GB2428754A

GB2428754A - Vehicle yaw control with tyre-road friction estimator

Info

Publication number: GB2428754A
Application number: GB0515790A
Authority: GB
Inventors: Matthew J Hancock; Francis Assadian
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2005-08-01
Filing date: 2005-08-01
Publication date: 2007-02-07
Anticipated expiration: 2025-08-01
Also published as: GB2428754B; GB0515790D0; DE102006031063A1

Abstract

A control module 12 comprises a friction estimator 18 that computes from information provided by on-board sensors an estimated value of the friction between a vehicle's tyres and the ground over which the vehicle is travelling. The friction estimator 18 has an output which is connected to an input of a target yaw rate generator 19 and from information supplied by a vehicle speed sensor (13, fig 1), steering angle sensor (17) and friction estimator 18, the target yaw rate generator computes a target (desired) yaw rate. A feedback controller 20 compares an actual vehicle yaw rate, measured by a yaw rate sensor (15), with the target yaw rate. The feedback controller 20, on comparing the two yaw rate values on its inputs detects if an oversteer condition exists. If this is the case an error signal is generated by the feedback controller 20 and this signal is applied to an electric motor (11) which controls a clutch (10) in an active limited slip differential (9) in order to control the yaw stability of the vehicle. In another embodiment the control module 12 includes a feedforward controller (26, fig 3) and a summer (25).

Description

ntroI system for a vehicle This invention relates to control systems for a

vehicle and particularly a yaw stability control system for a motor vehicle fitted with an "active" differential.

Standard open differentials were developed to allow the driven wheels of a vehicle to rotate at different speeds during vehicle cornering, while transmitting equal drive torque to each wheel. However, with the use of an active differential, the drive torque of the higher spinning wheel can be efficiently transferred to the lower spinning one. This results in a yaw movement, which can be used to stabilize a vehicle.

Under severe manoeuvres, e.g. high speed lane change, when the required contact patch tyre forces for keeping the vehicle on a desired trajectory exceed the available contact patch forces, the driver workload for keeping the vehicle on the desired path will drastically increase. Under these conditions, the average driver would not be able to cope with the requested workload. Hence, at every instant, the vehicle path tends to deviate more from the desired path. In terms of dynamic system analysis, under this condition, the

vehicle is unstable.

Vehicle dynamic instabilities manifest themselves in two ways; A first is referred to as understeer. This is the situation where the vehicle front tyre forces are saturated, and the driver is not capable of steering the vehicle in the desired direction. The second is referred to as oversteer. This occurs when the vehicle rear tyre forces are saturated, and the driver, at times, is required to apply countersteering to stabilize the vehicle.

Vehicle yaw and lateral dynamics are highly nonlinear with strong dependencies on road-tyre friction, longitudinal velocity, and lateral accelerations. Most control approaches utilize a large number of tuning parameters, which, as a result, make their implementation cumbersome.

A conventional vehicle yaw dynamics control system has been disclosed in Japanese Patent Provisional Publication No. 5-262156.

The conventional system includes a driving-force distribution adjusting mechanism which distributes a driving force output from an engine of. an automotive vehicle to left and right road wheels and adjusts torquedistribution between the left and right road wheels, a yaw.

rate sensor which detects an actual yaw rate of the automotive vehicle, a target yaw rate arithmetic-calculation means which arithmetically calculates a target yaw rate on the basis of input information, namely a steer-angle information data signal from a steer angle sensor and a vehicle speed information date signal form a vehicle speed sensor and a control means which controls the operation of the driving-force distributiOn adjusting mechanism.

The control means is constructed in a manner so as to set a controlled hydraulic pressure applied to the driving-force distribution adjusting mechanism, while performing feed-back control so that the actual yaw rate is approached to the target yaw rate.

The trend for the latest automotive technologies is toward developing new active systems, where high power electomechanical actuators are driven by low power control signals to influence certain vehicle characteristics. The benefits of these active systems are on-line modification of vehicle characteristics, increasing vehicle performance and stability, reducing the vehicle tuning efforts and reducing vehicle development costs.

In a first aspect, the present invention consists of a control system for controlling a drive torque transfer device arranged to distribute torque to a plurality of driven wheels of a vehicle, wherein the control system includes; means for estimating a value for the friction between a driven wheel of the vehicle and the ground, means for computing a target yaw rate using the estimated value of friction, and means for comparing the computed target yaw rate with a measured yaw rate to produce a drive torque transfer request signal for controlling said drive torque transfer device.

In a second aspect, the present invention consists of a method for controlling a drive torque transfer device arranged to distribute torque to a plurality of driven wheels of a vehicle wherein the method includes the steps of; estimating a value for the friction between a driven wheel of the vehicle and the ground, computing a target yaw rate using the estimated value of friction, and, comparing the computed target yaw rate with a measured yaw rate to produce a drive torque transfer request signal for controlling said drive torque transfer device.

In a third aspect, the invention consists of a vehicle incorporating a control system in accordance with said first aspect.

The invention is applicable to any vehicle with an active differential which may incorporate a friction clutch. The clutch may be actuated by an electric motor or by hydraulic or electromagnetic means, for example. -4.. .

Such an active differential transfers torque from a faster wheel to a slower one. Therefore generation of understeer can be performed during cornering for example when the measured yaw rate indicates an oversteer situation which needs to be stabilised i.e. application of locking torque speeds up the inside wheel.

Preferably the value for friction is estimated using information from at least one sensor providing information relating to at least one vehicle driving parameter, said at least one sensor being mounted on board the vehicle.

Preferably, the target yaw rate is computed using, additionally, signals from one or more on-board sensors which provide information relating to one or more vehicle driving parameter. In one example, such sensors provide information relating to steering angle and vehicle speed.

The means for comparing may also be configured to discriminate between an oversteer condition and an understeer condition and to produce a positive output only when oversteer is detected and a negative output if an understeer condition is detected.

The means for comparing is preferably further adapted to be able to distinguish whether the vehicle is negotiating a right-hand bend or a left-hand bend.

Optionally, the control system may further include means for applying a correction to the drive torque transfer request signal in order to compensate for the finite response time of the drive transfer device.

Some embodiments of the invention will now be described with reference to the drawings of which; Fig 1. is a diagrammatic representation of a vehicle including a yaw stability control system in accordance with an embodiment of the invention.

Fig 2. is a block diagram of a yaw stability control module in accordance with a first embodiment of the invention and Fig 3. is a block diagram illustrating operation of a yaw stability control module in accordance with a second embodiment.

With reference to Fig. 1, a vehicle 1 has four wheels, 2,3,4,5 and a powertrain 6, for providing drive to the rear wheels 4 and 5. The powertrain 6 comprises an engine 7 and a gearbox 8, for transmitting drive to a rear, active differential 9, which in turn, transmits drive torque to the rear wheels 4 and 5.

The differential 9 is a limited slip, active differential whose degree of locking can be set by a clutch pack 10, acting between the two output sides of the differential 9.

The operation of the clutch pack 10 is controlled by an electric motor 11. The electric motor 11, is driven by an output signal from an on-board control module 12, which calculates an optimum torque transfer request thereby permitting the differential to provide yaw stability to the vehicle 1.

The vehicle further includes a front wheel speed sensor 13, for measuring the speed of the vehicle An output from the wheel speed sensor 13, is connected to the control module 12.

A vehicle driving parameter sensor 14 and a yaw rate sensor 15, are also provided on the vehicle and an output from each of these sensors is connected to the control module 12.

The vehicle further includes a steering wheel 16, for steering the front wheels, 2, 3. A steering angle sensor 17 provides a steering angle signal which varies with the steering input from the driver. The steering angle sensor 17, has an output which is connected to the control module12.

Operation of the control module 12, will now be described with reference to Fig 2.

The control module 12 includes a friction estimator 18, a target yaw rate generator 19 and a feedback controller 20.

From information provided by the on-board sensors, the friction estimator 18, computes an estimated value of the friction between the vehicle's tyres and the ground over which the vehicle is travelling.

The friction estimator 18 has an output which is connected to the input of the target yaw rate generator 19.

From information provided by the vehicle speed sensor 13, steering angle sensor 17 and friction estimator 13, the target yaw rate generator 19, computes a target (desired) yaw rate for the vehicle.

The computed yaw rate is fed into the feedback controller 20 where it is compared with the actual vehicle yaw rate as measured by the yaw rate sensor 15.

The feedback controller, on comparing the two yaw rate values on its inputs detects if an oversteer condition exists. If this is the case an error signal is generated by the feedback controller 20. The signal is applied to the electric motor 11 for control of the operation of the differential 9. Depending upon the magnitude of the difference between the target and actual yaw rates, the feedback controller 20 can generate the appropriate control signal for the differential 9 for optimum torque distribution between the driven wheels. 4, 5.

Operation of a second embodiment of a control system will now be described with reference to Fig 3.

The control module 21 of Fig 3, includes a friction estimator 22, a target yaw rate generator 23, a feedback controller 24, a feed forward controller 25 and a summer. 26.

The friction estimator 22, target yaw rate generator 23 and feedback controller 24 all operate in the same fashion as their counterparts described with reference to fig 2 The additional components comprising the feedforward controller 25 and the summer 26 permit compensation for any slowness in response of the differential 9, its clutch pack 10, or motor 11. The feedlorward controller 25 has an input connected to the. output of the target rate generator.

Using the computed target yaw rate value and information from a stored model of vehicle parameters, the feedforwarcl controller 25, computes a correction signal which is summed in the summer 26 with the output error signal from the feedback controller 24.

The resulting summed value is then applied to the electric motor 11 for control of the operation of the differential 9 (See fig 1.)

Claims

1. A control system for controlling a drive torque transfer device arranged to distribute torque to a plurality of driven wheels of a vehicle, wherein the control system includes; means for estimating a value for the friction between a driven wheel of the vehicle and the ground, means for computing a target yaw rate using the estimated value of friction, and means for comparing the computed target yaw rate with a measured yaw rate to produce a drive torque transfer request signal for controlling said drive torque * transfer device.

2. A control system according to claim 1 in which the drive torque transfer device is a differential incorporating a friction clutch

3. A control system according to claim 2 in which the drive torque transfer device further incorporates an electric motor for actuating the friction clutch.

4. A control system according to any preceding claim in which the means for estimating the value for friction is adapted to estimate said value using information from at least one sensor providing information relating to at least one vehicle driving parameter, said at least one sensor being mounted on board the vehicle.

5. A control system according to any preceding claim in which the means for computing the target yaw rate is further adapted to compute said rate using information from a steering angle sensor and a vehicle speed sensor, both of said sensors being located on-board the vehicle.

6. A control system according to any preceding claim further including means for applying a correction signal to the drive torque transfer request signal in order to compensate for a finite response time of the drive torque transfer device.

7. A vehicle incorporating a control system in accordance with any preceding claim.

8. A method for controlling a drive torque transfer device arranged to distribute torque to a plurality of driven wheels of a vehicle wherein the method includes the steps of; estimating a value for the friction between a driven wheel of the vehicle and the * * ground, computing a target yaw rate using the estimated value of friction, and, * * comparing the computed target yaw rate with a measured yaw rate to produce a drive torque transfer request signal for controlling said drive torque transfer device.

9. A control system for controlling a drive torque transfer device, substantially as hereinbef ore described with reference to the drawings.