Bias-Repeatability Analysis of Vacuum-Packaged 3-Axis MEMS Gyroscope Using Oven-Controlled System

sensors
Article
Bias-Repeatability Analysis of Vacuum-Packaged 3-Axis MEMS
Gyroscope Using Oven-Controlled System
Hussamud Din , Faisal Iqbal , Jiwon Park and Byeungleul Lee *

 School of Mechatronics Engineering, Korea University of Technology and Education,
 Cheonan 31253, Republic of Korea
 * Correspondence: bllee@koreatech.ac.kr

 Abstract: The performance of microelectromechanical system (MEMS) inertial measurement units
 (IMUs) is susceptible to many environmental factors. Among different factors, temperature is one
 of the most challenging issues. This report reveals the bias stability analysis of an ovenized MEMS
 gyroscope. A micro-heater and a control system exploiting PID/PWM were used to compensate for
 the bias stability variations of a commercial MEMS IMU from BOSCH “BMI 088”. A micro-heater
 made of gold (Au) thin film is integrated with the commercial MEMS IMU chip. A custom-designed
 micro-machined glass platform thermally isolates the MEMS IMU from the ambient environment and
 is vacuum sealed in the leadless chip carrier (LCC) package. The BMI 088 built-in temperature sensor
 is used for temperature sensing of the device and the locally integrated heater. The experimental
 results reveal that the bias repeatability of the devices has been improved significantly to achieve the
 target specifications, making the commercial devices suitable for navigation. Furthermore, the effect
 of vacuum-packaged and non-vacuum-packaged devices was compared. It was found that the bias
 repeatability of vacuum-packaged devices was improved by more than 60%.

 Keywords: bias stability; gyroscope; inertial sensors; MEMS; micro-oven; temperature compensation

 1. Introduction
Citation: Din, H.; Iqbal, F.; Park, J.; In recent decades, microelectromechanical (MEMS) devices, precisely the inertial
Lee, B. Bias-Repeatability Analysis of measurement units (IMUs), have gained tremendous importance for their wide range of
Vacuum-Packaged 3-Axis MEMS applications. The global market survey shows a growing demand for inertial sensors due
Gyroscope Using Oven-Controlled to their improved performance, replacing their macro counterparts in many promising
System. Sensors 2023, 23, 256. applications such as consumer electronics, robotics, avionics, electronic stability control
https://doi.org/10.3390/s23010256 (ESC), global navigation satellite system (GNSS), and image stabilization [1–7]. Specifically,
 MEMS gyroscopes have been progressively famous for their ultra-high measurement
Academic Editor: Faisal
Mohd-Yasin
 sensitivity, high reliability, low power consumption, and low cost [2,8–10].
 The performance and accuracy of MEMS IMUs are affected by several parameters,
Received: 7 December 2022 such as temperature, ambient vibrations, and acoustic noise [11–14]. However, regulating
Revised: 21 December 2022 their temperature is complex and challenging [12,15,16]. Temperature-induced errors
Accepted: 23 December 2022 occur due to changes in the material properties of the selected material and electronic
Published: 26 December 2022
 parts. In MEMS sensors, temperature-induced errors result in several other errors, such as
 spring softening, resonant frequency error, cross-axis sensitivity, and bias instability [17–20].
 Among all the high-performance MEMS gyroscope specifications, bias stability is the first
Copyright: © 2022 by the authors.
 thing system designers must ensure to deliver performance [21]. The navigation grade,
Licensee MDPI, Basel, Switzerland. vacuum-packaged MEMS gyroscopes’ bias-repeatability improvement is significant.
This article is an open access article Different compensation techniques for temperature effects have been used to improve
distributed under the terms and the bias stability of MEMS gyroscopes. Such techniques include mechanical compensation,
conditions of the Creative Commons material variation, and electrostatic forces. However, it was noted that compensation is
Attribution (CC BY) license (https:// necessary for bias stability improvement and a controlled sensor temperature [22–24]. For
creativecommons.org/licenses/by/ temperature control, ovenization is an effective technique that has been implemented by
4.0/). attaching a local heater to the MEMS devices [25–28].

Sensors 2023, 23, 256. https://doi.org/10.3390/s23010256 https://www.mdpi.com/journal/sensors

Different compensation techniques for temperature effects have been used to im-
 prove the bias stability of MEMS gyroscopes. Such techniques include mechanical com-
 pensation, material variation, and electrostatic forces. However, it was noted that com-
Sensors 2023, 23, 256
 pensation is necessary for bias stability improvement and a controlled sensor temperature 2 of 12
 [22–24]. For temperature control, ovenization is an effective technique that has been im-
 plemented by attaching a local heater to the MEMS devices [25–28].
 This study presents a bias-repeatability and locally integrated heater performance
 This study presents a bias-repeatability and locally integrated heater performance
 analysis of an ovenized vacuum-packaged commercial MEMS gyroscope from Bosch
 analysis of an ovenized vacuum-packaged commercial MEMS gyroscope from Bosch “BMI
 “BMI 088” [29]. A PID/PWM-based conventional heater driving circuit [20] is integrated
 088” [29]. A PID/PWM-based conventional heater driving circuit [20] is integrated with the
 with the MEMS device. This device was vacuum packaged to isolate it from its surround-
 MEMS device. This device was vacuum packaged to isolate it from its surroundings and
 ings and reduce heat dissipation. The locally integrated heater keeps the device’s temper-
 reduce heat dissipation. The locally integrated heater keeps the device’s temperature stable,
 ature stable, i.e., above the highest environmental temperature. A previously developed
 i.e., above the highest environmental temperature. A previously developed LabVIEW-
 LabVIEW-based automatic test system was utilized for data gathering and analysis. An
 based automatic test system was utilized for data gathering and analysis. An NI device, NI
 NI device, NI 8452 [30], was utilized for sensor interfacing through I2C.
 8452 [30], was utilized for sensor interfacing through I2C.
 This article is arranged as follows: Section 2 presents the ovenization and vacuum
 This article is arranged as follows: Section 2 presents the ovenization and vacuum
 packaging of the commercial MEMS IMU “BMI 088”. Section 3 presents a detailed de-
 packaging of the commercial MEMS IMU “BMI 088”. Section 3 presents a detailed descrip-
 scription
 tion of theofexperiment’s
 the experiment’s design.
 design. Section
 Section 4 discusses
 4 discusses the experimental
 the experimental resultsresults
 of theof
 “BMIthe
 “BMI 088”. Finally, the paper is concluded in Section
 088”. Finally, the paper is concluded in Section 5. 5.

 2.
 2. Ovenization
 Ovenization andand Vacuum Packaging
 Vacuum Packaging
 Ovenization
 Ovenization andand vacuum
 vacuum packaging
 packaging aimaim to
 to provide
 provide aa constant
 constant temperature
 temperature toto the
 the
 MEMS device
 MEMS device for performance
 performance improvement. Figure 1 shows the block diagram of the
 improvement. Figure 1 shows the block diagram of
 oven-controlled
 oven-controlled system
 systemfor fora a3-axis
 3-axisMEMS
 MEMS gyroscope.
 gyroscope.TheTheproposed
 proposedoven-controlled
 oven-controlledsys-
 tem consists
 system of aof
 consists heater driving
 a heater circuit
 driving withwith
 circuit a PID/PWM
 a PID/PWM circuit. The PWM
 circuit. scheme
 The PWM is ad-
 scheme
 vantageous over conventional
 is advantageous over conventional schemes due todue
 schemes its low power
 to its consumption,
 low power high power
 consumption, high
 efficiency, small size,
 power efficiency, duty
 small cycle,
 size, dutyand digital
 cycle, andscheme
 digital operation. The dutyThe
 scheme operation. cycle of the
 duty fixed
 cycle of
 square wave signal controls the heater operating the current by switching
 the fixed square wave signal controls the heater operating the current by switching thethe power tran-
 sistors
 power ON and OFF
 transistors ONinstead
 and OFFof employing
 instead ofaemploying
 continuousacurrent supply.
 continuous The built-in
 current supply.tem-The
 built-in temperature
 perature sensor of the sensor
 MEMS of IMU
 the MEMS
 “BMI IMU
 088” “BMI 088”used
 has been has been used for temperature
 for temperature measure-
 measurement.
 ment. The LabVIEW
 The LabVIEW program program
 compares compares
 the IMUthe and
 IMUtarget
 and target temperatures,
 temperatures, whilewhile
 the
 the PID/PWM
 PID/PWM program
 program keepskeeps the heater
 the heater temperature
 temperature constant.
 constant.

 Figure 1.
 Figure Block Diagram
 1. Block Diagram of
 of the
 the Oven
 Oven-Controlled
 – ControlledSystem.
 System.

 A thin
 A thin film
 film of
 ofgold
 gold(Au)
 (Au)heater
 heateris is
 designed
 designedunder thethe
 under IMU chip
 IMU area,
 chip andand
 area, the “BMI 088”
 the “BMI
 IMU is mounted on it and assembled in a leadless chip carrier (LCC) package.
 088” IMU is mounted on it and assembled in a leadless chip carrier (LCC) package. A A custom-
 designed micro-machined glass platform and glass suspensions were used for isolation. The
 custom-designed micro-machined glass platform and glass suspensions were used for iso-
 glass isolation platform provides a smooth surface for the sensor and connecting lines, hav-
 ing excellent thermal and mechanical properties, as shown in Figures 2 and 3, respectively.
 The IMU has been connected through wire bonding to the metal pads, and the device
 has been sealed in a vacuum package, as shown in Figure 4. The vacuum level inside
 the package is kept at around 100 mTorr to achieve a high thermal resistance for heat loss
 compensation [31].

Sensors 2023, 23, 256 3 of 12
Sensors 2023, 23, 256 3 of 12

 lation. The glass isolation platform provides a smooth surface for the sensor and connect-
 lation.
 ing The
 lines, glass isolation
 having excellent platform provides
 thermal and a smooth
 mechanical surface for
 properties, the sensor
 as shown and connect-
 in Figures 2 and
Sensors 2023, 23, 256 ing lines, having
 3, respectively. excellent thermal and mechanical properties, as shown in Figures 23 of
 and12
 3, respectively.

 Figure 2. Heater Design with Glass Isolation Platform. (a) Heater design structure. (b) Glass Plat-
 form structure.

 Figure 2. Heater Design with Glass Isolation Platform. (a) Heater design structure. (b) Glass Plat-
 Figure
 Figure
 form 2. Heater
 2. Heater Design
 structure. Designwith
 withGlass
 GlassIsolation
 IsolationPlatform.
 Platform.(a)
 (a)Heater
 Heaterdesign
 designstructure. (b)(b)
 structure. Glass Platform
 Glass Plat-
 structure.
 form structure.

 Figure 3. Micro-Oven and IMU Integration in LCC Package.

 The IMU has been connected through wire bonding to the metal pads, and the device
 has been sealed in a vacuum package, as shown in Figure 4. The vacuum level inside the
 package is kept at around 100 to achieve a high thermal resistance for heat loss
 compensation
 Figure [31]. and IMU Integration in LCC Package.
 3. Micro-Oven
 Figure 3. Micro-Oven and IMU Integration in LCC Package.
 Figure 3. Micro-Oven and IMU Integration in LCC Package.
 The IMU has been connected through wire bonding to the metal pads, and the device
 The sealed
 has been IMU has in been connected
 a vacuum through
 package, wire bonding
 as shown in Figureto4.the
 Themetal pads,level
 vacuum and the device
 inside the
 has been sealed in a vacuum package, as shown in Figure 4. The vacuum level
 package is kept at around 100 to achieve a high thermal resistance for heat loss inside the
 package is kept
 compensation [31].at around 100 to achieve a high thermal resistance for heat loss
 compensation [31].

 Figure 4.
 Figure 4. Vacuum-Packed
 Vacuum-Packed Ovenized
 Ovenized MEMS
 MEMS IMU.
 IMU.

 3. Design of Experiment
 The commercial MEMS IMU “BMI 088” was interfaced using the “NI 8452” device
 through the I2C protocol using LabVIEW [30,32]. The experimental setup is shown in
 Figure 5.
 Figure 4. Vacuum-Packed Ovenized MEMS IMU.
 In4.this
 Figure work, a previously
 Vacuum-Packed developed
 Ovenized program based on the state machine and data
 MEMS IMU.
 analysis was utilized for automatic data gathering of bias-repeatability tests and data
 analysis programs to reduce human effort and test time. Figure 6 shows the LabVIEW
 program dashboard for the automatic test system, while Figure 7 shows the dashboard for
 the data analysis program using LabVIEW.

Sensors 2023, 23, 256 4 of 12
Sensors 2023, 23, 256 4 of 12

 3. Design of Experiment
 Initially, the heater performance and PWM duty cycle were analyzed. After that, the
 The commercial MEMS IMU “BMI 088” was interfaced using the “NI 8452” device
 bias-repeatability test was conducted for three non-vacuum and three vacuum-packed
 through the I2C protocol using LabVIEW [30,32]. The experimental setup is shown in Fig-
 ovenized devices. The resulting data were analyzed using the LabVIEW-based data
 ure 5.
 analysis program.

 Figure 5. Experimental Setup.

 In this work, a previously developed program based on the state machine and data
 analysis was utilized for automatic data gathering of bias-repeatability tests and data anal-
 ysis programs to reduce human effort and test time. Figure 6 shows the LabVIEW pro-
 gram dashboard for the automatic test system, while Figure 7 shows the dashboard for
 the data analysis program using LabVIEW.
 Figure 5. Experimental Setup.
 Figure 5. Experimental Setup.

 In this work, a previously developed program based on the state machine and data
 analysis was utilized for automatic data gathering of bias-repeatability tests and data anal-
 ysis programs to reduce human effort and test time. Figure 6 shows the LabVIEW pro-
 gram dashboard for the automatic test system, while Figure 7 shows the dashboard for
 the data analysis program using LabVIEW.

 Figure 6. LabVIEW –Figure
 Based Automatic Test
 6. LabVIEW-Based System.
 Automatic Test System.
 Figure 6. LabVIEW – Based Automatic Test System.

5 of 12
 Sensors 2023, 23, 256 5 of 12

 Figure 7. LabVIEW –Figure
 Based Data Analysis
 7. LabVIEW-Based Program.
 Data Analysis Program.

 3.1. Heater Performance and PWM Duty Cycle Analysis
 Initially, the heater
 Theperformance and sensor
 built-in temperature PWMofduty cycle
 the “BMI were
 088” analyzed.
 was utilized for theAfter that, the
 temperature
 bias-repeatability measurement
 test was conducted
 of the locallyfor three heater.
 integrated non-vacuum
 The PWMand threewith
 algorithm vacuum-packed
 a fixed square
 ovenized devices. The resulting data were analyzed using the LabVIEW-based(DIO)
 wave signal was used [33] for the locally integrated heater, and the digital I/O datalines
 anal-
 of the “NI 8452” device were used to control the current supply [30,32]. According to the
 ysis program. selected duty cycle and target temperature, the DIO line is turned ON and OFF by the
 PWM signal for heater switching and constant temperature. The relationship between
 3.1. Heater the duty
 Performance andcycle
 PWM and the maximum
 Duty Cycle achievable
 Analysis temperature was computed according to the
 experiment conditions listed in Table 1.
 The built-in temperature sensor of the “BMI 088” was utilized for the temperature
 measurement of the Tablelocally integrated
 1. Experiment Conditionsheater. The PWM algorithm with a fixed square
 and PID Parameters.

 wave signal was usedParameters
 [33] for the locally
 Values integrated heater, and the digital I/O (DIO) lines
 Remarks
 of the “NI 8452” device
 Outputwere used to
 Data Rate 100control
 Hz the current
 Correspond to loopsupply [30,32].
 time 10 msec, According
 minimum to the
 possible value
 Duty Cycle Range 0–80% Test range, where temperature rises to a maximum of 91 ◦ C
 selected duty cycle and target
 P
 temperature,
 0.4
 the DIO line Proportional
 is turnedgain ON and OFF by the
 PWM signal for heater switching
 I and0.365
 constant temperature. Integral The relationship
 gain between the
 D 0.074 Derivative gain
 duty cycle and the maximum achievable temperature was computed according to the ex-
 periment conditions listed in Table 1.
 The duty cycle varied from 0% to 80% at a DC voltage of 7 V. The duty cycle test
 was performed, and the data were gathered for 15 min with a delay interval of 5 min as
 Table 1. Experimenta Conditions andThe
 cool-down time. PIDstart
 Parameters.
 temperature varied every time because of the variations in
 environmental temperature and changes in the duty cycle.
 Values The time-domain data of the sensor Remarks
 temperature and duty cycle are shown in Figure 8.
ate 100 Hz The Correspond to loop time 10
 variation in the actual temperature is duemsec,
 to the air conditioning
 minimum in the room.
 possible value
 Furthermore, a duty cycle vs. maximum achievable temperature test was conducted
nge 0–80% for one Test range,and
 non-vacuum where temperature
 one vacuum rises to aas maximum
 device, respectively, shown in Figureof 9.91 C
 A temper-
 ◦
 0.4 ature change of 0.81 C/duty cycleProportional gainthe glass was used as a thermal
 was observed. Since
 0.365 Integral gain
 0.074 Derivative gain

Sensors 2023, 23, 256 6 of 12

Sensors 2023, 23, 256 6 of 12
 isolation platform, exhibiting excellent thermal properties, the study found that the heat
 loss conduction was reduced, and there is no significant change in temperature concerning
 the duty cycle.

 Figure 8. Duty Cycle vs. Maximum Temperature Analysis.

 Furthermore, a duty cycle vs. maximum achievable temperature test was conducted
 for one non-vacuum and one vacuum device, respectively, as shown in Figure 9. A tem-
 perature change of 0.81 C/duty cycle was observed. Since the glass was used as a ther-
 mal isolation platform, exhibiting excellent thermal properties, the study found that the
 heat loss conduction was reduced, and there is no significant change in temperature con-
 cerning the duty cycle.
 Figure 8. Duty Cycle vs. Maximum Temperature Analysis.
 Figure 8. Duty Cycle vs. Maximum Temperature Analysis.

 Furthermore, a duty cycle vs. maximum achievable temperature test was conducted
 for one non-vacuum and one vacuum device, respectively, as shown in Figure 9. A tem-
 perature change of 0.81 C/duty cycle was observed. Since the glass was used as a ther-
 mal isolation platform, exhibiting excellent thermal properties, the study found that the
 heat loss conduction was reduced, and there is no significant change in temperature con-
 cerning the duty cycle.

 Figure 9. Maximum Temperature vs. Duty Cycle of Vacuum and Non-Vacuum Devices.
 Figure 9. Maximum Temperature vs. Duty Cycle of Vacuum and Non – Vacuum Devices.
 3.2. Bias-Repeatability Analysis
 3.2. Bias-Repeatability Analysis Turn-on bias repeatability is measured by averaging all the data
 Bias-Repeatability:
 Bias-Repeatability: the
 gathered by turning sensorbias
 Turn-on ONrepeatability
 and OFF for specific time intervals.
 is measured The test
 by averaging is conducted
 all the data
 gathered by turning the sensor ON and OFF for specific time intervals. The test isconducted
 by turning OFF the power from the sensor. In the case of repeatability, the test is con-
 ductedseveral times.OFF
 by turning Thisthe
 kind of test
 power is difficult
 from forIn
 the sensor. humans
 the casetoofhandle withoutthe
 repeatability, an test
 automatic
 is
 test system.
 Before starting the bias-repeatability test, the bias vs. warm-up time was analyzed
 and was found to be 50 s, as shown in Figure 10. Turn-on bias repeatability or ON/OFF
 Figurebias was measured
 9. Maximum by averaging
 Temperature vs. Dutyall the data
 Cycle gathered
 of Vacuum andfor
 Non a –specific
 Vacuum interval
 Devices.by turning the
 sensor ON and OFF. The bias-repeatability test was conducted according to IEEE standards;
 3.2. Bias-Repeatability
 i.e., the samples Analysis
 in each data set were more significant than 10,000 [34–36]. Each device
 Bias-Repeatability: Turn-on bias repeatability is measured by averaging all the data
 gathered by turning the sensor ON and OFF for specific time intervals. The test is con-
 ducted by turning OFF the power from the sensor. In the case of repeatability, the test is

automatic test system.
 Before starting the bias-repeatability test, the bias vs. warm-up time was analyzed
 and was found to be 50 s, as shown in Figure 10. Turn-on bias repeatability or ON/OFF
 bias was measured by averaging all the data gathered for a specific interval by turning the
Sensors 2023, 23, 256
 sensor ON and OFF. The bias-repeatability test was conducted according to IEEE7stand- of 12
 ards; i.e., the samples in each data set were more significant than 10,000 [34–36]. Each
 device was tested for 24 hr at room temperature, while a 60 C controlled temperature
 was used in the for
 was tested integrated micro-oven.
 24 hr at room temperature, while a 60 ◦ C controlled temperature was used
 in the integrated micro-oven.

 Figure 10. Bias vs. Warm-up Time of the Ovenized Device.
 Figure 10. Bias vs. Warm – up Time of the Ovenized Device.
 During the bias-repeatability test, the sensor was turned ON for 30 min and then
 turned OFF for 30 min using the automatic test setup. For each device, 25 data sets were
 During the bias-repeatability test, the sensor was turned ON for 30 min and then
 gathered for a 24 hr test. The room temperature varied from 11 ◦ C to 20 ◦ C in winter and
 turned OFF for 30 min using the automatic test setup. For each device, 25 data sets were
 from 19 ◦ C to 28 ◦ C during summer. Six devices were tested, three of them were vacuum
 gathered
 packaged, 24 hr
 for a and test.
 three The
 were room temperature
 non-vacuum packaged. varied from 11 ℃
 The experimental to 20
 results are℃discussed
 in winter
 and from 19 ℃
 in Section 4.to 28 ℃ during summer. Six devices were tested, three of them were vac-
 uum packaged, and three were non-vacuum packaged. The experimental results are dis-
 cussed4.in
 Experimental
 Section 4. Results
 The 24 hr bias-repeatability test results for two devices (i.e., one vacuum packaged
 and one non-vacuum ◦
 4. Experimental Results packaged) at room temperature, as well as at a 60 C controlled
 temperature, are shown in Figure 11a,b, respectively. The target of this research project was
 The 24 hr bias-repeatability test results for two devices (i.e., one vacuum packaged
 to keep the bias repeatability lower than 5.5 mdps or 20◦ /hr.
 and one non-vacuum packaged)
 The experimental results at
 forroom temperature,
 a single as well in
 device, as depicted at a 6011,Cshow
 as Figure controlled
 that
 temperature, are shown
 bias repeatability wasinimproved
 Figure 11a,b, respectively.
 at a controlled The target
 temperature of this research
 compared project
 with the room
 was totemperature.
 keep the bias repeatability
 However, a few5.5
 lower than
 even though mdps or 20
 non-vacuum /hr. were improved with
 devices
 temperature control, the results are inconsistent with vacuum-packaged devices.
 Moreover, a series of bias-repeatability experiments were conducted on three vacuum-
 packaged and three non-vacuum-packaged devices to study and investigate further the
 effect of vacuum packaging and a thermal isolation platform on the MEMS gyroscope’s
 performance. The results of the three vacuum-packaged devices are shown in Figure 12a–c,
 which reveals that the target bias repeatability has been achieved with a controlled temper-
 ature at 60 ◦ C.
 Similarly, the results of non-vacuum-packaged devices are shown in Figure 13a–c,
 where it can be seen that bias repeatability in some devices was improved with temperature
 control but was still unsatisfactory compared with vacuum-packaged devices. The tempera-
 ture variations in both kinds of devices (i.e., vacuum-packaged and non-vacuum-packaged)
 have been reduced and controlled by the micro-oven, as seen in the bias-repeatability test
 summary in Table 2. The bias-repeatability and temperature variation results validate
 the significance of the ovenization and vacuum-packaging technique for the performance
 improvement of MEMS IMUs.

Sensors 2023, 23, 256 8 of 12
Sensors 2023, 23, 256 8 of 12

 (a) (b)
 Figure 11. Bias-Repeatability Analysis of Vacuum – and Non-Vacuum-Packaged Devices. (a) Bias-
 Repeatability of Vacuum-Packaged Device. (b) Bias-Repeatability of Non-Vacuum-Packaged De-
 vice.

 The experimental results for a single device, as depicted in Figure 11, show that bias
 repeatability was improved at a controlled temperature compared with the room temper-
 ature. However, even though a few non-vacuum devices were improved with tempera-
 ture control, the results are inconsistent with vacuum-packaged devices.
 Moreover, a series of bias-repeatability experiments were conducted on three vac-
 uum-packaged and three non-vacuum-packaged devices to study and investigate further
 the effect of vacuum packaging and a thermal isolation platform on the MEMS gyro-
 (a) performance. The results of the three vacuum-packaged
 scope’s (b) devices are shown in Fig-
 ureFigure
 12a–c,11.which reveals that the target bias repeatability has been achievedDevices.
 Bias-Repeatability Analysis of Vacuum – and Non-Vacuum-Packaged with a (a)
 con-
 Figure 11. Bias-Repeatability Analysis of Vacuum- and Non-Vacuum-Packaged Devices.Bias-
 (a) Bias-
 trolled temperature
 Repeatability at 60 C.
 of Vacuum-Packaged Device. (b) Bias-Repeatability of Non-Vacuum-Packaged De-
 Repeatability
 vice. of Vacuum-Packaged Device. (b) Bias-Repeatability of Non-Vacuum-Packaged Device.

 The experimental results for a single device, as depicted in Figure 11, show that bias
 repeatability was improved at a controlled temperature compared with the room temper-
 ature. However, even though a few non-vacuum devices were improved with tempera-
 ture control, the results are inconsistent with vacuum-packaged devices.
 Moreover, a series of bias-repeatability experiments were conducted on three vac-
 uum-packaged and three non-vacuum-packaged devices to study and investigate further
 the effect of vacuum packaging and a thermal isolation platform on the MEMS gyro-
 scope’s performance. The results of the three vacuum-packaged devices are shown in Fig-
 ure 12a–c, which reveals that the target bias repeatability has been achieved with a con-
 trolled temperature at 60 C.

 Sensors 2023, 23, 256 9 of 12

 (a) (b)

 (a) (b)

 (c)
 Figure 12. Bias-Repeatability Analysis of Vacuum-Packaged Devices at Room Temperature and 60
 Figure 12. Bias-Repeatability Analysis of Vacuum-Packaged Devices at Room Temperature and 60 ◦ C.
 °C. (a) Device 1. (b) Device 2. (c) Device 3.
 (a) Device 1. (b) Device 2. (c) Device 3.
 Similarly, the results of non-vacuum-packaged devices are shown in Figure 13a–c,
 where it can be seen that bias repeatability in some devices was improved with tempera-
 ture control but was still unsatisfactory compared with vacuum-packaged devices. The
 temperature variations in both kinds of devices (i.e., vacuum-packaged and non-vacuum-
 packaged) have been reduced and controlled by the micro-oven, as seen in the bias-re-
 peatability test summary in Table 2. The bias-repeatability and temperature variation re-

where it can be seen that bias repeatability in some devices was improved with tempera-
 ture control but was still unsatisfactory compared with vacuum-packaged devices. The
 temperature variations in both kinds of devices (i.e., vacuum-packaged and non-vacuum-
 packaged) have been reduced and controlled by the micro-oven, as seen in the bias-re-
 peatability test summary in Table 2. The bias-repeatability and temperature variation re-
Sensors 2023, 23, 256 sults validate the significance of the ovenization and vacuum-packaging technique for the 9 of 12
 performance improvement of MEMS IMUs.

Sensors 2023, 23, 256 10 of 12

 (a) (b)

 (c)
 Figure 13. Bias-Repeatability
 Figure 13. Bias-RepeatabilityAnalysis
 AnalysisofofNon-Vacuum-Packaged
 Non-Vacuum-Packaged Devices
 Devices at Room
 at Room Temperature
 Temperature and
 and◦ 60 °C. (a) Device 4. (b) Device 5. (c) Device
 60 C. (a) Device 4. (b) Device 5. (c) Device 6. 6.

 Table 2. Summary of the Bias-Repeatability Analysis.
 Table 2. Summary of the Bias-Repeatability Analysis.
 BMI088 Bias-Repeatability [ ]
 BMI088 Bias-Repeatability [mdps] Temperature
 Temperature Gyroscopes Temperature Remarks
 Device Temperature
 Gyroscopes Variations [°C]
 Variations [◦ C] Remarks
 Device Status X Y Z
 Status X Y Z
 Room Temp 12.868 5.899 3.499 1.598
 Device-1 Room Temp 12.868 5.899 3.499 1.598
 Device-1 60 ◦°C
 C 0.610
 0.610 2.803
 2.803 1.483
 1.483 0.002
 0.002

 Device-2
 Room Temp
 Room Temp 5.774
 5.774 5.711
 5.711 3.468
 3.468 1.744
 1.744 Vacuum-Packaged
 Device-2 60 ◦ C 1.283 2.387 2.092 0.001 Vacuum-Packaged
 60 °C 1.283 2.387 2.092 0.001
 Room Temp 4.734 9.458 1.817 1.766
 Device-3 Room60 ◦Temp
 C 4.734
 1.515 9.458
 1.131 1.817
 1.604 1.766
 0.001
 Device-3
 60 °C
 Room Temp 1.515
 15.050 1.131
 7.648 1.604
 1.150 0.001
 0.777
 Device-4 60 ◦Temp
 C 6.882 6.817 0.966 0.001
 Room 15.050 7.648 1.150 0.777
 Device-4 Room Temp 10.101 2.672 2.070 0.983 Non-Vacuum-Packaged
 Device-5 60
 60 ◦°C
 C 6.882
 1.541 6.817
 4.778 0.966
 1.834 0.001
 0.001
 Room Temp
 Room Temp 10.101
 3.539 2.672
 11.479 2.070
 2.459 0.983
 1.549 Non-Vacuum-
 Device-5
 Device-6 60 ◦°C
 60 C 1.541
 4.630 4.778
 9.370 1.834
 1.699 0.001
 0.001 Packaged
 Room Temp 3.539 11.479 2.459 1.549
 Device-6
 60 °C 4.630 9.370 1.699 0.001

 5. Conclusions
 This article presents a micro-oven controlled system with vacuum packaging for the
 commercial MEMS IMUs from BOSCH “BMI 088”. The temperature variations signifi-

Sensors 2023, 23, 256 10 of 12

 5. Conclusions
 This article presents a micro-oven controlled system with vacuum packaging for the
 commercial MEMS IMUs from BOSCH “BMI 088”. The temperature variations significantly
 impact the performance of MEMS IMUs, especially gyroscopes, and the bias stability is
 strongly dependent on temperature variations. The temperature variations have been
 reduced and compensated using a locally integrated heater in this research work.
 It was seen experimentally that the bias repeatability of the MEMS gyroscope is
 affected by a change in temperature. This was successfully compensated by controlling the
 temperature with the help of a designed micro-oven. The designed micro-oven has low
 power consumption, operating at 5 V as provided by the DIO lines of the “NI 8452” device,
 and the overall design has a small footprint area of 1.5 × 1.5 cm2 . Using a LabVIEW-based
 automatic test system, the device was interfaced and tested through an “NI 8452” device.
 This work has tested two types of ovenized MEMS IMUs—vacuum-packaged and
 non-vacuum-packaged. Both kinds of devices were tested at room temperature, as well
 as at a controlled temperature of 60 ◦ C. Experimentally, it was revealed that the bias
 repeatability of the devices had been improved. The target specifications were significantly
 achieved, making the devices suitable for navigation. Furthermore, a comparison of the
 vacuum-packaged and non-vacuum-packaged devices was performed, showing that the
 bias repeatability of the vacuum-packaged devices had been improved by more than 60%.
 It was also noted that temperature compensation is required, and vacuum packaging is
 mandatory for bias-repeatability improvement.
 The temperature variations and bias-repeatability results validated the significance and
 need for the vacuum-packaging and ovenization method for the performance improvement
 of MEMS IMUs. Moreover, the results and processing time have proven the significance
 of the developed automatic test system to increase and improve the mass production of
 MEMS IMUs.

 Author Contributions: Conceptualization, H.D., F.I. and B.L.; Formal analysis, H.D. and F.I.; Method-
 ology, H.D. and F.I.; Project Administration, B.L.; Software, H.D. and F.I.; Experiment, H.D., F.I. and
 J.P.; Supervision, B.L.; Writing—Original Draft, H.D., F.I. and J.P.; Writing—Review and Editing, H.D.,
 F.I., J.P. and B.L. All authors have read and agreed to the published version of the manuscript.
 Funding: This work is supported by the R&D program of the Korea Evaluation Institute of Industrial
 Technology (KEIT) assisted in the development of the IMU-embedded, 6-axis and 10-axis compound
 navigation system, integrating highly reliable inertial measurement unit [IMU], Global Navigation
 Satellite System [GNSS], magnetometer, and altimeter for manned/unmanned aircraft. This work
 was (partially) supported by the postdoctoral scholarship program of KoreaTech.
 Institutional Review Board Statement: Not applicable.
 Informed Consent Statement: Not applicable.
 Data Availability Statement: This research is a part of a classified project.
 Conflicts of Interest: The authors declare no conflict of interest.

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