DEVICE AND METHOD FOR ADJUSTING THE MEASURING RANGE AND SENSITIVITY OF MICROMECHANICAL SENSORS

Description

[0001] The invention relates to a sensor system and an associated method for adjusting the measuring range and increasing the sensitivity of a micromechanical sensor. The device can be integrated, in particular, into micromechanical strain gauges, accelerometers, or temperature sensors. The invention makes it possible, in particular, to capture a wide measuring range with a single micromechanical sensor element while simultaneously increasing the measurement resolution incrementally.

[0002] Micromechanical-electrical sensors, also known as MEMS (Micro Electro-Mechanical System) sensors, typically employ a chip-based technology where the sensors usually consist of a suspended mass between a pair of capacitive plates. When the sensor is moved or tilted, for example, this suspended mass creates a difference in electrical potential. This difference is then measured as a change in capacitance.

[0003] All types of sensors possess a characteristic curve that approximately represents their essential (measurement) characteristics, that is, their response function to a specific input or measured quantity. This characteristic curve, generally represented by a linear equation, e.g., in the form y(x) = a*x + b, does not always correspond ideally to the measurement being performed. The sensitivity corresponds to the parameter 'a' for the slope, and the offset corresponds to the y-intercept with the parameter 'b'. Adapting the characteristic curve to the current measurement range generally allows for an increase in measurement accuracy.

[0004] The sensitivity and measuring range of strain or acceleration sensors, for example, are usually adjusted electronically by the signal processing or evaluation electronics, for instance by increasing the resolution of an analog-to-digital converter or by changing the gain factor of an instrumentation amplifier. The sensitivity is thus only adjusted on the already acquired and converted signal. To increase the detectable measuring range, several sensor elements with different, especially adjacent, measuring ranges have been used, or the sensor element has been physically replaced.

[0005] EP 3 822 577 A1 describes a MEMS sensor which is used for mechanical strain measurement via two connecting elements, the displacement of which is amplified via a mechanical amplifier and converted into an electrical quantity via an electromechanical transducer.

[0006] The article "Improving the Sensitivity and Bandwidth of In-Plane Capacitive Microaccelerometers Using Compliant Mechanical Amplifiers" from the Journal of Microelectromechanical systems, doi: 10.1109 / JMEMS.2014.2300231, describes a method that can improve both the sensitivity and bandwidth of in-plane capacitive micromechanical accelerometers by using compliant mechanical amplifiers, thus avoiding the trade-off between sensitivity and bandwidth.

[0007] The article "A MEMS Accelerometer with an Auto-Tuning System based on an electrostatic anti-spring" from the 2021 IEEE 34th International Conference on Micro Electro Mechanical Systems (MEMS), doi: 10.1109 / MEMS51782.2021.9375285, describes a MEMS accelerometer with an auto-tuning system based on an electrostatic anti-spring. The sensitivity of the accelerometer can be automatically adjusted to change the dynamic range and bandwidth according to the acceleration inputs.

[0008] Therefore, the object of the invention is to provide a sensor or measuring system with which the aforementioned disadvantages are avoided and, in particular, the measuring accuracy or the measuring range and / or the sensitivity of a MEMS sensor are improved.

[0009] This problem is solved by a device according to claim 1 and a method according to claim 13. Preferred embodiments are found in the dependent claims.

[0010] The measuring principle implemented within the scope of the invention aims to adjust the measuring range and / or the sensitivity directly during signal acquisition with the hardware and thus by the sensor element itself, i.e., the sensor properties of the sensing element itself are changed.

[0011] According to the invention, a measuring system based on micromechanical elements is provided, comprising an energy converter which is configured to convert a physical signal into a displacement of the energy converter, a force generator and a movable runner which are configured to convert the displacement from the energy converter into a force on the movable runner, an electrostatic anti-spring which is mechanically connected to the runner, and an electromechanical signal converter which converts a displacement of the runner into an electrical signal, wherein the electrostatic anti-spring is configured to adjust the sensitivity of the measuring system via a control voltage.

[0012] The control voltage enables a shift in the measurement system characteristic and thus its adaptation to a specific application for an increase in the measurement range and / or the accuracy and / or the sensitivity of a MEMS-based measurement system.

[0013] A runner refers in particular to a mechanical element that is movably suspended or supported and can transmit forces or movements.

[0014] The advantages of the invention can be summarized as follows: The sensitivity of a micromechanical sensor can therefore be adjusted by applying a voltage. Furthermore, the measuring range can be shifted by applying a voltage.

[0015] Specific advantages of this strain sensor implementation include strain measurement via a single chip and the elimination of the need for a complex electrical amplifier circuit. The invention also enables a significant reduction in the additional evaluation electronics required.

[0016] Furthermore, no parasitic electrical effects such as electrical noise or high cross-sensitivity to temperature changes are possible, since a purely mechanical principle without resistance is involved. The sensors or measuring systems according to the invention also enable wafer-level manufacturing of the sensor chips and consequently mass production, thus making manufacturing cost-effective. Furthermore, no individual calibration or trimming, as required when trimming the base resistance of a strain gauge, is necessary.

[0017] The invention particularly comprises a measuring system consisting of an energy converter, such as a seismic mass, a force generator, a mechanical amplification mechanism, an electrostatic anti-spring, and optionally an actuator for offset compensation, as well as an electromechanical transducer. The energy converter is designed to convert a physical signal (e.g., pressure, strain, acceleration, temperature, etc.) into a displacement. The force generator, for example in the form of a spring, then converts this displacement into a force. Counterforces from the electrostatic anti-spring, the actuator for offset compensation, and the counterforces of other optional guide springs reduce or increase the deflection at the force generator.The resulting displacement of the force generator is typically amplified by a mechanical amplifier and ultimately transmitted, either directly or indirectly, to the electromechanical signal converter, which transforms the displacement into an electrically measurable signal, such as a change in capacitance. The exact arrangement of the components can vary.

[0018] In a preferred embodiment, the electrostatic anti-spring is bidirectional and / or has varying electrode spacing. Varying electrode spacing allows, in particular, the sensitivity of the entire measuring system to be changed by adjusting the control voltage. Bidirectionality means, in particular, that the properties of the anti-spring are nearly equivalent for both directions of movement or translation.

[0019] In another preferred embodiment, the electrostatic anti-spring is designed as a symmetrical electrostatic comb drive with pairs of curved and, optionally, additional parallel plate electrodes. In particular, the combination of curved and parallel plate electrodes allows for advantageous adjustment of the sensitivity of the measuring system via the control voltage.

[0020] In a preferred implementation of the measuring system, the runner is connected to an electrostatic actuator, which is configured to adjust the offset of the measuring system via a control voltage. The control voltage at the actuator thus enables an additional shift in the measuring system characteristic and therefore a further possibility for adaptation to a specific application, in particular for changing the measuring range and / or the accuracy and / or the sensitivity of a MEMS-based measuring system.

[0021] In a particularly preferred embodiment, the electrostatic actuator is designed as a symmetrical electrostatic comb drive with pairs of parallel plate electrodes. The characteristics of a comb drive with pairs of parallel plate electrodes allow for a relatively simple setting or correction of a constant offset for the measuring system by means of an applied control voltage.

[0022] Preferably, the runner is connected to a micromechanical amplifier. The micromechanical amplifier can increase the amplitude of a translation or displacement, particularly by means of a series connection of levers. In particular, the micromechanical amplifier converts an input translational motion into an output amplitude-amplified translational displacement by means of a series connection of levers.

[0023] In a particular embodiment, the runner has a return mechanism. This can be implemented electromechanically, but also purely mechanically, for example using spring elements.

[0024] In particular, the runner is held in place by means of guide springs. This also allows for a floating and / or movable suspension of the runner.

[0025] Preferably, the electromechanical signal converter is based on a change in capacitance, as is the case in particular with comb-like MEMS structures and is predominantly used.

[0026] The force generator can be easily implemented using a mechanical spring. Electromechanical implementations are also conceivable for converting a physical signal, particularly into a translation and / or displacement.

[0027] In a simple embodiment, the runner is a rigid mechanical element, for example a simple rod, to which other mechanical or MEMS elements can be coupled.

[0028] In a particularly preferred embodiment, the invention comprises a computer configured to perform offset compensation and / or sensitivity adjustment. For this purpose, the computer must acquire and evaluate signals from the measuring system and adjust the respective control voltage accordingly. This allows the adjustments to be made automatically.

[0029] The invention further comprises a method for adjusting the measuring range and / or increasing the sensitivity of a micromechanical measuring system, comprising the steps of: converting a physical signal into a displacement and / or translation by means of an energy converter, converting the displacement and / or translation from the energy converter into a force on a movable rotor by means of a force generator, and converting the displacement and / or translation of the rotor into an electrical signal by means of an electromechanical signal converter, wherein the sensitivity of the measuring system is adjusted via a control voltage of an electrostatic anti-spring, which is connected to the rotor and / or the force generator. The method can also provide for offset compensation of the measuring system via a control voltage of an electrostatic actuator, wherein the actuator is also connected to the rotor and / or the force generator.The control voltage at the actuator then allows for an additional shift in the measurement system characteristic and thus a further possibility for adaptation to a specific application.

[0030] Preferably, the method comprises the steps of measuring with a micromechanical measuring system, performing offset compensation, increasing sensitivity, and repeated measurements with the measuring system. This allows for an iterative improvement of the measurement accuracy or measuring range and / or sensitivity of a MEMS sensor or MEMS-based measuring system.

[0031] The invention further comprises the use of the aforementioned micromechanical measuring system according to one of the preceding claims in a strain sensor or acceleration sensor or temperature sensor.

[0032] The invention will now be explained in more detail using a preferred embodiment and with reference to the drawings.

[0033] The drawings show Fig. 1 a schematic basic principle for the iterative improvement of a measuring system according to a preferred embodiment of the invention, Fig. 2 an illustration of the basic principle for the iterative improvement of a measuring system according to the invention based on characteristic curves of the measuring system, Fig. 3 a preferred embodiment of the invention with a strain gauge sensor, Fig. 4 substitute diagrams to illustrate the forces present, Fig. 5 further substitute diagrams to illustrate the forces present, Fig. 6 a further embodiment of the sensor with a modified arrangement of the force generator, Fig. 7 a preferred embodiment of an electrostatic anti-spring, Fig. 8 a further preferred embodiment of an electrostatic anti-spring and Fig. 9 a preferred embodiment of the invention with an accelerometer.

[0034] The Figure 1This paper first presents a fundamental approach to improving the measurement characteristics of a micromechanical measurement system 1. The measurement system 1 is initially designed or configured for a very wide measurement range. In step 1, a coarse measurement is first performed across this wide range. Then, in step 2, the measurement range is shifted so that the measured value lies approximately in the middle of the new measurement range. In step 3, the sensor's sensitivity is increased, resulting in a more precise measurement resolution. The invention utilizes micromechanical components for shifting the measurement range (step 2) and increasing the sensitivity (step 3), allowing the adjustments to be made mechanically directly on the chip.

[0035] The basic concept of the device will be explained below using the example of a strain sensor and first in Figure 2This is illustrated. Specifically, it concerns a silicon-based micromechanical strain sensor. The strain sensor, for example in the form of a chip, is bonded to a test object and detects the strain present in the object, converting it into an electrically evaluable signal, in particular a change in capacitance. A key feature of the strain sensor is that the measuring range and / or the sensitivity can be influenced by applying at least one control voltage to a sensor according to the invention.

[0036] In a first step, the sensor converts the measurement signal X – here in the form of a strain amplitude – into a displacement signal y out. The displacement signal serves as the sensor's output and, following data acquisition, can be encoded via an encoder or converted into a capacitance change via a capacitive transducer. For an unknown strain amplitude, it is advantageous to cover as large a measurement range as possible with the sensor and thus initially determine the approximate amplitude of the measured quantity X (here, strain). Here, the measurement range is limited to a relatively high strain value, e.g., 5%. After the approximate strain amplitude has been determined in a first step (see...). Fig. 3In step 2, the offset is compensated for using a control voltage U offset, thus shifting the measuring range so that the measured value now lies in the middle or approximately in the middle of the measuring range of the adapted sensor. Consequently, measured values ​​that were previously at the edge or even outside the originally defined measuring range can now be captured. In step 3, the sensitivity of the sensor is increased by applying another control voltage US, in order to implement a more precisely resolved measurement after the coarse measurement from step 1.

[0037] Figure 3Figure 1 shows a possible embodiment of the strain sensor. The device comprises a mechanical differential amplifier 9 with two contact points 11 and 12, a force generator 3 in the form of a spring, an electrostatic actuator 5 for offset compensation, an electrostatic anti-spring 6, and an electromechanical signal converter 7. To detect strain, the sensor is connected to the test object via contact points 1 and 2. The strain ε of the test object is thus coupled into the sensor via the contact points. Strain on the test object causes a relative displacement of the two contact points. The mechanical differential amplifier 9 amplifies the relative displacement Δy by a gain factor A and introduces the amplified displacement yAMP into a force generator 3 – in this case, a spring.The increased displacement is reduced by counterforces of the guide springs 10, whereby the force generator 3 in the form of a spring is compressed and only transmits a reduced displacement y out < y AMP to the runner 4.

[0038] The equivalent circuit diagram in Figure 4 (Left) illustrates the springs at work and the resulting displacement. The displacement depends on the spring constant. k FG (power generator (3)) and k FF (guide springs 10): y out = y AMP k FG k FG + k FF

[0039] With increasing stiffness of the guide springs k FF consequently reduces the output shift y out and thus the sensitivity.

[0040] Provided that the electrostatic actuator 5 for offset compensation and the electrostatic anti-spring 6 are deactivated, i.e., provided that no voltage is applied to them, the displacement y output to the electromechanical signal converter 7.

[0041] The signal converter 7 translates the displacement y out into a change in an electrical state variable, usually a change in capacitance. The sensor now covers a wide measuring range with low sensitivity. If the measured value is at the edge of or slightly outside the measuring range, the electrostatic actuator 5 is activated for offset compensation by applying a voltage. The measuring range is shifted depending on the voltage U offset. A shift in the measuring range corresponds to a mechanical displacement of the rotor 4. The actuator for offset compensation generates a constant force for this purpose. F of depending on the applied voltage that is impressed on the power generator 3, whereby y out is shifted by an offset, see Figure 4 right. In simplified terms: y out = y AMP k FG k FG + k FF ± F of U offset k FG + k FF

[0042] Depending on the direction of the force F of will the postponement yThe offset is reduced or increased by the amount of the difference.

[0043] If the electrostatic anti-spring 6 is also activated, the equivalent circuit diagram is as follows: Figure 5 .

[0044] The spring stiffness of the guide spring k FF is now determined by the negative spring stiffness - k AS ( U s) of the electrostatic anti-spring is reduced. Thus, the restoring counterforce of the guide springs decreases and y The output displacement reaches higher displacements depending on the applied strain. Consequently, the sensitivity increases. For the initial displacement, the following simplified equation applies: y out = y AMP k FG k FG + k FF − k AS U S ± F of U offset k FG + k FF − k AS U S

[0045] Thus, the measuring range can be determined via the offset voltage Uoffset, as well as the sensitivity of the measurement via... U Set S.

[0046] Figure 6Figure 1 shows another embodiment of the strain sensor with a modified arrangement of the force generator 3 and without the electrostatic actuator 5 for offset compensation. The force generator 3 is moved upstream of the amplifier 9 in the sequence of operations. The stiffness compensation by the anti-spring thus takes place at the force generator before the displacement amplification. The lever amplifier increases the set stiffness at the anti-spring by the square of the amplification factor A, thereby exerting a particularly strong effect on the force generator 3.

[0047] A preferred embodiment of the electrostatic anti-spring is described in Figur 7A shown. The configuration comprises a symmetrical pairing of fixed curved electrodes 15 and fixed parallel electrodes 16 that are at a common first electrical potential (e.g., ground). Furthermore, movable inner electrodes 17 are required, which are connected to a second electrical potential (e.g., U sThe electrode gap d2(y) between the inner electrode 17 and the curved electrode 15 varies depending on the displacement y of the movable electrode. The anti-spring is subjected to a force depending on the displacement y and the applied voltage. U s an electrostatic force F AF ( y,U s ) generated. It is composed of the electrostatic force at the curved electrode. F 2 ( y,U s ) and the power F 1 ( y,U s ) at the parallel electrode: F AF y U s = F 1 y U s − F 2 y U s

[0048] Both individual forces act in opposite directions. The parallel electrodes 16 are used to compensate for a possible offset of the electrostatic force. F 2 ( y,U s ) to compensate within the anti-spring, as shown in the diagram in Figur 7A points to the right.

[0049] For the power F AF ( y,U s ) still applies: F AF y U s = ε 0 t d 1 U s 2 − ε 0 t d 2 y U s 2

[0050] Where the parameter t here is the thickness of the electrodes in the z-direction and ε 0 takes permittivity into account. Fig. 7B Figure 1 shows a curved electrode 15 in detail. The electrodes 15 are designed such that the electrode spacing d 2 (y) is proportional to the factor . a and the following relationship applies to the displacement factor b: d 2 y = a y + b

[0051] For the constant distance d The following still applies between inner electrode 17 and parallel electrode 16: d 1 = a b

[0052] By deriving the force F AF ( y,U s ) After the displacement y, the electrostatic stiffness of the anti-spring is thus obtained. k AF ( U s ) depending on the applied voltage Us to: k AF U s = − ε 0 t a U s 2 .

[0053] Figure 8Figure 1 shows another preferred embodiment of the electrostatic anti-spring. Instead of additional parallel electrodes 16, the stray field at the edge of the curved electrodes 15 is used here to compensate for a possible offset of the electrostatic forces (see diagram in Figure 1). Figure 8 right).

[0054] The Figure 9 Figure 1 shows another embodiment of a sensor with adjustable sensitivity and measuring range. Instead of strain, acceleration is detected. For this purpose, the sensor now comprises a seismic mass 13 instead of contact points. This mass is rigidly suspended by springs 14 and is connected via the force generator 3 to the input of a mechanical amplifier mechanism.

[0055] Acceleration sensors are typically selected according to the expected acceleration, so that the sensor's measuring range covers the expected acceleration. However, as the measuring range increases, the resolution of the sensor or measuring system generally decreases.

[0056] If an acceleration of 100.4g is expected, a 6-bit sensor with a measuring range of 0 to 128g is used, for example. The expected measured value is therefore at the edge of the measuring range. The sensor resolution corresponds to 2g / bit (= 128g / 2 6< )This means that the measured value of 100.4g is not fully resolved, but lies between the resolvable measured values ​​of 100g and 102g. In an equivalent embodiment of the sensor with the device according to the invention, with the same measuring range of 0 to 128g, the measured value of 100g±2g is initially roughly recorded. Now, however, the measuring range is shifted so that the value of 100g lies in the middle of the measuring range. The measuring range then lies between 36g and 164g. Now the sensitivity is increased by the anti-spring 6, e.g., by a factor of ten, which also narrows the measuring range. The measuring range now lies between 94g and 106g. The 6-bit resolution of the sensor now results in a resolution and step size of 0.2g / bit (=12.8g / 2 6< ). The measured value of 100.4g can now be resolved with the same bit depth. Reference symbol list

[0057] 1 Measuring system 2 Energy converter 3 Power generator 4 Rotor 5 Electrostatic actuator 6 Electrostatic anti-spring 7 Electromechanical signal converter 8 Micromechanical amplifier 9 Mechanical differential amplifier 10 Guide spring 11 Contact point 1 12 Contact point 2 13 Seismic mass 14 Spring 15 Curved electrode of a preferred embodiment of the anti-spring 16 Parallel electrode of a preferred embodiment of the anti-spring 17 Movable electrode of a preferred embodiment of the anti-spring

Claims

1. Measuring system (1) based on micromechanical elements, comprising - an energy converter (13), which is configured to convert a physical signal into a displacement of the energy converter (13), - a force generator (3) which is configured to convert the displacement of the energy converter (13) into a force on a movably mounted slider (4), and - an electromechanical signal converter (7) which converts a displacement of the slider (4) into an electrical signal, characterized in that the measuring system (1) comprises an electrostatic anti-spring (6), which is mechanically connected to the slider (4), wherein the electrostatic anti-spring (6) is configured to adjust a sensitivity of the measuring system (1) via a control voltage.

2. Measuring system according to claim 1, wherein the electrostatic anti-spring (6) is bidirectional and / or has varying electrode spacings.

3. Measuring system according to claim 1 or 2, wherein the electrostatic anti-spring (6) is configured as a symmetrical electrostatic comb drive with pairs of curved and parallel plate electrodes.

4. Measuring system according to one of the preceding claims, wherein the slider (4) is connected to an electrostatic actuator (5), wherein the electrostatic actuator (5) is configured to adjust an offset compensation of the measuring system (1) via a control voltage.

5. Measuring system according to claim 4, wherein the electrostatic actuator (5) is configured as a symmetrical electrostatic comb drive with pairs of parallel plate electrodes.

6. Measuring system according to one of the preceding claims, wherein the slider (4) is connected to a micromechanical amplifier (8).

7. Measuring system according to claim 6, wherein the micromechanical amplifier (8) converts an input-side translatory movement into an amplitude-amplified translatory displacement on the output side by means of a series connection of levers.

8. Measuring system according to one of the preceding claims, wherein the slider (4) is held by means of guide springs (10).

9. The measuring system according to any one of the preceding claims, wherein the electromechanical signal converter (7) is based on a capacitance change.

10. Measuring system according to one of the previous claims, wherein the force generator (3) is a spring.

11. A measuring system according to any one of the preceding claims, wherein the slider (4) is a rigid mechanical element.

12. A measuring system according to any one of the preceding claims, comprising a computer which is connected to the electrostatic actuator (5) and / or the electrostatic anti-spring (6) and / or the electromechanical signal converter (7) and is configured to perform the offset compensation and / or the sensitivity adjustment.

13. Method for adjusting the sensitivity and / or for adjusting the measuring range of a micromechanical measuring system (1), comprising the steps of - Converting a physical signal into a displacement of an energy converter (13) by means of the energy converter (13), - Converting the displacement from the energy converter (13) into a force on a movably mounted slider (4) by means of a force generator (3), and - Converting the displacement of the slider (4) into an electrical signal by means of an electromechanical signal converter (7), wherein the sensitivity of the measuring system (1) is adjusted via a control voltage of an electrostatic anti-spring (4), which is connected to the slider (4) and / or the force generator.

14. A method according to the preceding claim, comprising the steps of - Measure using the micromechanical measuring system (1), - Performing out offset compensation, - Increase sensitivity and - repeated measurement using the measuring system (1).

15. Use of the micromechanical measuring system according to one of the preceding claims in a strain sensor or acceleration sensor or temperature sensor.