Method and arrangement for ensuring road tracking up to a predefined lateral acceleration limit using a PID controller in a vehicle

Abstract

Methods for ensuring road tracking up to a predefined lateral acceleration limit in a vehicle having an autonomous steering function arranged to selectively apply a steering wheel overlay torque to a normal steering assistance torque in an electrical power assisted steering system of the vehicle are provided. Such methods include acquiring the predetermined lateral acceleration limit; acquiring a signal representing a current lateral acceleration of the vehicle; comparing the predetermined lateral acceleration limit with the acquired current lateral acceleration signal to obtain a controller error; setting a torque limit for the steering wheel overlay torque; and subjecting the controller error to a proportional-integral-derivative (PID) controller, which is arranged to provide the torque limit for the steering wheel overlay torque after setting the torque limit to the initial value.

Claims

1. A method for ensuring road tracking up to a predefined lateral acceleration limit in a vehicle having an autonomous steering function arranged to selectively apply a steering wheel overlay torque to a normal steering assistance torque in an electrical power assisted steering system of the vehicle, the method comprising: acquiring the predetermined lateral acceleration limit; acquiring a signal representing a current lateral acceleration of the vehicle; comparing the predetermined lateral acceleration limit with the acquired current lateral acceleration signal to obtain a controller error; setting a torque limit for the steering wheel overlay torque, if the controller error is equal to or greater than a predetermined first threshold, to the actual value of the steering wheel overlay torque as an initial value for the torque limit; and subjecting the controller error to a proportional-integral-derivative (PID) controller, which is arranged to provide the torque limit for the steering wheel overlay torque after setting the torque limit to the initial value.

2. The method according to claim 1, further comprising activating the PID controller when the controller error becomes equal to or greater than the predetermined first threshold, a driver is not interacting with a steering wheel of the vehicle and the I part of the PID controller is not in saturation.

3. The method according to claim 1, further comprising inactivating the PID controller when a driver is interacting with a steering wheel, the I part of the PID controller is in saturation or a switch off timer, which was started when the controller error dropped below the predetermined first threshold, has elapsed.

4. The method according to claim 1, further comprising reducing at least one of the P part, I part and D part of the PID controller when a predetermined time has elapsed after a driver interacted with a steering wheel and the controller error became equal to or greater than a predetermined second threshold that is greater than the predetermined first threshold.

5. The method according to claim 1, further comprising lowpass filtering the current lateral acceleration signal.

6. The method according to claim 1, further comprising using an Automatically Commanded Steering Function (ACSF) as the autonomous steering function.

7. The method according to claim 1, further comprising acquiring a signal representing a speed of the vehicle and tuning a P gain of the PID controller depending on the speed of the vehicle.

8. An arrangement for ensuring road tracking up to a predefined lateral acceleration limit in a vehicle having an autonomous steering function arranged to selectively apply a steering wheel overlay torque to a normal steering assistance torque in an electrical power assisted steering system of the vehicle, the arrangement further comprising: a closed loop controller having a comparator for comparing a predetermined lateral acceleration limit with an acquired current lateral acceleration signal of the vehicle to obtain a controller error; a lateral acceleration limiter arranged to set a torque limit for the steering wheel overlay torque, if the controller error is equal to or greater than a predetermined first threshold, to the actual value of the steering wheel overlay torque as an initial value for the torque limit; and a proportional-integral-derivative (PID) controller arranged to receive the controller error and provide the torque limit for the steering wheel overlay torque after setting the torque limit to the initial value.

9. The arrangement according to claim 8, further comprising activating the PID controller when the controller error becomes equal to or greater than the predetermined first threshold, a driver is not interacting with a steering wheel of the vehicle and the I part of the PID controller is not in saturation.

10. The arrangement according to claim 8, further comprising inactivating the PID controller when a driver is interacting with a steering wheel, the I part of the PID controller is in saturation or a switch off timer, which was started when the controller error dropped below the predetermined first threshold, has elapsed.

11. The arrangement according to claim 8, further comprising reducing at least one of the P part, I part and D part of the PID controller when a predetermined time has elapsed after a driver interacted with a steering wheel and the controller error became equal to or greater than a predetermined second threshold that is greater than the predetermined first threshold.

12. The arrangement according to claim 8, further comprising a lowpass filter arranged to filter a current lateral acceleration signal.

13. The arrangement according to claim 8, wherein the autonomous steering function is an Automatically Commanded Steering Function (ACSF).

14. The arrangement according to claim 8, further arranged to acquire a signal representing a speed of the vehicle and tune a P gain of the PID controller depending on the speed of the vehicle.

15. A vehicle comprising an arrangement for ensuring road tracking up to a predefined lateral acceleration limit in a vehicle having an autonomous steering function arranged to selectively apply a steering wheel overlay torque to a normal steering assistance torque in an electrical power assisted steering system of the vehicle, the arrangement further comprising: a closed loop controller having a comparator for comparing a predetermined lateral acceleration limit with an acquired current lateral acceleration signal of the vehicle to obtain a controller error; a lateral acceleration limiter arranged to set a torque limit for the steering wheel overlay torque, if the controller error is equal to or greater than a predetermined first threshold, to the actual value of the steering wheel overlay torque as an initial value for the torque limit; and a proportional-integral-derivative (PID) controller arranged to receive the controller error and provide the torque limit for the steering wheel overlay torque after setting the torque limit to the initial value.

Description

BRIEF DESCRIPTION OF FIGURES

(1) In the following, embodiments herein will be described in greater detail by way of example only with reference to attached drawings, in which:

(2) FIG. 1 schematically illustrates a vehicle that comprises an arrangement for ensuring road tracking up to a predefined lateral acceleration limit according to an example embodiment;

(3) FIG. 2 schematically illustrates a closed loop controller of an arrangement for ensuring road tracking up to a predefined lateral acceleration limit in a vehicle according to an example embodiment;

(4) FIG. 3 schematically illustrates diagrams depicting a controller error e, a steering wheel overlay torque T_PA and a torque limit T_LIM over time according to an example embodiment; and

(5) FIG. 4 schematically illustrates a flowchart of a method for ensuring road tracking up to a predefined lateral acceleration limit in a vehicle according to an example embodiment.

DETAILED DESCRIPTION

(6) FIG. 1 schematically illustrates a vehicle 1 including an arrangement 2 for ensuring road tracking up to a predefined lateral acceleration limit a_(lat,R79). Further, the vehicle 1 includes an autonomous steering function 3 and an electrical power assisted steering system 4.

(7) The autonomous steering function 3 may be an ACSF system arranged to selectively apply, to e.g., a steering column 5 or equivalent, a steering wheel overlay torque T.sub.PA to a normal steering assistance torque T.sub.a of the electrical power assisted steering system 4, i.e., applying an additional steering wheel torque T.sub.PA on top of what would have been provided by a base assist of the electrical power assisted steering system 4.

(8) In order to understand how a driver of the vehicle 1 experiences steering wheel overlay torque T.sub.PA from the autonomous steering function 3, such as an ACSF function, we first need to establish a model of the steering dynamics of the vehicle 1 steering system.

(9) The steering system dynamics can be modelled with newtons second law
J.sub.s{umlaut over (θ)}=B({umlaut over (θ)},θ)+βF.sub.r(θ,v,m,J.sub.v,c.sub.f,c.sub.r,l.sub.r)+γT.sub.a({umlaut over (θ)},{dot over (θ)},θ,v,T.sub.c)+γT.sub.PA+T.sub.c  (Eq.1)
In steady state: {umlaut over (θ)}=0, {dot over (θ)}=0
0=βF.sub.r(θ,v,m,J.sub.v,c.sub.f,c.sub.r,l.sub.r)+γT.sub.a(θ,v,T.sub.c)+γT.sub.PA+T.sub.c  (Eq.2)
θ: Steering wheel angle, J.sub.s: inertia of steering system, B: mechanical damping of steering system, β: mechanical ratio converting from lateral force on the wheels to torque on the steering wheel, F.sub.r: wheel forces, v: vehicle speed, m: vehicle mass, J.sub.v: vehicle inertia, c.sub.f: cornering stiffness front (tyre parameter), c.sub.r: cornering stiffness rear (tyre parameter), l.sub.r: distance from rear wheels axis to centre of gravity, γ: mechanical ratio converting from torque at the electrical power assist motor to torque on the steering wheel, T.sub.a: torque from the electrical power assist, T.sub.PA: overlay torque from the ACSF function, T.sub.c: steering wheel column torque.

(10) Equation 2 above shows the torque balance in a steady state cornering situation. The well-known kinetic bicycle model, e.g. as described by Rajamani, Rajesh, “Vehicle Dynamics and Control” Second Edition, Chapter 2 titled “Lateral Vehicle Dynamics”, Springer US, Published 2012, pp. 20-31, gives that for a given mass, inertia and tyres, the wheel forces are well approximated as linear in the lateral acceleration, as illustrated in Equation 3 below
F.sub.r(θ,v,m,J.sub.v,c.sub.f,c.sub.r,l.sub.r)≈k(m,J.sub.v,c.sub.f,c.sub.r,l.sub.r)a.sub.lat.  (Eq. 3).

(11) In steady state at the R79 UNECE acceleration limit a.sub.lat,R79 the following relation, Equation 4, approximately holds

(12) $\begin{array}{cc}k\left(m,J_v,c_f,c_r,l_r\right)a_{lat,R79}=-\frac{\gamma }{\beta }{T}_a\left(\theta ,v\right)-\frac{\gamma }{\beta }{T}_{PA,R79}& \left(\mathrm{Eq}.4\right)\end{array}$
where γT.sub.a (θ, v) corresponds to an active return functionality. An active return functionality applies a torque opposite to a steering wheel torque, in order to return the steering wheel angle to 0°. This functionality can be seen when the driver releases the steering wheel and the steering wheel slowly returns to a 0° steering wheel angle.

(13) Since the purpose of the ACSF function is to assist the driver of the vehicle 1 in tracking a path, both steering wheel column torque T.sub.c and torque T.sub.a from the electrical power assisted steering system 4 will be close to zero as long as a driver is not actively steering, overriding or assisting the ACSF function.

(14) In a steady state cornering situation overlay torque T.sub.PA from the ACSF function must counteract the wheel forces F.sub.r. If the driver of the vehicle 1 keeps his or her hands on the steering wheel and has a desire to stay in lane during a steady state cornering situation, a reduction of overlay torque T.sub.PA from the ACSF function must be balanced out by an increase in steering wheel column torque T.sub.c. The increase in steering wheel column torque T.sub.c is provided by the driver's hands which means that the driver will experience the reduction in overlay torque T.sub.PA from the ACSF function as if the ACSF function is pulling the steering wheel towards a path that leaves the lane.

(15) The proposed technical solution aims at saturating the ACSF overlay torque T.sub.PA at a torque limit close to a torque from the ACSF function T.sub.PA,R79, which in steady state results in the predefined R79 lateral acceleration, a.sub.lat,R79.

(16) If the overshoot in torque T.sub.PA from the ACSF function above T.sub.PA,R79 is small, the driver will get the experience that the ACSF function does it best to keep the road curvature but that the function is not strong enough and needs some torque assistance from the driver in order to keep the vehicle 1 in lane during a steep curve.

(17) A challenge is that the torque T.sub.PA,R79 from the ACSF function, which in steady state results in the predefined R79 lateral acceleration, a.sub.lat,R79, is strongly dependent on the mass, inertia, tyres, centre of gravity and electrical power assistance, T.sub.a, which all are parameters that vary between drive cycles and variants of the type of the vehicle 1.

(18) In order to be robust against variations in mass, inertia, centre of gravity and to tuning of the electrical power assistance, T.sub.a, feedback on lateral acceleration a.sub.lat is used to control the ACSF overlay torque limits close to the torque T.sub.PA,R79 from the ACSF function, which in steady state results in the predefined R79 lateral acceleration, a.sub.lat,R79.

(19) A block diagram of an example embodiment of the arrangement 2 for ensuring road tracking up to a predefined lateral acceleration limit a.sub.lat,R79 comprising a closed loop controller 6 suitable for use with the proposed method is illustrated in FIG. 2.

(20) The current lateral acceleration signal a.sub.lat is received from an acceleration sensor and is preferably lowpass filtered 10 to retrieve a smoother signal.

(21) A controller error e, which may also be denoted as lateral acceleration error, is obtained by subtracting 7 a predetermined lateral acceleration limit a.sub.lat,R79 from the lowpass filtered current lateral acceleration a.sub.lat, thus it holds e=a.sub.lat−a.sub.lat,R79. The controller error e then enters a lateral acceleration limiter 8. In addition, the lateral acceleration limiter 8 receives the actual steering wheel overlay torque T.sub.PA. The lateral acceleration limiter 8 further includes a proportional-integral-derivative, PID, controller 9. The lateral acceleration limiter 8 outputs a torque limit T.sub.LIM for the steering wheel overlay torque T.sub.PA.

(22) The operating principle of the lateral acceleration limiter 8 shall be explained with the help of FIG. 3. The upper diagram in FIG. 3 illustrates the controller error e over time. The lower diagram in FIG. 3 illustrates the steering wheel overlay torque T.sub.PA and the torque limit T.sub.LIM over time.

(23) The scenario shown in FIG. 3 is that the vehicle 1 drives straight on with a constant velocity. The vehicle 1 then enters a curve and the steering wheel overlay torque T.sub.PA ramps up smoothly, which leads to an increase of the lateral acceleration a.sub.lat and thus an increase of the controller error e.

(24) As long as the controller error e is below a predetermined first threshold, which is equal to zero in the present example embodiment, the lateral acceleration limiter 8 outputs a default value T.sub.LIM,default for the torque limit T.sub.LIM. As soon as the controller error e becomes equal to or greater than zero, the lateral acceleration limiter 8 sets the torque limit T.sub.LIM to the actual value of the steering wheel overlay torque T.sub.PA that is currently applied to the steering wheel servo motor. The instance in time when the controller error e becomes zero and the torque limit T.sub.LIM is set to the actual value of the steering wheel overlay torque T.sub.PA is indicated by a dashed line in FIG. 3.

(25) In addition, the actual value of the steering wheel overlay torque T.sub.PA, when the controller error e becomes zero, is saved and input into the PID controller 9 as an initial value. Starting from this instance in time, the PID controller 9 generates the torque limit T.sub.LIM for the steering wheel overlay torque T.sub.PA, which is output by the lateral acceleration limiter 8. The PID controller 9 uses the initial value as the starting value for generating the torque limit T.sub.LIM. The PID controller 9 controls the torque limit T.sub.LIM fast and smoothly without losing time.

(26) An advantage of the arrangement 2 illustrated in FIG. 2 is that there is no time delay to get the torque limit T.sub.LIM for the steering wheel overlay torque T.sub.PA and steady state is reached fast. In addition, overshoots are small and the PID controller 9 uses a smooth starting point. The PID controller 9 further has a dynamic and damping part next to the integral part.

(27) The PID controller 9 may generate an upper torque limit T.sub.LIM,upper and/or a lower torque limit T.sub.LIM,lower for the steering wheel overlay torque T.sub.PA. The upper torque limit T.sub.LIM,upper and/or the lower torque limit T.sub.LIM,lower can be input into a pinion angle controller. The pinion angle controller further receives the current pinion angle and a pinion angle request and uses the input values to generate a torque request, which is within the range limited by the upper torque limit T.sub.LIM,upper and/or the lower torque limit T.sub.LIM,lower.

(28) The PID controller 9 requires that only its P gain needs to be tuned for different speeds of the vehicle 1 according to the oscillatory method from Ziegler and Nichols, which method is well known to a skilled person.

(29) The PID controller 9 may be a digital controller. The PID controller 9 calculates a control deviation for consecutive time instances t and applies a correction based on proportional (P), integral (I) and derivative (D) parts in order to receive a command signal. The control deviation is the controller error e.

(30) The control deviations are summed up for each time t. Thus, the new sum of control deviations for each time t is calculated by adding the current control deviation to the old sum of control deviations:
sum_of_control_deviation_new=sum_of_control_deviation_old+control_deviation  (Ep. 5).

(31) The P, I and D parts are calculated by using the following equations, where k.sub.p, k.sub.i and k.sub.d denote the gains of the proportional, integral, and derivative parts, respectively, T.sub.s is the sample time, control_deviation is the control deviation currently measured and control_deviation_old is the previously measured control deviation:
p_part=k.sub.p*control_deviation  (Ep. 6)
i_part=k.sub.i*T.sub.s*sum_of_control_deviation_new  (Ep. 7)
d_part=k.sub.d*(control_deviation−control_deviation_old)/T.sub.s  (Eq. 8).

(32) The command signal is the sum of the P, I and D parts at time t:
command_signal=p_part+i_part+d_part  (Ep. 9).

(33) The torque limit T.sub.LIM, which can be the upper the torque limit T.sub.LIM,upper or the lower torque limit T.sub.LIM,lower, is calculated by subtracting the command signal from the initial value T.sub.LIM,init, which was saved when the controller error e became equal to or greater than zero:
T.sub.LIM=T.sub.LIM,init−command_signal  (Ep. 10).

(34) The PID controller 9 may get active when all of the following conditions are fulfilled:

(35) (1) the controller error e becomes equal to or greater than zero,

(36) (2) the driver of the vehicle 1 is not interacting with the steering wheel, for example, the driver does not exert torque on the steering wheel, and

(37) (3) the I part of the PID controller 9 is not in saturation.

(38) When all conditions (1)-(3) are fulfilled, the PID controller 9 gets active and outputs the torque limit T.sub.LIM for the steering wheel overlay torque T.sub.PA.

(39) The PID controller 9 may get inactive when at least one of the following is fulfilled:

(40) (1) the driver is interacting with the steering wheel, for example, the driver exerts torque on the steering wheel,

(41) (2) the I part of the PID controller 9 is in saturation, and

(42) (3) a switch off timer, which was started when the controller error e dropped again below zero, has elapsed.

(43) When the I part of the PID controller 9 is in saturation, the PID controller 9 may be not tuned correctly or it does not make sense to further control anymore.

(44) When the PID controller 9 is active, but the controller error e has been below zero for a longer time, no control is needed and the PID controller 9 shall be switched off. Therefore, the switch off timer is started when the controller error e drops below zero after the activation of the PID controller 9. When the controller error e becomes equal to or greater than zero again while the switch off timer is running, the switch off timer is turned off. However, when the controller error e remains below zero and the switch off timer elapses, the PID controller 9 is deactivated.

(45) When the PID controller 9 gets inactive, it stops outputting the torque limit T.sub.LIM for the steering wheel overlay torque T.sub.PA. After the PID controller 9 got inactive, the torque limit T.sub.LIM, i.e., the upper the torque limit T.sub.LIM,upper and/or the lower torque limit T.sub.LIM,lower, may ramp to its default value again to not disturb the angle controller.

(46) There are situations where the PID controller 9 has been activated, but then the curve on the road opens up, i.e., the radius becomes larger, and the lateral acceleration a.sub.lat goes below the predetermined lateral acceleration limit a.sub.lat,R79 again. But then the I part of the PID controller 9 runs in the opposite direction until it is saturated. When the PID controller 9 is needed again before it is saturated it may take some seconds until the PID controller 9 gets successful again. Therefore, when the PID controller 9 is active but there is no need to control because the controller error e is below zero, the switch off timer is started. Once the switch off timer has elapsed, the PID controller 9 is switched off until it is needed again. For example, the switch off timer may elapse after 3 seconds.

(47) The lateral acceleration limiter 8 may monitor whether the controller error e becomes equal to or greater than a predetermined second threshold that is greater than the predetermined first threshold, which is zero in the present example embodiment. Further, it is monitored when the driver stops interacting with the driving wheel. If the controller error e becomes equal to or greater than the predetermined second threshold, i.e., a high lateral acceleration a.sub.lat occurs, for example, at least 0.5 m/s.sup.2 over the predetermined lateral acceleration limit a.sub.lat,R79, and the driver does not interact with the driving wheel, an additional timer is started. When the timer has elapsed after a predetermined time, a smoother tuning of the PID controller 9 is activated by, for example, setting the P part and the D part to zero and setting the I part to half of its tuning value. This helps after a driver interaction because otherwise the PID controller 9 is too aggressive and it can cause an oscillation.

(48) FIG. 4 schematically illustrates a flowchart of a method 11 for ensuring road tracking up to the predefined lateral acceleration limit a.sub.lat,R79 in the vehicle 1 according to an example embodiment.

(49) In step 12 of the method 11, the predetermined lateral acceleration limit a.sub.lat,R79 is acquired.

(50) In step 13, a signal is acquired that represents the current lateral acceleration a.sub.lat of the vehicle 1.

(51) In step 14, the predetermined lateral acceleration limit a.sub.lat,R79 is compared with the acquired current lateral acceleration signal a.sub.lat to obtain the controller error e. In particular, the predetermined lateral acceleration limit a.sub.lat,R79 is subtracted from the acquired current lateral acceleration signal a.sub.lat to obtain the controller error e.

(52) In step 15, it is monitored whether the controller error e becomes equal to or greater than the predetermined first threshold, which is zero in the present example embodiment.

(53) If the controller error e is smaller than zero, the method 11 returns to step 13.

(54) If the controller error e is equal to or greater than zero, the method 11 proceeds to step 16 and the torque limit T.sub.LIM for the steering wheel overlay torque T.sub.PA is set to the actual value of the steering wheel overlay torque T.sub.PA as an initial value for the torque limit T.sub.LIM.

(55) In step 17, the controller error e is subjected to the PID controller 9, which then starts generating the torque limit T.sub.LIM for the steering wheel overlay torque T.sub.PA.

(56) Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Moreover, although the foregoing descriptions and the associated drawings describe exemplary embodiments in the context of certain exemplary combinations of elements and/or functions, it should be appreciated that different combinations of elements and/or functions may be provided by alternative embodiments without departing from the scope of the appended claims. In this regard, for example, different combinations of elements and/or functions than those explicitly described above are also contemplated as may be set forth in some of the appended claims. In cases where advantages, benefits or solutions to problems are described herein, it should be appreciated that such advantages, benefits and/or solutions may be applicable to some example embodiments, but not necessarily all example embodiments. Thus, any advantages, benefits or solutions described herein should not be thought of as being critical, required or essential to all embodiments or to that which is claimed herein. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.