Introduction to piezoelectric accelerometers

The introduction to piezoelectric accelerometers is the theme covered in this article.

Introduction

The accelerometer is a device used for the measuring vibrations The accelerometer is a device used for the. They can work from a variety of physical effects and are capable of measuring a wide range of acceleration values, logo having a very high range of applications. These devices are widely used in positioning systems, tilt sensors, as well as vibration and shock sensors or connected to Vibration analyzers. In our daily life we ​​are surrounded by accelerometers, applications being well known the orientation of mobile phone screens that adjust according to the angle they make in relation to the acceleration of gravity or the trigger sensors of “air-bags” of cars.

There are numerous types of accelerometers that use different types of physical effects to measure acceleration..

The most used for predictive maintenance in analysis vibrações, are piezoelectric accelerometers and will be discussed here.

Definition of Piezoelectricity

A Piezoelectricity consists of the capacity of some materials (namely crystals and some ceramics) to generate an electrical potential in response to the application of a mechanical force.. This can take the form of an electrical charge in the crystal matrix.. If the material is not short circuited, the electric charge induces a voltage in the material.

Introduction to piezoelectric accelerometers figure 1

Introduction to piezoelectric accelerometersThe Piezoelectric Transducer

The active element of all piezoelectric devices is a piezoelectric material element. There are many different sensor construction solutions based on different crystals and materials.

The types of piezoelectric sensor conditioning used today are:

  • Voltage Mode (with various trade names. mares, LIVM, ICP, Piezotron, Isotron) where there is electronics mounted on the transducer itself;
  • Charge mode that are connected to an external charge amplifier

Each has its advantages and disadvantages, which will be discussed below..

Because we use piezoelectric sensors?

The reasons why we use piezoelectric accelerometers are as follows.:

  • reduced dimension
  • light
  • with two wires (mares)
  • large dynamic range
  • wide temperature range
  • wide frequency range
  • very low noise
  • Simple signal conditioning
  • economic implementation
  • Structural Integrity Assessment

Construction solutions for piezoelectric sensors with integrated electronics (voltage mode)

The main constructive solutions for piezoelectric accelerometers are as follows:

Characteristic
Integrated electronicsYes No need for signal conditioning; more economical. Upper temperature limit imposed by included electronics (Typical example: 120 ºC) Many modern measuring equipment include IEPE power supply.Cannot withstand higher temperatures
Material piezoeltricoQuartzCeramic. more sensitive
Piezoelectric sensor assemblyto compressionWhen cutting Less sensitive to base deformation.

The sensor with piezoelectric material can be mounted by compression or by cutting as shown below. Mounting to the cut has the advantage of being less sensitive to deformations of the accelerometer base.

Introduction to piezoelectric accelerometers figure 2

 Input/Output Characteristics of Voltage Mode Sensors (mares)

The main features of IEPE accelerometers are as follows.

Entry: 18 – 24 Volts DC, 2-20mA constant current; input values ​​other than those mentioned may damage the sensor

Exit: voltage proportional to acceleration or vibration

Note: The voltage mode sensor or IEPEE is a two-wire device. (signal/power and ground)

Introduction to piezoelectric accelerometers figure 3

Introduction to piezoelectric accelerometersTypical IEPE system

Modern vibration analyzers are all powered for this type of accelerometer.

Introduction to piezoelectric accelerometers figure 4

BIAS voltage levels in typical IEPE systems

Introduction to piezoelectric accelerometers figure 1 – Frequency response of IEPE piezoelectric accelerometers

The low frequency response is controlled by the Discharge Time Constant (DTC)

  • Factory adjusted and cannot be changed.
  • Can be changed at manufacture.

The high frequency response is a function of the resonant frequency (natural) do sensor.

Filters in the sensor or signal conditioning can also affect the sensor's frequency response..

  • Highly sensitive sensors typically have a low resonant frequency..
  • Low sensitivity sensors typically have a high resonant frequency..

Download Time Constant (DTC)

Discharge Time Constant - time required for sensor output voltage, to download to 37% of its original value, in response to an abrupt ascent, in step, of input acceleration.

  • Determines the low frequency response of the sensor
  • The low frequency response is approximately equal to:

                                Point -3dB = .16/DTC

                               Point -5% = Point -3dB x 3

Construction solutions for piezoelectric sensors without integrated electronics (charging mode)

The constructive solutions for piezoelectric sensors without integrated electronics are as follows (charging mode).

  • They connect to a charge amplifier that converts the accelerometer signal into Volts
  • Usually with ceramic sensor
  • No electronics included
  • Suitable for high temperatures, such as monitoring gas turbines (for example up to 700°C)
  • Expensive signal conditioning
  • Low noise cables required for most applications
  • Load amplifiers are usually more difficult to use (ex.: sensitivity adjustment, etc.)

Below you can see the high temperature CA901 accelerometer, that works up to 700ºC..

Introduction to piezoelectric accelerometers – Mounting the accelerometer

How you set up an accelerometer is very important because it can influence its frequency response..

Mounting Types

  • Mount with leg
    • optimal frequency response
  • Adapter glued with epoxy glue, leg mounted sensor
    • good frequency response
  • Magnetic base
    • Comfortable, limited frequency response
  • Magnetic base in adapter glued with epoxy glue
    • Best method for mounting with magnetic base

  • Frequency Response Effects of Montage
  • dynamic range

    Range of levels the accelerometer can measure, which can be expressed in different ways

    The full scale of IEPE sensors is typically ±5 Volts output signal

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