E NVIRONMENTAL S TRESS S CREENING
6.5. Case study A: Test plan and Measurement setup setup
6.5.2. Proposed test plan: vibration stress profiles
This work proposes a non-standard procedure specifically tuned to reflect the operative condition of the MEMS inertial module, taking into account the real environmental stresses of the field of application. From a vibration point of view, the operating conditions of electronic devices installed on cars, motorcycles, or on low-cost commercial UAVs are comparable and could be reasonably approximate as equal.
The proposed vibration test plan is composed of four kinds of tests: a sinusoidal vibration profile, a random vibration test, a vibration step-test and a sine-on-random vibration profile.
6.5.2.1. Random vibration test
The random vibration profile provides the possibility to study the behavior of the device under test at different frequencies simultaneously, emulating the
Time 0
1
Stress applica�on
3 min 3 min 3 min 3 min
BEFORE Rep. 1ZONE
ZONETEST Rep. 1
AFTER Rep. 1ZONE
BEFORE Rep. 2ZONE
ZONETEST Rep. 2
AFTER Rep. 2ZONE
Test Execu�on Test Execu�on
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operating conditions of these devices (in terms of vibration) when they are installed and used in real scenarios.
Since there are no specific standards available for the device and applications studied in this work, the test setup was settled comparing several test profiles included in many international standards:
• IEC 60068-2-64 [230] is the European standard published by International Electrotechnical Commission (IEC) in 2008. The general standard regulates the environmental testing, while section 2-64 is about random vibration.
• MIL-STD-810G [231] is a milestone for environmental testing developed by the U.S. Department of Defense in 2008. It is currently the guideline for the majority of the laboratory tests.
• JESD22-B103B.01 (2016) [232] is an international standard published by “Jedec solid state technology association” in 2016. It proposes testing procedures for microelectronics devices focusing on vibration at variable frequency.
• ETSI EN 300 019-2-5 [233] developed by the European Telecommunications Standards Institute (ETSI) in 2002. It deals with the environmental testing area for different kinds of telecommunications equipment installed in vehicles.
• ISO 16750-3 (2003) [234] provided by the International Organization for Standardizations in 2003. It covers the environmental testing of electrical and electronic components installed on road vehicles.
• ANSI C136.31 [235] is an American standard developed in 2010 that covers the vibration test methods for roadway and area luminaires.
During the random test, the IMU is vibrated using normally distributed random vibrations. The standards mentioned above use the Acceleration Spectral Density (ASD) to characterize a random vibration profile over a frequency domain. According to the international standard IEC 60068-2-64 [230] ASD is defined as “the mean-square value of that part of an acceleration signal passed by a narrow-band filter of a center frequency, per unit bandwidth, in the limit as the bandwidth approaches zero and the averaging time approaches infinity”.
The developed test profile is divided into two zones: a low-frequency span, including the range from 5 Hz to 20 Hz and a high-frequency span from 20 Hz up to 500 Hz. In the first frequencies span, the Acceleration Spectral Density is defined as follow:
𝐴𝐴𝑆𝑆𝐷𝐷𝐿𝐿𝐿𝐿 = 2 𝑚𝑚2� = 0.02 𝑙𝑙𝑐𝑐3 2� 𝐻𝐻𝑧𝑧 (6.1)
157 Instead, in the high-frequency span 𝐴𝐴𝑆𝑆𝐷𝐷𝐻𝐻𝐿𝐿 decreases as -3 dB/oct. Fig. 6.4 shows a graphical representation of the developed profile.
Fig. 6.4. Proposed random vibration test profile showing the trend of the Acceleration Spectral Density in the considered range of frequencies. The data tips are located in
the most significant points of the profile.
In compliance with the standard mentioned above, both axes in the figure are illustrated on a logarithmic scale. Even if the trend illustrated in Fig. 6.4 is quite representative of the profile, other parameters must be defined in order to provide a complete and exhaustive description of the test. The Root Mean Square Acceleration 𝑎𝑎𝑅𝑅𝑀𝑀𝑆𝑆 (expressed using the RMS value of the gravitational acceleration g) is a useful parameter that measures the amount of acceleration impressed to the DUT during the test. In fact, it can be monitored continuously during the test, and consequently, it can be used by the shaker controller to ensure that the demanded profile will be fulfilled. It is defined as the RMS value of the square root of the surface area below the Acceleration Spectral Density trend through the frequency domain. The random vibration profile defined above results in a Root Mean Square Acceleration of 𝑎𝑎𝑅𝑅𝑀𝑀𝑆𝑆= 1.2864 𝑙𝑙.
The ASD test level shall be applied within a tolerance of ±3 𝑘𝑘𝐵𝐵 of the nominal value at any frequency, allowing for the instrument and random error, referred to the specified ASD. The RMS acceleration levels shall not deviate more than
±10% of the nominal value defined above.
The test profile is repeated along X, Y, and Z axes, with a time duration of 30 minutes on each axis and a peak displacement RMS 𝑘𝑘𝑅𝑅𝑀𝑀𝑆𝑆= 1.85 𝑚𝑚𝑚𝑚. The controller samples the profile and provides 400 different spectral lines to the shaker that implement the test.
The random vibration test in this work has been developed to simulate the effects of the vibration induced by several factors in a motorcycle or a drone,
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such as a gust of wind, wind buffeting, motor propulsion, a sudden brake, road bumps, linear acceleration or deceleration of the entire vehicle, path harshness, etc.
6.5.2.2. Sinusoidal vibration test
The sinusoidal vibration test is used to investigate any mechanical weakness and/or degradation in the device under test, to demonstrate the mechanical robustness of the specimen and/or to study its dynamic behavior. In order to achieve these objectives, the test is based on a continuous sweep of frequencies changed exponentially with time.
The sinusoidal test plan was developed based on the procedures included in the international standard IEC 60068-2-6 [236], which is a European standard published by the International Electrotechnical Commission (IEC) in 2009.
This standard provides general guidelines for sinusoidal vibration testing of commercial devices; therefore, the AEC-Q100-rev.H [237] is used to customize the test on the field of application. In fact, the AEC-Q100 is an international standard that contains a set of qualification tests for integrated circuits used on automotive applications.
The test is based on a continuous frequency swept of sinusoidal stimuli, and it is divided into two-zone:
• A first zone from 20 Hz to the so-called “cross-over frequency” in which the severity of the test is expressed as constant displacement. In this phase the amplitude of the acceleration increases according to the frequency of the stimuli. In the proposed test plan, this zone is characterized by 1 mm peak-to-peak displacement.
• A second zone from the cross-over frequency up to 2 kHz in which the severity of the test is expressed as constant acceleration, while the displacement is uncontrolled. In the proposed test plan, the test has been performed with three different service conditions: 2 g - 4 g - 8 g peak acceleration.
The value of displacement amplitude is related to the value of acceleration amplitude in such a manner that the magnitude of vibration is the same at the cross-over frequency. In this way, the frequency range may be swept continuously, changing from constant displacement to constant acceleration and vice versa at the cross-over frequency.
For this reason, every sinusoidal vibration test is generally characterized by a different cross-over frequency so that the above-mentioned relationship is
159 fulfilled. In particular, the cross-over frequencies of the three service conditions are reported in TABLE VI.III along with the other parameters of the test severity.
TABLE VI.III
COMPLETE TEST SEVERITY OF SINUSOIDAL VIBRATION PROFILES. SERVICE
CONDITION
PEAK
ACCELERATION
PEAK-PEAK DISPLACEMENT
CROSS-OVER FREQUENCY
FREQUENCY RANGE
S1 8 g 1 mm 63 Hz [20 - 2000] Hz
S2 4 g 1 mm 45 Hz [20 - 2000] Hz
S3 2 g 1 mm 32 Hz [20 - 2000] Hz
The test is performed according to a constant sweep rate performed in a logarithmic manner at 1 decade/minute speed rate. The sweep is performed four times, from the minimum to maximum, and returns to the minimum frequency. Considering the above information, a complete sweep is performed in 4 minutes, leading to a complete test which lasts 16 minutes.
The complete test is repeated in each of the orientation axes X, Y, and Z. A tolerance level of +/- 10% on the test being performed, either displacement or acceleration, is allowed.
Fig. 6.5 illustrates the service condition S3 of the sinusoidal vibration test proposed in this work, highlighting the most significant points using data tips.
Fig. 6.5. Sinusoidal vibration test profile: service condition S3. The data tips are located in the most significant points of the profile.
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The three service conditions S1, S2, and S3 are represented in Fig. 6.6, where the dotted lines stand for the cross-over frequencies 𝑓𝑓𝑆𝑆1, 𝑓𝑓𝑆𝑆2 and 𝑓𝑓𝑆𝑆3 related to the conditions S1, S2, and S3 respectively.
The first test to be performed is the service condition S3. If no failures are exposed after the test, then severity could be increased moving on the condition S2, and finally to condition S1.
Fig. 6.6. Representation of the three sinusoidal vibration profiles S1, S2 and S3
6.5.2.3. Vibration step-test
The objective of this test profile is to characterize the frequency behavior of the MEMS-based IMUs subjected to a sinusoidal vibration at different frequencies, maintaining a constant peak acceleration. Starting from the widely known sinusoidal vibration profile (as described in section 6.5.1.1.), a customized test profile is presented in this section based on a frequency step-up of a sinusoidal stimulus over time to carefully investigate the frequency response of the inertial platforms. The developed test plan is a sort of vibration step-test, where the physical quantity that step up is not the peak acceleration but is the frequency of the stimulus. Using this test profile, it is possible to achieve information about the frequency response of the IMU under test and, at the same time, it is possible to investigate its ability to withstand a constant vibration stimulus over a large frequency span. The latter could also provide significant information on the reliability performances of the IMUs, which represents a critical requirement in many application fields. As mention above, the classical sinusoidal sweep vibration test is defined and illustrated in the international standards IEC 60068-2-6 (2009) [236]. Based on the test profile proposed in
20 fS3 fS2 fS1 100 200 1000 2000
Frequency [Hz]
1 2 4 8 10
Peak accelera�on [g]
Vibra�on profile S3 Vibra�on profile S2 Vibra�on profile S1
161 section 6.5.1.1., the vibration step test adopted in this work consists of a single sweep from 40 Hz to 2000 Hz analyzing a single frequency for a fixed period of holding time. After such a period of time, the frequency will be increase of a fixed frequency step. An extract of the vibration profile proposed in this work is illustrated in Fig. 6.7, considering only the subrange 40 – 200 Hz.
Fig. 6.7. Extract of the vibration step-test profile in the frequency range 40 - 200 Hz.
The severity of the complete test is the following:
• Minimum frequency: 40 Hz
• Maximum frequency: 2000 Hz
• Frequency step: 20 Hz
• Vibration peak: 2 g
• Type of vibration stimuli: Sinusoidal
• Holding time at each step: 25 s
• Number of cycles: 1
• Number of axes: 3
6.5.2.4. Sine-on-Random vibration test
Each one of the test profiles presented in the previous sections is based on a single type of vibration stress. However, according to the international standard ISO 16750-3:2003 [234] (Road vehicles- Environmental conditions and testing for electrical and electronic equipment – Part 3: Mechanical loads), the vibration endured by an electronic device mounted on a road vehicle can be divided into two types:
• Sinusoidal vibration: it could be one of the spectral components from the acceleration due to the vehicle's motion, or it could be caused by
0 25 50 75 100 125 150 175 200 225
Time [s]
20 40 60 80 100 120 140 160 180 200 220
Frequency [Hz]
Constant Peak Accelera�on = 2 g
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unbalanced mass forces, vibration induced by the pulsation of the intake air, etc.…
• Random noise from all the other vibration sources of an engine (e.g., closing of valves), the noise created by the gearwheels' friction, random vibration induced by rough-road-driving.
Hence, ISO 16750-3:2003 [234] suggests performing the test as a combined sine and random test in compliance with International Standard IEC 60068-2-80 [238]. The latter defines "swept frequency sinusoidal vibration on wideband random vibration" or simply Sine-On-Random as one or more sinusoids swept over a frequency range and superimposed on random vibration.
In other words, this kind of vibration test is based on the application of two different vibration stimuli at the same time: wideband random vibration stress and a sinusoidal vibration stimulus. This test requires the definition of a composite vibration severity, consisting of swept frequency sinusoidal components on a random background. In some instances, the sinusoidal stimulus could be maintained fixed instead of being swept over a frequency range.
Consequently, both sinusoidal and random vibration severities have been customized on the actual operating condition related to the application field (automotive and low-cost UAVs).
Regarding the sinusoidal vibration, this stimulus is a low-frequency sinusoid with a vibration peak of 1 g. Both frequency and amplitude are maintained constant over the testing time. This vibration stands for a hypothetical IMU input signal, which must be acquired without distortion since it represents the acceleration that the positioning algorithms must process. The truthfulness of this vibration has been proven in [239], [240] in which a suitable measurement system has been proposed to evaluate the driver's exposure to vibration during a ride on a motorcycle. The paper highlights that the vibration analysis band for a motorcycle varies from 0.25 Hz to 20 Hz.
As a consequence, four different service conditions (SC) have been developed to recreate different scenarios well representative of the automotive application's actual vibration.
For the sake of simplicity, the service conditions are based on a single sinusoid, which is not the real vibration experienced in the automotive field. Instead, it is only a single spectral component of the actual vibration.
Three SCs are based on a single sinusoid with constant frequency (i.e., 5 Hz – 10 Hz – 15 Hz), while the last one is based on a continuous frequency sweep from 5 Hz to 20 Hz.
163 The wideband random vibration is based on the excitation of all the frequencies in a defined spectrum at any given time. This test is extremely useful since vibrations found in everyday life scenarios are not repetitive or predictable like sinusoidal waveforms.
The severity of random vibration is described providing an ASD value over a frequency range. The proposed test plan is based on a constant density value 𝐴𝐴𝑆𝑆𝐷𝐷 = 0.01 𝑙𝑙2/𝐻𝐻𝑧𝑧 over the frequency range from 200 Hz to 2 kHz. This represents a wideband gaussian white noise that could distort the low-frequency signal related to the monitored item's actual motion. Because of the ODR of the DUTs, a 50 Hz antialiasing filter is introduced on the IMU (see section 2.3).
Consequently, the random vibration should be completely cut-off by the antialiasing filter embedded in the considered devices (for more information see section 2.3).
Several standards agree that the vibration endured by an electronic device for automotive application is a wideband stimulus with a maximum frequency of up to 2 kHz. The frequency range is strictly related to the exact deployment of the DUT.
The most significant standards taken into account during the development of the proposed test plan are the following:
• ISO 16750-3 published by the International Organization of Standardization in 2003 [234].
• IEC 60068-2-64 published by the International Electrotechnical Commission in 2008 [230].
• ETSI EN 300 019-2-5 published by the European Telecommunications Standards Institute in 2002 [233].
• AEC-Q100-rev.H published by the Automotive Electronics Council in 2014 [237].
TABLE VI.IV summarizes the normative references used to identify the frequency range of the random vibration concerning installation type.
Most of the standards agree that devices mounted on-road vehicles are subjected to random vibration up to 2 kHz. This is why the proposed test plan includes a random vibration over the frequency range from 200 Hz to 2 kHz, although the ODR of the sensor is 119 Hz.
Additional information regarding the test are illustrated in the following:
• Test duration: 10 minutes.
• Axes involved: X, Y, and Z.