Abstract:
A method for monitoring and limiting the output or the torque of a drive motor in a drive train of a road motor vehicle, including the steps of: determining a permissible maximum value of the output or the torque as a function of a signal from a driver input sensor; determining an actual value of the output or the torque, and comparing the actual value to the maximum value and triggering a measure that limits the output or the torque as a function of the comparison result. The method is characterized by the repeated formation of a difference from the actual value and the maximum value, formation of a sum of values of a function of the difference, comparison of a value of the sum to a threshold value, and initiation of the measures if the value of the sum is greater than the threshold value. An independent claim is directed to a control device, which is programmed to implement the method.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and control device for monitoring and limiting the torque in a drive train of a road motor vehicle. 
       BACKGROUND INFORMATION 
       [0002]    A method and a control device of this type are both discussed in DE 195 36 038 A1. 
         [0003]    In modern combustion engines, performance-determinative actuators such as a throttle valve are no longer mechanically coupled to a driver-input sensor. Nowadays, a driver-input sensor typically converts a torque request from the driver into an electric signal, from which the control device determines one or a plurality of actuating variables for one or a plurality of performance-regulating final control elements. Malfunctions in the signal-processing chain between the driver-input sensor and the final control elements can therefore induce the combustion engine to generate more torque than the driver desired. To prevent this, in the known subject matter a permissible maximum value of the power output or the torque is determined as a function of a signal from a driver-input sensor, an actual value of the power output or the torque is determined, the actual value is compared to the maximum value, and a measure limiting the power output or the torque is triggered as a function of the comparison result. 
         [0004]    In such subject matter, a limitation of the torque acting in the drive drain is initiated when the actual value of the torque exceeds the maximum value determined from the driver input. As an alternative, a limitation is initiated when an actual value of the output transmitted by the drive train exceeds a corresponding maximum value determined from the driver input. In a further development of both alternatives, the limitation is to be initiated when the maximum value is exceeded for longer than a predefined time. 
         [0005]    Basic possibilities for limiting the torque are restrictions of the air supply into the combustion chambers of the combustion engine, restrictions of the fuel supply, and reductions in the ignition timing efficiency. The ignition timing efficiency is understood as the quotient from the torque at a particular ignition angle and the torque at an ignition angle that is optimal for the torque development. If the actual value of the torque exceeds a maximally permitted value derived from the driver input, the road motor vehicle could accelerate to an undesired extent or accelerate to a greater extent than desired, which could lead to dangerous driving situations. 
         [0006]    Once such an exceedance is detected, a throttle valve serving as air-mass actuating element is usually switched into a de-energized state. Mechanical restoring forces then adjust the throttle valve to a minimum opening position in which the combustion engine continues to run at a considerably reduced torque. However, a switch-off of the combustion engine is avoided so that functions such as steering support or braking support, for example, which require a running combustion engine, continue to be available. 
         [0007]    Because of the safety relevance, the monitoring and limiting must, for one, respond in a relatively sensitive manner. For another, erroneously triggered limitations must be avoided since they considerably restrict the drivability of the road motor vehicle and could themselves lead to critical driving situations, for example in a passing maneuver. 
       SUMMARY OF THE INVENTION 
       [0008]    Against this background, it is an object of the exemplary embodiments and/or exemplary methods of the present invention to provide a method and a control device with whose aid the safety-critical torque developments are detectable even more reliably and by which the risk of faulty and thus unnecessarily undertaken torque-limiting measures is reduced. According to the exemplary embodiments and/or exemplary methods of the present invention, the objective is achieved by the features delineated in the independent claims. 
         [0009]    The following advantages result in comparison with current methods and devices for monitoring the torques of the drives: In the known method, the actual value of the current torque was compared to a permissible value. If the permissible value is exceeded, a time counter was activated. If the permissible value was undershot, the time counter was reset. If the value of the time counter exceeded a threshold, an error reaction was triggered. The extent of the exceedance was not evaluated. In a real driving situation, however, a high exceedance has a more critical effect than only a slight exceedance. 
         [0010]    In contrast, the exemplary embodiments and/or exemplary methods of the present invention has the advantage that exceedances are recorded in quantitative terms as well and, by the sum operation, are also taken into account in quantitative terms in the decision as to whether or not a fault reaction is to be triggered. Furthermore, the summation (or integration) also provides a value that has a more direct relevance to the effect of the exceedance on the vehicle: A quantitatively greater exceedance causes greater acceleration than only an only smaller exceedance. In the invention, the fault reaction in the case of the greater exceedance is triggered earlier than in the case of the smaller exceedance. Furthermore, a summation or integration of output values instead of torque values has the advantage that the exceedance is able to be evaluated in terms of its effect at the driven wheels regardless of a currently set transmission ratio in the drive train. 
         [0011]    If a force transmission to the wheels is taking place, the sum of the power or torque contributions exceeding the threshold value and impermissibly output by the drives describes the impermissible change of the kinetic energy of the vehicle due to a fault, in an approximation that is sufficiently accurate for the control device monitoring. This allows a better evaluation of the effect of the fault on the vehicle. The evaluation of cyclically faulty drive torques is improved. The realization of the control device monitoring in systems having hybrid drive, made up of different combinations of electric machines and combustion engines, is simplified. The consideration of the moments of inertia of switch-selectable drives in the monitoring of the control devices in systems having a hybrid drive is simplified. More specifically, the modeling errors are reduced since, unlike in the case of a direct consideration in the permissible torque or the output, rotational speed gradients need not be taken into account. 
         [0012]    Further advantages result from the dependent claims, the description and the attached figures. 
         [0013]    It is understood that the aforementioned features and the features yet to be explained hereinafter may be used not only in the indicated combination, but also in other combinations or by themselves, without departing from the scope of the present invention. 
         [0014]    Exemplary embodiments of the present invention are shown in the drawing and explained in detail in the following description. 
         [0015]    Matching reference numerals denote the same elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows a drive train of a road motor vehicle together with a control of the drive train. 
           [0017]      FIG. 2  shows the forming of various performance quantities of the drive train in the control device. 
           [0018]      FIG. 3  shows a first exemplary embodiment of the present invention in the form of program structures that are implemented within the control device. 
           [0019]      FIG. 4  shows a development in which the power outputs are summed up. 
           [0020]      FIG. 5  shows a drive train that differs from the already described drive train by an electric machine, which is able to operate as additional drive motor and/or as generator. 
           [0021]      FIG. 6  shows the forming of various performance quantities of the drive train from  FIG. 5  in the control device of  FIG. 5 . 
           [0022]      FIG. 7  shows a development in the form of program structures that are implemented within the control device of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In detail,  FIG. 1  shows a drive train  10  of a road motor vehicle together with a control of drive train  10 . Drive train  10  in particular has a combustion engine  12 , which is coupled to remainder  16  of the drive train via a clutch  14 . Clutch  14  may be a friction clutch actuated automatically or by a driver, or a fluid clutch. Remainder  16  of drive train  10  represents additional elements usually provided for the torque transmission between the wheels and drive of the road motor vehicle, such as gear trains and shafts. 
         [0024]    In the subject matter of  FIG. 1 , the drive torque is generated solely by combustion engine  12 . To control its torque output, combustion engine  12  is equipped with final control elements  18  by which a charge of the combustion chambers of combustion engine  12  and/or the quality of the charge, i.e., a mixture ratio of fuel and air, for example, and/or the time sequence of the combustion are/is able to be controlled or influenced, for instance by shifting the moments of ignition. 
         [0025]    The control of combustion engine  12  is implemented by a control device  20 . To form actuating variables S_G_ 12 , control device  20  in the subject matter of  FIG. 1  processes signals from sensors  22  (measuring variables M_G_ 12 ),  24  and  26 , the operating parameters of combustion engine  12  such as the aspirated air mass, rotational speed n_ 12 , excess-air factor Lambda, signal FW from a driver-input sensor  24 , and—optionally—operating parameters of the rest of drive train  16 , which are provided by a sensor  26 , such as a rotational speed of a clutch n_K, for example. 
         [0026]      FIG. 2  represents the formation of various performance quantities of drive train  10  in control device  20 . Control device  20  is designed, especially programmed, to control drive train  10  and, in particular, to monitor the torque generation and/or the power output of combustion engine  12  in the process, the control device also taking its own actuating variables S_G_ 12  into account in the monitoring. For this purpose, control device  20  includes, among others, program structures  28 ,  30  and  32  illustrated in the form of blocks. 
         [0027]    Program structure  28  determines from measuring variables M_G_ 12 , which are provided by sensors  22  and represent performance variables of combustion engine  12 , e.g., the quantity and quality of the combustion chamber charge, and additionally also from driver input FW, actuating variables S_G_ 12  for final control elements  18  of combustion engine  12 . If all involved components function correctly, the combustion engine will generate a correct torque M_ist_ 12  according to the requirements. 
         [0028]    In program structure  30 , the value of torque M_ist_ 12  actually generated by the combustion engine as a function of driver input FW is modeled from measured variables M_G_ 12  and/or actuating variables S_G_ 12  of combustion engine  12 . Modeling means a calculation within control device  20 . In this context, an essential piece of actuating-variable information is, for example, ignition angle ZW, which is usually not detected as measured variable M_G_ 12  and thus is usually available only as one of actuating variables S_G_ 12 . 
         [0029]    Parallel to forming the value of modeled M_ist_ 12  in block  30 , a maximally permitted value M_zul for the torque generated by combustion engine  12  is formed in program structure  32  from driver input FW or at least as a function of driver input FW. Instead of the torque, it is also possible to model the output of combustion engine  12 . The same applies to the determination of a maximally permitted value. 
         [0030]    If all involved components function correctly, then modeled actual value M_ist_ 12  must always be smaller than maximally permitted value M_zul. On the other hand, if M_ist_ 12  is greater than M_zul, a malfunction of control device  20  or final control element  18  usually has occurred. 
         [0031]      FIG. 3  shows a first exemplary embodiment of the present invention in the form of program structures that are implemented within control device  20 . Just like the developments of  FIGS. 4 and 7 ,  FIG. 3  thus discloses individual method and device aspects of different developments of the invention introduced here. 
         [0032]    In block  34 , a difference dM=M_ist_ 12 −M_zul is repeatedly formed from modeled actual value M_ist_ 12  and maximally permitted value M_zul. In block  36 , a sum of a function of values of difference dM will then be formed. In the development of  FIG. 3 , this function f is identity f(dM)=dM. That is to say, direct values of difference dM are added. In block  38 , the value of the sum is then compared to a specified threshold value SW, which is supplied to block  38  by block  40 . Block  40  represents a memory cell or a memory area of control device  20 , in which a specified fixed value SW or a dependency SW=SW (performance quantity) of performance quantities of combustion engine  12  and/or remainder  16  of drive train  10  is stored. 
         [0033]    If the sum determined in block  36  exceeds threshold value SW, then an error reaction is output in block  42 . A typical error reaction consists of reducing the combustion chamber charge to a predefined minimum value. In one development, this is done in such a way that a throttle valve serving as charge actuator is no longer triggered to open, so that mechanical restoring forces drive it into a minimum opening position at which combustion engine  12  provides no more than a very low torque. Combustion engine  12  is not completely switched off, however, in order not to deactivate steering-support or brake-force support functions. 
         [0034]    Whereas  FIG. 3  represents a development in which torque values in the form of difference dM are summed up,  FIG. 4  discloses a development in which power outputs are summed up. For this purpose, difference dM formed in block  34  is multiplied in block  44  by two times the π of rotational speed n of combustion engine  12 . Product 2πn is supplied by block  46  in this instance. The subsequently summed-up values therefore represent a function f=2πndM of difference dM, and consequently represent power output values. 
         [0035]      FIG. 5  shows a drive train  50  that differs from already described drive train  10  by an electric machine  52 , which is able to operate as additional drive motor and, in one development, as generator as well. Like combustion engine  12 , electric machine  52  is controlled by a control device  54 . As an alternative, it is also possible to provide a separate control device  54  for the control of electric machine  52 , which is connected to control device  20  via a bus system. This analogously applies to the control of clutch  14 , which is likewise controlled by control device  20  in the development of  FIG. 5 . Like control device  20  as well, control device  54  is designed, especially programmed, to control drive train  50  and to monitor its torque- and engine-speed-determinative functions, the control device also taking its own actuating variables S_G_ 12 , S_G_ 52  into account in the monitoring. 
         [0036]    When clutch  14  is disengaged, electric machine  52  serves as drive motor on its own. When clutch  14  is engaged, combustion engine  12  operates as drive motor, either alternatively or additionally. In an exemplary embodiment, when clutch  14  is engaged, electric machine  52  is able to be operated as generator, which is driven by the rolling road motor vehicle by combustion engine  12  or via remainder  16  of the drive train. In one exemplary embodiment, electric machine  52  also serves as starter for combustion engine  12 . 
         [0037]      FIG. 6  represents the formation of various performance quantities of drive train  50  in control device  54 . In block  56 , actuating variables SG_ 12  are formed for the control of final control elements  18  of combustion engine  12 . To this extent, block  56  corresponds to block  28  from  FIG. 2 . One difference to block  28  consists of the fact that block  56  additionally takes an actual value of torque contribution M_ist_ 52  of electric machine  52  into account as input variable. The torque contribution generated by electric machine  52  reduces the torque contribution that is to be supplied by combustion engine  12 . 
         [0038]    Like block  30  from  FIG. 2 , block  30  is used for modeling an actual value M_ist_ 12  of the torque generated by combustion engine  12 . 
         [0039]    Block  58  is provided to determine actuating variables SG_ 52  for the control of electric machine  52 . To this end, block  58  processes as input variables driver input FW, measuring variables M_G_ 52 , which reflect operating parameters of electric machine  52 , such as its rotational speed n_ 52 , and actual value M_ist_ 12  of the torque contribution provided by combustion engine  12  and modeled in block  30 . The torque contribution provided by combustion engine  12  reduces the torque contribution that is to be supplied by the electric machine. 
         [0040]    In block  60 , actual value M_ist_ 52  of the torque contribution supplied by electric machine  52  is modeled from measured variables M_G_ 52 . 
         [0041]    Like block  32  of  FIG. 2 , block  62  is used to determine a marginally still permitted maximum value M_zul for the torque acting in drive train  50 . In contrast to block  32  of  FIG. 2 , maximum value M_zul may also be a negative value, by which the brake torque or the brake power of electric machine  52  is restricted during generator operation. 
         [0042]    Drive train  50  from  FIG. 5  represents an example of a drive train in a road motor vehicle in which either combustion engine  12  or electric machine  52 , or both simultaneously, are used as drive motors. The hybrid drive thus realized is designed in such a way that the power outputs of combustion engine  12  and electric machine  52  at clutch  14  add up. Combustion engine  12  is able to be decoupled by disengaging clutch  14 . 
         [0043]    In such a hybrid drive, during a ride in which the propulsion initially is provided solely by electric machine  52 , for example, combustion engine  12  is to be started. Stationary combustion engine  12  is to be started with the aid of electric machine  52 , which also is utilized for the drive, by engaging clutch  14 . In the process, the driving power transmitted to the wheels of the road motor vehicle should not vary, or should vary as little as possible. To this end, both a torque loss, determined in quasi-stationary manner, of combustion engine  12  and also a torque required for accelerating combustion engine  12  may be taken into account. 
         [0044]      FIG. 7  shows a development in the form of program structures which are executed within control device  54  and allow consideration of the torque influences that arise from a start of combustion engine  12 . 
         [0045]    In block  64 , a difference dM_ 52 −M_ist  52 −M_zul of the actual value of the torque of electric machine  52  and maximally permitted torque value M_zul depending on driver input FW is formed. In one development, the difference is then weighted in block  66  by rotational speed n_ 52  of electric machine  52 . The result thus represents a deviation of the actual output generated by electric machine  52  in drive train  50  from a limit value of permitted power outputs in drive train  50 . 
         [0046]    As long as combustion engine  12  makes no torque contribution, only a zero is to be added in logic operations  72  and  74 . Furthermore, when clutch  14  is engaged according to  FIG. 5 , only a zero is to be added in logic operation  72 , so that the moment of inertia of combustion engine  12  is not taken into account in this state. This behavior can be achieved by comparing the amount of rotational speed difference dn to thresholds S 1  and S 2  as switching condition for switches  70  and  68 . Rotational speed difference dn is formed from rotational speeds n_ 12  of combustion engine  12  and n_ 52  of electric machine  52 . 
         [0047]    The drawing should be read in such a way that switches  70 ,  68  are switched from the illustrated switching position to the alternative switching position when the individual statement written above switches  70 ,  68  is true. Therefore, switch  70  is switched over when the force transmission is achieved at interrupting clutch  14 . Switch  68  is switched over following the beginning of a start of combustion engine  12  (instant t=0) until a force transmission is achieved at interrupting clutch  14 . The force-transmission rotational speed threshold values S 1 , S 2  may have different values. 
         [0048]    Consequently, if combustion engine  12  is stationary, only the torque of electric machine  52  is analyzed. 
         [0049]    In block  76 , the previously formed deviation from the permitted value is weighted by the 2π-fold multiple of a sampling period T. In this way the deviation formed in block  76  gets the physical dimension of an energy. In block  78 , a sum of the values formed for one sampling period T in each case is formed across a plurality of sampling periods T and compared in block  73  to a threshold value SW, which is made available by a block  75 . When threshold value SW is exceeded, an error reaction is triggered in block  77 .  73 ,  75 ,  77  therefore correspond to blocks  38 ,  40 ,  42  discussed earlier in the text with reference to  FIG. 4 . 
         [0050]    In the subject matter of  FIG. 7 , as well, a torque limitation is therefore triggered as error reaction if threshold value SW is exceeded. It is understood that the torque limitation is implemented by interventions in the drive motor that is active in each instance. As long as only electric machine  52  is producing torque, the limitation intervention has to take place for electric machine  52 . If combustion engine  12  is active in addition, then the torque limitations are able to be triggered alternatively or additionally by interventions in combustion engine  12 . 
         [0051]    If combustion engine  12  is connected in addition when electric machine  52  is running, then the torque required to accelerate combustion engine  12  is able to be taken into account by the lower branch of the structure of  FIG. 7 . To this end, switch  68  is first closed as a function of the comparison of engine speed difference dn with threshold value S 2 . In one development, it is closed when t&gt;0, i.e., following a beginning of a start of the combustion engine at instant t=0, and for as long as rotational speed difference dn exceeds a threshold value S 2 , which is the case when the clutch is sliding or disengaged. In block  80 , engine speed n_ 12  (t=0) is squared. In analogous manner, rotational speed n_ 12  (t=kT) is squared at a later instant t=kT in block  82 . T is a sampling period, and k the current number of the sampling periods. 
         [0052]    In block  84  the difference of the squared rotational speeds is formed. A block  86  is used to multiply this difference by 2 times the π 2  of the moment of inertia J_ 12  of combustion engine  12  (block  89 ). In other words, the energy required to modify the rotational speed of combustion engine  12  is thereby determined in block  86 . Added to this energy in block  88  are frictional losses WLoss (kT) at clutch  14 , which are made available by a block  90 . Depending on the development, frictional losses WLoss (kT) are approximated by a fixed value or by a characteristics map, which is addressed via the rotational speed difference dn across clutch  14 . 
         [0053]    The torque contribution of combustion engine  12  generated by the combustion engine by combustions following the frictional connection of clutch  14  is taken into account by the upper branch in  FIG. 7 , which becomes active as a function of the comparison of rotational speed difference dn with threshold value S 1 . 
         [0054]    In the development of  FIG. 7 , actual torque value M_ist_ 12  of combustion engine  12  is therefore multiplied by its rotational speed n_ 12  in step  90 , and added to power output difference dM_ 52 *n_ 52  via switch  70  to be closed upon frictional connection of clutch  14 , and logic operation  74 . As a result, the value formed in block  76  is smaller than zero only if, e.g., given equality of rotational speeds of n_ 52  and n_ 12 , the sum of actual values M_ist_ 52  of electric machine  52  and M_ist_ 12  of combustion engine  12  is smaller than permitted limit value M_zul.