DE102006020521A1 - Air pressure measurement arrangement for use in motor vehicle`s tire, has electronic controlling/measuring device determining movement absorption along predetermined distance, where absorption indicates measure for ambient pressure - Google Patents

Abstract

The arrangement has a capacitively measuring acceleration sensor (1) to evaluate change of a capacity of capacitor components (3, 5) as acceleration, where the capacitor elements are adjustable against each other by outside force effect. An electronic controlling/measuring device (100) produces a force effect in the capacitor components to adjust the components at a predetermined distance against each other. The controlling/measuring device determines a movement absorption along the distance, where the movement absorption indicates the measure for the ambient pressure. An independent claim is also included for a method for the pressure measurement utilizing a pressure measuring arrangement.

Description

State of technology

The Sensor technology is taking over nowadays especially in automotive engineering a high priority. To the operating conditions of the engine to be able to control optimally For example, lambda probes are used, knock sensors to the regulate optimal combustion pressure, and tire pressure sensors, to determine the air pressure in the tires of the vehicle.

For the release of the Auxiliary equipment, in particular safety devices of the motor vehicle, become common Acceleration sensors used.

So far, sensors have been established as acceleration measurement sensors that are designed using semiconductor technology and in surface micromechanics technology. Such a sensor for capacitive absorption of an acceleration is in DE 196 37 265 A1 disclosed. In DE 103 50 536 B3 an evaluation method for measuring signals of a capacitive measuring acceleration sensor is specified. An in DE 196 37 265 A1 revealed and an in DE 103 50 536 B3 The illustrated capacitive acceleration measuring element usually consists of a self-supporting seismic mass, which is connected to the substrate via mechanical spring elements. Attached to the seismic mass are finger electrodes which form a movable comb structure. During acceleration, the seismic mass is moved and the comb structures are displaced by the movement of the seismic mass, resulting in a measurable change in the capacities in the region of the comb structures. This change specifies the measure of the acceleration.

In case of DE 196 37 265 A1 two comb structures of a sensor array are connected in parallel, one increasing their capacity during the acceleration process while the capacity of the other is reduced. In this form of the acceleration sensor, the differential capacitive measuring principle is applicable, where the difference of the capacitances is divided by the sum of the capacitances in order to determine the change of the capacitance and to evaluate it as acceleration.

By the use of by surface microtechnology designed and manufactured sensor structures, the structures become so small that common laws for the description of the viscosity of gases in the macro-area are no longer applicable. That is, in the area These microstructures of acceleration sensors are therefore different laws for the Description of gas viscosity. If, as in the concrete case, the structural dimensions between individual Electrodes are in the range of the mean free path of gas molecules are the common ones Navier-Stokes equations describing fluid mechanics of gases no longer applicable. Because the mean free path with declining pressure is growing this applies at lower pressure for larger distances.

investigations have shown that an existing gas film has an attenuation the structures of the sensor elements exerts, which is pressure-dependent. They are set forth in "The Influence of gas-surface interaction on gas film accelerometer "by Timo Veijola, Heikki Kuisma, Juha Lahdenpera, Helsinki University of Technology, Department of Electrical Communications Engineering Circuits Laboratory in Elsevier Science SA 1998, 0924-42447 / 98. The invention uses these findings.

The The object underlying the invention is an arrangement and to provide a method of pressure measurement to an accelerometer to open up further fields of application.

These Task is carried out according to the invention an arrangement for pressure measurement according to claim 1 and a Method for pressure measurement according to claim 9 solved.

further developments The invention will become apparent from dependent claims 2 to 8 and 10.

Advantageous let yourself according to the invention Use accelerometers in a pressure measuring arrangement, where there existing capacitor structures under ambient pressure defined to be moved against each other. From the damping of the defined movement becomes a measure of the ambient pressure by determining a dynamic change in capacity certainly. Thus, an acceleration sensor as a pressure sensor becomes advantageous operated.

at a development of the arrangement will be at least two in one another cross-comb structures used as capacitor elements because so Advantageously, acceleration sensors can be used which have a Kapazitätsauswerteprinzip are evaluable.

Advantageous be in a development of the sensor arrangement according to the invention as semiconductor materials for the construction of the sensor common semiconductor materials such as silicon, germanium, indium or gallium arsenide, because manufacturing processes for Such semiconductor structures are widely established and this inexpensive applicable are.

Advantageously, in a development that Sensor element for coupling with the ambient pressure on a flexibly configured silicon membrane, so that the internal pressure in the sensor corresponds to the external pressure, because in this way a coupling to the ambient pressure is made possible without polluting particles can enter the sensitive comb structures. Also, moisture is kept away from the inside of the sensor, whereby the dielectric constant remains constant inside, so that falsification of the measurement results is prevented.

A Such membrane can be used in the construction of micromechanical structures The sensor can be easily incorporated with and therefore requires no additional Effort in the production of the sensor.

Farther Advantageously, in a further development of the sensor arrangement according to the invention the connection to the ambient pressure via porous silicon material or semiconductor material happen, with the pore size such is dimensioned that only a coupling to the gas pressure is made possible, without moisture or dirt particles can enter.

Advantageous are in a development of the arrangement, the second means for Generation of a force on a capacitor element as a means for generating a potential difference between the capacitor elements executed because in this way from the outside by applying a potential a force is exerted on the movable sensor element in the interior can be moved and defined this without any additional constructive measures in the area of the sensor for the introduction of an external force would be required.

Especially is advantageous in a development of the inventive arrangement a mechanical stop mounted inside the sensor, the the movement of the movable capacitor element is limited so that especially for the Pressure measurement limited distances are specified, which defines to go through. In this way are exactly traceable movements can be displayed inside the sensor element.

Especially is advantageous in a development of the sensor arrangement according to the invention in the area of the acceleration sensor also a temperature sensor provided, over Temperature compensation of viscosity improves the measurement result.

Advantageous are in a development of the arrangement means for switching off the pressure measurement exists because in this way a combined Pressure measuring and accelerometer sensor is inexpensive and easy to display.

Advantageous the invention sets an acceleration measuring sensor for pressure measurement because it eliminates the need for a special sensor.

Advantageous Such an acceleration measuring sensor can be used for altitude measurement used, for example, in bicycle tachometers, altitude measurement applications when climbing or in the field of mobile telephony.

Advantageous In a further development of the method, the arrangement can be operated in time-division multiplexing be a parallel operation of an acceleration sensor as To allow pressure measuring sensor without requiring a separate pressure measuring sensor.

in the The invention will be described below with reference to figures and exemplary embodiments further explained.

1 shows a first embodiment of an inventive arrangement for pressure measurement.

2 shows a further embodiment of a pressure measuring arrangement according to the invention.

3 shows the relationship between damping and pressure in the area of microscopic structures or small pressures.

4 shows a further embodiment of a pressure measuring arrangement according to the invention.

1 shows an arrangement for pressure measurement in the form of a first embodiment of the invention, which consists of an acceleration sensor 1 and an electronic control / measuring device 100 consists. The acceleration sensor 1 is manufactured here via surface micromechanics. It comprises two capacitor units 3 . 5 , which are each formed as an interdigital capacitor. The two interdigital capacitors 3 . 5 with finger electrodes 7 . 9 point fingers 11 on. This shows the electrodes 7 . 9 a comb structure on. It can be clearly seen in the figure that the individual fingers 11 opposite electrodes 7 . 9 at least partially intertwined. So one finger of the second electrode emerges 9 in a space between two fingers 11 a first electrode 7 one. In this comb structure, a capacitor unit is interposed between a finger of the second electrode 9 and a neighboring finger of the first electrode 7 educated. The capacity of this capacitor unit depends on the one hand on the height of the structure and on the distance d between the two fin gladly off and on the other hand of a finger length t, which the finger 11 dips into the space. The two second electrodes 9 are with a seismic mass 13 coupled, at its two ends parallel to the fingers 11 extending stiffening processes 15 has, at the ends turn spring rods 17 are attached. Two spring bars each 17.1 . 17.2 respectively. 17.3 . 17.4 are anchoring 19 attached, which is central to the second electrode 9 and the opposite extensions 15.1 and 15.2 is provided. Thus, the unit is made of electrodes 9 , seismic mass 13 and extensions 15 on the spring bars 17 Spring stored and deflected in the x direction. In contrast, the electrodes 7 in their position about also centrally arranged anchorages 21 fixed.

Now an acceleration directed in the x-direction acts on the sensor 1 one, becomes the seismic mass 13 together with the second electrodes 9 deflected in the x-direction due to its inertia. In this case, the immersion depth t in the condenser unit is reduced here 3 while the immersion depth t in the condenser unit 5 increases. However, the distance d between the fingers remains constant here. This results in a total capacity change, which is proportional to the acceleration, since only a linear dependence of the immersion depth t is present. To prevent a deflection of the seismic mass with the second electrodes 9 occurs in the y-direction, the extensions are 15 so dimensioned and designed that they cause a stiffening.

Furthermore, there are attacks 107 . 108 to recognize the movement of the seismic mass 13 in positive or negative x-direction limit so that no short circuit between charged parts, such as electrode fingers occurs. Furthermore, there are first contact points 91 . 71 and second contact points 92 . 72 for the respective first and second capacitor units provided with the control / measuring device 100 at connections 101 respectively. 102 can be coupled. Likewise is in the range 105 an opening provided to the sensor interior, which is filled, for example, with porous silicon, or having a thin silicon membrane, which is adapted to allow a balance of the pressure level between the interior of the sensor and the outside space.

A defined pressure measurement can, for example, be such that the seismic mass 13 about applying a potential only to the first contact points 92 or 72 or second contact points 71 or 91 or to all contact points to the right stop in the positive x-direction 108 is spent and then, for example, by reversing the potential Rich direction a defined movement in the direction of the left stop in the negative direction 107 is generated, via the control / measuring device 100 the dynamic damping and so the pressure is determined.

It is clear to the person skilled in the art that the movement is achieved by individual application of the potentials to the first contact points 91 . 71 or second contact points 92 . 72 through positive, negative potentials or combined application of the potentials to the existing contact points 91 . 71 . 92 . 72 can be effected. It is important in this context that a coordinated motion sequence can be specified, which can not be influenced by external acceleration effects on the inert mass, so that measurement conditions that are always traceable can always be found for the pressure measurement.

The damping the movement from the left stop to the right stop or vice versa is dependent on the pressure inside the sensor. The time-dependent measurement this damping can, for example, over a difference between the two partial capacities can be determined. So can the pressure can be accurately determined.

Advantageous can be calibrated in advance of the sensor by placing in a measuring chamber a certain pressure at a given temperature is set and several measuring runs carried out be, with the later Measuring process is used and then the known pressure as a measure of this Pressure is set.

It is also conceivable that an electrostatic force does not go beyond the electrodes 7 . 9 is introduced, but that separate means for the defined movement of the seismic mass are provided, which are controllable from the outside. Conceivable in this context would be piezo-mechanical elements.

2 shows a further arrangement for pressure measurement according to an embodiment of the invention.

The operation of the acceleration sensor 1 differs from the previous example in that the unit of seismic mass 13 and second electrode 9 was separated. Here are the two seismic masses 13 with the second electrodes 9 between the two fixed first electrodes 7 arranged. Four spring bars 17 run between the two seismic masses 13 and are at one end at a common anchorage 21 attached and at the other end to connecting webs 15 that the two seismic masses 13 connect. This embodiment offers the best compensation of thermal expansion stresses, since the seismic mass only contacts the substrate at one point. Of course, other sensor forms conceivable. There is also the advantage here that the housing 1000 can be applied around an existing acceleration measurement structure and that the sensor thus has to be structurally modified only slightly in order to be used as a pressure sensor. The measuring procedure is analogous as described above.

η_eff: Effektive Viskosität; η: Viskosität der Luft; Kn: Knudsenzahl. Es gilt weiter:

λ₀: Mittlere freie Weglänge der Gasmoleküle beim Druck p₀; p₀: Luftdruck (Referenzdruck); p: Umgebungsdruck; d: typische geometrische Abmessung (zum Beispiel Abstand zwischen Elektrodenfingern). 3 shows the pressure dependence of the damping of a micromechanical structure. The invention is based on the principle of pressure dependence of the air damping in micromechanical structures, wherein the damping is proportional to the effective viscosity. The following equations apply here:

η _eff : Effective viscosity; η: viscosity of the air; Kn: Knudsen number. The following applies:

λ ₀ : mean free path of the gas molecules at pressure p ₀ ; p ₀ : air pressure (reference pressure); p: ambient pressure; d: typical geometric dimension (for example, distance between electrode fingers).

In the mean free path of λ ₀ = 64 nm for air at p ₀ = 0.1 MPa and a typical distance of d = 2 microns between the electrode fingers results in 3 illustrated dependency of the damping of the pressure. By targeted movement of the seismic mass, for example, between the two boundaries 108 and 107 Such a damping profile can be determined. He discusses the changing capacity of the capacitor units with the electrodes 9 . 7 determine. The ambient pressure results from the above equations, since all quantities are known. Advantageously, the measurement of acceleration and pressure can take place in a temporal multiplex method. The capacitive evaluation principle can be used for both measurements. Because of this combination chip area can be saved.

A temperature dependence of the viscosity can be measured by a temperature sensor 20 be compensated on the chip. One in the control / measuring device 100 Stored characteristic field is used for correction.

4 shows as an example a plan view of a further acceleration sensor as shown in FIG DE 103 50 536 is disclosed.

In contrast to the exemplary embodiments shown above, movement of the seismic mass takes place here in a detection direction 14 instead of. The reference numerals are otherwise used the same as in the other embodiments.

In plan view are the electrodes 7 . 9 . 12 to recognize with separately designated electrode fingers. Exemplary are the electrode fingers 11 the first electrode 7 indicated separately. The electrodes 7 . 9 . 12 form interlocking comm structures with their fingers.

The moving second electrodes 9 are here as center electrodes left and right on a seismic mass 13 appropriate. They are electrically connected to each other and via mechanical spring elements 17 and an insulating layer connected to a substrate. The second electrodes 9 are in detection direction 14 deflectable. For this purpose, analogously to the other embodiments, for example, a potential between the second electrode 9 and the other electrodes 9 and 12 be generated.

The second electrodes 9 are each of two fixed outer electrodes 7 and 12 surround. All connected fingers 11 the first electrode 7 , belong to a first capacitor unit C1 and form a fixed comb structure. This applies analogously to the fingers of the electrode belonging to a second capacitor unit C2 12 , The first electrodes 7 . 12 the two solid comb structures and the second electrodes 9 at least partially interlock. They are arranged such that a deflection of the electrodes of the movable comb structure with second electrodes 9 along the direction 14 perpendicular to the side surfaces of the fingers 11 he follows.

The evaluation of the electrical conditions in the micromechanical structure is carried out with the control / measuring device 100 in which the following output signal U _{out is formed} from the capacitor units C1 and C2: _Uout = (C1-C2) / (C1 + C2) * U _ref

Here, U _{ref is} a reference voltage of the evaluation circuit. As a result of this evaluation, the output signal is linearized in relation to the deflection of the movable structure.

There are stops to prevent the electrodes from touching and causing a short circuit 107 . 108 in the area of the spring elements 17 , They cause a maximum deflection for the movable structure is specified. Thus, even at maximum deflection by the evaluation circuit, an output signal can be detected.

Claims (10)

Arrangement for measuring pressure with: a) a capacitively measuring accelerometer, in which a change in a capacitance of at least two capacitor elements displaceable against one another by external force ( 3 . 5 ) can be evaluated as acceleration; b) first means for producing ambient pressure in the region of the capacitor elements ( 3 . 5 ); c) second means ( 100 ) for generating an external first force on the capacitor elements ( 3 . 5 ) to shift this first predetermined distance against each other; d) third resources ( 100 ) for determining a motion damping along the first path, wherein the motion damping indicates a measure of the ambient pressure. Arrangement according to Claim 1, in which the semiconductor sensor has at least two intermeshing comb structures ( 9 . 11 . 12 ) as capacitor elements. Arrangement according to one of the preceding claims, in which the first means ( 105 ) are designed as a membrane made of semiconductor material. Arrangement according to one of Claims 1 to 2, in which the first means ( 105 ) are designed as a porous semiconductor material. Arrangement according to one of the preceding claims, in which the second means ( 100 ) are carried out as means for generating a potential difference between the capacitor elements, the an electrostatic force between the capacitor elements ( 9 . 11 . 12 ) produce. Arrangement according to one of the preceding claims, in which at least one mechanical stop ( 107 . 108 ) for a movable capacitor element ( 7 . 9 . 10 ) is present for setting the first distance. Arrangement according to one of the preceding claims, in which a temperature sensor ( 20 ) is available. Arrangement according to one of the preceding claims, wherein the fourth means ( 100 ) are available to turn off the pressure measurement to switch the sensor as an acceleration sensor. Method for pressure measurement, in which a pressure measuring arrangement according to one of claims 1 to 8 is used, wherein in a first step a capacitor element ( 7 . 9 . 10 ) is moved via an external force, and during the movement, a time-dependent motion damping is determined, and in which in a further step from the temporal damping curve, an ambient pressure is determined. The method of claim 9, wherein between pressure measurement and acceleration measurement is switched back and forth.