# Patent

## Title
Method for shunt detection in sensors

## Abstract

For monitoring a sensor using differential voltage evaluation for detecting a short circuit to ground and/or to supply voltage UB, a first resistor and a second resistor R1, R2 are assigned to the sensor, a sum voltage is determined from voltages UP, UM at terminals of the sensor, the sum voltage is compared to supply voltage UB, a ratio a of determined sum voltage to supply voltage UB 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.

## Background
BACKGROUND OF THE INVENTION
The present invention relates to a method for shunt detection in sensors.
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.
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.
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 UB 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 UB, and detection of an interruption.
SUMMARY OF THE INVENTION
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.
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 UB, comprising the steps of assigning a first resistor R1 and a second resistor R2 to the sensor; determining a sum voltage based on voltages UP, UM at terminals of the sensor; comparing said sum voltage to the supply voltage UB; calculating a ratio a of the determined sum voltage to the supply voltage UB; and depending on a value for the ratio a, detecting an occurrence of a shunt at the sensor.
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 R1 and R2 are the same.
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 R1 and R2.
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
FIG. 1 shows a schematic depiction of the sensor circuitry with resistors R1 and R2 assigned to said sensor,
FIG. 2 shows the course of ratio a when a shunt is present with resistance RM,
FIG. 3 shows ratio a when a shunt is present with resistance RP,



Table I is a table of values for ratios a that occur when a shunt is present with resistance RM according to the depiction in FIG. 2, and
Table II shows the course of ratio a when a shunt is present with resistance RP, with a table of values.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The illustration shown in FIG. 1 is a schematic depiction of a sensor arrangement connected to a supply voltage source and two resistors R1 and R2.
A sensor 1, indicated in FIG. 1 as a variable sensor resistor Rs, is connected to a supply voltage source, indicated as supply voltage UB, and is also connected to ground 4. A resistor R2 is connected upstream of sensor 1, resistor R2 being connected between ground 4 and a negative terminal 3 of the sensor. In addition, a first resistor R1 is connected downstream of sensor 2, said first resistor R1 being connected between a positive terminal 2 and supply voltage UP.
A shunt at positive terminal 2 is symbolized by shunt resistance RM, which is also connected to ground 4; a shunt at negative terminal 3 of sensor 1 is indicated by shunt resistance RM at the negative terminal, and shunt resistance RM is also connected to ground 4. Shunt resistances RM and RP 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, RM and RP are considered to be real ohmic resistance. The values of resistances RM and RP are not known. UP, UM and UB 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.
The following different cases can occur, based on FIG. 1:
Case 1 (no shunt resistance present):
In this case, voltage UM 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 UP is present at positive terminal 2 of sensor 1, said voltage UP being defined by the following relationship:





  
    
      
        
          U
          P
        
        =
        
          
            U
            B
          
          ⁢
          
            
              
                R
                2
              
              +
              
                R
                S
              
            
            
              (
              
                
                  R
                  S
                
                +
                
                  R
                  1
                
                +
                
                  R
                  2
                
              
              )
            
          
        
      
    
    
      
        [
        2
        ]
      
    
  




Ratio a of the sum voltage to supply voltage is defined by:





  
    
      
        a
        =
        
          
            
              U
              p
            
            +
            
              U
              M
            
          
          
            U
            B
          
        
      
    
    
      
        [
        3
        ]
      
    
  




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
        
      
    
  




When R1=R2=R, ratio a is always=1.

Case 2 (with Shunt Resistance RM at Negative Terminal 3):

For simplicity, it is assumed that:





  
    R
    X
  
  =
  
    
      
        R
        2
      
      ⁢
      •
      ⁢
      
        
      
      ⁢
      
        R
        M
      
    
    =
    
      
        
          R
          2
        
        ⁢
        
          R
          M
        
      
      
        
          R
          2
        
        +
        
          R
          M
        
      
    
  




Ratio a is obtained, based on the equation above, as follows:





  a
  =
  
    
      
        2
        ⁢
        
          R
          X
        
      
      +
      
        R
        S
      
    
    
      
        R
        S
      
      +
      
        R
        1
      
      +
      
        R
        X
      
    
  




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
                  
                
                )
              
            
          
        
      
    
  




If R1=R2 =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
            
          
          )
        
      
    
  




For the case in which RM=∝, ratio a is equal to 1.

Case 3 (with Shunt Resistance RP at Positive Terminal 2)

In this case, in which shunt resistance RP 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
      
    
  




As a result, UP, i.e, voltage UP 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
        ]
      
    
  




Voltage UM present at negative terminal 3 is defined as:





  
    
      
        
          U
          M
        
        =
        
          
            U
            P
          
          ⁢
          
            
              R
              2
            
            
              
                R
                2
              
              +
              
                R
                S
              
            
          
        
      
    
    
      
        [
        2
        ]
      
    
  




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
      
    
  




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
              
            
            )
          
        
      
    
    =
  




For the case in which first resistance R1 is equal to second resistance R2, the following applies:

R1=R2=R:

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
            
          
          )
        
      
    
  




For the case in which RP has the value ∝, ratio a of sum voltage to supply voltage is equal to 1.

If the shunt resistances, i.e, RM and RP, are connected to supply voltage UP, the relationships become reversed, so that values >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:

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”.

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<0.95 or a>1.05.

In the illustration shown in FIG. 2, ratio a with a shunt having shunt resistance RM is plotted against various values for shunt resistance RM. 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 R1 and second resistance R2 are both 1 kOhm.

The values that occur in this case are listed in Table I. The individual values for sensor resistance RS of 50 Ohm to 2 kOhm are listed in the first row of the table of values. The values for shunt resistance RM of 0 to 50000 are listed in the column to the far left.

In the illustration in FIG. 3, the course of ratio a with a shunt having shunt resistance RP at positive terminal 2 of sensor 1 is shown. In the case depicted in FIG. 3, sensor resistance RS 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 R1 and second resistance R2 are also each equal to 1 kOhm in the curve traces shown in FIG. 3.

The values of ratio a that occurs with a shunt having resistance RP at positive terminal 2 that belong to FIG. 3 are shown in Table II. As with Table I, the changing shunt resistance RP from 0 to 50000 Ohm is listed in the column to the far left, and variable sensor resistances RS 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 RP 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 RM and with a shunt resistance RP—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.

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 RM and shunt resistance RP are ≧10 kOhm.

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 Rs






















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

















TABLE II





Variable Sensor Resistance Rs






















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
0.97402597


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

## Claims

1. A method for monitoring a sensor using differential voltage evaluation for detecting a short circuit to ground and/or to supply voltage UB, comprising the steps of assigning a first resistor R1 and a second resistor R2 to the sensor; determining a sum voltage based on voltage UP at a first terminal of the sensor and a voltage UM at a second terminal of the sensor; comparing said sum voltage to the supply voltage UB; calculating a ratio a of the determined sum voltage to the supply voltage UB; depending on a value for the ratio a, detecting an occurrence of a shunt at the sensor; and evaluating an operation of the sensor based on the occurrence or non-occurrence of the shunt at the sensor.

2. A method as defined in claim 1; and further comprising selecting the ratio




  a
  =
  
    
      
        U
        P
      
      +
      
        U
        M
      
    
    
      U
      B
    
  




equals 1 when the values for the first resistance R1, and the second resistance R2, are the same.

3. A method as defined in claim 1; and further comprising determining an occurrence of a shunt at a negative terminal to ground at the sensor by a ratio a based on a relationship:




  a
  =
  
    
      
        
          R
          M
        
        ⁡
        
          (
          
            
              2
              ⁢
              R
            
            +
            
              R
              S
            
          
          )
        
      
      +
      
        RR
        S
      
    
    
      
        
          R
          M
        
        ⁡
        
          (
          
            
              2
              ⁢
              R
            
            +
            
              R
              S
            
          
          )
        
      
      +
      
        R
        ⁡
        
          (
          
            R
            +
            
              R
              S
            
          
          )
        
      
    
  




with R1=R2=R.

4. A method as recited in claim 1; and further comprising determining an occurrence of a shunt at a positive terminal to ground at the sensor by a ratio a based on a relationship:




  a
  =
  
    
      
        R
        P
      
      ⁡
      
        (
        
          
            2
            ⁢
            R
          
          +
          
            R
            S
          
        
        )
      
    
    
      
        
          R
          P
        
        ⁡
        
          (
          
            
              2
              ⁢
              R
            
            +
            
              R
              S
            
          
          )
        
      
      +
      
        R
        ⁡
        
          (
          
            R
            +
            
              R
              S
            
          
          )
        
      
    
  




with R1=R2=R.

5. A method as defined in claim 3; and further comprising determining an occurrence of a shunt at the negative terminal to UB at the sensor by 1/a.

6. A method as recited in claim 4; and further comprising determining an occurrence of a shunt at the positive terminal to UB at the sensor by 1/a.

7. A method as defined in claim 1; and further comprising indicating by values of 0.95≦a≦1.05 that a shunt is not present.

8. A method as defined in claim 1; and further comprising indicating by values of a<0.95 or a<1.05 that a shunt is present at a positive terminal or a negative terminal of the sensor.

9. A method for monitoring a sensor using differential voltage evaluation for detecting a short circuit to ground and/or to supply voltage, comprising the steps of assigning a first resistor and a second resistor to the sensor; determining a sum voltage based on a voltage at a first terminal of the sensor and a voltage at a second terminal of the sensor; comparing said sum voltage to the supply voltage; calculating a ratio of the determined sum voltage to the supply voltage; depending on a value for said ratio, detecting an occurrence of a shunt at the sensor; evaluating an operation of the sensor based on the occurrence or non-occurrence of the shunt at the sensor; determining limits of said ratio at which a sensor evaluation should not take place any more; and stopping performing the sensor evaluation if values for said ratio are outside of a range defined by said limits.