Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 4M

• Vibrations are a common phenomena of mechanical structures that can be detrimental to many systems.
• Hard disk drives, for example, are extremely sensitive to both external vibrations1 and to airflow-induced vibration of the metal suspensions that carry the drive’s read/write heads.2
• The suspension vibrations prevent the accurate positioning of the head, resulting in an erroneous reading/writing signal; the external vibration may even cause heads to crash into the disk, a catastrophic failure for hard disk drives.
• Hence, vibration monitoring is a key to insuring system robustness and enhancing overall performance for applications of this kind.
• Various methodologies have been developed to measure vibrations.
• Laser vibrometers compare the frequency shift between the outgoing and reflected laser beam and the corresponding vibration velocity is evaluated.
• These instruments can take very accurate measurement if the measured surface is reasonably reflective and the laser beam is properly aligned.
• The non-contact nature of this measurement is the major advantage over other types of vibration measurements.
• However, these instruments are too bulky to fit into small machineries. Another methodology measures strain induced by the mechanical vibrations of the structure.
• In this case, sensors are applied to the mechanical structure to be monitored. Assuming the presence of sensors has a negligible effect on the structure, the true strain can be measured by monitoring the electrical signals passing through the sensors, and related to vibration of the structure.
• Piezoresistive and piezoelectric materials are the most common sensor materials used in this kind of measurement.
• Vibration is a deterministic or stochastic continuous force or displacement excitation of a device that may be due to the environment in which it must function.
• Shock is a single event or pulse applied to a device that may be due to the environment in which it must function.
• Typically, vibration and shock are measured in acceleration (g).
• Shock and vibration environments are generally specified by the power spectral density (PSD) of the vibration environment or the shock response spectra (SRS) of the shock environments.
• Vibration and shock environments could be an issue to MEMS devices by material damage or stiction due to the MEMS surfaces being damaged or forced together.
• A number of shock and vibration studies for MEMS devices have been performed which shows minimal effects on the microscale MEMS device.
• Mems inertial sensors have survived shock environments as high as 120,000g.
• The type of damage observed has been packaging damage and the movement of particulate.