WO2005068959A2

WO2005068959A2 - High performance integrated electronic for mems capacitive strain sensors

Info

Publication number: WO2005068959A2
Application number: PCT/US2005/000257
Authority: WO
Inventors: Wen H. Ko; Jun Guo; Darrin J. Young; Michael Suster; Nattapon Chaimanonart
Original assignee: Case Western Reserve University
Priority date: 2004-01-05
Filing date: 2005-01-05
Publication date: 2005-07-28
Also published as: WO2005068959A3

Abstract

An electronic strain sensing assembly comprising a capacitive strain sensor configured to sense a strain and output a capacitance change representative of the sensed strain. A driving clock provides a clock signal configured to drive the strain sensor at a high frequency. A differential charge amplifier is configured to receive the capacitance change, amplify the capacitance change and output an amplified signal. A mixer is configured to receive the amplified signal, mix the amplified signal with a signal equal to the clock signal and output a mixed signal. A low-pass filter is configured to receive and low-pass filter the mixed signal to provide a strain signal representative of the sensed strain.

Description

HIGH PERFORMANCE INTEGRATED ELECTRONICS FOR MEMS CAPACITIVE STRAIN SENSORS

BACKGROUND

[0001] The present invention relates to capacitive sensors, and particularly to electronics systems for capacitive sensors.

[0002] Strain sensors are widely used in various applications such as point-stress and torque sensing for ballbearings, rotating shafts and blades, and the like. In general, metal foil and semiconductor piezoresistive strain sensors are typically used to measure different strains or pressures. Conventional strain sensors made of metal foils and semiconductor piezoresistive elements also suffer from a limited sensitivity, large temperature dependence and turn-on drift, and incompatibility to standard CMOS integration, thus inadequate for high-performance applications. These techniques generally only achieve a strain resolution of 10 microstrain (μe), which offers a dynamic range of 20dB.

[0003] In order to achieve reliable industrial strain sensors with high sensitivity, large dynamic range (for example, 80 dB) over a large bandwidth, capacitive micro- electro-mechanical system ("MEMS") strain sensors are developed. Capacitive strain sensors have key advantages such as minimum temperature dependence and turn-on drift, low temperature drift, high sensitivity, low noise, and large dynamic range. For example, some capacitive strain sensors offer a sensitivity of about 0.1 microstrain (μe) and a dynamic range of about 10,000 or lOOdB. However, conventional interfacing circuits is either not compatible with these capacitive strain sensors or fail to achieve these stringent system performance requirements.

SUMMARY

[0004] Accordingly, there is a need for improved interface circuits for the capacitive strain sensors to achieve stringent system performance requirements, such as high sensitivity, large signal bandwidth, low power dissipation, small area, and reliable telemetry capability for power coupling and data transmission. For example, the interface circuit will provide low-noise and low-power CMOS interface electronics for MEMS capacitive strain sensor.

[0005] Since a small and accurate capacitance value is typically generally difficult to measure, the interface circuit is also configured to convert a change of the sensor capacitance value, due to an input strain, to an output voltage either in an analog or digital format. Furthermore, the interface circuit will optionally include low noise CMOS telemetry circuits. The low noise CMOS telemetry circuits can convert an incoming RF signal to a stable DC power supplying the interface electronics, and wirelessly transmit the sensor information to a nearby receiver.

[0006] Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 shows a simplified schematic of a capacitive MEMS strain sensor with mechanical amplification according to the invention;

[0008] FIG. 2 shows four fabricated sensors connected in parallel according to the invention;

[0009] FIG. 3 shows an embodiment of an electronic strain sensing system according to the invention;

[0010] FIG. 4 shows a second embodiment of an electronic strain sensing system with a low noise CMOS telemetry circuit;

[0011] FIG. 5 shows a detailed telemetry circuit of the electronic strain sensing system shown in FIG. 4;

[0012] FIG. 6 shows an embodiment of a chip incorporating core electronics according to the invention; [0013] FIG. 7 shows a board incorporating MEMS sensor chip bonded to a stainless steel substrate according to the invention;

[0014] FIG. 8 shows a measured output voltage versus an applied input strain with the interface circuit shown in FIG. 3 according to the invention; and

[0015] FIG. 9 shows a measured output noise spectral density with the interface circuit shown in FIG. 3 according to the invention.

DETAILED DESCRIPTION

[0016] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms "connected," "coupled," and "mounted" and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms "connected" and "coupled" and variations thereof are not restricted to physical or mechanical connections or couplings.

[0017] High-performance miniature strain sensing modules consisting of sensors and interface electronics are highly critical for advanced industrial applications, such as point-stress and torque sensing for ballbearings, rotating shafts and blades, and the like. Stringent performance requirements with a high sensitivity of 0.1 microstrain (μe) over a wide bandwidth and a large dynamic range of 80 dB are demanded for these applications. Conventional strain sensors made of metal foils and semiconductor piezoresistive elements, however, offer only a limited sensitivity, large temperature dependence and turn-on drift, and incompatibility to standard CMOS integration, and thus are inadequate for high-performance applications. Furthermore, capacitive strain sensors are attractive due to a number of key features such as high sensitivity, minimum temperature dependence and turn-on drift, low noise, large dynamic range, and potential monolithic integration with CMOS.

[0018] FIG. 1 shows an exemplary capacitive MEMS strain sensor 100 with mechanical amplification. Particularly, the capacitive MEMS strain sensor 100 includes three amplifying buckled beams, C_top 104, C_COm 108, and C_bottom 112 for detecting small input strains. Each of the buckled beams 104, 108, and 112, has a plurality of comb fingers 116 positioned at a structural center of the buckled beam 104, 108, 112. An externally applied strain introduces a small lateral displacement Δx. The lateral displacement results in a further enhanced beam deflection along a vertical axis, upward for the C_top 104 and Cbottom 112 beams and downward for the center beam C_COm 108. The C_top 104 and C_bottom 112 beams or electrodes thus form a set of linear differential capacitors (C_s+and C_3-) with respect to the C_com electrode, serving as a common reference electrode. Sensitivity of the capacitive MEMS strain sensor 100 can be optimized by carefully selecting parameters such as buckling angle 120, number of fingers, gauge length, beam thickness, gap size and the like. In one embodiment, the capacitive MEMS strain sensor 100 has a buckling angle 120 of approximately 6°, a structural thickness of 20μm, a minimum air gap size of 3.6μm, and a lateral gauge length of 1 mm with 37 sets of center-positioned fingers 116. Of course, other dimensions and numbers can also be used depending on operation requirements.

[0019] FIG. 2 shows four fabricated sensors 200 connected in parallel according to the invention. The sensors are preferably fabricated by using DRIE on SOI substrates followed by releasing a plurality of microstructures, however, other fabrication techniques may also be utilized. The fabricated sensors 200 connected in parallel achieve a nominal capacitance value of 0.44pF with a differential sensitivity of 25aF per O.lμe. Therefore, these sensors 200 can detect small signal change.

[0020] FIG. 3 shows an embodiment of an electronic strain sensing system 300. Specifically, the electronic strain sensing system 300 includes a low-noise CMOS integrated sensing electronics interfaced with a capacitive MEMS strain sensor module 308. Also, the sensor electronics of the module 308 are preferably fabricated using a 1.5μm CMOS process and consume 1.5mA current from a 3N-power supply. Furthermore, the electronic strain sensing system 300 can be configured to achieve different sensing resolution over a particular bandwidth. For example, the electronic strain sensing system 300 in one embodiment has a sensing resolution of 25aF over a lOKHz bandwidth, which corresponds to a minimum strain of O.lμe, and a dynamic range of about 80 dB. The MEMS sensor module 308, modeled with a pair of differential capacitors and as described earlier, are driven by the same clock signal 312 with amplitude such as 3V. The MEMS sensor module 308 is also interfaced with a differential charge amplifier 316, which converts a sensor capacitance change to an output voltage. In the embodiment, a clock frequency of 1 MHz is chosen to modulate the sensor information away from the 1// noise of the amplifier 316 to achieve a high sensitivity. Of course, other clock frequencies can also be used depending on applications and requirements.

[0021] Furthermore, an input common-mode feedback ("ICMFB") circuit 320 is incorporated with the amplifier 316 to minimize an input common-mode shift caused by the driving clock 312, and hence, suppressing any offset signal due to parasitic capacitance mismatch and drift over time. The amplifier 316 is preferably a low noise amplifier with an input referred noise spectral density of 5nV/(Hz° ⁵). Output of the amplifier 316 is then mixed by a same clock signal and low-pass filtered with a low pass filter 324 to obtain a desired strain information. Particularly, a fully balanced analog multiplier 328 is employed for mixing to minimize clock feed through, thus ensuring an adequate output voltage swing. In the embodiment, the low pass filter 324 is a second-order low-pass filter with a cut-off frequency of 150KHz. The second order low-pass filter has a linear phase characteristic to ensure an undistorted time- domain signal waveform over a large bandwidth.

[0022] Output of the low-pass filter 324 can also be digitized for data telemetry and processing. For example, FIG. 4 shows a second embodiment of an electronic strain sensing system 400 with a low noise CMOS telemetry circuit 428. Similar to FIG. 3, the MEMS differential strain sensor module 408, modeled as C_s+ and C_3-, is driven by a square clock signal 412, and interfaced with a low-noise charge amplifier 416 to convert the sensor capacitance change to an output voltage. The square wave clock signal has a typical frequency on the order of MHz (1MHz, for example) to modulate the sensor information to the clock frequency, thus effectively bypassing the low frequency noise, such as 1 //noise, contributed from the amplifier 416. Output of the amplifier 416 is further mixed with the same clock signal to demodulate the sensor information down to base band, at which the output is filtered by a low pass filter 420 to retrieve the sensor information. In one embodiment, a signal range from 76uN and 760mN over a lOKHz bandwidth can be achieved at the low pass filter output, which corresponds to an input strain ranging from a minimum of O.lμe to a maximum of lme over the bandwidth. A signal-to-noise ratio (SΝR) of 10 dB can be obtained at the minimum input strain. The low SΝR performance is achieved through minimizing the electrical thermal noise contribution from the electronic circuits. Output voltages of the low pass filter 420 can be further digitized through a second order delta-sigma analog-to-digital ("ΣΔ A/D") converter 424. For example, the ΣΔ A/D converter 424 can be configured to achieve a desired resolution of 13 bits. The ΣΔ A/D converter 424 architecture is chosen for its flexibility in resolution through adjusting the clock rate. However, other A/D converter that can achieve a desired resolution and satisfy the sampling clock rate can also be used. The digitized output data from the converter 424 will be transmitted to a nearby receiver for signal analysis through a telemetry circuit 428. In particular, the telemetry circuit 428 is configured to employ a frequency-shift keying (FSK) transmitter for data telemetry. The system 400 consumes a DC power of 5mW from a 3 V-power supply.

[0023] FIG. 5 shows a system diagram of the telemetry circuit 428 of the electronic strain sensing system 400 shown in FIG. 4. Particularly, the telemetry circuit 428 includes a matched coil loop network or coupled coil loop 448 that couples RF power of any external RF signal source into the CMOS telemetry circuit 428. A typical coupling distance of the loop 448 is between 1 to 6 inches, however, other distances are also possible depending on applications and requirements. The coupled RF signal is first rectified by a CMOS-based full wave rectifier 452 and followed by a voltage regulator 456 providing a stable DC supply of about 3 V with a current driving capability of 2mA. The DC supply of 3N and 2mA are generally sufficient to supply power to the interface system 400 shown in FIG. 4. The supply voltage achieves a low noise of 12μV over lOKHz bandwidth and a peak to peak ripple of 2.5 mV 50 MHz.

[0024] FIG. 6 shows an embodiment of a chip 600 incorporating core electronics according to the invention occupying an area of 0.6 mm x 1.7 mm. Specifically, the chip 600 includes a low-pass filter 606, a mixer 612, a charge amplifier 616, a bandgap reference 620, and an oscillator 624. FIG. 7 shows a prototype system 648 incorporating a MEMS sensor chip 652 bonded to a stainless steel substrate. Specifically, the MEMS sensor chip 652 is first bonded to a stainless steel substrate and then wire bonded to the sensing electronics 600 as shown in FIG. 6 to form the prototype system 648. The stainless steel substrate is generally subjected to a four- point testing setup for system characterization.

[0025] FIG. 8 shows a plot 700 of a measured output voltage 704 of the system 648 versus an applied input strain 708 with the interface circuit shown in FIG. 3. Particularly, FIG. 8 also indicates that the prototype system 648 achieves a maximum input signal of lOOOμe, corresponding to an output voltage of 380 mV with a linearity of 1.5%. FIG. 9 shows a second plot 720 of a measured output noise spectral density with the interface circuit shown in FIG. 3. Particularly, FIG. 9 shows that the prototype system 648 achieves a low noise floor of 375nV/(Hz° ⁵), which is equivalent to a minimum detectable signal of 37.5μV over lOKHz bandwidth, thus a dynamic range of 80 dB.

[0026] Various features and advantages of the invention are set forth in the following claims.

Claims

What is Claimed

1. An electronic strain sensing assembly comprising: a capacitive strain sensor configured to sense a strain and output a capacitance change representative of the sensed strain; a driving clock providing a clock signal configured to drive the strain sensor at a high frequency; a differential charge amplifier configured to receive the capacitance change, amplify the capacitance change and output an amplified signal; a mixer configured to receive the amplified signal, mix the amplified signal with a signal equal to the clock signal and output a mixed signal; and a low-pass filter configured to receive and low-pass filter the mixed signal to provide a strain signal representative of the sensed strain.

2. The electronic strain sensing assembly of claim 1 further comprising an input common-mode feedback circuit associated with the amplifier and configured to minimize an input common-mode shift caused by the driving clock.

3. The electronic strain sensing assembly of claim 1 wherein the high frequency is a frequency on the order of MHz.

4. The electronic strain sensing assembly of claim 3 wherein the high frequency is equal to approximately 1 MHz.

5. The electronic strain sensing assembly of claim 1 wherein the capacitive strain sensor is a MEMS sensor module.

6. The electronic strain sensing assembly of claim 1 wherein the amplifier is a low noise amplifier with an input referred noise spectral density of about 5nV/(Hz° ⁵).

7. The electronic strain sensing assembly of claim 1 wherein the low-pass filter is a second-order low-pass filter with a cut-off frequency of 150KHz.

8. The electronic strain sensing assembly of claim 8 wherein the second-order low-pass filter has a linear phase characteristic.

9. The electronic strain sensing assembly of claim 1 further comprising an A/D converter configured to receive the strain signal and convert the strain signal to a digitized output signal.

10. The electronic strain sensing assembly of claim 9 further comprising a telemetry circuit configured to receive the digitized output signal and transmit the digitized output signal to a receiver.

11. The electronic strain sensing assembly of claim 10 wherein the telemetry circuit is configured to employ a frequency-shift keying transmitter for data telemetry.

12. The electronic strain sensing assembly of claim 10 wherein the telemetry circuit includes a matched coil loop network or coupled coil loop configured to couple an RF power signal of an external RF signal source into the telemetry circuit.

13. The electronic strain sensing assembly of claim 12 wherein the RF power signal is rectified by a CMOS-based full wave rectifier and a voltage regulator to provide a stable DC supply.

14. A method of sensing a strain including the steps of: sensing a strain on a strain sensor and outputting a capacitance change representative of the sensed strain; providing a clock signal configured to drive the strain sensor at a high frequency; amplifying the capacitance change and outputting an amplified signal; mixing the amplified signal with a signal equal to the clock signal and outputting a mixed signal; and low-pass filtering the mixed signal to provide a strain signal representative of the sensed strain.

15. The method of claim 14 wherein the high frequency is a frequency on the order of MHz.

16. The method of claim 15 wherein the high frequency is equal to approximately 1 MHz.

17. The method of claim 14 further comprising the step of converting the strain signal to a digitized output signal.

18. The method of claim 17 further comprising the step of providing a telemetry circuit configured to receive the digitized output signal and transmit the digitized output signal to a receiver.

19. The method of claim 18 further comprising the step of providing a matched coil loop network or coupled coil loop configured to couple an RF power signal of an external RF signal source into the telemetry circuit.