WO2012149958A1

WO2012149958A1 - Method for determining a change in a value of capacitance of a capacitive component, and apparatus

Info

Publication number: WO2012149958A1
Application number: PCT/EP2011/057031
Authority: WO
Inventors: Dirk Scheibner; Arno Steckenborn; Oliver Theile; Jörg HASSEL
Original assignee: Siemens Aktiengesellschaft
Priority date: 2011-05-03
Filing date: 2011-05-03
Publication date: 2012-11-08

Abstract

The invention relates to a method for determining a change in a value of a capacitance (C_L) of a capacitive component (1) having the following steps: - an electrical circuit arrangement (2) which comprises the capacitive component (C_L) and a quartz oscillator (3) is produced, wherein the capacitive component (1) acts as a load capacitance for the quartz oscillator (3); - the quartz oscillator (3) is made to oscillate at a first frequency (f₀); - the capacitance (C_L) of the capacitive component (1) is changed; - when the capacitance has changed (C_L±ΔC_L), a deviation in the oscillation frequency (Δf) of the quartz oscillator (3) from the first frequency (f₀) is determined; and - the change in the value of the capacitance (C_L±ΔC_L) of the capacitive component (1) is determined from the deviation in the oscillation frequency (Δf).

Description

description

Method for determining a change in a value of a capacitance of a capacitive component and device

The invention relates to a method for determining a change of a value of a capacitance of a capacitive Bauele ^¬ ments comprising the steps of: forming an electrical circuit comprising the capacitive component and a quartz oscillator, wherein the capacitive component acts as a load capacitance of the crystal oscillator, and placing the quartz oscillator in vibration with a first frequency. The invention also relates to a device having an electrical circuit arrangement, which comprises a capacitive component and a quartz oscillator, wherein the capacitive construction ^¬ element acts as a load capacitance of the quartz oscillator, wherein the capacitive element is formed so that a value of it to its own capacity is variable , There are known capacitive sensors, where a change in a capacity as a measure to be determined measured variable is he ^¬ range covered. With such sensors, for example, distances, strains, levels or inclinations can be measured. Capacitive sensors in the context of silicon micromechanics can be realized in a particularly simple and space-saving manner.

An example here are so-called micromechanical force ^¬ sensors, which are designed as a MEMS (microelectromechanical system) component. Various methods of capacitance measurement are known here. Thus, the time can be measured, which is needed to Aufla ^¬ tion of a given measurement capacity. Here, a comparison with a known Referenzkapa ^¬ capacity usually takes place. However, the measuring capacitance can also be used as a frequency-determining element in a resonant circuit, wherein an evaluation of the oscillation frequency can be used to deduce the measuring capacitance. If the measuring frequency is known, the impedance of the capacitance can also be measured. The method of charging the measuring capacitance with a constant current has the disadvantage that dynamic measurements of the change of the measuring capacity are only possible in the case of very low frequencies. For the charging time, a slight change in the measuring capacity is assumed. In this way, integration times of 100ms, for example, are just possible. For fast dynamic measurements (time ^¬ scale much smaller than 100ms), this method is therefore not suitable.

In addition, the evaluation of capacitive sensors with high accuracy poses a challenge since usually only a very small change in capacitance must be detected with a comparatively large basic capacitance.

It is an object of the invention to provide a method and a device by means of which changes in a measuring capacity can be measured even easier.

This object is achieved by a method having the features of claim 1, and a device, wel ^¬ che having the features of claim 5, solved. An inventive method for determining a change of a value of a capacitance of a capacitive Bauele ^¬ ment and comprising the steps of:

• Forming an electrical circuit arrangement which comprises the capacitive component and a quartz oscillator, wherein the capacitive component acts as a load capacitance of the quartz oscillator;

• oscillating the quartz oscillator at a first frequency;

• changing the capacitance of the capacitive device;

· If the capacity changes, determine a deviation of the

Oscillation frequency of the quartz oscillator from the first frequency; • determining the change in the value of the capacitance of kapa ^¬ zitiven component from the deviation of the oscillation frequency ^¬. In particular, the method makes it possible to draw precise conclusions about the change in the capacitance of the capacitive component by means of frequency evaluation. Crystal oscillators are inexpensive and yet allow a high-precision frequency ^¬ measurement and thus also measuring the capacitance of the capacitive device. The method can be carried out in connection with a comparatively simple circuit arrangement in very compact components. By returning a capacity change to a frequency change, a highly accurate measurement method is created since times and thus frequencies can be determined with high precision.

The capacitive component may in particular be an electrical capacitor, in particular a plate capacitor. By the capacitive component is then provided in particular a measuring capacity whose

Capacity value is variable. The coupling with the quartz oscillator, in particular in a series circuit of capacitive component and quartz oscillator, allows the use of the so-called pulling effect for determining capacitance in the context of the method. The frequency at which the crystal oscillator oscillates, namely, determined by a load capacitance and thus by the capacitive component, which acts as a straight Lastkapa ^¬ capacity. By changing the load capacity, it comes in particular to change the frequency of the quartz oscillator, which is measurable. The capacitive component can in particular also act as trim capacitor. Changes in capacitance of the capacitive component can be caused for example by changes in a dielectric and / or distance ^¬ changes between electrodes.

The capacitive component or the measuring capacitance acts as a load capacitance of the quartz oscillator. A change in the Lastka ^¬ capacity causes a frequency change of the quartz oscillator. This change in frequency is again a measure of the change in capacitance and of a measured variable to be determined.

Preferably, the method comprises the following additional steps:

• forming a second electrical circuit arrangement comprising a second capacitive component with egg ^¬ ner capacitance whose value is known, and a second quartz oscillator so that an oscillation frequency of the second crystal oscillator is determined by the capacitance of the second capacitive component, at least partially;

• oscillating the second crystal oscillator with an output oscillation frequency;

Changing the capacitance of the capacitive component by a first and a second means;

Changing the capacitance of the second capacitive element exclusively by the second means;

• If the capacitance of the second capacitive component has changed, determine a final oscillation frequency of the second one

Quartz oscillator;

Determining the proportion of the change of the value of the capacitance of the capacitive component, which is caused by the first means, from the deviation of the oscillation frequency and the output oscillation frequency and the

Final vibration frequency.

The second electrical circuit arrangement then provides, in particular, a reference system for the actually measuring electrical circuit arrangement. In particular it can be provided that the first means by which the Ka ^¬ capacity of the capacitive component is changed is explicitly correlated to a quantity to be measured. For example, it may be provided that the capacitive component is designed as a plate capacitor, wherein the first means may be to change the capacitance via a variation of the plate spacing in order to be able to infer a path change or a force change. With this In particular, no change in the capacitance of the second capacitive component should take place in the first means. In contrast, the second capacitive device detects particular exclu ^¬ Lich changes by a second agent, which also acts on the first capacitive device. In such With ^¬ tel it may order a change in an environmental ^¬ control, such as a change in temperature and / or a change in a Betauungsgrades act in particular. Such an environmental influence can then be particularly similar to the electrical circuit arrangement and the second electrical

Affect circuit arrangement. By means of the measuring method with the aid of the second electrical circuit arrangement, it is then ensured that these environmental influences, which cause an unintended change in capacitance at the actual measuring capacitance, can be calculated out. In this way, disturbing influences can be taken into account during the measurement process. The change in the capacitance of the capacitive ^¬ tive component can be made even more precise. Then preferably takes place, determining the proportion of Ände ^¬ tion of the value of the capacitance of the capacitive component since ^¬ by that a frequency signal which is provided by the electrical circuitry, and a further frequency signal, which is provided by the second electrical circuitry, are mixed to a mixed signal with a beat and the beat is detected. The mixing can be effected in particular by multiplication of the two frequency signals, which in particular produces an amplitude-modulated signal whose beat can be determined. By means of a demodulation of the overall signal, it is possible in particular to deduce the deviation of the oscillation frequency and consequently the change in the capacitance of the capacitive component. This type of frequency evaluation is extremely convenient, accurate and easy to perform.

A device according to the invention comprises an electrical circuit arrangement, which comprises a capacitive component and a quartz oscillator, wherein the capacitive component ment acts as a load capacitance of the quartz oscillator, wherein the capacitive element is designed so that a value of a capacity that is its own variable. The erfindungsge ^¬ Permitted device also comprises an evaluation unit which is designed to determine a conditional the capacitance of the capacitive component by a change in deviation of the oscillation frequency of the quartz oscillator of a first frequency and to determine from the change in the value of the capacitance of the capacitive component.

Preferably, the capacitive component comprises at least two partial capacitive components, each of which acts as a load capacitance of the quartz oscillator. In particular, this makes it possible to realize a differential measuring capacitance, with both partial components of the measuring capacitance in each case being able to be used as a load capacitor, and thus a significantly greater pulling effect being achievable. The measuring accuracy can be increased overall. For example, lead already low ex ^¬ changes of state of the plates of a plate capacitor formed as a capacitive element in large changes in the oscillation frequency, which are easy to detect. It can be provided that the capacity change takes place in a similar manner on both partial components. However, it can also be provided that the capacity of the one sub-component decreases, while one and the same

Measuring process, the capacity of the other sub-component magnified ^¬ ßert.

Preferably, the capacitive component comprises at least two units, by which an electrical capacitor is formed together, and the changing of the capacitance of the kapa ^¬ zitiven device is effected by changing a distance to ^{¬ at} least two units to each other. The Kondensa ^¬ tor can hereby be excluded forms in particular as a plate capacitor. In this way, a highly sensitive distance measuring device is realized. Namely, the change in the distance can cause a capacitance change, which in turn leads to a frequency change on the quartz oscillator. The frequency Change is then measured and it can be deduced on the Abstandsände ^¬ tion. This embodiment also allows the realization of a device by means of which forces can be measured. Thus, it can be provided that a force change is correlated with a change in travel via the hook's law, the change in travel being detected in turn via the capacitance and frequency change according to the present method. The measuring device thus created is very universally applicable for all possible measuring processes in which a distance is to be determined.

Preferably, then, a first of the at least two units as a displacement element is positionally variable relative to a second of the at least two units, and the evaluation unit is adapted to change a value of a relative distance between the first and the two from the change in the value of the capacitance of the capacitive component ^¬ th unit to determine. The displacement element may in particular be a plate of a plate capacitor arranged in a positionally variable manner.

Preferably, at least the capacitive component and the quartz oscillator are formed in a micromechanical component, in particular a silicon-based MEMS component. In this way, a highly compact capacitance or distance ^{meter is} created ^¬ ser, which is also very inexpensive to produce.

Preferably the apparatus comprises a second electrical circuit arrangement comprising a second capacitive Bauele ^¬ element having a capacitance whose value is known, and a second quartz oscillator, an oscillation frequency of the second crystal oscillator is determined by the capacitance of the second capacitive component, at least partially, wherein the evaluation unit is designed to change the capacitance of the capacitive component by a first and by a second means and when changing the capacitance of the second capacitive component exclusively by the Second means a portion of the change in the value of the capaci ^¬ ability of the capacitive component, which is conditioned by the first ^Mit¬ tel, from the deviation of the oscillation frequency of the quartz oscillator and a change in the Schwingungsfre- frequency of the second quartz oscillator to determine.

Preferably, the device then comprises a housing, wherein both the electrical circuit arrangement and the second electrical circuit arrangement are formed within the housing. This embodiment is particularly before ^¬ geous since it is ensured by that both the electrical circuit and second electrical circuit arrangement almost identical environmental conditions, such as temperature changes or humidity changes are exposed. Then, a very meaningful reference system is created by the second electrical scarf ^¬ tion arrangement, with which the measurement accuracy of the electrical circuit can be improved. The ^{dargestell ¬} with reference to the inventive method ^¬ preferred embodiments and their advantages apply correspondingly for the device according to the invention.

Reference to exemplary embodiments, the invention is explained in more detail below. Show it:

1 shows a series connection of a drawing capacitor with egg ^¬ nem quartz oscillator and an associated Ersatzschal ^¬ tion;

2 shows a circuit arrangement with a quartz oscillator and a drawing capacitor;

3 shows a diagram which describes the relationship between a load resonance offset and the value of a capacitance for different oscillating crystals; 4 shows an electrical circuit arrangement for a device according to a first embodiment;

5 shows an electrical circuit arrangement for a device according to a second embodiment; and

6 shows a reference circuit. In the figures, identical or functionally identical elements are provided with the same reference numerals.

1 shows a simple schematic series connection of a capacitive component, which is designed as a drawing capacitor 1 with an electrical capacitance C _L , and a vibrating ^¬ quartz 3. In this arrangement, the drawing capacitor 1 acts as a load capacitance of the quartz crystal 3. A change in the capacity ^¬ C _L causes a change in frequency of the quartz crystal 3. The frequency change in the quartz crystal 3 is thus a measure of the change in the capacitance C _{L of} the pull capacitor 1. It is said that with the draw capacitor, the frequency of the quartz crystal 3 can be quasi pulled (so-called Ziehef- effect ). The quartz crystal 3 is an electronic component, which can be described by the equivalent ^{switching ¬} image also shown in Figure 1 with a resonant circuit 4. The oscillating circuit 4 is formed via a parallel connection of a capacitance Co and a series circuit of a capacitance Ci, a resistance Ri and an inductance Li. The pull capacitor ^¬ 1 can then be the resistor R _L assign, for the following applies:

2 shows a circuit 2 which comprises the series circuit known from FIG. The variability of the draw con- sators 1 is shown explicitly in the diagram. The so-formed ^¬ series circuit is connected at its ends with an input and output of an amplifier 5, and with resistors R. In this arrangement, the effect of pulling the oscillation frequency of the quartz crystal 3 can be utilized in the measurement method. For the signal and frequency from ^¬ evaluation a microcontroller can additionally be provided.

3 shows the so-called load resonance offset LO as a function of the capacitance C _L. The capacitance C _L is plotted on the x-axis in picofarads, while the load resonance offset is plotted on the y-axis in parts per million. The ver ^¬ different curves belong to different Co and Ci capacitance values by which FIG 1 are given replacement data for the quartz oscillator 3 according to. The diagram can be read such that the load capacitance C _L virtually warps the resonance frequency of the quartz crystal 3. The context applies:

LO

2 (C ₀ + C _Z )

Also in the figures 4 and 5 again this relationship is illustrated. According to FIG. 4, a drawing capacitor 1 is likewise provided in the circuit 2, whose capacitance changes ± AC _L lead to frequency changes + Δf on the quartz crystal 3. This frequency change + Af is made around a frequency fo.

Similarly, circuit 2 behaves according to the circuit diagram shown in FIG. In it, however, the capacitive component of two capacitive components in the form of draw capacitors la and lb. In this way, a differential Messka ^¬ capacity is realized in which both pulling capacitors la and lb respectively used as a load capacitor of the oscillating quartz 3 ^¬ to. In this way, a larger pulling effect can be achieved, which is expressed in more pronounced frequency changes Δf. In the illustrated embodiments, the drawing capacitor 1 (or la and lb) is formed as a plate capacitor, wherein one of the two plates is formed with respect to the other plate positionally variable in their distance. This movable plate is in turn connected to the system to be measured. If there is a change in position of the movable plate, for example by the fact that the system to be measured is moved, then this leads to ei ^¬ ner change in plate separation in the plate capacitor. This, in turn a change in capacitance AC _L, which is detected within the framework of the method ^¬ men over the frequency change Af. With the embodiment shown in FIG 5 even minimal changes in position can be detected very accurately. A further improvement in the measurement accuracy can be achieved by using a reference circuit 2 ', as shown by way of example in FIG. Indeed, in ^¬ play, the circuit 2 of Figure 4 and the reference acoustic ^¬ device 2 'of Figure 6 in a common housing of a measurement device before. With the drawing capacitor 1 of FIG 4, which is designed as a plate capacitor, now a Ab ^¬ change in state to be measured. However, while the distance measurement results in the change of environmental variables, namely, for example, a temperature change or a change in the humidity within the enclosure, which is a modified Be ^¬ condensation allowed the electronic components of the circuit 2 and the reference circuit 2 'conditionally. Even without a change in distance, this changes the dielectric constant of the medium within the drawing capacitor 1. Consequently, the change in capacitance AC _{L also} contributes to the change in these environmental variables. If the reference system shown in FIG. 6 were not provided, this change in the environmental influences would be erroneously interpreted as a change in distance. With the reference circuit 2 ', however, these environmental influences can be compensated metrologically. For this purpose, 'also an amplifier 5, a reference quartz 3' comprising the Re ^¬ conference circuit 2 and a reference capacitor 1 'with refer- capacity C _R. The reference capacitor 1 'is not formed variable, so it has a constant plate spacing. The reference capacity C _R is known.

If the change in the environmental variables now has the same effect on the circuit 2 and the reference circuit 2 ', then a correlated change in the oscillation frequency of the quartz oscillator 3 and the reference oscillator quartz 3' due to the environmental variables is to be expected. It is now envisaged that

Oscillation frequency of the quartz crystal 3 with the oscillation frequency of the reference oscillating quartz 3 'to compare. A similar frequency shift or frequency change Af is detected. By mixing (multiplication) of the two frequency signals, an amplitude-modulated signal is produced, which identifies a beat with the frequency change of the measuring oscillator or oscillating quartz 3. This measurement ^¬ size proportional frequency change is used to evaluate demodulates (rectification and low-pass filtering).

By varying the load capacity for capacitive signal evaluation and the compensation of environmental influences by mixing with a reference oscillator with the same center frequency, the following advantages result:

• simple signal evaluation by frequency evaluation, e.g. by means of a microcontroller;

• easier circuit than with the time measuring method; There is in addition to the oscillator further circuit complexity for forming to switch the capacity.

• the crystals to be drawn are inexpensive and allow a cost-reduced implementation of the circuit; and

• Environmental influences affect both oscillators resp.

Oscillating crystals 3 and 3 'and can be compensated so.

The circuits shown in Figures 2, 4, 5 and 6 are in the form of a MEMS device. This is used in the exemplary embodiment in a capacitive sensor in weighing technology. Even very slight changes in weight, which only lead to small changes in the spacing in the drawing capacitor 1, are caused by the proposed detuning of the vibration frequency. Query of the quartz crystal 3 precisely detected. There is a MEMS strain sensor created.

Reference sign list

1, la, lb draw condenser

1 'reference capacitor

2 circuit

2 'reference circuit

3 quartz crystal

3 'reference vibrating quartz

4 resonant circuit

5 amplifiers

c _L capacity

CR reference capacity

AC _L capacity change

AC _R capacity change fo frequency

Af frequency change

Ci capacity

Co capacity

i resistance

RL Resist

R resistance

Li inductance

LO load resonance offset

Claims

claims

A method of determining a change in a value of a capacitance (C _L ) of a capacitive device (1) comprising the steps of:

- Forming an electrical circuit arrangement (2), which comprises the capacitive component (C _L ) and a quartz oscillator (3), wherein the capacitive component (1) acts as Lastka ^¬ capacity of the quartz oscillator (3);

- putting the quartz oscillator (3) in oscillation at a first frequency (f ₀ );

characterized by the steps:

- changing the capacitance (C _L ) of the capacitive component (1);

in the case of a changed capacitance (C _L ± AC _L ), determining a deviation of the oscillation frequency (Af) of the quartz oscillator (3) from the first frequency (fo);

- Determining the change in the value of the capacitance (C _L ± AC _L ) of the capacitive component (1) from the deviation of the oscillation frequency (Af).

2. The method according to claim 1,

characterized by the steps:

- Forming a second electrical circuit arrangement {2 '), which comprises a second capacitive component (1') with a capacitor (C _R ) whose value is known, and a second quartz oscillator (3 '), so that a Schwingungsfre ^¬ frequency of the second crystal oscillator (3 ') through the capacitance (C _R) of the second capacitive component (1') is at least defined in part ^¬ example;

- putting the second crystal oscillator (3 ') in oscillation with an output oscillation frequency;

- Changing the capacitance (C _L ) of the capacitive device (1) by a first and by a second means;

Changing the capacitance (C _R ) of the second capacitive component (1 ') exclusively by the second means;

- If the capacitance (C _R ) of the second capacitive component (1 ') has changed, determining an end oscillation frequency of the second quartz oscillator (3'); - Determining the proportion of the change in the value of the capacitance (C _L ) of the capacitive component (1), which is caused by the first means, from the deviation of the Schwingungsfre ^¬ frequency (Af) and the output oscillation frequency and the final oscillation frequency.

3. The method according to claim 2,

characterized in that

the second means comprises a change in an environmental influence, in particular a change in a temperature and / or a change in a degree of condensation.

4. The method according to claim 2 or 3,

characterized in that

determining the proportion of change in the value of the capacitance (C _L) of the capacitive component (1) takes place in that a frequency signal which is provided by the electrical switch ^¬ processing arrangement (2), and another Fre ^¬-frequency signal, which of the second electrical circuit arrangement (2 ') is provided, mixed into a mixed signal with a beat and the beat is determined.

5. Device with an electrical circuit arrangement (2), which comprises a capacitive component (1) and a Quarzos ^¬ zillator (3), wherein the capacitive component (1) acts as a load capacitance of the quartz oscillator (3), wherein the kapa ^¬ citive component (1) is designed so that a value of its own capacity (C _L ) is variable, characterized by

to determine an evaluation unit which is adapted to one of the capacitance by a change (C _L ± AC _L) of the capacitive Bauele ^¬ ments (1) induced deviation of the oscillation frequency (Af) of the quartz oscillator from a first frequency (fo) and from this the Change the value of the capacitance (AC _L ) of the capacitive device (1) to determine.

6. Apparatus according to claim 5, characterized in that

the capacitive component (1) at least two capacitive devices ^¬ part (la, lb) which act respectively as Lastkapazi ^¬ ty of the quartz oscillator (3).

7. Apparatus according to claim 5 or 6,

characterized in that

the capacitive component (1) ^encompasses at least two units, by means of which an electrical capacitor is jointly formed, and changing the capacitance (C _L ± AC _L ) of the capacitive component (1) by changing a distance of the at least two units from one another is effected.

8. Apparatus according to claim 7,

characterized in that

a first of the at least two entities is constructed as a displacement element position is variable relative to a second of the at least two units, and the evaluation unit is designed, from the change of the value of the capacitance (AC _L) of the capacitive component (1) a change in a Re ^¬ lativabstands between the first and second units.

9. Device according to one of claims 5 to 8,

characterized in that

at least the capacitive component (1) and the Quarzoszilla ^¬ gate (3) are formed in a micromechanical component.

10. Device according to one of claims 5 to 9,

marked by

a second electrical circuit arrangement {2 ') including a second capacitive component (1' comprises) with a capacitance (C _R) whose value is known, and a second Quarzos ^¬ zillator (3 '), wherein an oscillation frequency of the second crystal oscillator ( 3 ') (by the capacitance C _R) of the second capacitive component (1') at least partly determined, wherein the evaluation unit is designed in Än ^¬ alteration of the capacitance (C _L ± AC _L) of the capacitive component (1) by a first and by a second means and Ände ^¬ tion of the capacitance (C _R ± AC _R) of the second capacitive Bauele ^¬ member (1 ') (exclusively by the second means an on ^¬ part of the change in the value of the capacitance C _L ) of the capacitive component (1), which is conditioned by the first means, from the deviation of the oscillation frequency (fo ± Af) of the quartz oscillator (3) and a change in the Schwingungsfre ^¬ frequency of the second quartz oscillator (3 ') to determine.

11. The device according to claim 10,

with a housing, wherein both the electrical Schaltungsan ^¬ order (2) and the second electrical Schaltungsanord ^¬ tion (2 ') are formed within the housing.