Abstract:
For an automatic transmission ( 2 ), a safety system is proposed in which a virtual reduction ratio is calculated when a driver moves the selector lever from a drive position to neutral and then from neutral back to a drive position. The virtual reduction ratio is then compared with the reduction ratio resulting from a shift characteristic field. In case of divergence, the virtual reduction ratio is assigned to a characteristic field having a linearly constant reduction ratio surrounded by a reduction ratio range. The electronic control unit ( 5 ) selects as a set reduction ratio, the one containing in its range the virtual reduction ratio.

Description:
The invention concerns a safety system for an electrohydraulically controlled automatic transmission preferably driven by an internal combustion engine and whose reduction ratio is determined via a shift characteristic field by an electronic control unit depending at least on a throttle valve position and a transmission output speed wherein a driver can engage the control of the automatic transmission by means of a selector lever. 
     BACKGROUND OF THE INVENTION 
     It is known from the prior art that a driver prevents certain shifts by the position of a selector lever. For example, if the selector lever is in the three position, then shifts to the fourth and fifth gears are prevented. Typical cases of this are downhill drives. As relevant prior art can be taken into consideration in this connection, e.g., the book  Pkw - Automatgetriebe  by H. Dach and P. Kopf, Moderne Industrie Editing House, 1994, pages 58-61. 
     In the practice the following problem can now arise: If a driver at the end of the downhill drive inadvertently moves the selector lever from the three position to the neutral position, then there exists no more frictional connection between the transmission output and the transmission input. If at the same time the driver has actuated the accelerator pedal, then the speed of the internal combustion engine will very quickly increase in relation to the governed speed. If the driver now detects his error, he will try to shift the selector lever to a forward drive position. In other words, in the first shift from the three position to neutral, the clutch for the third gear will be opened. In the second shift from the neutral position to a forward drive position, the clutch for the third gear again closes. In this closing process, however, the speed of the internal combustion engine or the transmission input speed is forced from a high level to a low level, hence the third gear here. This elevated speed difference leads to a strong loading of the engaging clutch. 
     SUMMARY OF THE INVENTION 
     In view of the above, the invention is base on the problem of reducing the load of the engaging clutch for the above described case. 
     According to the invention, the problem is solved by the electronic control unit testing while driving whether there has occurred a first change on the selector lever from a drive position to a neutral position followed by a second change from the neutral position to the drive position, and the electronic control unit determining a virtual reduction ratio upon detection of the first change and the selector lever being in neutral position. The virtual reduction ratio is calculated from the quotient of the transmission input speed to the transmission output speed. With detection of the second change and a selector lever being in a drive position, the electronic control unit compares the virtual reduction ratio with the reduction ratio resulting from the shift characteristic field. In case of a divergence, the virtual reduction ratio is assigned to a characteristic field which has lines of constant reduction ratio surrounded by a reduction ratio range. As a consequence of this, the electronic control unit will determine a set reduction ratio whose range contains the virtual reduction ratio. The solution, according to the invention, offers the advantage that the same clutch, such as for the third gear, is not first opened and then closed again. The case can arise, that instead of the clutch for the third gear, the clutch for the second gear is closed. Thereby becomes reduced the speed difference or the shifting work of the engaging clutch. 
     In development of this, it is proposed that the reduction ratio ranges do not overlap and the electronic control unit forms a difference from the virtual reduction ratio and the possible reduction ratios of the automatic transmission when the virtual reduction ratio is between two ranges. As a consequence of this, the electronic control unit will select a set reduction ratio in which the difference results in a minimum. 
     It is proposed that upon detection of the first change and the selector lever being in the neutral position, the further curve of the virtual reduction ratio is forecast. In development of this, it is proposed that, at a first and second moment, a transmission input speed value be found and a gradient of the transmission input speed be determined therefrom. From the gradient of the transmission input speed, at the first moment, is now calculated a transmission input speed value that is to be expected at a third moment. From the transmission input speed value expected, the virtual reduction ratio is then forecast at the third moment. This development, according to the invention, takes into consideration the case when the driver immediately after having activated the neutral position, detects his error and simultaneously releases the accelerator pedal and shifts the selector lever to a forward drive position. Due to the release of the accelerator pedal, the speed of the internal combustion engine, or the transmission input speed, quickly becomes reduced. Since signal transit times appear between the issuance of a shift command by the electronic control unit and the actual closing of the clutch, it is possible, by applying the solution according to the invention, to obtain the advantage of a more accurate selection of the set reduction ratio. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment is shown in the drawings wherein: 
     FIG. 1 is a system diagram; 
     FIGS. 2A through 2C is a solution according to the prior art; 
     FIG. 3 is a first characteristic field; 
     FIG. 4 is a second characteristic field; and 
     FIG. 5 is a solution according to the invention 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a system diagram with the following main parts: internal combustion engine  1 , automatic transmission  2  and electronic control unit  5 . The internal combustion engine  1  drives the automatic transmission  2  via an input shaft  6 . The input shaft  6  is non-rotatably connected with a hydrodynamic converter  7  and drives the impeller  8  thereof. The hydrodynamic converter  7  is known to consist of the impeller  8 , a turbine wheel  9  and a stator  10 . Parallel to the hydrodynamic converter  7  is shown, without a reference numeral, a converter bridge clutch. When the converter bridge clutch is actuated, the turbine shaft rotates at the same speed as the input shaft  6 . The turbine shaft leads further into a composite planetary transmission  11  consisting of two planetary gear pairs, the clutches, and brakes B to F. By an adequate clutch/brake combination, it is possible to adjust a gear step or a reduction ratio according to the following table: 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Gear 
                 B 
                 E 
                 C 
                 D 
                 F 
               
               
                   
                   
               
             
             
               
                   
                 1 
                 x 
                   
                   
                   
                 x 
               
               
                   
                 2 
                   
                 x 
                   
                   
                 x 
               
               
                   
                 3 
                 x 
                 x 
               
               
                   
                 4 
                   
                 x 
                 x 
               
               
                   
                 R 
                 x 
                   
                   
                 x 
               
               
                   
                   
               
               
                   
                 x: clutch/brake activated  
               
             
          
         
       
     
     Hereinafter the designation clutch will be used. But as it can clearly be seen from the table, thereby are meant the clutches and clutch/brake activated in the respective gear. 
     Since the mechanical part, the output results from the differential  12  and the two axle half shafts  13 A and  13 B, are not relevant for a better understanding of the invention, a detailed description is omitted. 
     The clutches and brakes B to F are controlled or regulated by the electronic control unit  5  via the hydraulic control unit  4 . In the hydraulic control unit  4  are placed electromagnetic servo components and hydraulic valves. The hydraulic control unit  4  is usually an integral part of the automatic transmission  2 . The function blocks micro-controller  14 , memory  15 , function block calculation  17 , and function block control servo components  16 , all of the electronic control unit  5 , are shown in extensively simplified form. The memory  15  is usually designed as EPROM, EEPROM, or buffered RAM. The data relevant to the transmission are deposited in the memory  15 . The function block control servo components  16  serves to control the electromagnetic servo component in the hydraulic control unit  4 . The function block calculation  17  serves to calculate the data relevant to shifting. The latter are determined from the input parameters  18  to  21 . Input parameters  20  are, e.g., the signal of an accelerator pedal or throttle valve position, the temperature of the hydraulic fluid, etc. A selector lever is shown with the reference numeral  21 . Such a selector lever usually has the positions P, R, N, D, three, two and one. By said selector lever, the driver can lock in certain reduction ratios of the transmission. For example, if the selector lever is in the three position, a shift to the fourth or fifth gear is prevented. The transmission input speed  18  (nT) and the transmission output speed  19  (nAB) are shown as additional input parameters for the control unit  5 . 
     The internal combustion engine  1 , shown in FIG. 1, is controlled by an electronic motor control unit  3 . The latter is connected, via a bidirectional interface, with the electronic control unit  5  of the automatic transmission  2 . Via said interface, shown without reference numeral in FIG. 1, motor operating parameters, for example, can be transmitted or a motor engagement performed by the electronic control unit  5 . 
     Turning now to FIGS. 2A to  2 C, these figures show: 
     FIG. 2A the state of the selector lever (pos); 
     FIG. 2B the curve of the throttle valve position (DKI) and 
     FIG. 2C the curve of the transmission input speed (nT). 
     Two examples are shown in FIGS. 2B and 2C. The first example, with reference numeral  22 , comprises the curve of the throttle valve position in FIG.  2 B and corresponding thereto, with reference numeral  24 , the curve of the transmission input speed in FIG.  2 C. The second example comprises a curve of the throttle valve position according to reference numeral  23 , and a curve of the transmission input speed with reference numeral  25 , both shown in dotted line. 
     First example: 
     At the t 1  moment, the driver actuates the selector lever and inadvertently shifts it from the three position to the neutral position. To this belongs the curve of FIG.  2 A. It is assumed that during this operation the driver keeps the accelerator pedal at a constant value such as 10%, as shown in FIG. 2B with reference numeral  22 . As a result of this first change, that is, from the three position to the neutral position, the frictional connection between transmission output and transmission input is dissolved, since the clutch for the third gear is open. Consequently, the transmission input speed nT, reference numeral  24  in FIG. 2C, increases. In this example, the transmission input speed nT reaches the governed speed value of 6,500 revolutions. At the t 3  moment, the driver detects his inadvertence and changes the selector lever from the N position to the forward drive D position. At the t 4  moment, point A, the now engaging clutch of the third gear begins to close so that as a result thereof, the transmission input speed nT decreases. The temporary offset between t 3  and t 4  results from signal transit times between the output command of the electronic control unit  5  and the filling operation of the third gear clutch. At the t 6  moment, the clutch is closed and the transmission input speed nT has again reached the initial speed value of 2,000 revolutions. As results from the transmission input speed curve, reference numeral  24  in FIG. 2C, is that the closing clutch must eliminate said speed difference, the amount here being 4,500 revolutions. This means a high load for the clutch. 
     Second example: 
     At the t 1  moment, the driver shifts the selector lever from the three position to the neutral position. In this example, it is assumed that at the t 2  moment, the driver detects his inadvertence due to the sharply increasing transmission input speed, reference numeral  25  in FIG. 2C, and as a reaction thereto, releases the accelerator pedal. For this case, the curve of the accelerator pedal position is provided with the reference numeral  23 . Due to the inherent dynamics of the internal combustion engine, the transmission input speed nT does not immediately diminish after the t 2  moment. At the t 3  moment, the driver shifts the selector lever from the N position to the D position. As a result of this, the curve of the transmission input speed nT in point B will change. At the t 5  moment, as a result of the closed clutch of the third gear, the transmission input speed has again reached the output speed of 2,000 revolutions. As can be seen from this example, the clutch must here, like in the first example, eliminate a small speed difference. Nevertheless, this means an increased load for the clutch. 
     In FIG. 3 is shown a first characteristic field. It shows as abscissa values the torque (MM) produced by the internal combustion engine and as ordinate values, the reduction ratios of the automatic transmission  2 . Parallel to the abscissa are extend dotted lines of constant reduction ratio designated here as i( 3 ), i( 2 ), etc., which correspond to the gear steps of the automatic transmission. The lines of the constant reduction ratio are surrounded by a reduction ratio range. To the line of constant reduction ratio i( 3 ) belongs the reduction ratio range B 3  with the lower limit  26  and the upper limit  27 . To the line of constant reduction ratio i( 2 ) belongs the reduction ratio range B 2  with the lower limit  28  and the upper limit  29 . 
     The course of the process is the following: The selector lever is in the  3  position. It is assumed that the third gear is active which is designated in FIG. 3 as point P 1 , which is on the line i( 3 ). If a first change is now effected, that is, the driver inadvertently shifts the selector lever from the  3  position to the neutral position, the clutch for the third gear opens. Since now there no longer exists any mechanical connection between transmission output and transmission input, the speed of the internal combustion engine or the transmission input speed nT will sharply increase. According to the invention, the process proposes that the electronic control unit will now determine a virtual reduction ratio from the quotient of the transmission input speed nT, to the transmission output speed nAB, iV=nT/nAB. This is shown with the change at point P 2 . 
     If now a second change takes place, that is, the driver detects his error and shifts the selector lever from the neutral position to a forward drive position, then according to the prior art as described above in FIGS. 2A through 2C, the transmission input speed nT is led back to the initial value. The reason for this is that the clutch for the third gear is closed and now a strong connection again exists between the transmission output and the transmission input. The process, according to the invention, now provides that the electronic control unit  5  determines a set reduction ratio whose reduction ratio range contains the virtual reduction ratio. In FIG. 3, this is the reduction ratio range B 2  with the set reduction ratio i( 2 ). In other words, in this example what is activated is not the clutch for the third gear, but the clutch for the second gear. The speed difference for the clutch of the second gear here is less thus this clutch must perform less shifting work. 
     If the calculated virtual reduction ratio is outside the range limits, e.g. the point P 4 , then the electronic control unit calculates a difference from the virtual reduction ratio and the possible reduction ratios of the automatic transmission. Thereafter, the electronic control unit determines a set reduction ratio whose difference results in a minimum. In FIG. 3, this would be the reduction ratio i( 2 ). 
     Additionally, it can be provided that upon detection of the first change of the selector lever  21  being in the neutral position, the further curve of the virtual reduction ratio iV can be forecast. To this end, at a first and second moment, t 1  and t 2 , a transmission input speed value nT is detected from which a gradient of the transmission input speed GRAD=nT(t 1 )−nT(t 2 )/(t 1 −t 2 ) is determined. From the GRAD gradient and the transmission input speed nT at the first moment (t 1 ), a transmission input speed value to be expected is then calculated at a third moment t 3 , nT(t 3 )=nT(t 1 )−GRAD(t 3 −t 1 ) and therefrom the virtual reduction ratio iV is forecast at the third moment iV=nT(t 3 )/nAB. The third moment, t 3 , is determined from the moment at which the second change is detected by the electronic control unit  5  and from a signal transit time resulting from the temporary deceleration between output command of the electronic control unit  5  and the actually self-adjusting fixed reduction ratio i(K). 
     A second characteristic field is shown in FIG.  4 . It differs from the first characteristic field of FIG. 3 in that the range limits overlap. To the line of constant reduction ratio i( 3 ) belongs the range B 3  with the lower limit  26  and upper limit  27 . To the line of constant reduction ratio i( 2 ) belongs the range B 2  with the lower limit  28  and the upper limit  29 . If a first change takes place, here again from the  3  position and the point P 1 , the curve of the transmission input speed nT changes in the direction of point P 2 . The difference from the first characteristic field now consists in that the electronic control unit does not select the reduction ratio as a set reduction ratio until the virtual reduction ratio has left the hysteresis band which resulted from the overlapping ranges. As shown in FIG. 4, the virtual reduction ratio exceeds, at point A, the range upper limit of the reduction ratio i( 3 ). In other words, starting from point A, after the second change has taken place, the electronic control unit determines as a set reduction ratio the second gear as i( 2 ) with the point P 3 . 
     Should the transmission input speed change, departing from point P 2  in the direction of point P 1 , it would be defined as set reduction ratio i( 3 ) when falling below the line  28 . 
     In FIG. 5 is shown the application of the process according to the invention. Turning now to FIGS. 5A through 5C, these figures show. 
     FIG. 5A the signal of the selector lever; 
     FIG. 5B the curve of the throttle valve position; and 
     FIG. 5C the curve of the transmission input speed nT. 
     In each of FIGS. 5B and 5C are shown two examples. In the first example it is assumed that the driver has kept the accelerator pedal at a constant 10% value. In FIG. 5B, this curve is shown with the reference numeral  22 . To this example belongs in FIG. 5C, the corresponding curve of the transmission input speed nT with the reference numeral  24 . In the second example, it is assumed that the driver, after the transmission is in neutral, detects his inadvertence and releases the accelerator pedal. In FIG. 5B, this curve is shown in dotted lines with the reference numeral  23 . To this example belongs in FIG. 5C, the transmission input speed curve with the reference numeral  25 , also shown in dotted lines. 
     First example: 
     At the t 1  moment, the driver inadvertently moves the selector lever from the  3  position to the N position. Since the friction connection now no longer exists between the transmission output and the transmission input, the transmission input speed nT, reference numeral  24 , will sharply increase. In this example, it has been assumed that the transmission input speed nT reaches the governed speed of 6,500 revolutions. At the t 3  moment, the driver will now move the selector lever from the N position to the D position. Due to the signal transit time, the time period t 3  to t 4  elapses before the clutch of the set reduction ratio begins to close. At the t 6  moment, the clutch is closed and the transmission input speed nT has reached the new synchronization point. This can be, for ex., the synchronization point of the second gear. 
     Second example: 
     At the t 1  moment, the driver inadvertently moves the selector lever from the  3  position to the N position. Thereby the transmission input speed nT, reference numeral  25 , begins to increase very sharply. At the t 2  moment, the driver detects his inadvertence and releases the accelerator pedal, signal curve  23 , thus starting from the moment the curve of the transmission input speed nT does not substantially increase any more. At the t 3  moment, the second change takes place, that is, the driver moves the selector lever from the N position to the D position. At point B, the curve of the transmission input speed changes in the direction of the new synchronization point. The new synchronization point is reached at the t 5  moment. Likewise, this can be the second gear. As results from the comparison of FIG. 5 with FIG. 2, the clutch to be engaged after the second change has to perform substantially reduced shifting work. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Reference numerals 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  1 
                 internal combustion engine 
               
               
                   
                  2 
                 automatic transmission 
               
               
                   
                  3 
                 electronic motor control unit 
               
               
                   
                  4 
                 hydraulic control unit 
               
               
                   
                  5 
                 electronic control unit 
               
               
                   
                  6 
                 input shaft 
               
               
                   
                  7 
                 hydrodynamic converter 
               
               
                   
                  8 
                 impeller 
               
               
                   
                  9 
                 turbine wheel 
               
               
                   
                 10 
                 stator 
               
               
                   
                 11 
                 composite planetary transmission 
               
               
                   
                 12 
                 differential 
               
               
                   
                 13A 
                 axle half shaft 
               
               
                   
                 13B 
                 axle half shaft 
               
               
                   
                 14 
                 micro-controller 
               
               
                   
                 15 
                 memory 
               
               
                   
                 16 
                 function block control servo 
               
               
                   
                   
                 components 
               
               
                   
                 17 
                 function block calculation 
               
               
                   
                 18 
                 transmission input speed 
               
               
                   
                 19 
                 transmission output speed 
               
               
                   
                 20 
                 input parameters 
               
               
                   
                 21 
                 selector lever 
               
               
                   
                 22 
                 DKI curve, Example 1 
               
               
                   
                 23 
                 DKI curve, Example 2 
               
               
                   
                 24 
                 nT curve, Example 1 
               
               
                   
                 25 
                 nT curve, Example 2 
               
               
                   
                 26 
                 lower limit 
               
               
                   
                 27 
                 upper limit 
               
               
                   
                 28 
                 lower limit 
               
               
                   
                 29 
                 upper limit