Why Do We Need Accelerometers?
Vibration and shock are present in all areas of our daily
may be generated and transmitted by motors, turbines, machine-tools,
bridges, towers or by the human body, for example.
While some vibrations are desirable, others may be disturbing
destructive. Consequently, there is often a need to understand the
causes of vibrations and to develop methods to measure and prevent them.
The sensors we manufacture serve as a link between vibrating
structures and electronic measurement equipment.
What is Measured?
The commonly used quantity for the measurement of vibration is
acceleration. It has the Standard International unit m/s² (meters per
second squared). Sometimes also the non-SI unit gravitational
acceleration (g) is used for acceleration (1g is about 9.81 m/s²).
For some applications, for example in machine monitoring,
velocity (mm/s) or vibration displacement (µm, mm) are measured.
Velocity can be derived from acceleration by single integration,
displacement by double integration. Integrators can be implemented by
an analog circuit or a software routine.
While we have an idea of the order of magnitudes of
displacement and velocity, it may be difficult to imagine acceleration:
- Accelerations below 0.001 m/s² are measured in seismic
- A racing car driver can experience 50 m/s². Most humans
lose consciousness at around 60 m/s².
- A car accident of 100 m/s² will break human bones while 300
m/s² are sufficient for a seatbelt to break ribs.
- A laptop dropping onto a concrete floor from a height of 1
m may endure as much as 20,000 m/s².
- Accelerations beyond 100,000 m/s² are found in ballistics
and explosion tests.
The Advantages of Piezoelectric Sensors
The accelerometers Metra has been manufacturing for over 40
utilize the phenomenon of piezoelectricity. "Piezo" is from the Greek
word meaning to squeeze. When a piezoelectric material is stressed it
produces electrical charge. Combined with a seismic mass it can
generate an electric charge signal proportional to vibration
The active element of Metra’s accelerometers consists of a
selected ceramic material with excellent piezoelectric properties
called Lead-Zirconate Titanate (PZT). Specially formulated PZT provides
stable performance and long-term stability. High stability similar to
quartz accelerometers is achieved by means of an artificial aging
process of the piezoceramic sensing element. The sensitivity of
ceramics compared to quartz materials is about 100 times higher.
Therefore, piezoceramic accelerometers are the better choice at low
frequencies and low acceleration.Piezoelectric accelerometers are
widely accepted as the best choice for measuring absolute vibration.
Compared to the other types of sensors, piezoelectric accelerometers
have important advantages:
- Extremely wide dynamic range, almost free of noise -
suitable for shock measurement as well as for almost imperceptible
- Excellent linearity over their dynamic range
- Wide frequency range, high frequencies can be measured
- Compact yet highly sensitive
- No moving parts - no wear
- Self-generating - no external power required
- Great variety of models available for nearly any purpose
- Integration of the output signal provides velocity and
The following table shows other common types of vibration
sensors compared to piezoelectric accelerometers:
- Measures static acceleration
- Limited resolution because of resistive noise
- Only for low and medium frequencies
- Supply voltage required
- Only for low frequencies
- Measures static acceleration (for tilt measurement)
- Cheap manufacturing with semiconductor technology
- Limited resolution
- Relatively fragile
- Only for low frequencies
The piezoelectric principle requires no external energy.
Only alternating acceleration can be measured.
This type of
accelerometers is not capable of a true DC response, e.g. gravitation
The high impedance sensor output needs to be converted into a
low impedance signal first. In the case of IEPE compatible transducers
this is the task of the built-in electronics. This electronic circuit
is powered by the connected instrument. This can be a simple supply
unit, for instance Metra’s M28,
or the signal conditioners M32,
and M208. For sensors
with charge output, an external charge
amplifier is required, for instance Model M68 or IEPE100.For processing
the sensor signal, a variety of equipment can be used. Common are, such
However, the capability of such equipment would be wasted without an
accurate sensor signal. In many cases the accelerometer is the most
critical link in the measurement chain. To obtain precise vibration
signals some basic knowledge about piezoelectric accelerometers is
- Time domain equipment, e.g. RMS and peak value meters
- Frequency analyzers
- PC instrumentation
Proceed to chapter Piezoelectric Principle