A method for controlling the engagement and disengagement of an all-wheel drive in a vehicle

Abstract: The proposed control device is particularly suitable for a vehicle with disengageable all-wheel drive, with double differential transmission, and makes it possible to engage and disengage the same all-wheel drive by means of coupling means separable as a function of the speed of the vehicle, and of the slip between the front wheels and the rear wheels, steering angle, acceleration of the wheels.

Description: The present invention relates to a method of controlling the engagement and disengagement of an all-wheel drive in a vehicle, i.e. of automatically engaging and disengaging the all-wheel drive (four driving wheels), as an alternative to the two-wheel drive (two driving wheels), and vice-versa, as a function of the conditions, or of characteristic parameters, of travelling of the vehicle. Control devices for carrying out a method of this type are already known. For example, those disclosed in EP-A-0213654 and EP-A-0212721, automatically engage and disengage the all-wheel drive by means of an electromagnetic coupling, as a function of the speed of the vehicle and of the acceleration and deceleration of the constantly engaged wheels. In particular, these devices control the two-driving-wheel travelling mode until a pre-established travelling speed of the vehicle is reached, with the possibility of engaging the all-wheel drive if, in this situation, accelerations of the constantly engaged wheels higher than a reference limit occur, such as, e.g., on low-adhesion terrains. These devices show the advantage of a considerable rapidity of response, because the acceleration of the wheels is a characteristic quantity which can be detected extremely quickly; but it does not constitute a control parameter which makes it possible to discriminate the type of terrain, and its adhesion value, because the angular acceleration of the wheels is a function of the applied driving torque only, and shows very high initial peaks, with which it is difficult to select the available adhesion values. Control devices are also known, such as the control device disclosed in FR-A-2,552,034, which use the difference between the front wheel speed and the rear wheel speed as a control parameter. By using this parameter, it is possible to select the intervention of the device as a function of the type of terrain, hence of the available adhesion, and on the basis of the applied driving torque. But the solution proposed in the cited French document does not result to be fully satisfactory, because it just provides the detection of the value of this parameter to compare it to a limit value, and does not take into account the speed difference existing between the front wheels and the rear wheels, when the vehicle runs along a bend at a low speed, nor does it take into account the influence of the vehicle speed on the slip behaviour. A further device for controlling engagement and disengagement of an all-wheel drive is known from FR-A-2 577 485. This document discloses a method having the features of the preamble of the claim. According to this known method the detected values of the speed of the wheels and of the steering angle are first adapted to take account of mechanical imprecisions and wear, and the corrected values are further processed and filtered in part to obtain values which are then compared with allowable limit values to determine whether the all-wheel drive is to be engaged or not. This document also teaches to correct the speed difference between the front and rear wheels when the vehicle is running on a bend. A high number of manipulations of the detected values is thus performed which complicates the practical employment of the method and may reduce the rapidity of action of intervention of the all-wheel drive. The purpose of the present invention is to improve the known device as hereinbefore described, which uses the wheel speed difference as a control parameter, by providing a method of controlling with precision and rapidity the action of engagement or disengagement of the all-wheel drive and optimizing it by discriminating the wheel drive mode essentially on the basis of a sole preselected parameter represented by a preselected vehicle speed limit value. This purpose is achieved by a method having the features recited in the claim. Characteristics and advantages of the invention are now illustrated by referring to the hereto attached figures 1 through 3, wherein a preferred form of practical embodiment of the invention is illustrated for exemplifying purposes. In Figure 1, the transmission is shown of an all-wheel drive vehicle, with a block diagram schematically illustrating the means for carrying out the method of the present invention; in Figure 2, a detail of Figure 1 is shown on an enlarged scale; in Figure 3, a flow chart is shown, which relates to the control logic of the method. Figure 1 shows a cover 10 of the clutch (not shown) and a gear box 11 of a speed gear of a vehicle equipped with an engine provided with at least one throttle valve, the vehicle being further provided with a steering mechanism and with a disengageable all-wheel drive. The main shaft of the engine driven speed gear is indicated with 12, and an output shaft is indicated with 13. Also shown is one of the rods 14, which are actuated by the gear shift lever, for the purpose of engaging the different transmission ratios. With the shaft 13, a bevel pinion 15 is integral; said bevel pinion 15 inmeshes with a ring gear 16 of a first differential, indicated in its entirety by 18; in this specific case, the differential 18 is permanently connected to the output shaft 13 and transmits the motion to the wheels of the front axle, which hence are constantly engaged driving wheels. The shaft 13 is connected, by means of splined couplings 17 and 19 (Fig.2), with a hub 20, a ferrule 21 and a shaft 22, which latter is connectable by disengageable connection means 24 in the form of an electromagnetic coupling, and by a universal joint 25, to a transmission shaft 23. As it is shown in detail in Figure 2, the shaft 22 is connected by means of a coupling 26 with the core 27 of a coil 28 housed in a body 29. Integral with the core 27 is a pulley 30, provided with a peripheral ring of front teeth 31, and opposite to the pulley 30 a plate 32 is positioned, which constitutes the armature of the electromagnetic assembly and is also provided with a peripheral ring of front teeth 34. The plate 32 is axially movable, by being connected by a splined coupling 33 with a flange 34', in its turn fastened by a screw 35 to a spider of the universal joint 25. Figure 2 further shows bearings 36 and 37 of the shaft 13, a cover 38 of the gear box 11, bearings 39 and 40 of the shaft 22, a box 41 of the disengageable connection means 24, which is fastened to the cover 38 by screws 42, and sealing rings 43 and 44. In Figure 1 there are further shown a first section 46, a second section 47, and a third section 48 of the shaft 23. An intermediate support of the shaft is shown at 56. Universal couplings 49 and 50 connect the three sections of the shaft 23. The end section 48 of the shaft 23 is integral with a pinion 51,which inmeshes with a ring gear 52 of a second differential 53, which is selectively connectable by the disengageable connection means 24 to the output shaft 13,and which is connected with the wheels of the rear axle which, in this specific case, are the auxiliary wheels. Figure 1 further shows bearings 54 and 55 of the third section 48 of the shaft 23 and a box 53a of the rear axle. The coil 28 of the disengageable connection means 24 is connected, by means of a lead 57, with actuator means 58 of a control device, indicated in its entirety by 60. The actuator means 58 are operatively connected with a microcomputer control unit 61, through an actuation line 59 and through a feedback line 80. The control unit 61 comprises memory means and processing means and controls the engagement and disengagement of the all-wheel drive as will be seen later on. Figure 1 further schematically shows the front driving wheels 62 and 63, constantly engaged, of the vehicle, with which respective tachometrical wheels 64 and 65, equipped with peripheral slots are rigid. Sensors 66 and 67 are operatively associated with the wheels 64 and 65. The tachometrical wheels 64, 65 and the sensors 66, 67 define first detector means detecting the angular speed of the pair of driving wheels 62, 63. The numerals 68 and 69 indicate the auxiliary rear wheels of the vehicle to which respective tachometrical wheels 70 and 71 and sensors 72 and 73 are associated, which define second detector means detecting the angular speed of the auxiliary wheels 68, 69. The signals of angular speed of the wheels, supplied by the detectors, come, through lines 74,75,76 and 77, to the control unit 61. To the control unit 61 also arrive signals 78 which are indicative of the vehicle steering angle (δ) detected by third detector means, and signals 79 of the torque Cm applied to the wheels 62,63,68,69, detected by fourth detector means. In permanent memory means of the control unit 61, correction values (Sc) of the speed difference between the driving and the auxiliary wheels are stored, determined as a function of the speed (v) of the vehicle, e.g., lower than 10 m/sec, and of the steering angle (δ) of the wheels. The stored correction values take into account the difference in angular speed between the pair of driving wheels 62,63 and the pair of auxiliary wheels 68,69 when the vehicle is running on a bend. In the same memory, there are stored also allowable values (Si) of the speed difference, e.g. comprised within the range of from 5% to 10% of the average angular speed of the wheels, and determined as a function of the speed (v) of the vehicle and of the torque applied to the wheels (Cm). In the same memory there are further stored limit values of vehicle speed (v1), of vehicle acceleration (a1) and of steering angle (δ1), as a function of the vehicle speed (v). The control unit 61 is programmed to perform the operating sequences provided for the purpose of controlling the engagement and disengagement of the disengageable connection means 24, as it results from the flow chart of Figure 3. The microcomputer of the control unit 61 verifies, through the signal from the line 80, the state of the engagement of the all-wheel drive AWD detected by a fifth detector means operatively connected to the actuator means 58. The microcomputer computes,from the signals of angular speed of wheels 62,63,68,69,the values of angular speed (ωa) of the front driving wheels and the values of angular speed (ωp) of the rear auxiliary wheels, the values of acceleration of the wheels and of the vehicle. The microcomputer computes, from the above-said signals of angular speed (ωa) and (ωp), the vehicle speed (v) and verifies in a first sequence whether the all-wheel drive AWD is engaged or disengaged (see box 81 in Fig.3). In case the all-wheel drive AWD is disengaged, the microcomputer verifies in a second sequence whether the vehicle speed (v) is greater than a preselected limit value (v1) (box 82 in Fig.3), e.g. 10 m/sec, stored in the memory of the control unit 61. If the verification of the second sequence is yes, the microcomputer controls the disengageable connection means 24 to engage the all-wheel drive AWD (box 83 in Fig. 3), by rendering the shafts 13 and 23 rotatably rigid with each other. If, on the contrary, the verification of the second sequence is no, i.e. the speed of the vehicle is less than the limit speed value (v1), the microcomputer computes the speed difference (S) between the angular speed of the front wheels 62,63, constantly engaged, and that relating to the auxiliary rear wheels 68,69, from the respective signals, S = (ωa - ωp). If the vehicle is running along a bend at a low speed, e.g. lower than 10 m/sec, the microcomputer 61 reads from its memory the correction values (Sc) of the speed difference, as a function of the actual vehicle speed (v) and of the actual steering angle (δ), then corrects the computed value of speed difference (S), by subtracting from it the respective correction value (Sc), and compares the corrected value (S-Sc) of speed difference with the allowable value (Si) of speed difference, e.g., comprised within the range of from 5% to 10% of the average angular speed of the wheels, which is to read from the memory. The microcomputer verifies in a third sequence if the corrected value (S-Sc) is greater than the allowable limit value (Si) of speed difference (box 84 in Fig.3). If the verification of the third sequence is yes, the microcomputer controls the engagement of the all-wheel drive AWD (box 85 in Fig.3); if, on the contrary, the verification of the third sequence is no, i.e. the corrected value is less than the allowable value (Si), the all-wheel drive AWD is maintained disengaged, and the vehicle operates with the two-wheel drive TWD only (box 86 in Fig.3). On the contrary, if in the first sequence (box 81 in Fig.3) it is verified that the all-wheel drive AWD is engaged, the microcomputer 61 verifies in a further sequence whether the speed (v) of the vehicle is less than the limit value (v1) of the vehicle speed (box 87 in Fig.3). If the verification of said further sequence is no, the all-wheel drive AWD is maintained in the engagement position (box 88 in Fig.3). If the verification of said further sequence is yes, the microcomputer verifies in a subsequent sequence whether the acceleration (a) of the vehicle is less than the prefixed limit value (a1) (box 89 in Fig.3). If the verification of said subsequent sequence is no, the all-wheel drive AWD is maintained in the engagement position (box 88 in Fig.3). If the verification in said subsequent sequence is yes, the microcomputer verifies in a next sequence whether the steering angle (δ) is greater than the limit value (δ1) (box 90 in Fig.3), and if the verification in said next sequence is yes, the microcomputer controls the disengagement of the all-wheel drive AWD and engages the two-wheel drive TWD (box 91 in Fig.3), whilst, if the verification is no, the microcomputer keeps the all-wheel drive AWD engaged (box 88 in Fig.3). From the above disclosed computations the correction values (Sc) are missing, if the vehicle runs along a straight road, or along a bend at a medium, or at a high speed. As the control parameter, instead of the speed difference (S), the % speed difference (Sr) could be used, which is defined as the difference between the front wheel speed value and the rear wheel speed value, expressed as the ratio to the front wheel speed value The above disclosed device could be also accomplished with two tachometrical wheels only, the one integral with the shaft 13, and the other integral with the shaft 23, so to detect, by means of these latter, the revolution speed of respectively the driving wheels 62 and 63, constantly engaged, and of the auxiliary wheels 68 and 69.

Claims: A method of controlling the engagement and disengagement of an all-wheel drive in a vehicle equipped with an engine which is provided with at least one throttle valve, an engine driven speed gear, a wheel steering mechanism, a pair of driving wheels (62, 63) of a first axle driven by a first differential (18) and a pair of auxiliary wheels (68, 69) of a second axle driven by a second differential (53), wherein the first differential (18) is permanently connected to the output shaft (13) of the speed gear and the second differential (53) is selectively connectable by disengageable connection means (24) to the output shaft (13) of the speed gear, a control unit (61), including memory means and being operatively connected to actuator means (58) of the disengageable connection means (24), first detector means (64 - 67) detecting the angular speed of the pair of driving wheels (62, 63), second detector means (70 - 73) detecting the angular speed of the auxiliary wheels (68, 69), third detector means detecting a physical quantity indicative of the wheel steering angle (δ), fourth detector means detecting torque applied to the wheels (62,63; 68,69), the first, second, third and fourth detector means being operatively connected with the control unit (61) to supply respective signals of angular speed of the wheels (62,63; 68,69), the steering angle (δ) and of the torque (Cm) applied to the wheels (62,63; 68,69) to the control unit (61), the control unit (61) comprising processing means and controlling the engagement and disengagement of the all-wheel drive in the steps of: processing the speed difference (S; Sr) in the angular speed between the pair of driving wheels (62, 63) and the pair of auxiliary wheels (68, 69) from the respective signals, processing a correct value (S-Sc; Sr-Sc) of the speed difference as a difference between said calculated speed difference (S; Sr) and correction values (Sc) stored in the memory, the stored values taking into account the difference in angular speed between the pair of driving wheels (62, 63) and the pair of auxiliary wheels (68, 69) when the vehicle is running on a bend, processing from the signals indicative of the angular speed (ωa, ωp) of the driving and the auxiliary wheels (62,63; 68,69) the vehicle speed (v) and the vehicle acceleration (a), comparing the corrected value (S-Sc; Sr-Sc) of the speed difference with an allowable value (Si) of said speed difference, the allowable values (Si) being associated to values of vehicle speed (v), characterised in that the state of the engagement of the all-wheel drive (AWD) is detected by a fifth detector means operatively connected to the actuator means (58) and the control unit (61), whereby the engagement and the disengagement is controlled by the control unit (61) in the following steps: in a first sequence (81), it is verified whether the all-wheel drive (AWD) is engaged or disengaged; if the all-wheel drive (AWD) is disengaged, it is verified in a second sequence (82), whether the vehicle speed (v) is greater than a limit value (v1) of vehicle speed stored in the memory, the limit value (v1) being associated to the vehicle speed (v); if the verification of the second sequence (82) is yes, the all-wheel drive (AWD) is controlled into engagement position (83); if the verification of the second sequences (82) is no, it is verified in a third sequence (84), if the corrected value (S-Sc; Sr-Sc) of the wheel speed difference is greater than the allowable value (Si) of speed difference, the correction values (Sc; Sr) being further associated to values of vehicle speed (v) and steering angle (δ) and the allowable value (Si) of speed difference being further determined as a function of torque (Cm) applied to the wheels (62,63; 68,69); if the verification of the third sequence (84) is yes, the all-wheel drive (AWD) is controlled into engagement position (85); if the verification of the third sequence (84) is no, the all-wheel drive (AWD) is maintained in the disengagement position (TWD; 86); if in the first sequence (81) it is verified that the all-wheel drive (AWD) is engaged, it is verified in a further sequence (87), whether the vehicle speed (v) is less than the limit value (v1) of vehicle speed; if the verification of said further sequences (87) is no, the all-wheel drive (AWD) is maintained in the engagement position (88); if the verification of said further sequence (87) is yes, it is verified in a subsequent sequence (89), whether the vehicle acceleration (a) is less than a limit value (a1) of vehicle acceleration stored in the memory, the limit value (a1) being associated to values of vehicle speed (v); if the verification of said subsequent sequence (89) is no, the all-wheel drive (AWD) is maintained in the engagement position (88); if the verification of said subsequent sequence (89) is yes, it is verified in a next sequence (90), whether the steering angle (δ) is greater than a limit (δ1) of the steering angle stored in the memory, the limit value (δ1) being associated to values of vehicle speed (v); if the verification of said next sequence (90) is no, the all-wheel drive (AWD) is maintained in the engagement position (88); if the verification of said next sequence (90) is yes, the all-wheel drive (AWD) is controlled into the disengagement position (TWD; 91).

Inventors: RUSPI MASSIMO, TRAVAGLIO GIANCLAUDIO, VIRGILIO UGO, RUSPI, MASSIMO, TRAVAGLIO, GIANCLAUDIO, VIRGILIO, UGO
Applicants: FIAT AUTO SPA, FIAT AUTO S.P.A.
Application year: 1988
Main classification: B60K23
IPC codes: F16D48, B60K23