DE10318602A1 - Adjusting device for signal-noise ratio between signal level and noise level in test signal emits test signal dependent on detected measurable variable - Google Patents

Abstract

A signal-noise ratio depends on first and second influencing variables (IV). A detecting device (14) records a value for a first IV. An adjusting device (18) relies on the value of the first IV to adjust a value for a second IV in order to adjust the signal-noise ratio in this way. A memory (16) stores a look-up table that assigns each value of the second IV to multiple values for the first IV. Independent claims are also included for the following: (a) a measuring system with an adjusting device for signal-noise ratio; (b) a method for adjusting a signal-noise ratio between a signal level and a noise level in a test signal; (c) a method for adjusting signal-noise ratio in a tire pressure testing system; (d) a computer program with program code for carrying out the method of the present invention.

Description

The The present invention relates to an apparatus and a Method for setting the signal / noise ratio in a measuring system, being the signal to noise ratio depends on several factors.

The Signal / noise ratio, with a view to logarithmic representation, often as Signal-to-noise ratio is an important parameter in all types of measurements. Often the signal to noise ratio can be around the price of an increase the expenditure on equipment, an increase in the time required, one increased Power requirements or a deterioration of other parameters can be increased. It however, there are numerous applications in which a predetermined one or one if possible high signal / noise ratio with one if possible low outlay on equipment with little time requirement and little Power requirements must be achieved. These include measurements with battery operated measuring systems, for example measuring systems for recording and monitoring the tire pressure in vehicle tires.

A measure to achieve a low power requirement and a high signal / noise ratio is it as excitation voltage for a sensor to use the battery voltage immediately and both upon generation of an excitation voltage higher than is the battery voltage, as well as a stabilization of the excitation voltage (a value that is lower than the battery voltage). This means that the power requirement and the outlay on equipment are one Voltage generator, a charge pump, a voltage stabilization circuit or similar Circuits saved. A disadvantage is that the operating voltage or the battery voltage then usually an immediate Influence on the signal-to-noise ratio the measurements of the measuring system Has. An influence of the Operating voltage or battery voltage also exists in numerous cases where possible, for example simple circuits or circuits optimized for their power requirements to generate the excitation voltage of the sensor or one Supply voltage of an amplifier or another downstream component can be used. This is often the case especially when the excitation voltage to be generated should be as large as possible within the limits of the operating voltage in order to achieve the greatest possible Generate sensor signal.

A further influencing variable, the usually the noise level or the signal level and thus that Signal / noise ratio affected is the temperature of the sensor, the downstream amplifier and further downstream circuits. In particular, the temperature an essential factor that determines the size of the white noise.

Either the battery voltage as well as the temperature or others, not mentioned above Are influencing factors in many cases cannot be influenced, can but vary widely. For example, the battery voltage increases a battery throughout its life and especially towards the end their lifespan with increasing discharge. Often the battery voltage also depends on the temperature of the battery, which in turn is determined by the Ambient temperature affected becomes. The temperature of the sensor and the downstream electronic Circuits is also a function of ambient temperature, which can vary by many 10 Kelvin in many applications.

at the one already mentioned above Reifendruckmeßsystem for example, a capacitive sensor is arranged in a capacitive measuring bridge, whose excitation voltage is the same as the battery voltage for the reasons mentioned or is obtained from it with a simple circuit. In this case, the battery voltage is used as a proportionality factor directly into the size of the measurement signal, the output voltage of the measuring bridge, or at least influences it. The temperature of the tire and thus the measuring system can be high in winter are below 0 ° and for example down to -30 ° C and below be. In strong and direct sunlight, the temperature can in summer, for example, up to 70 ° C and above. While driving there is still warming by friction with the road, by friction or flexing work within of the tire and through waste heat of the braking system. Is within this extreme temperature range consequently the thermally induced noise of the measuring system fluctuates greatly subjected.

In order to achieve a predetermined minimum signal-to-noise ratio under all operating conditions, that is to say both with a fully charged battery and with a largely discharged battery and both at low and at very high temperatures of the measuring system, the measuring system and its parameters are produced Conveniently designed for a worst-case scenario or for the worst case of operating conditions. However, during most of the battery life, the battery voltage is significantly higher than at the end of the life. Furthermore, the temperature of the measuring system is generally low or moderate and only in exceptional cases assumes the values assumed in the worst-case scenario. For example, it is usually assumed that a car with a lifespan of 10 years is parked more than 90% of this time. The measuring system and its parameters, for example a bias or bias voltage, a bias voltage or bias, the bandwidth or cut-off frequency of a filter etc., are therefore oversized for most of the operating time of the measuring system. However, this oversizing is also accompanied by an unnecessarily high power requirement or an unnecessarily high outlay on equipment.

The The object of the present invention is to provide a device a method and a computer program for adjusting the signal / noise ratio in a measuring system to accomplish.

This Object is achieved by a device according to claim 1, a method solved according to claim 13 or a computer program according to claim 21.

The The present invention provides an apparatus for adjusting the Signal / noise ratio between a signal level and a noise level of a measurement signal in a measurement system for recording a measured variable and outputting it the measurement signal dependent on the measured variable, where the signal / noise ratio of a first influencing variable and of depends on a second influencing variable. The Device includes a detection device for detecting a value of the first influencing variable and one Setting device for setting a value of the second influencing variable depending on Value of the first influencing variable to thereby the signal-to-noise ratio adjust.

The present invention provides a method for adjusting the signal-to-noise ratio between a signal level and a noise level of a measurement signal in a measurement system, which detects a measurement variable and generates the measurement signal dependent on the measurement variable, the noise level being dependent on a first influencing variable and a second Influencing variable depends on the following steps:
Detecting a value of the first influencing variable; and
Setting a value of the second influencing variable depending on the value of the first influencing variable, in order to thereby set the signal / noise ratio.

The The present invention is based on the finding that the signal / noise ratio of a measuring system usually depends on several influencing factors. Some of these factors are through the measuring system not or not without further ado or not without other disadvantageous Consequences or can only be influenced with great effort. These include, for example often ambient temperatures, supply voltages or currents. This Influencing factors hereinafter referred to as the first influencing variables. Other factors are through the measuring system can be influenced easily or with reasonable effort. To counting for example often filter parameters, amplifier bandwidths, measuring times or bias currents. These influencing factors are hereinafter referred to as second influencing variables.

The The present invention is based on the idea of one or more the influenceable second influencing factors are not as in the prior art for a rarely actually the worst case scenario that occurs should be preset while the operation of the measuring system each set so that a predetermined minimum signal-to-noise ratio exceeded or preferred is achieved exactly. The value of the first influencing variable or the values of the first influencing variables are recorded or measured. One or more of the second influencing variables will then become so changed that this Signal / noise ratio of the measuring system the predetermined minimum value of the signal-to-noise ratio reached or exceeded.

On Advantage of the present invention is that it is an unnecessarily high one Effort to achieve an unnecessary high signal-to-noise ratio is avoided. Since the signal / noise ratio achieved with a Energy or power requirement of the measuring system is correlated, results significant energy or performance savings. this in turn has, for example, in a battery-operated measuring system a corresponding extension the life of the battery.

One or more second influencing variables are preferably set on the basis of a (mathematical) model of the signal / noise ratio. Is from this mathematical model a mathematical algorithm, in the simplest case a simple formula or equation, which is carried out in order to determine the value to be set from the recorded values of the influencing variables which cannot be influenced by the measuring system or the values to be set of the influencing variables which can be changed by the measuring system. Alternatively, tuples are stored in a two-dimensional or multidimensional look-up table or look-up table from values of the influencing variables which cannot be influenced by the measuring system and which are to be set, the tuples preferably also being derived or obtained from the mathematical model. Alternatively, the mathematical algorithm or lookup table is determined empirically.

The Influence of Influencing factors on is the signal to noise ratio generally well understood and theoretically described. The use according to the invention a lookup table or a mathematical algorithm particularly simple and reliable for a precise setting of the signal / noise ratio and at the same time to minimize the power requirement of the measuring system.

According to one preferred aspect, the present invention provides a measurement system with the device according to the invention.

According to one Another preferred aspect of the invention provides a computer program with program code for implementation of the method according to the invention, if the program runs on a computer.

preferred Further developments of the present invention are defined in the subclaims.

preferred embodiments of the present invention will hereinafter be described with reference to the accompanying Figures closer explained. Show it:

1 is a schematic representation of a preferred embodiment of the present invention;

2 is a schematic circuit diagram of a measuring system in which the present invention can be used; and

3 is a schematic circuit diagram of another measuring system in which the present invention can be used.

1 is a schematic representation of a preferred embodiment of the present invention. A measuring system 10 is designed and provided to detect a measured variable and to output a signal dependent on the measured variable. The measured variable is, for example, a pressure of a fluid, ie a liquid or a gas, a force, an acceleration, a temperature, a location, a distance, an angle, a voltage, a current, a charge, a frequency, a field strength of an electrical or magnetic field, an intensity of an electromagnetic radiation, a particle flux density etc. The signal generated by the measuring system depending on the measured variable has a signal level and noise with a noise level.

The signal level or the noise level and thus the signal / noise ratio of the measuring system 10 become essential by operating conditions of a section 12 of the measuring system 10 affected. These operating conditions are referred to below as influencing variables since they affect the signal / noise ratio of the measuring system 10 influence. The section 12 stands for the one or those parts of the Meßsy stems 10 , which contribute in particular to the generation of the noise of the measuring system. The section 12 includes, for example, a sensor, a preamplifier, a filter, a main amplifier, a comparator, an analog-to-digital converter, etc.

The signal level or the noise level of the measuring system 10 is influenced by several factors. Some of these influencing variables cannot be adjusted or influenced by the measuring system, or not significantly, or not at all, or not at reasonable expense. These influencing variables are referred to below as first influencing variables. They include, for example, the measuring system 10 Operating or battery voltage supplied from the outside, a temperature of the sensor or a temperature of the preamplifier of the measuring system 10 , which depends essentially on the temperature of the environment of the measuring system 10 depends.

Other Are influencing factors adjustable. These are referred to below as second influencing factors. The adjustable influencing variables include, for example Filter parameters, amplifier bandwidths, measurement times, Bias currents, Bias voltages, Sampling frequencies, the number of individual measurements over which is averaged, etc.

A detection device 14 is with the section 12 of the measuring system 10 connected in order to detect a current value of the influencing variable which cannot be set by the measuring system or current values of the influencing variables which cannot be set by the measuring system. The detection device 14 includes, for example, a temperature sensor that is thermally connected to the section 12 is coupled to the temperature of the section 12 to measure, a temperature sensor that measures the ambient temperature, or a voltage sensor to detect the operating or battery voltage that is supplied to the measuring system. The detection device 14 preferably further comprises devices for amplifying, filtering, processing or digitizing a detection signal generated during the detection of the influencing variable.

An institution 16 is designed and provided to determine a value to be set for a second influencing variable as a function of the detected value of the first influencing variable. According to a variant of the present invention, the device comprises 16 a memory in which a look-up table is stored or stored. A value of the second influencing variable is assigned to a plurality of values of the first influencing variable in the look-up table, and if one of the values of the first influencing variable entered in the look-up table is present, the assigned value of the second influencing variable is to be set. In the case of a limited number of entries, the next largest or the next smaller entry is always selected in such a way that a signal / noise ratio above the desired or predetermined minimum value is set with the given step size.

According to another variant, the device comprises 16 a calculation device which is provided and designed to use a mathematical algorithm to calculate the data generated by the detection device 14 determined value of the first influencing variable to determine a value to be set for the second influencing variable. In the simplest case, the mathematical algorithm is an equation or a formula or a mathematical function, the input variable of which is the detected value of the first influencing variable and the output variable of which is the value to be set of the second influencing variable.

The table entries the lookup table or the mathematical algorithm are preferably off a mathematical model of the signal-to-noise ratio derived or gained, the mathematical model dependency the signal-to-noise ratio from the first influencing factors and the modeled second influencing variables. Alternatively, the entries are the lookup table or the mathematical algorithm empirically certainly.

An adjustment device 18 is provided and designed to by the device 16 set certain value to be set for the second influencing variable. The setting device controls this 18 for example the cut-off frequency, the order or another filter parameter of a filter which is that of the measuring system 10 generates a signal dependent on the measured variable or an intermediate signal from which the signal is obtained is filtered. Another possible second influencing variable, the value of which is determined by the setting device 18 is the bandwidth or the gain of an amplifier. Further examples of second influencing variables are bias currents, voltages, measuring times, the number of measurements, the individual results of which are averaged, etc. If the first influencing variable is temperature, the second influencing variable also includes a supply voltage, an excitation voltage of the sensor or the operating voltage of the measuring system 10 in question.

The detection device 14 , the establishment 16 and the adjustment device 18 are connected to one another by one or more lines or by a bus in order to transmit or exchange the detected value of the first influencing variable and the value to be set for the second influencing variable. The following is an example of how the detection device works 14 , the establishment 16 and the setting device 18 described.

The sensor of the measuring system 10 detects the measured variable in such a way that a maximum signal / noise ratio results under the current operating conditions. In order to ensure with simple means that the signal has the greatest possible amplitude, the operating voltage or the battery voltage that is used by the measuring system is used as the sensor excitation voltage 10 is supplied from the outside, or the sensor excitation voltage is obtained from the operating voltage by a simple circuit, as a result of which it is dependent on the latter.

The measuring system 10 detects the operating voltage as a first one, from the measuring system 10 unchangeable influencing variable. In many systems, in particular in battery-operated systems, the detection of the operating voltage means no additional effort, since it is required or required anyway. The temperature of the measuring system is also optional 10 or from its section 12 as another first influencing variable. This also generally means no additional effort, since the temperature is recorded for temperature compensation or for compensation of a temperature response of the sensor or the measuring system 10 is often mandatory.

Typically, the output voltage of a sensor and thus the signal level are those of the measuring system 10 dependent on the measured variable generated signal proportional to the excitation voltage of the sensor. This applies, for example, to resistive measuring bridges or capacitive sensors. The signal level therefore decreases as the operating voltage decreases. To compensate for this, one is set by the adjuster 18 adjustable second influencing variable is changed so that the influence of the decreasing operating voltage on the signal / noise ratio is compensated or the signal / noise ratio is kept constant or greater than a predetermined minimum value.

With increasing absolute temperature of the measuring system 10 and especially its section 12 the amplitude of the white noise or the noise level also increases. This also reduces the signal-to-noise ratio. Therefore, one is preferred by the setting device 18 adjustable second influencing variable changed so that the influence of temperature on the signal / noise ratio is compensated.

Conversely, when the operating conditions improve, that is to say an increase in the operating voltage or a decrease in the temperature, one is carried out by the adjusting device 18 adjustable influencing variable is changed so that the signal-to-noise ratio does not become unnecessarily high, but assumes the predetermined minimum value or is only slightly above it. This saves power that would otherwise be used for an unnecessarily high sampling rate, an unnecessarily long measuring time, etc.

An advantage of the present invention is that the reference or excitation voltage of the sensor can be selected to a maximum, ie equal to the battery or operating voltage, since it can be accepted that it will decrease with decreasing operating voltage. Another advantage is that the detection of temperature and operating voltage in battery-operated sensor systems or measuring systems are generally provided anyway and can therefore also be used by the present invention. With a large operating voltage and thus a large reference or excitation voltage, the desired signal / noise ratio is achieved with a shorter measuring duration or a low power consumption or power requirement in comparison to a conventional worst case dimensioning. This will extend the life of a battery that the measuring system 10 supplied with power, significantly extended. Likewise, at a low temperature compared to a worst case dimensioning, the desired signal / noise ratio is achieved with a low power consumption or power requirement or a shorter measuring time. Especially in systems with long idle times, during which the temperature of the measuring system 10 is low, the battery life can be significantly extended in this way. Examples of this are applications in cars that only reach high temperatures during operation or in strong sunlight and therefore rarely.

2 Fig. 3 is a schematic circuit diagram of a measurement system in which the present invention can be used. The measuring system comprises a capacitive pressure sensor with two variable capacitances C _S1 , C _S2 . The two capacitances C _S1 , C _{S2 of} the pressure sensor are dependent on a fluid pressure, ie a gas or liquid pressure to which the pressure sensor is exposed. The two variable capacitances C _S1 , C _{S2 of} the pressure sensor form diametrically opposite branches of a bridge circuit 30 , The other two branches of the bridge circuit, which are also diametrically opposite one another 30 are formed by reference capacitors whose reference characters use their unchangeable capacitances C _R1 , C _R2 .

A first knot 32 the bridge circuit 30 between the first reference capacitor C _R1 and the second variable capacitance C _{S2 of} the sensor is via a first switch 34 alternatively connectable with two reference voltages + Vref, –Vref of the same amount and opposite sign. A second knot 36 the bridge circuit 30 between the first reference capacitor C _R1 and the first variable capacitance C _{S1 of} the sensor is via a first coupling capacitor C _DAC1 , the reference _{symbol of} which is again equal to its capacitance, and a second changeover switch 38 connectable either with a positive compensation voltage + Vdac or with a negative compensation voltage –Vdac. The compensation voltages + Vdac, -Vdac have the same amount and opposite sign. She who generated by one or two digital-to-analog converters, not shown. A third knot 40 the bridge circuit 30 between the first variable capacitance C _{S1 of} the sensor and the second reference capacitor C _R2 is via a third switch 42 connectable either with the positive reference voltage + Vref or the negative reference voltage –Vref. A fourth knot 44 is between the second reference capacitor C _R2 and the second variable capacitance C _{S 2} via a second coupling capacitor C _DAC2 and a fourth switch 46 connectable either with the positive compensation voltage + Vdac or the negative compensation voltage –Vdac.

The second knot 36 the bridge circuit 30 and the first coupling capacitor C _DAC1 are also with a non- _inverting input 48 an OPV or differential amplifier 50 connected. The fourth knot 44 the bridge circuit 30 and the second coupling capacitor C _DAC2 are also with an inverting input 52 of the differential amplifier 50 connected. A first exit 54 of the differential amplifier 50 is with a first signal output 56 connected to the measuring system. The first exit 54 of the differential amplifier 50 is also via a first switch 58 with the non-inverting input 48 of the differential amplifier 50 , the second knot 36 the bridge circuit 30 and the first coupling capacitor C _DAC1 connectable. A second exit 60 of the differential amplifier 50 is with a second signal output 62 connected to the measuring system. The second exit 60 of the differential amplifier 50 is also via a second switch 64 with the inverting input 52 of the differential amplifier 50 , the fourth knot 44 the bridge circuit 30 and the second coupling capacitor C _DAC2 connectable.

The first variable capacitance C _{S1 of} the sensor and the capacitance of the first reference capacitor C _R1 are selected so that their difference is as small as possible, and the second variable capacitance C _{S2 of} the sensor and the capacitance of the second reference capacitor C _R2 are selected such that their Difference is as small as possible. The differences between the first variable capacitance C _{S1 of} the sensor and the capacitance of the first reference capacitor C _R1 and between the second variable capacitance C _{S2 of} the sensor and the capacitance of the second reference capacitor C _R2 are dependent on the pressure detected by the sensor.

The switch 34 . 38 . 42 . 46 and the switches 58 . 64 are controlled synchronously and periodically alternately switched by a clock signal. Each clock period of the clock signal consists of a first time period clock1 and a second time period clock1, which together form the clock period. The first switch connects within the first time period (clock1) of each clock period 34 the first knot 32 the bridge circuit 30 with the negative reference voltage –Vref, the second switch 38 connects the first coupling capacitor C _DAC1 with the negative compensation voltage –Vdac, the third switch 42 connects the third node 40 the bridge circuit 30 with the positive reference voltage + Vref, the fourth switch 46 connects the second coupling capacitor C _DAC2 with the positive compensation voltage + Vdac, the first switch 58 connects the first exit 54 of the differential amplifier 50 with the non-inverting input 48 of the differential amplifier 50 , the second knot 36 the bridge circuit 30 and the first coupling capacitor C _DAC1 , and the second switch 64 connects the second output 60 of the differential amplifier 50 with the inverting input 52 of the differential amplifier 50 , the fourth knot 44 the bridge circuit 30 and the second coupling capacitor C _DAC2 . During the second period (clock2) of each clock period, the first switch connects 34 the first knot 32 the bridge circuit 30 with the positive reference voltage + Vref, the second switch 38 connects the first coupling capacitor C _DAC1 with the positive compensation voltage + Vdac, the third switch 42 connects the third node 40 the bridge circuit 30 with the negative reference voltage –Vref, the fourth switch 46 connects the second coupling capacitor C _DAC2 with the negative compensation voltage -Vdac, the first output 54 of the differential amplifier 50 is from the non-inverting input 48 of the differential amplifier 50 , the second knot 36 the bridge circuit 30 and the first coupling capacitor C _DAC1 isolated, and the second output 60 of the differential amplifier 50 is from the inverting input 52 of the differential amplifier 50 , the fourth knot 44 the bridge circuit 30 and the second coupling capacitor C _DAC2 isolated.

The function of the measuring system is first explained below without the compensation voltages ± Vdac. The switching of the first switch by the synchronous and periodically alternating 34 and the third switch 42 generated square wave voltage between the first node 32 and the third knot 40 the bridge circuit 30 created between the second node 36 and the fourth knot 44 an AC voltage, its amplitude and its relative sign related to the voltage between the first node 32 and the third knot 40 the bridge circuit 30 depends on the sign and amount of the difference between the first variable capacitance C _{S1 of} the sensor and the capacitance of the first reference capacitor C _R1 and on the amount and sign of the difference of the second variable capacitance C _S2 and the capacitance of the second reference capacitor C _R2 .

In the first time period clock1 of each clock period, or in the clock phase in which the switches 58 . 64 in the feedback branches of the differential amplifier 50 are closed, is the voltage between the inputs 48 . 52 of the differential amplifier 50 Zero, since the feedback causes a virtual GND or ground potential to be established. In this clock phase, the sensor capacitors or the variable capacitances C _S1 , C _{S1 of} the sensor are charged against this virtual GND node. Only in the second time period clock1 of each clock period or in the clock phase in which the switches 58 . 64 in the feedback branches of the differential amplifier 50 open, a voltage or a potential difference arises at the output 36 . 44 the bridge circuit 30 and thus between the entrances 48 . 52 of the differential amplifier 50 , This voltage is then with the open circuit gain of the differential amplifier 50 strengthened.

This difference becomes apparent during the second period of each clock period in which the switches 58 . 64 are open through the differential amplifier 50 strengthened. Between the signal outputs 56 . 62 The measuring system thus creates a periodically variable voltage, the phase of which is related to that of the changeover switch 34 . 38 . 42 . 46 and the switches 58 . 64 controlling clock signal indicates the sign of the capacitance differences mentioned.

The function of the measuring system is based on the fact that the capacitance differences are more or less compensated for by the compensation voltages ± Vdac coupled in via the coupling capacitors C _DAC1 , C _DAC2 . The compensation voltages required for compensation are proportional to the detuning of the bridge circuit and thus to the measured variable. One with the exits 56 . 62 connected control changes, for example in accordance with the successive approximation method, an approximation number and thus the compensation voltages ± Vdac generated by the digital-analog converter (not shown) until the compensation voltages ± Vdac compensate for the capacitance differences (apart from an inevitable quantization error). The approximation number that is then present (apart from the inevitable quantization error mentioned) is proportional to the pressure applied to the sensor.

_Charge differences on the variable capacitances C _S1 , C _{S2 of} the sensor are compensated for by charges on the coupling capacitors C _DAC1 , C _DAC2 . Therefore, the digital-to-analog converter can alternatively also be implemented by the compensation voltage Vdac remaining constant and the coupling capacitors C _DAC1 , C _DAC2 by capacitors with binary capacitance capacitors that _{can be} connected in parallel, ie with capacitance _ratios 1: 2: 4: 8: ... : 2 ^{n are} formed. The sums of the capacitors connected in parallel then form programmable or variable capacitances of the coupling capacitors C _DAC1 , C _DAC2 .

In the 2 Circuit shown together with the digital-to-analog converter, not shown, and the control, also not shown, is part of a tire pressure measuring system, for example. The changes in the variable capacitances C _S1 , C _S2 of the sensor generated by pressure changes within the measuring range of the sensor are generally very small compared to the values of the variable capacitances C _S1 , C _S2 averaged over the measuring range or relative to the capacitances of the reference capacitors C _R1 , C _R2 . The noise bandwidth of the in 2 The circuit shown is therefore generally determined by the capacitors of the measuring bridge or their average capacitance C ₀ . The signal / noise ratio SNR _ADC measured in dB or the logarithmic signal / noise ratio of the in 2 Analog-digital converter shown is approximately

Here, k is the Boltzmann constant, T the absolute temperature of the measuring system, ΔC the change in the variable capacitances C _S1 , C _{S2 of} the sensor due to the measured variable (here: due to the pressure) and C ₀ the sum of the quantity not dependent on the measured variable or unchangeable portion of a variable capacitance C _S1 or C _{S2 of} the sensor and the (likewise unchangeable) capacitance of the respective reference capacitor C _R1 or C _R2 .

In the case of a tire pressure sensor, measurements are made at very long time intervals (for example every 0.5 s) in comparison to the possible sampling rate of the analog-digital converter. A measurement typically takes less than 1 ms. In order to reduce the noise level, the measurement of the tire pressure can therefore be repeated several times at each sampling time and an average of the results of the repeated measurements can be formed. The signal-to-noise ratio SNR _{averaged for} the averaged measurement depends on the number N of measurements whose individual results are averaged as follows:

This equation can be solved for the number N of averaged measurements,

If the detection device 14 If the reference voltage V _ref and the temperature T are detected, the number N of averaged individual measurements can thus be set for each measurement point in such a way that the _averaged signal-to-noise ratio SNR is always constant or always (at least) assumes a predetermined minimum value SNR _should ,

This equation shows that under favorable conditions (high reference voltage V _ref and low temperature T) the number N of the averaged individual measurements is small. The number N of averaged individual measurements only assumes a high value under unfavorable conditions (low reference voltage V _ref and high temperature T). In contrast, in conventional worst-case dimensioning, the number N of averaged individual measurements would always have the high value, even under favorable conditions, which, according to the present case, they actually only have in the worst case, a minimum reference voltage V _ref and a maximum temperature T. having.

On conventional dimensioned measuring system would therefore take far too many measurements and therefore unnecessary have high power requirements. If you choose the reference voltage Vref depends on the operating voltage, there is a worst Case and thus the maximum number N of the averaged individual measurements only at the minimum operating voltage and a maximum temperature. The minimum operating voltage only occurs at the end of the service life the battery on and therefore only during a small proportion of the operating time of the measuring system. Also for the temperature can be assumed that this over the Lifetime of a tire pressure measurement system rarely reaches the maximum (worst case). From the last equation it can be seen that the Number N of the averaged individual measurements for operating voltages above the minimum operating voltage, and for temperatures below the maximum temperature, exponentially decreases. From the reduction the number N of averaged individual measurements is followed by a reduction the number N of the averaged individual measurements and a corresponding one Energy savings.

3 is a schematic circuit diagram of a measurement system according to another embodiment of the present invention. This measuring system has a bridge circuit 80 from four bridge impedances 82 . 84 . 86 . 88 on, of which at least one component of a sensor and is variable by a measured variable detected by the sensor. The bridge circuit 80 has four nodes 92 . 94 . 96 . 98 on. The first knot 92 is connected to a supply voltage Vdd, the third node 96 is connected to ground Gnd. An analog-to-digital converter (ADC; ADC = analog-to-digital converter) 100 detects a voltage between the second node 94 and the fourth knot 98 the bridge circuit 80 , There is a first entrance 102 of the analog-digital converter 100 with the second knot 94 the bridge circuit 80 connected and a second input 104 with the fourth knot 98 the bridge circuit 80 connected. The analog-to-digital converter converts the voltage between the second node 94 and the fourth knot 98 the bridge circuit 80 into a digital signal or a number represented by the digital signal. The analog-digital converter gives this number 100 at its exits 106 . 108 out. The exits 106 . 108 of the analog-digital converter 100 are with a digital filter 110 or with its inputs 112 . 114 connected.

The measuring system 3 carries out several individual measurements at a single measurement time, ie the analog-digital converter 100 converts the tension between the second node several times in succession 94 and the fourth knot 98 the bridge circuit 80 into digital signals. Because at least one of the bridge impedances 82 . 84 . 86 . 88 in the bridge circuit 80 The detuning of the bridge circuit also depends on the measured variable of the sensor 80 and thus the tension between the second node 94 and the fourth knot 98 and with that from the analog-digital converter 100 at its exits 106 . 108 generated digital signal from the measured variable of the sensor. The filter 110 filters the majority of them from the analog-to-digital converter in succession 100 generated digital signals, for example by means of averaging dung.

The sampling rate or sample frequency fs of the analog-digital converter 100 and the cut-off frequency fg (corresponding, for example, to a time period within which the digital signal of the analog-digital converter 100 can be averaged) and are therefore second influencing variables that can be set by the measuring system in the sense of the present invention.

The bridge impedances 82 . 84 . 86 . 88 the bridge circuit 80 out 3 are preferably resistive elements or resistors. In this case, as shown, there will be a constant voltage between the first node 92 and the third knot 96 the bridge circuit 80 applied to a constant voltage between the second node which is dependent on this at a constant measured variable 94 and the fourth knot 98 the bridge circuit 80 to obtain. Alternatively, the bridge impedances 82 . 84 . 86 . 88 capacitive components or capacitors. In this case, it is preferably between the first node 92 and the third knot 96 the bridge circuit 80 like the one in 2 illustrated embodiment, an AC voltage is applied to between the second node 94 and the fourth knot 98 the bridge circuit 80 to obtain an alternating voltage, its amplitude and relative phase position in relation to the alternating voltage between the first node 92 and the third knot 96 the detuning of the bridge circuit 80 and thus represents the value of the measured variable.

Alternatively, the bridge impedances 82 . 84 . 86 . 88 inductive components or coils. In this case, the first node is preferred 92 and the third knot 96 the bridge circuit 80 an alternating current is applied or driven, which is an alternating voltage between the second node 94 and the fourth knot 98 causes the amplitude and phase of the detuning of the bridge circuit 80 and thus indicates the current value of the measured variable.

Also for that in 3 Embodiment shown applies that the measurement signal from the excitation voltage of the bridge circuit 80 depends. Furthermore, the signal / noise ratio of the measuring system is dependent on the excitation voltage and the temperature. These influences are compensated for, for example, by changing the ratio of the sampling rate to the filter cutoff frequency in order to achieve a signal / noise ratio that is independent of the operating conditions.

10

measuring system

12

section

14

detector

16

Facility

18

adjustment

CS1

first variable capacity of the sensor

CS2

second variable capacity of the sensor

30

bridge circuit

C _R1

first reference capacitor

C _R2

second reference capacitor

32

first node

34

first switch

36

second node

C _DAC1

first coupling capacitor

38

second switch

40

third node

42

third switch

44

fourth node

C _DAC2

second coupling capacitor

46

fourth switch

48

inverting entrance

50

differential amplifier

52

inverting entrance

54

first output of the differential amplifier 50

56

first Signal output of the measuring system

58

first switch

60

second output of the differential amplifier 50

62

second Signal output of the measuring system

64

second switch

80

bridge circuit

82

bridge impedance

84

bridge impedance

86

bridge impedance

88

bridge impedance

92

node

94

node

96

node

98

node

100

Analog to digital converter

102

first input of the analog-digital converter 100

104

second input of the analog-digital converter 100

106

first output of the analog-digital converter 100

108

second output of the analog-digital converter 100

110

filter

112

first input of the filter 110

114

second input of the filter 110

Claims (21)

Device for setting the signal / noise ratio between a signal level and a noise level of a measurement signal in a measurement system ( 10 ) for detecting a measured variable and outputting the measurement signal dependent on the measured variable, the signal / noise ratio depending on a first influencing variable and a second influencing variable, with the following features: a detection device ( 14 ) for detecting a value of the first influencing variable; and an adjustment device ( 18 ) for setting a value of the second influencing variable depending on the value of the first influencing variable, thereby setting the signal / noise ratio. Device according to Claim 1, in which the adjusting device ( 18 ) is designed to set the value of the second influencing variable in such a way that the signal / noise ratio assumes or exceeds a predetermined value. Apparatus according to claim 1 or 2, further comprising: a memory ( 16 ) for storing a look-up table which in each case assigns a value of the second influencing variable to a plurality of values of the first influencing variable, the setting device ( 18 ) is also designed to access the memory in order to read from the look-up table the value of the second influencing variable which corresponds to that obtained by the detection device ( 14 ) value is assigned to the first influencing variable, and by the value from the look-up table ( 16 ) set the read value of the second influencing variable. Apparatus according to claim 1 or 2, further comprising a calculation device ( 16 ) for calculating the by the setting device ( 18 ) value of the second influencing variable to be set from the value determined by the detection device ( 14 ) recorded value of the first influencing variable by means of a mathematical algorithm. The apparatus of claim 4, wherein the mathematical Algorithm on a mathematical model of dependency the signal-to-noise ratio from the first influencing variable and from is based on the second influencing variable. Device according to one of claims 1 to 5, wherein the first Influencing variable one through the measuring system is not adjustable size. Device according to one of Claims 1 to 6, in which the first influencing variable is a supply voltage or a temperature of the measuring system ( 10 ), the detection device ( 14 ) a device for detecting the supply voltage or the temperature of the measuring system ( 10 ) is. Device according to one of Claims 1 to 7, in which the second influencing variable is a filter parameter of a filter ( 110 ) for filtering the signal dependent on the measured variable, a measuring time during which the measuring system ( 10 ) the measured variable is recorded, a number of recordings of the measured variable, via which the measuring system ( 10 ) averaged, or a sampling rate for detecting the measured variable, the setting device ( 18 ) is a device for setting the supply voltage, the filter parameter, the measuring time, the number of acquisitions or the sampling rate. Device according to one of claims 1 to 8, wherein the measuring system ( 10 ) capacitive or resistive or inductive sensor components or a capacitive or a resistive or an inductive measuring bridge ( 30 . 80 ) which has an impedance component (C _S1 , C _S2 , 82 . 84 . 86 . 88 ), whose impedance depends on the measured variable. Measuring system ( 10 ) with a device according to one of claims 1 to 9. Measuring system ( 10 ) according to claim 10, wherein the measuring system ( 10 ) is a tire pressure measuring system for measuring an air pressure in a tire of a vehicle. Measuring system ( 10 ) according to claim 11, wherein the first influencing variable is the operating voltage or the operating temperature, and wherein the second influencing variable is a number N of individual measurements over which the averaging takes place. Method for setting the signal / noise ratio between a signal level and a noise level of a measuring signal in a measuring system ( 10 ), which detects a measured variable and generates the measurement signal dependent on the measured variable, the signal / noise ratio depending on a first influencing variable and a second influencing variable, with the following steps: detecting a value of the first influencing variable; and setting a value of the second influencing variable depending on the value of the first influencing variable, to thereby set the signal / noise ratio. The method of claim 13, wherein the step setting one step of setting the value of the second Influencing variable such that this Signal / noise ratio assumes or exceeds a predetermined value. The method of claim 13 or 14, further comprising a Reading step to be set in the setting step Value of the second influencing variable from a Look-up table, each of a plurality of values of the first influencing variable assigns a value to the second influencing variable. The method of claim 13 or 14, further comprising a Computing step to be set in the setting step Value of the second influencing variable from the value captured at the step of capturing the first influencing variable by means of of a mathematical algorithm. Method according to one of claims 13 to 16, wherein the Noise level of the measurement signal from the first influencing variable or depends on the second influencing variable. Method according to one of claims 13 to 17, wherein the Signal level of the measurement signal further from the first influencing variable or depends on the second influencing variable. Method according to one of claims 13 to 18, wherein the Signal / noise ratio also depends on a third influencing variable. Method of adjusting the signal-to-noise ratio a tire pressure measuring system to one of the operating voltage and operating temperature of the tire pressure measuring system independent The predetermined value with a method according to claim 19, wherein the Measured variable of tire pressure is, the first influencing variable is the operating voltage is, the second influencing variable is a Number N of individual measurements over which the averaging is carried out and the third influencing variable is the operating temperature is. Computer program with program code for carrying out the Procedure according to a of claims 13 to 20 if the program runs on a computer.