SHIFT CONTROL SYSTEM FOR AUTOMATIC TRANSMISSION

Abstract: A shift control system for an automatic transmission of a vehicle includes a steering angle sensor (3) for sensing a steering amount and producing a steering amount signal, a brake sensor (7) for sensing a depression of a brake pedal and producing a brake signal representing the frequency of brake applications, and a control unit for determining a degree of correlation between the steering amount signal and the brake signal, and modifying the shift pattern so as to adapt the shift schedule to driving conditions in narrow streets or the like when the degree of correlation is high.

Description: BACKGROUND OF THE INVENTION The present invention relates to a shift control system for an automatic transmission. Japanese Patent Provisional Publications Nos. 59-200840 and 60-132159 show conventional shift control systems having a steering angle sensor. Each of these control systems is designed to improve the acceleration feeling at the later stage of cornering by effecting a downshift when the steering wheel is returning toward the neutral position, and the throttle opening degree is increasing. GB.A- 2174/58 discloses a control system having a steering wheel angle sensor and a brake sensor, as per the preamble of claim 1. However, these conventional control systems are unsatisfactory in that shifts between first and second speeds are too frequent and annoying in some situations. In a narrow street in a residential area, for example, shifts between first and second speeds occur so frequently that the drive feeling is made worse by shift shock. SUMMARY OF THE INVENTION It is an object of the present invention to provide a shift control system which can improve the drive feeling by preventing unwanted automatic shifts. According to the present invention, a shift control system for an automatic transmission of a vehicle comprises an actuating means for shifting the automatic transmission in accordance with a shift control signal, a basic sensor means for sensing at least a vehicle speed of the vehicle, and a basic controlling means for determining a desired speed of the automatic transmission corresponding to the vehicle speed according to a predetermined shift control mode, and producing the shift control signal to achieve the desired speed of the transmission. The shift control system of the present invention further comprises an additional sensor means, a discriminating means, and a changing means. The additional sensor means comprises a first additional sensing means for sensing a steering amount of the vehicle and producing a first data signal, and a second additional sensing means for sensing a condition of a brake system of the vehicle and producing a second data signal. The discriminating means is a means for determining a degree of correlation between the first and second data signals. The changing means is a means for changing the shift control mode of the basic controlling means from a normal mode to a special mode and vice versa in dependence upon the degree of correlation. Fig. 1 shows one example of a basic arrangement of the present invention. A shift control system of this example includes at least an actuating means 101, a basic sensor means 102, and a basic controlling means 103. The shift control system further includes at least an additional sensor means 104, a discriminating means 105 and a changing means 106. These means are connected with one another as shown by solid lines in Fig. 1. In an illustrated embodiment of the present invention, the basic sensor means 102 comprises a vehicle speed sensor and a throttle opening sensor, and the additional sensor means 104 comprises a steering angle sensor, a brake sensor, and first and second accumulating means for sampling and accumulating values of output signals of the steering angle sensor and the brake sensor. The discriminating means 105 may comprise first and second storing means, a calculating means for calculating the degree of correlation, a deciding means for producing a discrimination signal, and first and second comparing means, as shown by broken lines in Fig. 1. In this example, the first data signal is a signal representing the steering amount of the vehicle, and the second data signal is a signal representing the frequency of brake applications, or the ratio of working time of the brake system to a predetermined total time. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing a basic arrangement of the present invention. Fig. 2 is a schematic view of an automatic transmission shift control system according to one embodiment of the present invention. Figs. 3 and 4 are diagrams showing shift patterns used in the control system of Fig. 2. Fig. 5 is a flow chart showing a shift control process of the control system shown in Fig. 2. Fig. 6 is a timing chart for showing effects of the shift control system of Fig. 2. DETAILED DESCRIPTION OF THE INVENTION Figs. 2-6 show one embodiment of the present invention. A shift control system of this embodiment has a sensor group 1 including a steering amount sensor 3, an accelerator sensor 5, a brake sensor 7 and a vehicle speed sensor 9. The steering amount sensor 3 of this embodiment is a steering wheel angle sensor for sensing a steering angle ϑ of a steering wheel 2 of the vehicle. The accelerator sensor 5 of this embodiment is a throttle sensor for sensing a depression degree of an accelerator pedal of the vehicle or sensing an opening degree TVO of a throttle valve of the vehicle. The brake sensor 7 of this embodiment is a brake switch for sensing a position of a brake pedal 6 of the vehicle, and producing a brake operation signal S BK which is "0" when the brake pedal is not depressed at all, and which is "1" when the brake pedal is depressed in any degree. The vehicle speed sensor 9 of this embodiment is arranged to sense the vehicle speed V by sensing a revolution speed of one of road wheels 8 of the vehicle. Alternatively, the vehicle speed sensor may be arranged to sense a revolution speed of a meter cable within a speedometer, or to sense a revolution speed of an output shaft of an automatic transmission. The shift control system further includes a control unit 10. The control unit 10 of this embodiment comprises a microcomputer including an I/O buffer section 10a, a ROM section 10b, a RAM section 10c and a CPU 10d. The I/O buffer section 10a is a section for receiving the steering angle ϑ, the throttle opening degree TVO, the brake operation signal S BK ,and the vehicle speed V, and outputting a shift control signal S SEL . The ROM section 10b is a read only storage device for storing a shift control program, a primary shift pattern PT-P, and a secondary shift pattern PT-S. The RAM section 10c is a read/write storage device for storing arrays of data items. The CPU 10d is a control section for executing the shift control program by transferring data and other information to and from the I/O buffer section 10a, the ROM section 10B and the RAM section 10c. The CPU 10d determines the shift position of the automatic transmission by using one of the shift patterns PT-P and PT-A, and produces the shift control signal S SEL . An automatic transmission 11 of this embodiment has a hydraulic circuit 11a and a transmission mechanism 11b having a planetary gear system. The hydraulic circuit 11a receives the shift control signal S SEL and controls the gear mechanism 11b in accordance with the shift control signal S SEL . Fig. 3 shows the primary shift pattern PT-P of this embodiment which is stored in the ROM section 10b. (Fig. 3 shows only upshift lines, and lines for downshift are omitted.) The primary shift pattern PT-P is designed for normal vehicle operations. In this example, the shift schedule is determined by two parametes, the throttle opening TVO and the vehicle speed V. Fig. 4 shows the secondary shift pattern PT-S of this embodiment stored in the ROM section 10b. (In Fig. 4, lines for downshift are omitted.) The secondary shift pattern PT-S is a shift pattern specifically suited to narrow streets in residential areas, and other similar situations. The main difference between the main and secondary patterns resides in that the zone of the second speed (D₂) is widened especially in the region in which TVO is small, and V is low. Although this embodiment employs a three speed transmission, it is possible to employ a four speed automatic transmission. Fig. 5 shows the shift control program of this embodiment stored in the ROM section 10b. When an ignition key of the vehicle is turned on, the control unit 10 is supplied with power, and a first request is made for timer interrupt, then the CPU 10d starts executing the shift control program. At a first step P₁, the CPU 10d initializes a predetermined field of the RAM section 10c used for storing data arrays. At a next step P₂, the CPU 10d reads the various input data from the sensor group 1 such as ϑ, TVO, S BK and V. At a step P₃, the CPU 10d determines accumulated quantities as follows: An accumulated quantity AH of the steering angle ϑ is given by; AH = AH₋₁ + Dϑ where AH₋₁ is a value of AH determined in the last repetition, Dϑ is an absolute value of a difference between ϑ and ϑ₋₁ (Dϑ = | ϑ - ϑ₋₁ |), and ϑ₋₁ is a value of ϑ obtained in the last repetition. An accumulated quantity D B of brake applications is given by an equation (2). D B = D B-1 + S BK where D B-1 is a value of D B determined in the last repetition. An accumulated quantity ATVO of the throttle opening is given by an equation (3). ATVO = ATVO₋₁ + TVO where ATVO₋₁ is a value of ATVO determined in the last repetition. An accumulatd quantity U of the vehicle speed is given by an equation (4). U = U₋₁ + V where U₋₁ is a value of U determined in the last repetition. At a step P₄, the CPU 10d determines whether an elapsed time T reaches a predetermined time length T₁ (2 seconds, for example). The elapsed time T is measured by a timer which was initialized at the step P₁. If T is smaller than T₁, the CPU 10d returns to the step P₂. In this way, the CPU 10d repeats the steps P₂ and P₃ until the elapsed time T reaches the predetermined time length T₁. Consequently, the CPU 10d obtains instantaneous values of ϑ, S BK , TVO and V at regular intervals, and accumulates a predetermined number of the values for each of ϑ, S BK , TVO and V in the form of the accumulated quantity. This predetermined number is equal to T₁/T s where T s is a sampling period which is a period of repetitions of the reading step P₂. At a step P₅, the CPU 10d increments (increases by one) an indicator I (which was initialized at the step P₁). The indicator I is used to distinguish elements of each data array. At a step P₆, the CPU 10d stores the current value of each accumulated quantity at a position specified by the indicator I in the corresponding one of four data arrays, DA(STR,I), DA(B,I), DA(V,I) and DA(TVO,I). When I = 1, for example, the CPU 10d stores the values of the accumulated quantities AH, D B , ATVO and U in the first column positions of the arrays DA(STR,I), DA(B,I), DA(TVO,I) and DA(V,I), respectively. Then, the CPU 10d clears each accumulated quantity. At a step P₇, the CPU 10d determines whether the elapsed time T reaches a predetermined time length T₂. The time length T₂ is set equal to T₁ x k, where k is a number of elements of each array. For example, k =200, and each array includes a time series of 200 data items. Each array is in the form of a matrix having only one row and k columns (or a row vector). The CPU 10d repeats the steps P₂-P₇ until the elapsed time T reaches T₂. When T becomes equal to T₂, the CPU 10d has stored 200 values of each accumulated quantity in one array in sequence of time, and completed a data table of a matrix having four rows and k columns. Each element of the matrix is a value of one accumulated quantity which is a sum of a predetermined number (T₁/T S ) of sampled values of ϑ, S BK , TVO or V. At a step P₈, the CPU 10d determines an average vehicle speed V AV which, in this embodiment, is defined as; At a step P₉, the CPU 10d compares the average vehicle speed V AV with a predetermined reference level L₁ corresponding to a predetermined low vehicle speed. The reference vehicle speed level L₁ is set equal to a value corresponding to a vehicle speed slightly higher than vehicle speeds normally achieved in narrow streets. If the average vehicle speed V AV is not higher than L₁, then the CPU 10d proceeds from the step P₉ to a step P₁₀. At the step P₁₀, the CPU 10d determines an average throttle opening degree TVO AV defined as; At a step P₁₁, the CPU 10d compares the average throttle opening degree TVO AV with a predetermined reference opening level L₂ corresponding to a predetermined small throttle opening degree. The reference opening level L₂ is set equal to a value corresponding to a throttle opening degree slightly higher than throttle opening values achieved in narrow streets. If both answers of the steps P₉ and P₁₁ are negative, then the CPU 10d calculates quantities K₁ and K₂ by using the following equations (7) and (8) at a step P₁₂. In these equations, X₁′ is an average of AH during T₂, and given by; The quantity X₂′ is an average of D B during T₂, and given by: In the equations (7) and (8), the elements of the data array DA(STR,I) are represented by X n1 , and the elements of the data array DA(B,I) are represented by X n2 , where n corresponds to I(I = 1,2,3 .... k), and n is any whole number from 1 to k. At a step P₁₃, the CPU 10d calculates a correlation coefficient K between steering operation and braking operation according to the following equation (11). k = K₁ K₂ The thus-obtained correlation coefficient K becomes high when the steering and braking operations are both frequent, and exhibits a peculiar tendency when the vehicle is running along a narrow street requiring frequent steering and braking operations. Furthermore, the control system of this embodiment is arranged to perform the calculation of the correlation coefficient K only when the vehicle speed V is in a low speed range determined by the step P₉, and at the same time, the throttle opening degree is in a low opening range determined by the step P₁₁. The following table shows values of the correlation coefficient K obtained in various driving conditions. As known from the table 1, the frequencies of steering operations and brake applications become higher, and the correlation coefficient K becomes higher in the order of I, II, III .... VI. Therefore, the shift control system of the invention is arranged to distinguish driving conditions by monitoring the correlation coefficient. At a step P₁₄, the CPU 10d compares the current value of the correlation coefficient with a predetermined reference coefficient value L₃. In this embodiment, the reference coefficient value L₃ is set equal to an appropriate value between the value of K obtained in the driving situation IV of the table 1, and the value of K obtained in the driving situation V. If the correlation coefficient K is higher than the reference value L₃, then the CPU 10d selects the secondary shift pattern PT-S at a step P₁₅, and determines the desired shift position of the automatic transmission by using the shift pattern PT-S at a step P₁₆. In the secondary pattern PT-S, the zone of the second speed (D₂) is broadened as shown in Fig. 4, and the tendency to select the second speed is increased. At the step P₁₆, the CPU 10d delivers the shift control signal S SEL to achieve the determined speed, to the automatic transmission 11. Therefore, in the driving situations (V) and (VI), this control system can prevent unwanted upshifts and downshifts by holding the transmission gear mechanism 11b at the second speed, and improve the drive feeling, and the controllability of the vehicle. On the other hand, the CPU 10d selects the normal shift pattern PT-P at a step P₁₇ when V AV is higher than L₁(i.e., the answer of the step P₉ is affirmative), when TVO AV is higher than L₂ (i.e., the answer of the step P₁₁ is affirmative) and when K is equal to or smaller than L₃ (i.e., the answer of the step P₁₄ is negative). In this way, the control system of this embodiment is arranged to increase the tendency that a predetermined lower speed is selected, when the frequencies of the steering operations and brake applications are both high. Therefore, this control system holds the second speed as shown by a solid line in a graph (c) of Fig. 6, by preventing undesired upshifts and downshifts as in a broken line characteristic of a conventional shift control system. The control system of this embodiment employs the secondary shift pattern in addition to the normal shift pattern. However, it is possible to prevent undesired shifts by modifying TVO and V so as to change the shift schedule of PT-P without using another shift pattern.

Claims: 1. A shift control system for an automatic transmission of a vehicle, comprising; actuating means (11a) for shifting said automatic transmission in accordance with a shift control signal, basic sensor means (102) for sensing a vehicle speed (v) of said vehicle, basic controlling means (103) for determining a desired shift position of said automatic transmission corresponding to said vehicle speed in accordance with a predetermined shift control mode, and producing said shift control signal to achieve said desired shift position, additional sensor means comprising first additional sensing means (3) for sensing a steering amount of said vehicle and producing a first data signal, and second additional sensing means (7) for sensing a condition of a brake system of said vehicle and producing a second data signal, characterized by discriminating means (105) for determining a degree of correlation between said first and second data signals, and changing means (106) for changing said shift control mode of said basic controlling means in dependence upon said degree of correlation. 2. A shift control system according to Claim 1 wherein said discriminating means (105) comprises first storing means for storing a plurality of values of said first data signal obtained at time intervals, and second storing means for storing a plurality of values of said second data signal obtained at time intervals. 3. A shift control system according to Claim 2 wherein said discriminating means (105) further comprises calculating means for determining said degree of correlation which is a correlation coefficient between said values stored in said first storing means and said values stored in said second storing means. 4. A shift control system according to Claim 3 wherein said discriminating means (105) further comprises deciding means for comparing said correlation coefficient with a predetermined coefficient value, and produces a discrimination signal when said correlation coeficient is higher than said predetermined coefficient value, and said changing means (106) changes said control mode of said basic controlling means when said discrimination signal is present. 5. A shift control system according to Claim 4 wherein said discriminating means further comprises first comparing means for comparing said vehicle speed (v) with a predetermined speed level, and said discriminating means (105) produces said discrimination signal when said correlation coefficient is higher than said coefficient value and at the same time said vehicle speed is lower than said predetermined speed level. 6. A shift control system according to Claim 5 wherein said basic sensor means comprises an accelerator sensing means (5) for sensing an accelerator opening degree (TVO) by sensing a position of a movable element (4) of an accelerating system of said vehicle, and said discriminating means (105) further comprises second comparing means for comparing said accelerator opening degree (TVO) with a predetermined opening level, said discriminating means (105) producing said discrimination signal only when said accelerator opening degree is smaller than said predetermined opening level. 7. A shift control system according to Claim 6 wherein said first additional sensing means comprises a steering angle sensor (3) for sensing a steering angle ϑ of said vehicle and producing a steering angle signal, and wherein said second additional sensing means comprises a brake sensor (7) for sensing a position of a brake pedal (6) of said vehicle and producing a brake signal (SBK) which is in a first signal state when said brake pedal is not depressed and in a second signal state when said brake pedal is depressed. 8. A shift control system according to Claim 7 wherein said first additional sensing means further comprises first accumulating means for sampling values of said steering angle signal ϑ at regular intervals of a predetermined sampling period, and determining values of an accumulated quantity which is a sum of a predetermined first number of successively sampled values of said steering angle signal, at regular intervals of a predetermined first time length, and wherein said second additional sensing means further comprises second accumulating means for sampling values of said brake signal at regular time intervals of said sampling period, and determining, at regular intervals of said first time length, values of an accumulated quantity which is a sum of said predetermined first number of successively sampled values of said brake signal. 9. A shift control system according to Claim 8 wherein said first storing means stores a predetermined second number of values of said accumulated quantity of said steering angle signal ϑ, and said second storing means stores said predetermined second number of values of said accumulated quantity of said brake signal (SBK).

Inventors: TAKAHASHI, HIROSHI
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1989
Main classification: B60K41
IPC codes: F16H61, F02D41, F16H59, A23B4