GB2343016A

GB2343016A - Method of and control means for controlling vehicle speed in curved travel

Info

Publication number: GB2343016A
Application number: GB9923899A
Authority: GB
Inventors: Johannes Schmitt
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1998-10-20
Filing date: 1999-10-08
Publication date: 2000-04-26
Anticipated expiration: 2019-10-08
Also published as: GB2343016B; JP2000127798A; DE19848236A1; GB9923899D0

Abstract

A method for controlling the speed of a vehicle prior to travelling around a bend comprises the steps of determining a target speed (V<SB>soll</SB>) by an iterative process in dependence on the vehicle actual speed (VFZ), the maximum transverse acceleration (ay<SB>max</SB>) and curve radius (R) of the bend. The target speed approaches a limit speed at which the curve can be travelled through safely. The speed of the vehicle is controlled in dependence on this target speed (V<SB>soll</SB>) and the actual speed (VFZ).

Description

2343016 METHOD OF AND CONTROL MEANS FOR CONTROLLING VEHICLE SPEED IN

CURVED TRAVEL The present invention relates to a method of and control means for controlling the speed of a vehicle for travel along a curved path.

Systems for limiting the speeds of vehicles are known. A travel speed limitation, in which the travel speed is limited to safe values during curvilinear travel, is known from DE-A 42 05 978 (US 5 485 381). In that case, the radius or degree of curvature of a path lying ahead is determined by navigation equipment for land vehicles from the stored geographic data. In one embodiment, a transverse acceleration of the vehicle is computed from the instantaneous speed of the vehicle and the radius of a curve lying ahead and compared with a preset maximum value, and a warning signal is generated in the case of expected exceeding of the maximum transverse acceleration. In another embodiment, a limit speed at which the relevant curved section of road can be travelled through without danger is computed on the basis of the degree of curvature, subject to consideration of the coefficient of friction of the vehicle and road surface and optionally also further environmental data. By fixing the limit speed, the transverse acceleration of the vehicle is also limited to an non-critical value. In order to set the limit speed, the speed is reduced in good time before travelling on the curved section. In that case, a retardation value which ensures that the limit speed value is reached at a target point at the entry to the curve, is computed in dependence on the initial speed of the vehicle and in dependence on the distance from the curved section.

Both procedures described in the afore-mentioned publication ensure that no instability of the vehicle occurs during a curvilinear travel. In order to reach the computed limit speed at the curve entry, a speed course, which leads to attainment of the limit speed at the desired point, is computed on the basis of the actual speed, the limit speed, a desired retardation and the distance from the curve.

There remains a need for a control by which, for example, a safe limit speed can set in automatically at the entry to a curve.

According to a first aspect of the invention there is provided a method for limiting the speed of a vehicle, wherein a limit speed, which ensures a safe curvilinear travel, is 2 determined on the basis of the radius of a curve to be travelled through, wherein the speed of the vehicle is limited to this limit speed before the curve, characterised in that a target speed is ascertained by an iterative process on the basis of a preset maximum transverse acceleration, the actual vehicle speed and the radius of the curve, wherein this target speed approaches the limit speed, and the vehicle speed is controlled on the basis of the actual speed and the target speed.

By means of a method exemplifying the invention, skidding of the vehicle in the curve may be able to be prevented even when the original travel speed was too high. A limit speed is maintained by which the curvilinear travel can be managed without risk of instabilities. It is advantageous in this case that this limit speed sets in automatically without expensive computations, in particular when the speed of the vehicle is too high for the curve concerned. This is because an approach to this limit speed takes place by iterative computation of the limit speed.

It is of particular advantage if, the greater actual vehicle speed, the smaller the limit speed computed in each iteration step. An appropriate braking of the vehicle before the curve is thereby ensured. Through use of a suitable regulator, the vehicle is then initially retarded more strongly in order to gently approach the actual speed limit value directly before entry into the curve.

It is particularly advantageous to provide a speed reduction by engine torque reduction and/or braking torque increase, in particular in the case of an actual speed exceeding the target speed.

For preference, the coefficient of friction is detected and taken into consideration in the computation of the limit speed. It is also advantageous, during the computation of the limit speed, to take into consideration a wet travel surface recognised by, for example, a rain sensor. It is particularly advantageous if the instant of initiation of the action to reduce the travel speed is preset in dependence on the distance from the curve, the travel speed and the coefficient of friction. In dependence on these magnitudes, the intensity of the retarding action can also be varied.

For preference, the vehicle speed is limited only until reaching of the curve crest point, the limitation being cancelled thereafter so that the vehicle can be accelerated to the extent 3 desired by the driver. This leads to comfortable travelling behaviour accustomed to the driver, since the dangerous region of the curvilinear travel is so managed that an instability of the vehicle is virtually prevented.

The distance from the curve as well as the radius of the curve to be travelled through can be supplied by a navigation system and/or a GPS receiver.

For preference, the reduction in the vehicle speed is performed by action on the engine, downward gear change and/or clutch disengagement. Preferably, also, the driver is informed of the action.

According to a second aspect of the invention there is provided control means for limiting the speed of a vehicle, comprising a control unit which on the basis of the curve radius of the curve to be travelled through computes a limit speed ensuring a safe curvilinear travel, and a speed limiter which limits the speed of the vehicle to this speed before the curve, characterised in that the control unit comprises means to ascertain a target speed by an iterative process on the basis of a preset maximum transverse acceleration, the actual vehicle speed and the radius of the curve, wherein this target speed approaches the limit speed, and the limiter controls the vehicle speed on the basis of the actual speed and the target speed.

An example of the method and embodiment of the control means according to the invention will now be more particularly described with reference to the accompanying drawings, in which:

Fig. 1 is a schematic block diagram of a control unit for the controlling of a braking system and/or the drive train of a vehicle; Fig. 2 is a flow chart illustrating the steps of a method exemplifying the invention; and Figs. 3a and 3b are time diagrams illustrating aspects of steps in the method.

Referring now to the drawings, there is shown in Fig. 1 a control unit 10 for the control of the drive unit of a motor vehicle and/or its braking system. The control unit 10 essentially 4 comprises an input circuit 14, a microcomputer 12, an output circuit 16 and a communications system 18 connecting these components. Different input lines, or a data bus, by way of which signals indicative of different operating magnitudes of the vehicle are supplied, are connected to the input circuit 14. Signals which represent the wheel speeds of the vehicle and are provided by measuring devices 26 to 30 are fed by way of input lines 20 to 24 to the input circuit 14. Other operational magnitudes of the vehicle, such as from its braking system, drive unit and so forth, provided by measuring devices 42 to 46 are fed to the circuit 14 by way of input lines 36 to 40. Signal magnitudes of that kind can be, for example, engine rotational speed, steering angle, yaw rate and so forth, such as may be used in conjunction with the control of the braking system and/or the drive unit. These operational magnitudes can be measured directly or be computed in the microcomputer 12 from other sensor signals. The control unit 10 is also connected by way of an input line 52 with a navigation system 54 or a GPS receiver. A magnitude, which corresponds with the radius of curvature of a curve lying ahead of the vehicle, preferably the smallest radius of curvature of the curve, and the distance between the actual position of the vehicle and this curve, are fed by way of the input line 52 to the control unit 10. An example of the formation and computation of these signals is present in the aforementioned prior art publication. This computation takes place, in one example, in the microcomputer 12 of the control unit 10 and in another example in the system 54. The output circuit 16 is linked to output lines which control the drive unit and/or the braking system of the vehicle. At least one setting element 50 which influences the power of the drive unit, for example an electrically controllable throttle flap, is controlled by way of an output line 48 and at least one setting element 58 which controls the braking force in at least one wheel brake, for example a valve arrangement for traction control or electricalmechanical brake setters, are controlled by way of the output line 56.

The control unit 10 represents a control unit for control of the power of the drive unit of the vehicle and/or for control of the braking system within the scope of traction regulation or regulation of dynamic travel range. In modem vehicle control systems, control units of that kind are interlinked with each other by way of bus systems. This also applies to the interlinking of such a control unit with a vehicle navigation system or a GPS-receiver so that the smallest radius of a curve that is approaching and the distance therefrom or magnitudes from which these values can be computed are made available to the control unit 10. According to the procedure described below, a permissible limit speed for at least one friction value is computed from these data and the vehicle speed is so regulated by engine torque reduction and/or braking torque increase that the limit speed is reached at the entry to the curve. The intensity and the instant of the actions is dependent on the distance, the vehicle speed and optionally also the coefficient of friction of the road surface. The coefficient of friction is ascertained by means of friction value recognition. The limit speed reduces with reducing friction value. In another example, a rain sensor is deployed and delivers a signal on the basis of which wetness is recognised. In the case of recognised wetness, the limit speed is similarly reduced.

The speed control is based on the following relationships:

co = VFZ / R (1) co-= ay / VFZ (2) in which (o is the angular speed of the vehicle, R is the (smallest) curve radius, VFZ is the vehicle speed and ay is the transverse acceleration.

For computation of the limit speed, on the basis of the distance, which is dependent on the vehicle speed and in a given case the coefficient of friction, from the curve to be travelled through, a target speed is determined by the afore-mentioned equations as a function of the maximum transverse acceleration preset in dependence on the coefficient of friction, of the communicated curve radius and of the vehicle speed ascertained or measured on the basis of the wheel speed signals. This target speed is compared with the actual speed and, in the case of greater actual speed than the target speed, the vehicle speed is reduced by braking action and/or engine torque reduction. This process is iterative, wherein the limit speed increases with reducing vehicle speed until an equilibrium state is reached between the two magnitudes. This equilibrium state determines the limit speed at which the curve can be travelled through safely. This vehicle speed, which is reached at the curve entry, is maintained up to the curve crest point, whereafter the limitation is cancelled and the vehicle can be accelerated. The exact starting point at which the speed limitation sets in before the curve is dependent on the vehicle speed and the coefficient of friction, wherein the distance at which the afore-described procedure is initiated is likewise applied in dependence on the mentioned magnitudes. In a preferred example, the limitation is performed by a limitation regulator with at least one proportional component so that the intensity of the action is dependent on the magnitude of the deviation between the target speed value and the actual speed value. In another example, the intensity of the 6 action is dependent, for example by choice of regulator constants, on the distance, the speed and the road surface friction value, wherein, for example, the constants of the limitation regulator become greater as the distance becomes smaller and at higher speed and become smaller with reducing coefficient of friction of the road surface.

A preferred example of the described method is illustrated in Fig. 2, which shows steps in a computer program. This program is initiated and run through at preset time intervals when the necessity of an action before a curve is recognised, i.e. the distance from the curve to be travelled through falls below the limit value computed for the action. In a first step 100, a curve radius R and the vehicle speed VFZ are read in. Thereafter, in a step 102, the angular speed is computed according to the above equation (1) and, in a step 104, a target speed (V,,,,) for the travel through the curve is determined (equation 2, Vsdj = aym,/co) on the basis of the angular speed and the preset maximum transverse acceleration. It is then checked in a step 106 whether the target speed Vs,11 is smaller than the actual speed VFZ. If this is the case, the vehicle speed is reduced in a step 108. If this is not the case, it is checked in a step 110 whether the target speed is greater than the actual speed. If this is not the case, the vehicle speed is maintained in a step 112 and in the other case the vehicle speed is increased in a step 114, in a given case in dependence on the coefficient of friction, and then limited in a step 116 by the present wish of the driver (for example accelerator pedal setting). After the steps 108, 112 and 116, it is checked in a step 118 whether the curve crest point is reached. This takes place in a preferred example on the basis of a comparison of the actual position of the vehicle with the position of the curve crest point through comparison of successive radii, which after the crest become greater, or by determination of the curve centre point. If the curve crest point is not reached, the program is repeated at step 100. Otherwise the limitation is cancelled in a step 120, the motor vehicle being accelerated in a given case in dependence on the coefficient of friction and the program being terminated.

This manner of procedure is indicated by the diagrams of Figs. 3a and 3b. Fig. 3a shows the course over time of the vehicle speed and Fig. 3b shows a bivalent indication of whether curvilinear travel is present or not. The vehicle is assumed to travel at a certain speed. At the instant To, the distance, which is preset for limiting the speed according to the procedure described above is reached, whereafter the vehicle speed is reduced according to Fig. 3a until the instant T1, the beginning of the curvilinear travel, where the limit value is then reached. This speed is maintained within the curve until the instant T2, 7 after which the limitation is cancelled following attainment of the crest point of the curve (for example, curve centre point) and the vehicle is accelerated. At the instant T3, the curvilinear travel has ended and the vehicle is accelerated at the wish of the driver.

The method can be further clarified with the aid of the following numerical example, in which there is assumed a vehicle speed of 100 kilometres per hour = 27.7 metres per second, a radius of 50 metres and a maximum transverse acceleration of 10 metres per second per second. According to the individual program sequences, the following results:

First run-through: (o = VFZ/R = (27.7m/s/50m) = 0.55 I/s Vw, = ay./w = 1 Orn/s/0.55 I/s = 18 m/s = 65 km/h Since VFZ is greater than V,,.,,, the speed is reduced.

Next run-through at VFZ = 90 km/h: (o = VFZ/R = (25m/s/50m) = 0.5 I/s Vwl = ay,,,,x/co = 1 Om/s2/0.5 I/s = 20 m/s = 72 km/h Since VFZ is greater than Vs,,,,, the speed is reduced.

Next run-through at VFZ = 80 kmfh: (o = VFZ/R = (22.3m/s/50m) = 0.45 I/s V,01 = aym,,x/o = I Orn/s/0.45 I/s = 22.2 m/s = 80 km/h Since VFZ is the same as Vs,11, the speed is maintained.

The described method can be applied in conjunction with a traction regulation with engine action, dynamic travel range regulation, for example an ESP system, or another stability system which makes available an engine action.

In addition or alternatively to the engine action, the reduction in the vehicle speed can be performed by changing-down a gear and/or by declutching.

When the above-mentioned action is activated and the vehicle speed is reduced during curvilinear travel, the ddver is informed about the action, for example by way of a warning lamp.

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Claims

1 A method of controlling the speed of a vehicle for travel along a curved path, comprising the steps of determining a limit speed for safe travel of the vehicle along the path in dependence on the radius of curvature of the path, determining a target speed, which approaches the limit speed, for travel of the vehicle along the path by an iterative process in dependence on the radius of curvature of the path, the actual speed of the vehicle and a predetermined maximum transverse acceleration of the vehicle, and controlling the speed of the vehicle in dependence on the relationship of the actual speed and the target speed.

2. A method as claimed in claim 1, wherein the step of controlling comprises reducing the vehicle speed by at least one of reduction in vehicle drive torque and increase in vehicle braking torque in response to exceeding of the target speed by the actual speed.

3. A method as claimed in claim 1 or claim 2, wherein the step of determining the target speed is carried out before travel of the vehicle along the path.

4. A method as claimed in any one of the preceding claims, wherein the step of determining the target speed is initiated in dependence on distance of the vehicle from the start of the curved path.

5. A method as claimed in any one of the preceding claims, comprising the step of predetermining the maximum transverse acceleration in dependence on at least one of the coefficient of friction between the vehicle and a surface on which the vehicle is travelling and recognised wetness of the environment of the vehicle.

6. A method as claimed in any one of the preceding claims, comprising the step of determining the radius of curvature of the path by means of at least one of a navigation system and a receiver for global positioning system signals.

7. A method as claimed in claim 6 when dependent on claim 4, comprising the step of determining said distance of the vehicle by means of at least one of the navigation system and the receiver.

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8. A method as claimed in any one of the preceding claims, wherein the step of controlling comprises controlling the speed during travel of the vehicle along the path until and only until the vehicle has reached a predetermined crest point of the path.

9. A method as claimed in any one of the preceding claims, wherein the step of controlling is initiated at a time and with an intensity dependent on at least one of the distance of the vehicle from the start of the path, the actual speed of the vehicle and the coefficient of friction between the vehicle and a surface on which the vehicle is travelling.

10. A method as claimed in claim 1, wherein the step of controlling comprises reducing the vehicle speed by at least one of action on the vehicle engine, downward gear change of the vehicle transmission and disengagement of the vehicle clutch.

11. A method as claimed in any one of the preceding claims, comprising the step of providing an indication of the controlling of the vehicle speed to the vehicle driver.

12. A method as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

13. Control means for controlling the speed of a vehicle for travel along a curved path, comprising determining means for determining a limit speed for safe travel of the vehicle along the path in dependence on the radius of curvature of the path and determining a target speed, which approaches the limit speed, for travel of the vehicle along the path by an iterative process in dependence on the radius of curvature of the path, the actual speed of the vehicle and a predetermined maximum transverse acceleration of the vehicle, and speed limiting means for controlling the speed of the vehicle in dependence on the relationship of the actual speed and the target speed.