TEMPERATURE AND SENSITIVITY ANALYSIS ON MEMS VIBRATION SENSORS WITH DIFFERENT PRINCIPLES OF OPERATION

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1 TEMPERATURE AND SENSITIVITY ANALYSIS ON MEMS VIBRATION SENSORS WITH DIFFERENT PRINCIPLES OF OPERATION Stanislav Klusáček and Jiří Fialka CEITEC - Central European Institute of Technology, Brno University of Technology, Technicka 3082/12, Brno, Czech Republic stanislav.klusacek@ceitec.vutbr.cz Zdeněk Havránek, Petr Beneš and Stanislav Hasík Centre for Research and Utilization of Renewable Energy / Faculty of Electrical Engineering and Communication, Brno University of Technology, Technicka 3082/12, Brno, Czech Republic This article describes the experimental research on qualitative parameter validation of MEMS-based vibration sensors. The aim of the paper is to compare three MEMS vibration sensors with different principles of operation; the investigated samples represent major principles used in currently available MEMS accelerometers, namely the capacitive (STMicroelectronics LIS3L06AL), thermal (MEMSIC MXA6500M), and piezoresistive (Panasonic AGS61331) approaches. The actual comparison of the examined objects was performed using primary and secondary methods for the calibration of vibration sensors. In the first experiment, we measured the sensitivity and frequency characteristics within the operating range of the accelerometers, applying the primary methods to compare the investigated MEMS objects. At the following stage, the results yielded from the monitoring of temperature effects and their analysis based on the secondary calibration methods were utilized to verify the sensitivity and temperature dependences of the MEMS accelerometers. Within the paper, the measured characteristics are compared to the related catalog datasheets, and the possibilities of suppressing the temperature effects are discussed to finalize the overall presentation of the problem. 1. Introduction At present, MEMS-based vibration sensors are widely utilized within various branches of electrical engineering, including the production of consumer electronics, complex measuring systems, automobile components, and space technologies. A major factor behind such popularity of these elements consists in the relatively low purchase costs, which actually made the sensors fully applicable even in highly specialized areas, such as the development of systems for precise navigation. The central advantage of these sensors consists in that the embedded electronics ensure reliable processing output signal; thus, the signal does not require further modification. Alternatively, the signal can be effectively included in superior, or master, systems. Yet there still ICSV22, Florence (Italy) July

2 remain several problematic aspects to be solved, and one of these lies in securing thermal stabilization in the sensors. The authors of this paper were motivated by the overall effort to examine the thermal stability and output signal dependence in differently conceived MEMS accelerometers (capacitive; thermal; piezoresistive). The resulting knowledge of the temperature dependence can be employed as a tool to suppress spurious temperature effects influencing the sensors; for example, actual the temperature sensitivity can be corrected in such a manner that a temperature sensor is placed in the vicinity of the vibration sensor, thus ensuring efficient temperature monitoring around the accelerometer. Subsequently, sensor temperature models will enable us to adjust the sensor s sensitivity and to secure more accurate measurement within a wide range of temperatures. 2. Measuring methods To verify reliably the characteristics of the three MEMS-related principles, we utilized two different measuring systems. While a SPEKTRA CS-18 calibration system (Fig. 4a) was used to facilitate exact calibration of the vibration sensors sensitivity at the stabilized temperature of 24 C, the measurement of the temperature characteristics within the range of between -20 C and 60 C relied on a calibration chain by Brüel&Kjær and a CTS temperature test chamber (Fig. 4b). As the structure of the applied MEMS-based sensors involves multiple axes, their correct orientation was secured via a duralumin cube having several assembly holes on its sides. 2.1 Measured Samples Piezoresistive accelerometer AGS61331 The product is a triaxial piezoresistive MEMS accelerometer (with a voltage output) manufactured by Matsushita-Panasonic; the sensor does not enable any additional suppression of temperature-related or other spurious effects. C: 333 mv/g (33,98 mv/m.s -2 ) ±6 % Temperature sensitivity: max. ±9 % in range -20 C - 70 C Acceleration measurement range: ±3 g Power supply voltage: 3 VDC (min. 2.7 V max. 3.6 V) Offset C: 1.5 V ±4 % Offset voltage vs. temperature: max. ±8 % in the range of -20 C - 70 C Cross axis sensitivity: max. ±6 % C: max. ±2 % Frequency range: DC Hz Thermal accelerometer MXA6500M This is a biaxial vibration sensor manufactured by MEMSIC; the product exploits the thermal principle of acceleration sensing and its transfer to the voltage output. The sensor contains, for each axis, two grids of thermocouples in differential connection and also two complementary circuits to suppress the noise and the effect of ambient temperature. C: 500 mv/g (51,02 mv/m.s -2 ) ±5 % Temperature sensitivity: max. ±10 % in the range of -40 C - 85 C Acceleration measurement range: ±1 g Power supply voltage: 3 VDC (min. 2.7 V max. 3.6 V) Offset C: 1.25 V ± 4 % Offset voltage vs temperature: typically ±1,5 mg/ C (±0,75mV/ C) Cross axis sensitivity: typically ±1,5 % C: max. ±1 % Frequency range: DC - 17 Hz (limited by filter) ICSV22, Florence, Italy, July

3 2.1.3 Capacitive accelerometer LIS3L06AL This product is a triaxial capacitive vibration sensor (with a voltage output) manufactured by STMicrelectronic. The sensor comprises capacitive sensing elements in differential connection and supplementary functions such as self-testing or two-way manual range selection. The actual capacity-to-voltage conversion is nevertheless performed by only a single charge amplifier via a multiplexer for all three axes. The sensor does not contain any other circuits to suppress the effect of ambient temperature. C: 200 mv/g (20.41 mv/m.s -2 ) ±10 % Temperature sensitivity: typically ±0,01 %/ C in the range of -40 C - 85 C Acceleration measurement range: ±6 g Power supply voltage: 3 VDC (min. 2.4 V max. 3.6 V) Offset C: 1.5 V ±6 % Offset voltage vs. temperature: typically ±0,5 mg/ C (±0.1 mv/ C) Cross axis sensitivity: typically ±4 % C: max. ±1,5 % Frequency range: DC Hz Assembling and Fixation of the Samples For the investigated vibration sensors, we fabricated printed circuit boards of mm (Fig. 1) mountable on the calibration device to which the sensors were soldered. The PCBs comprised only the most indispensable components, such as feeding-related filter capacitors; thus, we considered merely the voltage signal outputs of all three axes and feeding as recommended by the manufacturer. Each PCB was fixed with four screws to a duralumin block (a cube of 50 mm at the edge). (a) (b) (c) Fig. 1. The PCBs for all measured MEMS sensors: (a) Piezoresistive accel. AGS61331; (b) thermal accel. MXA6500M; (c) capacitive accel. LIS3L06AL 2.2 Measurement Setup The block diagram of the measurement chain set up to monitor the temperature characteristics of the sensors is presented in Fig. 2. A duralumin unit (block) with the examined sensors was fixed on a shaker (Fig. 3a) and placed in a CTS temperature test chamber (Fig. 3b). The actual positioning of the measured sensor for the calibration of all three sensitivity axes was performed via adjustment of the duralumin cube in axes X, Y, and Z. The signals were led to the input channels of a PULSE frequency analyzer, which enabled us to measure the transverse sensitivity of the sensor, too. As the reference sensor we used a calibrated PCB 356B18 sensor and attached it to the block (Fig. 3). The supply voltage of all the sensors was maintained at 3V, and the temperature range was established between -20 C and +60 C, with the step value of 10 C. For each temperature value (stabilization ICSV22, Florence, Italy, July

4 time: 60 minutes), we defined the frequency spectra of sensitivities in the main and transverse axes. The sensor spectrum were measured using FFT analysis, with the applied frequency ranges of between 1 Hz and 3,2 khz. 1. Shaker (Robotron 11077) 2. Duralumin block 3. Reference power supply (Agilent E3631A) 4. Power Amplifier (TIRA BAA120) 5. PULSE analyzer (Brüel&Kjær) 6. PC with the PULSE LabShop SW 7. Temperature chamber (CTS T-65/50) Fig. 2. An outline of the measurement laboratory. (a) Fig. 3. A detailed view of the measurement setup: (a) Duralumin block with the measured/reference sensors mounted on the shaker; (b) the shaker in the temperature chamber. (b) (a) (b) Fig. 4. The applied excitation methods and instruments: (a) A SPEKTRA CS-18 calibration system; (b) a calibration apparatus by Brüel&Kjær ICSV22, Florence, Italy, July

5 2.3 Results of the experimental investigation Temperature sensitivity analysis Within this stage, we focused mainly on comparing the sensitivities of the vibration sensors from the perspective of their thermal stabilities. Three types of MEMS accelerometers were measured, namely those based on the piezoresistive, temperature, and capacitive approaches. To compare the temperature sensitivities, we indicate the sensitivity relative errors caused by temperature. The reference value corresponds to a sensor s sensitivity at 24 C, and it is therefore identical with the value obtained via calibration of the sensors performed with the applied SPEKTRA CS-18 calibration system. Relative error of sensitivity [%] 4,0 3,0 2,0 1,0 0,0-1,0-2,0-3,0-4,0-5, Temperature [ C] LIS3L06AL AGS61331 MXA6500M Fig. 5. The MEMS samples and their sensitivity relative errors induced by temperature (axis X). Relative error of sensitivity [%] 4,0 3,0 2,0 1,0 0,0-1,0-2,0-3,0-4,0-5, Temperature [ C] LIS3L06AL AGS61331 MXA6500M Fig. 6. The MEMS samples and their sensitivity relative errors induced by temperature (axis Y) Transverse sensitivity analysis Generally, the determination of transverse sensitivity constitutes a difficult task because the sought value is influenced not only by the noise of the sensor but also by any resonance of the shaker. The manufacturers commonly specify only the maximum percentage value of transverse sensitivity at 25 C. As, for the given reason, it was problematic to determine the temperature dependence of transverse sensitivity, we established the maximum sensitivity of this type within the temperature range of between -20 C and 60 C. Tables 1, 2, and 3 indicate the transverse sensitivity values for all examined MEMS accelerometers in individual axes; the percentages refer to the amount of sensitivity ICSV22, Florence, Italy, July

6 transferred from the main sensitivity axis, which is acted upon by acceleration, to a subordinate axis. Table 1. The transverse sensitivities of AGS61331 AGS61331 Main axis X Main axis Y Main axis Z Transverse axis Axis Y Axis Z Axis X Axis Z Axis X Axis Y Transverse sensitivity [mv/m.s 2 ] 0,858 0,580 0,290 0,422 0,615 0,926 Main axis / Transverse axis [%] 2,5 1,7 0,8 1,2 1,8 2,7 Table 2. The transverse sensitivities of MXA6500M MXA6500M Main axis X Main axis Y Transverse axis Axis Y Axis X Transverse sensitivity [mv/m.s 2 ] 0,566 0,799 Main axis / Transverse axis [%] 1,1 1,5 Table 3. The transverse sensitivities of LIS3L06AL LIS3L06AL Main axis X Main axis Y Main axis Z Transverse axis Axis Y Axis Z Axis X Axis Z Axis X Axis Y Transverse sensitivity [mv/m.s 2 ] 0,314 0,257 0,499 0,240 0,339 0,300 Main axis / Transverse axis [%] 1,7 1,4 2,6 1,3 1,8 1, Temperature analysis of the sensor offset The temperature dependence of voltage offset in all the MEMs accelerometers was determined only statically, and therefore no acceleration (including its gravitational form) acted on the individual axes; otherwise, the measurement would have been affected by the actual temperature sensitivity of the sensor. The measured offset values of the piezoresistive and temperature sensors in individual axes are shown in Tabs. 4 and 5. In the capacitive MEMS accelerometer, the manufacturer indicates the typical temperature-related offset change of 0.5 mg/ C, which at the approximate sensitivity of 0.2 V/g and the range of 6 g corresponds to an offset change around 0,1 mv/ C. Such a low value of temperature-related offset change was measurable only with difficulty. Table 4. The sensor offsets of AGS61331 AGS61331 Temperature [ C] Offset in axis X [V] 1,49 1,49 1,50 1,50 1,50 1,50 1,50 1,50 1,50 Offset in axis Y [V] 1,49 1,49 1,49 1,49 1,49 1,49 1,49 1,49 1,49 Offset in axis Z [V] 1,51 1,51 1,52 1,53 1,53 1,54 1,53 1,52 1,51 Table 5. The sensor offsets of MXA6500M MXA6500M Temperature [ C] Offset in axis X [V] 1,27 1,27 1,27 1,26 1,25 1,25 1,24 1,24 1,23 Offset in axis Y [V] 1,26 1,26 1,26 1,26 1,26 1,25 1,25 1,25 1,25 ICSV22, Florence, Italy, July

7 2.3.4 Temperature analysis of the resonant frequency The data of the resonance frequency of a sensor can be easily obtained from the measured frequency spectra. For the piezoresistive and capacitive sensors, the behavior of the temperature dependence of the resonant frequencies is shown in Figs. 7 and 8; in the temperature sensor, however, the resonant frequency data or other frequency characteristics cannot be obtained, because the output consists in low-pass frequency characteristics. Resonant frequency in axis X,Y [Hz] Resonant frequency in axis X,Y [Hz] f(x) = 0,314x + 963,225 osa X Lineární (osa X) f(x) = 0,207x + 983,728 osa Y Lineární (osa Y) f(x) = 0,693x ,428 osa Z Lineární (osa Z) Temperature [ C] Fig. 7. The temperature dependence of the resonant frequencies of AGS61331 Ga á sost e o a č o točtu s ače S3 06 atepotě osa X Lineární (osa X) osa Y Lineární (osa Y) osa Z Lineární (osa Z) f(x) = - 0,942x , f(x) = - 0,214x , f(x) = - 0,196x , Temperature [ C] Fig. 8. The temperature dependence of the resonant frequencies of LIS3L06AL Resonant frequency in axis Z [Hz] Resonant frequency in axis Z [Hz] 3. Conclusion This paper discusses experiments with piezoresistive, thermal and capacitive MEMS-based vibration sensors, and the authors concentrate predominantly on comparing the sensors in term of their overall thermal stability. The MXA6500M accelerometer, which is built upon the thermal principle, exhibits a narrow frequency range; its sensitivity markedly decreases already at 5 Hz. Despite the integrated temperature adjustment components, this product is influenced by ambient temperature to a relatively significant degree. The temperature-related sensitivity change corresponds ICSV22, Florence, Italy, July

8 to approximately 0.07 or 0.08 %/ C as the sensitivity bias at 24 C. The offset of MXA6500M exhibits considerable temperature dependence, too; this factor is then expressed as ±0,40 mv/ C. In piezoresistive accelerometer AGS61331, the sensor nonlinearity reaches up ±0,4 % in all axes, assuming the frequency range of approximately 100 Hz. The temperature sensitivity of this sensor is significant only at temperatures below 20 C, and commonly it does not exceed 0.05 %/ C. The temperature dependence of the offset is not manifested markedly. In the LIS3L06AL capacitive accelerometer, the sensor nonlinearity oscillates around ±0,5 % in all axes and at the frequency range of up to 200 Hz. The temperature sensitivity of the capacitive sensor amounts to approximately 0.02 %/ C as the bias from 24 C, which constitutes the lowest dependence among the measured sensors. The temperature dependence of the offset is insignificant. The values measured in the given principles corresponded to the data provided in the sensor catalog sheets. ACKNOWLEDGEMENTS This paper was realized at CEITEC the Central European Institute of Technology, with the research infrastructure supported by the project CZ.1.05/1.1.00/ financed from the European Regional Development Fund. Further support was provided via project No. CZ.1.07/2.3.00/ , EXCELLENT TEAMS of Brno University of Technology. The completion of this paper was also made possible by grant No. FEKT-S The research of new control methods, measurement procedures and intelligent instruments in automation financed from the internal science fund of Brno University of Technology. The authors acknowledge financial support from the Ministry of Education, Youth and Sports under projects No. LO1210 Energy for Sustainable Development (EN-PUR) solved at the Centre for Research and Utilization of Renewable Energy, Brno. REFERENCES 1 HASÍK, S. Kvalitativní srovnání MEMS snímačů vibrací. Brno: Brno University of Technology, Faculty of Electrical Engineering and Communication, s. Thesis Supervisor: Stanislav Klusáček, Ph.D. 2 ČSN ISO Methods for the calibration of vibration and shock transducers -- Part 11: Primary vibration calibration by laser interferometry. Praha: Czech Metrology Institute, ČSN ISO Methods for the calibration of vibration and shock transducers -- Part 21: Vibration calibration by comparison to a reference transducer. Praha: Czech Metrology Institute, The "family" of SPEKTRA Shock Exciters. SPEKTRA [online] [cit ]. Dostupné z: 5 RYDEN, Bjorn. Temperature Compensating MEMS Accelerometers with Programmable Signal Conditioners. Measurement Specialities, Inc., [cit ] Accessible from: 6 ACAR, Cenk a Andrei M SHKEL. Experimental evaluation and comparative analysis of commercial variable-capacitance MEMS accelerometers. Journal of Micromechanics and Microengineering. ed. 13, No. 5, p DOI: / /13/5/315. Accessible from: ICSV22, Florence, Italy, July