Whether the smart infrastructure structure is healthy is the key!
With the advent of digitalization or the Internet of Things, sensors are becoming central to industrial infrastructure applications. In this case, applications rely on MEMS for condition monitoring and structural health monitoring, and with these new applications come very specific standards regarding performance and reliability.
Smart Infrastructure
Leveraging digitalization to create smart infrastructure offers many benefits, including increased capacity, efficiency, and reliability. Smart infrastructure can provide more and more targeted services to customers and users without increasing investment or resources. In addition, interconnected infrastructure can collect data to help design and implement future infrastructure more effectively. Introducing intelligence into infrastructure can also effectively solve the main challenge of maintenance.
MEMS sensors play a decisive role in structural health monitoring.
They can be used to measure changes in inclination, vibration analysis, and linear or circular motion – even under extreme conditions. Such sensors make it possible to perform predictive maintenance, make better use of available resources and help avoid service failures and interruptions. Analog Devices has deep expertise and has invested heavily in developing MEMS technologies that support smart infrastructure applications.
ADXL35x MEMS Accelerometer Family
ADI introduces a new series of low noise, low power, 3-axis accelerometers -
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ADXL354 to provide analog output;
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ADXL355 with digital output and programmable ranges of ±2 g, ±4 g, and ±8 g;
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ADXL356 to provide analog output;
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ADXL357 with digital output and ±10 g, ±20 g, ±40 g programmable ranges
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These devices can be used in IMUs (Inertial Measurement Units), platform stabilization systems, inclinometers, and predictive maintenance systems. These mid-range and high-range sensors are designed for some of the most demanding sensor applications, such as seismic analysis, industrial and infrastructure predictive maintenance systems.
Advanced features to support structural health monitoring
For condition monitoring and structural health monitoring, the measurement range is an important parameter. For example, in applications where the peak acceleration is several g, a sensor with a 2 g range is sufficient. However, these devices often operate in environments where they are subject to strong vibrations and shocks, which can lead to sensor saturation. Once saturated, it is no longer possible to measure the correct acceleration. The result is data loss until the device returns to normal operation. In this case, a 40 g sensor can be used. The probability of reaching saturation is lower, and even in the presence of high mechanical noise, the required information can be extracted with appropriate signal processing.
Sensors in many infrastructure applications may be remote or difficult to access, so wireless sensor networks are the best solution. This makes low power consumption another key consideration. The ADXL35x devices consume only 21μA in standby mode; in measurement mode, the analog output devices consume 150μA and the digital output devices consume 200μA. When the host microcontroller is in sleep mode, the FIFO memory in the ADXL355/ADXL357 stores data. When the memory is full, an interrupt wakes up the microcontroller to transfer the data and perform the required operation. Once the microcontroller completes the transfer, it returns to low-power sleep mode, ensuring that the system power consumption is very low.
Typically, low power consumption comes at the expense of other performance features, such as speed and noise. In the ADXL35x accelerometers, the noise spectral density is 20μg/√Hz for low-g devices and 80μg/√Hz for high-g devices. In addition, the internal architecture helps optimize the sensitivity of the accelerometer. Figure 2 shows the block diagram of these analog and digital devices. The signal from the sensor enters the filtering block and is then processed by the subsequent circuits. After the filter and before the output, there is a buffer and a 32 kΩ resistor, where further analog filtering can be performed. The digital device has an additional programmable digital filter.
The cutoff frequency of the low-pass filter is adjusted according to the output data rate, and a high-pass filter can also be inserted to achieve a bandpass function. For condition monitoring, vibration spectrum analysis is the main tool, so a high bandwidth is important to capture a larger number of harmonics. The mechanical resonance frequency of the sensor is about 5.5 kHz, but the frequency response is mainly determined by the anti-aliasing filter with a cutoff frequency of 1.5 kHz. Finally, in order to provide the required resolution, a 20-bit ∑-∆ converter is used to perform the analog-to-digital conversion. Due to these characteristics, these accelerometers can also be used to record seismic events.
ADXL356 and ADXL357 Functional Block Diagram
Stability is very important if you want to monitor the structural health of buildings, bridges, tracks, high-voltage towers or any other element of infrastructure. What you want to measure here is the drift of the structure, which should not be confused with the drift of the measurement device.
The long-term stability of the sensor is related to the mechanical stress. Any mechanical stress experienced during the soldering phase can result in electrical offsets. The stress may change over time, causing an offset drift. This can be misinterpreted as a change in tilt or other structural parameters. To avoid this problem, special attention needs to be paid to the chip attach operation.
Package selection is also important. These accelerometers are packaged in a robust 14-lead LCC ceramic package, which offers far better protection against mechanical stress than the plastic packages widely used in consumer electronics applications. The ceramic package also ensures a high degree of sealing against the ingress of moisture and particles, which helps improve long-term stability.
During normal operation, the device may be affected by various environmental conditions, especially temperature and humidity, which may affect its performance. For humidity, the hermetic package of the sensor (such as the 14-lead LCC) ensures stable operation of the device even under the most adverse conditions. The operating temperature range is –40°C to +125°C, which means that the device can work well even at extreme temperatures. In addition, special attention should be paid to minimize the offset drift, which is the most critical parameter. On all three axes, the maximum drift of the low-g accelerometer ADXL354/ADXL355 is ±0.15 mg/°C, and the maximum drift of the high-g accelerometer ADXL356/ADXL357 is ±0.75 mg/°C. In addition, these accelerometers are equipped with an integrated temperature sensor, which can be used for thermal compensation of drift.
Table 1. Key features of the new family of MEMS accelerometers
in conclusion
Today, the demand for MEMS sensors has expanded beyond consumer electronics applications. New opportunities are being created in the industrial and infrastructure markets. In these areas, reliability and performance are key factors. ADI's research has been focused on developing solutions that can achieve high performance and reliable operation in extreme environmental conditions. The ADXL354, ADXL355, ADXL356, and ADXL357 are the new series of high-g accelerometers - ADXL1001 (±100 g), ADXL1002 (±50 g), ADXL1004 (±500 g) ultra-low noise and 24 kHz ultra-high bandwidth products.
Intelligence changes industry, give a thumbs up to ADI MEMS technology!
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