COVER STORY
Successfully Integrating MEMS Accelerometers into High Precision Applications
Dr Panos Ioakim, Field Applications Engineer at Anglia M 8 June 2021
EMS accelerometers have evolved to deliver high performance in a small footprint at relatively low cost. They are available in numerous varieties and diverse specifications making them suitable for wide range
of applications, from ultra-low power wearables, to high bandwidth condition monitoring. With their ever-increasing capability, they are now on the verge of challenging their much more expensive and physically larger counterparts who have long been established in industry as the golden standard. Dr Panos Ioakim, Field Applications Engineer at Anglia, discusses their characteristics and gives an insight into overcoming the design challenges for the successful integration of MEMS accelerometers into instrument grade products. Selecting a MEMS accelerometer for an application primarily involves making decisions based on dynamic range, bandwidth of operation, current consumption, number of axes, sensitivity, and noise density. With over 50 different products currently
Components in Electronics
available from Analog Devices offering a wide variety of operational characteristics and capabilities, selecting the appropriate device requires a methodical selection process. Aiding this selection process, MEMS Inertial Sensors Selection Tables are available on the Analog Devices website, providing product segmentation by application, as is a more detailed parametric Product Selection Table, offering finer selection by attribute. Since the selection process is mostly informed by target application criteria however, arriving at an appropriate list of parts is by no means a daunting exercise. For static operations such as tilt measurement for example, the focus would typically be on devices with low bandwidth, low dynamic range and good noise and sensitivity characteristics, yielding a selection of devices such as the ADXL103, ADXL203, and the ADIS16003, to mention but a few. Worth noting is that for some applications, highly integrated solutions also exist, for example the ADIS16203 inclinometer able to provide out-of-the-box 0.025° resolution, offering a quick time to market. For high resolution specialist applications, the ADXL354 and ADXL355 offer superior performance with noise densities down to 22.5µg/√Hz over analogue and digital outputs respectively.
For mixed dynamic and static applications such as bio-sensing wearables, prosthetics and patient monitoring where a larger dynamic range and bandwidth are required at the lowest power consumption possible, the focus would mainly be on the micro-power range of devices, such as the ADXL362 and ADXL363 which outperform in this area, offering 200Hz bandwidth and up to 8g at just 3uA operational current, whilst also incorporating 270nA motion- activated wake up and 10nA standby modes. Conversely, for applications requiring wide bandwidths and higher dynamic ranges, the ADXL100X series would be aptly suited, offering bandwidths up to 24 kHz on fast analogue outputs, whilst for lower bandwidths, the digital output ADCMXL1021-1 would suit applications to 10 kHz. For products specifically employing frequency domain analysis, more specialised modules are also available, such as the ADIS16228 which
provides on-chip tri-axial FFT, storage, and a flat frequency response up to 5 kHz.
Although selecting the correct device for an application based on specific performance criteria is important, integrating a chosen device successfully into a product is critical for high precision applications as it requires careful management of the many inherent characteristics of these sensors.
Fig. 1 MEMs internal inertial structure
MEMS accelerometers are predominantly based on the physical properties of the mass-spring topology, where an inertial micro-mass is suspended by a set of polysilicon springs and is free to move within limits determined by the physical geometry of the sensor, in one, two, or three dimensions (Fig. 1). Fingers extending from the inertial mass locate between fingers fixed to the substrate in a mesh to create movable differential capacitive elements in each axis. Accelerations in the x or y directions in this example, induce a force upon the inertial mass resulting in a motion relative to the fixed substrate, thus offsetting the fingers within the mesh from
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