Abstract:
A measuring device is disclosed for capacitive pressure and/or temperature measurement, particularly for tire pressure control systems, having at least one sensor, which has a capacitive measuring element to detect a state value, which is applied at an output-side measuring node of the measuring element, with at least one A/D converter operating according to the dual-slope method, with a charging/discharging circuit, for mutual charging and discharging of the measuring element and for generating a sawtooth-shaped measuring potential at the measuring node as a measure for the capacitance of the measuring element, with a period counter, which determines the periods of the measuring potential, and with a clock counter, which determines the cycles of a clock signal, which lie within the duration of at least one period of the measuring potential. The invention relates to a measuring method for capacitive pressure and/or temperature measurement.

Description:
[0001]     This nonprovisional application claims priority under 35 U.S.C. § 119(a) on German Patent Application No. DE 102005061090, which was filed in Germany on Dec. 21, 2005, and which is herein incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a measuring device and a measuring method for capacitive pressure and/or temperature measurement, such as, for example, for tire pressure control systems. Although it can be used in principle in any pressure and temperature measuring devices, the present invention and its underlying situation will be described below with reference to wireless pressure and temperature measuring devices with use of transponders and here particularly for tire pressure control systems. For a general background on transponder technology, reference is made to the “RFID-Handbuch” (RFID Handbook) by Klaus Finkenzeller, Hanser Verlag, 3rd updated edition, 2002.  
         [0004]     2. Description of the Background Art  
         [0005]     Transponders are microelectronic elements, which are designed to be able to store information. For this purpose, transponders have an antenna array to enable contactless reading out and modification of the stored information. In the case of transponders, an electromagnetic signal transmitted by the base station is received by the transponder and demodulated.  
         [0006]     Active, semipassive, and passive transponders are differentiated depending on the design of their energy supply. In contrast to active transponders, passive transponders do not have their own energy supply, so that the passive transponder obtain the energy necessary for demodulating and decoding the received electromagnetic signal from the electromagnetic signal itself. This occurs by absorption modulation of the transmitted electromagnetic waves. In currently employed passive 110-140 kHz transponder systems, this is realized in the electromagnetic near field by inductive coupling. The energy range achieved thereby is within the range of a few centimeters to about half a meter and depends inter alia on the specific national HF regulations.  
         [0007]     Passive transponders are employed in the field of contactless communication for identification (RFID=Radio Frequency Identification), in tire pressure control systems, and the like. In the case of a tire pressure control system, typically piezoresistive or capacitive pressure sensors are employed for determining tire-specific parameters, particularly tire pressure and tire temperature, whereby the present invention relates also to capacitively designed pressure sensors.  
         [0008]     The absolute value of the capacitance of the pressure sensor, which corresponds to the actual tire pressure of the automobile tire, is to be determined by the capacitive pressure sensor. The tire temperature is determined in a similar way with the internal reference capacitance. In this case, the temperature-dependent charge of the reference capacitor is evaluated. A voltage is applied to the pressure sensor to measure this capacitance. The charge stored in the capacitor of the pressure sensor, which thereby contains information on the actual tire pressure, is subsequently determined in an AD converter using suitable converter methods. The challenge in determining the actual tire pressure from the charge stored by this capacitor now is the highest measuring accuracy possible, a resolution of less than one per thousand being required in current tire pressure control systems.  
         [0009]     A/D converters used today or conventional corresponding converter methods in fact perhaps achieve such a high accuracy in the resolution, but do not take into account the integratability of the pressure sensor. In particular, the capacitive pressure sensor for tire pressure measurements as a component of an integrated circuit, which is disposed on a wheel of a vehicle or vulcanized into its rubber material, must be as small as possible and therefore made area-optimized. A/D converters and capacitive pressure sensors in use today, which are designed for a high accuracy, are not suitable here or only suitable conditionally when designed for this. Moreover, these have a relatively long converter time to determine the capacitance of the capacitive pressure sensor and thereby the actual tire pressure. A long converter time, however, is directly associated with a high energy consumption, which is essential to avoid, however, particularly in tire pressure control systems in which the transponder must obtain the energy from the electromagnetically transmitted signal.  
         [0010]     A pressure and temperature sensor with wireless transmission of measured data for microelectronic circuits is described in the article by Werner Schulz, “A wireless integrated pressure sensor” in the journal Elektronik, 7/2000, page 54. As a result of the low power consumption, the current supply can be obtained here from the field of a base station up to a meter away.  
         [0011]     In the German textbook by Tietze and Schenk “Semiconductor Circuit Technology,” 11th edition, pages 1059-1060, a generally known so-called dual-slope method is described, furthermore, by means of which the capacitance of a capacitor can be determined.  
       SUMMARY OF THE INVENTION  
       [0012]     It is therefore an object of the present invention to provide a method for capacitance measurement for a capacitive pressure sensor, which provides in particular as high a resolution as possible. Another object is to provide a corresponding method for capacitance measurement with as low an energy consumption as possible.  
         [0013]     Accordingly, provided are a measuring device for capacitive pressure and/or temperature measurement, particularly for tire pressure control systems, having at least one sensor, which has a capacitive measuring element to detect a state value, which is applied at an output-side measuring node of the measuring element, with at least one A/D converter operating according to the dual-slope method, with a charging/discharging circuit, for mutual charging and discharging of the measuring element and for generating a sawtooth-shaped measuring potential at the measuring node as a measure for the capacitance of the measuring element, with a period counter, which determines the periods of the measuring potential, and with a clock counter, which determines the cycles of a clock signal, which lie within the duration of at least one period of the measuring potential.  
         [0014]     A measuring method for the capacitive pressure measurement, particularly for tire pressure control systems, particularly by the measuring device of the invention, in which a measurement of a measuring potential of a capacitive element to be measured is repeated multiple times, to average out an uncorrelated noise, which occurs due to an exceeding or falling of the measuring potential above the top reference threshold or below the bottom reference threshold, respectively.  
         [0015]     A dual-slope measuring method is can be used for measuring the capacitance of the capacitive pressure sensor. The particular advantage of this type of dual-slope measuring method, compared with other measuring methods, is its higher measuring accuracy. To improve this measuring accuracy even further, in the method of the invention, the measurement of the charge in the capacitor of the capacitive pressure sensor and thereby for determining the capacitance is repeated multiple times, to average out in this way the uncorrelated noise of the reference thresholds, which is typically inherent to the dual-slope measuring method. This noise results because a particular rising or falling slope of the measuring signal is not switched precisely at the reference thresholds, as is ideally the case, but continues slightly beyond the specific reference threshold. The particular advantage in the measuring device of the invention or in the corresponding measurement method therefore is the very high measuring accuracy due to the multiple execution of the dual-slope measuring method. In this way, a single undesirable charging is theoretically distributed among several measuring cycles, as a result of which measuring accuracies are averaged out.  
         [0016]     The particular advantage here is also that a lower supply voltage can be used. by this measuring method, because a significantly lower supply voltage is required for the comparatively low slope in the case of the employed clock signal versus the measuring signal.  
         [0017]     In order to correct further the error in switching at the reference thresholds, preferably two correction measurements can be made before the actual measurement. In these correction measurements, first a simple capacitance value and then a double capacitance value are measured. The difference between these two correction measurements indicates the lag, which must be taken into account afterwards in the calculation of the actual capacitance value of the capacitive pressure sensor. The capacitance values of the single capacitance value and of the double capacitance value are known very precisely and therefore can be used as reference for the actual capacitance measurement. In this way, a dual-slope method divided into several cycles can be corrected and also expanded as desired without the switching errors adding up.  
         [0018]     In an preferred, an oscillator is provided which generates an oscillator clock signal on the output side, whose frequency is at least higher than the frequency of the measuring potential.  
         [0019]     In another embodiment, the period counter on the output side outputs a first counter reading signal as a measure for the counted periods of the measuring potential. The clock counter on the output side outputs a second counter reading signal as a measure for the counted cycles of the clock signal. Typically, but not necessarily, the period counter and/or the clock counter is/are made as an up- or down-counter. These increment or decrement by plus one their counter readings with each period of the measuring potential or with each cycle of the clock signal.  
         [0020]     In a preferred development, the charging/discharging circuit has at least one switchable current source to generate the sawtooth-shaped measuring potential. For this purpose, the charging/discharging circuit in each case can have at least one first current source for charging a capacitive element to be measured in each case and at least one second current source in each case for discharging the capacitive element to be measured in each case. For a mutual charging and discharging, at least one controllable switch is disposed between each current source and the measuring node. Typically, the controllable switch is driven by a control signal derived from the measuring potential in such a way that either the first or the second current source is activated thereby.  
         [0021]     In an embodiment, the charging/discharging circuit, to generate the sawtooth-shaped course of the measuring potential, can have a threshold switch, which is connected downstream of the measuring node, which compares the measuring potential with a predefined top and bottom threshold value and which changes the control signal to drive the controllable switches with the exceeding of the measuring potential above the top threshold value or falling of the measuring potential below the lower threshold value in such a way that the charging/discharging process of the element to be measured is thereby reversed.  
         [0022]     In another embodiment, the sensor has a capacitor array, which contains, in addition to the measuring element, to determine the state value, at least one capacitive reference element as reference for the measuring element.  
         [0023]     In yet another embodiment, the capacitor array further can have at least two correction capacitors, which are provided for averaging out an uncorrelated noise, which arises when the measuring potential exceeds a top reference threshold or falls below a bottom reference threshold. Typically, a first correction capacitor has a double capacitance value compared with the second correction capacitor.  
         [0024]     Typically, however, but not necessarily, the state value is a tire-specific parameter, particularly the tire pressure or the tire temperature. For this purpose, the sensor can be a capacitive pressure sensor to determine a pressure.  
         [0025]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:  
         [0027]      FIG. 1   a  is a block diagram of a measuring device according to an embodiment of the invention, to determine a capacitance value of a capacitive pressure sensor;  
         [0028]      FIG. 1   b  is a signal-time course of the measuring potential to show the measuring method, according to the invention, of the measuring device of  FIG. 1 ;  
         [0029]      FIG. 2  is a signal-time diagram of a single period of the measuring signal;  
         [0030]      FIG. 3  is a signal-time diagram of two cycles of a clock signal;  
         [0031]      FIG. 4  is a signal-time diagram of two cycles of the clock signal. 
     
    
     DETAILED DESCRIPTION  
       [0032]     In the figures of the drawing, the same and substantially functionally identical elements, features, and signals, if not specified otherwise, are provided with the same reference characters.  
         [0033]      FIG. 1  shows a block diagram of a measuring device that determines a capacitance value of a capacitive pressure sensor. The measuring device of the invention is used to determine tire-specific parameters by a capacitive pressure sensor. Tire-specific parameters are taken to mean, e.g., the actual tire pressure or the actual tire temperature.  
         [0034]     This measuring device according to an embodiment of the invention is designated with the reference character  10  in  FIG. 1   a . Measuring device  10  contains a capacitor array  19  with a capacitive pressure sensor  11 , whose capacitance CP can vary within the range of typically CP=5-20 pF, depending on the tire pressure to be measured. This capacitance CP is determined by measuring device  10  as a measure for the tire pressure. For this purpose, capacitor array  19  further has a reference capacitor  12  and two correction capacitors  13 ,  14 . Reference capacitor  12 , which is to be used as a reference for the measurement, has a reference capacitance value of, for example, CREF=8 pF. The tire temperature is determined using the same measuring method with CREF=8 pF. For this purpose, two measurements are also necessary as in the tire pressure determination. Hereby, the charge at CREF is compared with a temperature-dependent and temperature-independent value.  
         [0035]     The two correction capacitors  13 ,  14  have the purpose of correcting a measurement error and the accompanying measurement accuracies during switching at the specific reference thresholds. First correction capacitor  13  has a capacitance value of, e.g., COR 0 =1 pF. Second correction capacitor  14  has double the capacitance value compared with first correction capacitor  13 , therefore in the shown example of COR 1 =2 pF.  
         [0036]     Measuring device  10  has two supply terminals  17 ,  18 . First supply terminal  17  has a first supply potential VDD, for example, a positive DC voltage potential VDD, whereas second supply terminal  18  is supplied with a second supply potential GND, for example, a reference potential GND.  
         [0037]     Capacitors  11 - 14  of capacitor array  19  are each connected on one side to second supply terminal  18 . Furthermore, a controllable switch  15  is provided, by which at least one of these capacitors  11 - 14  can be connected to measuring node  16 . A measuring potential UC, which thereby corresponds to the charge stored in the connected capacitors  11 - 14 , is therefore applied at the measuring node  16 .  
         [0038]     For the measuring method of the invention with use of a dual-slope measuring method, these capacitors  11 - 14  of capacitor array  19  must be mutually charged and again discharged. A current source array  25  is provided for this purpose. Current source array  25  has two current sources  20 ,  21 . First current source  20  is used to charge capacitors  11 - 14  connected in each case and is disposed for this purpose between first supply terminal  17  and a tap  22 , which forms the output of current sources  20 ,  21 . Tap  22  is connected to measuring node  16 . For charging a specific capacitor  11 - 14 , further, a controllable switch  23  is provided, across which with a closed switch  23 , current source  20  generates a charging current IL to charge a specific capacitor  11 - 14 . Second current source  21 , which is connected to second supply terminal  18 , is used to discharge a specific capacitor  11 - 14  with a discharge current IE, provided that a controlled switch  24 , disposed between second current source  21  and tap  22 , is closed.  
         [0039]     Controllable switches  23 ,  24  can be made, for example, as simple MOSFET transistors (CMOS technology) or as bipolar transistors (bipolar technology).  
         [0040]     The measuring device of the invention  10  contains further a threshold circuit  30 , which predetermines an internally predefined top and bottom threshold value UH, UL for the measuring potential UC. Threshold circuit  30  takes up the measuring potential UC and compares this measuring potential UC with the top and bottom threshold value UH, UL. The measuring potential UC′ provided at the output of threshold circuit  30  is used via feedback path  31  to drive the two controllable switches  23 ,  24 , whereby this signal UC′ is used for the direct driving of controllable switch  23  and via an inverter  32  to drive controllable switch  24 .  
         [0041]     First, with a closed switch  23  a capacitor  11 - 14 , to be measured, of capacitor array  19  is charged with charging current IL. The corresponding, slowly rising charging of capacitor  11 - 14  leads to an increase in measuring potential UC, which is compared with the top threshold value UH in threshold circuit  30 . If the measured potential UC exceeds the top threshold value UH, this leads to a signal inversion in the output signal UC′, which immediately causes switch  23  to be opened and switch  24 , which was initially open, to be closed. In this way, the appropriate capacitor  11 - 14  of capacitor array  19  is discharged with discharge current IL. The corresponding, slowly declining charge and thereby declining measuring potential UC is compared with the bottom threshold value UL in threshold circuit  30 . If the measured potential UC falls below the bottom threshold value UL, this leads to a signal inversion in the output signal UC, which immediately causes switch  24  to be opened again and switch  23  to be closed. The measuring potential UC thereby describes a sawtooth-shaped measuring course, as shown in  FIG. 1   b.    
         [0042]     Furthermore, a period counter  33  is provided, which determines the individual periods  38  of the sawtooth-shaped signal UC. At output  36  of period counter  33 , a counter reading signal XPER can therefore be tapped. Counter reading signal XPER indicates how many complete periods  38  of signal UC were counted. In the example in  FIG. 1   a , a total of four and a half periods  38  of measuring signal UC during the time period T=TB−TA are shown.  
         [0043]     Furthermore, an oscillator  34  is provided which provides an oscillator clock signal OSC on the output side. The individual cycles  39  of the clock signal OSC are also counted in a measuring counter  35  specially provided for this. At output  37  of measuring counter  35 , therefore, a counter reading signal XCNT can be tapped, which indicates how many cycles  39  of clock signal OSC were counted within a predefined time period.  
         [0044]      FIG. 1   b  shows the course of the measuring potential UC with use of a signal-time diagram. Reference character  38  here indicates an individual period of duration T 1  of measuring potential UC. Period counter  33  is designed to determine all periods  38  of measuring potential UC within the measuring duration T. This measuring duration T results from the time between the start of the measurement at time TA and up to the stopping of the measurement at time TB. The starting time TA is typically selected so that it is selected at a fixedly predefined time relative to the measuring potential UC, for example, to its maximum (or alternatively also to its minimum). The stop condition TB depends in each case on which of the capacitors  11 - 14  are to be measured. The stop condition TB is determined particularly from the number of counted periods  38  of measuring potential UC or from the counted cycles  39  of the clock signal OSC.  
         [0045]     The measuring method of the invention to determine the capacitance CP of capacitive pressure sensor  11  will be described next with use of  FIGS. 2-4 .  
         [0046]      FIG. 2  shows a signal-time diagram in the region of a single period of measuring potential UC to illustrate the operation mode of period counter  33  and measuring counter  35 . The beginning of period  38  is designated here with TX and the end with TY. Within this time period T 1 =TY−TX, a total of 15 cycles  39  of clock signal OSC are provided. Measuring counter  35  measures these cycles, whereas periods  38  are counted by period counter  33 . In an embodiment, the measuring potential UC has an amplitude maximum REF 1  and an amplitude minimum REF 3 , which correspond to the upper and lower threshold value UH or UL, respectively. The clock signal OSC has an amplitude maximum REF 2  and the amplitude minimum REF 3 . The amplitude maximum REF 2  here is significantly smaller than the amplitude maximum REF 1 .  
         [0047]     In the dual-slope measuring method, only a single charging and discharging are carried out. This is shown with use of the measuring curve for period  38 . In so doing, however, inaccuracies can occur due to an uncorrelated noise in region  40  of the top and bottom reference thresholds REF 1 , REF 3 . This uncorrelated noise typically leads to a reduction in the accuracy of this dual-slope measuring method, as explained herein above. In the measuring method of the invention, the individual cycles  39  of the clock signal OSC, which are present within the single period  38  of the measuring signal UC, are measured. In fact, here as well there is an uncorrelated noise of the specific reference thresholds REF 2 , REF 3 . However, here a slope method and particularly a dual-slope method are carried out multiple times. The uncorrelated noise of the specific reference thresholds REF 2 , REF 3  is averaged out by means of this multiple run of the dual-slope method. The advantage here is also that a lower supply voltage can be used by this measuring method, because for the comparatively small slopes a significantly lower supply voltage VDD-GND is required in the case of the clock OSC signal versus the measuring signal UC.  
         [0048]      FIGS. 3 and 4  show two signal-time diagrams to illustrate a switching error Δt 1  and Δt 2  for the reference thresholds REF 2 , REF 3 , which can arise in the dual-slope method. To be able to determine this switching error Δt 1  and Δt 2  as accurately as possible, two correction measurements are made by using correction capacitors  13 ,  14 . In this way, a switching error in the capacitance measurement to determine the sensor capacitance CP of pressure sensor  11  can be compensated. The switching errors are also not added up by the multiple carrying out of the dual-slope method for several periods  38  or several cycles of the clock signal OSC.  
         [0049]     The method of the invention is to be explained below with use of these  FIGS. 3 and 4 .  
         [0050]     During charging and discharging with a current source, the time course of a voltage U(t) at any capacitor depending on the current I and time t can be calculated as follows: 
 
 U(   t )=1 /C*I*t  
 
         [0051]     First, only one period (see  FIG. 4 ) is examined. In two correction capacitances C 2 , C 1 , of which one is twice as large as the other, therefore for the correlation 
 
 C   2 =2 *C   1 , 
 
         [0052]     the following correlation results theoretically, because the measuring time increases only by the capacitance to be measured, but the switching errors enter only once regardless of the capacitance to be measured: 
 
 t   C2 =2 *t   C1  
 
 t   1C2 =2 *t   1C1  
 
 t   2C2 =2 *t   2C1 , 
 
         [0053]     The actual measurement time per period results from: 
 
 t   C1   =t   1C1   +Δt   1   +t   2C1   +Δt   2  
 
 t   C2   =t   1C2   +Δt   1   +t   2C2   +Δt   2  
 
         [0054]     Due to the double run time of capacitance C 1  compared with the run time of capacitance C 2 , thereby two periods are covered, as a result of which in comparison with capacitance C 2  the switching error in the second period of capacitance C 1  must be subtracted. Because capacitance behaves proportional to time (see above), the following applies: 
 
 t   C2 =2 *t   C1   −Δt   1   −Δt   2  
 
 t   1C2   +Δt   1   +t   2C2   +Δt   2 =2 *t   1C1   +Δt   1 +2 *t   2C1   +Δt   2  
 
         [0055]     Thus, switching errors Δt 1  and Δt 2  can be subtracted from both sides of the equations and the following correlation results: 
 
 t   1C2   +t   2C2 =2 *t   1C1 +2 *t   2C1  
 
         [0056]     The pure capacitance-dependent measuring time remains: 
 
 t   1C2   +t   2C2 =2*( t   1C1   +t   2C1 ) 
 
         [0057]     For the actual pressure and temperature measurements, the following six measurements are now performed, which are listed in short form in the following Table 1.  
                                                         TABLE 1                                       Stop   Period number   Counter reading           Description   condition   (period counter)   (measuring counter)                                    1st   Correction measurement   Periods = 128   COR0_PER   COR0_CNT       measured   (COR0)       value       2nd   Correction measurement 1   Periods = 64   COR1_PER   COR1_CNT       measured   (COR1)       value       3rd   Pressure measurement   Cycles &gt;2048   P_REF_PER   P_REF_CNT       measured   (P_REF)       value   reference capacitor       4th   Pressure measurement   Cycles &gt;1024   P_MEAS_REF   P_MEAS_CNT       measured   (P_MEAS)       value   pressure sensor       5th   Temp. measurement   Cycles &gt;2048   T_REF_PER   T_REF_CNT       measured   (T_REF)       value   referenced capacitor       6th   Temp. measurement   Cycles &gt;1024   T_MEAS_PER   T_MEAS_CNT       measured   (T_MEAS)       value   pressure sensor                  
 
         [0058]     The first column shows the successive six measuring steps for the pressure and temperature measurement. The second column indicates which measured value is to be measured, whereby the specific correction capacitance is to be measured with COR 0 , COR 1 . With P_REF and P_MEAS, the specific reference pressure and measuring pressure are to be determined with use of reference capacitor  12  or a pressure sensor (measuring sensor)  11 . With T_REF and T_MEAS, the particular reference temperature and measuring temperature are to be determined. The third column shows the necessary period count or clock count and thus the stop condition, which are counted by period counter  33  or measuring counter  35  and in which a specific measurement is regarded as ended. The fourth column shows the particular measurement result based on the period count measured by period counter  33  and the fifth column shows the corresponding counter reading of the corresponding counter  35 . The measurement values designated with “PER” should signal that the appropriate period count was measured here. The values given with “CNT” indicate that the result is from the specific counter reading.  
         [0059]     The following standardizations were performed for the measurements:  
       COR_NOM   =       COR0_CNT   -   COR1_CNT       COR0_PER   -   COR1_PER           
         P_REF   ⁢   _NOM     =       P_REF   ⁢   _CNT       P_REF   ⁢   _PER           
         P_MEAS   ⁢   _NOM     =       P_MEAS   ⁢   _CNT       P_MEAS   ⁢   _PER           
         T_REF   ⁢   _NOM     =       T_REF   ⁢   _CNT       T_REF   ⁢   _PER           
         T_MEAS   ⁢   _NOM     =       T_MEAS   ⁢   _CNT       T_MEAS   ⁢   _PER           
 
         [0060]     The following corrections were calculated furthermore for the measurements: 
 
P_REF_COR=P_REF_NOM−COR_NOM 
 
P_MEAS_COR=P_MEAS_NOM−COR_NOM 
 
T_REF−COR=T_REF_NOM−COR_NOM 
 
T_MEAS_COR=T_MEAS_NOM−COR_NOM 
 
         [0061]     Using the values measured in the above Table 1 and the corresponding standardizations and corrections, the capacitance CP and the temperature TP of the capacitive pressure sensor  11  can be calculated as follows.  
       CP   =         C   0     *       P_MEAS   ⁢   _COR       P_REF   ⁢   _COR       *     C   gradient       +     C   offset           
       TP   =         T   0     *       T_MEAS   ⁢   _COR       T_REF   ⁢   _COR       *     T   gradient       +     T   offset           
 
         [0062]     The constants are determined, for example, with 9 measuring runs. The sensor capacitance CP is to be between 5 and 20 pF and to be measured at 233 and 398 Kelvin. The basic setting for determining the above constants is as follows: 
 
 C   0 =1;  C   gradient =1 ; C   offset =0 
 
 T   0 =1 ; T   gradient =1 ; T   offset =0 
 
         [0063]     The constants C 0 , C gradient , C offset  in the above equations can be calculated as follows for the capacitance measurement:  
         C   0     =     Mean   ⁢           ⁢   of   ⁢           ⁢     (             20   ⁢           ⁢   pF       Mean   ⁢       P_MEAS   ⁢   _COR       P_REF   ⁢   _COR       ⁢   of   ⁢           ⁢   20   ⁢           ⁢   pF   ⁢           ⁢   measurements                     8   ⁢           ⁢   pF       Mean   ⁢       P_MEAS   ⁢   _COR       P_REF   ⁢   _COR       ⁢   of   ⁢           ⁢   8   ⁢           ⁢   pF   ⁢           ⁢   measurements       ;                 5   ⁢           ⁢   pF       Mean   ⁢       P_MEAS   ⁢   _COR       P_REF   ⁢   _COR       ⁢   of   ⁢           ⁢   5   ⁢           ⁢   pF   ⁢           ⁢   measurements             )           
 
         C   gradient     =           20   ⁢           ⁢   pF       C   0       /     [     Mean   ⁢       P_MEAS   ⁢   _COR       P_REF   ⁢   _COR       ⁢   of   ⁢           ⁢   20   ⁢           ⁢   pF   ⁢           ⁢   measurements     ]             5   ⁢           ⁢   pF       C   0       /     [     Mean   ⁢       P_MEAS   ⁢   _COR       P_REF   ⁢   _COR       ⁢   of   ⁢           ⁢   5   ⁢           ⁢   pF   ⁢           ⁢   measurements     ]             
 
         C   offset     =     mean   ⁢           ⁢   of   ⁢           ⁢     (       20   ⁢           ⁢   pF     -     [           Mean   ⁢           ⁢     (       C   0     *       P_MEAS   ⁢   _COR       P_REF   ⁢   _COR       *     C   gradient       )                 of   ⁢           ⁢   20   ⁢           ⁢   pF   ⁢           ⁢   measurements           ]     +     8   ⁢           ⁢   pF     -             [           Mean   ⁢           ⁢     (       C   0     *       P_MEAS   ⁢   _COR       P_REF   ⁢   _COR       *     C   gradient       )                 of   ⁢           ⁢   8   ⁢           ⁢   pF   ⁢           ⁢   measurements           ]     +     5   ⁢           ⁢   pF     -     [           Mean   ⁢           ⁢     (       C   0     *       P_MEAS   ⁢   _COR       P_REF   ⁢   _COR       *     C   gradient       )                 of   ⁢           ⁢   5   ⁢           ⁢   pF   ⁢           ⁢   measurements           ]       )               
 
         [0064]     Furthermore, the constants T 0 , T gradient , T offset  in the above equations for the temperature measurement can also be calculated as follows:  
         T   0     =     Mean   ⁢           ⁢   of   ⁢           ⁢     (                 398   ⁢           ⁢   K       Mean   ⁢       T_MEAS   ⁢   _COR       T_REF   ⁢   _COR       ⁢   of   ⁢           ⁢   398   ⁢           ⁢   K   ⁢           ⁢   measurements                     300   ⁢           ⁢   K       Mean   ⁢       T_MEAS   ⁢   _COR       T_REF   ⁢   _COR       ⁢   of   ⁢           ⁢   300   ⁢           ⁢   K   ⁢           ⁢   measurements       ;                 233   ⁢           ⁢   K       Mean   ⁢       T_MEAS   ⁢   _COR       T_REF   ⁢   _COR       ⁢   of   ⁢           ⁢   233   ⁢           ⁢   K   ⁢           ⁢   measurements             ⁢     
     ⁢     T   gradient       =           398   ⁢           ⁢   K       T   0       /     [     Mean   ⁢       T_MEAS   ⁢   _COR       T_REF   ⁢   _COR       ⁢   of   ⁢           ⁢   398   ⁢           ⁢   K   ⁢           ⁢   measurements     ]             233   ⁢           ⁢   K       T   0       /     [     Mean   ⁢       T_MEAS   ⁢   _COR       T_REF   ⁢   _COR       ⁢   of   ⁢           ⁢   233   ⁢           ⁢   K   ⁢           ⁢   measurements     ]                   
 
         T   offset     =     mean   ⁢           ⁢   of   ⁢           ⁢     (       398   ⁢           ⁢   K     -     [           Mean   ⁢           ⁢     (       T   0     *       T_MEAS   ⁢   _COR       T_REF   ⁢   _COR       *     T   gradient       )                 of   ⁢           ⁢   398   ⁢           ⁢   K   ⁢           ⁢   measurements           ]     +     300   ⁢           ⁢   K     -             [           Mean   ⁢           ⁢     (       T   0     *       T_MEAS   ⁢   _COR       T_REF   ⁢   _COR       *     T   gradient       )                 of   ⁢           ⁢   300   ⁢           ⁢   K   ⁢           ⁢   measurements           ]     +     233   ⁢           ⁢   K     -     [           Mean   ⁢           ⁢     (       T   0     *       T_MEAS   ⁢   _COR       T_REF   ⁢   _COR       *     T   gradient       )                 of   ⁢           ⁢   233   ⁢           ⁢   K   ⁢           ⁢   measurements           ]       )               
 
         [0065]     Although the present invention was described more closely above with use of a specific and preferred exemplary embodiment, it is not limited thereto, but can be changed and modified as desired within the scope of the invention. In particular, it is understood that the selected numerical data are to be taken only as examples, but are not intended to limit the invention. This also applies to the selected circuit elements and circuit parts, which were shown as simply as possible for the sake of better clarity. It is understood that the measuring device of the invention can be expanded and modified as desired.  
         [0066]     Of course, the invention is also not exclusively suitable for a tire pressure control system and also not necessarily exclusively suitable for determining tire-specific parameters. Rather, the invention can be advantageously used in other measuring devices and for other converter methods.  
         [0067]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.