GB2372020A - Haptic controller for electrically-assisted power steering in road vehicles - Google Patents

Abstract

An electric assisted steering system for a motor driven road vehicle, having assist torque signal generating means which generates an assist torque signal for the steering system in response to the driver's applied torque and sensed vehicle speed to reduce the driver's steering effort. A yaw rate haptic torque is generated which is based upon vehicle rate error and is arranged to be added to the torque assist signal such that, when the yaw rate error builds up corresponding to increasing steering instability (e.g. understeer or oversteer) of the vehicle, the haptic torque added to the torque assist signal reduces the effective road reaction feedback sensed by the driver in advance of any actual vehicle stability loss whereby to allow the driver to correct appropriately in good time before terminal steering instability is reached.

Description

DESCRIPTION HAPTIC CONTROLLER FOR ROAD VEHICLES The present invention relates to electric assisted steering systems (EAS) in motor driven road vehicles and is concerned in particular with a control system in a road vehicle adapted to provide steering torque compensation or haptic torque based on the measured vehicle yaw.

Electric assist steering systems are well known in the art. Electric assist steering systems that use, for example, a rack and pinion gear set to couple the steering column to the steered axle, provide power assist by using an electric motor to either apply rotary force to a steering shaft connected to a pinion gear, or apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (a) a driver's applied torque to the vehicle steering wheel, and (b) sensed vehicle speed.

Other known electric assist steering systems include electro-hydraulic systems in which the power assist is provided by hydraulic means under at least partial control of an electrical or electronic control system.

It is an object of the present invention to improve the haptic information that the driver receives from the steering system.

In accordance with the present invention, there is provided a power assisted steering system for a motor driven road vehicle, the system including assist torque signal generating means arranged to generate an assist torque signal for the steering system in response to the driver's applied torque and sensed vehicle speed and effective to reduce the driver's steering effort, and a means for generating a haptic torque based upon vehicle yaw rate error which is arranged to be added to the torque assist signal such that when the yaw rate error builds up, corresponding to increasing steering instability of the vehicle, the haptic torque added to the torque assist signal reduces the effective road reaction feedback sensed by the driver in advance of any actual vehicle stability loss whereby to allow the driver to correct appropriately in good time before terminal steering instability is reached.

Such a system has the advantage of drawing steering instability conditions (e. g. understeer or oversteer) to the attention of the driver.

Preferably, the assist torque signal generating means comprises an electric motor.

Yaw rate error can be established by comparing an estimated yaw rate derived from measured values of steering angle and vehicle longitudinal velocity, with measured vehicle yaw rate.

Preferably, the yaw rate error is saturated, if necessary, to prevent excessive demand and scaled by a gain map.

The gain is preferably controlled in accordance with yaw rate error, such that a low yaw rate error results in a relatively low gain and a high yaw rate error results in a relatively large gain so as to increase the assist torque from the power steering and make the steering feel light to the driver.

In some embodiments, a plurality of gain maps are provided, the most suitable to comply with the prevailing conditions being arranged to be selected automatically from a judgement of road surface conditions based on measured data, such as measured yaw rate error and column torque.

The haptic torque is preferably established by scaling the steering column torque using the scaled yaw rate error, the haptic torque being added to the torque assist to provide an output for driving the electric motor.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a block diagram illustrating one embodiment of a control system in accordance with the present invention; and Fig. 2 shows examples of controller gain maps that can be used in the present invention.

Referring first to Fig. 1, the vehicle steer angle and longitudinal velocity are input to element 10 where an estimate is established of the yaw rate demanded by the driver of the vehicle. The yaw rate estimation is based for example on the steady state understeer equation, expressed as:

th hi where Vx is the vehicle longitudinal velocity, I is the wheel base, Vch is the vehicle characteristic speed, Ôsw is the handwheel angle and Gs is the gain of the steering system from road wheels to handwheel. This estimated value is then passed through a first order low pass filter, tuned to give the estimate similar lag to the vehicle. Careful selection of the break point in this filter allows the point where the steering goes light in relation to the loss of steer authority to be controlled.

The resulting estimated yaw rate is compared at 12 with a signal representative of the actual vehicle yaw rate, as measured by a Vehicle Stability Controller (VSC) or similar sensor, to generate a yaw rate error signal on line 14. The yaw rate error is then saturated at 16 to prevent excessive demand and scaled by a gain map 18.

The saturation block 16 prevents the yaw rate error from reaching too high a level. Experience has shown that if the yaw error is allowed to increase too much, then this can excite an instability in the EAS system. This may be dependent to some extent on the particular characteristics of the EAS system fitted to the vehicle, but the saturation also prevents the system producing excessive torques in the event of an error. It may be necessary sometimes to tune the value of this saturation in dependence upon the surface that the vehicle is on. A possibility is to use column torque and yaw rate error as indices into a look-up table.

The gain at 18 is varied in accordance with the yaw rate error. A low yaw rate error indicates the linear regime referred to above in which the vehicle is operating at constant forward speed and before terminal understeer is reached, and where therefore a low gain is required. On the other hand, a high yaw rate error is indicative of excessive understeer, and therefore a large gain is required to increase the assist torque from the power steering and make the steering feel light.

The haptic torque on line 20 is established by using the scaled yaw rate error signal from the gain map 18 to scale at 28, the column torque (Tcol) which has been low pass filtered at 22 to prevent exciting unstable modes in the power steering. By scaling the column torque at 28, rather than adding to it, the controller is prevented from entering a region where it may attempt to drive the steering system against the driver. In this manner, the inherent self-centring of the steering is maintained. If the driver releases the steering wheel, then the column torque falls to zero, and the haptic torque also falls to zero.

The output from the controller is thus determined by multiplying at 28 the column torque (Tcol) with the output from the gain map 18. As described above, the column torque is low pass filtered at 22, again to prevent excitation of the EAS unstable modes. Careful design of this filter 22 may show that is possible to guarantee stability and allow the removal of the saturation element 16. The result of the multiplication at 28 is then added to the assist torque (Tassist) generated from the power steering controller, and fed to the power steering (EAS) motor.

The"Abs"blocks 24 and 26 in Fig. 1 are included so that the absolute values, or magnitudes, of the input signals are taken.

The yaw rate error in the arrangement of Fig. I is scaled by a gain map at 18 in establishing the final output from the controller. The gain value is dependent on the value of the yaw rate error and also on the characteristics of the surface on which the vehicle is running, ie. high Mu or low Mu.

An example of a controller gain map which can be used is shown in Fig. 2.

For low yaw rate error, the scaling is negative, therefore reducing the assist torque and making the steering feel slightly heavier. The aim is to produce a torque that increases with handwheel angle. For high yaw rate errors, the gain is much higher, giving a large positive output that greatly increases the assist torque and makes the handwheel feel light. The shape of these maps can be varied to produce the desired feel in the steering system.

A single map can be used or, preferably, a plurality of maps can be available, the most suitable of which to suit the prevailing circumstances can be selected automatically from a judgement of road surface conditions based, for example, on the measured yaw moment error and column torque.

Improvement in the haptic information that the driver receives is provided in the abovedescribed system by altering the torque in the steering column in two ways.

In the broadly linear operating region at constant forward speed and before terminal understeer is reached, the torque in the steering column is gradually increased as the handwheel angle is increased. This provides the driver with a haptic indication via the handwheel of the amount of lateral acceleration on the vehicle. When terminal understeer is reached such that additional handwheel angle fails to increase the vehicle yaw rate, the torque in the steering column is greatly reduced. This causes the handwheel to become very light, providing the driver with an indication that the limit of traction has been reached. Tuning of the controller allows this drop in torque to happen slightly ahead of the actual loss of traction, providing the driver with a short but usable response time before steer authority is lost.

Traditional power steering systems attempt to control the torque applied by the driver to within limits. This can easily lead to a system where there are none of the haptic features described above and may have little or no change in torque with vehicle dynamic state. The design of the steering geometry also has a significant effect on the feel of the steering. If the geometry is such that there is no build up of steering torque, or drop in torque once the limit is reached, then no simple power steering system will be able to put that feel back. The present system considers what yaw rate the driver is demanding and the actual yaw rate of the vehicle to determine what the torque im the steering system should be. The assist torque generated by the power steering system is then adjusted accordingly. The proposed system therefore produces steering feel which is independent of the power steering system and the steering geometry.

Claims (8)

1. An electric assisted steering system for a motor driven road vehicle, the system including assist torque signal generating means arranged to generate an assist torque signal for the steering system in response to the driver's applied torque and sensed vehicle speed and effective to reduce the driver's steering effort, and a means for generating a haptic torque based upon vehicle yaw rate error which is arranged to be added to the torque assist signal such that when the yaw rate error builds up, corresponding to increasing steering instability of the vehicle, the haptic torque added to the torque assist signal reduces the effective road reaction feedback sensed by the driver in advance of any actual vehicle stability loss whereby to allow the driver to correct appropriately in good time before terminal steering instability is reached.

2. A steering system as claimed in claim 1, wherein the assist torque signal generating means comprises an electric motor.

3. A steering system as claimed in claim 2, wherein yaw rate error is established by comparing an estimated yaw rate derived from measured values of steering angle and vehicle longitudinal velocity, with measured vehicle yaw rate.

4. A steering system as claimed in claim 3, wherein the yaw rate error is saturated, if necessary, to prevent excessive demand and scaled by a gain map.

5. A steering system as claimed in claim 4, wherein the gain is controlled in accordance with yaw rate error, a low yaw rate resulting in a relatively low gain and a high yaw rate resulting in a relatively large gain so as to increase the assist torque from the power steering and make the steering feel light to the driver

6. A steering system as claimed in claim 4 or 5, wherein a plurality of gain maps are provided, the most suitable to comply with the prevailing conditions being arranged to be selected automatically from a judgement of road surface conditions based on measured data.

7. A steering system as claimed in claim 4. 5 or 6, wherein the haptic torque is established by scaling the steering column torque using the scaled yaw rate error, the haptic torque being added to the torque assist to provide an output for driving the electric motor.

8. An electric assisted steering system substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.