Method for controlling road vehicle by micro-slip of clutch

Abstract

A control method for a road vehicle (1) having a clutch (16) which connects an internal combustion engine (4) to a drive wheel (3) and is arranged upstream of a servo-assisted gearbox (7); the control method comprises the following steps: checking whether the tires of the drive wheels (3) are approaching the grip limit; and opening the clutch (16) when the tires of the drive wheels (3) are approaching the grip limit, so that the clutch (16) transmits torque to the drive wheels (3) with constant and non-zero slip of the clutch (16).

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This patent application claims the priority of Italian Patent Application No. 102019000017531 filed on September 30, 2019, the disclosure of which is incorporated herein in its entirety by reference. Technical Field

[0003] The invention relates to a control method for a road vehicle.

[0004] The invention is advantageously used in a drive train having a dual-clutch servo-assisted gearbox, to which explicit reference will be made in the following description without loss of generality. Background Art

[0005] The powertrain with a dual-clutch servo-assisted gearbox comprises: a pair of main shafts, which are coaxial, independent of each other and inserted into each other; two coaxial clutches, each of which is designed to connect the corresponding main shaft to the drive shaft of the internal combustion engine; and at least one secondary shaft, which transmits movement to the drive wheels and can be coupled to the main shaft by a corresponding gear train, each defining a gear.

[0006] During a gear shift, the current gear couples the countershaft to one main shaft, while the subsequent gear couples the countershaft to the other main shaft; as a result, the gear shift is performed by alternating the two clutches, ie by opening the clutch associated with the current gear and simultaneously closing the clutch associated with the subsequent gear.

[0007] It has been found that when the tires of the drive wheels approach the limit of their grip (usually when a low gear is engaged in the gearbox and the grip of the road surface is insufficient, for example due to the presence of water), a phenomenon known as "stick-slip" often occurs, which is characterized by a continuous and gradual loss and recovery of grip; in other words, the tires of the drive wheels grip the ground along a short section, the tires lose grip and slip along a subsequent section, the tires regain grip along another subsequent section, and so on.

[0008] The phenomenon known as "stick-slip" is disadvantageous because it generates vibrations in the drive train that are transmitted to the internal combustion engine, which in turn transmits these vibrations to the frame via the dampers used to fasten the internal combustion engine to the frame. The vibrations triggered by the phenomenon known as "stick-slip" almost always cause metallic noises, which are particularly annoying to the driver (because it sounds like something is broken in the road vehicle); in addition, in certain particularly unpleasant cases, the vibrations triggered by the phenomenon known as "stick-slip" can be amplified by structural resonances, which can generate torque peaks that can damage components of the drive train and the internal combustion engine (fatigue fracture after a given time or sudden fracture also with particularly strong impact stresses).

[0009] Patent application US2003183031A1 describes an automatic transmission in which the movement of the vehicle can be started in the forward or backward direction and a vehicle driving mode suitable for slippery roads can be automatically reached. For this purpose, a driving wheel slip detector is provided for detecting whether the driving wheel is slipping; a secondary clutch is used to engage with one of the gear trains other than the first gear and reverse gear trains in the gear train; and a controller is used to connect the secondary clutch when the shift position is in the first gear or reverse gear and when the driving wheel slip detector detects that the driving wheel is slipping. Summary of the invention

[0010] The object of the present invention is to provide a method for controlling a road vehicle which avoids the adverse consequences of the above-mentioned phenomenon known as "stick-slip" and which is at the same time easy and economical to implement.

[0011] According to the invention, there is provided a method for controlling a road vehicle having a clutch connecting an internal combustion engine to drive wheels and arranged upstream of a servo-assisted gearbox, the control method comprising the following steps:

[0012] Check whether the drive wheel tires are close to the limit of grip; and

[0013] When the tires of the drive wheels approach the limit of grip, the clutch is opened so that the clutch transmits torque to the drive wheels with constant and non-zero slip of the clutch.

[0014] Therein, the tires of the drive wheels are considered to be approaching a grip limit when: in a servo-assisted gearbox, a gear belonging to a predetermined group of gears is engaged; and the torque generated by the internal combustion engine exceeds a torque threshold.

[0015] The appended claims describe preferred embodiments of the invention and form an integral part of the description. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention will now be described with reference to the accompanying drawings showing non-limiting embodiments of the invention, in which:

[0017] Figure 1 is a schematic plan view of a rear-wheel drive road vehicle provided with a powertrain having a dual-clutch servo-assisted gearbox and controlled according to the control method of the present invention;

[0018] Figure 2 yes Figure 1 A schematic diagram of the powertrain; and

[0019] Figure 3 is a block diagram of the control logic implemented in the control unit of the powertrain. DETAILED DESCRIPTION

[0020] exist Figure 1 In the figure, reference numeral 1 generally denotes a road vehicle, in particular a car, having two front driven (i.e. non-driven) wheels 2 and two rear driven wheels 3. In the front position there is an internal combustion engine 4 having a drive shaft 5 which generates a torque which is transmitted to the drive wheels 3 via a drive train 6. The drive train 6 comprises a dual clutch servo-assisted gearbox 7 arranged in a rear wheel drive assembly and a transmission shaft 8 connecting the drive shaft 5 to the input of the dual clutch servo-assisted gearbox 7. The dual clutch servo-assisted gearbox 7 is connected in a gear train manner to a self-locking differential 9, from which a pair of half shafts 10 originate, each integral with the drive wheels 3.

[0021] The road vehicle 1 comprises a control unit 11 of an engine 4, which controls the engine 4, a control unit 12 of a powertrain 6, which controls the powertrain 6, and a bus line 13, which is made, for example, according to the CAN (Car Area Network) protocol, extends over the entire road vehicle 1 and allows the two control units 11 and 12 to communicate with each other. In other words, the control unit 11 of the engine 4 and the control unit 12 of the powertrain 6 are connected to the bus line 13 and can therefore communicate with each other by means of information sent via the bus line 13. Furthermore, the control unit 11 of the engine 4 and the control unit 12 of the powertrain 6 can be directly connected to each other via a dedicated synchronization cable 14, which is capable of directly transmitting signals from the control unit 12 of the powertrain 6 to the control unit 11 of the engine 4 without delays caused by the bus line 13. Alternatively, the synchronization cable 14 may not be present and all communications between the two control units 11 and 12 may be exchanged using the bus line 13.

[0022] according to Figure 2, the dual-clutch servo-assisted gearbox 7 comprises a pair of main shafts 15, which are coaxial, independent of each other and inserted into each other. In addition, the dual-clutch servo-assisted gearbox 7 comprises two coaxial clutches 16, each of which is designed to connect the corresponding main shaft 15 to the drive shaft 5 of the internal combustion engine 4 through the intervention of the transmission shaft 8; each clutch 16 is an oil bath clutch and is therefore pressure-controlled (i.e., the degree of opening / closing of the clutch 16 is determined by the pressure of the oil in the clutch 16); according to an alternative embodiment, each clutch 16 is a dry clutch and is therefore position-controlled (i.e., the degree of opening / closing of the clutch 16 is determined by the position of the movable element of the clutch 16). The dual-clutch servo-assisted gearbox 7 comprises a single secondary shaft 17 connected to the differential 9, which transmits movement to the drive wheels 3; according to an alternative and equivalent embodiment, the dual-clutch servo-assisted gearbox 7 comprises two secondary shafts 17, both of which are connected to the differential 9.

[0023] The dual-clutch servo-assisted transmission 7 has seven forward gears (1st gear I, 2nd gear II, 3rd gear III, 4th gear IV, 5th gear V, 6th gear VI and 7th gear VII) indicated by Roman numerals and a reverse gear (indicated by R). The main shaft 15 and the countershaft 17 are mechanically coupled to each other through a plurality of gear trains, each of which defines a corresponding gear and includes a main gear 18 fixed to the main shaft 15 and a counter gear 19 fixed to the countershaft 17. In order for the dual-clutch servo-assisted transmission 7 to operate correctly, all odd-numbered gears (1st gear I, 3rd gear III, 5th gear V, 7th gear VII) are coupled to the same main shaft 15, while all even-numbered gears (2nd gear II, 4th gear IV and 6th gear VI) are coupled to another main shaft 15.

[0024] Each main gear 18 is connected to the corresponding main shaft 15 by a spline so as to always rotate in an integral manner with the main shaft 15, and is permanently meshed with the corresponding sub-gear 19; on the other hand, each sub-gear 19 is installed on the countershaft 17 in an idling manner. In addition, the dual-clutch servo-assisted transmission 7 includes four synchronizers 20, each of which is coaxially installed on the countershaft 17, disposed between two sub-gears 19, and designed to be operated to alternately fix the corresponding two sub-gears 19 to the countershaft 17 (that is, alternately make the corresponding two sub-gears 19 angularly integral with the countershaft 17). In other words, each synchronizer 20 can move in one direction to fix one sub-gear 19 to the countershaft 17, or can move in the other direction to fix another sub-gear 19 to the countershaft 17.

[0025] The dual-clutch gearbox 7 comprises a single layshaft 17 connected to the differential 9 , which transmits the movement to the drive wheels 3 ; according to an alternative and equivalent embodiment, the dual-clutch gearbox 7 comprises two layshafts 17 each connected to the differential 9 .

[0026] according to Figure 1 , the road vehicle 1 comprises a passenger compartment accommodating a driving position for a driver; the driving position comprises: a seat (not shown); a steering wheel 21; an accelerator pedal 22; a brake pedal 23; and two paddle shifters 24 and 25, which control a dual-clutch servo-assisted gearbox 7 and are connected to opposite sides of the steering wheel 21. The upshift paddle shifter 24 is operated by the driver (by means of a short pressure) to request an upshift (i.e., to engage a new gear that is higher than and adjacent to the current gear), while the downshift paddle shifter 25 is operated by the driver (by means of a short pressure) to request a downshift (i.e., to engage a new gear that is lower than and adjacent to the current gear).

[0027] In use, the control unit 12 of the drivetrain 6 checks whether the tires of the drive wheels 3 are close to the grip limit and then (slightly) opens the clutch 16 so that the clutch 16 transmits torque to the drive wheels 3 with a non-zero constant slip of the clutch 16. In other words, due to the slip, the rotation speed ω1 (at Figure 2 ) is greater than the speed ω2 of the driven disc 27 of the clutch 16 (in Figure 2 Therefore, the slip of the clutch 16 is equal to the absolute value of the rotational speed ω1 of the driving plate 26 of the clutch 16 and the rotational speed ω2 of the driven plate 27 of the clutch 16.

[0028] Generally speaking, the slip of the clutch 16 (i.e., the difference between the rotation speed ω1 of the driving plate 26 and the rotation speed ω2 of the driven plate 27) is in the range of 70 to 180 rpm in absolute value, and in the range of the rotation speed ω2 of the internal combustion engine 4 in relative value. E Therefore, if the tire of the driving wheel 3 approaches the grip limit, the clutch 16 is controlled to obtain the target slip ratio S of the clutch 16. T , which is usually established at the design stage and stored in the memory of the control unit 12 of the powertrain 6, the absolute value range is between 70 and 180 rpm, and the relative value range is between the speed ω of the internal combustion engine 4 E According to a feasible implementation, the target slip rate S of the clutch 16 is between 0.5% and 1.2%. T may not always be constant, but may depend on the gear engaged in the servo-assisted gearbox 7, the speed ω of the internal combustion engine 4 E and / or the torque T generated by the internal combustion engine 4 E In this case, the target slip rate S of the clutch 16 is provided in the memory of the power transmission system 6 of the control unit 12 is stored. TA map of values ​​of (usually refined experimentally with the aid of road tests).

[0029] If the following three situations occur at the same time, the control unit 12 of the powertrain system 6 will consider that the tire of the drive wheel 3 is close to the grip limit:

[0030] 1. In the servo-assisted gearbox 7, a gear belonging to a predetermined group of gears (including the lowest gear, i.e., generally speaking, first gear I, second gear II and third gear III) is engaged;

[0031] 2. Speed ​​ω of the internal combustion engine 4 E within a predetermined speed range (e.g., between 4000 and 8000 rpm); and

[0032] 3. Torque T generated by the internal combustion engine 4 E The torque threshold is exceeded.

[0033] According to a possible embodiment, the torque threshold is always constant. Alternatively, the control unit 12 of the powertrain 6 can determine the level of grip of the road surface on which the road vehicle 1 is located, and thus it can determine the torque threshold each time based on the level of grip of the road surface; in general, information about the level of grip is available via the bus line 13, because it is estimated (in a known manner) and shared by the brake control unit.

[0034] according to Figure 3 In a possible embodiment schematically shown in FIG. 1 , the control unit 12 of the powertrain 6 determines a target slip ratio S of the clutch 16. T , periodically determine the actual slip rate S of the clutch 16 R , detecting the torque T generated by the internal combustion engine 4 E , should be transmitted by clutch 16 and (slightly) less than the engine torque T E The clutch torque T C , control the clutch 16 to transmit the clutch torque T C And according to the target slip ratio S of the clutch 16 T The actual slip rate S of the clutch 16 R The difference between the clutch torque T changes periodically C .

[0035] In other words, the control unit 12 of the powertrain 6 changes the clutch torque T C , in order to obtain the target slip rate S of the clutch 16 T , that is, the clutch 16 always has the target slip ratio S T .

[0036] It should be noted that since the clutch 16 is controlled based on the (oil) pressure, the clutch torque T C Convert to the corresponding pressure value.

[0037] according to Figure 3 In the preferred embodiment shown, the control unit 12 of the powertrain 6 implements feedback control to adjust the clutch torque T C ; In feedback control, the control error ε S Equal to the target slip rate S of the clutch 16 T The actual slip rate S of the clutch 16 R In particular, feedback control requires the use of a subtractor block 28, which calculates the target slip ratio S of the clutch 16. T The actual slip rate S of the clutch 16 R The difference between the control error ε S , and it is also necessary to use a PID controller 29, which receives the control error ε S As an input value, the clutch torque T is provided for controlling the clutch 16. C As the output value. Figure 3 , a calculator block 30 is provided, which calculates the actual slip rate S of the clutch 16 that realizes the difference between the rotation speed ω1 of the driving plate 26 and the rotation speed ω2 of the driven plate 27 R .

[0038] Due to the clutch torque T C is the engine torque T E The PID controller 29 also receives the engine torque T generated by the internal combustion engine 4. E ; In fact, the clutch torque T C Must be (slightly) less than the engine torque T E .

[0039] In other words, the above control mode requires the clutch torque T C The actual slip rate S of the clutch 16 is managed by a small change R , so that the engine torque T generated by the internal combustion engine 4 E This control mode (involving only the clutch 16 and not the internal combustion engine 4) is effective (i.e., it enables the clutch 16 to always be at the target slip ratio S T ), but on the other hand tends to transmit vibration torque to the drive wheel 3 (because the clutch torque T transmitted by the clutch C is continuously variable), which may cause longitudinal vibrations in the movement of the road vehicle 1 that may be perceived by the driver.

[0040] To avoid this disadvantage (ie, to keep the torque transmitted to the drive wheels 3 more constant), the control unit 12 of the powertrain 6 may operate differently, that is, the control unit 12 of the powertrain 6 determines the target slip ratio S of the clutch 16. T , detecting the engine torque T generated by the internal combustion engine 4 E , set up should be transmitted by clutch 16 and equal to the engine torque T E The clutch torque T C (thus, it is constant as long as the driver does not operate the accelerator pedal 22 in a different manner), controlling the clutch 16 to transmit the clutch torque T C (which remains constant as long as the driver does not operate the accelerator pedal 22 in a different manner), detecting the rotation speed ω2 of the driven plate 27 of the clutch 16, and setting the target slip rate S of the clutch 16 by T The target speed ω of the driving plate 26 of the clutch 16 is determined by adding the speed ω2 of the driven plate 27 of the clutch 16. 1-T and controls the internal combustion engine 4 to change (with small changes) the engine torque T generated by the internal combustion engine 4 E , thereby obtaining the target speed ω of the driving plate 26 of the clutch 16 1-T .

[0041] In other words, the above-mentioned alternative operating mode requires the engine torque T generated by the internal combustion engine 4 to be E The actual slip rate S of the clutch 16 is managed by a small change R , so that the clutch torque T C remains constant (obviously, as long as the driver's application of the accelerator pedal 22 is constant).

[0042] The above control methods have different advantages.

[0043] First, the control method described above avoids the adverse consequences of the phenomenon known as "stick-slip" (which in some cases cannot be avoided by itself) because the clutch 16 operating under a specified non-zero constant slip condition prevents torque vibrations introduced into the powertrain 6 through the phenomenon known as "stick-slip"; in other words, when the clutch 16 is operated under a specified non-zero constant slip condition, the clutch 16 acts like a mechanical low-pass filter, which effectively and efficiently blocks torque vibrations introduced into the powertrain 6 through the phenomenon known as "stick-slip". As a result, the torque vibrations introduced into the powertrain 6 through the phenomenon known as "stick-slip" are stopped in the clutch 16 and are not transmitted to the internal combustion engine 4 (in a non-marginal manner and without consequences).

[0044] Furthermore, the control method described above is easy and economical to implement, since only limited memory space and low computing power are required to execute the method.

[0045] Reference numerals list

[0046] 1 Road vehicles

[0047] 2 Front wheels

[0048] 3 Rear wheels

[0049] 4 Engine

[0050] 5 Drive shaft

[0051] 6 Powertrain

[0052] 7 Gearbox

[0053] 8 Drive shaft

[0054] 9 Differential

[0055] 10 Half shaft

[0056] 11 Engine control unit

[0057] 12 Powertrain control unit

[0058] 13 Bus lines

[0059] 14 Sync Cable

[0060] 15 Spindle

[0061] 16 Clutch

[0062] 17 Countershaft

[0063] 18 Main gear

[0064] 19 Secondary gear

[0065] 20 Synchronizer

[0066] 21 Steering Wheel

[0067] 22 Accelerator pedal

[0068] 23 Brake pedal

[0069] 24 Upshift paddle shifters

[0070] 25 Downshift paddle shifters

[0071] 26 Driver disk

[0072] 27 Driven plate

[0073] 28 Subtractor Frame

[0074] 29 PID Controller

[0075] 30 Calculator Box

[0076] ω1 Speed

[0077] ω2 Speed

[0078] ω E Speed

[0079] S R Actual slip rate

[0080] S T Target slip rate

[0081] ε S Control error

[0082] T C Clutch torque

[0083] T E Engine torque

Claims

1. A method for controlling a road vehicle (1) having a clutch, the clutch (16) connecting an internal combustion engine (4) to drive wheels (3) and being arranged upstream of a servo-assisted gearbox (7), the method comprising the following steps: Check whether the tires of the drive wheels (3) are close to the grip limit; as well as When the tire of the driving wheel (3) approaches the grip limit, the clutch (16) is opened so that the clutch (16) transmits the torque to the driving wheel (3) with constant and non-zero slip of the clutch (16), The tire of the driving wheel (3) is considered to be close to the grip limit in the following cases: A gear belonging to a predetermined group of gears is engaged in the servo-assisted gearbox (7); and The engine torque (T E ) exceeds the torque threshold.

2. The control method according to claim 1, characterized in that: Due to slip friction, the rotation speed (ω1) of the driving plate (26) of the clutch (16) is greater than the rotation speed (ω2) of the driven plate (27) of the clutch (16).

3. The control method according to claim 2, characterized in that: The difference between the rotation speed (ω1) of the driving plate (26) and the rotation speed (ω2) of the driven plate (27) is in the range of 70 to 180 rpm.

4. The control method according to claim 2, characterized in that: The difference between the rotation speed (ω1) of the driving plate (26) and the rotation speed (ω2) of the driven plate (27) is equal to the rotation speed (ω E ) is in the range of 0.5% to 1.2%.

5. The control method according to claim 1, characterized in that: If the speed (ω) of the internal combustion engine (4) E ) is within a predetermined speed range, the tire of the drive wheel (3) is also considered to be close to the grip limit.

6. The control method according to claim 1, characterized in that: The control method further comprises the following steps: determining a level of grip of a road surface on which a road vehicle (1) is positioned; and The torque threshold is determined according to the grip level of the road surface.

7. The control method according to claim 1, characterized in that: The control method further comprises the following steps: Determine the target slip rate (S) of the clutch (16) T ); Detecting the engine torque (T E ); The clutch torque (T C );as well as The clutch (16) is controlled to transmit the clutch torque (T C ).

8. The control method according to claim 7, characterized in that: The target slip rate (ω) of the clutch (16) is read from the memory of the control unit (12) of the power transmission system (6). E ).

9. The control method according to claim 7, characterized in that: The control method further comprises the following steps: The actual slip rate (S) of the clutch (16) is periodically determined. R ); The torque (T E ) Smaller clutch torque (T C );as well as According to the target slip rate (S) of the clutch (16) T ) and the actual slip rate (S R ) periodically changes the clutch torque (T C ).

10. The control method according to claim 9, characterized in that: Change the clutch torque (T C ) to obtain the target slip rate (S) of the clutch (16) T ).

11. The control method according to claim 9, characterized in that: The clutch torque (T C ), in the feedback control the control error (ε S ) is the target slip ratio (S) of the clutch (16) T ) and the actual slip rate (S R ) between the two.

12. The control method according to claim 11, characterized in that: Clutch torque (T C ) is controlled by the receiving error (ε S ) is used as the input value of the PID controller (29) to change.

13. The control method according to claim 7, characterized in that: The control method further comprises the following steps: The clutch (16) should transmit the torque (T E ) equal clutch torque (T C ); Detecting the rotation speed (ω2) of the driven disc (27) of the clutch (16); By setting the target slip rate (S) of the clutch (16) T ) is added to the rotation speed (ω2) of the driven disk (27) of the clutch (16) to determine the target rotation speed (ω) of the driving disk (26) of the clutch (16). 1-T );as well as Controlling the internal combustion engine (4) to change the engine torque (T E ) to obtain the target rotation speed (ω) of the driving plate (26) of the clutch (16) 1-T ).

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