FR3146638A1 - A braking system and a vehicle - Google Patents

Abstract

A method for dynamically braking a rear wheel of a vehicle equipped with an electric parking brake, by implementing an RWU braking mode, the RWU braking mode being ensured by controlling the parking brake in compliance with the following rules, while the slip (13) of the wheel is measured: - if the slip (13) is less than a first predetermined threshold (3), progressive application of the brake until the slip (13) becomes greater than this first threshold (3), - if the slip (13) is greater than a second predetermined threshold (4), greater than the first threshold (3), progressive release of the brake until the slip (13) becomes less than this second threshold (4), - if the slip (13) is greater than the first threshold (3) and less than the second threshold (4), maintaining the application at its current level, the method being characterized in that it consists in executing at least the following three steps during each dynamic braking request: - test: application of a test application at a level of application of predetermined test, then waiting for an observation time, then measuring the slip (13), - selection: if the slip (13) measured is greater than a predetermined trigger value, selection of an intermittent braking mode, otherwise, selection of the RWU braking mode and - braking: actuation of the parking brake in the mode selected during the selection step. Figure for abstract: figure 7

Description

A braking system and a vehicle

The present invention relates to the technical field of electric parking brakes, and in particular to the field of using electric parking brakes during dynamic braking.

As described in US 7,744,166, a parking brake is implemented electrically and/or automatically by an electromechanical unit, also called an electric actuator, associated with one or more of the vehicle's wheels. Thus, a user of the vehicle wishing to engage the parking brake simply needs to press a push button, located for example on the dashboard near the steering wheel, to trigger or terminate the application of the parking brake around the vehicle's wheel.

We already know the RWU braking mode, for "Rear Wheel Unlocker". It consists of using the parking brake of the rear wheels to accompany dynamic braking of the vehicle. The goal is to apply the brakes of the rear wheels without locking them. More precisely, in the event of a wheel locking, the brake is released to unlock the wheel, then it is re-tightened, etc. This control mode is inspired by the hydraulic anti-lock braking system marketed under the ABS brand, but differs in that it uses the electric control of the brake motor.

Since the parking brake has been electrically controlled, the presence of means to implement the RWU braking mode has become a common requirement of automobile manufacturers.

More precisely, the RWU braking mode consists of performing the following operations in a loop:
- if the slippage is below a first predetermined threshold, called the “low threshold”, progressive application of the brake until the slippage becomes above this low threshold,
- if the slip is greater than a second predetermined threshold, called the “high threshold”, greater than the low threshold, progressive release of the brake until the slip becomes lower than this high threshold,
- if the slip is greater than the low threshold and less than the high threshold, maintaining the tightening at its current level.

The thresholds depend on the vehicle model. They must be set empirically by carrying out full-scale tests on each vehicle model or at least on the model in a range that is most representative of it. These parameters are set so that the vehicle complies with the legal requirements both on wet roads, considered slippery, and on dry roads, considered grippy. In short, setting the parameters for the RWU braking mode amounts to an intermediate calibration that achieves a compromise between very different braking conditions.

One problem with the RWU braking mode is that this compromise is not compatible with a search for braking optimization according to the road condition. Another problem is that the parameters of the RWU braking mode, in particular the thresholds, must be set empirically for each new vehicle model.

An object of the invention is a method for dynamically braking a rear wheel of a vehicle equipped with an electric parking brake, by implementing an RWU braking mode, the RWU braking mode being ensured by controlling the parking brake in compliance with the following rules, while the slippage of the wheel is measured:
- if the slip is less than a first predetermined threshold, progressive application of the brake until the slip becomes greater than this first threshold,
- if the slip is greater than a second predetermined threshold, greater than the first threshold, progressive release of the brake until the slip becomes lower than this second threshold,
- if the slip is greater than the first threshold and less than the second threshold, maintaining the tightening at its current level,
the method being characterized in that it consists of executing at least the following three steps during each dynamic braking request:
- test: application of a test tightening at a predetermined test tightening level, then waiting for an observation period, then measuring the slip,
- selection: if the measured slip is greater than a predetermined trigger value, selection of an intermittent braking mode, otherwise, selection of the RWU braking mode and
- braking: actuation of the parking brake in the mode selected during the selection step.

An advantage of the method according to the invention is that the braking mode selected for actuating the parking brake is adapted to the road condition. If the road is dry, RWU braking is applied, while if the road is wet, intermittent braking is applied. Another advantage is that the parameters such as trigger value and thresholds are valid for a vehicle category, i.e. valid for different vehicle models with similar masses and dimensions. The scope of each category is, however, limited.

In this description, dynamic braking request means a braking command actuated by the driver of the vehicle – human being, robot or artificial intelligence – according to the needs of the traffic, while the vehicle is in motion.

In this description, slip is understood to mean that the translation speed of the wheel is greater than its rotation speed multiplied by its diameter. A method for measuring slip is known from document FR2903063A1.

According to a particular mode of implementation of the dynamic braking method, the measurement of the slip of a rear wheel consists of a comparison of the rotation speeds of the rear wheel and of a front wheel located on the same side of the vehicle as the rear wheel. This comparison can be expressed as a percentage of speed.

According to a particular implementation mode of the dynamic braking process, the observation duration of the test step is between 20ms and 100ms.

According to a particular mode of implementation of the dynamic braking method, the intermittent braking mode is carried out at a braking level higher than the test braking level.

According to a particular mode of implementation of the dynamic braking process, the intermittent braking mode consists of executing the following steps in a loop:
- additional waiting for a predetermined latency period,
- braking in accordance with the following rules, while the wheel slip is measured:
> if the slip is below the first threshold, apply moderate tightening for a short braking period,
> if the slip is greater than the second threshold, apply a progressive loosening until the slip becomes less than the second threshold,
> if the slip is greater than the first threshold and less than the second threshold, maintain the tightening at its current level.

According to a particular implementation mode of the dynamic braking process, the latency duration is between 20ms and 100ms.

According to a particular implementation mode of the dynamic braking process, the brief braking duration is between 10ms and 100ms.

According to a particular implementation mode of the dynamic braking process, the first threshold is between 0.01 and 0.05.

According to a particular implementation mode of the dynamic braking process, the second threshold is between 0.03 and 0.07.

According to a particular mode of implementation of the dynamic braking method, the predetermined trigger value is zero.

The choice between zero and a non-zero but low value, e.g. 0.01, for the trigger value depends mainly on the parking brake.

According to a particular mode of implementation of the dynamic braking method, different sets of parameters for the RWU braking mode and/or the intermittent braking mode have been previously prepared, making it possible to brake differently depending on the road grip state and therefore to apply different RWU braking modes and/or intermittent braking adapted to these different grip states, and the method comprises a preliminary step of choosing a set of parameters according to the slip measured during the test step, from among these different sets of parameters.

An advantage of using different parameter sets is therefore that we optimize the braking mode that will be applied depending on the road grip condition, whether this braking mode is of the RWU type or of the intermittent type.

According to a particular mode of implementation of the dynamic braking method, in the case where, during RWU braking, the slip is greater than the first threshold and less than the second threshold, then the progress of the method continues by returning to the test step.

An advantage of returning to the test stage, if conditions permit, is to select a parameter set more suited to the road condition, for example because the vehicle has left a wet area of the roadway.

Another object of the invention is a braking system, characterized in that it comprises means capable of implementing the method as described above.

Another subject of the invention is a motor vehicle, characterized in that it comprises at least one braking system according to the invention.

Brief description of the figures

The invention will be better understood from reading the attached figures, which are provided as examples and are not limiting in any way, in which:

is a graphical representation of the electric current flowing in a parking brake and of the slippage of a wheel equipped with this brake, for RWU braking according to the state of the art, in a first example of dynamic braking on a dry road,

is a graphical representation of the electric current flowing in a parking brake and of the slippage of a wheel equipped with this brake, for RWU braking according to the state of the art, in a second example of dynamic braking, on a wet road,

is a graph of the measurements taken on a vehicle during dynamic braking, during RWU braking in the case of the second example,

is a graphical representation of the electric current and the slip during a test step of a method according to the invention, on a dry road,

is a graphical representation of the electric current and the slip during a braking step of a method according to the invention, on a dry road,

is a graphical representation of the electric current and slip during a test step of a method according to the invention, on a wet road,

is a graphical representation of the electric current and the slip during a braking step of a method according to the invention, on a wet road,

is a graph of the measurements taken on a vehicle during dynamic braking, during braking according to the method of the invention, on a wet road.

Detailed description

On the and the following figures 3 to 7, two curves have been drawn one above the other. The top curve represents, as a function of time, the current 1 flowing in the motor of an electric rear wheel brake (not shown).

This current 1 translates the resistance encountered by the motor, therefore in particular the clamping force exerted by this brake on the wheel when the clamping member (not shown) is in contact with the friction member (not shown).

The curve below represents, as a function of time, the slip 2 of the wheel. The time scales are synchronized between the two curves, so that the effect of current 1 on slip 2 at a given time is directly visible.

At time t0, no braking force is applied. The wheel does not slip at all but rolls on the road.

At time t1, we start to apply the brake according to the RWU braking mode, which we recall respects the following rules:
- if slip 2 is less than a first predetermined threshold 3, called “low threshold 3”, progressive application of the brake until slip 2 becomes greater than this first threshold 3,
- if slip 2 is greater than a second predetermined threshold 4, called “high threshold 4”, greater than low threshold 3, progressive release of the brake until slip 2 becomes lower than the high threshold,
- if slip 2 is between the two thresholds, maintaining the tightening at its current level.

The electric current 1 of the motor passes through a peak 5 which corresponds to the brake being released and set in motion. The current 1 then remains at a plateau 6, which corresponds to the movement of parts used to take up the functional clearance between the clamping member and the friction member of the brake, until at time t2, the brake reaches a position of contact of its clamping member against its friction member, a position from which a clamping force occurs. The wheel then begins to undergo a slowdown which results in a slip 2.

The low threshold 3 and the high threshold 4 have been previously set depending on the vehicle model.

As the brake application increases, the slowing down of the wheel increases. When at time t3, the slip 2 of the wheel crosses the low threshold 3, the brake application force is stopped. The application maintained in place, advantageously by an irreversible mechanism for converting the rotational movement of the motor into a translational movement of a brake pad application member on a brake disc, continues to produce its effect and the slip 2 of the wheel continues to increase. It crosses the high threshold 4 at time t4.

When slip 2 crosses the high threshold 4, a brake release movement is initiated by operating the brake motor in the opposite direction. Current 1 flowing in the opposite direction in the brake motor passes through a peak 5 corresponding to the start of motion. Slip 2 of the wheel eventually stops increasing, then decreases.

At time t5, when slip 2 passes below high threshold 4, the release is stopped and the brake is maintained. Wheel slip 2 stabilizes by remaining between low threshold 3 and high threshold 4.

Thus, the parking brake contributed to the dynamic braking of the vehicle by preventing the wheel from locking.

On the , the same two synchronized curves showing the electric current 1 of the motor and the slip 2 of the wheel are shown, on an example of dynamic braking in which the road is wet, therefore more slippery than in the first example. A snowy or icy road could also be considered slippery.

We apply the same operations as described previously, namely:
- from time t0 to time t1, brake application until time t3, when slip 2 exceeds low threshold 3,
- at time t4, release of the brake when slip 2 exceeds high threshold 4.

A difficulty arises in this second example, due to the fact that:
1/ stopping the tightening at time t3 does not prevent slip 2 from continuing to rise very significantly, well beyond the high threshold 4 (dotted box containing t2, t3 and t4), and
2/ releasing the brake causes slip 2 to pass well below the high threshold 4, which is crossed at time t5, but also below the low threshold 3, which is crossed at time t6 (dotted box containing t5 and t6), so that the brake is released again.

In this example, the brake is applied and released several times, so the wheel is locked and unlocked several times, without slip 2 ever stabilizing in the desired interval between low threshold 3 and high threshold 4.

On the , three curves are shown superimposed on the same time scale, reflecting measurements taken during dynamic braking.

The top curve shows the variations of the electric current 7 in the brake motor.

The middle curve shows the variations of the wheel slip 8.

The curve below shows the variations in the rotation speed 9 of the wheel.

As explained with reference to the , because the wheel is constantly blocked, then unlocked, without reaching a moderate slip level 8 between the low threshold 3 and the high threshold 4:
- the current 7 in the motor reaches numerous peaks 10 to set said motor in motion,
- the slide 8 oscillates for a long time between extreme values 11 and zero,
- the maximum rotation speed 9 of the wheel decreases in an approximately linear manner, thanks to the successive small braking operations, but each of these braking operations results in the wheel locking, since its speed is almost zero between two releases.

Braking does occur, of course, but in a way that the invention allows to be optimized.

On the , the changes in current 12 and slip 13 during implementation of the method according to the invention are shown.

The operations at times tt0, tt1 and tt2 are the same as in the state of the art for times t0, t1 and t2.

At time tt2, wheel braking begins to occur.

Unlike the previous examples, the tightening is not increased here until the low threshold 3 is crossed. It increases only for a short observation period, up to a time tt3, at which time the slip 13 has not yet reached the level that was reached at time t3 in the previous examples. The slip 13 at time tt3 is either zero, if the road is sufficiently grippy, or very low, if the road is sufficiently slippery, but it is in all cases very small and significantly lower than the low threshold 3 of slip 13. In order to ensure that the slip 13 at time tt3 is very small, either the difference between times tt2 and tt3 or a cap on the current 12 delivered to the motor during this test step has been chosen in advance.

The braking applied at time tt3 is therefore at a predetermined test clamping level. The clamping is then maintained at the same level, without releasing the brake, until time tt4, spaced tt3 apart by a predetermined observation time.

At the end of the observation period, i.e. at time tt4, the effect of the tightening on the wheel is felt and a slight slip 13 of the wheel is measured. Here again, the slip 13 at time tt4 is either zero, if the road is sufficiently grippy, or slight, if the road is sufficiently slippery, but it is in all cases moderate and lower than the low threshold 3 of slip 13. In order to guarantee that the slip 13 at time tt3 is moderate, the observation period which defines time tt4 was previously chosen. At time tt4, the slip 13 is observed.

A choice is then made during the selection step: if the slip 13 measured at time tt4 is greater than a predetermined trigger value 14, an intermittent braking mode is selected, otherwise, the classic RWU braking mode is selected.

In the example of the , the slip 13 at time tt4 is always zero, therefore less than the trigger value 14. We therefore choose the RWU braking mode.

Finally, the wheel is braked according to the braking mode thus selected, i.e. here the RWU braking mode. The braking operation according to the RWU mode is not described again here since it is the subject of the . It is observed on the , where RWU braking follows the testing and selection steps just described.

In the example of the , we apply the same testing and selection steps as in the example in Figures 4 and 5.

The difference between these two examples appears at time tt4, where the slip is no longer zero. It is greater than the trigger value 14 but still moderate and less than the low threshold 3, thanks to the prior settings of tt3 and tt4.

In this case, the selection step which follows the test step leads to retaining the intermittent braking mode, the progress of which is illustrated by the .

Intermittent braking consists of the following two stages of waiting and braking:
- waiting for a predetermined latency period of 16,
- braking in accordance with the following rules, while the wheel slip is measured:
> if the slip is below the low threshold 3, apply moderate tightening for a short braking period,
> if the slip is greater than the high threshold 4, apply a progressive loosening until the slip becomes lower than the high threshold 4,
> if the slip is greater than the low threshold 3 and less than the high threshold 4, maintain the tightening at its current level.

At time tt4, in application of the intermittent braking mode which is triggered, the waiting step is carried out for a predetermined latency period 16: the clamping is maintained as is, without modification, and the slip is measured at the end of the latency period 16, at time tt5. The difference between times tt4 and tt5 corresponds to the latency period 16.

The latency duration 16 allows to observe the effect of the tightening on the brake and to avoid that, as in the examples of and 3, the delayed effect of braking does not accumulate with its increase and causes an overflow of the slip.

Thus, at time tt5, braking is applied in accordance with the following rules, while wheel slip continues to be measured:
- if the slip is below the low threshold 3, application of moderate tightening for a short braking duration,
- if the slip is greater than the high threshold 4, apply a progressive loosening until the slip becomes lower than the high threshold 4,
- if the slip is greater than the low threshold 3 and less than the high threshold 4, maintaining the tightening at its current level.

In the present case, at time tt5, the slip is below the low threshold 3. A moderate clamping is therefore applied which results in the injection of a current which actuates the brake motor, first at peak 17, then decreasing, until time tt6 from which the increase in clamping ceases. The motor is simply left in its clamping position.

Finally, in accordance with the definition of the intermittent clamping mode, the waiting step is resumed for the latency period 16, to give the wheel time to undergo the effects of the moderate braking it has just undergone.

The slip measurement is performed again at the end of the latency period 16, at a time tt7. In this case, the latency period 16 between tt6 and tt7 is equal to the latency period 16 between tt4 and tt5.

We note, at time tt7, that the slip has increased to a value between the low threshold 3 and the high threshold 4.

According to the definition of intermittent tightening mode, the tightening is maintained at its current level, without moving the motor.

Then the intermittent clamping mode loop resumes, waiting for latency duration 16, then applying the appropriate action.

On the , which is similar to the , we see the measurements of:
- current 19 in the motor,
- 20 wheel slip and
- wheel rotation speed 21.

It can be seen that thanks to the intermittent tightening mode, the slip 20 of the wheel is better controlled and its speed no longer oscillates between zero – wheel blocked – and its maximum – free wheel –. On the contrary, the wheel is almost never blocked and its rotation speed 21, limited in a more restricted interval, decreases more quickly.

Thus, under the same conditions as in the example in figures 2 and 3, with the intermittent braking mode we obtain a vehicle stop in 18 seconds instead of 32 seconds, with a braking distance of 132 meters instead of 241 meters with an initial speed of 53 km/h.

The invention is not limited to the embodiments presented and other embodiments will become apparent to those skilled in the art.

List of references

1: Current
2: Sliding
3: First threshold
4: Second threshold
5: Peak
6: Plateau
7: Current
8: Sliding
9: Rotation speed
10: Peaks
11: Extreme values
12: Current
13: Slide
14: Trigger value
16: Latency duration
17: Peak
19: Current
20: Slide
21: Rotation speed
t0: No braking force applied
t1: Start of brake application
t2: Contact of the clamping member against the friction member
t3: Crossing of low threshold 3
t4: Crossing of high threshold 4
t5: Crossing of high threshold 4
t6: Crossing of low threshold 3
tt0: No braking force is applied
tt1: Start of brake application
tt2: Contact of the clamping member against the friction member
tt3: Zero or very low slip
tt4: End of observation time
tt5: Slip below low threshold 3
tt6: Stopping the increase in tightening
tt7: Slippage between low threshold 3 and high threshold 4

Claims (14)

Method for dynamic braking of a rear wheel of a vehicle equipped with an electric parking brake, by implementing an RWU braking mode, the RWU braking mode being ensured by controlling the parking brake in compliance with the following rules, while the slip (13) of the wheel is measured:
- if the slip (13) is less than a first predetermined threshold (3), progressive application of the brake until the slip (13) becomes greater than this first threshold (3),
- if the slip (13) is greater than a second predetermined threshold (4), greater than the first threshold (3), progressive release of the brake until the slip (13) becomes lower than this second threshold (4),
- if the slip (13) is greater than the first threshold (3) and less than the second threshold (4), maintaining the tightening at its current level,
the method being characterized in that it consists of executing at least the following three steps during each dynamic braking request:
- test: application of a test tightening at a predetermined test tightening level, then waiting for an observation period, then measuring the slip (13),
- selection: if the measured slip (13) is greater than a predetermined trigger value (14), selection of an intermittent braking mode, otherwise, selection of the RWU braking mode and
- braking: actuation of the parking brake in the mode selected during the selection step. Dynamic braking method according to claim 1, in which the measurement of the slip (13) of a rear wheel consists of a comparison of the rotational speeds of the rear wheel and of a front wheel located on the same side of the vehicle as the rear wheel. Dynamic braking method according to any one of claims 1 and 2, in which the observation duration of the test step is between 20ms and 100ms. A dynamic braking method according to any one of claims 1, 2 and 3, wherein the intermittent braking mode is performed at a braking level higher than the test braking level. Dynamic braking method according to any one of claims 1, 2, 3 and 4, in which the intermittent braking mode consists of performing the following steps in a loop:
- additional waiting for a predetermined latency period (16),
- braking in accordance with the following rules, while the wheel slip (13) is measured:
> if the slip (13) is lower than the first threshold (3), application of moderate tightening for a short braking period,
> if the slip (13) is greater than the second threshold (4), application of a progressive loosening until the slip (13) becomes less than the second threshold (4),
> if the slip (13) is greater than the first threshold (3) and less than the second threshold (4), maintaining the tightening at its current level (12). Dynamic braking method according to claim 5, in which the latency duration (16) is between 20ms and 100ms. Dynamic braking method according to any one of claims 5 and 6, in which the brief braking duration is between 10ms and 100ms. Dynamic braking method according to any one of claims 5, 6 and 7, in which the first threshold (3) is between 0.01 and 0.05. Dynamic braking method according to any one of claims 5, 6, 7 and 8, in which the second threshold (4) is between 0.03 and 0.07. A dynamic braking method according to any one of claims 1, 2, 3, 4, 5, 6, 7, 8 and 9, wherein the predetermined trigger value (14) is zero. Dynamic braking method according to any one of claims 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10, in which different sets of parameters for the RWU braking mode and/or the intermittent braking mode have been previously prepared, making it possible to brake differently depending on the road grip state and therefore to apply different RWU braking modes and/or intermittent braking modes adapted to these different grip states, and the method comprises a preliminary step of choosing a set of parameters according to the slip measured during the test step, from among these different sets of parameters. Dynamic braking method according to any one of claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11, in which, in the case where, during RWU braking, the slip is greater than the first threshold and less than the second threshold, then the course of the method continues by returning to the test step. Braking system, characterized in that it comprises means capable of implementing the method according to any one of claims 1 to 12. Motor vehicle, characterized in that it comprises at least one braking system according to claim 13.