DE102017216009A1 - Method for testing a micromechanical baseband sensor - Google Patents

Abstract

A method of testing a micromechanical baseband sensor (100), comprising the steps of:
- introducing electrostatic forces to a ground electrode (CM) of the baseband sensor (100), the ground electrode (C _M ) being deflected in a defined manner;
- Defines a long wait; and
- Defined evaluation of a geometric displacement of the ground electrode (CM) after the waiting time (t _NK ).

Description

The invention relates to a method for testing a micromechanical baseband sensor. The invention further relates to a micromechanical baseband sensor.

State of the art

Micromechanical capacitive sensors (accelerometers, pressure sensors, etc.) are normally operated on a single frequency during normal operation. Capacitive MEMS acceleration sensors consist of fairly simple structures. This is ultimately a plate capacitor, which is detuned by external acceleration influences.

1. a) eine feststehende Masse/Kondensatorplatte
2. b) eine mittels Federn aufgehangene Kondensatorplatte, die durch eine externe Beschleunigung geometrisch verschoben wird
3. c) ein Gehäuse mit definiertem Innendruck, in welcher sich der Sensor befindet

In principle, the MEMS acceleration sensor can be divided into three components:

1. a) a fixed mass / capacitor plate
2. b) a suspended by means of springs capacitor plate, which is geometrically displaced by an external acceleration
3. c) a housing with a defined internal pressure, in which the sensor is located

The sensors mentioned are usually tested for sufficient internal pressure, but an influence of insufficient internal pressure is usually not or only with great effort can be determined. Although the internal pressure could be generally determined by a noise power density measurement as well, such a measurement is conventionally not performed due to environmental influences (e.g., self-movement of the adjustment engine).

Disclosure of the invention

It is an object of the present invention to provide an improved method for testing a micromechanical baseband sensor.

• - Einbringen von elektrostatischen Kräften auf eine Masseelektrode des Basisbandsensors, wobei die Masseelektrode definiert ausgelenkt wird;
• - Definiert langes Warten; und
• - Definiertes Auswerten einer geometrischen Auslenkung der Masseelektrode nach Ablauf der Wartezeit.

The object is achieved according to a first aspect with a method for testing a micromechanical baseband sensor, comprising the steps:

• - Applying electrostatic forces on a ground electrode of the baseband sensor, wherein the ground electrode is deflected defined;
• - Defines a long wait; and
• - Defined evaluation of a geometric deflection of the ground electrode after the waiting time.

As a result, an indirect method of detecting a noise power density which can be detected by an internal pressure of the baseband sensor is provided in this way. In this case, an electrostatic force is generated, which leads to Sensorauslenkung, after which time the sensor is given so that the mechanical mass can swing back. From the geometric deflection of the mass can be closed to the pressure prevailing in the sensor internal pressure.

• - eine MEMS-Einrichtung mit einer beweglichen Masseelektrode und einer definierten Anzahl von Ausleseelektroden, die mit der beweglichen Masseelektrode zusammenwirkbar ausgebildet werden, um elektrische Ladungsverschiebungen auf den Ausleseelektroden aufgrund einer Bewegung der Masseelektrode zu erfassen; und
• - einer Ansteuereinrichtung, die ausgebildet ist, die Masseelektrode mit wenigstens zwei unterschiedlichen elektrostatischen Kräften anzusteuern und die Auslenkung der Masseelektrode nach einer definierten Wartezeit definiert auszuwerten.

The object is achieved according to a second aspect with a micromechanical baseband sensor, comprising:

• a MEMS device having a movable ground electrode and a defined number of sense electrodes cooperatively formed with the movable ground electrode for detecting electrical charge shifts on the sense electrodes due to movement of the ground electrode; and
• - A drive means which is adapted to drive the ground electrode with at least two different electrostatic forces and to evaluate the deflection of the ground electrode defined after a defined waiting time.

Preferred embodiments of the method for testing a micromechanical baseband sensor are the subject of dependent claims.

A preferred embodiment of the method provides that the waiting is carried out until the ground electrode has reached a defined percentage of its full scale. In this way, it is advantageously possible to evaluate the deflection of the ground electrode according to defined mathematical principles.

A further preferred development of the method is characterized in that the waiting is carried out until the ground electrode has reached about 30% to about 70%, preferably about 50% of its full scale. This provides favorable geometric information regarding the average deflection of the ground electrode.

A further preferred embodiment of the method is characterized in that a deflection of the ground electrode after the defined waiting time is converted into an internal pressure of the baseband sensor. In this way, an efficient check of the internal pressure of the micromechanical baseband sensor can be carried out, which represents an important parameter during normal operation.

A further preferred embodiment of the method provides that the method can be initiated by a user at any time during the use of the baseband sensor. This advantageously offers the user the option of proposing methods at any time during the operation of the micromechanical baseband sensor. A quick analysis of the baseband sensor is favorably supported this way.

Further advantageous embodiments of the method provide that the method for a capacitive acceleration sensor or a capacitive pressure sensor is performed. This provides useful use cases for the method.

The invention will be described below with further features and advantages with reference to several figures in detail. Identical or functionally identical elements have the same reference numerals therein. The figures are particularly intended to illustrate the principles essential to the invention and are not necessarily to scale. For better clarity, it can be provided that not all the figures in all figures are marked.

Disclosed device features result analogously from corresponding disclosed method features and vice versa. This means, in particular, that features, technical advantages and embodiments relating to the micromechanical baseband sensor result analogously from corresponding embodiments, features and technical advantages relating to the method for testing a micromechanical baseband sensor and vice versa.

• 1 eine prinzipielle Darstellung eines konventionellen mikromechanischen kapazitiven Beschleunigungssensors;
• 2 ein zeitliches Diagramm zum Erläutern eines Messens mittels eines konventionellen mikromechanischen kapazitiven Beschleunigungssensors;
• 3 ein zeitliches Diagramm zum Erläutern eines vorgeschlagenen Prüfens eines mikromechanischen kapazitiven Beschleunigungssensors; und
• 4 einen prinzipiellen Ablauf einer Ausführungsform eines Verfahrens zum Prüfen eines vorgeschlagenen mikromechanischen Basisbandsensors.

In the figures shows:

• 1 a schematic representation of a conventional micromechanical capacitive acceleration sensor;
• 2 a timing diagram for explaining a measurement by means of a conventional micromechanical capacitive acceleration sensor;
• 3 a timing diagram for explaining a proposed testing of a micromechanical capacitive acceleration sensor; and
• 4 a basic sequence of an embodiment of a method for testing a proposed micromechanical baseband sensor.

Description of embodiments

1 shows a schematic diagram of a conventional micromechanical baseband sensor 100 in the form of a micromechanical capacitive acceleration sensor. One recognizes a MEMS device 10 with a movable ground electrode CM which is movable along an x-axis. It also recognizes readout electrodes C0N . c0p between which a crest of the ground electrode CM is mobile. It goes without saying that this is only a schematic representation and that in practice significantly more readout electrodes C0N . c0p are provided.

As a result, by moving the ground electrode CM between the readout electrodes C0N . c0p the capacitance between the readout electrodes C0N . c0p and the ground electrode CM influenced and There may be electrical charges via connecting lines 12 . 13 in the form of an electrical current to an evaluation circuit 21 a control or evaluation 20 be transmitted.

Ideally form the ground electrode CM and the readout electrodes C0N . c0p an ideal plate capacitor, which is detuned in the case of an acceleration sensor from external acceleration influences. For an ordinary acceleration measurement are the readout electrodes C0N . c0p at a fixed electrical potential V _BIAS , while the ground electrode CM is applied with an alternating voltage VCM, wherein the resulting electric charge flow is measured and evaluated to determine an acceleration value.

The electrical release voltages result in the following electrostatic force: $$F_{elec}=\frac{1}{2}\frac{C_1}{d_1}{\left(V_{C0N}-V_{CM}\right)}^2-\frac{1}{2}\frac{C_2}{d_2}{\left(V_{C0P}-0\right)}^2=-\frac{1}{2}\frac{{\text{C}}_{\text{2}}}{{\text{d}}_{\text{2}}}{\text{V}}_{\text{DISPLACE}}{}^2$$

This means that in normal operation by the equality of the electrical voltages at the readout electrodes C0N . c0p two oppositely equal forces are generated, cancel each other out and thereby in normal operation of the baseband sensor 100 have no influence.

mit den Parametern:

F_ACC

durch externe Beschleunigung aufgebrachte Kraft

K

Federkonstante

Δx

geometrische Auslenkung der Masseelektrode CM

Masse(CM)

physikalische Masse der Masseelektrode CM

a

externe zu messende Beschleunigung

In normal operation, the deflection is thus, to a first approximation, independent of internal electrostatic forces, thus dominated by the spring constant, according to which: $$F_{ACC}=K*\Delta x=Masse\left(CM\right)*a$$

with the parameters:

F _ACC

force applied by external acceleration

K

spring constant

Ax

geometric deflection of the ground electrode CM

Mass (CM)

physical mass of the ground electrode CM

a

external acceleration to be measured

1. (i) Ausüben einer asymmetrischen elektrostatischen Kraft auf den Sensor
2. (ii) Vermessen der daraus resultierenden Bewegung der Masseelektrode CM

Self-test functionality, as implemented in conventional acceleration sensors, works according to the following principle:

1. (i) applying an asymmetric electrostatic force to the sensor
2. (ii) measuring the resulting movement of the ground electrode CM

2 shows a principal time sensor excitation and measurement sequence designed according to the above principle:

Shown are time profiles of electrical voltages at the readout electrodes C0N . c0p and at the ground electrode CM of the micromechanical baseband sensor 100 ,

One recognizes in the 2 two excitation phases A and a measurement phase M , where the measurement phase M temporally directly to the first excitation phase A followed. In the excitation phases A, the readout electrodes C0N . c0p placed on GND or on V _DISLACE potential and the ground electrode CM is set to GND potential. In the measuring phase M, the readout electrodes C0N . c0p is applied to the potential VBIAS and it is applied to the ground electrode CM a readout signal VREAD placed.

mit den Parametern:

F_elec

auslenkende elektrostatische Kraft

V_DISPLACE .

effektive elektrische Auslenkspannung

d1, d2

Abstände der Ausleseelektroden von der Masseelektrode

C1, C2

skalierende Konstanten

In the excitation phase A acts on the ground electrode CM a deflecting force according to the following mathematical relationship: $$F_{elec}=\frac{1}{2}\frac{C_1}{d_1}{\left(0\right)}^2-\frac{1}{2}\frac{C_2}{d_2}{\left(V_{DISPLACe}-0\right)}^2=-\frac{1}{2}\frac{{\text{C}}_{\text{2}}}{{\text{d}}_{\text{2}}}{\text{V}}_{\text{DISPLACE}}{}^2$$

with the parameters:

F _elec

deflecting electrostatic force

V _DISPLACE .

effective electrical deflection voltage

d1, d2

Distances of the readout electrodes from the ground electrode

C1, C2

scaling constants

The force is due to the spring constant of the springs, with which the ground electrode CM of the baseband sensor 100 suspended, a corresponding geometric deflection of the ground electrode CM force, which in turn by the electronic evaluation device 20 of the baseband sensor 100 is measured.

Thus, the electrostatic deflection measurement is conventionally performed immediately after the excitation, which is required to cause the baseband sensor to reverberate 100 into his "electrostatically unrestrained basic position".

A disadvantage of this approach is that it can not be seen in this way, whether a correct internal pressure within the micromechanical baseband sensor 100 prevails. For example, it could happen that due to an error in the manufacturing process in sealing the baseband sensor 100 the internal pressure is reduced, resulting in erroneous operating characteristics of the baseband sensor 100 pulls.

It is therefore proposed that the measurement of the ground electrode CM of the baseband sensor 100 not immediately after the excitation phase A is performed, but only after waiting for an adjustable zero force time T _NK ,

3 shows a schematic representation of the proposed principle. It can be seen that now after the first excitation phase A a so-called zero-force time T _NK is waited, in which the earth electrode CM after its deflection can swing back to a defined degree of its maximum deflection. This is designed individually for sensors, it was determined the zero force time T _NK should be so long that the ground electrode CM in this time can swing back to about 30% to about 70%, preferably to about 50% of their maximum excursion. It should be noted that the zero force time T _NK in 3 is not true to scale, it should preferably be at least about fifty times the measurement time M.

$${\beta }_{\mathrm{1,2}}=-\delta \pm \sqrt{{\delta }^2-{\omega }_0{}^2},\delta \rangle {\omega }_0$$

mit den Parametern:

t

Zeit

x

Geometrische Auslenkung

C₁, C₂

Konstanten zur Skalierung

β

Rechengröße zusammengesetzt aus Eigenkreisfrequenz und Dämpfung („Systemdynamik“)

ω₀

Eigenkreisfrequenz des ungedämpften Systems

δ

Abklingkonstante

Only then will be in the measurement phase M the geometric deflection of the ground electrode CM evaluated. The corresponding transfer function of the baseband sensor 100 is described with sufficient accuracy by a second-order transfer function with strong damping, wherein a time response of such a system can be described mathematically as follows: $$x\left(t\right)=C_1\times e^{\beta 1\times t}+C_2\times e^{\beta 2\times t}$$

$${\beta }_{1.2}=-\delta \pm \sqrt{{\delta }^2-{\omega }_0{}^2}.\delta >{\omega }_0$$

with the parameters:

t

Time

x

Geometric deflection

C ₁ , C ₂

Constants for scaling

β

Calculated variable composed of natural angular frequency and damping ("system dynamics")

ω ₀

Natural angular frequency of the undamped system

δ

decay constant

The decay constant δ is a measure of the damping of the natural angular frequency of the undamped system. Thus, by the appropriate dimensioning of the zero force time T _NK using the above equations (4), (5) with t = T _NK, the internal pressure of the micromechanical base band sensor can be determined.

Both dynamic properties and Brownian noise in the baseband sensor are largely determined by the internal pressure of the baseband sensor.

The proposed measuring method makes it possible to implement a self-test for the baseband sensor which can check the baseband sensor for a changed internal pressure. Thus, reliable functionality of the sensor operation is advantageously supported.

Furthermore, this method is useful for identification of defective produced MEMS in a final measurement in which the parts are calibrated and tested for their function.

Advantageously, the proposed method can be initiated by a user at any time in the operation of the baseband sensor, giving the user a quick overview of the proper functionality of the baseband sensor. This is especially important in applications involving virtual reality, where the micromechanical acceleration sensor is required to respond very quickly to external accelerations.

It is also conceivable, the proposed measurement method automated during normal operation, periodically at appropriate intervals or exclusively after a system start of the micromechanical baseband sensor 100 perform once.

Advantageously, the proposed method can be controlled both by hardware and by software.

4 shows a basic sequence of a proposed method for testing a micromechanical baseband sensor 100 ,

In one step 200 becomes an introduction of electrostatic forces on a ground electrode CM of the baseband sensor 100 performed, wherein the ground electrode C _M is deflected defined.

In one step 210 is maintained for a long time.

In one step 220 a defined evaluation of a geometric deflection of the ground electrode CM after expiry of the waiting period t _NK carried out.

Although the proposed method has been described above solely in connection with a capacitive micromechanical acceleration sensor, it goes without saying that the method also applies to other types of micromechanical baseband sensors (ie sensors without internal mechanical excitation), such as pressure sensors, capacitive and resistive sensors can be used, in which a micromechanical mass element can be deflected defined.

The person skilled in the art can also realize embodiments of the invention which are not disclosed or are only partially disclosed, without deviating from the gist of the invention.

Claims (10)

A method of testing a micromechanical baseband sensor (100), comprising the steps of: - applying electrostatic forces to a ground electrode (CM) of the baseband sensor (100), the ground electrode (C _M ) being deflected in a defined manner; - Defines a long wait; and - Defined evaluation of a geometric deflection of the ground electrode (CM) after the expiration of the waiting time (t _NK ). Method according to Claim 1 wherein the waiting is carried out until the ground electrode (C _M ) has reached a defined percentage of its full scale. Method according to Claim 1 or 2 wherein the waiting is carried out until the ground electrode (C _M ) has reached about 30% to about 70%, preferably about 50% of its full scale. Method according to one of the preceding claims, wherein a deflection of the ground electrode (C _M ) after the defined waiting time is converted into an internal pressure of the baseband sensor. The method of any one of the preceding claims, wherein the method may be initiated by a user at any time during use of the baseband sensor (100). Method according to one of the preceding claims, wherein the method is carried out for a capacitive baseband sensor. Method according to Claim 6 The method is used for a capacitive acceleration sensor. Micromechanical baseband sensor (100), comprising: - A MEMS device having a movable ground electrode (CM) and a defined number of readout electrodes (C0N, C0P), which are cooperatively formed with the movable ground electrode (CM) to electrical charge shifts on the readout electrodes (C0N, C0P) due to movement to detect the ground electrode (CM); and - A drive device (20) which is designed to control the ground electrode (CM) with at least two different electrostatic forces and to evaluate the deflection of the ground electrode (CM) defined after a defined waiting time. Micromechanical baseband sensor (100) after Claim 8 , characterized in that the micromechanical baseband sensor (100) is a micromechanical capacitive acceleration sensor or a micromechanical pressure sensor. Computer program product for carrying out the method according to one of Claims 1 to 7 when running on an electronic driver (20) or stored on a computer-readable storage medium.

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