Method for controlling braking of a vehicle

Abstract

The invention provides a method for controlling braking of a vehicle (1) driving along a downhill portion of a road, the vehicle comprising a propulsion arrangement (2, 3), for the propulsion of the vehicle, the method comprising dividing the road portion into a plurality of sections (RS0-RS2), the sections comprising a first section (RS1), and a second section (RS2) following, in the direction of travel of the vehicle, immediately upon the first section (RS1), determining, for the road portion, a road portion control strategy, with a condition that braking on the road portion is done at least partly by means of the propulsion arrangement (2, 3), wherein determining the road portion control strategy comprises determining a speed (SD21), on the second section (RS2), with an aim to minimize the time travelled on the second section, and/or, where the propulsion arrangement comprises an internal combustion engine (2), and a gearbox (3), determining a gear selection (GS2) on the second section (RS2), with an aim to minimize the time travelled on the second section, and wherein determining the road portion control strategy comprises determining, for the first section (RS1), a first section control strategy, with an aim to minimize the time travelled on the first section, and with an aim to provide a vehicle speed at the end of the first section (RS1) which is the same as said determined speed (SD21) on the second section (RS2), and/or to provide a gear selection at the end of the first section which is the same as said determined gear selection (GS2) on the second section (RS2), the method further comprising controlling the vehicle (1) according to the determined road portion control strategy.

Claims

1. A method for controlling braking of a vehicle driving along a downhill portion of a road, the vehicle comprising a propulsion arrangement, for the propulsion of the vehicle, the method comprising: dividing the road portion into a plurality of sections, the sections comprising a first section, and a second section following, in a direction of travel of the vehicle, immediately upon the first section, determining, for the road portion, a road portion control strategy, with a condition that braking on the road portion is done at least partly by means of the propulsion arrangement, wherein determining the road portion control strategy comprises: determining a speed, on the second section, to minimize time travelled on the second section, and/or, where the propulsion arrangement comprises an internal combustion engine, and a gearbox, determining a gear selection on the second section, to minimize the time travelled on the second section, and wherein determining the road portion control strategy comprises determining, for the first section, a first section control strategy, to minimize the time travelled on the first section, and to provide a vehicle speed at an end of the first section which is the same as said determined speed on the second section, and/or to provide a gear selection at the end of the first section which is the same as said determined gear selection on the second section, wherein the method further comprising controlling the vehicle according to the determined road portion control strategy.

2. The method according to claim 1, wherein the speed, on the second section, is determined before the first section control strategy is determined.

3. The method according to claim 1, wherein the first section control strategy minimizes the time travelled on the first section, and provides a speed at the end of the first section which is the same as said determined speed on the second section.

4. The method according to claim 1, wherein the first section has a constant declination.

5. The method according to claim 1, where the vehicle comprises service brakes, wherein determining the road portion control strategy is done with the condition of avoiding the use of the service brakes on the road portion.

6. The method according to claim 1, wherein determining the road portion control strategy comprises determining, for the second section, a second section control strategy, the second section control strategy determination comprising the determination of the second section speed.

7. The method according to claim 6, wherein the second section control strategy is determined before the determination of the first section control strategy.

8. The method according to claim 6, wherein determining the second section control strategy determination comprises determining, for the second section, a second section speed profile.

9. The method according to claim 8, wherein said determined speed on the second section occurs at a plurality of positions within the first 10% of the second section.

10. The method according to claim 1, wherein determining the first section control strategy comprises determining, for the first section, a first section speed profile.

11. The method according to claim 1, wherein the gear selection, on the second section, is determined before the first section control strategy is determined.

12. The method according to claim 11, wherein the gear selection strategy on the first section, and/or the gear selection on the second section is determined at least partly based on data indicative of the maximum available braking force for each gear, as a function of the vehicle speed.

13. The method according to claim 1, wherein determining the road portion control strategy comprises determining, for the second section, a second section control strategy, the second section control strategy determination comprising the determination of the second section speed, and the determination of the second section gear selection.

14. The method according to claim 1, wherein determining a strategy for a setting, on the first section, and/or a setting, on the second section, of a device for controlling an engine braking force of the engine.

15. The method according to claim 1, wherein the first section control strategy, the second section speed, and/or the second section gear selection, is determined in dependence on an engine exhaust temperature, and/or a de-rating of the engine in dependence on the exhaust temperature.

16. The method according to claim 1, wherein determining the first section control strategy comprises determining a plurality of speed profile candidates, and selecting, for the first section, a first section speed profile, from the profile candidates.

17. The method according to claim 1, wherein: determining the first section control strategy comprises determining, for the first section, a first section speed profile, wherein the sections comprise a preceding section, preceding, in the direction of travel of the vehicle, immediately before the first section, wherein determining the road portion control strategy comprises determining, for the preceding section, a preceding section control strategy, to minimize the time travelled on the preceding section, and to provide a vehicle speed at the end of the preceding section which is the same as a speed of the first section speed profile, occurring at one or more positions within the first 10% of the first section.

18. A computer program comprising program code for performing the method according to claim 1 when said program code is run on a computer, or a group of computers.

19. A control unit configured to perform the steps of the method according to claim 1.

20. A vehicle comprising a control unit according to claim 19.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

(2) In the drawings:

(3) FIG. 1 shows a vehicle in the form of a truck.

(4) FIG. 2 depicts parts of an engine of the vehicle in FIG. 1.

(5) FIG. 3 is a flow diagram depicting stages in a method according to an embodiment of the invention.

(6) FIG. 4a shows a schematic vertical cross-section along a route travelled by the vehicle in FIG. 1, with the altitude a as a function of the horizontal distance travelled s.

(7) FIG. 4b shows a reference speed profile, and a determined speed profile for the vehicle in FIG. 1, with the vehicle speed v as a function of the horizontal distance travelled s.

(8) FIG. 5 shows a diagram indicating, for each gear of a gearbox of the vehicle in FIG. 1, the maximum available engine braking force, as a function of the vehicle speed.

(9) FIG. 6 is a flow diagram depicting stages in a method according to a more general embodiment of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

(10) FIG. 1 depicts a heavy-duty vehicle 1 in the form of a truck. The vehicle comprises a drivetrain. The drivetrain comprises a propulsion arrangement 2, 3, for the propulsion of the vehicle. The propulsion arrangement comprises an internal combustion engine 2, and a gearbox 3. The gearbox is in this embodiment an automatic transmission. The gearbox has in this example twelve gears, but a different number of gears is of course possible. A control unit CU is provided to control the engine 2 and the gearbox 3. It is understood that the control unit CU may be provided as a single physical unit, or as a plurality of physical units. The vehicle further comprises service brakes, (not shown).

(11) As can be seen in FIG. 2, the engine in this example comprises six cylinders arranged in a line. In this example the engine is a diesel engine. However, the invention is equally applicable to other types of engines, such as Otto engines.

(12) The engine comprises a turbocharger 205, an air guide 202 with a charge air cooler 206, and a fuel system (not shown). The exhaust conduit arrangement comprises two exhaust guides 203, each arranged to guide a gas flow from a respective group of the cylinders, to a turbine of the turbocharger 205. In each exhaust guide, an adjustable exhaust flow restriction element 204 is provided, in the form of an exhaust throttle valve. The exhaust flow restriction elements 204 are controllable by the control unit CU. Each of the flow restriction elements 204 is arranged to provide a plurality of levels of the exhaust flow restriction depending on the adjustment by the control unit CU of the respective exhaust flow restriction element 204.

(13) The control unit CU is arranged to determine a requested engine braking torque. The control unit CU is further arranged to control an actual engine braking torque, towards the requested engine braking torque, by means of the exhaust flow restriction elements 204. For this control, the control unit CU obtains values of the engine rotational speed, which may be determined by means of an engine speed sensor. The control unit CU may be provided with stored data correlating values of the engine torque and the engine rotational speed with settings for the exhaust flow restriction elements 204. Further input parameters in the engine break control may be the pressure in the exhaust guides, and/or the air flow in the air guide 202, determined e.g. by means of an air flow sensor (not shown). Further, for the engine brake control, the control unit CU may be arranged to access stored data correlating values of the engine torque and the engine rotational speed with desired values of the air guide pressure.

(14) Alternatively, a single exhaust guide, with a single exhaust flow restriction element 204, may be arranged to guide exhaust gases from all cylinders of the engine. In some embodiments, an exhaust flow restriction element 204 may be provided downstream of the turbine of the turbocharger 205. In further embodiments, the engine braking torque may be controlled by means of a variable geometry turbocharger.

(15) FIG. 3. depicts steps in a method, according to an embodiment of the invention, for controlling engine braking of a vehicle 1 driving along a downhill portion of a road.

(16) Reference is made also to FIG. 4a. The method comprises dividing S1 the road portion into a plurality of sections RS0-RS2. The sections comprise what is referred to herein as a first section RS1, and a second section RS2 following, in the direction of travel of the vehicle, immediately upon the first section RS1. The division of the road portion is made such that each section RS0-RS2 has a substantially constant declination. Thus, stretches forming a transition between two road inclinations, form divisions between adjacent sections RS0-RS2. Further, the division of the road portion is made so that curves, entailing a vehicle speed restriction, also form divisions between adjacent sections. The road portion division may be done at least partly based on map data.

(17) Reference is made also to FIG. 4b. The method may further comprise obtaining a reference speed profile SPr for the road portion. The reference speed profile SPr may be a stored speed profile. The reference speed profile SPr may have recorded during a previous travel of the vehicle 1, or another vehicle, on the road portion.

(18) The method further comprises determining, for the road portion, a road portion control strategy. The road portion control strategy is determined with a condition that braking on the road portion is done by means of the propulsion arrangement 2, 3 only. Thus, determining the road portion control strategy is done with the condition of avoiding the use of the vehicle service brakes on the road portion. As suggested above, in some embodiments, where the vehicle comprises a retarder, the road portion control strategy may be determined with a condition that braking on the road portion is done by means of the propulsion arrangement, and the retarder only.

(19) The road portion control strategy is determined with a calculation that is opposite to the vehicle travel direction. Thus, a speed profile for a road section RS1 is determined partly based on a previously determined speed profile for a road section RS2 following immediately after the road section RS1, for which a speed profile is to be determined.

(20) Thus, the road portion control strategy determination comprises determining S3, for the second section RS2, a speed profile, referred to herein as a second section speed profile. Determining the second section speed profile comprises S2 determining a speed SD21, on the second section RS2, with an aim to minimize the time travelled on the second section. Thus, the determined speed SD21 is the highest speed that the vehicle engine brake allows in the second section. Similarly to what will be described below regarding the first section, the second section speed profile is adapted to a speed for the following, third section. Thereby, an end portion of the second section RS2 will have a speed profile, which gradually adjusts, from the highest allowed speed SD21, to said third section speed. Thus, an initial part of the second section will have constant speed profile with the highest allowed speed SD21. In this example, said highest allowed speed SD21 on the second section occurs at all positions within the first 75% of the second section RS2.

(21) The road portion control strategy determination further comprises determining S3, for the second section RS2, a gear selection strategy, referred to herein as a second gear selection strategy. Determining the second gear selection strategy comprises determining a gear selection GS2 on the second section RS2. This is done similarly to how it is done for the first section RS1, as described below. In this example, as can be seen in FIG. 4a, the gear selection GS2 on the second section RS2 is the 5th gear.

(22) Subsequently, a first section control strategy is determined S4 for the first section RS1, with an aim to minimize the time travelled on the first section, and to provide a vehicle speed at the end of the first section RS1 which is the same as said determined speed SD21 on the second section RS2. Determining the first section control strategy comprises determining, for the first section, a first section speed profile SP1. It should be noted that in some embodiments, the second section speed profile does not have to be determined before the first section speed profile SP1 is determined. In some embodiments, the first section speed profile SP1 may be determined upon merely determining said speed SD21 on the second section RS2.

(23) Further, the first section control strategy determination comprises determining S4, for the first section RS1, a first section gear selection strategy, with an aim to provide a gear selection at the end of the first section which is the same as said determined gear selection GS2 on the second section RS2. It should be noted that in some embodiments, the second section gear selection strategy does not have to be determined before the first section gear selection strategy is determined. In some embodiments, the first section gear selection strategy may be determined upon merely determining said gear selection GS2 on the second section RS2.

(24) The determinations of the first section speed profile SP1, and the first section gear selection strategy, are in this embodiment performed as follows:

(25) Forces acting on the vehicle are determined. This could be done using a vehicle model. The vehicle model could be stored in the control unit CU. A force pulling the vehicle in the direction of travel is a component of gravity {right arrow over (F.sub.g)}(t) acting along the road, which component is dependent on the road declination. Opposite to the direction of travel there is a resistive force {right arrow over (F.sub.r)}(t), resisting the vehicle motion. The resistive force could be the result of a plurality of contributors, such as air drag, rolling resistance, driveline and engine frictions, etc. It is understood that the resistive force may be dependent on the vehicle speed. The effective force accelerating the vehicle downhill is the difference between said gravity component and the resistive force. In order to keep the vehicle speed constant, an engine braking force {right arrow over (F.sub.b)}(t) of the engine needs to be equal to said difference. This could be expressed as follows:
{right arrow over (F.sub.g)}(t)+{right arrow over (F.sub.r)}(t)+{right arrow over (F.sub.b)}(t)=0  (1)

(26) The braking force {right arrow over (F.sub.b)}(t) is dependent on the gear selection, as exemplified below. In addition, the braking force {right arrow over (F.sub.b)}(t) may be dependent on an engine oil temperature, limitations due to the road being slippery, the engine or output shaft speed, etc. The dependence on such operational circumstances may be included in the vehicle model.

(27) Determining the first section speed profile SP1 may be achieved by minimizing a cost function. Minimizing the cost function may involve finding a first section speed profile SP1, which maximizes the integral of the speed profile, while avoiding the use of the service brakes. The integral of the speed profile SP1 is the area under the line SP1 in FIG. 4b. Thus, the integral may have the distance travelled s as an independent variable. It should be noted that such a cost function may be used without the involvement of the reference speed profile SPr.

(28) Alternatively, the first section speed profile SP1 may be achieved by the minimization of the following cost function:

(29) $\begin{array}{cc}\underset{{\varphi }_i,p_1,p_2}{\min }.Math.{\int }_{s=0}^{S_d}\Phi \left(s\right)\mathrm{ds}-{\int }_{s=0}^{S_d}\varphi \left(s|{\varphi }_i,p_1,p_2\right)\mathrm{ds}.Math.& \left(2\right)\end{array}$
Where
ϕi is the entry (initial) speed on the first section RS1,
ϕ(⋅), i.e. the function in the first integral, is the reference speed profile SPr on the first section RS1;
ϕ(⋅), i.e. the function in the second integral, is the final (optimized) speed profile, i.e. the first section speed profile SP1,
(p.sub.1, p.sub.2)∈.sup.2×1 are cardinals pairs used to define a function of the first section speed profile SP1,
s is the distance travelled, i.e. an independent variable indicating the displacement within the first section.

(30) Preferably, the cost function is solved by a numerical optimization method. In this embodiment, this optimization method comprises a stochastic/analytical hybrid algorithm with the following steps:

(31) Based at least partly on the vehicle mass, and the declination of the first section RS1, the gravity component {right arrow over (F.sub.g)}(t), the resistive force {right arrow over (F.sub.r)}(t) are determined. The resistive force may be determined based on a speed of the reference speed profile. In some embodiments, the resistive force could be approximated as speed independent, e.g. by ignoring the air drag contribution. As follows from equation (1) above, the absolute value of the required engine braking force {right arrow over (F.sub.b)}(t) is equal to the sum of the gravity component {right arrow over (F.sub.g)}(t), and the resistive force {right arrow over (F.sub.r)}(t), (one being positive and the other negative).

(32) Reference is made also to FIG. 5. Due to the declination being constant within the first section RS1, it is possible to find a constant gear and speed pair for the vehicle. The sum of the gravity component {right arrow over (F.sub.g)}(t), and the resistive force {right arrow over (F.sub.r)}(t) gives a value of the required engine braking force {right arrow over (F.sub.b)}(t) on the vertical axis in the diagram in FIG. 5. The diagram in FIG. 5 indicates, for each gear, the maximum available braking force, as a function of the vehicle speed. As can be seen increasing gears generally give a lower engine braking forces. As can be seen in the diagram, in this example, the highest gear, which will keep the vehicle at a constant speed in the first section RS1 is the 7.sup.th gear. Thus, a gear selection GS1 on the first section RS1 is the 7.sup.th gear.

(33) Determining the first section speed profile comprises determining a speed SD12, on the first section RS1, with an aim to minimize the time travelled on the first section. the determined speed SD12 is the highest speed that the vehicle engine brake allows in the first section. In this example, as can be seen in FIG. 5, the maximum braking force for each gear is, at an upper speed interval of the full speed interval for the respective gear, essentially constant, or less increasing with the speed, compared to at a lower speed interval of the full speed interval for the respective gear. Said upper speed interval for the 7.sup.th gear is indicated with the interval end values V1 and V2. In this embodiment, the highest allowed speed SD12 is selected to be within the upper speed interval V1-V2. The highest allowed speed SD12 may be at the upper end of the upper speed interval V1-V2.

(34) As can be seen in FIG. 5, the required engine braking force is below the maximum braking force for the 7.sup.th gear. To keep the required engine braking force, a setting, on the first section RS1, of the adjustable exhaust flow restriction element 204, is determined.

(35) As understood, the determined first section highest allowed speed SD12, and the determined second section highest allowed speed SD21, are different. Also, the gear selection GS1 on the first section RS1 is not the same as the gear selection GS2 on the second section RS2. The speed profile of the first section is determined so as to provide a vehicle speed at the end of the first section RS1 which is the same as said highest allowed speed SD21 on the second section RS2. Thereby, the first section highest allowed speed SD12 becomes an initial speed on the first section.

(36) Further, the gear selection strategy of the first section is determined so as to provide a gear selection at the end of the first section RS1 which is the same as the gear selection GS2 on the second section RS2. Therefore, the optimization method for the first section speed profile includes, as boundaries, the second section highest allowed speed SD21, and the second section gear selection GS2, at the end of the first section RS1.

(37) The optimization method involves determining, or generating, a plurality of speed profile candidates for the first section RS1. Each speed profile candidate includes a respective intermediate speed SD11 at a position between the start and the end of the first section RS1. The intermediate speed SD11 is different from one profile candidate to another. For the first section, the first section speed profile SP1 is selected from the profile candidates. The criteria for this selection is to minimize the time travelled on the first section. Each speed profile candidate may be associated with a respective gear selection strategy. The optimization may take into account that a gear change will increase the speed of the vehicle. This may be taken into account for selecting between a gear strategy which changes from one gear to another via one or more intermediate gears, and a gear strategy which changes directly from the one gear to the other gear.

(38) The optimization method may involve the use of CMA-ES (Covariance Matrix Adaptation Evolution Strategy). Alternatively, another strategy could be used for the optimization method, such as Cross-Entropy, or NSGA-II. The plurality of speed profile candidates for the first section RS1 may be randomly generated. The amount of generated speed profile candidates may be 50-200, e.g. 100. From the generated speed profiles, a sub-group is selected. The amount of speed profiles in the sub-group could form e.g. a quarter to half of the generated speed profiles. E.g. the amount of selected speed profiles could be 30. The selected speed profiles could be such that they provide the lowest values of the cost function mentioned above. In some embodiments, the selected speed profiles could be the speed profiles having the highest integral in the distance domain. Based on the selected sub-group of speed profiles, a further set of speed profiles is generated. Another selection is made from the speed profiles in the further set. The optimization method could involve a plurality of such iterations.

(39) As can be seen in FIG. 4a, the second section RS2 has a higher declination than the first section RS1. As can be seen in FIG. 4b, the first section speed profile SP1 has decreasing speeds at the end of the first section RS1.

(40) Subsequently, a control strategy for a preceding section RS0, preceding, in the direction of travel of the vehicle, immediately before the first section RS1, is determined. The preceding section control strategy is determined similarly to the first section control strategy, described above. Thus, the preceding section control strategy is determined with an aim to minimize the time travelled on the preceding section RS0, and to provide a vehicle speed at the end of the preceding section which is the same as said initial speed SD12 of the first section speed profile SP1.

(41) As stated, the division of the road portion may be made so that curves, entailing vehicle speed restrictions, also form divisions between adjacent sections. Thus, a curve, dividing two adjacent sections, may have a speed restriction. Thereby, a preceding of the adjacent sections may obtain a speed profile, such that the speed at the end of the preceding section is the speed according to the speed restriction. Further, a following of the adjacent sections may obtain a speed profile, such that the speed at the start of the following section is the speed according to the speed restriction. Thereby, the speed profile may have increasing speeds, after the start of the following section, up to a highest allowed speed on the following section.

(42) When the control strategies for all sections RS0-RS2 have been determined, they will together form S5 the control strategy for the entire road portion RS0-RS2. Thereupon, the vehicle 1 is controlled S6 according to the determined road portion control strategy.

(43) Many modifications may be made with the scope of the invention. For example, one or more section control strategies may be determined in dependence on an engine exhaust temperature, and/or a de-rating of the engine in dependence on the exhaust temperature.

(44) Alternative computations methods for the optimization method may be used, such as Evolutionary computation, Genetic Algorithms, Monte-Carlo, Tabu-Search, Neural networks, or Dynamic programing.

(45) FIG. 6 shows steps in a method, according to a more general embodiment of the invention, for controlling braking of a vehicle driving along a downhill portion of a road, the vehicle comprising a propulsion arrangement, for the propulsion of the vehicle. The method comprises dividing S1 the road portion into a plurality of sections, the sections comprising a first section, and a second section following, in the direction of travel of the vehicle, immediately upon the first section. The method further comprises determining, for the road portion, a road portion control strategy, with a condition that braking on the road portion is done by means of the propulsion arrangement. Determining the road portion control strategy comprises determining S21 a speed on the second section, with an aim to minimize the time travelled on the second section. Determining the road portion control strategy comprises determining S41, for the first section, a first section control strategy, with an aim to minimize the time travelled on the first section, and to provide a vehicle speed at the end of the first section which is the same as said determined speed on the second section. The method further comprising controlling S6 the vehicle according to the determined road portion control strategy.

(46) It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.