Page 29 - EngineerIT March 2022

MEASUREMENT

number T4171010004-001 from TE. This connector is IP67 rated
for water and dust resistance and includes a STEP file from TE
that can easily be integrated into the enclosure design file. This
connector can be used with an M12-to-M12 cable, such as the
TAA545B1411-002 from TE.
A good mechanical mounting is critical to ensure the best
transfer of vibration and avoid resonances that may affect
performance. A good mounting is typically achieved using a stud
threaded to both sensor enclosure and monitored equipment.
The stainless steel model shown in Figure 16 includes a solid
7 mm base with an industry-standard ¼”-28 threaded hole for
mounting the stud attachment to the monitored equipment.
The enclosure measures 24 mm in diameter and includes a
hexagonal 25 mm base, which can be used to torque the sensor
into the monitored equipment. The total height of the enclosure
with M12 connector can be varied between 48 mm and 57
mm, depending on manufacturing tolerances and assembly of Figure 18. FEM mesh detail and relative deformation of the enclosure
internal wiring or soldering options from the connector to the
MEMS PCB. For example, at least 5 mm of height is needed if Modal simulation
using a straight wire connection between the M12 cap and the Before modal simulation, one solid body should be created using
MEMS PCB. the components shown in Figure 17. This will provide a simulation
Figure 17 shows an exploded view of one possible assembly model that closely matches an assembled and welded sensor.
option for the enclosure, M12 connector, and MEMS PCB. The A fine mesh should be selected for accurate FEM numerical
MEMS PCB can be assembled to the enclosure wall using M3 simulation, especially for the connector geometry. The fine
screws, then attached to the M12 connector, and then finally span angle centre ANSYS mesh option should be selected for
the two enclosure pieces can be laser welded together. The PCB best performance. Figure 18 shows the FEM mesh and relative
is vertically mounted as shown, with the ADXL1002 MEMS axis deformation of the enclosure after simulation.
of sensitivity aligned vertically with the z-axis of the enclosure. The gradual gradient from blue to orange and red in Figure 18
A vertical mount is also important from a system measurement illustrates the larger relative structural deformation at the top of
perspective, as this orientation is usually required for measuring the enclosure and the connector.
bearing faults (for example, radial vibration measurements) on Figures 19 and 20 show the FEM results for first natural
motors. frequency with significant MPF (greater than 0.1 for the ratio of
effective mass to total mass of the system) vs. total sensor height
for the z-axis. The z-axis performance is critical, with 19.38 kHz
for first significant natural frequency when the enclosure height
is at 52 mm. For 48 mm total height the performance improves
to 22.44 kHz. A 50 mm height enclosure will give around 21 kHz
performance.

Figure 17. One possible assembly concept for MEMS sensor PCB, M12
connector and enclosure Figure 19. First significant natural frequency (z-axis) vs. enclosure height.

EngineerIT | March 2022 | 27