GB2502802A

GB2502802A - Steering and yaw control for low speed, cruise control and low friction terrain

Info

Publication number: GB2502802A
Application number: GB1210064.0A
Authority: GB
Inventors: Andrew Fairgrieve; James Kelly
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2012-06-07
Filing date: 2012-06-07
Publication date: 2013-12-11
Also published as: GB201210064D0

Abstract

A method and system provide steering in cruise control, in low friction conditions, at low speed and in low range. The speed of individual vehicle wheels is repeatedly and automatically adjusted to ensure that the actual rate of turn of the vehicle approaches the theoretical rate of turn as demanded by the vehicle driver. It is noted that off-road cruise control has been proposed whereby a vehicle may maintain a pre-set speed over variable terrain. Such a system has the advantage that the vehicle driver, particularly a novice driver, can concentrate upon steering whilst allowing vehicle systems to automatically select a suitable transmission ratio and engine speed. Without the invention, the vehicle 11b may be prone to understeer and not following a theoretical vehicle path 13 associated with the steering wheel input of the driver.

Description

Improvements in Vehicle Steering

FIELD OF THE INVENTION

This invention relates to vehicle steering and particularly, but not exclusively, to steering in low traction conditions such as mud or snow. Aspects of the invention relate to a method, to a system and to a vehicle.

BAG KG ROUND

Vehicle steering relies upon friction between the vehicle tyres and the ground. Where high levels of friction are available, such as in normal highway driving, understeer and oversteer are seldom encountered. However where friction is reduced, as in mud or snow, the rate of turn of a vehicle may vary substantially from that indicated by steering angle, and in consequence the vehicle may understeer or oversteer.

In such circumstances, the experience of the vehicle driver plays an important part in retaining control of the vehicle and achieving the intended rate of turn.

A vehicle may include a cruise control mode whereby a pre-set speed is maintained regardless of topography. Gruise control systems are widely used in highway driving, generally above about 30 kph, where steering angles are typically small. Autonomous cruise control (AGC) systems permit one vehicle to follow another at a pre-determined separation whilst accommodating speed variations of the leading vehicle.

Off-road cruise control has been proposed whereby a vehicle may maintain a pre-set speed over variable terrain. Such a system has the advantage that the vehicle driver, particularly a novice driver, can concentrate upon steering whilst allowing vehicle systems to automatically select a suitable transmission ratio and engine speed. Thus the work rate of the vehicle driver may be substantially reduced. However in the case of high steering angles on low friction surfaces some side slip (understeer) may occur, so that the vehicle does not follow the intended path. An inexperienced driver may not know how to best control the vehicle in such circumstances, in order to make progress whilst avoiding risk or damage.

Mere disengagement of a cruise control mode in difficult conditions places the inexperienced driver under a greater workload, since engine speed and transmission ratio must be controlled in addition to steering.

I

What is required is a system and method of providing for a prescribed rate of turn of a vehicle in low friction conditions and which is also suitable for use whilst in a cruise control mode.

S SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a method of steering a vehicle in cruise control mode, and comprising substantially continually: detecting steering angle; calculating a theoretical rate of turn of the vehicle appropriate to the detected steering angle; and controlling individually the speed of rotation of vehicle wheels to achieve the theoretical rate of turn.

Cruise control mode is intended to describe a vehicle system for automatically maintaining a selected vehicle speed regardless of topography.

According to a second aspect of the invention, there is provided a method of steering a vehicle at low speed, and comprising substantially continually: detecting steering angle; calculating a theoretical rate of turn of the vehicle appropriate to the detected steering angle; and controlling individually the speed of rotation of vehicle wheels to achieve the theoretical rate of turn.

By low speed we mean less than about 50 kph. Low speed may be determined by a settable threshold of vehicle speed or by engagement of, for example, a low range in a vehicle transmission. In the latter case, the invention may be automatically enabled upon selection of a low range, and the low range may be manually or automatically selected.

According to a third aspect of the invention there is provided a method of steering a vehicle in a low transmission range, and comprising substantially continually: detecting steering angle; calculating a theoretical rate of turn of the vehicle appropriate to the detected steering angle; and controlling individually the speed of rotation of vehicle wheels to achieve the theoretical rate of turn.

A low transmission range provides a lower final drive gear ratio at the wheels, and is typically available for off-road use where lower maximum vehicle speed can be accepted.

According to a fourth aspect of the invention there is provided a method of steering a vehicle in low friction conditions and comprising substantially continually: detecting steering angle; calculating a theoretical rate of turn of the vehicle appropriate to the detected steering angle; and controlling individually the speed of rotation of vehicle wheels to achieve the theoretical rate of turn.

Low friction may be indicated by wheel slip of greater than 20%.

The methods of the invention may provide an open loop system whereby vehicle wheel speeds are determined according to the theoretical rate of turn. Feedback is provided by active input to the steering wheel by the vehicle driver -in other words driver demand may change the theoretical rate of turn according to change of steering angle; the driver may for example increase the steering angle if the vehicle rate of turn does not match driver expectation.

In all variants of the invention, closed loop control may be achieved by determining a current rate of turn of the vehicle, and controlling individually the speed of rotation of vehicle wheels such that the current (real time) rate of turn approaches the theoretical rate of turn.

The methods of the invention may also be used in combination, so that a plurality of conditions may require to be met for implementation of the method. For example cruise control and low friction, or cruise control and low range may be a required combination.

More than two conditions may apply in a desirable combination, which may be vehicle specific.

Methods of the invention rely upon an iterative approach, and sample the inputs of steering angle and current rate of turn at an appropriate refresh rate, for example 10Hz or greater.

The speed of rotation of individual wheels may be directly reduced, by braking, or may be adjusted by directing more or less torque to the relevant wheel driveshaft. Torque variation may rely upon automatically increasing engine output torque and/or by using techniques of biasing torque to one or more vehicle axles, or to one or other wheel side of a single axle.

Torque bias may for example be achieved by controlling one or more differential gears so that the output shafts thereof deliver different torques to the respective wheels. This torque may be redistributed between driven wheels of the vehicle, and the available torque may be automatically increased or reduced by adjustment of the torque output of the vehicle motor.

The vehicle motor may be an internal combustion engine, an electric motor or a combination thereof.

The methods of the invention are also applicable to vehicles having electric wheel motors.

Aspects of the invention are applicable to both on-road and off-road driving, and may be implemented automatically by a vehicle system upon detection of certain adverse conditions.

Alternatively, or in addition, the methods may be selected or de-selected manually by a vehicle driver.

Adverse conditions may be any circumstance in which side slip is present or is predicted.

Such conditions may be detected automatically by on-board vehicle systems, and include for example detection of wheel slip, and failure of the vehicle to follow the prescribed path. In each case the adverse condition may be subject to a threshold being exceeded, for example wheel slip exceeding a predetermined percentage, and the threshold may be variable depending on the terrain or upon driver implementation of a terrain mode of the vehicle.

Thus an off-road mode may be automatically detected or manually selected, and in consequence certain thresholds may be determined, from e.g. a look-up table in a memory of a processor. Different thresholds may apply for different off-road terrain conditions, such as sand, mud, snow or rocks -and these terrains may be detected automatically by a suitable on-board vehicle system.

In the case of on-road low speed driving, where wheel slip is typically very low, the speed of individual vehicle wheels is monitored. These speeds may be compared with steering angle to ensure that they remain within a narrow permissible range (it being understood that inside wheels turn more slowly than outside wheels during cornering). A plurality of wheel speeds may be interpolated to allow determination of vehicle speed.

In the case of off-road driving, significant wheel slip may occur and accordingly an alternative vehicle speed reference is required where the invention is applied to a cruise control mode.

In one embodiment, the method is applied to a cruise control mode whereby vehicle reference speed is determined without reference to vehicle wheel speed(s), for example by use of GAS positioning, fixed object radar, sonar laser, camera recognition of terrain passing under the vehicle, or any other suitable technique, or any combination of these techniques.

In one embodiment, the method of the invention comprises a closed loop control system.

Thus in one example one or more inside wheels of the vehicle is braked to counter understeer. Such braking may be incorporated within a closed loop control system, whereby the theoretical and current rates of turn are continually compared, braking being applied in proportion to the deviation to be corrected. Closed loop control may operate at any suitable refresh rate, for example 10Hz or greater.

In the alternative an open loop control system may be employed in the invention. Using the same example one or more inside vehicle wheels may be braked until the vehicle steering angle is changed -the vehicle driver thus provides a control input, and a recalculation of deviation from the intended path follows. A further braking input may follow if the vehicle deviation is not successfully corrected.

The invention allows a cruise control mode to be adopted, so that the driver can concentrate on steering the vehicle whilst maintaining progress in challenging terrain.

The theoretical rate of turn may be represented as a vehicle body yaw and side slip which can be mapped to the slip angles and speeds of individual wheels. If required, an instant reference position of the vehicle may repeatedly or continuously be provided by any known technique, for example by GAS positioning, fixed object radar, sonar laser, or camera recognition of terrain passing under the vehicle. Reference position may be useful in some applications of the invention.

The expected vehicle yaw rate, as a consequence of steering angle, may be repeatedly compared with an actual (measured) yaw rate in order to determine side slip, and thus to permit calculation of a correction factor whereby vehicle wheel speed is adjusted.

Vehicle speed may be determined by an averaging technique of the rotational speeds of two or more vehicle wheels, for example in conjunction with the reference position techniques noted above. The averaging technique may be applied to undriven wheels and/or to non-steering wheels. However it will be understood that vehicle speed, as such, is optional, and not a necessary input for correcting side slip according to the invention.

Thus the invention allows for optimised steering response by providing that individual wheel speeds are appropriate to the steering angle, and particularly where the steering wheels are driven. The vehicle driver retains full control of steering, and the vehicle automatically forces S appropriate wheel speeds by braking, or by applying more or less drive torque to the intent that the desired vehicle path is followed, particularly in low speed off-road conditions where cruise control is activated.

The invention has the advantages of reducing tyre wear, due to restricting unnecessary wheel spinning as a result of excess drive torque, and of reducing damage to the terrain caused by spinning wheels.

In an embodiment of the invention, a hybrid vehicle may provide increased torque to one or more wheels via an electric traction motor, in order to supplement an internal combustion engine.

In one embodiment an electronic control unit of the vehicle determines the vehicles target speed according to a cruise control input, detects the instant steering angle, and calculates the expected yaw angle of the vehicle on the assumption that sufficient friction is available to permit the vehicle wheels to follow the path dictated by the steering angle. The expected yaw angle is compared with a detected yaw angle, and convergence is initiated by controlling individual wheel speeds.

In another embodiment, the ECU determines the theoretical individual wheel speeds required for a prescribed rate of turn corresponding to the instant steering angle, and compares these theoretical wheel speeds with actual wheel speeds. Individual wheel speeds are in consequence controlled to ensure convergence, by braking or directing increased torque to the or each respective wheel driveshaft.

Vehicle yaw may be sensed by any suitable technique, including OPS, fixed object radar, camera recognition of terrain and on-board accelerometers or a gyro. Wheel speed may be determined in any conventional manner, including use of systems associated with anti-lock braking.

In one embodiment, the invention is implemented in response to detection of an off-road condition, for example by reference to selection of an off-road vehicle operating condition, selection of a low transmission range, selection of a raise of suspension setting, or detection of high suspension travel. The invention may also be implemented upon detection of wading by any suitable wading sensor.

Within the scope of this application it is expressly intended that the various aspects, S embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. For example, features described with reference to one embodiment are applicable to all embodiments, unless such features are incompatible. i0

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-Fig. 1 illustrates the turning path of a vehicle in plan on a high friction surface; Fig. 2 illustrates the turning path of a vehicle in plan on a low friction surface; and Fig. 3 illustrates the instant angle between desired direction of travel and actual direction of travel.

DETAILED DESCRIPTION

With reference to Fig. 1 a vehicle 11 is illustrated in a straight ahead condition, with the steering road wheels 12 parallel. By turning the vehicle steering wheel, the wheels will generally adopt a non-parallel condition and cause the vehicle to follow a curve, as illustrated by the dotted line path 13 and vehicle 11 a.

The representation of Fig. 1 implies a high friction surface between the road wheels and the ground, so that side slip of the vehicle is negligible -in consequence the vehicle follows the intended path very closely.

Fig. 2 illustrates what may happen on a low friction ground surface such as snow or sand. In this case, and somewhat dependent on vehicle speed, the vehicle 11 b will tend to slip sideways as the components of turning force are resolved. The vehicle will follow a less tight curve (so-called understeer) and does not follow the theoretical vehicle path 13 associated with the steering wheel input of the driver.

The instant relative directions of the vehicle are shown in Fig. 3 where expected direction 21, and actual direction 22 are separated by a side slip angle 23.

It will be understood that in the case of Fig. 1, the steering wheels 12 will have different S rotational speeds as they follow the curved path 13. These speeds can be directly computed by a processor from information regarding vehicle steering geometry and steering angle, or provided in a look-up table of a memory of an electronic control unit. Theoretical road wheel speeds may be compared with actual road wheel speeds, for example provided by a conventional anti-lock braking system in order to give confidence that the vehicle is following the intended path.

In the case of a side slip (Fig. 3), the rate of turn of the vehicle may be increased by speeding up the outer road wheel relative to the inner road wheel, or by slowing the inner road wheel with respect to the outer road wheel, or both. Thus the vehicle may be forced to more closely approach the theoretical path 13, and correct the side slip tendency.

Speeding up of one or more vehicle wheels is a useful option in low friction conditions, since traction may be gained by deliberately forcing a degree of slip in the range 5-20%, depending upon the nature of the terrain. A different degree of slip may be desirable in for example mud, gravel, sand and snow, and may also be varied depending on the quality of the terrain, for example soft or hard-packed snow.

Adjustment of differential road wheel speed may be made on a steering axle, a non-steering axle, or both.

Wheel speed adjustment may be provided by any suitable means typically by braking one wheel of an axle or by varying the torque applied to one wheel of an axle. In the latter case torque variation may be by adjustment of the torque split in a differential gear of an axle, and may be accompanied by an increase or a reduction of vehicle motor output torque. The latter may be accomplished by variation of a motor torque/speed map within a suitable control processor of the vehicle.

Vehicle position relative to the intended path may be detected or calculated in any suitable manner, so that a reference position may be compared with a current position, and a correction applied automatically. In a simplified open-loop system correction is applied by the vehicle driver changing the steering angle. GPS positioning systems, fixed object radar, or camera recognition techniques are suitable together with analysis of relative wheel speeds of non-steering axles.

Although understeer is illustrated in Fig. 2, the invention is equally applicable to oversteer, in which condition the vehicle turns through a greater angle than demanded by the vehicle driver.

Terrain type may be detected automatically by an on-board vehicle system, or may be manually selected by a vehicle driver based on visual appearance. Where quality influences a degree of forced wheel slip, the vehicle driver may be presented with options, for example soft, medium or hard-packed sand. Such options may however be resolved by automatic vehicle systems responsive to terrain recognition.

Claims

Claims 1. A method of steering a vehicle in cruise control mode, and comprising substantially continually: S detecting steering angle; calculating a theoretical rate of turn of the vehicle appropriate to the detected steering angle; and controlling individually the speed of rotation of vehicle wheels to achieve the theoretical rate of turn. i0
2. A method according to claim 1, and further comprising determining a current rate of turn of the vehicle; and controlling said speed of rotation such that the current rate of turn approaches the theoretical rate of turn.
3. A method of steering a vehicle at low speed, and comprising continually: detecting steering angle; calculating a theoretical rate of turn of the vehicle appropriate to the detected steering angle; and controlling individually the speed of rotation of vehicle wheels to achieve the theoretical rate of turn.
4. A method according to claim 3, and further comprising determining a current rate of turn of the vehicle; and controlling said speed of rotation such that the current rate of turn approaches the theoretical rate of turn.
5. A method of steering a vehicle in a low transmission range, and comprising substantially continually: detecting steering angle; calculating a theoretical rate of turn of the vehicle appropriate to the detected steering angle; and controlling individually the speed of rotation of vehicle wheels to achieve the theoretical rate of turn.
6. A method according to claim 5, and further comprising determining a current rate of turn of the vehicle; and ii controlling said speed of rotation such that the current rate of turn approaches the theoretical rate of turn.
7. A method of steering a vehicle in low friction conditions and comprising substantially continually: detecting steering angle; calculating a theoretical rate of turn of the vehicle appropriate to the detected steering angle; and controlling individually the speed of rotation of vehicle wheels to achieve the theoretical rate of turn.
8. A method according to claim 7, and further comprising determining a current rate of turn of the vehicle; and controlling said speed of rotation such that the current rate of turn approaches the theoretical rate of turn.
9. A method according to any preceding claim, wherein the speed of rotation of a steering road wheel is controlled.
10. A method according to claim 9 wherein the speed of rotation of all road wheels is controlled.
11. A method according to any preceding claim, wherein the speed of rotation of vehicle wheels is controlled by braking one or more of said wheels.
12. A method according to any preceding claim, wherein the speed of rotation of vehicle wheels is controlled by variation of driving torque thereof.
13. A method according to claim 12, wherein driving torque to one output of a differential gear is reduced.
14. A method according to claim 12, wherein driving torque to one output of a differential gear is increased.
15. A method according to claim 13 or claim 14, wherein driving torque is varied by adjustment of output torque of the vehicle motor.
16. A method according to any preceding claim, wherein rate of turn of the vehicle is determined by a GPS system.
17. A method according to any preceding claim, and having a refresh rate of 10Hz or S greater.
18. A steering system of a vehicle having a cruise control mode, the system including a processor having inputs of steering angle and rotational speed of steering wheels, the processor being adapted in cruise control mode to repeatedly calculate theoretical vehicle yaw with respect to steering angle, and to control individually vehicle wheel speed to achieve said theoretical yaw.
19. A system according to claim 18, wherein said processor is further adapted to compare theoretical vehicle yaw with actual vehicle yaw and to control individually vehicle wheel speeds such that actual vehicle yaw approaches theoretical vehicle yaw.
20. A steering system operable at low speed, and including a processor having inputs of steering angle and rotational speed of steering wheels, the processor being adapted to repeatedly calculate theoretical vehicle yaw with respect to steering angle, and to control individually vehicle wheel speed to achieve said theoretical yaw.
21. A system according to claim 20, wherein said processor is further adapted to compare theoretical vehicle yaw with actual vehicle yaw and to control individually vehicle wheel speeds such that actual vehicle yaw approaches theoretical vehicle yaw.
22. A steering system operable in a low vehicle transmission range, and including a processor having inputs of steering angle and rotational speed of steering wheels, the processor being adapted to repeatedly calculate theoretical vehicle yaw with respect to steering angle, and to control individually vehicle wheel speed to achieve said theoretical yaw.
23. A system according to claim 22, wherein said processor is further adapted to compare theoretical vehicle yaw with actual vehicle yaw and to control individually vehicle wheel speeds such that actual vehicle yaw approaches theoretical vehicle yaw.
24. A steering system operable upon detection of low friction terrain, and including a processor having inputs of steering angle and rotational speed of steering wheels, the processor being adapted to repeatedly calculate theoretical vehicle yaw with respect to steering angle, and to control individually vehicle wheel speed to achieve said theoretical yaw.
25. A system according to claim 24, wherein said process is further adapted to compare theoretical vehicle yaw with actual vehicle yaw and to control individually vehicle wheel speeds such that actual vehicle yaw approaches theoretical vehicle yaw.
26. A steering system according to any of claims 18-25, wherein said processor is adapted to control engine output torque.
27. A steering system according to any of claims 18-26, wherein said processor is adapted to control vehicle wheel speed by braking.
28. A steering system according to any of claims 18-27, wherein said processor is adapted to determine driving torque applied to opposite wheels of an axle.
29. A vehicle incorporating the steering system of any of claims 18-28.