Accelerometers

Accelerometers

where the inertia force is applied to the piezoelectric element as a shear strain or as a tensile strain.

 

For an accelerometer, acceleration is the signal that is being measured (the measurand).

 

Hence, accelerometer sensitivity is commonly expressed in terms of electrical charge per unit acceleration or voltage per unit

acceleration.

 

Acceleration is measured in units of acceleration due to gravity (g), and charge is measured in Pico coulombs (pC), which are units of 10-12 coulombs (C).

 

Typical accelerometer sensitivities are 10 pC/g and 5 mV/g. Sensitivity depends on the piezoelectric properties, the way in which the inertia force is applied to the piezoelectric element (e.g., compressive, tensile, shear), and the mass of the inertia element. If a large mass is used, the reaction inertia force on the crystal will be large for a given acceleration, thus generating a relatively large output signal. Large accelerometer mass results in several disadvantages, however. In particular:

 

1.       The accelerometer mass distorts the measured motion variable (mechanical loading effect).

2.       A heavy accelerometer has a lower resonant frequency and hence a lower useful frequency range.

 

For a given accelerometer size, improved sensitivity can be obtained by using the shear strain configuration. In this configuration, several shear layers can be used (e.g., in a delta arrangement) within the accelerometer housing, thereby increasing the effective shear area and hence the sensitivity in proportion to the shear area.

 

Another factor that should be considered in selecting an accelerometer is its cross-sensitivity or transverse sensitivity Cross-sensitivity is present because a piezoelectric element can generate a charge in response to forces and moments (or, torques) in orthogonal directions as well.

 

The problem can be aggravated due to manufacturing fracturing irregularities of the piezoelectric element, including material unevenness and incorrect orientation of the sensing element, and due to poor design.

Cross-sensitivity should be less than the maximum error (percentage) that is allowed for the device (typically 1%).

 

The technique employed to mount the accelerometer on an object can significantly affect the useful frequency range of the accelerometer. Some common mounting techniques are:

 

1.       Screw-in base

2.       Glue, cement, or wax

3.       Magnetic base

4.       Spring-base mounts

5.       Hand-held probe

 

Drilling holes in the object can be avoided by using the second through fifth methods, but the useful range can decrease significantly when spring-base mounts or hand-held probes are used (typical upper limit of 500 Hz).

 

The first two methods usually maintain the full useful range (e.g., 5 kHz), whereas the magnetic attachment method reduces the upper frequency limit to some extent (typically 3 kHz).