A METHOD FOR ESTIMATING VEHICLE MOTION STATE DURING A VEHICLE MANEUVER

Abstract

A method in a vehicle for estimating vehicle motion state during a vehicle maneuver, comprising; obtaining a trigger signal indicating an onset of the vehicle maneuver, selecting a sub-set of wheels on the vehicle to be in a free-rolling condition, measuring one or more parameters related to revolution of the sub-set of wheels in free-rolling condition, and estimating the vehicle motion state based on the measured parameters.

Claims

1. A method in a vehicle for estimating vehicle motion state during a vehicle maneuver, wherein the vehicle motion state comprises vehicle position, the method comprising; obtaining a trigger signal indicating an onset of the vehicle maneuver, selecting a sub-set of wheels on the vehicle to be in a free-rolling condition, wherein the free-rolling condition comprises reduced breaking, measuring one or more parameters related to revolution of the sub-set of wheels in free-rolling or reduced braking condition, and estimating the vehicle motion state based on the measured parameters.

2. The method according to claim 1, wherein the vehicle maneuver is a safe stop maneuver requiring a controlled deceleration of the vehicle below a maximum deceleration capacity of the vehicle.

3. The method according to claim 1, wherein the vehicle maneuver is a sensor calibration maneuver, whereby one or more sensor systems of the vehicle are arranged to be calibrated against the estimated motion state data.

4. The method according to claim 1, wherein the subset of wheels is selected as the wheels on a selected wheel axle.

5. The method according to claim 4, wherein the selected wheel axle is the wheel axle associated with least vertical load compared to other wheel axles of the vehicle.

6. The method according to claim 4, wherein the selected wheel axle is a liftable wheel axle, wherein the method comprises lowering the liftable wheel axle prior to measuring the parameters.

7. The method according to claim 4, wherein the selected wheel axle is a wheel axle associated with low rotational inertia compared to one or more other axles of the vehicle.

8. The method according to claim 4, comprising adjusting a suspension air pressure associated with the selected axle by an amount.

9. The method according to claim 1, wherein the subset of wheels is selected from different wheel axles of the vehicle and from different sides of the vehicle.

10. The method according to claim 1, wherein wheels comprised in the subset of wheels is selected in dependence of vehicle stability, whereby the subset of wheels is selected to give an acceptable impact on vehicle stability depending on maneuver scenario.

11. The method according to claim 1, wherein the subset of wheels is selected as a single wheel.

12. The method according to claim 1, wherein the vehicle motion state comprises any of; global or relative position coordinates, vehicle heading, vehicle trajectory curvature, vehicle longitudinal speed, vehicle longitudinal acceleration, and vehicle yaw rate.

13. The method according to claim 1, comprising controlling the vehicle to execute the maneuver based on the estimated motion state and on a preferred maneuver track to be followed.

14. The method according to claim 1, comprising verifying a deceleration value in the motion state against a preferred deceleration value for the maneuver.

15. The method according to claim 1, further comprising estimating a road banking grade and/or a road slope based on the measured parameters and on obtained accelerometer data associated with the vehicle.

16. The method according to claim 1, further comprising estimating a road friction coefficient by comparing wheel revolution characteristics between the selected wheels in free-rolling condition and other wheels where braking or acceleration forces are applied.

17. A computer program comprising program code means for performing the steps of claim 1 when said program is run on a computer.

18. A computer readable medium carrying a computer program comprising program code means for performing the steps of claim 1 when said program product is run on a computer.

19. A control unit for estimating vehicle motion state during a vehicle maneuver, the control unit being 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

[0027] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples. In the drawings:

[0028] FIG. 1 schematically illustrate vehicle maneuvers;

[0029] FIG. 2 shows an example tractor and semi-trailer combination vehicle;

[0030] FIG. 3 schematically illustrates a tractor vehicle;

[0031] FIG. 4 is a flow chart illustrating methods;

[0032] FIG. 5 schematically illustrates a control unit; and

[0033] FIG. 6 shows an example computer program product.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

[0034] The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain aspects of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments and aspects set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

[0035] It is to be understood that the present invention is not limited to the embodiments described herein 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.

[0036] Herein, a vehicle motion state is assumed to describe the present state of the vehicle in terms of, e.g., position, velocity, acceleration, and turn-rate. The vehicle motion state may also comprise other quantities such as current road friction properties, current braking capacity depending on road conditions and the like. In general, vehicle motion state comprises any of; global or relative position coordinates, vehicle heading, vehicle trajectory curvature, vehicle longitudinal speed, vehicle longitudinal acceleration, vehicle yaw rate, and various environment variables. Vehicle motion states in general, vehicle motion state estimation based on sensor input signals, and the like is known and will therefore not be discussed in more detail herein.

[0037] Herein, a semi-trailer may refer to the trailer part of a tractor and semitrailer combination vehicle, or it may refer to the whole tractor and semitrailer combination vehicle. The vehicle powering the semi-trailer is referred to as the towing vehicle or as the tractor vehicle.

[0038] In order to ensure that a vehicle motion management (VMM) system can always control the motion of a highly automated vehicle, redundancy is often provided to cover at least parts of the vehicle control system. During normal vehicle operation, sensors such as radar sensors, lidar sensors, global positioning system (GPS) sensors and the like provide input data to a primary control unit which bases its control decisions on the sensor input.

[0039] If the sensor input signals are lost for some reasons, or if the primary control unit brakes down, the secondary redundant control system kicks in. In case of sensor signal outage, the vehicle is essentially blind and must operate for a limited time duration without sensor input. A safe stop maneuver is then executed to stop the vehicle in a controlled manner. The most important vehicle motion states that need to be estimated during a safe stop maneuver are: global position in two coordinates (x and y or longitudinal and latitudinal position), vehicle heading, trajectory curvature, vehicle longitudinal speed and yaw rate. Knowing these quantities, the vehicle can follow the last known path, or a path determined based on old data, and thus be less likely to exit a safe driving area.

[0040] Known approaches to estimate these vehicle motion state quantities in the event of loss of global localization information (from GPS, camera, etc) is to use inertial sensor information from accelerometers and angular rate sensors, along with wheel speed information to perform ‘dead-reckoning’ based on the last known global position of the vehicle.

[0041] There are several possible sources of error that can lead to large drifting of the estimated position which are common in this approach. The acceleration and angular rate signals must be integrated in order to obtain position and speed info. Any offset or error in a signal will be integrated over time, resulting in larger and larger errors in position and speed estimates. The sensor signals are also generally noisy and contain both biases and errors from, e.g., gravity influence, heat, and the like. To make matters worse, during braking and propulsion of the vehicle, relative slip or skidding will occur between the tires and the road surface. In this situation the wheel rotational speeds will not equal the vehicle's actual speed over ground. This will result in large errors in vehicle speed estimate and longitudinal position, especially when the road surface is slippery.

[0042] FIG. 1 shows an example scenario 100 where an articulated vehicle 110 loses sensory input at a location (A). A safe stop maneuver is therefore triggered, with the purpose to steer 130 the vehicle 110 onto a road shoulder and bring the vehicle to a full stop at location (C) in a controlled manner. A dangerous situation would result if the vehicle 110 veers 140 into the opposite lane 120 where it may collide with meeting traffic. Such veering may be the result of accumulated errors during dead reckoning, especially if the road surface is slippery.

[0043] FIG. 2 illustrates the articulated vehicle 110 in more detail. The vehicle comprises a towing truck 210 and a trailer unit 220. The towing truck here comprises three wheel axles, where one wheel axle may be a liftable wheel axle, i.e., it may be raised such that the wheels leave the ground when it is not needed to carry weight. One or more of the wheel axles may comprise actively driving wheels. Often, all wheels are capable of decelerating the vehicle 110.

[0044] As will be discussed in more detail below, certain sub-sets of wheels may be selected to be in a free-rolling condition, or at least be in a condition associated with a reduced braking or acceleration operation compared to other wheels on the vehicle 110. Such a selection may comprise selecting the wheels on a single axle 230, or selecting wheels on different axles 240a, 240b, or even selecting a single wheel 240c.

[0045] The nominal maximum braking performance of a truck on a dry road is about 7 m/s.sup.2. On a wet road this number may be lowered to about 4 m/s.sup.2. Truck combinations typically have more than 2 axles, e.g., for a tractor and semitrailer combination there are typically 5 or 6 axles. It therefore becomes possible to take advantage of the fact that the maximum deceleration request for a safe stop will be limited to about 3 m/s.sup.2, which acceleration can be achieved even in wet road conditions, while leaving two wheels on the vehicle unbraked, i.e., in a free-rolling condition. For example, if the vehicle has 5 equally laden axles, and the nominal maximum deceleration that can be achieved with all wheels active is 5 m/s.sup.2 (limited by tire-road friction) then with one axle unbraked a deceleration of ⅘*5 can still be achieved, i.e., 4 m/s.sup.2 which is well above the 3 m/s.sup.2 requirement for a safe stop maneuver.

[0046] In other words, suppose that a safe stop maneuver is requiring a deceleration of 3-4 m/s.sup.2 and that maximum truck deceleration is 7-8 m/s.sup.2. Then, with a three axle truck, a whole axle can be unbraked or the diagonal wheels unbraked on a two axle system.

[0047] In case wheels on different axles are selected, then it may be necessary to compensate the wheel speeds for inner and outer radius.

[0048] In a vehicle with many axles such as the vehicle 110 shown in FIGS. 1 and 2, any of the axles could be selected to ‘free-roll’. The decision criteria as to which axle to select could for example be based on any of; [0049] axle load, e.g., selecting axle with lowest load, or with the least impact on vehicle stability, [0050] the axle whose position is of most interest to the path follower algorithm,

[0051] Instead of ‘free rolling’ two wheels on the same axle, for some vehicle layouts it may be advantageous to ‘free-roll’ wheels at opposite corners of the vehicle (e.g. the front left and rear right). This may result in a more stable braking situation, particularly in the case of a short wheel base tractor unit. When combined with knowledge of the vehicle's wheelbase length this may also provide a more accurate position estimate. The wheel speed information can optionally be fused with accelerometer and/or angular rate sensor information, e.g. in a Kalman filter or a particle filter, to improve dead reckoning accuracy further.

[0052] The wheel speed information could be used as the only sensor information for blind stop navigation, which is an advantage since it yields a system with very low cost. Steering angle information from, e.g., a steering angle sensor, could also be combined with the sensors mentioned above to further improve the estimate of heading angle.

[0053] FIG. 3 schematically illustrates a vehicle 210 in a turn scenario 300, where the vehicle is executing a turn with curvature 1/R. The vehicle comprises a control unit 320 arranged to perform the herein disclosed techniques. Further details of the control unit 320 will be discussed below in connection to FIG. 5. Here, the wheels on axle 310 has been selected to be in a free rolling condition and are being used to estimate vehicle motion state. The leftmost wheel on the axle is associated with a velocity V.sub.1, while the rightmost wheel is associated with a velocity V.sub.2. A vehicle motion state parameter V, which is a velocity in longitudinal direction, i.e., in the forward direction of the vehicle, is to be estimated.

[0054] Wheel rotation speeds such as V.sub.1 and V.sub.2 can be measured by a wheel speed sensor (VSS) which is often implemented by some type of tachometer. It is a sender device used for reading the speed of a vehicle's wheel rotation, which usually consists of a toothed ring and a pickup device. A common wheel speed sensor system consists of a ferromagnetic toothed reluctor ring (also known as a tone wheel) and a sensor which can be either passive or active. Wheel speed sensors are known and will not be discussed in more detail herein.

[0055] With reference to FIG. 3, according to an example of the herein disclosed techniques, a method for carrying out a safe stop maneuver is as follows:

[0056] Select an axle 310 on the main towing unit (the truck), such as the axle with the least amount of vertical load. Keep the brakes on this axle disabled throughout the safe stop maneuver.

[0057] Use the free-rolling wheels on the unbraked axle to measure the longitudinal vehicle speed-over-ground directly, e.g. taking the average of the two speeds V.sub.1 and V.sub.2.

[0058] Use this measured vehicle speed V to estimate: longitudinal vehicle position (integrate V), as well and the longitudinal acceleration of the vehicle (differentiate V).

[0059] Use the difference between the wheel speeds measured on the free-rolling axle, and the vehicle's known wheel base, to estimate the curvature of the vehicle's current trajectory. The curvature 1/R at the center of the free-rolling axle is given by

[00001]$R=\frac{L}{2}\frac{V_1+V_2}{V_2-V_1},$

where L is me axle wheel distance also shown in FIG. 2, V.sub.1 is the measured velocity of the first wheel, and V.sub.2 is the measured velocity of the second wheel. These geometrical relationships are known and will not be discussed in more detail here.

[0060] Yaw rate {dot over (Ψ)} can then be calculated as

[00002]$\stackrel{.Math.}{\Psi }=\frac{V}{R}.$

This yaw rate estimate can of course be ‘fused’ with any available yaw rate sensors on the vehicle to improve accuracy, and then integrated to get the heading angle of the vehicle during the course of the safe stop. Herein, by fusing two estimates is meant that the two estimates are used to jointly estimate a common value. Fusing commonly amounts to weighting the two values based on relative accuracy. Sensor fusion algorithms are known and will not be discussed in more detail here.

[0061] If a rear axle of the vehicle is braked, the position and orientation of the vehicle can for example be calculated at each time step during the safe stop using common equations:

Ψ.sub.t=Ψ.sub.t-1+dt{dot over (Ψ)},

x.sub.t=x.sub.t-1+dt(V.sub.t cos(Ψ)),

y.sub.t=y.sub.t-1+dt(V.sub.t sin(Ψ)),

a.sub.x=(V.sub.t−V.sub.t-1)/dt,

[0062] where dt is the timestep of the estimation algorithm, x.sub.t represents latitudinal position at time t, y.sub.t represents longitudinal position at time t, V.sub.t represents vehicle velocity at time t, and a.sub.x is acceleration in latitudinal direction. The directions x and y are indicated in FIG. 3.

[0063] The position and heading angle Ψ can be used directly by a path follower to follow a desired blind stop trajectory.

[0064] The a.sub.x value and V.sub.t can be used by the brake system. These are important inputs to ensure correct deceleration values are reached, and to ensure that the antilock braking algorithm can function properly.

[0065] The techniques disclosed herein can also be used to estimate a road banking grade and/or a road slope based on the measured parameters and on obtained accelerometer data associated with the vehicle.

[0066] FIG. 4 is a flow chart illustrating various methods which generalize the above discussed example operations. There is illustrated a method in a vehicle 110, 210 for estimating vehicle motion state during a vehicle maneuver. The method comprises obtaining S1 a trigger signal indicating an onset of the vehicle maneuver. This trigger signal may, e.g., be obtained from an external source, from an interface operable by a driver in the cabin such as an emergency brake button, or it may be generated internally in the control unit 320 following sensor signal outage. The trigger signal may also be received from a remote source via, e.g., wireless signal transmission over a cellular network or the like.

[0067] According to some aspects, the vehicle maneuver is a safe stop maneuver requiring a controlled deceleration of the vehicle below a maximum deceleration capacity of the vehicle.

[0068] According to some other aspects, the vehicle maneuver is a sensor calibration maneuver, whereby one or more sensor systems of the vehicle are arranged to be calibrated against the estimated motion state data. Both safe stop maneuvers and sensor calibration maneuvers were discussed above.

[0069] The method also comprises selecting S2 a sub-set of wheels on the vehicle to be in a free-rolling condition or at least in a reduced braking condition. It is appreciated that wheels in free-rolling condition may be preferred in order to minimize error sources due to tire slippage. However, the method does not require fully free-rolling wheels, but also functions well in case a braking operation with reduced magnitude is actuated via the selected sub-set of wheels.

[0070] The sub-set of wheels may be selected differently depending on circumstances and wanted effects; for instance, according to some aspects, the subset of wheels is selected S21 as the wheels on a selected wheel axle 230. By selecting two or more wheels on both sides of the vehicle, track curvature, yaw rate, and the like can be estimated, as was illustrated and discussed in connection to FIG. 3 above. The selected wheel axle 230 may be the wheel axle associated with least vertical load compared to other wheel axles of the vehicle. This is a preferred choice in case loss in braking capability is to be minimized, since the wheel axle with least load is likely to yield less braking force effect compared to a wheel axle which is more heavily loaded.

[0071] The selected wheel axle 230 may also be a liftable wheel axle. In this case, the method comprises lowering S3 the liftable wheel axle prior to measuring the parameters. The liftable wheel axle is an ‘extra’ wheel axle which is only used in case the vehicle is heavily loaded. Thus, in case it is not already in use, it can be lowered and used to estimate vehicle motion state without significant impact on, e.g., vehicle braking capability or the like.

[0072] The selected sub-set of wheels may further comprise wheels from different wheel axles of the vehicle and from different sides of the vehicle 240a, 240b. This type of selection may improve on vehicle stability during the maneuver, which is an advantage.

[0073] It is appreciated that the subset of wheels may comprise any number of wheels, from one wheel and up. Thus, according to some aspects, the subset of wheels is selected S24 as a single wheel. According to other aspects, e.g., when calibrating sensors, the sub-set of wheels can be selected as all wheels of the vehicle.

[0074] According to some aspects, wheels comprised in the subset of wheels is selected S23 in dependence of vehicle stability, whereby the subset of wheels is selected to give an acceptable impact on vehicle stability depending on maneuver scenario.

[0075] The method also comprises measuring S5 one or more parameters related to revolution of the sub-set of wheels in free-rolling condition and estimating S6 the vehicle motion state based on the measured parameters.

[0076] Of course, should some sensor input signals be available, sensor fusion algorithms can be applied in order to fuse information from the dead reckoning system based on the free-rolling subset of wheels with the information obtained from the available sensors. Consequently, it is appreciated that the disclosed techniques are also applicable in cases where sensor input signals from, e.g., radar sensors, lidar sensors, vision sensors and GPS is available.

[0077] According to some aspects, the method comprises adjusting S4 a suspension air pressure associated with the selected axle 230 by an amount. For instance, suspension air pressure may be adjusted so as to reduce load on a selected wheel axle, thereby reducing negative impact on braking capability of the vehicle. According to other aspects, wheel axle load on a selected wheel axle may be increased in order to increase friction between free-rolling wheels and the road surface, thereby further reducing errors due to wheel slip.

[0078] In addition to estimating vehicle motion state, the method may also comprise controlling S7 the vehicle 110 to execute the maneuver based on the estimated motion state and on a preferred maneuver track to be followed. This controlling may comprise using a last known path, or recent information about vehicle surroundings together with the estimated vehicle motion state in order to execute a controlled safety stop maneuver. For instance, with reference again to FIG. 1, when the vehicle loses sensor input signal at location (A), the environment some distance ahead of the vehicle is likely known. Thus, the control unit 350 knows how the road extends some distance in front of the vehicle 110 at location (A) and can therefore determine a suitable path 130 for the safe stop maneuver. The control unit can then control steering and braking in order to follow the determined path 130 based on the estimated travelled distance from location (A), vehicle velocity, and vehicle turn rate. Thus, a safe stop maneuver from location (A) to location (B) following path 130 can be executed despite the vehicle being blind in the sense that, e.g., GPS, vision, radar, and/or lidar sensor input is no longer available. Consequently, veering into the more dangerous path 140 is avoided despite sensor signal outage.

[0079] According to some other aspects, the method comprises verifying S8 a deceleration value in the motion state against a preferred deceleration value for the maneuver. This means that the vehicle can obtain information about if the current deceleration is reasonable, or if adjustments to braking action should be made in order to increase or decrease vehicle deceleration during the maneuver.

[0080] According to some further aspects, the method further comprises estimating S9 a road banking grade and/or a road slope based on the measured parameters and on obtained accelerometer data associated with the vehicle. If combined with accelerometer data from, e.g., an inertial measurement unit (IMU), the speed measured from the selected wheels can be used to estimate road banking grade/slope.

[0081] According to other aspects, the method comprises estimating S10 a road friction coefficient by comparing wheel revolutions between wheels in free-rolling condition and wheels where braking or acceleration forces are applied. In case the free-rolling wheels are behaving differently with respect to, e.g., revolution speed compared to the braking wheels, then a slippery road surface can be suspected. Also, if no significant difference in wheel revolution characteristics are measured between the free-rolling wheels and the braking wheels, then it can be inferred that road friction conditions are favorable for braking operations.

[0082] FIG. 5 schematically illustrates, in terms of a number of functional units, the components of a control unit 320 according to embodiments of the discussions herein. Processing circuitry 510 is provided using any combination of one or more of a suitable central processing unit CPU, multiprocessor, microcontroller, digital signal processor DSP, etc., capable of executing software instructions stored in a computer program product, e.g. in the form of a storage medium 530. The processing circuitry 510 may further be provided as at least one application specific integrated circuit ASIC, or field programmable gate array FPGA.

[0083] Particularly, the processing circuitry 510 is configured to cause the control unit 320 to perform a set of operations, or steps, such as the methods discussed in connection to FIG. 4. For example, the storage medium 530 may store the set of operations, and the processing circuitry 510 may be configured to retrieve the set of operations from the storage medium 530 to cause the control unit 320 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuitry 510 is thereby arranged to execute methods as herein disclosed.

[0084] The storage medium 530 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

[0085] The control unit 320 may further comprise an interface 520 for communications with at least one external device, such as the antenna array comprising the phase controllers and the mechanically rotatable base plate. As such the interface 520 may comprise one or more transmitters and receivers, comprising analogue and digital components and a suitable number of ports for wireline or wireless communication.

[0086] The processing circuitry 510 controls the general operation of the control unit 320, e.g., by sending data and control signals to the interface 520 and the storage medium 530, by receiving data and reports from the interface 520, and by retrieving data and instructions from the storage medium 530. Other components, as well as the related functionality, of the control node are omitted in order not to obscure the concepts presented herein.

[0087] The control unit 320 optionally comprises a heading detection unit, such as a compass or GPS module. The control unit may also comprise an IMU. The input from these sensors may be fused with the estimates obtained from the wheel rotation measurements.

[0088] FIG. 6 illustrates a computer readable medium 610 carrying a computer program comprising program code means 620 for performing the methods illustrated in FIG. 4, when said program product is run on a computer. The computer readable medium and the code means may together form a computer program product 600.