Abstract:
For monitoring a sensor using differential voltage evaluation for detecting a short circuit to ground and/or to supply voltage U B , a first resistor and a second resistor R 1 , R 2  are assigned to the sensor, a sum voltage is determined from voltages U P , U M  at terminals of the sensor, the sum voltage is compared to supply voltage U B , a ratio a of determined sum voltage to supply voltage U B  is then calculated, and depending on the value for the ratio a, the occurrence of a shunt at the positive terminal and/or negative terminal of the sensor is detected.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a method for shunt detection in sensors.  
         [0002]     Resistance-dependent semiconductor components (PTC elements) are used to record temperature values, which are evaluated via an analog input of a microprocessor. To increase accuracy, a difference evaluation is carried out, by way of which interfering ground effects can be eliminated. The temperature is required internally in the control devices to enable calculations to be carried out in the control device.  
         [0003]     The method for performing a differential voltage evaluation is very widespread, in the automotive industry in particular, for use in the evaluation and determination of engine and transmission temperatures.  
         [0004]     To prevent faulty temperature readings by temperature sensors, the operating method of temperature sensors is monitored. If temperatures are recorded in a faulty manner, erroneous calculations are carried out in control devices and incorrect characteristic curves are used. Generally speaking, a differential evaluation is carried out to detect a short circuit to ground, to U B  and an interruption. It has not been possible to detect shunts with methods used so far to monitor sensors. Shunts result in alterated differential voltage and, therefore, to a faulty temperature reading. As such, it is not sufficient to merely monitor temperature sensors for short circuit to ground, to U B , and detection of an interruption.  
       SUMMARY OF THE INVENTION  
       [0005]     Accordingly, it is an object of the present invention to provide a method for shunt detection in sensors, which eliminates the disadvantages of the prior art.  
         [0006]     In keeping with these objects and with others which will become apparent hereinafter, one feature of the present invention resides, briefly stated, in a method for monitoring a sensor using differential voltage evaluation for detecting a short circuit to ground and/or to supply voltage U B , comprising the steps of assigning a first resistor R 1  and a second resistor R 2  to the sensor; determining a sum voltage based on voltages U P , U M  at terminals of the sensor; comparing said sum voltage to the supply voltage U B ; calculating a ratio a of the determined sum voltage to the supply voltage U B ; and depending on a value for the ratio a, detecting an occurrence of a shunt at the sensor.  
         [0007]     With the solution proposed according to the present invention, a shunt is detected at a sensor, e.g, a temperature sensor, by calculating the voltage values at the sensor. When the resistances at the positive and negative terminals of the sensor are the same, the sum of the voltage at the sensor is equal to the supply voltage. A ratio a of the sum voltage to supply voltage is therefore equal to 1, provided that R 1  and R 2  are the same.  
         [0008]     This circumstance provides an opportunity for diagnostics to be improved, namely the diagnostics of shunt resistances in particular. The following tolerance intervals, for example, for the value of ratio a are practical choices for use in a diagnosis evaluation: As long as the value for a fluctuates between 0.95 and 1.05, a shunt is not present. A shunt error is detected as soon as the ratio a falls below 0.95 or exceeds 1.05, for example. Instead of the values of 0.95 and 1.05 for ratio a stated here as an example, values such as 0.9 and 1.1, respectively, can be selected, for instance; this depends on the accuracy requirement placed on a temperature sensor, for example. Depending on the requirements and specifications on the accuracy of the temperature reading, ratio a can be specified in a defined manner as a function of resistances R 1  and R 2 .  
         [0009]     The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  shows a schematic depiction of the sensor circuitry with resistors R 1  and R 2  assigned to said sensor,  
         [0011]      FIG. 2  shows the course of ratio a when a shunt is present with resistance R M ,  
         [0012]      FIG. 3  shows ratio a when a shunt is present with resistance R P , 
     
    
       [0013]     Table I is a table of values for ratios a that occur when a shunt is present with resistance R M  according to the depiction in  FIG. 2 , and  
         [0014]     Table II shows the course of ratio a when a shunt is present with resistance R P , with a table of values.  
       DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     The illustration shown in  FIG. 1  is a schematic depiction of a sensor arrangement connected to a supply voltage source and two resistors R 1  and R 2 .  
         [0016]     A sensor  1 , indicated in  FIG. 1  as a variable sensor resistor R s , is connected to a supply voltage source, indicated as supply voltage U B , and is also connected to ground  4 . A resistor R 2  is connected upstream of sensor  1 , resistor R 2  being connected between ground  4  and a negative terminal  3  of the sensor. In addition, a first resistor R 1  is connected downstream of sensor  2 , said first resistor R 1  being connected between a positive terminal  2  and supply voltage U P .  
         [0017]     A shunt at positive terminal  2  is symbolized by shunt resistance R M , which is also connected to ground  4 ; a shunt at negative terminal  3  of sensor  1  is indicated by shunt resistance R M  at the negative terminal, and shunt resistance R M  is also connected to ground  4 . Shunt resistances R M  and R P  are understood herein below to mean effective resistances, said shunt resistances being caused, e.g, by a conductive contamination at the sensor or leakage currents at a defective sensor cable. In the calculations shown below, R M  and R P  are considered to be real ohmic resistance. The values of resistances R M  and R P  are not known. U P , U M  and U B  are measured in the entire system, and the results are used to calculate ratio a. The value of ratio a is then compared with defined limits.  
         [0018]     The following different cases can occur, based on  FIG. 1 :  
         [0000]     Case 1 (No Shunt Resistance Present):  
         [0019]     In this case, voltage U M  is present at negative terminal  3 , said voltage being defined by the following relationship:  
               U   M     =       U   B     ⁢       R   2       (       R   S     +     R   1     +     R   2       )                 [   1   ]             
 
 and, voltage U P  is present at positive terminal  2  of sensor  1 , said voltage U P  being defined by the following relationship:  
               U   P     =       U   B     ⁢         R   2     +     R   S         (       R   S     +     R   1     +     R   2       )                 [   2   ]             
 
         [0020]     Ratio a of the sum voltage to supply voltage is defined by:  
             a   =         U   p     +     U   M         U   B               [   3   ]             
 
         [0021]     When [1] and [2] are substituted in [3], ratio a is defined by:  
       a   =           (       R   2     +     R   S       )     +     R   2           R   S     +     R   1     +     R   2         =         2   ⁢     R   2       +     R   S           R   S     +     R   1     +     R   2               
 
         [0022]     When R 1 =R 2 =R, ratio a is always =1.  
         [0000]     Case 2 (with Shunt Resistance R M  at Negative Terminal  3 ):  
         [0023]     For simplicity, it is assumed that:  
         R   X     =         R   2     ⁢   •   ⁢           ⁢     R   M       =         R   2     ⁢     R   M           R   2     +     R   M               
 
         [0024]     Ratio a is obtained, based on the equation above, as follows:  
       a   =         2   ⁢     R   X       +     R   S           R   S     +     R   1     +     R   X             
 
         [0025]     Therefore:  
             a   =         2   ⁢     R   2     ⁢     R   M       +       R   S     ⁡     (       R   2     +     R   M       )               R   1     ⁡     (       R   2     +     R   M       )       +       R   S     ⁡     (       R   2     +     R   M       )       +       R   2     ⁢     R   M                       =           R   M     ⁡     (       2   ⁢     R   2       +     R   S       )       +       R   S     ⁢     R   2               R   M     ⁡     (       R   1     +     R   2     +     R   S       )       +       R   2     ⁡     (       R   1     +     R   S       )                     
 
         [0026]     If R 1 =R 2  =R, ratio a is equal to:  
       a   =           R   M     ⁡     (       2   ⁢   R     +     R   S       )       +     RR   S             R   M     ⁡     (       2   ⁢   R     +     R   S       )       +     R   ⁡     (     R   +     R   S       )               
 
         [0027]     For the case in which R M =∝, ratio a is equal to 1.  
         [0000]     Case 3 (with Shunt Resistance R P  at Positive Terminal  2 )  
         [0028]     In this case, in which shunt resistance R P  is present at positive terminal  2  of sensor  1  according to the illustration in  FIG. 1 , the following applies:  
         U   P     =       U   B     ⁢           R   P     ⁡     (       R   2     +     R   S       )           R   P     +     R   2     +     R   S                 R   P     ⁡     (       R   2     +     R   S       )           R   P     +     R   2     +     R   S         +   R             
 
         [0029]     As a result, U P , i.e, voltage U P  present at positive terminal  2 , is defined as:  
               U   P     =       U   B     ⁢         R   P     ⁡     (       R   2     +     R   S       )             R   P     ⁡     (       R   2     +     R   S       )       +       R   1     ⁡     (       R   P     +     R   2     +     R   S       )                     [   1   ]             
 
         [0030]     Voltage U M  present at negative terminal  3  is defined as:  
               U   M     =       U   P     ⁢       R   2         R   2     +     R   S                   [   2   ]             
 
         [0031]     Ratio a of sum voltage to supply voltage is defined as follows:  
       a   =           U   P     +       U   P     ⁢       R   2         R   2     +     R   S               U   B       =         U   P     ⁡     (     (     1   +       R   2         R   2     +     R   S           )     )         U   B             
 
         [0032]     Based on equation [1], ratio a is defined as follows:  
       a   =           R   P     ⁡     (       2   ⁢     R   2       +     R   S       )             R   1     ⁡     (       R   P     +     R   2     +     R   S       )       +       R   P     ⁡     (       R   2     +     R   S       )           =         
 
         [0033]     For the case in which first resistance R 1  is equal to second resistance R 2 , the following applies: 
 
R 1 =R 2 =R: 
 
         [0034]     Ratio a of sum voltage to supply voltage is therefore defined as follows:  
       a   =         R   P     ⁡     (       2   ⁢   R     +     R   S       )             R   P     ⁡     (       2   ⁢   R     +     R   S       )       +     R   ⁡     (     R   +     R   S       )               
 
         [0035]     For the case in which R P  has the value ∝, ratio a of sum voltage to supply voltage is equal to 1.  
         [0036]     If the shunt resistances, i.e, R M  and R P , are connected to supply voltage U P , the relationships become reversed, so that values &gt;1 result for ratio a. The values indicated below are practical choices for use in performing a diagnosis evaluation; these values are selected as examples only, however:  
         [0037]     For the case in which the value of ratio a is ≦1.05 but ≧0.95, i.e, 0.95≦a≦1.05, the diagnosis is “no shunt”.  
         [0038]     If the value of ratio a is less than 0.95 or greater than 1.05, the diagnosis is that a shunt error has occurred, i.e, a&lt;0.95 or a&gt;1.05.  
         [0039]     In the illustration shown in  FIG. 2 , ratio a with a shunt having shunt resistance R M  is plotted against various values for shunt resistance R M . The various curve traces are plotted for a variable sensor resistance of 50, 100, 200, 400, 600, 800, 1000, 1200, 1400, 1600, 1800 and 2 kOhm. The values for first resistance R 1  and second resistance R 2  are both 1 kOhm.  
         [0040]     The values that occur in this case are listed in Table I. The individual values for sensor resistance R S  of 50 Ohm to 2 kOhm are listed in the first row of the table of values. The values for shunt resistance R M  of 0 to 50000 are listed in the column to the far left.  
         [0041]     In the illustration in  FIG. 3 , the course of ratio a with a shunt having shunt resistance R P  at positive terminal  2  of sensor  1  is shown. In the case depicted in  FIG. 3 , sensor resistance R S  has values 50 Ohm, 100 Ohm, 200 Ohm, 400 Ohm, 600 Ohm, 800 Ohm, 1000 Ohm, 1200 Ohm, 1400 Ohm, 1600 Ohm, 1800 Ohm, and 2 kOhm, and ratio a is between 0.8 and 1. The values for first resistance R 1  and second resistance R 2  are also each equal to 1 kOhm in the curve traces shown in  FIG. 3 .  
         [0042]     The values of ratio a that occurs with a shunt having resistance R P  at positive terminal  2  that belong to  FIG. 3  are shown in Table II. As with Table I, the changing shunt resistance R P  from 0 to 50000 Ohm is listed in the column to the far left, and variable sensor resistances R S  of 50, 100, 200, 400, 600, 800 Ohm, 1 kOhm, 1.2 kOhm, 1.4 kOhm, 1.6 kOhm, 1.8 kOhm and 2 kOhm are listed in the top row in Table II. The course of ratio a with a shunt having shunt resistance R P  at positive terminal  2  is based on the values in Table II. It becomes clear that, similar to the depiction according to  FIGS. 2, 3 , the curve traces that occur for ratio a with a shunt resistance R M  and with a shunt resistance R P —which are selected to be between 0 and 50 kOhm—result in a large number of curve traces. All of them have a steep slope that transitions into an asymptotic course approaching 1.  
         [0043]     Values for ratio a for which a sensor evaluation should be carried out are between 0.95 and 1.05, as mentioned above. In other words, they lie in a range in which shunt resistance R M  and shunt resistance R P  are ≧10 kOhm.  
         [0044]     The exact limits of a at which a sensor evaluation must still take place depend on the accuracy requirements placed on the overall system. If values for a are outside this defined range, the influence of the shunt resistance on the overall system is so great that it no longer makes sense to perform the sensor evaluation and, if necessary, a sensor replacement value can be utilized instead. In this case, the sensor diagnosis detects a shunt error.  
                                                                                                                                               TABLE I                       Variable Sensor Resistance R s                                  Shunt   50   100   200   400   600   800   1000                    50000   0.99034283   0.99057493   0.99100719   0.99176277   0.99240122   0.99294781   0.99342105       45000   0.98928189   0.98963975   0.99001996   0.99085923   0.9915683   0.99217527   0.99270073       40000   0.98796906   0.98824912   0.98878924   0.98973306   0.9905303   0.99121265   0.99180328       35000   0.98626374   0.98659517   0.98721228   0.9882904   0.98920086   0.98997996   0.99065421       30000   0.98401279   0.98439938   0.98511905   0.98637602   0.98743719   0.98834499   0.98913043       25000   0.98087954   0.98134328   0.98220641   0.98371336   0.98498498   0.98607242   0.98701299       20000   0.97621879   0.97679814   0.97787611   0.9795709   0.98134328   0.98269896   0.98387097       15000   0.96855346   0.96932515   0.97076023   0.97326203   0.97536946   0.97716895   0.9787234       10000   0.96359629   0.95475113   0.95689655   0.96062992   0.96376812   0.96664495   0.96875       8000   0.94269341   0.94413408   0.94680851   0.95145631   0.95535714   0.95867769   0.96153846       6000   0.92509363   0.9270073   0.93055556   0.93670886   0.94186047   0.94623656   0.95       5000   0.91150442   0.9137931   0.91803279   0.92537313   0.93150685   0.93670886   0.94117647       4000   0.89189189   0.89473684   0.9   0.90909091   0.91666667   0.92307692   0.92857143       3000   0.86111111   0.86486486   0.87179487   0.88372093   0.89361702   0.90196078   0.90909091       2000   0.80582524   0.81132075   0.82142857   0.83870968   0.85294118   0.86486486   0.875       1000   0.67741935   0.6875   0.70588235   0.73684211   0.76190476   0.7826087   0.8       0   0.04761905   0.09090909   0.1666667   0.28571429   0.375   0.44444444   0.5                        Shunt   1200   1400   1600   1800   2000                            50000   0.99318477   0.99419954   0.99452355   0.99481328   0.99507389           45000   0.99316005   0.99356499   0.99392467   0.99424626   0.99453552           40000   0.99231951   0.99277457   0.99317872   0.99354005   0.99386503           35000   0.99124343   0.99176277   0.99222395   0.99263623   0.99300699           30000   0.9898167   0.99042146   0.99095841   0.99143836   0.99186992           25000   0.98783455   0.98855835   0.98920066   0.98777505   0.99029126           20000   0.9848926   0.98579545   0.98659517   0.98730964   0.98795181           15000   0.98007968   0.98127341   0.98233216   0.98327759   0.98412698           10000   0.97076023   0.97252747   0.97409326   0.9754902   0.97674419           8000   0.96402878   0.96621622   0.96815287   0.96987952   0.97142857           6000   0.95327103   0.95614035   0.95867769   0.9609375   0.96296296           5000   0.94505495   0.94845361   0.95145631   0.95412844   0.95652174           4000   0.93333333   0.9375   0.94117647   0.94444444   0.94736842           3000   0.91525424   0.92063492   0.92537313   0.92957746   0.93333333           2000   0.88372093   0.89130435   0.89795918   0.90384615   0.90909091           1000   0.81481481   0.82758621   0.83870968   0.84848485   0.85714286           0   0.54545455   0.58333333   0.61538462   0.64285714   0.66666667                      
 
         [0045]    
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                   
               
               
                 Variable Sensor Resistance R s   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Shunt 
                   
                   
                   
                   
                   
                   
                   
               
               
                 resistance 
                 50 
                 100 
                 200 
                 400 
                 600 
                 800 
                 1000 
               
               
                   
               
             
          
           
               
                 50000 
                 0.98985997 
                 0.98963242 
                 0.98920863 
                 0.98846787 
                 0.98784195 
                 0.98730606 
                 0.98684211 
               
               
                 45000 
                 0.98874598 
                 0.98849372 
                 0.98802395 
                 0.98720293 
                 0.98650927 
                 0.9591549 
                 0.98540146 
               
               
                 40000 
                 0.98735701 
                 0.98707403 
                 0.98654709 
                 0.98562628 
                 0.98484848 
                 0.98418278 
                 0.98360656 
               
               
                 35000 
                 0.98557692 
                 0.98625469 
                 0.98465473 
                 0.98360656 
                 0.98272138 
                 0.98196393 
                 0.98130841 
               
               
                 30000 
                 0.98321343 
                 0.98283931 
                 0.98214286 
                 0.98092643 
                 0.9798995 
                 0.97902098 
                 0.97826087 
               
               
                 25000 
                 0.97992352 
                 0.97947761 
                 0.97864769 
                 0.9771987 
                 0.97597598 
                 0.97493036 
                 0.974025797 
               
               
                 20000 
                 0.97502973 
                 0.97447796 
                 0.97345133 
                 0.97165992 
                 0.97014925 
                 0.96885813 
                 0.96774194 
               
               
                 15000 
                 0.96698113 
                 0.96625767 
                 0.96491228 
                 0.96256684 
                 0.96059113 
                 0.95890411 
                 0.95744681 
               
               
                 10000 
                 0.9512761 
                 0.95022624 
                 0.94827586 
                 0.94488189 
                 0.94202899 
                 0.93959732 
                 0.9375 
               
               
                 8000 
                 0.93982808 
                 0.93854749 
                 0.93617021 
                 0.93203883 
                 0.92857143 
                 0.92561983 
                 0.92307692 
               
               
                 6000 
                 0.92134831 
                 0.91970803 
                 0.91666667 
                 0.91139241 
                 0.90697674 
                 0.90322581 
                 0.9 
               
               
                 5000 
                 0.90707965 
                 0.90517241 
                 0.90163934 
                 0.89562239 
                 0.89041096 
                 0.88607595 
                 0.88235294 
               
               
                 4000 
                 0.88648649 
                 0.88421053 
                 0.88 
                 0.87282727 
                 0.86666667 
                 0.86153846 
                 0.85714286 
               
               
                 3000 
                 0.85416667 
                 0.85135135 
                 0.84615385 
                 0.8372093 
                 0.82978723 
                 0.82352941 
                 0.81818182 
               
               
                 2000 
                 0.791165 
                 0.79245283 
                 0.78571429 
                 0.77419355 
                 0.76470588 
                 0.75675676 
                 0.75 
               
               
                 1000 
                 0.66129032 
                 0.65625 
                 0.64705882 
                 0.63157895 
                 0.61904762 
                 0.60869565 
                 0.6 
               
               
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
               
             
          
           
               
                   
                 Shunt 
                   
                   
                   
                   
                   
               
               
                   
                 resistance 
                 1200 
                 1400 
                 1600 
                 1800 
                 2000 
               
               
                   
                   
               
             
          
           
               
                   
                 50000 
                 0.986436498 
                 0.98607889 
                 0.98576123 
                 0.98547718 
                 0.98522167 
               
               
                   
                 45000 
                 0.98495212 
                 0.98455598 
                 0.98420413 
                 0.98388953 
                 0.98360656 
               
               
                   
                 40000 
                 0.983102919 
                 0.98265896 
                 0.98226467 
                 0.98191244 
                 0.9815909 
               
               
                   
                 35000 
                 0.980735552 
                 0.98023046 
                 0.97978227 
                 0.97938144 
                 0.97902098 
               
               
                   
                 30000 
                 0.977596741 
                 0.97701149 
                 0.97649186 
                 0.9760274 
                 0.97560976 
               
               
                   
                 25000 
                 0.97323601 
                 0.9725490S 
                 0.97192225 
                 0.97137014 
                 0.97087379 
               
               
                   
                 20000 
                 0.966767372 
                 0.96590909 
                 0.96514745 
                 0.96446701 
                 0.96385542 
               
               
                   
                 15000 
                 0.956175299 
                 0.95505618 
                 0.9540636 
                 0.95317726 
                 0.96238095 
               
               
                   
                 10000 
                 0.935672515 
                 0.93406593 
                 0.93264249 
                 0.93137255 
                 0.93023256 
               
               
                   
                 8000 
                 0.920863309 
                 0.91891892 
                 0.91719745 
                 0.91566265 
                 0.91428571 
               
               
                   
                 6000 
                 0.897196262 
                 0.89473684 
                 0.89256198 
                 0.890625 
                 0.88888889 
               
               
                   
                 5000 
                 0.879120879 
                 0.87628866 
                 0.87378641 
                 0.87156963 
                 0.86956622 
               
               
                   
                 4000 
                 0.853333333 
                 0.85 
                 0.84705882 
                 0.84444444 
                 0.84210526 
               
               
                   
                 3000 
                 0.813559322 
                 0.80952381 
                 0.80597015 
                 0.8028169 
                 0.8 
               
               
                   
                 2000 
                 0.744186047 
                 0.73913043 
                 0.73469388 
                 0.73076923 
                 0.72727273 
               
               
                   
                 1000 
                 0.592592593 
                 0.5862069 
                 0.58064516 
                 0.57575758 
                 0.57142867 
               
               
                   
                 0 
                 0 
                 0 
                 0 
                 0 
                 0