Testing Approaches for Characterization and Selection of MEMS Inertial Sensors 2016, 2016, ACUTRONIC 1

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1 Testing Approaches for Characterization and Selection of MEMS Inertial Sensors by Dino Smajlovic and Roman Tkachev 2016, 2016, ACUTRONIC 1

2 Table of Contents Summary & Introduction 3 Sensor Parameter Definitions 5 Selection of Sensor Parameters for Testing 9 Test Methodology 14 Test Procedures 21 Test Results 29 Conclusion , ACUTRONIC 2

3 Content Summary The presentation is dealing with the question of selecting proper MEMS inertial sensors for your device/application: how to choose sensors that best fit your needs? Sensor Parameter definition This section will covers a variety of parameters that typically need to be characterized. Test methodology overview This section will cover how to determine parameter characterization. Sample test process A sample test report is reviewed demonstrating the parameters and methodology discussed previously. 2016, ACUTRONIC 3

4 Introduction Due to a large number of manufacturers of the inertial MEMS sensors it becomes obvious selection process is more complicated than ever before. While some of the characteristics of the sensors are easily compared (price, size, number of axes, etc.), performance parameters are typically only partially defined by the manufacturers, and can vary greatly from one to another and over different environments. Because the expected use of the products with the embedded inertial sensors (smartphones, wearables, smart helmets, VR sets, etc.) can vary greatly the best approach to select MEMS inertial sensors is by conducting characterization tests for a defined set of parameters. In this presentation we are describing the process of testing inertial MEMS sensors (accelerometers and gyroscopes), selection of the parameters and choosing test procedures to collect relevant data. Once the data is collected and the product application is considered selection of the appropriate sensors can be made. 2016, ACUTRONIC 4

5 Sensor Parameter Definitions Inertial sensors measure object s orientation and position in space Gyroscopes (gyros), accelerometers and magnetometers are considered inertial sensors Typically inertial sensor parameters can be divided into two groups: dynamic and static. Static measurements include: Noise and zero input offset information Dynamic tests include Scale factor error and linearity, cross-axis sensitivity, misalignment, full scale range and bandwidth testing Most of these parameters can be tested over temperature to identify any temperature sensitivity. 2016, ACUTRONIC 5

6 Sensor Parameter Definitions In order to properly define each parameter we recommend using IEEE 2700 Standard for Sensor Performance Parameter Definitions by the IEEE Standards Association In 2012, Intel Corp and Qualcomm Technologies, Inc. published a document to establish an industry standard minimum set of MEMS performance parameters: Standardized Sensor Performance Parameter Definitions (Rev 1) Later, that working standard become an officially sanctioned standard by IEEE: IEEE Standard for Sensor Performance Parameter Definitions Currently work for is underway for the next edition of the standard For more information on inertial parameters IEEE Standards dedicated to inertial sensors only are released and maintained by Gyro and Accelerometer panel 2016, ACUTRONIC 6

7 Some of the Sensor Parameter Definitions from IEEE 2700 Noise The smallest measurable change in rotation rate expressed as the root mean square (RMS) and calculated as the standard deviation of a minimum of 10,000 sample points under vibration isolation and zero rotational input Allan Variance Allan variance is a time domain analysis technique used to determine the character of the underlying random processes that give rise to the data noise. It helps identify the source of a given noise term present in the data Zero Rate Bias Zero rotation rate output deviation from expected zero rotation rate output value for each sensing axis 2016, ACUTRONIC 7

8 Some of the Sensor Parameter Definitions from IEEE 2700 Sensitivity The change in rotational rate input corresponding to 1 least significant bit (unit in which digital values are counted) change in output. Non-Linearity Error Maximum deviation of measured output from the best fit straight line Cross-Axis Sensitivity Ratio of the measured rotation rate for an axis to the input rotational rate along each axis orthogonal to the measured axis. Full Scale Range Peak to peak measurement range of the sensor per each orthogonal axis. 2016, ACUTRONIC 8

9 Selection of Sensor Parameters for Testing So which tests are appropriate for your sensor? It all depends on the application! In AHRS systems at least zero rate bias (over temperature) and sensitivity/nonlinearity should be tested they are the biggest error contributors to the final orientation angles Noise and Allan variance measurements should be performed to identify goodness of the sensors, impact of noise on error budget, and long term tendency of the sensor For high speed rotation/high dynamic applications, bandwidth and full scale range are important they define sensors ability to track the motion and identify the point at which the saturation of the output occurs 2016, ACUTRONIC 9

10 Selection of Sensor Parameters for Testing It is very important to keep in mind the end use of the device as that information can drastically decrease the complexity and duration of the tests In general, all MEMS sensors, including inertial, are temperature sensitive and performance will be affected by variations in temperature If the device is known to work only over a defined temperature range then testing should be performed over that range Environments with high h vibration content t also negatively impact performance; if a device containing inertial sensors is expected to encounter high levels of vibration, then sensors should be tested over vibration environments 2016, ACUTRONIC 10

11 Selection of Sensor Parameters for Testing When doing thermal testing most companies perform testing at a limited number of discrete temperature points Most often 3 temperature points are used (maximum, minimum and room temperature) However, if the device is expected to be used over a broader temperature range it may be appropriate to perform thermal testing over the entire range while slewing, at a predetermined temperature rate, from one set point to another. This step could be used during characterization of the sensors 2016, ACUTRONIC 11

12 Selection of Sensor Parameters for Testing Benefits of doing more thorough thermal testing It allows for screening of the unusual behavior of the sensors that would otherwise be undetected by testing at a few discrete temperature points Slewing at a predefined ramp rate, for example 1 or 2 C/min, could mimic the real environment that the device sensor will experience If you choose to collect data during the ramp up or down you will get a better model of sensor performance between the set temperatures. 2016, ACUTRONIC 12

13 Test Selection Test Parameters Test Description Test Conditions Units As a recognized leader in the inertial sensor testing industry, ACUTRONIC has developed their own standard suite of tests and conditions to comply with the standardized parameters specified in IEEE 2700 standard In order to provide a direct product test performance comparison, ACUTRONIC applies their own, standard set of test conditions to every test product. The list of tests, conditions, and parameters is provided here: Noise and Allan Variance Static Output for Sensor Measured over Long Time Duration At room temperature, no physical stimulus, all axis Zero Input Bias Static Sensor Over -40 to +85 C, deg/s, g Output with no all axes physical stimulus applied Sensitivity Non-Linearity Error Cross-Axis Sensitivity Change in Sensor Output per Change in Physical Input Maximum deviation in sensor output from a best fit line % change in offaxis output to primary sensitive axis Over -40 to +85 C, all axes Over -40 to +85 C, all axes Over -40 to +85 C, all axes deg/s RMS, g RMS deg/s/ Hz deg/s/lsb, g/lsb % of Full Scale Range % of Full Scale Range Full Scale Range Peak to Peak Over -40 to +85 C, deg/s, g measurement all axis equipment range of sensor permitting output 2016, ACUTRONIC 13

14 Test Methodology Several hardware platforms that can be used to test both the dynamic and static properties of MEMs sensors. It is possible to use 1-, 2- or 3-axis rate table test systems. Since the market trend is towards full 6 (9) degrees of freedom IMU, a 2-axis rate table provides enough flexibility to perform almost all test groups on all axes in a reasonable time frame. This section discusses test principles applied to such IMUs. 2-AXIS RATE TABLE ACUTRONIC AC , ACUTRONIC 14

15 Test Methodology Static Tests Zero Input Bias For gyros, the zero input offset tests can be done with sensitive axis in any orientation and no rotational motion of the rate table. The data can be collected over a 3 minute period at each chosen temperature. At least 5 temperature points should be used, both extremes, ambient and two more temperature in between the 3 others. For accelerometers, the sensitive axis has to be placed parallel to Earth s gravity thus providing a zero G input. As such, the motion platform would have to be repositioned at least once to align the 3rd IMU axis parallel to the Earth s gravity. The same temperature profile can be used as for gyros. Both accelerometers and gyros can be tested at the same time during the static tests with the exception being one axis of an accelerometer that would need additional sampling time after proper alignment. 85 ºC 45 ºC 25 ºC Temperature Cycle 0 ºC -40 ºC 2016, ACUTRONIC 15

16 Test Methodology Static Tests Noise Characteristics These tests should be run at higher sensor sampling rates, as close to sensor ODR as possible. Over a long period of time (3 to 6 hours) to allow enough data for statistical analysis. No reorientation of IMU is needed and data sampling for accelerometers and gyroscopes can be done at the same time. Noise is calculated in terms of the smallest change of rotation measurements in a rotation-resistant environment with no input rotation. The Allan Variance computation calculates the random variation in the sensor output due to white noise, random walk, bias instability, etc. All the above tests can be done over temperature as well thus making this a very lengthy portion of the testing. 85 ºC 45 ºC 25 ºC Temperature Cycle 0 ºC -40 ºC 2016, ACUTRONIC 16

17 Test Methodology Dynamic Tests The dynamic tests typically consist of non-linearity, sensitivity, full scale range and bandwidth of a sensitive axis. In an IMU, it is also common to find factors such as cross axis sensitivity between sensitive axes which shows mechanical misalignment. All of the above mentioned tests should also be performed over temperature to expose any temperature sensitivity. Conveniently, the results for each test parameter can be obtained from a common motion profile performed on a 2 axis rate and positing table as long as all axis are sampled in the IMU. For gyroscopes, the motion profile involves performing 5 to 10 rotation at a given rate going from negative maximum rate to positive maximum rate with a preset increment of some value of deg/s. For accelerometers, the motion profile is even simpler and consists of a series of position indexes in the field of Earth s gravity for each sensitive axis. As little as 4 index positions can be used per sensitive axis to adequately characterize the test t parameters for that axis. 85 ºC 45 ºC 25 ºC Temperature Cycle 0 ºC -40 ºC 2016, ACUTRONIC 17

18 Test Methodology Dynamic Tests One parameter that takes an exception to a single motion profile is full scale range. For accelerometers, that have a full scale range above 1G. Centrifugal force has to be used in order to obtain data at higher Gs. Centrifugal force is defined as the product of the radius and the square of the angular rate. Figure to the right shows an IMU at a radius to center of rotation. In this case Y axis will experience the G forces. For gyroscopes, remounting of the x and y axis maybe necessary to be in plane with table axis if the tilting axis is not able to achieve the same rates as the table axis. In the figure to the right z axis is in plane with the table axis. 2016, ACUTRONIC 18

bandwidth (in 100s of Hz) specification, the best tool for this job would be a linear and an

19 Test Methodology Dynamic Tests Another parameter that usually takes an exception to using a rate table as a test platform all together is bandwidth Since most accelerometers and gyros have higher bandwidth (in 100s of Hz) specification, the best tool for this job would be a linear and an angular vibration platform for accelerometers and gyroscopes respectively ACUTRONIC 105-AVT 2016, ACUTRONIC 19

Test Methodology Dynamic Tests Bandwidth The test

of a logarithmic or linear frequency sweep from 5

the IMU output at maximum sampling rate possible

20 Test Methodology Dynamic Tests Bandwidth The test profile for each type of inertial sensor consists of a logarithmic or linear frequency sweep from 5 to 2000 Hz on each motion platform while recording the IMU output at maximum sampling rate possible (as close to sensor ODR as possible). 2016, ACUTRONIC 20

21 Test Procedure Following the methodology described in previous slides ACUTRONIC has tested three 3-axis MEMs gyroscopes from Bosch Sensortec, InvenSense and STMicroelectronics The devices used were BMG160, MPU-3050 and L3G3200D Bandwidth tests were not performed on these devices 2016, ACUTRONIC 21

22 Test Procedure Model Manufacturer # Devices Available* Gyroscope Evaluation/Shuttle Board BMG160 Bosch Sensortec 3 MPU InvenSense Inc. 4 L3G3200 D STMicroelectronics , ACUTRONIC 22

Test Procedure UUT Orientation ACUTRONIC 2-Axis AC277 Rate Table Mounted Bosch BMG160 Axis 1 - CW

Negative (-) Z Positive (+) Axis 2 - CCW - Positive (+)* Y Positive (+) Axis 2 - CW - Negative

Positive (+) *These results assume ACUTROL Axis 1 is at 0 º. **The AC277 only has two axes.

23 Test Procedure UUT Orientation ACUTRONIC 2-Axis AC277 Rate Table Mounted Bosch BMG160 Axis 1 - CW - Positive (+) Z Negative (-) Mounted Orientation Onboard Chip Orientation Axis 1 - CCW - Negative (-) Z Positive (+) Axis 2 - CCW - Positive (+)* Y Positive (+) Axis 2 - CW - Negative (-)* Y Negative (-) Axis 3 - CCW - Positive (+)** X Negative (-) Axis 3 - CW - Negative (-)** X Positive (+) *These results assume ACUTROL Axis 1 is at 0 º. **The AC277 only has two axes. In order to test the 3 rd gyroscope axis, we rotated the ACUTROL Axis 1 by 90º and used daxis , ACUTRONIC 23

Test Procedure UUT Orientation ACUTRONIC 2-Axis AC277 Rate Table Mounted Invensense MPU3050 Mounted Orientation Onboard Chip Orientation Axis 1 - CW

Negative (-) Axis 3 - CCW - Positive (+)** X Negative (-) Ai Axis 3 - CW - Negative (-)** X Positive (+) *These results assume ACUTROL Axis 1 is at

24 Test Procedure UUT Orientation ACUTRONIC 2-Axis AC277 Rate Table Mounted Invensense MPU3050 Mounted Orientation Onboard Chip Orientation Axis 1 - CW - Positive (+) Z Negative (-) Axis 1 - CCW - Negative (-) Z Positive (+) Axis 2 - CCW - Positive (+)* Y Positive (+) Axis 2 - CW - Negative (-)* Y Negative (-) Axis 3 - CCW - Positive (+)** X Negative (-) Ai Axis 3 - CW - Negative (-)** X Positive (+) *These results assume ACUTROL Axis 1 is at 0 º. **The AC277 only has two axes. In order to test the 3 rd gyroscope axis, we rotated the ACUTROL Axis 1 by 90º and used daxis , ACUTRONIC 24

Axis 1 - CW - Positive (+) Z Negative (-) Axis 1 - CCW - Negative (-) Z

Negative (-)* Y Positive (+) Axis 3 - CCW - Positive (+)** X Positive (+)

ACUTROL Axis 1 is at 0 º. **The AC277 only has two axes.

25 Test Procedure UUT Orientation ACUTRONIC 2-Axis AC277 Rate Table Mounted STMicroelectronics L3G3200D Mounted Orientation Onboard Chip Orientation Axis 1 - CW - Positive (+) Z Negative (-) Axis 1 - CCW - Negative (-) Z Positive (+) Axis 2 - CCW - Positive (+)* Y Negative (-) Axis 2 - CW - Negative (-)* Y Positive (+) Axis 3 - CCW - Positive (+)** X Positive (+) Ai Axis 3 - CW - Negative (-)** XN Negative () (-) *These results assume ACUTROL Axis 1 is at 0 º. **The AC277 only has two axes. In order to test the 3 rd gyroscope axis, we rotated the ACUTROL Axis 1 by 90º and used daxis , ACUTRONIC 25

Test Procedure The testing is divided into three sessions: First Session: 65 ºC 25 ºC Temperature Cycle 1. Data is recorded during all stages of the test at 50 Hz via I2C.

45 ºC 0 ºC b) The temperature cycles negative and then positive stopping at the following key temperatures in Celsius: 25, -20, -40, 0, 45, 85, 65, 25. At each stable temperature, a soak of 1.

26 Test Procedure The testing is divided into three sessions: First Session: 65 ºC 25 ºC Temperature Cycle 1. Data is recorded during all stages of the test at 50 Hz via I2C. a) All devices are mounted on a rate table inside a temperature 85 ºC chamber. Starting at 25 degrees C, the temperature is controlled at +/-1 C/min. Data is recorded during the transition periods. 45 ºC 0 ºC b) The temperature cycles negative and then positive stopping at the following key temperatures in Celsius: 25, -20, -40, 0, 45, 85, 65, 25. At each stable temperature, a soak of 1.5 hours with no rotational input is recorded. -20 ºC -40 ºC b) After each soak, before progressing to the next temperature step, the following input rates are applied to the devices for 5 revolutions per rate: to 2000 deg/s (at +/-100 deg/s intervals) for Z axis to 500 deg/s (at +/- 100 deg/s intervals) for X and Y axes. 2016, ACUTRONIC 26

a) The devices are physically rotated to test each device axis at

27 Test Procedure Second session: 2. Data is recorded during each segment of the test at 50 Hz via I2C. a) The devices are physically rotated to test each device axis at the full scale range of +/-2000 deg/s. Data is recorded for 2.5 minutes at each extreme. 2016, ACUTRONIC 27

28 Test Procedure Third session: 3. Data is recorded at device native frequencies of 1 khz via I2C. a) Devices are placed in a temperature and rotational resistant environment. Data is recorded for 90 minutes with no input rotation. 2016, ACUTRONIC 28

29 Test Procedure Results Noise Tests RMS Noise calculated for each type of sensor 2016, ACUTRONIC 29

30 Test Procedure Results Noise Tests Allan Variance calculated for each type of sensor More data on: Noise and Allan Variance Zero Input Bias Sensitivity Non-Linearity Error Cross-Axis Sensitivity Performance over temperature available from ACUTRONIC 2016, ACUTRONIC 30

31 Test Procedure Results Dynamic Tests Full Scale Range for each type of sensor 2016, ACUTRONIC 31

32 Test Procedure Results Dynamic Tests Cross-Axis Sensitivity for each type of sensor BMG160 MPU3050 L3G3200D Parameter Units Mean Mean Mean Noise gyrox deg/s RMS Noise gyroy deg/s RMS Noise gyroz deg/s RMS AllVar gyrox deg/s/ Hz AllVar gyroy deg/s/ Hz AllVar gyroz deg/s/ Hz Zero Rate Bias gyrox deg/s ±2.63 ±1.744 ±2.139 Zero Rate Bias gyroymore data on: deg/s ±2.593 ±0.880 ±0.671 Zero Rate Bias gyro Z deg/s ±3.010 ±1.582 ±1.203 Noise and Allan Variance Zero Input Bias Sensitivity Non-Linearity Error Cross-Axis Sensitivity Performance over temperature available from ACUTRONIC ZeroRateBiasTempCoef gyrox deg/s/degc /d ± ± ± ZeroRateBiasTempCoef gyroy deg/s/degc ±0.005 ±0.023 ±0.014 ZeroRateBiasTempCoef gyroz deg/s/degc ±0.001 ±0.106 ±0.023 Sensitivity gyrox deg/s/lsb Sensitivity gyroy deg/s/lsb Sensitivity gyroz deg/s/lsb SensitivityTempCoef gyrox %/degc ±0.010 ±0.016 ±0.023 SensitivityTempCoef gyroy %/degc ±0.003 ±0.010 ±0.027 SensitivityTempCoef gyroz %/degc ±0.001 ±0.003 ±0.008 Integral NLE gyrox % FSR ± Integral NLE gyroy % FSR ± Integral NLE gyroz % FSR ± CrossAxis gryoxy % ±0.075 ±0.522 ±1.235 CrossAxis gyroxz % ±1.781 ± ±0.452 CrossAxis gyroyz % ±0.649 ±3.719 ±0.627 CrossAxis gyroyx % ±0.077 ±0.775 ±1.041 CrossAxis gyrozx % ±1.708 ±0.714 ±0.701 CrossAxis gyrozy % ±0.427 ±2.051 ± , ACUTRONIC 32

33 Conclusion Testing Approaches Integrator of MEMS inertial sensors should select testing parameters based on expected application of the device containing MEMS inertial sensors IEEE 2700 Standard should be used to define parameters that are to be tested Test methodology needs to be defined, Acutronic can help Based on test results, developers and integrators can make sensor selection based on specific needs for that t application (i.e. low noise, low bias, high h sensitivity, etc.) This type of testing provides additional differentiation for the MEMS sensors selection process (in addition to cost, size and power requirements) 2016, ACUTRONIC 33

34 Conclusion Testing Approaches For example, in the data set one type of sensor (InvenSense MPU3050) has significantly lower Noise RMS values than the other two sensors If RMS Noise is a very important parameter for the given application this sensor would have been the choice for that application Testing also allows comparison of actual live sensors data to manufacturer s published specifications It provides valuable measurement of quality of performance With that t in mind ACUTRONIC has set up a lab (its Lab) to provide testing ti services to encourage wider application of the IEEE 2700 Standard its Lab enables such testing for all types of inertial MEMS sensors 2016, ACUTRONIC 34

35 Thank you for your attention! For more information contact: Dino Smajlovic , ACUTRONIC 35

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