Transmission control system for and method of automotive vehicle with continuously variable automatic transmission

[0002] 1. Field of the Invention

[0003] The present invention relates to a transmission control system for and method of an automotive vehicle employing a continuously variable automatic transmission, and specifically to techniques for optimally controlling a speed ratio, which states the ratio of the transmission input to output speed, of an automobile continuously variable automatic transmission even under particular vehicle driving conditions, for example when restarting after the vehicle experiences wheel spin at drive road wheels during the starting period on uphill roads with wheel spin, or when restarting after the vehicle experiences wheel spin at drive wheels during the starting period on low-μ roads.

[0004] 2. Description of the Prior Art

[0005] Japanese Patent Provisional Publication Nos. 8-178042 and 4-54371 have disclosed a belt-type continuously variable automatic transmission, often abbreviated to “CVT”. Generally, the speed ratio of the CVT is electronically controlled in accordance with a predetermined speed-change pattern by means of an electronic transmission control unit. The predetermined speed-change pattern is pre-stored as a preprogrammed characteristic map in the memory (ROM) of the transmission control unit. A desired speed necessary for the speed-change control action is generally based on vehicle speed and engine load. As a control parameter corresponding to the vehicle speed, a rotational speed of the output shaft of the CVT (i.e., a drive-wheel speed) is generally used.

[0006] When the vehicle starts from the standstill state on low-μ roads, such as wet, icy, or snow roads, particularly when starting on low frictional roads or during uphill driving, there is a tendency for wheel spin to occur. Owing to the wheel spin, the drive-wheel speed rises rapidly, while the rise of vehicle speed is not so much. In spite of such less rise of the vehicle speed, the electronic transmission control unit begins to perform the speed-change control action according to the speed-change ratio based on the engine load and the drive-wheel speed (regarded as the vehicle speed), as if the actual vehicle speed rises rapidly. That is, the transmission control unit would generate an upshift command due to the rapid rise of drive-wheel speed. As a result of this, the speed ratio tends to unreasonably shift toward the minimum speed ratio line (or the highest-speed line) in the predetermined speed-change pattern. Under these conditions, if the accelerator pedal is released by the driver to prevent the undesired wheel spin occurring at the drive road wheels, the vehicle may come to a standstill without adequate return of the speed ratio of the CVT to a relatively large value closer to the maximum speed ratio line (or the lowest-speed line). Thus, the speed ratio tends to be fixed undesiredly to a relatively small speed ratio closer to the minimum speed ratio line. Assuming that the vehicle is restarted from the standstill state with the speed-change ratio fixed to a certain small value closer to the minimum speed ratio line, the vehicle cannot be easily restarted owing to the small speed ratio.

[0007] In an electronically-controlled continuously variable automatic transmission, it is desirable to provide a satisfactory speed ratio control action even under the previously-discussed particular vehicle driving conditions, such as when restarting on the uphill roads owing to wheel spin, or when restarting on low-μ roads, such as wet, icy, or snow roads, owing to wheel spin.

[0008] Accordingly, it is an object of the invention to provide a transmission control system of an automotive vehicle with a continuously variable automatic transmission which avoids the aforementioned disadvantages of the prior art.

[0009] In order to accomplish the aforementioned and other objects of the present invention, a transmission control system of an automotive vehicle with a continuously variable automatic transmission, comprises a transmission control system for an automotive vehicle with a continuously variable automatic transmission whose speed-change ratio is defined as a ratio of transmission input to output speed and is changeable continuously depending on vehicle speed and engine load, comprises a hydraulic actuator producing a regulated line pressure and a speed-change control pressure necessary to adjust the speed ratio hydraulically, and an electronic transmission control unit including a data processing section detecting an acceleration of a drive wheel of the vehicle, comparing the acceleration of the drive wheel to a predetermined acceleration value, and inhibiting an upshifting action by preventing a speed ratio of the continuously variable automatic transmission from being upshifted toward a speed ratio less than a predetermined upshift limiting threshold.

[0010] According to another aspect of the invention, a transmission control method of an automotive vehicle with a continuously variable automatic transmission whose speed-change ratio is defined as a ratio of transmission input to output speed and is changeable continuously depending on vehicle speed and engine load, comprising the steps of: detecting an acceleration of a drive wheel of the vehicle; comparing the acceleration of the drive wheel to a predetermined acceleration value; and inhibiting an upshifting action by preventing a speed ratio from being shifted toward a speed ratio less than a predetermined upshift limiting threshold.

[0011] FIG. 1 is a system block diagram illustrating an embodiment of a speed-change control system according to the invention.

[0012] FIG. 2 is a flow chart of the speed-change control routine corresponding to the CVT control of the embodiment.

[0013] FIG. 3 is a characteristic map illustrating a predetermined speed-change pattern based on the target primary pulley rotational speed (N_pri*) and the vehicle speed (VSP), which map is used in the automatic speed-change control routine shown in FIG. 2.

[0014] FIGS. 4 and 5 are timing charts, prepared to comparing the improved CVT control with the conventional CVT control under particular driving condition, that is, when restarting owing to wheel spin during the starting period on low-μ and uphill roads, and indicating their simulation results under the particular driving condition where the vehicle starts on low-μ and uphill roads, and then stops with the accelerator pedal released after the occurrence of wheel spin, and thereafter restarted.

[0015] Referring now to the drawings, particularly to FIG. 1, there is shown a power train of an automotive vehicle employing a CVT control device and an electronic transmission control system,. necessary for the speed-change control executed by the CVT control of the invention. As seen in FIG. 1, the power train is constructed by an engine 1 and a continuously variable automatic transmission (CVT) 2. Reference sign 3 denotes a throttle valve 3. As may be appreciated from FIG. 1, the transmission control system of the invention is exemplified in the automotive vehicle with a belt-type continuously variable automatic transmission (CVT) which uses a segmented steel belt 9 running in a pair of variable-width pulleys 7 and 8, namely primary and secondary—driving and driven—pulleys whose effective diameters are continuously variable to provide varying pulley ratios (or speed ratios). The primary pulley 7 has a driven connection with the output shaft of the engine 1 via the torque converter 6. Torque transmitted from the primary pulley 7 to the secondary pulley 8 is transmitted via the final gear set 10 to the differential gear 11. Then, the torque transmitted to the differential gear 11 is further transmitted via axle drive shafts to the left and right drive wheels. As is generally known, the V groove of the primary pulley 7 is constructed by a stationary flange and an adjustable flange axially slidable on linear ball bearing splines for varying the width of the V groove of the primary pulley by hydraulic pressure P_pri(often called a “primary pulley pressure” or a “speed-change control pressure”). Similarly, the V groove of the secondary pulley 8 is constructed by a stationary flange and an adjustable flange axially slidable on linear ball bearing splines for varying the width of the V groove of the secondary pulley by spring bias plus hydraulic pressure P_sec(often called a “secondary pulley pressure” or a “regulated line pressure”) in the smaller-diameter cylinder. The two hydraulic pressures P_priand P_secare generated from the hydraulic actuator 12 to the respective cylinders of the primary and secondary pulley units, in response to a command signal representative of a target speed ratio i_ptwhich is calculated as a ratio (N_pri*/N_sec) of a desired primary pulley rotational speed N_pri* to a secondary pulley rotational speed N_sec. A transmission controller 13 is provided for feedback-controlling, so that the actual speed ratio i_pof the CVT 2 is adjusted to the target speed ratio i_pt. Generally, the controller 13 of the CVT control system comprises a microcomputer. As seen in FIG. 1, the input interface of the controller 13 receives various input-information signals from engine/vehicle sensors, namely a throttle opening sensor 16, a secondary pulley rotational speed sensor 18, a vehicle speed sensor 19, an inhibitor switch 20, a drive-wheel speed sensor 21, and an anti-skid brake system (ABS) 22. On the basis of these sensor signal values, the controller 13 determines or arithmetically calculates the target speed ratio i_pt. The throttle opening sensor 16 is provided for monitoring the throttle opening TVO of the throttle 3 of the engine 1. The vehicle speed sensor 19 is usually located near the transmission output shaft to detect the rotational speed of the transmission output shaft. Usually, the rotational speed of the transmission output shaft is used as a vehicle speed VSP. The inhibitor switch 20 is used to detect the more-recent selected range of the CVT 2. The drive-wheel speed sensor 21 is provided for detecting a drive-wheel speed V_w. Also, the input interface of the controller 13 is connected to the ABS system 22, for detecting whether the skid control system is in operative or in inoperative. In FIG. 3, there is shown the characteristic map illustrating the speed-change control characteristics which are achieved by the continuously variable automatic transmission control system of the present invention. In FIG. 3, the axis of ordinate indicates a target primary pulley rotational speed N_pri*, whereas the axis of abscissa indicates the vehicle speed VSP. Suppose that the vehicle speed (VSP) of the axis of abscissa is identical to the transmission output shaft speed N_sec, since the transmission output shaft speed N_secis generally regarded as the vehicle speed (VSP) or the vehicular body speed (VB) . In the map shown in FIG. 3, the upper polygonal line indicates the maximum speed ratio line, whereas the lower polygonal line indicates the minimum speed ratio line. The right-hand moderately inclined part of the lower polygonal line means a predetermined lower limit (the minimum speed ratio) of the speed-change ratio, whereas the left-hand steeply inclined part of the upper polygonal line means a predetermined upper limit (the maximum speed ratio) of the speed-change ratio. The intermediate zone, bordered between the upper and lower polygonal lines, corresponds to a usual speed-change control enable zone. As seen in FIG. 3, the speed-change line tends to shift from the lower polygonal line toward the upper polygonal line, such that the desired primary pulley rotational speed N_pri* is set at a higher revolution speed, as the vehicle speed VSP (N_sec) increases. Additionally, as appreciated from the arrow indicated in FIG. 3, the speed-change line is designed to shift from the lower polygonal line toward the upper polygonal line in accordance with the increase in the engine load, commonly detectable as a throttle opening (TVO) or an accelerator pedal depression (APS). In the shown embodiment, the throttle opening TVO detected by the throttle opening sensor 16 is used as the engine load. As clearly seen in FIG. 3, in the transmission control system of the invention, an upshift inhibition zone, bordered by the two straight lines L₁and L₂and indicated by the hatched line, is provided in close vicinity to the minimum speed ratio line, so as to inhibit the upshifting action of the CVT within the upshift inhibition zone. In other words, within the upshift inhibition zone, only the upshifting action is prevented or inhibited, but the downshifting action is allowed. In order to ensure a proper driving torque under particular driving conditions, such as when restarting after starting on uphill roads with wheel spin, or when restarting after starting on low-μ roads with wheel spin, it is advantageous to provide the previously-noted upshift inhibition zone in close vicinity to the minimum speed ratio line (see the hatched zone close to the lower polygonal line of FIG. 3). In the transmission control method of the invention, either one of the usual speed-change operating mode and the upshift inhibition mode is suitably selected depending on predetermined necessary conditions, as will be more fully described later by reference to the flow chart shown in FIG. 2. In FIG. 3, the gradient (N_pri*/VSP) of the boundary line L₁means a certain target speed ratio i_pt(=N_pri*/N_sec), since a transmission output speed (N_sec) is generally used as the vehicle speed VSP. This certain target speed-change ratio i_ptcorresponding to the gradient of the boundary line L₁(the upper limit of the upshift inhibition zone) is an upshift-inhibition zone upper threshold value i_p* (simply an upshift limiting threshold value i_p*). Particularly, at the beginning of the upshift inhibition operating mode, the target speed-change ratio corresponding to the gradient of the boundary line L₁means a predetermined initial value i_pi* of the upshift limiting threshold i_p*. The lowermost straight line L₂(the lower limit of the upshift inhibition zone) is identical to the right-hand inclined part of the lower polygonal line (the minimum speed ratio line). When the upshift inhibition mode is selected, the upshift limiting threshold i_p* functions to prevent shifting of the speed ratio i_ptoward the minimum speed ratio side (overdrive) by preventing or inhibiting the upshifting action within the upshift inhibition zone. That is, the provision of the gradual relaxation or release (see the flow from step 38 via step 40 to step 41) of the upshift limiting threshold is effective to prevent the rapid shifting of the actual speed-change ratio i_ptoward the minimum speed ratio side (overdrive), and consequently to attain the speed-change control action with as small shift-shock as possible. In the shown embodiment, at the beginning of the upshift inhibition mode, the value of the threshold i_p* is first set at the initial value i_pi* that provides a driving torque enough to travel on uphill road having a desired gradient. As seen in steps 31 and 32, the upshift limitation mode initiates, when the following specified conditions are satisfied. One specified condition is a condition where the shift lever position of the continuously variable automatic transmission is the D range. The other specified condition is a condition where the wheel acceleration dVW/dt (time rate of change of the drive-wheel speed VW) is above a predetermined wheel-spin threshold value α_spin. The predetermined wheel-spin threshold α_spinis set at a value greater than an acceleration value corresponding to the standard longitudinal acceleration of the vehicle, which acceleration is produced during the usual vehicle driving. In the shown embodiment, although the two necessary conditions (that is, the CVT put into the D rangeand dVW/dt≧α_spin) are used to initiate the upshift inhibition mode (or to provide the upshift inhibition zone), according to the transmission control system of the invention, at least the latter necessary condition of dVW/dt≧α_spinis required to initiate the upshift inhibition mode. As described in detail later, in the shown embodiment of the transmission control system according to the invention, the automatic speed-change control routine of FIG. 2 is so designed that the upshift limitation or the upshift inhibition for the speed-change ratio i_pis gradually relaxed or released by the flow from step 38 via step 40 to step 41. That is, during the gradual relaxation mode for upshift limitation, the value of the upshift limiting threshold i_p* is gradually reduced from the initial set value i_p* toward a preset low value i_pssubstantially corresponding to the minimum speed ratio line (the lowermost straight line L₂corresponding to the lower limit of the upshift inhibition zone), so that the upshift inhibition zone is gradually decreased or narrowed. It is preferable that the gradual relaxation or moderately decrease in the upshift limiting threshold i_p* is not made under a particular condition where the vehicle is running at an excessively low speed that the drive-wheel speed VW, regarded as the vehicle speed (VSP) or the vehicular body speed (VB), is below a predetermined low speed. This is because the stopping time of the vehicle is very short during driving (during crawling) at excessively low vehicle speeds. The predetermined low speed means a predetermined vehicle-speed threshold value V_lownecessary to determine whether the vehicle is substantially in the standstill state. The predetermined threshold value V_lowwill be hereinafter referred to as a “predetermined standstill-decision vehicle speed”. The electronic transmission control unit decides that the vehicle is substantially in the standstill state, when the drive-wheel speed VWis below the predetermined threshold value V_low. Thus, the driver must restart the vehicle, after the accelerator pedal is released once owing to the occurrence of wheel-spin which may occur at the drive wheels under particular operating conditions, for example during uphill driving, or during starting period on low-μ roads. In order for the vehicle to easily restart on the low-μ road or on the uphill road, where the vehicle may experience wheel spin at drive wheels, the vehicle requires the reasonable magnitude of driving torque transmitted to the drive wheels. To achieve this, a comparatively large speed ratio i_pis required during the restarting period on the uphill road or on the low-μ road. Thus, in order to prevent the speed ratio i_pfrom shifting toward the preset value i_ps(overdrive) during driving (during crawling) at an excessively low vehicle speed below the predetermined standstill-decision threshold V_low, it is advantageous to inhibit the previously-noted gradual relaxation in the upshift limitation and to keep the upshift inhibition mode (see the flow from step 37 to step 34 in FIG. 2), when the vehicle is substantially in the standstill state. Furthermore, in the transmission control system according to the invention, as seen from step 36 in FIG. 2, the upshift limitation (the upshift inhibition) is forcibly removed or released completely rapidly when the anti-skid brake system (ABS) is in the operative state, since the wheel speed can be quickly recovered during skid control (ABS control) by way of the decreased speed-change ratio, resulting from the forcible release of upshift inhibition (see the flow from step 36 to step 39 in FIG. 2). As seen from the viewpoint of the skid control, it is advantageous to rapidly stop the gradual relaxation in upshift inhibition during the skid control. Details of the automatic transmission control routine based on the system according to the invention are described hereunder with reference to the flow chart of FIG. 2. The arithmetic processing (the automatic transmission control routine) shown in FIG. 2 is executed as time-triggered interrupt routines to be triggered every predetermined time intervals, such as 10 msec.

[0016] In step 31, a test is made to determine whether the selected range of the CVT is “D”. When the answer to step 31 is in the negative (NO), that is, when either one of the other ranges (e.g., L range, P range, N range) except the D range is selected, the routine jumps to step 39. In step 39, the usual speed-change control action (or the usual speed-change operating mode) is performed. Conversely, when the answer to step 31 is in the affirmative (YES), the routine proceeds to step 32. In step 32, the acceleration α(=dVW/dt) of the drive wheel is compared to a predetermined acceleration value α_spin. When the answer to step 32 is affirmative (YES), that is, in case of dVW/dt≧α_spin, step 33 occurs. In step 33, the upshift limiting threshold i_p* is set at the predetermined initial set value i_pi*. Then, the upshift inhibition mode is entered at step 34. In step 34, the usual speed-change control action is allowed within the speed-change control action permission zone defined by i_pt≧i_p*, whereas only the upshifting action is inhibited within the upshift inhibition zone defined by i_pt<i_p*. During the starting period of the vehicle, the speed ratio i_p(or the target speed ratio i_pt) is first set essentially at the maximum speed ratio identical to the maximum gradient of the upper polygonal line of FIG. 3. Thereafter, the target speed ratio i_pttends to gradually reduce in accordance with the increase in the wheel speed VWand the increase in the vehicle speed VSP (the vehicular body speed VB). Under these conditions, if the two necessary conditions (i.e., the CVT put into the D range and dVW/dt≧α_spin) are satisfied, the upshift inhibition mode initiates through step 34. That is, the speed-change control action that the target speed ratio i_ptbecomes less than the threshold i_p*, is prevented or inhibited by the upper limit L₁(the threshold i_p*) of the upshift inhibition zone, as soon as the target speed ratio i_ptis reduced down to the threshold i_p*. Particularly during the starting period on the low-μroad or on the uphill road, the provision of step 34 is effective to avoid the speed ratio i_pfrom entering into the upshift inhibition zone defined by the two lines L₁(determined by the threshold i_p*) and L₂(including the right-hand inclined part of the lower polygonal line). Thereafter, in step 35, the same test as step 31 is made to check whether the current selected range is maintained in the D range. When the answer to step 35 is negative, the routine proceeds to step 39 at which the usual speed-change operating mode is executed. On the contrary, when the answer to step 35 is affirmative, step 36 occurs. In step 36, a check is made to determine whether the anti-skid brake system (ABS) is in its inoperative state. When the answer to step 36 is negative, the routine flows to step 39. On the contrary, when the answer to step 36 is affirmative, that is, when the ABS is in the inoperative state, step 37 occurs. In step 37, a test is made to determine whether the drive-wheel speed VWis below the predetermined threshold value V_low. When the answer to step 37 is affirmative, that is, in case of VW>V_low, the routine returns from step 37 to step 34 to continue the upshift inhibition mode. Conversely, when the answer to step 37 is negative, that is, in case of VW>V_low, the routine flows to step 38. In step 38, the threshold i_p* is compared to a predetermined small speed ratio i_ps. The predetermined small speed ratio i_psis preset at a value substantially corresponding to the minimum speed ratio line (the lowermost straight line L₂corresponding to the lower limit of the upshift inhibition zone). As may be appreciated, the difference between the threshold i_p* and the preset small speed ratio i_psdefines a releasing time duration at the gradual relaxation mode for upshift limitation. The proper setting of the predetermined small speed ratio i_psis important to prevent rapid release in the previously-discussed upshift limitation, and to avoid shift-shock resulting from the rapid shifting of the speed ratio i_ptoward the minimum speed ratio. Also, the proper setting of the preset initial value of the threshold i_p* is important to ensure a reasonable upshift limiting action under the previously-discussed particular driving conditions, and to insure adequate recovery of the speed ratio of the CVT toward a value closer to the maximum speed ratio line. Although the predetermined small speed ratio i_pscan be set at a value corresponding to the minimum speed ratio line (the lowermost straight line L₂), there is a possibility that the recovery of the speed ratio i_pto the usual speed ratio given during the usual speed-change operating mode is retarded. For the reasons set forth above, it is more preferable to set the predetermined small speed ratio i_psat a value somewhat lower than the minimum speed ratio corresponding to the minimum speed ratio line (L₂). In step 38, when the answer to step 38 is negative, that is, when the predetermined threshold i_p* is below the predetermined small speed ratio i_ps, the routine proceeds to step 40. In step 40, the same check as step 32 is made. In step 40, when the condition dVW/dt≧α_spinis satisfied, the routine flows to step 33 and as a result the upshift limiting threshold i_p* is set again at the initial set value i_pi*. This step 40 is effective to prevent the previously-noted gradual releasing mode (or the gradual releasing control) for the upshift limiting threshold i_p* and to continue the upshift inhibition mode with the upshift limiting threshold i_p* set at the initial set value i_pi*, when wheel-spin reoccurs. Returning to step 38, when the answer to step 38 is affirmative, that is, in case of i_p*≦i_ps, the routine proceeds to step 39. When the answer to step 40 is negative, that is, when the condition of dVW/dt<α_spinis satisfied, step 41 occurs. In step 41, the current value of the threshold i_p* is decreased or decremented by a predetermined decrement or a predetermined time rate-of-change Δi_pof the threshold value i_p*. Owing to repetition of the loop defined by the flow from step 34 through steps 35, 36, 37, 38, 40 and 41, again to step 34, the threshold value i_p* can be gradually reduced toward the predetermined small speed ratio i_ps, so that the upper limit L₁of the upshift inhibition zone is shifted toward the smaller limit defined by the predetermined small speed ratio i_ps, and thus the upshift inhibition zone is gradually narrowed or reduced. In this manner, the upshift inhibition mode can be gradually relaxed or released by way of repetition of the loop defined by the flow from step 34 through steps 35, 36, 37, 38, 40 and 41, again to step 34. On the other hand, by way of repetition of the loop defined by the flow from step 34, through steps 35, 36 and 37, again to step 34, the upshift inhibition mode is continued with no change in the threshold i_p*.

[0017] Referring now to FIGS. 4 and 5, there are simulation results representative of the relationship among the throttle opening TVO (considered to the engine load), the speed ratio i_p, the drive-wheel speed VW, and the vehicular body speed V_B(considered to be equivalent to the vehicle speed VSP and the transmission output shaft speed N_sec), respectively based on the improved CVT control and the conventional CVT control according to the invention. The simulation test is based on the assumption that wheel spin takes place at drive wheels during the starting period on the low-μ and uphill road and additionally the skid control system is in the inoperative state.

[0018] As seen in FIG. 5, according to the conventional CVT control, at the time t₁when the vehicle is started with depression of the accelerator pedal under a condition where the CVT is put into the D range, the drive wheels begin to spin due to the low-μ and uphill road. Irrespective of when the actual vehicle speed V_Bis not so much owing to the wheel spin, the rotational speed of the secondary pulley (the transmission output shaft speed N_sec) rises rapidly. Thus, owing to the acceleration slip, the drive-wheel speed VWbegins to rise rapidly from the time t₁. Due to the rapid rise in the transmission output shaft speed N_sec, the conventional transmission controller performs a speed-change control, so that the speed ratio i_pshifts toward a low speed ratio. At the time t₂when the driver releases the accelerator pedal to avoid the wheel spin, the throttle opening TVO drops to a predetermined low value (0/8) corresponding to a substantially closed position of the throttle 3. Thus, the drive-wheel acceleration value α(=dVW/dt) shifts to the negative sign and the drive-wheel speed VWreduces rapidly. As a result, the speed ratio i_pbegins to increase and the vehicle begins to decelerate from the time t₂. After this, the vehicle comes to a standstill at the time t₃, because the vehicle speed is excessively low due to the low friction coefficient of the low-μ and uphill road and the accelerator pedal is undepressed. In the standstill state, the change-speed control action is not performed due to the mechanism or construction of the CVT. Thus, after the vehicle is stopped once at the time t₃, the speed ratio i_premains unchanged and is fixed to a comparatively low value (closer to overdrive) with no return of the speed ratio i_pto an adequate high value. Thereafter, when restarting with depression of the accelerator pedal at the time t₄, it is difficult to easily restart the vehicle on the low-μ and uphill road with the speed-change ratio fixed to the comparatively low value.

[0019] On the contrary, as seen in FIG. 4, according to the improved transmission control of the invention, when the vehicle is started at the time t₁with the accelerator pedal depressed under a condition where the CVT is put into the D range, the drive wheels begin to spin due to the low friction factor of the low-μ and uphill road. Assuming that the acceleration α(=dVW/dt) of the drive wheel becomes above the predetermined acceleration value (the predetermined wheel-spin threshold value) α_spinwith a brief response delay (t_spin−t₁) due to the rapid depression of the accelerator pedal at the time t_spinjust after the time t₁, the routine of FIG. 2 flows from step 32 to step 33. Thus, as soon as the condition of dVW/dt≧α_spinis sensed at step 32, the upshift inhibition zone defined by i_pt<i_p* is provided. At this time t_spin, the threshold i_p* of the upshift inhibition zone is set at the predetermined initial value i_pi* (see step 33 of FIG. 2). The predetermined acceleration value α_spincan be hereinafter referred to as a “wheel-spin decision acceleration value”. As indicated by the broken line in the intermediate timing chart for variations in the speed-change ratio i_pand changes in the threshold i_p* of the upshift inhibition zone, the threshold i_p* of the upshift inhibition zone is held at the initial set value i_pi* for the time period from the time t_spinto the time t_s1, until the drive-wheel speed VWexceeds the predetermined standstill-decision vehicle speed V_low. The time t_s1will be referred to as a “start-decision times”, since the controller determines that the vehicle begins to run when the drive-wheel speed VWexceeds the standstill-decision vehicle speed V_low. The time t_s2when the drive-wheel speed VWbecomes below the standstill-decision vehicle speed V_lowwill be referred to as a “standstill-decision time”. The time t_s3when the vehicle is restarted after the vehicle comes to the standstill state at the time t₃, will be referred to as a “restart-decision time”. As seen in FIG. 4, according to the flow from step 37 through steps 38, 40, 41, 34, 35 and 36 again to step 37, for the time period from the start-decision time t_s1to the standstill-decision time t₂, the threshold i_p* of the upshift inhibition zone is gradually moderately reduced from the initial set value i_pi* with the predetermined decrement Δi_p(see step 41). Thereafter, the threshold i_p* of the upshift inhibition zone is held at the value obtained at the standstill-decision time t₂, until the restart-decision time has been reached. After the restart-decision time t_s3, the threshold i_p* of the upshift inhibition zone is moderately reduced again with the predetermined decrement Δi_p(see step 41). Finally, the threshold i_p* of the upshift inhibition zone is reduced down to the preset low value i_ps. In this manner, the controlled threshold i_p* of the upshift inhibition zone properly inhibits or limits the upshift action as indicated by the broken line of the intermediate timing chart of FIG. 4. Note that the CVT control system of the present invention ensures an effective upshift inhibition action during the starting period on the low-μ and uphill road (particularly see the time period between t₁and t₂). The two-dotted line β indicated in the intermediate timing chart of FIG. 4 corresponds to the characteristic curve of the speed ratio i_pindicated by the solid line in the intermediate timing chart of FIG. 5. As can be appreciated from the two-dotted line β, the speed ratio i_ptends to upshift rapidly toward overdrive, since the conventional system does not provide any upshift inhibition action. In case of the conventional system with no upshift inhibition control, as indicated by the two-dotted line, the speed ratio i_pis fixed to or held at a comparatively low after the accelerator pedal is released at the time t₂. On the other hand, in case of the improved system of the present invention with upshift inhibition control, as indicated by the solid line of the central timing chart of FIG. 4, the speed ratio i_pcan be held at a comparatively high speed ratio γ remarkably higher than the previously-discussed speed ratio δ by virtue of the proper upshift inhibition control, after the accelerator pedal is released at the time t₂. This is because the drop of the speed ratio i_pis limited to the threshold i_p* of the upshift inhibition zone, as remarkably seen from the substantially middle time period of t_s1and t_s2(see the line segment of the speed ratio (i_p) indicative solid line conforming to the threshold (i_p*) indicative broken line). That is, the smaller limit of the speed ratio i_pis limited to the controlled threshold i_p* of the upshift inhibition zone, so that the speed ratio i_pis prevented or inhibited from being shifted toward the highest-speed side (overdrive). Owing to the speed ratio i_pheld at the comparatively high value, when the accelerator pedal is depressed again at the time t₄, the vehicle can be easily restarted even on the low-μ and uphill road. After restarting at the time t₄, when the vehicular body speed VBexceeds the predetermined standstill-decision vehicle speed V_lowat the restart-decision time t₃, the routine proceeds from step 37 to step 38. Assuming that the condition of i_p*≦i_psis not yet satisfied, the routine flows from step to step 40. When the drive-wheel acceleration value α(=dVW/dt) is less than the wheel-spin decision acceleration value α_spin, the routine proceeds from step 40 via step 41 to step 34. Thus, after the restart-decision time t_s3, the threshold value i_p* of the upshift inhibition zone is decremented by the predetermined decrement Δi_pSuppose that the conditions of steps 35, 36, and 36 are all satisfied and the conditions of steps 37 (VW≦V_low) and 40 (dVW/dt≧α_spin) are unsatisfied for a while from the time t₃, the loop defined by steps 34, 35, 36, 37, 38, 40, and 41 is repeated. Therefore, the threshold value i_p* is gradually moderately reduced down to the preset small value i_ps. In this manner, the previously-discussed upshift inhibit gradual-releasing action begins again from the time t_s3. After this, as soon as the threshold value i_p* becomes below the preset low value i_ps, the routine jumps from step 38 to step 39, so as to execute the usual speed-change operating mode. In this manner, the routine terminates.

[0020] In the embodiment, the predetermined wheel-spin decision acceleration value α_spinis set at a lower limit of a high, unrealizable acceleration range which lower limit is determined for all sorts of automotive vehicles. The accuracy of estimation of occurrence of wheel-spin, based on the condition of dVW/dt≧α_spin(see steps 32 and 40) is enhanced. Thus, the upshift inhibiting action can be timely effectively executed, without sacrificing fuel economy. Also, in the embodiment, the initial set value i_pi* of the threshold i_p* of the upshift inhibition zone is set at a critical speed ratio enough to produce a driving torque necessary to start the vehicle under a predetermined condition defined by an uphill road having a specified friction coefficient and a specified rising gradient. The frequency of the upshift inhibiting action is reduced to the minimum. Only when the upshift inhibiting action is required actually, the upshift inhibition mode can be timely performed. This prevents unreasonably increased fuel consumption and avoids unreasonably increased noise resulting from the use of high engine speed range during the upshift inhibition mode. Furthermore, as appreciated from the time period between t_s1and t_s2, and from the time point t₃, the gradual relaxation mode for upshift limitation (upshift inhibition) is executable with the predetermined decrement Δi_p. This avoids a rapid change in the speed ratio i_p. Moreover, during the standstill decision when the answer to step 37 is affirmative (YES) and the condition of VW≦V_lowis satisfied, the loop defined by the flow from step 34, through steps 35, 36 and 37, again to step 34 is repeated, and thus the threshold i_p* of the upshift inhibition zone is held at a certain value obtained or set at the standstill-decision time t₂, as seen from the time period between t_s2and t_s3. The recovery of the threshold i_p* to the preset low value i_psis inhibited or prevented. In this manner, the gradual relaxation mode for upshift inhibition can be timely temporarily stopped by the provision of step 37, to attain a smooth restart after the vehicle has been stopped once due to wheel spin. Although it is not shown in the timing chart of FIG. 4, as appreciated from the flow from step 40 to step 33, when the drive-wheel acceleration value α(=dVW/dt) exceeds the predetermined wheel-spin decision acceleration value α_spinagain before the threshold i_p* of the upshift inhibition zone is recovered to the predetermined small value i_ps(corresponding to substantially overdrive), in the transmission controller 13, the routine proceeds from step 40 to step 33, so that the threshold value i_p* is reset to the initial set value i_pi*. This facilitates restart even when the wheel spin reoccurs during the gradual relaxation mode for upshift inhibition. Moreover, as appreciated from the flow from step 36 to step 39, the controller rapidly releases the upshift inhibition mode so that the operating mode is rapidly shifted to the usual speed-change operating mode to permit shifting of the speed ratio to the minimum speed ratio line (overdrive) during the skid control. The adjustment of the speed ratio to such a comparatively small value is effective to reduce the rotational inertia of the drive wheels. Therefore, it is advantageous to switch from the upshift inhibition mode to the usual speed-change operating mode, when the skid control begins due to wheel lock (so-called deceleration slip) resulting from unreasonably hard braking action and thus the wheel-cylinder pressure of the road wheel subjected to the skid control decreases in accordance with the pressure-reduction mode. This facilitates recovery of the drive wheels to their rotating states. The system of the invention can timely performs the upshift inhibition mode without sacrificing the performance of skid control of the ABS system.

[0021] The contents of Application No. TOKUGANHEI 9-200029, filed Jul. 25, 1997, in Japan is hereby incorporated by reference.

[0022] In the embodiment, although the transmission control system of the invention is exemplified in a belt type continuously variable automatic transmission (CVT), it will be appreciated that the technique of the invention is applicable to the other types of a continuously variable transmission, such as a toroidal CVT. Typical details of such a toroidal CVT, being set forth, for example, in U.S. Pat. No. 4,955,246 issued Sep. 11, 1990 to Masaki Nakano and assigned to the assignee of the present invention, the teaching of which are hereby incorporated by reference.

[0023] While the foregoing is a description of the preferred embodiments carried out the invention, it will be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the scope or spirit of this invention as defined by the following claims.