Page 31 - EngineerIT March 2022

MEASUREMENT

The Figure 24 y, x face and four M2.5 holes are constrained for
modal simulation. The z direction is the weakest link in the design,
even at sub-20 mm heights. Figure 25 shows one of the FEM modal
simulation dominant modes, which illustrates the larger relative
structural deformation at the top of the enclosure.
The z direction stiffness can be increased by increasing the
wall thickness. For example, if 2 mm wall thickness is used, the z
direction’s first significant natural frequency is at 14.76 kHz. When
a 3 mm wall thickness is used, this increases to19.83 kHz. As shown
in Figure 26, using a 3.5 mm wall thickness provides greater than
21 kHz natural frequency in the z direction.

Adding epoxy in the enclosure
Epoxy resin can be added to vibration sensor enclosures to hold
hardware PCBs in a fixed position, and to prevent movement of
connectors and internal wiring.
To study the effects of epoxy resin on the natural frequency of
an enclosure, a simple FEM model was created with a 40 mm × 40
mm hollow, stainless steel cube of fixed 2 mm wall thickness. The
cube was filled with 36 mm × 36 mm epoxy resin. The height of the
enclosure was increased from 40 mm to 80 mm to 100 mm, and
alternate simulations were performed with and without the epoxy
fill. The FEM simulations were performed with the x, y surface as
the fixed constraint.

Figure 23. The ADcmXL3021 and interface circuits are housed in separate
enclosures

Design and modal analysis
As shown previously, a rectangular shape is a good approach to
achieve similar natural frequency performance across three axes in
comparison to a cylindrical shape. In Figure 23, the ADcmXL3021
is placed in a small, hollow, rectangular enclosure with a tiny PCB
to interface between the ADcmXL3021 flex cable and industrial
connector. A small profile M8 connector, such as TE 7-1437719-5, Figure 25. Dominant mode from simulation for the hollow enclosure used to
can be used with the model. The rectangular enclosure includes house the ADcmXL3021.
four M2.5 mounting holes, to provide a fixed mounting to
equipment. The total enclosure size is 40.8 mm × 33.1 mm × 18.5
mm. Critically, the z-axis height is 18.5 mm, which helps to achieve
higher frequency modes.

Figure 24. Hollow enclosure used to house the ADcmXL3021. Figure 26. First significant natural frequency vs. wall thickness for the z-axis.

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