Page 30 - EngineerIT March 2022

MEASUREMENT

ADcmXL3021 is not suitable for use with long cables. The SPI
output needs to be converted for long cable driving, using
industrial ethernet or RS-485 transceiver circuits.
Based on the studies in this article, it’s not possible to
place the ADcmXL3021, the RS-485 or ethernet PCB, and
a connector in the same enclosure and achieve 21 kHz
resonant frequencies across all three (x, y, and z) axes.
The combination of components will result in a minimum
enclosure size like that shown previously in Figure 2 (40
mm × 43 mm × 37 mm). Figure 2 provided a first significant
natural frequency of around 10 kHz to 11 kHz across
three axes. In addition, Figure 2 was not simulated using
a connector, which will increase the effective height and
reduce the natural frequencies further.
If a simple rectangular aluminum shape is simulated using
FEM, with 23.7 mm × 27 mm × 12.4 mm dimensions (like the
ADcmXL3021) and 2 mm wall thickness, the first significant
natural frequency exceeds 21 kHz across all axes.
When the 12.4 mm height is doubled and tripled to
Figure 20. First significant natural frequency vs. enclosure height (x, y, and provide space for additional circuitry, the natural frequency
z axes)
reduces significantly, as shown in Figure 22. Even with just a
12.4 mm allowance for additional circuitry, the first significant
Triaxial 10 kHz MEMS sensor with 21 kHz resonance natural frequency falls below 15 kHz.
Controlling the natural frequency of an enclosure design across
three axes is a more difficult task compared to a single axis A distributed system
sensor, particularly when 21 kHz performance is required. Instead of trying to fit all components into one rectangular
enclosure, a distributed system like Figure 23 is suggested.
ADcmXL3021 Using this concept, the ADcmXL3021 is housed in an IP67 rated
Fortunately, Analog Devices has developed the ADcmXL3021 enclosure, with SPI data routed over a short distance (less than
±50 g, 10 kHz, triaxial, digital output MEMS vibration sensing 10 cm) to a separate IP67 enclosure which houses the cable
module, as shown in Figure 21. The ADcmXL3021 is available in interface PCBs with ethernet or RS-485 transceiver, as well as
a 23.7 mm × 27.0 mm × 12.4 mm aluminum package with four associated power supply IC and other circuitry.
mounting flanges to support installation with standard M2.5 Using this approach the geometry is significantly reduced,
machine screws. The ADcmXL3021 package aluminum material and the problem of matching the enclosure’s natural
and geometry support resonant frequencies of greater than 21 frequency to that of the ADcmXL3021 is significantly easier.
kHz across x, y, and z axes.

Adding the ADcmXL3021 to an IP67 rated enclosure
An IP67 rated (water and dust proof) enclosure and
connector are required for placing the ADcmXL3021 in
industrial environments. In addition, the SPI output from the

Figure 21. ADcmXL3021 triaxial digital output MEMS in aluminum package
with flex connector Figure 22. Increasing the height of a shape like the ADcmXL3021

EngineerIT | March 2022 | 28