Page 28 - EngineerIT March 2022

MEASUREMENT

structure less rigid, so it makes sense that the natural frequency Consider a scenario where the enclosure needs to
will reduce. However, using simulation it’s clear that the reduction accommodate a sensor and circuit, with a defined 60 mm
in first natural frequency is not dramatic, and changing diameters height, and 43 mm × 37 mm breadth and width. Using a
can still result in a first natural frequency in the tens of kHz. rectangular shape and changing the orientation of fixed
constraint (equipment attachment) can help to boost
Will changing the orientation of the sensor enclosure performance. The rectangular enclosure shown in Figure 14 has
increase performance? multiple attachment holes, so the enclosure can be mounted to
The previous sections of this article show that increasing the height equipment in various orientations. If the enclosure is mounted
of the enclosure will result in a reduced first natural frequency. on the x, z face, then the effective height of the enclosure is 60
It was also shown that using a cylindrical enclosure shape is mm. However, if the enclosure is mounted on the y, z face, then
recommended, instead of using rectangular shapes. However, there the effective height is only 37 mm. This approach can be used
are some cases where rectangular shapes are useful. for a rectangular enclosure, but is not feasible using a cylinder’s
curved surface.
Figure 15 shows that by changing the enclosure orientation,
the x-axis first resonant frequency can be boosted, and the y-axis
is better when compared to a cylinder. The z-axis first resonant
frequency is higher for the y, z fixed orientation compared to the
x, z fixed orientation, with almost double the frequency mode.
However, the cylinder performs the best by far for z-axis natural
frequency. A rectangular shape is a good approach to have similar
performance across three axes in comparison to a cylindrical
shape.

Single-axis 11 kHz MEMS sensor with 21 kHz resonance
Based on the simulation and analytical results presented in this
article, a cylindrical enclosure will perform best for housing a
single-axis ADXL1002 MEMS sensor with 21 kHz resonance. The
MEMS sensor axis of sensitivity should be orientated to take
advantage of the cylindrical enclosure’s first natural frequency
performance in the z-axis.

Enclosure prototype and assembly concept
Figure 14. Rectangular enclosures can be constrained on the x and z axes, or The simulation models presented so far have excluded connector
on the y and z axes to reduce height
choices and their influence on the natural frequencies of the
enclosure design. Figure 16 shows an M12 4-wire connector, part

Figure 15. First significant natural frequency vs. cylinder or rectangular
shape orientation. Figure 16. Enclosure prototype

EngineerIT | March 2022 | 26