Limited-slip differential control system

Abstract: A control system for controlling a differential limiting force of a limited slip differential of a vehicle includes a section for sensing a turning radius of the vehicle, a section for sensing a centripetal acceleration of the vehicle, and a controller section which increases an engagement force of a differential limiting clutch of the limited slip differential in accordance with the turning radius and the centripetal acceleration.

Description: BACKGROUND OF THE INVENTION The present invention relates to a differential limiting force control system for controlling a differential limiting force for limiting a differential action between right and left drive wheels of a vehicle by controlling an engagement force of a differential limiting clutch of a limited slip differential. More specifically, the present invention relates to a differential limiting force control system responsive to a turning radius and a centripetal acceleration of a vehicle. JP-A-6167629 discloses one conventional example. A control system of this example is arranged to increase an engagement force of a differential limiting clutch as the vehicle speed increases. JP-A-61102321 (see also US-A-4679436) discloses another conventional control system, which is arranged to sense an accelerating condition of a vehicle, and to increase a clutch engagement force of a differential limiting clutch in a rapid accleration. However, these conventional control systems are unsatisfactory in that the clutch engagement force is controlled in accordance with only the vehicle speed, or the condition of acceleration. If control characteristics are adapted to a high friction coefficient road condition so that the clutch engagement is made relatively strong, then these conventional systems tend to cause rear end swing of the vehicle by a steep increase of the differential limting force during driving on a low friction coefficient road surface, and produce strong understeer at the time of "turn-in" during turn at a tight corner on a road surface of a low friction coefficient. If the control characteristics are adapted to a low friction coefficient road condition by making the clutch engagement force relatively weak, then the differential limiting force becomes so low during driving on a high friction coefficient road surface, that spin of an inside wheel of the turn is increased remarkably, and the ability of acceleration during turn is degraded. It is possible to adjust the control characteristics in accordance with the friction coefficient of a road surface, However, it is difficult to accurately sense the friction coefficient. JP-A-63071428 proposes a control system using a centripetal acceleration to adjust a control based on the vehicle speed and accelerating condition in such a manner that the differential limiting force is increased as the centripetal acceleration increases. However, this control system is still unsatisfactory in the following points. In a high speed straight ahead operation, the vehicle speed is high but the centripetal acceleration is low. In such a situation, the differential limiting force is insufficient, so that the stability against side wind is poor. This system decreases the differential limiting force almost to zero in a low speed straight ahead operation. Therefore, this system tends to produce spin during driving on a split mu road having different friction coefficients between a contact surface of the left wheels and a contact surface of the right wheels. A differential limiting force control system according to the preamble of claim 1 is known from the earlier EP-A-0310094 , the content of which is comprised in the state of the art pursuant to Article 54 (3) EPC for the contracting states DE and FR. This known system comprises a control means for controlling the roll rigidity of the vehicle in such a way that a roll rigidity is larger on the front wheel side and smaller on the rear wheel side in response to a cornering condition in which a driving force is distributed more to a cornering outside driving wheel than to a cornering inside driving wheel. FR-A-2434969 shows a differential limiting force control system having a clutch disposed between right and left drive wheels of a vehicle for varying a differential limiting force between the right and left drive wheels in accordance with a control signal. The differential permits to have a maximum of adhesive power at starting from a rest and at straight forward movement. The differential limiting force is suppressed or reduced after entering a curve and modulated as a function of the radius of the curve. SUMMARY OF THE INVENTION The present invention has the object to provide a differential limiting force control system which allows to influence the steering characteristic of a vehicle in a predetermined manner and which especially ensures the vehicle stability against disturbance such as a cross-wind. This object is solved according to the invention by a control system according to claim 1. Preferred embodiments of said control system are subject matter of the subclaims. As shown in Fig. 1, a differential limiting force control system according to the present invention comprises sensor means 1, controller means 2 and differential limiting clutch means 3. The clutch moans is disposed between right and left drive wheels of a vehicle for varying a differential limiting force in accordance with a control singal. The sensor means comprises first sensing means 101 for determining a turning radius of the vehicle, and second sensing means 102 for sensing a centripetal acceleration of the vehicle. The controller means 2 produces the control signal in accordance with signals supplied from the sensor means 1. The controller means 2 is arranged to determine a command magnitude which increases as the turning radius increases and which increases as the contripetal acceleration increases. The command magnitude is represented by the control signal, and corresponds to the differential limiting force. When the vehicle is in a straight ahead movement of a low or medium vehicle speed, the turning radius is infinite, and the controller means 2 maximizes the differential limiting force by increasing the command magnitude to a maximum. Therefore, the control system according to the present invention enables the vehicle to safely negotiate a split mu road by increasing the differential limiting force. Thus, the present invention can improve the vehicle's ability of negotiating adverse road conditions. At an early stage of a turn at low or medium vehicle speeds, the turning radius is decreased with angular displacement of the steering wheel from a straight ahead position, and accordingly, the controller means 2 decreases the command magnitude toward zero. In this case, the centripetal acceleration remains within a low and medium range. Thus, the control system of the present invention decreases the understeer tendency. At a late stage of a low or medium speed turn, the centripetal acceleration increases while the turning radius is held approximately constant. Therefore, the controller means 2 increases the command magnitude again, and improve the traction ability during turn by preventing inside wheel spin and providing sufficient driving torque. The turning radius is infinite in a high speed straight ahead operation, and the centripetal acceleration is very high in a high speed cornering operation. Therefore, the controller means 2 improve the stability of the vehicle at high speeds by increasing the differential limiting force. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing a basic arrangement of a differential limiting force control system according to the present invention. Fig. 2 is a schematic view of a rear wheel drive vehicle equipped with a differential limiting force control system of one embodiment of the invention. Fig. 3 is a flowchart showing a control process performed by a controller of the control system of the embodiment. Fig. 4 is a graph showing a characteristic of a command magnitude, employed in the embodiment. Fig. 5 is a graph showing a characteristic between a centripetal acceleration and a turning radius. Fig. 6 is a graph showing another characteristic which can be employed in place of the characteristic of Fig. 4. Fig. 7 is a graph showing still another characteristic which can employed. DETAILED DESCRIPTION OF THE INVENTION One embodiment of the present invention is shown in Figs. 2-4. AS shown in Fig. 2, a vehicle of this embodiment is a rear wheel drive vehicle. The vehicle includes an engine 10, a transmission 11, a propeller shaft 12, a differential 13, right and left drive shafts 14 and 15, right and left rear wheels 16 and 17, and right and left front wheels 18 and 19. The differential 13 includes a differential limiting clutch 20. In this embodiment, the differential limiting clurch 20 is a multiple disc wet type friction clutch. An external oil pressure source 30 is connected through a control valve 31 to the differential limiting clutch 20. The control valve 31 is capable of varying an engagement force of the differential limiting clutch 20 by varying an oil pressure supplied to the clutch 20 in response to a control signal (i). A differential limiting force T between the right and left rear wheels 16 and 17 is controlled by varying the engagement force of the differential limiting clutch 20. A sensor 33 is disposed near the front right wheel 18 for sensing a front right wheel rotational speed NFR. A sensor 34 is disposed near the front left wheel 19 for sensing a front left wheel rotational speed NFL. A controller 32 is an electronic control circuit for controlling the differential limiting force by producing the control signal (i). The controller 32 is arranged to perform functions shown in Fig. 2. The controller 32 has an input circuit 320 for receiving the signals from the front wheel speed sensors 33 and 34, a section 321 for calculating a centripetal acceleration Yg, a section 322 for calculating a turning radius R of the vehicle, a first function generator 323 for producing a first command signal representing a first command magnitude T₁ which increases as the absolute value of the centripetal acceleration Yg increases, a second function generator 324 for producing a second command signal representing a second command magnitude T₂ which increases as the turning radius R increases, an adder 325 for determining a total command magnitude Ttotal by adding the first and second command magnitudes T₁ and T₂ (T = T₂ + T₂) , and a valve driver circuit 326 for producing the control signal (i) corresponding to the total command magnitude Ttotal. The controller 32 delivers the control signal (i) to the control valve 31. The control valve 31 increases the differential limiting force, that is the clutch engagement force of the differential limiting clutch 20, as the total command magnitude Ttotal increases, and decreases the differential limiting force as Ttotal decreases. Fig. 3 is a flowchart showing a control process performed by the controller 32. At a step 100, the controller 32 reads the front right wheel speed NFR and the front left wheel speed NFL sensed by the sensors 33 and 34. Then, the controller 32 calculates the turning radius R at a step 101, and calculates the centripetal acceleration Yg at a step 102, by using the front right wheel speed NFR and the front left wheel speed NFL. The turning radius R and the centripetal acceleration Yg are obtained by using the following equations. V = {(NFL + NFR)/2}/r ψ = K1·|NFL - NFR| In these equations, V is a vehicle speed, ψ ˙ is a yaw rate of the vehicle, K1,K2 and K3 are proportional constants which are determined by the specifications of the vehicle, and r is a tire radius. At a step 103, the controller 32 determines the first command magnitude T₁ from the centripetal acceleration Yg obtained at the step 102, and determines the second command magnitude T₂ from the turning radius R obtained at the step 101. The first command magnitude T₁ is determined according to a predertimed characteristic function in which the first command magnitude T₁ increases as the absolute value of the centripetal acceleration Yg increases. The second command magnitude T₂ is determined according to a predetermined characteristic function in which the second command magnitude T₂ increases as the turning radius R increases. In this embodiment, the first command magnitude T₁ is proportional to the absolute value of the centripetal acceleration Yg, and the second command magnitude T₂ is proportional to the turning radius R. At a step 104, the controller 32 determines the total command magnitude Ttotal by adding the first command quantity T₁ and the second command quantity T₂ (Ttotal = T₁ + T₂) . At a step 105, the controller 32 delivers the control signal (i) representing the total command magnitude T, to the control valve 31. Thus, the engagement force of the differential limiting clutch 20 is controlled so as to obtain the differential limiting force equal to the total command quantity Ttotal. In this way, the controller 32 varies the total command magnitude in accordance with the centripetal acceleration Yg and the turning radius R, as shown in Fig. 4. In a graph of Fig. 5, the turning radius R is expressed along an x-axis, the centripetal acceleration Yg is expressed along a y-axis, and the vehicle speed is taken as a prameter. The control system is operated as follows: When the vehicle is in a straight ahead movement of a low or medium vehicle speed: In this case, the turning radius R is infinite because of the straight ahead operation, and accordinly the controller 32 produces the control signal (i) to increase the differential limiting force T to a maximum. Therefore, even on a split mu road having different friction coefficients under the right and left wheels, the control system ensures safe and stable operation by limiting the differentional action between the right and left wheels 16 and 17 as in a differential lock state. When the vehicle is in a turning movement of a low or medium vehicle speed: At an early stage of a turning movement of a low or medium vehicle speed, the turning radius R is decreased as the steering wheel is turned away from the straight ahead position, and the controller 32 produces the control signal (i) to make the differential limiting force T closer to zero, or reduce the differential limiting force T to zero. For example, the total command magnitude Ttotal is decreased toward zero, from 4 to 3, then from 3 to 2, and so on. At this stage, the centripetal acceleration Yg is not high, but remains within a low or medium range. In this way, the control system decreases the tendency to understeer. At a later stage of the low or medium speed turning movement, the turning radius R is held approximately constant, but the centripetal acceleration Yg increases. Therefore, the control system increases the differential limiting force T again. For example, the total command magnitude Ttotal is incresed from zero to 1, from 1 to 2, and so on. At the later stage of a turn, the control system prevent spin of the inside wheel by increasing the differential limiting force, and improve the traction ability during turn. When the vehicle speed is high: The turning radius R becomes infinitely great when the vehicle is in a high speed straight ahead movement, and the centripetal acceleration Yg becomes very high when the vehicle is in a turning movement of a high vehicle speed. In either case, the controller 32 makes the differential limiting force high when the vehicle speed is high. For example, the controller 32 produces the control signal (i) corresponding to the total command magnitude equal to 4. Therefore, the control system ensures the vehicle stability against disturbance such as a cross wind during straight ahead high speed operation, with the high differential limiting force. During a high speed turn, the control system increases the tendency to understeer by producing a moment toward understeer with the differential limiting force, so that stable cornering behavior is obtained. It is optional to employ a characteristic shown in Fig. 6, or a characteristic shown in Fig. 7, in place of the characteristic shown in Fig. 4. In the case of Fig. 6, the command magnitude is determined by using concentric circles having a common center lying on the origin. For example, the control system is arranged to determine the command magnitude in accordance with the turning radius R and the centripetal acceleration Yg so that the command magnitude is increased in proportion to a distance in a two dimentional coordinate system, from a predetermined fixed point, such as the origin, to a point whose x coordinate is equal to a value obtained by multiplying the turning radius by a predetermined constant, and whose y coordinate is equal to a value obtained by multiplying the centripetal acceleration by a predetermined constant. With the characteristic of Fig. 6, the control system can prevent the command maginitude from being increased by the vehicle speed when the vehicle speed is low or medium and the centripetal acceleration Yg is high. Similar effects are obtained by employing the characteristic of Fig. 7. In the characteristic of Fig. 7, each line is flat in a region in which the turning radius R is small, and the centripetal acceleration Yg is high. It is possible to determine the turning radius from a steering angle by using a sensor for sensing the steering angle, instead of the turning radius sensing means of this embodiment. Furthermore, it is possible to determine the centripetal acceleration by using a sensor directly sensing the centripetal acceleration. Alternatively, it is possible to calculate the centripetal acceleration from the vehicle speed and the turning radius.

Claims: A differential limiting force control system comprising; clutch means (320) disposed between right and left drive wheels (16,17) of a vehicle for varying a differential limiting force between the right and left drive wheels (16,17) in accordance with a control signal, sensor means (33,34) comprising means for determining a urning radius (R) of the vehicle, and means for determining a centripetal acceleration (Yg) of the vehicle, and controller means (32) for producing said control signal in accordance with signals sent from said sensor means, said controller means determining a command magnitude (Ttotal which increases as said turning radius (R) increases, and which increases as said centripetal acceleration (Yg) increases, and wherein said controller means (32) comprises a first function generating means (321) for determining a first command signal (T₁) whose magnitude increases as an absolute value (|Yg|) of said centripetal acceleration (Yg) increases, and second function generating means (322) for producing a second command signal (T₂)magnitude increases as said turning radius (R) increases, and wherein said controller means (32) further comprises adding means (325) for determining said command magnitude (Ttotal) by adding said first and second command signals (Ttotal = T₁ + T₂) . A control system according to claim 1 wherein the magnitude of said first command signal (T₁) is proportional to the absolute value (|Yg|)of said centripetal acceleration (Yg), and the magnitude of said second command signal (T₂) is proportional to said turning radius (R). A control system according to Claim 2 wherein said sensor means (33,34) comprises a front right wheel speed sensor (33) for sensing a front right wheel (18) rotational speed (NFR) of the vehicle, and a front left wheel speed sensor (34) for sensing a front left wheel (19) rotational speed (NFL) of the vehicle, first calculating means (321) for calculating said centripetal acceleration (Yg) from said front right wheel speed (NFR) and said front left wheel speed (NFL), and second calculating means (322) for calculating said turning radius (R) from said front right wheel speed (NFR) and said front left wheel speed (NFL). A control system according to Claim 3 wherein said drive wheels (16,17) are rear wheels of the vehicle.

Inventors: IMASEKI TAKASHI, KOBARI YUJI, IMASEKI, TAKASHI, KOBARI, YUJI
Applicants: NISSAN MOTOR, NISSAN MOTOR CO., LTD.
Application year: 1988
Main classification: B60K23
IPC codes: B60K23, F16D48, B62D9