provide regenerative braking by taking motor power to charge the battery.

[0055] In this embodiment an anti-lock braking system (ABS) controller 330 is shown which receives signals from wheel speed sensors 332, 334, 336 and 338. From these signals a vehicle speed is calculated which is sent to the VSC 320. The ABS controller also controls braking of the wheels through brakes 342, 344, 346 and 348 and braking is provided both from a driver input from a brake pedal and on command from the VSC without driver input.

[0056] FIG. 4 shows a graph of torque [Nm] vs. speed [kph] according to an embodiment of the present invention. Road load is shown in line 410 which indicates the torque required to overcome resistance on a level road, it will be appreciated that over the low speeds shown this line is almost straight as aerodynamic resistance has little effect. The road load will also vary when the vehicle is driving over a gradient or when the terrain surface is soft although these effects are not shown. Line 420 shows the creep torque which is generated by the torque converter in a creep control mode of the vehicle. The curve is simplified in this figure. At point 422, representing the vehicle at rest, the torque converter provides a creep torque of 600 Nm. This torque causes the vehicle to accelerate. At point 426 the torque converter torque curve crosses the road load curve and so at this point the drive torque balances the resistance, such that the vehicle no longer accelerates. The vehicle speed at this point is known as the creep speed. Conveniently the slope of the torque converter curve is steep across the intersection 426 so small changes in road resistance have little effect on the creep speed.

[0057] Line 430 shows a braking torque which may be applied by the friction brakes or by the traction motor (where provided) during creep control mode. This provides 300 Nm of braking (negative torque) at rest (point 432) ramping to 0 Nm at 3 kph (point 434). Line 440 shows the combination of lines 420 and 430 and is the net creep torque experienced by the vehicle. The net creep torque at rest is 300 Nm ramping up to 425 Nm at 3 kph (point 444) then following curve 420 to achieve the same creep speed of 4.8 kph. This modification allows a gentler acceleration from rest when the driver releases the brake pedal which is appropriate for a terrain surface which has a low coefficient of friction. Advantageously this open loop control method can prevent wheel slip which is known to polish a surface and thereby further reduce the available friction. While this embodiment shows an ICE with a torque converter, it will be appreciated that an electric drive system may be able to provide the same output line 440 without the need for braking torque, since the traction motor controller and inverter can control the electric motor to generate substantially any desired drive torque from zero.

[0058] During engine warmup the ICE may be controlled to a higher engine speed which would affect the creep speed. This invention would allow regenerative braking to control the creep torque relationship produced to maintain a consistent creep speed despite an elevated idle speed.

[0059] FIG. 5 shows a graph of torque [Nm] vs. speed [kph] according to an embodiment of the present invention. Road load is shown in line 510 which indicates the torque required to overcome resistance on a level road as described above. Lines 520, 530 and 540 show net creep torque for different terrain modes although the number of modes and the curves are simplified for clarity. It is known that some vehicles include a system in which a driver can select one of a plurality of modes suitable for driving on respective surfaces or terrains. In other vehicles, the system can automatically select a terrain mode by determining the terrain on which the vehicle is driving based on inputs from one or more vehicle sensors. Example terrain modes include grass, gravel, snow, mud, sand and rocks. Some terrain modes may be suitable for driving on more than one terrain.

[0060] Line 520 shows the creep torque for a “general” terrain mode and this may be the unmodified torque converter characteristic at idle.

[0061] Line 530 shows the creep torque for a sand terrain mode which has several features which differ from the general mode. At rest, the creep torque has increased from 600 Nm (point 522) to 775 Nm (point 532) in order to overcome the higher surface friction due to the soft sand terrain. The creep speed has also increased from 4.8 kph (point 526) to 6.9 kph (point 536). Although the higher surface resistance of sand is not shown, this creep speed may be 6.5 kph if the surface resistance were 300 Nm. A higher creep speed is possible because travelling on sand rarely requires tight manoeuvring and it makes acceleration from the creep speed more convenient as the wheels are less likely to dig into the surface from the higher creep speed. The additional torque required to achieve curve 530 may be provided by the traction motor using battery power or by increasing the engine idle speed. In some instances, both may be applied to achieve the desired curve. The VSC may provide control signals to the ECM, TMC and ABS to achieve the most efficient method of providing the required torque. This may depend on the state of charge of the battery and the driveline configuration. At speeds above 7 kph the curve 530 provides a lower overrun torque 538 than the general curve 520. This is again due to the higher surface resistance of sand, so the desired vehicle deceleration does not require as much overrun torque. This modification may be provided by the traction motor as before.

[0062] Line 540 shows the creep torque for a grass, gravel & snow (GGS) terrain mode. Grass, gravel and snow generally have a lower surface friction coefficient than sand and thus in this case the creep torque at rest is lower than the general mode at point 542 to provide a gentle pull away from rest without wheel spin. The torque then approaches the general curve and achieves the same creep speed at point 526. The curve may alternatively be calibrated to achieve a lower creep speed because it may be advantageous to manoeuvre more slowly on an icy surface. In this case the negative torque required to achieve curve 540 (i.e. a reduced net drive torque compared to the general curve 520) may be provided by the traction motor using battery power, by application of the brakes or by decreasing the engine idle speed. The VSC may provide control signals to the ECM, TMC and ABS to achieve the most efficient method of providing the required torque.

[0063] FIG. 6 shows a state diagram 600 for controlling the control system according to an embodiment of the present invention. Two states are shown: creep control mode on 610 and creep control mode off 620. The transition from creep mode off to creep mode on may require that the driver demand, either from an accelerator pedal input or from an automated vehicle control system such as a cruise control setting, is less than the creep torque. This means that creep mode will engage when the driver releases the accelerator at low speeds. Additionally, the creep mode may only transition from off to on when the creep mode switch is on and this allows the driver to select whether creep is required. The transition from creep mode on to off may require the driver demand to be greater than the creep torque or for the creep switch to be off. This allows the driver to accelerate from creep by pressing the accelerator to demand a higher torque or by switching off creep.

[0064] When the creep mode is off 620 the drive torque depends on driver demand. When creep torque is on the drive torque depends on the selected creep torque map which, as explained above, may depend on the determined terrain mode. Within the creep mode on state the selection of creep torque maps is made according to the terrain mode 630 from a range of maps shown 632.

[0065] It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.