Introduction

While some vibrations are desirable, others may be disturbing or even 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 distribute serves 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, vibration 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 surveys.
• 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 accelerometers

The accelerometers IDS Innomic offers 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 acceleration.

The active element of our accelerometers consists of a carefully 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 vibration.
• 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 displacement.

The following table shows other common types of vibration sensors compared to piezoelectric accelerometers:

Which instrumentation?

The piezoelectric principle requires no external energy. The sensor acquires the energy transmitted to the subsequent instrument, also the integrated amplifier, from the acceleration during measurement process. Only alternating acceleration can be measured. It is offen called vibraation. This type of accelerometers is not capable of a true DC response, e.g. gravitation acceleration.

The high impedance sensor output needs to be converted into a low impedance signal first. In the case of IEPE accelerometers this is the task of the built-in electronics. This electronic circuit is powered by the connected instrument. Suitable are for example the signal conditioners M28, M32, M68 and M208. The InnoBeamer also offers constant current supply, gain and filtering. For sensors with charge output, an external charge amplifier is required, for instance a model from the M68-series.

For processing the sensor signal, a variety of equipment can be used. Common are, such as:

• Time domain equipment, e.g. RMS and peak value meters,
• Frequency analyzers,
• Recorders,
• PC instrumentation.
  

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 required.

Copyright (C) 2008 Metra Mess- und Frequenztechnik, edited by IDS Innomic.
Publication allowed, if we are named  as source and a short information about the publication purpose
was sent to us.