Patent Application
20220396312
This patent application claims priority from Italian patent application no. 102021000015206 filed on Jun. 10, 2021, the entire disclosure of which is incorporated herein by reference.
The invention relates to a method to control a road vehicle with independent motors acting upon wheels of a same axle and to a relative road vehicle.
Self-locking differentials are known, which are configured to change torque distribution so as to encourage the transmission of power to the wheel rotating more slowly (avoiding stalls or slips due, for example, to the lifting of a wheel or to the slip thereof on a surface with a reduced friction, for example a wet surface). These devices generally comprise a clutch system configured to limit the torque transmitted to the drive wheel rotating more quickly, to the advantage of the one rotating more slowly.
Furthermore, in recent years, in order to increase vehicle stability, safety or performances, different types of electronically controlled self-locking differentials have been developed.
Patent application WO2004087453A1 discloses a road vehicle provided with an electronically controlled self-locking differential, whose percentage of locking is controlled by a control unit in order to try and stabilize (namely, make more stable and, hence, safe) the road vehicle.
According to patent application WO2004087453A1, while driving along a bend, the electronic control unit progressively increases the percentage of locking of the self-locking differential (namely, it “closes” the clutch of the self-locking differential in order to transfer a greater quantity of torque to the drive wheel rotating more slowly, namely to the wheel on the inside of the bend) in case of release of the accelerator pedal to stabilize the road vehicle.
According to patent application WO2004087453A1, while driving along a bend, the control unit progressively decreases the percentage of locking of the self-locking differential (namely, it “opens” the clutch of the self-locking differential in order to transfer a greater quantity of torque to the drive wheel rotating more quickly, namely to the wheel on the outside of the bend) in case of pressing of the accelerator pedal to improve both the stability of the road vehicle and the acceleration performances along the bend; in particular, the reduction in the percentage of locking of the self-locking differential is proportional to the lateral acceleration of the road vehicle, to the driving speed of the road vehicle, to the torque delivered by the engine and/or the gear engaged in the transmission.
According to patent application WO2004087453A1, while driving along a bend at a substantially constant speed, the control unit estimates the state of grip of the drive wheels to the road surface and consequently cancels the percentage of locking of the self-locking differential, when the state of grip of the drive wheels to the road surface is far from the grip limit, progressively increases the percentage of locking of the self-locking differential, when the state of grip of the drive wheels to the road surface gets close to the grip limit, and finally reduces the percentage of locking of the self-locking differential up to the zero value, when the state of grip of the drive wheels to the road surface is very close to the grip limit.
Document WO2015146445A1 describes a device to control the activation of a motor, which is capable of reducing the rattling noise of the gears generated in a speed reduction gear without increasing the dimensions of the speed reduction gear and the number of components.
With advent of electric cars, the concept of differential was maintained even in those cases in which there are different motors, for example one for the front axles and one for the rear axle. In some cases, furthermore, in case the drive wheels of a same axle are operated by different actuators, software was implemented in order to imitate the behaviour of an electronically controlled self-locking differential.
All examples discusses above, for they use a physical differential, either an electronically controlled one or a software-simulated one, determine or define a bond between the difference of angular speed of the drive wheels belonging to the same axis. In particular, these bonds cause a lack of optimization in terms of performances (part of the torque is anyway transmitted to the wheel with a greater angular speed) or comfort, further avoiding the possibility of opposing possible undesired yaws due to adverse conditions of the ground or generated by the failure of an actuator.
The object of the invention is to provide a method to control a road vehicle with independent motors acting upon wheels of a same axle and to a relative road vehicle, said control method not suffering from the drawbacks described above, being easy and economic to be implemented and, in particular, being capable of maximizing performances while driving on a track, without making the road vehicle unstable.
According to the invention there are provided a method to control a road vehicle with independent motors acting upon wheels of a same axle as well as a relative road vehicle, according to the appended claims.
The invention will now be described with reference to the accompanying drawings showing a non-limiting embodiment thereof, wherein:
In
The powertrain system 4 comprises at least two electric motors 5, each independently operating a respective drive wheel 3. The electric motors 5 are preferably arranged in a transversely central, longitudinal rear position. Each one of said electric motors 5 is mechanically connected (by means of reduction or transmission elements 6) to the respective wheel 3 through a respective axle shaft 7, which is integral to a respective rear drive wheel 3.
In some non-limiting cases which are not shown herein, each drive wheel 3 is independently operated by a respective axle shaft 7. In particular, the control method comprises the step of controlling the torque TL, TR delivered to each respective axle shaft towards the first drive wheel 3 or towards the second drive wheel 3 as a function of a torque requested by the driver and independently of the difference in angular speed between the first and second wheel 3. More precisely, the torque delivered to the axle shafts 7 can be delivered by one single (combustion or electric) actuator of the powertrain system 4.
Each wheel 2 or 3 is mechanically connected to a frame of the road vehicle 1 by means of a suspension 8 (partially shown in
The road vehicle 1 comprises an electronic control unit 10 (“ECU”), which, among other things, adjusts the behaviour of the road vehicle 1 both while it drives along a straight road and while it drives along a bend by acting, as described more in detail below, upon the torque delivered by the electric motors 5 to the drive wheels 3 and, if necessary, in collaboration with the shock absorbers 9 of the suspensions 8. The control unit 10 can physically consist of one single device or of several devices separate from one another and communicating with one another through the CAN network of the road vehicle 1.
In known vehicles, as mentioned above, an electronically controlled self-locking differential is present or simulated. In these devices, when the locking clutch is completely open (namely, the percentage of locking is zero), the self-locking differential is completely free and the torque is equally divided between the two rear drive wheels (namely, each rear drive wheel receives 50% of the total torque, regardless of its rotation speed); on the contrary, by closing the locking clutch (namely, by increasing the percentage of locking), the self-locking differential starts locking and the torque delivered to the rear drive wheel rotating more slowly is progressively increased (namely, the rear drive wheel rotating more slowly receives more torque than the rear drive wheels rotating more quickly). In these devices, the difference in the torque that can be delivered to each drive wheel is a function of the difference in the angular speed of the drive wheels. Furthermore, the differential tends to re-establish the aforesaid balance in the division of the torque between the two drive wheels, hence forbidding a differentiation in the torque delivered to the two drive wheels in case the drive wheels have the same angular rotation speed.
On the contrary, according to the control method described below, the control unit 10, in particular a controller CTR on the inside of the latter, is configured to control the torque delivered by each electric motor 5 to the respective drive wheel 3 as a function of a torque Cin requested by the driver (for example, through an accelerator pedal) and independently of the angular speed difference between the drive wheels 3. In this way, for example based on a driving mode selected by the driver in order to prefer “performances” or “ease of driving”, the torque delivered by the motors 5 to the wheels 3 can be differentiated irrespective of the angular speed conditions of the drive wheels 3. In particular, thanks to the elimination of the aforesaid bond between the difference in angular speed and the difference in torque delivered to the wheels 3, it is possible to improve the maintenance of a straight trajectory on a bumpy or non-flat road.
According to
In particular, the control unit 10 determines the variation β′ of the attitude angle β and calculates the torque to be delivered to each drive wheel 3 as a function of the variation β′ of the attitude angle.
According to the preferred embodiment shown in
Advantageously, though not necessarily, the variation β′ of the attitude angle β is calculated without using a numerical derivative of the attitude angle β of the vehicle, but is calculated through a non-linear combination of yaw rate ψ′, lateral acceleration (detected or determined by means of known models of the road vehicle 1) and linear speed Vx of the vehicle 1. In this way, it is possible to avoid the occurrence of those approximations that would naturally be necessary in calculating the variation β′ of the attitude angle β deriving the attitude angle β itself in time.
According to the preferred, though non-limiting embodiments of
Advantageously, though not necessarily, the control unit 10, in particular the controller CTR, limits (by means of a suitable slip control SLC) the delivery of the torque TR, TL from each motor 5 to the respective drive wheel 3 in case a target delivery (resulting from the sum of a yawing contribution ΔTNL and an adverse-yawing contribution ΔTLIN, as described more in detail below) exceeds a limit determined by the grip of a respective tyre installed on one of the drive wheels 3. In particular, said limit is calculated according to known techniques as a function of the properties of the tyre, of the longitudinal speed Vx of the vehicle 1 and of the angular speeds VRR and VRL of the right drive wheel 3 and of the left drive wheel 3, respectively (
As mentioned above, advantageously, though not necessarily, the controller CTR processes the torque TR, TL to be delivered to each drive wheel (right and left wheel, respectively) by adding the (in particular, linear) yawing contribution ΔTNL and the (in particular, non-linear) adverse-yawing contribution ΔTLIN to one another. In particular, the yawing contribution ΔTNL and the adverse-yawing contribution ΔTLIN each indicate a torque difference between the two drive wheels 3 belonging to the same (rear) axle RA. More precisely, the yawing contribution ΔTNL and the adverse-yawing contribution ΔTLIN are values in Nm that indicate, of a total input torque FZM of the rear axle, what difference there has to be in the delivery of torque between the right drive wheel 3 and the left drive wheel 3.
Advantageously, though not necessarily, the adverse-yawing contribution ΔTLIN is calculated (by the controller CTR) as a function of the variation β ′ of the attitude angle β of the road vehicle 1, of the yaw rate ψ′ and of the longitudinal speed Vx of the road vehicle (1) itself.
In particular, the adverse-yawing contribution ΔTLIN results from the sum of a contribution RT based on the variation β′ of the attitude angle β and of a contribution SL based on the yaw rate ψ′. More precisely, the contribution RT provides an adverse-yawing contribution to the final control output, which counters more sudden trajectory variations. These variations are typically caused by a road input disturbance d or by transient dynamics, both linear and non-liner ones. In other words, the contribution RT relates to the area of the dynamics behaviour of the road vehicle 1 concerning straightness and transients.
On the other hand, the contribution SL based on the yaw rate ψ′ provides an adverse-yawing contribution to the final output useful to modulate a lateral stationary response of the road vehicle 1 relative to steering inputs (SW) provided by the driver. Therefore, by means of this control, the steering wheel torque perceived by the driver can be changed, as well. When modulating (calibrating) the lateral stationary response of the vehicle, the multiplying coefficient of the yaw rate ψ′ depends on the longitudinal speed Vx and depends on the requests of the driver by means of the table LUT (lookup table).
Advantageously, though not necessarily, the yawing contribution ΔTNL is calculated (by the controller CTR) as a function of the lateral dynamics VDL of the vehicle (
In particular, the relation between TR−TL and MZ is preferably determined, in an unequivocal manner, by the equations of the dynamics of the vehicle (namely, not by the controller CTR). In detail, said relation translates itself into a multiplying gain resulting from car track width t and wheel radius R, wherein MZ=0.5*(TR−TL)/R*t.
In particular, the yawing contribution ΔTNL results from the product between the distribution FZ % of the vertical forces between the drive wheels 3 and the total input torque FZM of the rear axle. In detail, the distribution FZ % of the vertical forces between the drive wheels 3 indicates the percentage of vertical force acting upon the right and left drive wheels 3 relative to the total force acting upon the rear axle RA. Block % indicated in
In some non-limiting cases, the control unit 10 is configured to control the torque TR delivered by the respective motor 5 to the right drive wheel 3 so as to compensate for a possible failure of the motor 5 acting upon the left drive wheel 3 and vice versa.
Advantageously, though not necessarily, alternatively or in addition, the control unit 10 is configured to monitor one or more actuators other than the motors 5 (for example the suspensions 8, in particular the shock absorbers 9) and to control the torques TR, TL delivered to the right drive wheel 3 and/or the left drive wheel 3 so as to compensate for possible failures of said one or more actuators, so as to maintain the target trajectory T of the road vehicle 1.
According to a non-limiting embodiment, the control unit 10 estimates the trajectory T followed by the road vehicle 1 using the measures provided in real time by a triple-axis gyroscope and by a GPS tracking unit; in particular, the trajectory T is determined by integrating twice in time the accelerations measured by the triple-axis gyroscope and the measures provided by the GPS tracking unit are used to cyclically cancel the position errors occurring during the integration process. Furthermore, the control unit 10 estimates the driving speed V of the road vehicle 1 in the centre of gravity B using the measures provided in real time by the triple-axis gyroscope; in particular, the speed V of the road vehicle 1 in the centre of gravity B is determined by integrating once in time the accelerations measured by the triple-axis gyroscope (making sure that the driving speed V of the road vehicle 1 in the centre of gravity B actually is tangent to the trajectory T followed by the road vehicle 1, otherwise, in case of a significant deviation, at least one further iteration of the calculation is carried out making corrections to the parameters used).
While driving along a bend, the control unit 10 determines in real time (for example, as described below) the variation β′ of the actual (real) attitude angle β of the road vehicle 1.
According to a possible (though non-binding) embodiment, the control unit 10 cyclically estimates (for example, with a frequency of at least some dozens of Hz and in a known manner) a grip of the wheels 2 and 3 to the road surface, determines a radius of curvature of the trajectory T of the road vehicle 1 (namely, determines a curvature degree of the trajectory T) and determines the driving speed V of the road vehicle 1. As a function of the grip of the wheels 2 and 3 (hence, of the stability of the road vehicle 1), of the radius of curvature of the trajectory T and of the driving speed V, the control unit 10 cyclically determines the variation β′ of the attitude angle.
The control unit 10 detects or calculates the speed β′ of variation of the attitude angle β and changes the torque delivered to the drive wheels 3 by the respective electric motors 5.
In the non-limiting embodiment of
Following the processing of the model VDL, the control unit 10 sends to the controller CTR a plurality of inputs, among which there are:
Furthermore, the controller CTR is provided with the input torque Ti, namely the request for driving or braking torque provided based on the condition of at least one pedal (for example, accelerator pedal).
The controller CTR provides, as an output towards the motors 5 of the motorized (rear) axle RA, the torques TR, TL to be delivered, sending their difference TR−TL to the proportionality factor G.
In particular, as mentioned above, the final output of the control (before the check of the slip control SLC) is calculated as the sum of the contributions handling each area of dynamic behaviour of the car (yawing and adverse-yawing contributions).
To sum up, the total torque FZM requested (or Ti), thanks to the control described so far, is divided between the torques TR and TL directed to the drive wheels 3 without bonds due to the difference in the angular speed thereof.
Furthermore, there is provided a calibration of the controller CTR processing the right gains, thresholds, loading tables, etc. (for example, K for β or the lookup table LUT for Vx or in the slip control SCL), so as to pursue target car dynamics, which maximize know performance indicators (lap time on track) and driving pleasure. This calibration allows, in each moment and based on the conditions of the car, the more important contribution ΔTNL, ΔTLIN to prevail.
The control method described above has different advantages.
First of all, the advantage relative to a system with a mechanical differential lies in the removal of the bond between torques and angular speed of the wheels of the same axle.
Furthermore, the aforesaid method maintains the stabilizing ability of a mechanical differential with a limited slip, exploiting, at the same time, the potentialities offered by the freedom to freely deliver different torques to the drive wheels, even in conditions in which they have the same angular speed (which is not possible with known systems).
In addition, the control described above easily maintains a straight trajectory, even on a bumpy or inclined road, as well as manages, for example as a function of the position of a hand lever or based on the driver's requests, the yawing and adverse-jawing contributions, so as to favour, for example, the first one, in case performances are the target, or the second one, in case a simplified driving of the car is the target.
Moreover, the control method described above is particularly safe, for it counters a yaw that is undesired by the driver and is caused by possible failures detected in other actuators, promptly and effectively intervening in case of need.
Finally, the control method described above is simple and economic to be implemented in a road vehicle provided 1 with a motor for each drive wheel, since it does not require the addition of any physical component (in fact, the differentials can be removed, thus facilitating the maintenance and the flexibility of the vehicle 1), and can completely be carried out via software. It should be pointed out that the control method described above does not use either a high computing ability or a large memory space and, therefore, it can be implemented in a control unit with no need for updates or boosts.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102021000015206 | Jun 2021 | IT | national |
