Page 42 - EngineerIt May 2021

MEASUREMENT AND INSTRUMENTATION

In order to make it easy to mount MEMS sensors to assets,
Analog Devices created a mounting cube as shown in Figure
7. A central mounting hole with a diameter of 0.2 in. and a
surrounding lip of 0.38 in., which is 0.3 in. deep, allows for a
#10 machine screw to be used for fixing the mounting block to
the asset. Breakout boards for multiple accelerometer families
(EVAL-ADXL100XZ, EVAL-ADXL35XZ, and EVAL-ADXL37XZ)
are compatible with this mechanical mount. By using this
mounting block and fixing it securely to the mechanical source,
the accelerometer frequency response can be evaluated in a
controlled environment.
The mounting fidelity of the accelerometer is critical when
capturing high frequency events. This was verified on a vibration
shaker table and is shown in Figure 8 with the resulting frequency
response shown in Figure 9.

Figure 6: (a) MEMS B cross-axis sensitivity at roughly 2.5% and
(b) zoomed x-axis.

Solutions for mechanically attaching MEMS Figure 7: Mechanically optimised mounting block for
sensors to machines accelerometer breakout boards (EVAL-XLMOUNT1).
Piezo-electric accelerometers are the vibration sensors most
commonly used today. They have standard interfaces like IEPE
and 4 mA to 20 mA, as well as various mounting methods
like stud, magnet and adhesive. For MEMS accelerometers
to compete with the long-standing gold standard for vibration
sensing, it is imperative to not only match their performance
but to make it easy to attach MEMS sensors to assets. This
has been a pain point for MEMS accelerometer customers for
some time now. Multiple resonances will exist in any vibration
measurement setup, and it is important to minimise their effect
on measurements. Above 1 kHz bandwidth, mounting becomes
critical as resonances can affect measurements significantly.
For piezo-electric accelerometers, there are a myriad
of mounting strategies, with stud mount offering the widest
frequency response all the way down to handheld probe tips with
the lowest frequency response range. MEMS accelerometers do
not have a susceptibility to magnets and can and have been used Figure 8: Vibration measurement test setup board attached to
successfully with magnetic mounting strategies. shaker table using an aluminum block.

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