Page 33 - EngineerIT March 2022

MEASUREMENT

Designing a good mechanical enclosure for MEMS
accelerometers requires an understanding of modal analysis.
Modal analysis provides the natural frequencies in the axis of
interest for a vibration sensor enclosure. In addition, the mode
participation factor (MPF) will enable the designer to decide if a
frequency can be ignored in a design.
Both material characteristics and geometry need to be
considered when designing a vibration sensor enclosure to meet
natural frequency targets. Enclosure height needs to be minimised
to achieve higher natural frequencies. Reducing wall thickness or
enclosure diameter both have secondary effects on the enclosure
natural frequencies.
Cylindrical shapes with higher cross-sectional areas are better
designed for higher rigidity and natural frequencies across all axes,
compared to rectangular shapes. Rectangular shapes offer more
options in sensor orientation and equipment attachment, compared
Figure 28. Effect of mounting technique on sensor resonance.
to cylindrical shapes. Rectangular shapes are useful in maintaining
similar natural frequency performance across three axes.
For most cases, adding epoxy resin will aid the natural
frequency performance for a vibration sensor enclosure solution.
Using stud or adhesive mounting provides the best usable
frequency range for a vibration sensor, while using magnetic or
adhesive pads reduces sensor performance. n

References
1. ANSYS Innovation Courses: Modal Analysis. ANSYS, 2021.
Figure 29. Mounting techniques for vibration sensors. 2. Stephen Timoshenko. Vibration Problems in Engineering, 4th
edition. John Wiley and Sons Inc., New York, 1974.
performed using the default bonded contact constraint. This is where 3. Leszek Majkut. “Free and Forced Vibrations of Timoshenko Beams
the bottom of the vibration sensor – and, in particular, the ¼”-28- Described by Single Difference Equation.” Journal of Theoretical
and Applied Mechanics, Vol. 47, No. 1, January 2009.
inch mounting hole – is designated as a fixed constraint using ANSYS. 4. Mohammad Hossein Abolbashari, Somayeh Soheili, and Anoshirvan
The constraint type is default bonded or a bolted connection. Farshidianfar. “Flexural Vibration of Propeller Shafts Using
Simulation of adhesive contact is an advanced topic Distributed Lumped Modeling Technique.” The 16th International
and requires ANSYS cohesive zone modeling (CZM) and an Congress on Sound and Vibration, Krakow, 2009.
understanding of contact mechanics. For accuracy, the ANSYS CZM 5. Saida Hamioud and Salah Khalfallah. “Spectral Element Analysis
requires parameters input based on lab test data. For example, of Free - Vibration of Timoshenko Beam.” International Journal of
the article “Direct Measurement of the Cohesive Law of Adhesives Engineering, 2018.
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Using a Rigid Double Cantilever Beam Technique” can be used 6. Olivier A. Bauchau and James I. Craig. Structural Analysis: With
for inputs to ANSYS. If you don’t find experimental data published Applications to Aerospace Structures, Springer, 2009.
for your chosen adhesive, then you will need to do some lab 7. TN17: Installation of Vibration Sensors, Wilcoxon Sensing
Technologies, 2018.
measurements. In addition, the correct contact formulation needs 8. A. Khayer Dastjerdi, E. Tan, and F. Barthelat. “Direct Measurement
to be set up in ANSYS, with guidance provided in short courses of the Cohesive Law of Adhesives Using a Rigid Double Cantilever
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such as Fundamental Topics in Contact. Finally, the CZM and modal Beam Technique.” Society for Experimental Mechanics, May 2013.
techniques then need to be combined within the ANSYS workbench. 9. ANSYS Innovation Courses: Fundamental Topics in Contact.
ANSYS Maxwell can be used to simulate magnetic fields. However, ANSYS, 2021.
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as magnetic forces are non-contact forces (they push or pull objects 10. Ansys Maxwell: Low Frequency EM Field Simulation. ANSYS, 2021.
without “solid” contact), generating an appropriate contact constraint
for numerical modal analysis is not possible. Modal analysis can be About the Author
performed with bonded, frictionless, frictional, and no separation Richard Anslow is system applications engineer with the
contacts. CZM contact may also be possible, as mentioned previously. Connected Motion and Robotics Team within the Automation
and Energy Business Unit at Analog Devices. His areas of
Conclusion expertise are condition-based monitoring and industrial
A good mechanical enclosure design for a MEMS accelerometer communication design. He received his B.Eng. and M.Eng.
will ensure that high quality vibration data for CbM is extracted degrees from the University of Limerick, Limerick, Ireland. He can
from the monitored asset. be reached at richard.anslow@analog.com.

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