EP1917504A1

EP1917504A1 - Hot-film type air-flow sensor that uses frequency modulated signal detection

Info

Publication number: EP1917504A1
Application number: EP06763655A
Authority: EP
Inventors: Bernhard Opitz; Ulrich Wagner; Carsten Raudzis; Axel Franke
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2005-08-16
Filing date: 2006-06-13
Publication date: 2008-05-07
Also published as: JP2009505086A; US20090314079A1; US7861586B2; JP4934137B2; WO2007020115A1; DE102005038597A1

Abstract

The invention relates to a method for measuring an air flow that flows in a principal flow direction (120), in addition to a hot-film type air-flow sensor (110), which can be used to carry out said method. The method and the hot-film type air-flow sensor (110) are particularly suitable for use in the induction tract of an internal combustion engine. The hot-film air-flow sensor (110) comprises a sensor chip (112) with a chip surface, over which the air mass flows. Said chip surface has in turn a measuring surface (118) with a central air-flow sensor circuit (122) comprising at least one central heating element (124) and at least two temperature probes (126, 128). According to the method, the central heating element(s) (124) is or are periodically heated using a frequency ?. At least two measuring signals (142, 144) are detected by the temperature probes (126, 128). The measuring signals (142, 144) and/or at least one differential signal between the two measuring signals (142, 144) is or are demodulated using the frequency ?.

Description

Hcißfilmluftmasscnmcsscr with frcqucnzmodulicrtcr signal acquisition

The invention relates to a hot film air mass meter for measuring an air mass flow flowing with a main flow direction using a frequency-modulated signal detection and a method for operating such a Heißfllmlufhnassenmessers. Such Heißfiluuftmassenmesser be used in particular in the intake of an internal combustion engine.

State of the art

In many processes, for example in the field of process engineering, chemistry or mechanical engineering, defined gas mass, in particular an air mass, must be supplied. These include, in particular, combustion processes which take place under controlled conditions. An important example is the combustion of fuel in internal combustion engines of motor vehicles, in particular with subsequent catalytic emission control. Various types of sensors are used to measure the air mass flow rate.

A sensor type known from the prior art is the so-called hot film air mass meter (HFM), which is described for example in DE 196 01 791 A1 in one embodiment. Such a hot-film air mass meter usually uses a sensor chip which has a thin sensor membrane, for example a silicon sensor chip. On the sensor membrane is typically arranged at least one heating resistor, which is surrounded by two or more temperature measuring resistors (temperature sensors). In an air flow, which is guided over the membrane, the temperature distribution changes, which in turn can be detected by the temperature measuring resistors and can be evaluated by means of a control and evaluation circuit. So, for example, from a resistance difference the temperature measuring resistors, an air mass flow can be determined. Various other variants of this type of sensor are known in the art.

A problem of this type of sensor, which is known for example from DE 101 11 840 C2, is that contaminations of the sensor type can frequently occur, for example contamination by oil, other liquids or other types of contaminants. The sensor chip is usually used directly in the intake tract of the internal combustion engine or in a bypass to the intake tract of the internal combustion engine. During operation of the internal combustion engine, oil may be deposited on the sensor chip and, in particular, on the sensor membrane. This oil precipitate can lead to an undesired influence of the measurement signal of the sensor chip, in particular since an oil film acts on the surface of the sensor chip on the thermal conductivity of the surface, which leads to the falsification of the measurement signals. The oil contamination can also occur during or shortly after switching off the internal combustion engine, for example a diesel engine. This is the case in particular when, after switching off the internal combustion engine, an overpressure present in a crankcase is reduced via a crankcase ventilation into the intake tract of the internal combustion engine (and thus, for example, into the bypass duct of the hot film air mass meter). In this case, oil vapor or oil mist is often carried. DE 101 11 840 C2 therefore proposes a method for preventing soiling on a sensor chip using an auxiliary heater. The sensor chip has a sensor area and an additional heater arranged outside the sensor area. This additional heater is electrically heated so that in the area of the additional heater thermal gradient vortices occur, which lead to precipitation of the contaminants of the flowing medium in the region of the additional heater away from the region of the sensor area.

However, the arrangement disclosed in DE 101 11 840 C2 and the disclosed method are in practice associated with various operating modes of the internal combustion engine with disadvantages. A disadvantage, for example, is that a localization of the Thermogradientenwirbel, which is intended by the device disclosed in DE 101 11 840 C2, in practice is hardly possible. Due to the high thermal conductivity of the silicon, the heat generated by the additional heater spreads easily over the entire chip, resulting in a "Smeared" temperature distribution and thus leads to a Auflieizung the entire chip.

The problem of contamination of the membrane or sensor surface is exacerbated by thermodynamic effects. Thus, it is known that drops of liquid which have a gradient in their surface tension experience a force in the direction of the higher surface tension. This usually results in a movement of the drop from a low to a high surface tension. In particular, this gradient can be caused by a temperature gradient on a surface on which the liquid droplet is applied. The droplet is usually moved by the temperature gradient from a warmer area of the surface to a colder area of the surface. This effect is z. Levich, "Physicochemical Hydrodynamics", Prentice-Hall, N.J., 1962, p.373 and p.380.

As described above, typical hot film air mass meters are constructed to include a sensor membrane (eg, a silicon membrane) of low thermal conductivity and a surrounding chip land. During operation of the hot-film air mass meter, therefore, a temperature gradient and, accordingly, a liquid wall usually builds up at the edges of the sensor membrane, ie at the boundary to the surrounding chip mainland. Due to the air flow, this liquid wall can be entrained in whole or in part, so that oil droplets can reach the sensor membrane and influence the measurement there. Furthermore, the liquid wall causes an increase in the thermal conductivity at the edge of the sensor membrane, which can lead to a distortion and drift of the measurement signal.

Advantages of the invention

A hot-film air mass meter is therefore proposed for measuring an air mass flow flowing in a main flow direction, in particular in the intake tract of an internal combustion engine, and a method for operating such a hot-film air mass meter which avoids the disadvantages of the devices and methods known from the prior art. - A -

The hot-film air mass meter has a sensor chip with a chip surface that can be overflowed by the air mass flow. A "main flow direction" may be understood to mean, for example, a main transport direction of the air mass flow at the location of the hot air mass meter, for example at the location of a sensor chip For example, as described above, the sensor chip may be a silicon chip, the chip surface in turn has a measurement surface and a mainland surface, whereby this division of the chip surface should in particular be effected in such a way that that the sensor chip has at least an order of magnitude lower transverse conductivity in the area of the measuring surface than in the area of the mainland surface .This reduction of the transverse conductivity can be achieved in various ways e known from the prior art and described above, sensor chips are used with a sensor membrane, which has a thickness of only a few microns. In this case, the low thermal conductivity (about 0.02 W / m K) of the air surrounding the sensor membrane is utilized. Alternatively, one or more porous regions may be formed in the sensor chip as a measuring region with a measuring surface facing the air mass flow, for example by a porosification of a silicon chip. In this way, measuring ranges can be produced which have transversal conductivities of 0.1 to 2 W / m K, due to the enclosed air cavities, compared to a silicon substrate with a thermal conductivity of about 156 W / m K.

On the measuring surface of the hot-film air mass meter, a central Heißfileinuftmassenmersserschaltung is applied. By "applied" is meant an application directly to the measurement surface or, alternatively or additionally, an application below the surface overflowed by the air mass flow, the hot film air mass sensor circuit being completely or partially covered by an additional layer, for example For example, the at least one central heating element and the at least two temperature sensors may be designed as strip conductors or conductor loops Other configurations are also possible and are known to the person skilled in the art The outer dimensions of the central hot-film air mass meter circuit may be, for example, a transmitter Define the sensor area on the measurement surface. Thus, the measurement surface may be divided into a sensor area and an area outside the sensor area.

According to the invention, the hot-film air mass meter is operated according to the method described below, wherein the hot-film air mass meter has the additional devices required in addition to carrying out the method according to the invention in one of its embodiments. The described method steps do not necessarily have to be performed in the order shown. Furthermore, additional, not shown, process steps can be performed. Also, process steps can be repeated or parallel in time.

A basic idea of the present invention is to spatially separate the above-described thermal displacement effects of soiling as a result of heating the measuring surface (approximately temporally approximately constant) from the actual measuring of the air mass flow. In this case, a periodic heating by means of the central heating element is used for measuring the air mass flow, wherein the spatial heating due to the thermal inertia of the measuring surface does not extend into the region of contamination, but is essentially limited to a central region within the measuring surface. For the evaluation of the measurement, a frequency modulation method is used, for example a modified "lock-in method", which additionally minimizes environmental influences, for example the above-described interference by impurities, on the signal detection Thus, a thermal, "DC" component (temporally approximately constant) to displace the contaminants and a thermal "AC" component (periodic) can be used to measure the air mass flow.

The basic idea of this modulation method is thus to periodically design the thermal stimulation by the at least one central heating element and to take into account only the portions which lie in the frequency range of the periodic stimulation in the subsequent measurement signal detection in the frequency spectrum. In this way, noise, such as noise or thermal drift, largely "hidden." To this end the at least one central heating element with a frequency ω periodically heated. For example, this can take place in that the at least one central heating element has at least one heating resistor, wherein the periodic heating takes place by applying a voltage with frequency ω / 2 to the at least one heating resistor. Since the heating power is proportional to the square of the voltage, this results in a periodic heating with a frequency ω.

In order to limit the periodic heating by the central heating element to an ambient region of the central heating element, it has proven to be advantageous to heat the frequency in a range of 100 Hz to 100 kHz and more preferably in a range between 100 Hz and 10 kHz to choose. The typical materials used for sensor chips (eg, silicon) and typical dimensions of the measurement surface (eg, having a width perpendicular to the main flow direction of 300-800 microns, preferably about 450 microns, with approximately central arrangement of the central heating element) causes this choice the frequency that the temperature increase by the periodic heating does not extend to the edge region of the measuring surface. Thermal signals in this frequency range are not or only poorly propagated in the chip materials in this frequency range or are strongly attenuated. Because of this damping, the spatial temperature profile of such a periodic heating therefore decreases exponentially, for example, with the distance from the thermal exciter (that is to say the at least one central heating element). The exact course of this decay depends heavily on the chip structure used, which is usually a complex layer structure.

The fact that the temperature increase due to the periodic heating does not extend or only weakly extends to the edge area of the measuring surface has two decisive advantages: First, the temperature profile of the periodic heating used for the measurement of the air mass flow is not superimposed or marginally superimposed the air flow caused by contaminants accumulated there, which typically have an extent of approximately 100 m parallel to the main flow direction (ie into the region of the measurement surface). Thus, the actual measurement actually detects substantially the main flow of the air mass flow, without significant influence by turbulence caused by impurities. On the other hand, the temperature profile of the periodic heating used to measure the air mass flow tion is not or only insignificantly influenced by changes in the thermal conductivity of the measuring surface in the edge region of the measuring surface, which are also caused by the impurities occurring there. Both fluidically and thermally, the heating by the at least one central heating element is thus largely decoupled from the disturbances by impurities occurring in the edge region of the measuring surface.

This is in contrast to the known from the prior art methods and devices in which usually the measurement of the air mass flow in the middle of the measuring surface with a uniform heating, for example, at 0 Hz occurs. Such low-frequency or temporally constant heaters cause the temperature profile to propagate into the edge region of the measuring surface where it is influenced by changes in the thermal conductivity and by flow turbulence. Both effects are integral for the entire measurement surface at 0 Hz.

At least two measuring signals are recorded by means of the at least two temperature sensors. As described above, these at least two temperature sensors can be, for example, at least two temperature measuring resistors. Preferably, at least one of these at least two temperature sensors upstream of the at least one central heating element with respect to the main flow direction of the air mass flow and at least a second of the at least two temperature sensor downstream of the at least one central heating element with respect to the main flow direction. The measurement signals are "raw signals" which are usually superimposed with disturbances, for example disturbances due to temperature drift or noise.Thus, an immediate evaluation of these at least two measurement signals, for example by a subtraction, is generally associated with errors according to the basic idea of the Er Therefore, at least two measuring signals are now demodulated with the frequency ω, ie the same frequency with which the periodic heating of the at least one central heating element takes place This means that the measuring signals are demodulated directly or that, alternatively or additionally, at least one difference is first formed from these measuring signals, which is subsequently demodulated, it being understood that under "Signals" here nic It is necessary to understand immediately the corresponding signals, but also that For example, a gain, a multiplication by a factor, a smoothing or the like can take place. For example, instead of a difference signal, a signal proportional to the difference signal can also be used or generated.

A "demodulation" is to be understood as a method in which only the signals within the frequency range of the "excitation", in this case the periodic heating of the at least one central heating element, are detected and evaluated. The term "at the frequency ω" does not necessarily mean that a frequency range of "zero width" is used, but a frequency range of finite width may be used, the width of the frequency range depending on the experimental setup. Furthermore, in addition to a demodulation with a frequency which exactly corresponds to the excitation frequency of the heating, also an analog demodulation using higher harmonics can be used. Such methods are those skilled in other fields of technology, for. As the high frequency technology, known. The demodulation can take place, for example, by mathematically multiplying the signal or signals to be demodulated by a periodic signal having a frequency corresponding to a decoding frequency. In the case of this multiplication, the periodic measuring signal produces time-constant, ie non-periodic, portions as well as higher-frequency components In this multiplication, the other frequency range is converted exclusively into high-frequency components, but not into non-periodic components, so that the actual measurement signal can be filtered out using a low-pass filter, wherein higher-frequency components are discarded.

The demodulated measurement signals thus generated and / or the at least one demodulated difference signal can now be used for the evaluation, that is, for example, for calculating an air mass flow. The described method and the device can additionally be developed according to the invention advantageously. For example, additionally at least one sum signal may be formed, wherein the at least one sum signal has at least one sum of the at least two demodulated measurement signals and / or at least one demodulated sum of the at least two measurement signals. In turn, therefore, it is optionally first possible to generate a summation and then a demodulation or, first of all, a demodulation and closing a summation done. It is particularly advantageous if this at least one sum signal is used to control the at least one central heating element. Thus, for example, the heating of the at least one central heating element can be regulated in such a way that the at least one sum signal is essentially constant over time. For example, this can be done by regulating the amplitude of the heating, for example by regulating the amplitude of a periodic heating voltage. By "substantially constant over time" it can be understood, for example, that a time deviation of not more than 10%, preferably not more than 7%, and particularly preferably a control deviation of less than 1%, is still tolerable.

The described demodulation method alone already has a high insensitivity to interference. An improvement of the usefulness of the method described above has proven to be a further development of the method in which the demodulation method described above is additionally supported by additionally controlling the temperature of the measuring surface in the region of the central hot-air mass flow meter circuit. For this purpose, the hot-film air mass meter may additionally comprise at least one tempering element, which is operated according to the invention such that the sensor chip is maintained in the region of the central hot-film air mass meter circuit during operation of the hot-film air-mass meter at a substantially constant basic temperature profile. As described above, this time-constant basic temperature profile forms the "DC" component of the thermal heating of the measurement surface, which ensures that the contaminants are displaced into the edge region of the measurement surface only low-frequency changes in the basic temperature profile occur. The deviations from this basic temperature profile over the time of operation of the Heißfileinuftmassenmessers should be, for example, not more than 20%, preferably not more than 10%. This "basic temperature profile" is preferably exclusively due to the at least one tempering element, wherein preferably not caused by the at least one central heating element caused periodic temperature fluctuations or only slightly in the basic temperature profile.

For example, the additionally at least one tempering element may have a Peltier element or preferably at least one additional heating resistor. The at least one Temperierelement is controlled with a corresponding control circuit such that the substantially constant Grundtemperaturprofll is maintained, which, as described above, by the periodic heating of at least one central heating element caused periodic temperature fluctuations in the regulation of the basic temperature profile should remain substantially unaccounted for. This can be done, for example, by using one or more of the at least two measuring signals of the at least two temperature sensors and / or at least one additional measuring signal of at least one control temperature sensor to control the basic temperature profile, ie for controlling the at least one temperature control element. These measuring signals can, in order to "hide" the influence of the periodic heating of the at least one central heating element, for example, be filtered by a low pass having a cutoff frequency below the heating frequency of the at least one central heating element and / or half of this heating frequency (ω / 2) By means of this at least one tempering element, it is additionally possible to reduce disturbing influences by impurities, for example by deposition of an oil film, in the region of the central hot-film air mass meter circuit, in particular to ensure that the "temperature background" (ie the basic temperature profile) is constant over time and exhibits slight fluctuations in the frequency domain of the demodulation method described above.

In principle, an arbitrary local course of the essentially constant basic temperature profile can be used. In order to simplify the above-described formation of differential signals (ie in particular to simplify the electronic circuit which is required to form the differential signals), the basic temperature profile in the region of the central hot-film air mass meter circuit can, for example, have a substantially constant temperature profile. In this case, "substantially constant" is to be understood as meaning that the temperature profile between the at least two temperature sensors used for the demodulation method does not change or changes only insignificantly, for example by no more than 20% set a "temperature plateau" between the at least two temperature sensors. The measuring signals of these at least two temperature sensors have due to this "plateau" a common "offset", which disappears when forming a difference. The disadvantage of a precisely flat "plateau", however, is that impurities occurring in the area of this plateau no longer experience any thermogradient force and thus are not displaced from this area, and it has therefore proven advantageous, albeit in the area of this "plateau" slight temperature gradient (for example 10 K / 100 microns), which displaces impurities to the edge region of the measuring surface. At the plateau area, stronger temperature gradients can then be added in the edge area of the measuring surface in order to effectively keep impurities away from the plateau.

In this case, the basic temperature profile is preferably set in such a way that the temperature in the region of the basic temperature profile (edge regions outside of the central hot-film air mass meter circuit can be disregarded) is above the ambient temperature. In the case of the above-described chip dimensions with a typical width of the measuring range of about 450 μm, it has proved to be advantageous if the temperature in the region of the basic temperature profile (or in the region of the central hot-film air mass meter circuit) is at least 40 K, preferably 80 K, and especially preferably at least 120 K above ambient temperature. For typical edge area widths (where edge areas are those areas where contamination-induced flow turbulence and contamination-related changes in thermal conductivity occur) of about 150 μm, this means preferred average temperature gradients of at least about 0.2-0.3 K / μm, advantageously at least about 0.5 K / μm and more preferably at least about 0.8 K / μm. The ambient temperature may be understood to mean, for example, a room temperature or preferably an operating temperature which, depending on the area of application of the hot-film air mass meter, may be subject to considerable fluctuations with respect to the room temperature.

This choice of basic temperature for an "over-temperature" compared to the ambient temperature has numerous advantages, for example, minimizing interference from ambient temperature fluctuations, but one major advantage is the "barrier effect" already described above: a "temperature ramp" is created around the central hot-film air mass meter circuit. Due to the thermogradient effect, this prevents impurities, in particular oil, from reaching the area of the hot-film air mass meter circuit the Heißfileinuftmassenmessers and the method described thus effectively protected against interference.

The described hot-film air mass meter and the described method in one of the described embodiments have numerous further advantages over conventional devices and methods in addition to the already described advantages. In particular, no absolute values of the temperature profile are detected, but only relative changes within a separately controlled temperature plateau. Contamination deposits outside this range therefore have no influence on the measuring signal. Furthermore, the hot-film air mass meter can be made insensitive to a resistance drift of the printed conductors by normalizing the output signal to the sum of the input voltages (sum signal). Another advantage is that the sensor chip of the Heißfileinuftmassenmessers can be made comparatively small and space-saving. In this way, the minimum required area for the measurement, that is, for example, the measuring surface or the sensor area, can be greatly reduced in comparison with the prior art. This is due in particular to the fact that, as described above, a "temperature ramp" can be built up around the measurement surface or around the sensor area due to the essentially constant basic temperature profile, so that contaminants arranged close to the conductor tracks of the central hot-film air mass meter circuit are effectively taken up by the central hot-film air mass meter circuit can be kept away.

drawing

Reference to the drawings, the invention will be explained in more detail below.

It shows:

Figure 1 shows a first embodiment of a Heißfileinuftmassenmessers;

FIG. 2A shows a temperature profile along the line A-A in FIG. 1 at flow velocity 0; and

FIG. 2B shows the temperature profile according to FIG. 2A at flow velocity> 0. embodiments

FIG. 1 schematically shows a preferred exemplary embodiment of a hot-film air knife 110 according to the invention. The hot-film air mass meter 110 has a sensor chip 112 (only partially shown) and a (also only partially shown)

Control and evaluation circuit 114 on. The sensor chip 112 can be used, for example, in the intake tract of a combustion machine or in a bypass channel to the intake tract of a combustion machine. Such devices are known for example from DE 196 01 791 Al. The sensor chip according to the embodiment in FIG. 1 has a chip mainland with a mainland surface 116 in the plane of the drawing (only partially shown). In this embodiment, it is assumed that the sensor chip 112 is a silicon sensor chip. Furthermore, the sensor chip 112 has a measuring area with a measuring surface 118 in the drawing plane. The measuring surface 118 is configured in this embodiment in the form of a rectangle whose longer sides are arranged perpendicular to a main flow direction 120 of an air mass flow. The sensor chip 112 has a thermal conductivity in the region of the measurement surface 118 which is 0.5 to 2 W / m K, compared to the surrounding mainland with 156 W / m K.

Conductor tracks of a central hot-film air mass meter circuit 122 are arranged in the region of the measurement surface 118. This central Heißfileinuftmassenmesserschaltung 122 is composed in this embodiment of a central heating element 124 and two temperature sensors 126, 128 together. Here, the central heating element 124 is designated in this embodiment with R _h3 and designed as a conductor loop. Alternatively, the central heating element 124 may also be designed as a line heating element. The temperature sensors 126, 128 are designed as line elements and designated R _M and R _h ß. In this case, the temperature sensor 126 with respect to the main flow direction 122 to the central heating element 124 is located "upstream", the temperature sensor 128 "downstream". The temperature sensors 126, 128 are arranged so that they are arranged closely adjacent to a conductor track of the central heating element 124. The central hot-film air mass meter circuit 122 covers a sensor area 130 of the measurement surface 118. It can be seen that the sensor area upstream and downstream does not fully exhaust the measurement surface 118. Outside the sensor region 130, two additional heating elements 132, 134 are arranged parallel to the conductor tracks of the central hot-film air mass meter circuit 122 and perpendicular to the main flow direction 120. These additional heating elements 132, 134 constitute tempering elements, by means of which a substantially constant basic temperature profile can be set in the sensor region 130 of the measuring surface 118. These additional heating elements 132, 134 are also designated in FIG. 1 with Ru and R _h2 . Furthermore, an ambient temperature sensor 136, which is denoted by Ri _f in FIG. 1, is provided in the hot-film air mass meter 110. This ambient temperature sensor 136 can preferably be configured on the sensor chip 112 or, alternatively or additionally, as a sensor in the area of the control and evaluation circuit 114.

During operation of the hot film air mass meter 110, the additional heating elements 132, 134 are heated by means of control circuits 138, 140 to an excess temperature T ^* relative to the ambient temperature Tu. For the control circuits 138, 140 serve the measurement signals 142, 144 of the temperature sensors 126 and 128 as controlled variables. For example, the control circuits 138, 140 may be PI controllers, e.g. B. PI controller with a band-stop filter. Also, additional electronic elements may be provided, such as power sources and / or amplifiers. The effect of the temperature control according to FIG. 1 is shown in FIGS. 2A and 2B. 2A and 2B show a temperature profile along the section line AA in Figure 1. Figure 2A shows the case in which the air mass above the sensor chip 112 rests (air mass velocity 0, insofar the term "main flow direction" 128 is to be understood symbolically here). In contrast, FIG. 2B shows the case of a flow velocity> 0.

As can be seen from the comparison of FIGS. 2A and 2B, the heating temperatures T ^* at the location of the additional heating elements 132, 134 coincide with vanishing speed only in the case of an air mass flow. At finite flow velocity, however, the additional heating elements 132, 134 are heated differently to compensate for the removal by the air mass flow and to keep the temperature at the location of the temperature sensor 126, 128 constant at the value T _P. At an ambient temperature Tu of approximately 20 ^° C., the temperature T _P is usually heated to a value of approximately 150 ^° C. by means of the additional heating elements 132, 134. Between the temperature sensors 126, 128 a basic temperature turprofil 146, which in this simple embodiment, approximately flat, so constant, runs. Thus, this basic temperature profile 126 within the sensor region 130, which is approximately bounded by the outer edges of the temperature sensors 126, 128, has the constant value T _P. In practice, however, the formation of an absolutely flat basic temperature profile 126 is technically difficult to realize and unfavorable due to the lack of temperature gradient in this area (see above). Accordingly, the area between the temperature sensors 126, 128 may be configured, for example, with a slight increase in temperature toward the center of the measurement surface 128, in order to achieve a displacement effect here as well.

The central heating element 124 is now subjected to an alternating voltage U _em = A-sin (ωt / 2). Thereby, since the heating power of the central heating element 124 is proportional to the square of the applied voltage U _em , the central heating element 124 is heated with a heating power proportional to sin (ωt). This periodic heating with the frequency sin (ωt) is symbolically denoted by reference numeral 148 in FIG. The circuit required to be charged with the alternating voltage U _em is indicated symbolically in FIG. 1 by the reference numeral 150. As indicated by the reference numerals 152, 154 in FIG. 2A, the periodic Auflieizung 148 of the central heating element 124 leads to local temperature increases. These local temperature increases "grow out" periodically with a frequency ω from the basic temperature profile 146 and each have their maximum at the location of the tracks of the central heating element 124. The temperature sensors 126, 128, which are arranged adjacent to the tracks of the central heating element 124 immediately outside capture These local temperature increases 152 and 154, respectively. In this case, the heating circuit 150 and the input voltage with which the central heating element 124 is heated are dimensioned in terms of their amplitude and their frequency such that the local temperature increases 152, 154 predominantly determine the temperature profile within the temperature range from the additional heating elements 132, 134. Since the measuring signals 142, 144 of the temperature sensors 126, 128 simultaneously serve to measure the local temperature increases 152, 154 and to regulate the heating power of the additional heating elements 132, 134 via the control circuits 138, 140 If the measuring signals 142, 144 of the temperature sensors 126, 128 are additionally fed before feeding into the control circuit 138, 140 before being fed into the control circuits 138, 140 by low-pass filters 156, 158 are filtered to filter out the high frequency (frequency ω) components caused by the local temperature increases 152, 154.

Changes in the local temperature increases 152, 154 by the air mass flow are superimposed by the periodic changes of the local temperature increase 152, 154 (see FIGS. 2A and 2B) and can be used to determine the flow velocity of the air mass flow. This influence of the flow velocity on the measurement signals 142, 144 is determined by means of a synchronous demodulator 160, which operates with a demodulation frequency ω. The synchronous demodulator 160 is shown greatly simplified in FIG. In a technical realization, it can be constructed, for example, of complex value. The synchronous demodulator 160 has two frequency mixers 162, 164 into which the measurement signals 142, 144 of the temperature sensors 126, 128 are respectively mixed with a demodulation signal 166 of the frequency ω. Subsequently, high-frequency components are filtered out in low-pass filters 168, 170, so that demodulated measurement signals 172, 174 are formed from the original measurement signals 142, 144 of the temperature sensors 126, 128. Subsequently, a difference signal can be formed from these demodulated measuring signals 172, 174 (denoted by reference numeral 176 in FIG. 1) or a summation signal (reference numeral 178). This embodiment is preferred over an embodiment in which first difference or sum signals are formed, with subsequent demodulation.

As shown in FIG. 2B in comparison with FIG. 2A, the local temperature increases 152, 154 are deformed as a result of the air mass flow, which flows with the main flow direction 120 via the sensor chip 112. This has, as shown in Figure 2B, the effect of measuring a higher temperature at the location of the downstream temperature sensor 128 than at the location of the upstream temperature sensor 126. Thus, the difference signal 176 reflects the flow velocity of the air mass flow and it may become out of this Difference signal 176, this flow rate can be calculated. In this case, the summation signal 178 can be used to normalize the difference signal 176, for example by dividing the difference signal 176 by the summation signal 178. In this way, if necessary, existing drifts in the resistance values of the resistors 126, 128 can be compensated. Furthermore, non-linear temperature effects can be suppressed by a control loop (not shown in FIG. 1) which utilizes the summation signal 178. Thus, through readjustment tion of the amplitude of the heating circuit 150 using the sum signal 178 as an input signal, a readjustment of the periodic heating 148 to the effect that the sum signal 178 is kept constant.

Furthermore, the influence of contamination by oil droplets 180 is symbolically shown in FIGS. 1 and 2A. Due to the temperature gradient effect described above, these oil droplets 180 mainly accumulate at the transition between the mainland surface 116 and the measurement surface 118. In this region, ie at the boundary of the measurement surface 118, there is a particularly pronounced temperature gradient, as shown in FIG. 2A. The oil droplets 180 cause the thermal conductivity of the sensor chip 112 to increase in this range. The temperature profile is thereby "smeared out", which is shown symbolically in FIGURE 2A by the dashed temperature profile 182 in comparison to the continuous temperature profile 184 without contamination by oil droplets 180. However, it is shown that the sensor region 130 according to the invention is formed by the control circuits 138, 140 temperature stabilization as described above, this influence of the oil droplets 180 is eliminated, so that the oil droplets 180 have no thermal influence on the measurement signals 142, 144 as long as the temperature profile modulated with the demodulation frequency ω (ie the local temperature increases 152, 154 ) does not protrude beyond the sensor region 130. As the modulation frequency ω increases, the region in which the local temperature increases 152, 154 become noticeable decreases due to the thermal inertia of the measurement surface 118. Since liquid contaminants such as the oil droplets 180 accumulate predominantly on the edge of the measuring surface 118, the hot air mass meter 110 according to the invention thus has a considerably higher resistance to contamination. The displacement of the contaminant from the measurement surface 118 requires a certain DC component of the temperature increase on the measurement surface 118, which, as described above, is achieved by controlling the additional heating elements 132, 134 to the temperature T _P. In addition, a DC component of the power loss of the central heating element 124 can be noticeable here. As part of a design optimization, all the resistors and printed conductors 124, 126, 128, 132, 134 can be brought very close together, so that the total contamination-sensitive area of the hot-air mass meter 110 is further reduced.

Claims

claims

Anspruch [en] A method for measuring an air mass flow flowing with a main flow direction (120) with a hot-air mass meter (110), in particular in the intake tract of a

Internal combustion engine, wherein the Heißfileinuftmassenmesser (110) has a sensor chip (112) with an overflowed by the air mass flow chip surface, the chip surface having a measuring surface (118), wherein on the measuring surface (118) has a central Heißhilaluftmassenmesserschaltung (122) with at least one central Heating element (124) and at least two temperature sensors (126, 128) is applied, characterized by the following method steps: the at least one central heating element (124) is heated periodically at a frequency ω; at least two measuring signals (142, 144) of the at least two temperature sensors (126, 128) are detected; and the measurement signals (142, 144) and / or at least one difference signal of the at least two measurement signals (142, 144) are demodulated with the frequency ω.

2. Method according to the preceding claim, characterized in that the heating takes place at a frequency in the range of 100 Hz to 100 kHz and preferably in the range of 1 kHz and 10 kHz.

3. The method according to any one of the preceding claims, wherein the Heißfileinuftmassenmesser (110) additionally comprises at least one tempering (132, 134), characterized in that the sensor chip (112) in the region of the central Heißfiluuftmassenmesserschaltung

(122) during operation of the Heißfileinuftmassenmessers (110) by the at least one tempering (132, 134) is maintained at a substantially constant time constant temperature profile (146).

4. Method according to the preceding claim, characterized in that for controlling the basic temperature profile (146) one or more of the at least two measuring signals (142, 144) of the at least two temperature sensors (126, 128) and / or at least one additional measuring signal of at least one control temperature sensor is used.

5. The method according to the preceding claim, characterized in that by the periodic heating of the at least one central heating element (124) caused periodic temperature increases (152, 154) when controlling the basic temperature profile (146) are essentially disregarded, in particular by using at least a blocking filter.

6. The method according to any one of the three preceding claims, characterized in that in addition an ambient temperature is detected, wherein the basic temperature profile (146) is selected so that the temperature in the region of the base temperature profile (146) and / or in the region of at least two temperature sensors ( 126, 128) is at least 40 K, preferably at least 80 K, and more preferably at least 120 K above the ambient temperature.

7. Method according to claim 1, wherein additionally at least one sum signal is formed, wherein the at least one sum signal comprises at least one sum of the at least two demodulated measuring signals and / or at least one of the summed signals a demodulated sum of the at least two measuring signals (142, 144).

8. The method according to the preceding claim, characterized in that the heating of the at least one central heating element (124) is regulated such that the at least one sum signal (178) is substantially constant in time.

9. The method according to one of the two preceding claims, characterized in that the at least one sum signal (178) for normalizing the measurement signals (142, 144) and / or the at least one demodulated difference signal (176) is used.

10. hot-film air mass meter (110) for measuring a with a main flow direction (120) flowing mass air flow, in particular in the intake tract of an internal combustion engine, with means for carrying out a method according to one of the preceding method claims.