Señales Y Sistemas

Piezoelectric Accelerometers
Theory and Application

Published by: Metra Mess- und Frequenztechnik © 2001

Contents
1. Introduction 1.1. Why Do We Need Accelerometers? 1.2. The Advantages of Piezoelectric Sensors 1.3. Instrumentation 2. Operation and Designs 2.1. Operation 2.2. Accelerometer Designs 2.3. Built-in Electronics 3. Characteristics 3.1. Sensitivity 3.2. Frequency Response 3.3.Transverse Sensitivity 3.4. Maximum Acceleration 3.5. Non-Vibration Environments 3.5.1. Temperature 3.5.2. Base Strain 3.5.3. Magnetic Fields 3.5.4. Acoustic Noise 4. Application Information 4.1. Instrumentation 4.1.1. Accelerometers With Charge Output 4.1.2. Accelerometers With Built-in Electronics 4.1.3. Intelligent Accelerometers to IEEE1451.4 4.2. Preparing the Measurement 4.2.1. MountingLocation 4.2.2. Choosing the Accelerometer 4.2.3. Mounting Methods 4.2.4. Cabling 4.2.5. Calibration
Notice: ICP is a registered trade mark of PCB Piezotronics Inc. #390 08.01

3 3 3 4 4 4 7 9 12 12 13 13 13 14 14 15 15 15 16 16 16 16 17 18 18 19 19 21 23

2

1. Introduction
1.1. Why Do We Need Accelerometers? Vibration and shock are present in all areas of our daily lives. They may begenerated and transmitted by motors, turbines, machine-tools, bridges, towers, and even by the human body. 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 sensor we manufacture serves as a link between vibrating structures and electronicmeasurement equipment. 1.2. The Advantages of Piezoelectric Sensors The accelerometers Metra has been manufacturing for over 40 years utilize the phenomenon of piezoelectricity. They generate an electric charge signal proportional to vibration acceleration. The active element of Metra accelerometers consists of a specially developed ceramic material with excellent piezoelectric properties.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, low output noise - suitable for shock measurement as well as for almost imperceptible vibration • Excellent linearity over their dynamic range • Wide frequency range • Compact yethighly sensitive • No moving parts - no wear • Self-generating - no external power required • Great variety of models available for nearly any purpose • Acceleration signal can be integrated to provide velocity and displacement

3

1.3. Instrumentation The piezoelectric principle requires no external energy. Only alternating acceleration can be measured. This type of accelerometer is notcapable of a true DC response, e.g. gravitation acceleration. The high impedance sensor output needs to be converted into a low impedance signal first. For processing the sensor signal a variety of equipment can be used, 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 wastedwithout 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.

2. Operation and Designs
2.1. Operation The active element of the accelerometer is a piezoelectric material. One side of the piezoelectric material is connected to a rigid postat the sensor base. A so-called seismic mass is attached to the other side. When the accelerometer is subjected to vibration a force is generated which acts on the piezoelectric element. This force is equal to the product of the acceleration and the seismic mass. Due to the piezoelectric effect a charge output proportional to the applied force is generated. Since the seismic mass is constant...