Electronic Design
Bring the Benefits of MEMS Accelerometers to Condition Monitoring

Bring the Benefits of MEMS Accelerometers to Condition Monitoring

Designers of applications that demand higher performance, such as condition monitoring for the IoT, are increasingly looking to MEMS devices for solutions.

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Ed Spence, Marketing Manager, Analog Devices Inc.

A growing number of condition-monitoring products now use a microelectromechanical-system (MEMS) accelerometer as the core sensor. These economical, highly integrated solutions help reduce the cost of deployment and ownership. As a result, they make it possible for more facilities and equipment to benefit from a condition-monitoring program.

Solid-state MEMS accelerometers hold many advantages over legacy mechanical sensors, but unfortunately, their use for condition monitoring has been restricted to applications that can tolerate the lower bandwidth of low-cost, standards-based smart sensors. In general, diagnostic applications require lower noise over higher frequency ranges and bandwidths beyond 10 kHz.

Low-noise MEMS accelerometers are available today with noise-density levels anywhere from 10 µg/√Hz to 100 µg/√Hz, but are restricted to a few kilohertz of bandwidth. Still, condition-monitoring product designers are using MEMS devices with “good enough” noise performance in their new product concepts—and for good reason.

Because MEMS devices are based on solid-state electronics and built-in semiconductor fabrication facilities, they offer compelling and valuable advantages for condition-monitoring products. Putting aside the performance factor for the moment, here are several of them:

Weight and Size

For airborne applications, such as health and usage monitoring systems (HUMS), weight translates to higher fuel costs—as much as thousands of dollars per pound. With multiple sensors typically deployed on a platform, a lighter sensor can mean lower fuel costs.

A current higher-performance, triaxial MEMS device in a surface-mount package, with a footprint of less than 6 × 6 mm, can weigh less than a gram. Plus, the small size and highly integrated nature of many MEMS products also enable the designer to shrink the size of the final package, reducing weight even more. In addition, the interface of a typical MEMS device is single supply, making it easier to manage and more easily lending itself to a digital interface that can ultimately help save on the cost and weight of cables, too.

Solid-state electronics can also impact the size of the transducer. A smaller-form-factor triaxial mounted on a printed circuit board (PCB) and inserted into a hermetic housing suitable for mounting and cabling on a machine can contribute to a smaller overall package. This offers more mounting and placement flexibility on the platform.

In addition, today’s MEMS devices can include significant amounts of integrated, single-voltage-supply signal-conditioning electronics, providing analog or digital interfaces with very low power to help enable battery-powered wireless products. For example, a recently developed high-resolution, high-stability triaxial accelerometer with an integrated sigma-delta analog-to-digital converter (ADC) has an effective resolution of 18 bits and an output data rate of 4 ksamples/s, and consumes less than 65 μA per axis.

The topology of a MEMS signal-conditioning circuit with both analog and digital output variations is common and opens up options to adapt the sensor to a wider variety of situations. This allows for a transition to digital interfaces commonly found in industrial settings.

In this scanning-electron-microscope image of an inertial MEMS accelerometer, polysilicon fingers are suspended in a depressurized cavity to enable movement. Electrical capacitance proportional to acceleration is measured by adjacent signal-conditioning electronics.

For example, RS-485 transceiver chips are widely available and open market protocols, such as Modbus RTU, are available to load into an adjacent microcontroller. A complete transmitter solution can be designed and laid out with surface-mount chips that can fit within relatively small PCB areas. These can then be inserted into packages that support environmental robustness certifications requiring hermetic or intrinsically safe characteristics.

Scalability

Perhaps one of the biggest advantages of MEMS-based sensors is the ability to scale up manufacturing. MEMS vendors have been shipping high volumes for mobiles, tablets, and automotive applications since 1990. This manufacturing capability, which resides in semiconductor fabrication facilities for both the MEMS sensor and signal-conditioning circuit chip, is now available to industrial and aviation applications as well, helping to lower overall cost while enhancing quality and reliability.

Stability

MEMS devices have proven to be very robust, even in challenging environments. Shock specifications of today’s generation of devices are stated to 10,000 g, but can actually tolerate much higher levels with no impact on sensitivity specifications. Sensitivity for a high-resolution sensor can be designed and tested on automatic test equipment (ATE) to be stable over time and temperature to 0.01°C. Overall operation, including offset shift specifications, can be guaranteed for wide temperatures ranges, such as –40 to +125°C. For a monolithic triaxial sensor with all channels on the same substrate, cross-axis sensitivity of 1% is commonly specified.

Finally, as a device designed to measure the gravity vector, a MEMS accelerometer has a dc response, maintaining the output noise density to near dc. It’s limited only by the 1/f corner of the electronic signal conditioning, which, with careful design, can be minimized to 0.01 Hz.

Proven Performance

The low cost and excellent reliability of MEMS sensors can enable safer, better-handling automobiles; thus, many automotive systems make extensive use of MEMS inertial sensors (see figure). With more than a billion sensors shipped for automotive applications over the last 25 years, MEMS inertial sensors have shown a high degree of quality and reliability. Systems with MEMS sensors can detect crashes from any direction and appropriately activate seatbelt tensioners and airbags to protect occupants. Gyros and high-stability accelerometers are also key sensors in vehicle safety controls.

The Future for MEMS and Condition Monitoring

Currently, there is tremendous interest and investment in MEMS technology for numerous applications requiring high performance. In addition to their array of attractive qualities, MEMS inertial sensors help alleviate many of the quality problems that plague other materials and architectures. MEMS inertial sensors are used in demanding consumer, aviation, and automotive applications, demonstrating a long track record of delivering high quality and reliability.

Has the time come for the MEMS to further penetrate applications demanding higher performance, such as condition monitoring? It’s fully expected that the performance of MEMS devices will continue to improve dramatically, providing more options for designers of condition-monitoring equipment and enabling a new generation of smart sensors, wireless sensors, and low-cost vertically integrated systems. Stay tuned for more on this subject in the near future.

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This file type includes high-resolution graphics and schematics when applicable.
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