GB2158912A - Automatic clutch control system - Google Patents

Abstract

An automatic clutch control system is disclosed where a first servo loop for controlling clutch actuating means when the engine is idling, a non-drive range (P,N of 20) of the transmission (14) having been selected when the vehicle is at standstill, includes means (1300) for storing an operating parameter related to the engine idle speed for example a target engine speed or the speed detector of an idle speed control system, and a second servo loop for controlling the clutch engagement to adjust the clutch transfer torque to a predetermined level. The second servo loop is effective when the engine is idling, a drive range (R,D,L of 20) having been selected when the vehicle is at standstill, and includes means (1300) for determining a target, for example an engine speed to give a creep torque, based on the stored operating parameter and means for controlling the clutch engagement to reduce the deviation of the parameter from the target. The control system may be used in conjunction with a transmission control system for a steplessly variable V-belt transmission, the inputs to the system being range selected (304), throttle opening (303), brake actuation (307), coolant temperature (306), engine speed (302) and shift reference (1298). <IMAGE>

Description

SPECIFICATION Automatic clutch control system The present invention relates to an automatic clutch control system.

Automatic clutch control systems for use in automotive vehicles are known. When the vehicle is to be started, the clutch must be brought into firm engagement smoothly. One proposed measure to meet this demand is to increase actuating pressure supplied to the clutch in response to engine speed. Although this measure is effective in smoothly engaging the clutch, there remains a difficulty in properly controlling the clutch engagement when the engine idles under a condition where a manual range selector lever is pointed to a drive range when the vehicle is at standstill.

This causes engine racing accompanied with the occurrence of substantial shocks upon clutch engagement when the vehicle is to move off from standstill or unintentional vehicle's starting or engine stall during vehile's starting.

U.S. patent application Ser. No. 543,838, filed October 20, 1 983 (which corresponds to EP 83 110 546.5) commonly assigned herewith discloses an automatic clutch control system for adjusting actuating pressure to a predetermined level so as to keep the clutch slightly engaged when the engine idles under a condition where the vehicle is at standstill.

This clutch control system is not satisfactory in failing solve the problems encounted during clutch engagement when the vehicle is to move off from standstill because the same slight engagement of the clutch can not be maintained under the presence of hydraulic or electronic errors in detecting the engine speed, the variations in friction characteristics of the clutch friction elements and the variations due to aging of them.

SUMMARY OF THE INVENTION An object of the present invention is therefore to improve an automatic clutch control system such that the clutch engagement is controlled such as to maintain the clutch transfer torque constant, even if there are variations which otherwise would affect the system, to cause the vehicle to creep in order to ensure smooth clutch operation into firm engagement required for the vehicle to travel.

According to the present invention, there is provided an automatic clutch control system for a motor vehicle including an engine, a transmission, and a clutch adapted to transfer torque between the engine and the transmission, the transmission having a drive range and a non-drive range, the automatic clutch control system comprising:: means for actuating the clutch; a first servo loop for controlling the operation of said actuating means, said first loop being effective when the engine idles under a condition where the transmission is rendered to select the non-drive range when the vehicle is at standstill; a second servo loop for controlling the operation of said actuating means in such a manner as to control the clutch engagement to adjust the transfer torque by the clutch to a predetermined level, said second loop being effective when the engine idles under a condition where the transmission is rendered to select the drive range when the vehicle is at standstill; said first servo loop including means for storing predetermined operating information related to idle operating condition of the engine;; said second servo loop including means for determining a target in response to said predetermined operating information stored by said storing means, and means for controlling the operation of said actuating means so as to control the clutch engagement in such a direction as to decrease the deviation from said target.

Another aspect of the present invention is in an automatic clutch control method for a motor vehicle including an engine, a transmission, a clutch adapted to transfer torque between the engine and the transmission, and means for actuating the clutch, the transmission having a drive range and a non-drive range, the automatic clutch control method comprising:: controlling the operation of said actuating means when the engine idles under a condition where the transmission is rendered to select the non-drive range when the vehicle is at standstill; controlling the operation of said actuating means in such a manner as to control the clutch engagement to adjust the transfer torque by the clutch to a predetermined level, when the engine idles under a condition where the transmission is rendered to select the drive range when the vehicle is at standstill; said first mentioned controlling step including storing predetermined operating information related to idle operating condition of the engine;; said second controlling step including determining a target in response to said predetermined operating information stored, and controlling the operation of said actuating means so as to control the clutch engagement in such a direction as to decrease the deviation from said target.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a motor vehicle drive train and an electronic control unit for implementing the automatic clutch control system according to the present invention; Fig. 2 is a schematic diagram of a continu ously variable transmission having the auto matic clutch mechanism; Figs. 3A and 3B, when combined, illustrate a hydraulic circuit diagram; Fig. 4 is a detailed block diagram of the electronic control unit; Fig. 5 is a flowchart of a clutch actuator (force motor) control routine; Fig. 6 is a graph depicting a family of torque versus engine speed characteristic curves representive of the engine perfor mance; Fig. 7 is a block diagram, similar to Fig. 7, of a motor vehicle drive train and a control unit for implementing the second embodiment of the invention; Fig. 8 is a detailed block diagram of the control unit;; Fig. 9 is a flowchart of a clutch actuator control routine of the second embodiment; Fig. 10 is a graph depicting a family of torque versus engine speed characteristic curves representing the performance of the engine; and Fig. 11 is a flowchart of a clutch actuator control routine of the third embodiment.

DESCRIPTION OF THE EMBODIMENTS Referring now more particularly to Fig. 1, reference numeral 10 designates a motor vehicle drive train comprising an engine 12, a forward and reverse clutch mechanism including a forward clutch 1004 and a reverse clutch 1024, and a continuously variable transmission 14. The engine output shaft 1002 drives transmission input shaft 1008 through the clutch mechanism 1004 (1024), and the transmission output shaft 1052 is connected to a differential 1070 (see Fig. 2) and then to driving wheels of a vehicle (not shown), having a brake pedal 16, in a conventional manner. A brake sensor 307 is responsive to the application of the brake 1 6 to detect whether the brake is applied or not, and the output of the brake sensor 307 is applied as an input to an electronic control unit 1 300.

Reference numeral 1 8 generally designates an engine throttle valve for controlling the power output of the engine 1 2. A throttle opening degree sensor 303 is responsive to the position or the opening degree of the throttle valve 1 8 and the output of throttle sensor 303, which is indicative of the throttle position, is applied to an input to the control unit 1 300. Since the intake manifold vacuum represents the engine load as the position of the throttle valve 1 8 does, an intake maniold vacuum sensor may be used instead of the throttle sensor.

An engine coolant temperature sensor 306 is responsive to the engine coolant temperature and generates an output signal when the engine coolant temperature is lower than a predetermined level, and the output of the engine coolant temperature sensor 306 is applied as an input to the control unit 1 300.

Reference numeral 20 designates a manually operable gear range selector in the passenger compartment of the vehicle, including a control lever 22 movable to one of the five designated positions. The positions of range selector 20 are similar to those found in conventional motor vehicles, the position "P" representing Park, the position "R" representing Reverse, the position "N" representing Neutral, the position "D" representing Drive, and the position "L" representing Low -- a forward drive range requiring high output torque. The positions "R", "D", and "L" constitute a drive range, whereas the positions "P" and "N" consititute a non-drive range. The control lever 22 controls in a conventional manner a spool 11 36 of a manual valve 1104 shown in Figs. 3A and 3B through a suitable linkage (not shown).

A shift position switch 304 develops an output signal indicative of the range position to which control lever 22 is pointed. Such output is connected as an input to the control unit 1300.

Engine revolution speed sensor 301 measures the number of ignition spark pulses to develop an output signal indicative of the engine speed. A vehicle speed sensor 302 measures the revolution speed of the transmission output shaft 1052 and develops an output signal indicative of the vehicle speed. These output signals are applied as inputs to the control unit 1 300.

Referring also to Fig. 3B, a shift reference switch 1 298 is responsive to the movement of a control rod 11 82 of a shift operating mechanism 111 2 and is turned ON to develop an output signal when the rod 11 82 has come to a position which causes the establishment of the largest reduction ratio.

Such output signal is applied as an input to the control unit 1 300. After turning ON the shift reference switch 1298, the rod 11 82 is movable further, and during this overstoke the shift reference switch 1 298 is kept turned ON to develop the output signal.

The control unit 1 300 is operative in response to each of the above-mentioned inputs to provide output control signals for transmission 14 and clutch 1004 (1024).

In Figs. 2, 3A-3B and 4, transmission 14, clutch 1004 (1024), control unit 1300 are shown in more detail.

Referring to Fig. 2, engine output shaft 1002 is connectable with a drive pulley 1006 via hydraulically operable forward clutch 1004 or reverse clutch 1 024. Forward clutch 1004 is adapted to be engaged for forward drive, while reverse clutch 1024 is adapted to be engaged for reverse drive. Forward clutch 1004 has a cylinder chamber 1036, i.e., a clutch engagement chamber, and reverse clutch 1024 has a cylinder chamber 1 038, i.e., a clutch engagement chamber.As shown in Figs. 3A and 3B, forward clutch cylinder chamber 1036 is connected with manual valve 1104 via fluid conduit 1142, while reverse clutch cylinder chamber 1038 is connected with the manual valve 1104 via fluid conduit 1144, reverse inhibitor valve 11 22 and fluid conduit 11 38. Manual valve 1104 receives regulated fluid pressure developed in fluid conduit 1140 and selectively admits the regulated fluid pressure to forward cylinder chamber 1036 when the range position "D" is selected or to reverse clutch cylinder chamber 1038 when the range position "R" is selected.Fluid conduit 1140 is connected via orifice 1 226 with throttle fluid pressure circuit 11 62 and is formed with drain opening or port 1 222 communicating with drain circuit 1 200. The rate of fluid discharge from fluid conduit 11 40 is controlled by start adjustment valve 111 8 which will be described later.

Referring back to Fig. 2, the detailed description of this Figure is found in a copending U.S. patent application Ser. No.

489,600, filed April 28, 1983 (see Fig. 24 thereof), which has corresponding European patent application 83 104 182.7 (publication no 0093413, published on November 9, 1983). That portion of the description of this co-pending application which relates to Fig.

24 is hereby incorporated by reference to complete the disclosure of Fig. 2.

Briefly, the rotation of the shaft 1002 is transmitted to axle or power output shafts 1076 and 1078, via drive pulley 1006, Vbelt 1050, and driven pulley 1051, when the forward clutch 1004 or the reverse clutch 1024 is engaged. Listing all of the elements shown in Fig. 2, there are shaft 1 002, forward clutch 1004, drive pulley 1006, shaft (or drive shaft) 1008, fluid pump 1010, drive gear 1012, driven gear 1014, rotary flume 1016, reservoir 1018, pitottube 1020, auxiliary shaft 1022, reverse clutch 1024, gears 1026, 1028, 1030, 1032 and 1034, clutch cylinder chambers 1036 and 1 038, fixed conical disc 1040, drive pulley cylinder chamber 1042, movable conical disc 1044, rotary flume 1046, fluid reservoir 1047, pitot tube 1048, V-belt 1050, driven pulley 1051, shaft (or driven shaft) 1052, fixed conical disc 1054, driven pulley cylinder chamber 1056, spring 1057, movable conical disc 1058, gear 1060, ring gear 1062, differential case 1064, pinion gears 1066 and 1068, differential 1070, side gears 1072 and 1074, and power output shafts 1076 and 1078.

Referring to Figs. 3A and 3B, hydro-electronic control system involving control unit 1 300 may be divided into an automatic clutch control system and a shift control system. The clutch control system comprises fluid pump 1010, line pressure regulator valve 1102, throttle valve 1114, manual valve 11 04, reverse inhibitor valve 11 22, start adjustment valve 1118, and control unit 1 300. The shift control system comprises fluid pump 1010, line pressure regulator valve 1102, shift control valve 1106, shift motor (stepper motor) 1110, shift operating mechanism 1112, maximum reduction ratio maintaining valve 1120, lubrication valve 1124, tank 11 30 and control unit 1 300. The detailed description of the illustrated -lOele- ments in Figs. 3A and 3B is found in the previously mentioned co-pending U.S. patent application Ser. No. 489,600 (see Figs. 25A and 25B). From comparison of Figs. 3A and 3B of the present application with Figs. 25A and 25B of the co-pending U.S. patent application, it will be noted that the clutch control system of the present application is not provided with a valve corresponding to a starting valve 111 6 shown in Fig. 25A of the copending application. For complete disclosure of the shift control system, a reference is made to another co-pending U.S. patent application Ser.No. 543,838 entitled "CON TROL SYSTEM FOR HYDRAULIC AUTO MATIC CLUTCH," filed October 20, 1983, particularly to Figs. 2A, 2B, 3, 9(a), 9(b) and 10-21 and the corresponding description.

This another co-pending U.S. patent application Ser. No. 543,838 has a corresponding European patent application 83 110 546.5, filed October 21, 1983 and it is hereby incorporated by reference. The following description, therefore, relates mainly to start adjustment valve 1118, control unit 1 300 and clutch control routine 500 (see Fig. 5).

The start adjustment valve 1118, which is an actuator for controlling clutch engagement or clutch transfer capacity, is in the form of a force motor 1 224 having a plunger 1 224a which is adapted to control the amount of fluid discharge from fluid conduit 1140 to drain port 1 222. Throttle pressure variable with engine induction manifold vacuum is supplied to fluid conduit 1140 via orifice 1226.The rate of fluid discharge from fluid conduit 1140 is inversely proportional to the intensity of electric current passing through force motor 1 224, so that fluid pressure (start pressure) within fluid conduit 11 40 is regulated by controlling the intensity of electric current passing through the force motor 1 224. The intensity of the electric current is controlled by control unit 1 300 in the later described manner. The regulated fluid pressure within oil conduit 11 40 is supplied, as start pressure, selectively to forward clutch 1004 or reverse clutch 1024 in accordance with the range position to which manual valve 1104 is pointed.

Referring now to Fig. 4, control unit 1 300 is described which contains the shift control routine for controlling actuation of the stepper motor 1110 and the clutch control routine 500 for controlling electric current passing through the force motor 1 224.

As shown in Fig. 4, control unit 1 300 receives, as inputs, electric signals from en gine revolution speed sensor 301, vehicle speed sensor 302, engine load detector in the form of throttle opening degree sensor (or intake manifold vacuum sensor) 303, shift position switch 304, shift reference switch 1298, engine coolant temperature sensor 306, and brake sensor 307. The vehicle speed sensor 302 serves as a vehicle stop detector in this embodiment. Output signals of engine revolution speed sensor 301 and vehicle speed sensor 302 are allowed to enter an input interface 311 after passage through wave shapers 308 and 309, respectively, and electric voltage signal from throttle opening degree sensor (or intake manifold vacuum sensor) 303 is converted at A/D converter 310 into digital signals before entering input interface 311.Control unit 1300 inciudes input interface 311, reference pulse generator 312, CPU (Central Processor Unit) 313, ROM (Read Only Memory) 314, RAM (Randam Access Memory) 315, and output interface 316, which are linked with each other by address bus 319 and data bus 320. Refer ence pulse generator 312 generates a refer ence pulse which actuates CPU 313. ROM 314 stores programs necessary for controlling stepper motor 1110 and force motor 224 and data necessary for controlling them. RAM stores various parameters necessary for pro cessing information from each of sensors and switches and those necessary for control.Out put signals from control unit 1 300 are sup plied to stepper motor 1110 and force motor 1 224 via amplifier 31 7 and electric current controller 318, respectively.

In Fig. 5, the flowchart of force motor control routine 500 is shown. The execution of this routine 500 is repeated after a predet ermined period of time has lapsed (viz., the execution of routine is repeated for a short period of time). First of all, throttle opening degree TH is read from throttle opening de gree sensor 303 (step 501), vehicle speed V is read from vehicle speed sensor 302 (step 503), and engine revolution speed N is read from engine revolution speed sensor 301 (step 504). Thereafter, decision is made in step 505 whether or not vehicle speed V is lower than or equal to a predetermined small value Vo. When vehicle speed V is lower than or equal to the predetermined value Vo, deci sion is made in step 507 whether or not throttle opening degree TH is less than or equal to a predetermined small value THo.

When throttle opening degree TH is less than or equal to the predetermined value THo, shift position is read from the shift position switch 304 (step 11 00). The fact that the above mentioned steps have been carried out in the order indicates that the engine is idling when the vehicle is at standstill. Thereafter, decision is made whether or not the position "N" is selected (step 1102), and decision is made whether or not the position "P" is selected (step 11 04) when the position "N" is not selected. When the position "N" or "P" is selected, the control goes to step 1106 where the force motor electric current signal I is set to 11 (11 = a small value that causes start pressure to allow the clutch to be released).

Thereafter, the engine revolution speed N is stored (step 1108), and the running average Nm of engine revolution speed N is determined by arithmetic operation (step 1110) of the equation Nm = Nm' - Nm'/a + Nm/a (where, Nm' = the running average given in the previous routine, and a = the number of sample). If desired, this step 1110 may be omitted to allow the use of stored engine revolution speed N for the subsequent jobs. Thereafter, the control goes to step 527 where the force motor electric current 11 is output to be supplied to the force motor 1 224. When it is decided in steps 1102 and 1104 that the position selected is not "N" nor "P" and thus one of the positions "D" (drive), "L" (low), and "R" (reverse) which fall in a drive range is selected, a target engine revolution speed Nt is determined by arithmetic operation (step 508).

In this step 508, what is determined as target Nt is an engine revolution speed at which the engine would operate if the clutch engagement be adjusted so as to cause the engine to produce a predetermined torque (i.e., a creep torque) Tc that is large enough to enable the vehicle to travel at a very low speed (i.e., a creep speed). This target engine revolution speed Nt satisfies a predetermined relationship with the running average Nm determined in step 111 0. As depicted in graph shown in Fig. 6, with the same engine load (no load in this graph), the torque versus engine speed characteristic curves are fixed and determined by the performance of each engine. Different characteristic curves are provided each corresponding to one of different values in the running average Nm.For example, when the running average Nm is a value as shown in Fig. 6, the fully drawn characteristic curve is given. Using this curve, a single value in engine speed which causes the engine to produce the predetermined creep torque Tc can be given as the target Nt.

If a higher value in engine speed is given as the new running average Nm, a broken line curve which corresponds to this new value is used to determined the target engine revolution speed Nt. In this manner, the target Nt is determined based on the running average Nm. In practice, the determination of the target Nm is made by arithmetic operation of equation representing the relationship illustrated in Fig. 6. Alternatively, the relationship illustrated in Fig. 6 is stored as table data in ROM 314 and this table is retrieved to determine target Nt.

After determining the target engine revolution speed Nt in step 508, the control goes to step 511 where decision is made whether or not actual engine revolution speed N is greater than a target engine revolution speed upper limit Nt + An (An = a very small value). When N is greater than Nt + An, the force motor electric current signal I obtained in the previous routine is increased by a small value Al and set as a new force motor electric current (step 513). Hereinafter, decision is made whether or not electric current signal I is less than or equal to a maximum electric current sigal lo (step 515). When I is less than or qaual to lo, the control goes to the step 527, while when I is greater than lo, I is set equal to lo (step 517) before executing the step 527 where the force motor electric current signal I is output.When, in step 511, N is less than or equal to Nt + An (Combining with the decision in step 511, this results in that Nt - An~NcNt + An. Viz., the actual engine revolution speed is between the upper and lower limits of the target engine revolution speed Nt.), the control goes to step 527 where the elctric current signal I obtained in the previous routine is output. When, in step 519, N is less than Nt -bn, the force motor electric current signal I is decreased by the small value Al and the result is set as a new electric current I (step 521). Thereafter, in order to prevent the electric current signal from becoming negative, decision is made whether ot not I is greater than or equal to 0 (zero) (step 523).When I is greater than or equal to 0 (zero), the control goes to step 527, while when I is less than 0 (zero), I is set to O (zero) before executing the step 527.

When it is decided that V is greater than Vo (step 505) or TH is greater than THo (step 507), the control goes to step 506 where target force motor electric current signal required for the vehicle to start moving is determined. In this case, the target force motor electric current signal I is determined based on engine revolution speed N and engine throttle opening degree TH and it increases corresponding to the engine output torque.

Thereafter, this target force motor electric current signal is output in step 527.

It will now be understood that when the actual engine revolution speed N is greater than the upper limit for target engine revolution speed Nt, force motor electric current signal I is increased to raise the start pressure to increase the clutch transfer torque so as to bring the engine revolution speed down, whereas when the actual engine revolution speed is less than the lower limit for target engine revolution speed Nt, force motor electric signal I is decreased to lower the start pressure to decrease the clutch transfer torque so as to allow the engine to speed up. As a result, the actual engine revolution speed N is maintained between the upper and lower limits for target engine revolution speed Nt. At the target engine revolution speed Nt, the engine output torque is closely adjusted to the predetermined torque Tc which causes the vehicle to travel at a very low speed or creep.

Immediately after depressing the accelerator pedal under this condition, the target force motor electric current signal is determined in step 506 which differ corresponding to engine output, and the start pressure increases corresponding to this target force motor electric current signal. This causes gradual and smooth clutch engagement, thus providing stable starting of the vehicle, free from engine racing, substantial shocks, and engine stall.

Since, the same creep torque Tc is maintained, unintentional vehicle's starting is prevented.

Although, in this embodiment, it is applied to a clutch associated with a continuously variable transmission, the present invention may be equally applied to an automatic clutch associated with a step automatic transmission or a manual transmission. Although, in the embodiment described, the clutch employed is an automatic clutch of the hydraulic pressure operated type, the present invention may be equally applied to an electromagnetic clutch provided clutch transfer capacity is varied by controlling electric signal.

It will now be appreciated that when engine idles under a condition where drive range position is selected when vehicle is at standtill, engine revolution speed is controlled based on idling speed obtained under a condition where nondrive range position is selected when vehicle is at stanstill, so that the clutch is held slightly engaged irrespective of variations in hydraulic pressure characteristic or friction characteristic of clutch facing or variations due to aging.

Referring to Figs. 7 and 8, a second embodiment is described. In this embodiment, the present invention is embodied in a motor vehicle drive train 10 having an engine 1 2 installed with a conventional idle speed control system (abbreviated as ISC hereinafter).

Typical examples of ISC are disclosed in U.S.

patent 4,345,557 (Ikeura), U.S. patent 3,964,457 (Coscia), U.S. patent 4,203,395 (Cromas et al.), U.S. patent 4,181,104 (Shinoda), and U.S. patent 4,191,051 (Kawata et al.). Reference numeral 321 designates an ISC's actuator position detector which is responsive to the operating position of actuator of ISC and develops an output signal G indicative of the operating position of the actuator. Such output enters an input interface 311 of control unit 1 300 via an A/D converter 310 as shown in Fig. 8. Figs. 7 and 8 are substantially identical to Figs. 1 and 4, respectively.

The second embodiment is different from the previously described first embodiment in the following respects. As mentioned above, in the second embodiment, ISC actuator position detector 321 is provided and the clutch engagement is controlled such that an error of the output signal G from target Gt decreases instead of using engine speed N which will be later described in connection with flowchart shown in Fig. 9.

In Fig. 9, the flowchart of force motor control routine designated by reference numeral 500' is described comparing with previously described control routine 500 for ease of understanding. Control routine 500' is substantially identical to control routine 500 shown in Fig. 5 except new job steps 504', 1108', 1110', 508', 511', and 519' are the substitutes for steps 504, 1108, 1110, 508, 511, and 519, respectively.

Referring to Fig. 9, after executing steps 500 and 503, signal G indicative of ISC's actuator operating position is read. Thereafter, steps 505, 507, 1100, 1102, and 1106 are executed before step 1108' where signal G indicative of ISC's actuator position is stored.

In the next step 1110', the running average is determined by arithmetic Qpeation of the equation Gm = Gm' - Gm'/a + Gm/a (where, Gm' = the running average given in the previous routine, and a = the numbner of sample). If desired, this step 1100 may be omitted (without cauculating the running average) to allow the use of stored G for the subsequent job. Thereafter, the control goes to step 527 where the force motor electric current signal il is output. When it is decided in steps 1102 and 1104 that the drive range position ("D" or "L" or "R") is selected, a target signal Gt is determined in step 508'.

What is determined as the target signal Gt is the operating position of the actuator of ISC which the actuator is pointed when the clutch engagement is so adjusted as to cause the vehicle to creep. This target Gt satisfies a predetermined relationship with the running average Gm. In the case where the engine revolution speed for no load engine idle operation is set to NI, the relationship between engine revolution speed and output torque at the same operating position of the ISC's actuator is determined by the performance of each engine and exibits the characteristic as shown by the fully drawn curve as shown in Fig. 10.

With the same engine speed NI, the operating position of the ISC's actuator (target Gt) which cause the engine to produce a predetermined creep torque Tc is determined by broken line curve. It follows that if the running average Gm changes to a new one, the corresponding target Gt to this new one can be determined by arithmetic operation of a suitable equation or by table look-up of table data reflecting the relationship shown in Fig.

10.

After determining the target signal Gt in step 508', the control goes to step 511' where decision is made whether or not signal G is greater than the target upper limit Gt + bg (Ag = a very small value). When the signal G is greater than Gt + Ag, force motor electric current signal I obtained in the previous routine is increased by a small valve Al and set as a new force motor electric current value (step 513). Thereafter, decision is made whether or not electric current signal I is less than or equal to a maximum allowable electric current signal lo (step 515). When I is less than or equal to Io, the control goes to step 527, whereas when I is greater than lo, I is set equal to lo (step 517) before executing the step 527 where the force motor electric current signal I is output.When, in step 511', the signal G is less than or equal to Gt + Ag, decision is made whether or not the signal G is less than a target signal lower limit Gt - Ag (step 519'). When G is greater or equal to Gt - bg, the control goes to step 527.

When, in step 519', G is less than Gt - Ag, force motor electric current signal I is decreased by small value Al and the result is set as a new electric current signal I (step 521).

Thereafter, in order to prevent the electric current signal I from becoming negative, decision is made whether or not I is greater than or equal to 0 (zero) (step 523). When I is greater than or equal to O (zero), the control goes to step 527, while when I is less than 0 (zero), I is set to O (zero) before executing the step 527. When it is decided that V is greater than Vo (step 505) or TH is greater than THo (step 507), the control goes to step 506 and then to step 527.

From the above description, it will be understood that when the signal G is greater than the upper limit for the target signal Gt, the force motor electric current signal I is increased to raise the start pressure to increase transfer torque so as to bring the signal G down, whereas when the signal G is less than the lower limit for target signal Gt, the force motor electric current signal I is decreased to lower the start pressure to decrease the clutch transfer torque so as to allow the signal G to increase. The engine output torque corresponding to the target signal Gt is set to such value Tc (creep torque) that the vehicle is allowed to travel at a very low speed.

Referring lastly to Fig. 11, a third embodiment according to the present invention is described.

This third embodiment is substantially identical to the above described second embodiment except that new steps 502, 1112, 1114 are added, and a step 508" is the substitute for step 508'.

Describing specifically, there are added step 502 where engine revolution speed N is read, another step 111 2 where the engine revolution speed N having read is stored, and still another step 1114 where the running average Nm of the engine revolution speed is determined by arithmetic operation of the same equation as used in step 1110 shown in Fig.

5. In step 508", a target ISC's actuator position signal Gt is determined based on the running average Gm, determined in step 1110 and the running average Nm, determined in step 1114.

This embodiment features in determining the target signal Gt based on the running averages Gm and Nm. This is effective in securely keeping the clutch slightly engaged even if the setting of idle speed is shifted to a high level to cope with the use of an air conditioner or the engine's warming up.

Claims (10)

1. An automatic clutch control system for a motor vehicle including an engine, a transmission, and a clutch adapted to transfer torque between the engine and the transmission, the transmission having a drive range and a nondrive range, the automatic clutch control system comprising: means for actuating the clutch; a first servo loop for controlling the operation of said actuating means, said first loop being effective when the engine idles under a condition where the transmission is rendered to select the non-drive range when the vehicle is at standstill;; a second servo loop for controlling the operation of said actuating means in such a manner as to control the clutch engagement to adjust the transfer torque by the clutch to a predetermined level, said second loop being effective when the engine idles under a condition where the transmission is rendered to select the drive range when the vehicle is at standstill; said first servo loop including means for storing predetermined operating information related to idle operating condition of the engine; said second servo loop including means for determining a target in response to said predetermined operating information stored by said storing means, and means for controlling the operation of said actuating means so as to control the clutch engagement in such a direction as to decrease the deviation from said target.

2. An automatic clutch control system as claimed in claim 1, wherein said predetermined operating information involves the engine revolution speed, and said target satisfies a relationship with said predetermined operating information, said relationship being predetermined for the engine.

3. An automatic clutch control system as claimed in claim 1, wherein said predetermined operating information involves the operating position of an idle speed control system for the engine, and the target satisfies a relationship with said predetermined operating condition information, said relationship being predetermined for the engine.

4. An automatic clutch control system as claimed in claim 1, wherein said predetermined information operating information involves the engine revolution speed and the operating position of an idle speed control system for the engine, and a target satisfies a relationship with said predetermined operating information, said relationship being predetermined for the engine.

5. An automatic clutch control method for a motor vehicle including an engine, a transmission, a clutch adapted to transfer torque between the engine and the transmission, and means for actuating the clutch, the transmission having a drive range and a non-drive range, the automatic clutch control method comprising:: controlling the operation of said actuating means when the engine idles under a condition where the transmission is rendered to select the non-drive range when the vehicle is at standstill; controlling the operation of said actuating means in such a manner as to control the clutch engagement to adjust the transfer torque by the clutch to a predetermined level, when the engine idles under a condition where the transmission is rendered to select the drive range when the vehicle is at standstill; said first mentioned controlling step including storing predetermined operating information related to idle operating condition of the engine;; said second controlling step including determining a target in response to said predetermined operating information stored, and controlling the operation of said actuating means so as to control the clutch engagement in such a direction as to decrease the deviation from said target.

6. An automatic clutch control method as claimed in claim 5, wherein said predetermined operating information involves the engine revolution speed, and said target satisfies a relationship with said predetermined operating information, said relationship being predetermined for the engine.

7. An automatic clutch control method as claimed in claim 5, wherein said predetermined operating information involves the operating position of an idle speed control system for the engine, and the target satisfies a relationship with said predetermined operating condition information, said relationship being predetermined for the engine.

8. An automatic clutch control method as claimed in claim 5, wherein said predetermined information operating information involves the engine revolution speed and the operating position of an idle speed control system for the engine, and a target satisfies a relationship with said predetermined operating information, said relationship being predetermined for the engine.

9. An automatic clutch control system substantially as hereinbefore described ,ith reference to anyone of the embodiments shown in the accompanying drawings.

10. An automatic clutch control method, substantially as hereinbefore described with reference to the accompanying drawings.