Accelerometer principle

• Generally, the accelerometer design consists of a proof (seismic) mass elastically attached to a base that is in a contact with a measured vibrating structure
• Movement of the accelerometer base exerts a force F on the seismic mass, which is translated into a displacement by the elastic support
• Value of this force is due to the Newtons law proportional to the mass of the seismic element and to the acceleration
• Corresponding displacement of the elastic support can be transformed by an appropriate physical principle to an electrical signal

Physical principles of accelerometers

• Piezoelectric (PE)
• Piezoresistive (PR)
• Capacitive (C)

Piezoelectric accelerometers

• Use a spring-mass system to generate a force proportional to the amplitude and frequency of vibration
• Force is applied to a PE element which produces a charge on its terminals that is proportional to the mechanical motion
• Advantage of PE materials: self-generating and do not require an external power source
• Crystals such as quartz are naturally piezoelectric when properly cut, but they have low sensitivity
• Ferroelectric materials are more commonly used because of their much higher charge sensitivity

Piezoresistive accelerometers

• Using strain gauge elements - electrical resistance is changed in proportion to applied mechanical stress
• Strain gauges are mechanically attached to cantilever beams and electrically connected in a Wheatstone bridge to produce an electrical signal proportional to vibratory motion
• Compared to wire gages, PR gages are virtually free of mechanical hysteresis and have several orders of magnitude greater sensitivity
• PR accelerometers offer the advantage of dc response
  This suits them to measurements of long duration pulses found in transportation vibration, automotive crash studies and blast testing
• Inherently low output impedance - because they utilize an external source of electrical energy
• Sensitivity is high enough for many applications - preamplification of the output is unnecessary
• Standard and miniature sizes are available.

Capacitive (electrostatic) accelerometers

• Using a capacitive sensor that is in most cases fabricated by micromachining techniques
• Individual sensor element is in its typical form made from single crystal silicon and then electrostatically bonded to form a parallel plate capacitive device
• Response to DC acceleration inputs
• Stable damping characteristics which maximize frequency response
• Ruggedness to withstand extremely high acceleration over range conditions
• Integral electronics provide a high level low impedance output signal which is stable over temperature
• Different designs of a capacitive accelerometer offer a large range of accelerations to be measured with
• Low g accelerometers intended for applications such as
  • trajectory monitoring
  • aircraft/vehicle structural evaluation
  • flutter testing

have the ability to provide low g measurements in the environment of high shock or vibration

• High g accelerometer designs are used in electronics systems for airbacks

Resonant accelerometers

• Represent an alternative approach to achieving high sensitivity and large dynamic range
• Use a resonant microstructure as a strain sensing element
• Frequency of a resonant structure can be made highly sensitive to compressive or tensile strain
• Resonant accelerometers that are commercially available use chemically milled quartz tuning forks as the resonant strain gauges
• If the entire accelerometer consisting of proof mass, suspension, and micromechanical resonators can be fabricated in silicon, then substantial cost savings can be achieved