Abstract:
A measuring element for a flow rate sensor is described, in which the number of terminals is minimized to reduce the dimensions of the measuring element.

Description:
BACKGROUND INFORMATION 
   A flow rate sensor for determining the air mass taken in by an internal combustion engine having two heating resistors and two reference temperature sensors is discussed in European Published Patent Application No. 0 955 524. Prior to the air flowing over the heating resistors, the temperature of the air flowing past the flow rate sensor is detected by the reference temperature sensors. Both heating resistors are used for measuring the air mass flowing over the flow rate sensor. The effect of the first heating resistor in the flow direction heating the air flowing over it is utilized here, which results in the second heating resistor in the flow direction needing less heating energy than the first heating resistor to reach a predefined temperature. The cooling of the first heating resistor in the flow direction results in reduced electrical resistance of this heating resistor, while the second heating resistor is only slightly cooled by the air heated by the first heating resistor, so that the electrical resistance of the second heating resistor, under what are otherwise the same starting conditions, is greater than the electrical resistance of the first heating resistor. From the difference of the temperature-dependent resistances of the first heating resistor and the second heating resistor or from the difference of the necessary for maintaining constant (excess) temperatures, conclusions are drawn with regard to the air mass flowing past the flow rate sensor. 
   It is further known that a temperature sensor may be assigned to each heating resistor to be used for measuring the temperature of the heating resistor. 
   This circuit system is thus made up of two heating resistors, two temperature sensors, and two reference temperature sensors having a total of 2 times 6=12 terminals. If these 12 terminals are to be accommodated on a chip, the size of the chip, among other things, is predetermined by the number of terminals. The costs of a chip, among other things, also depend on its size. 
   SUMMARY OF THE INVENTION 
   In a measuring element for a flow rate sensor according to the present invention, having a ground terminal and five additional terminals, and a reference temperature sensor for determining the ambient temperature, the reference temperature sensor being electrically connected to the ground terminal and the first terminal, having a first heating resistor, the first heating resistor being electrically connected to the ground terminal and the second terminal, having a first temperature sensor, the first temperature sensor being electrically connected to the ground terminal and the third terminal, having a second heating resistor, the second heating resistor being electrically connected to the ground terminal and the sixth terminal, and having a second temperature sensor, the second temperature sensor being electrically connected to the ground terminal and the fifth terminal; the number of terminals may be reduced to six so that the size of the chip on which the measuring element according to the present invention is situated and thus the manufacturing costs may be substantially reduced. 
   The reference temperature sensor and the first temperature sensor are advantageously part of a bridge circuit, a Wheatstone bridge in particular. It is likewise advantageous if the reference temperature sensor and the second temperature sensor are part of a bridge circuit, a Wheatstone bridge in particular. Due to the fact that the reference temperature sensor is used in both the first bridge circuit and the second bridge circuit, the number of electrical components and terminals may be reduced without compromising the functionality of the measuring element according to the present invention. 
   Furthermore, it has been found to be advantageous if a first bridge voltage is applied between the first terminal and the third terminal and the voltage applied to the first heating resistor is regulated as a function of the first bridge voltage. 
   In a similar manner it is advantageous if a second bridge voltage is applied between the first terminal and the fifth terminal and the voltage applied to the second heating resistor is regulated as a function of the second bridge voltage. 
   Difference amplifiers may be advantageously used for regulating the voltage applied to the first heating resistor and the second heating resistor; the bridge voltages may be adjusted via the offset voltage of the difference amplifiers or via the bridge resistors. 
   In an advantageous embodiment of the present invention, the reference temperature sensor may be composed of a first partial resistor and a second partial resistor connected thereto in series. 
   The operating mode of the measuring element according to the present invention is further improved if the temperature sensors have a much greater resistance than the heating resistors. 
   The manufacture of the measuring element according to the present invention is simplified if the measuring element has a substrate on which a resistive layer is situated, the heating resistors and the temperature sensors being structured out of this layer. The leads for contacting and connecting the heating resistors and the temperature sensors may simultaneously be structured out of the resistive layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the layout of a measuring element according to the present invention. 
       FIG. 2  shows the diagram of a flow rate sensor including a measuring element according to the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a chip  11  on which the measuring element according to the present invention is situated. Measuring element  11  is part of the flow rate sensor circuit shown in  FIG. 2 . The integration of measuring element  11  into the flow rate sensor circuit is explained in greater detail below based upon  FIG. 2 . 
   First it should be noted that measuring element  11  is designed as a chip which in turn is made up of a substrate  13  on which a resistive layer is applied. The components of measuring element  11 , described in detail below, are etched out of this resistive layer. 
   Measuring element  11  has a ground terminal  4  to which all components situated on substrate  13  are connected. 
   A first U-shaped heating resistor R H,up  is electrically connected to third terminal  3  via a printed conductor  15  and to ground terminal  4  via a printed conductor  17 . A first temperature sensor R HF,up  is situated within first heating resistor R H,up . First temperature sensor R HF,up  is electrically connected to third terminal  3  via a printed conductor  19  and to ground terminal  4  via a printed conductor  21 . 
   The flow direction of the air flowing over measuring element  11  whose mass flow is to be measured is indicated in  FIG. 1  by an arrow  23 . 
   A second heating resistor R H,down  and a second temperature sensor R HF,down  are situated downstream from first heating resistor R H,down . Second heating resistor R H,down  is electrically connected to sixth terminal  6  via a printed connector  25  and to ground terminal  4  via a printed conductor  27 . 
   Second temperature sensor R HF,down  is electrically connected to terminal  5  via a printed conductor  29  and to ground terminal  4  via a printed conductor  31 . 
   To prevent mutual interference of the temperature regulations of first heating resistor R H,up  and second heating resistor R H,down , printed conductors  21 ,  17 ,  27 , and  31 , which all end at ground terminal  4 , are designed as separate printed conductors as close as possible to ground terminal  4 . 
   Finally, measuring element  11  also includes a reference temperature sensor R LF  which, according to the exemplary embodiment in  FIG. 1 , is made up of two partial resistors R LF,1  and R LF,2  which are connected in series. Partial resistors R LF,1  and R LF,2  are electrically connected to first terminal  1  and ground terminal  4  via printed conductors  33 ,  35 , and  37 . 
   Of course, reference temperature sensor R LF  may also be made up of a resistor (not shown) which may be positioned, for example, where printed conductor  35  is situated now on substrate  13 . 
   The integration of measuring element  11  into the electric circuit of a flow rate sensor is illustrated and explained in the following based upon  FIG. 2 . The symbols used for the identification of the components of the measuring element in  FIG. 1  have been used for the diagram in  FIG. 2 . The same is true for the symbols identifying electrical terminals  1  through  6 . 
   As can be seen in  FIG. 2 , temperature sensor R LF  and first heating resistor R HF,up  together with a first bridge resistor R 1  and a second bridge resistor form a first Wheatstone bridge whose bridge voltage is applied between terminals  1  and  3 . This bridge voltage is supplied to a first difference amplifier  39  as an input voltage. The output voltage of first difference amplifier  39  is used for regulating the voltage applied to first heating resistor R H,up  and is thus used for regulating the temperature of first heating resistor R H,up . 
   Reference temperature sensor R LF  and second temperature sensor R HF,down  together with first bridge resistor R 1  and a third bridge resistor R 3  form a second Wheatstone bridge. The bridge voltage of this second Wheatstone bridge is applied to terminals  1  and  5  of measuring element  11  (see  FIG. 1 ). This second bridge voltage is supplied to a second difference amplifier  41  as an input voltage. The output voltage of second difference amplifier  41  is used for regulating the power output of second heating resistor R H,down . The output voltage of second difference amplifier  41  is applied between terminals  6  and  4  of measuring element  11 . 
   The output voltages of first difference amplifier  39  and second difference amplifier  41  are supplied to a subtraction element  43  which generates an output voltage U A  of the flow rate sensor therefrom. This output voltage U A  represents the output signal of the flow rate sensor. 
   The first circuit bridge and the second circuit bridge may be adjusted via the adjustable offset voltages of first difference amplifier  39  and second difference amplifier  41 . 
   Alternative adjustments may be performed by varying the resistances of bridge resistors R 1 , R 2 , and R 3 .