The piezoelectric sensor of vibration

 introduction

The piezoelectric sensor works on the basis of the seismic principle and the piezoelectric effect. Here quartz crystal and piezo ceramic replace the spring used in a Seismometer. The piezo material is fixed to the vibrating object on one side and to the seismic mass on the other. Vibrations forces lead to strain and compression on the piezo material. The piezoelectric effect describes the occurrence of an electric charge due to the change in length of the polarised materials. This charge is proportional to the acting force and can be tapped. Since the force is a product of mass and acceleration it can be easily computed. The piezo materials are very rigid, therefore damping might be necessary. This can be achieved by adding stoppers or immersing the parts in oil.

Vibration is the motion of a particle or a device or system of connected devices scattered around the balanced position. Most vibrations are undesirable in machines and equipment because they lead to increased loads, fatigue and energy loss, increased bearing loads, creating discomfort for passengers in vehicles and absorbing energy from the system. The rotating parts in machines must be carefully balanced to prevent vibration damage.

Vibration is a response of the system to internal or external impact, which causes it to fluctuate or pulsate. Although it is commonly believed that vibrations do damage to the equipment and the machinery, they do not. Instead, the damage is done by dynamic loads, which lead to fatigue and dynamic loads are caused by vibration. If a vibrating object can be seen in slow motion, it will be found running in different directions. Each vibration has two measurable variables that help to determine the vibration characteristics, how far (magnitude or intensity) and how fast (frequency) the subject is moving. The parameters used to describe this movement are displacement, frequency, amplitude and acceleration.

Working principle Piezoelectric Transducer

 Piezoelectric effect occurs in some crystalline substances - natural quartz, Rochelle salt, lithium sulphate, some ceramics and more. When such a crystal is placed in an electric field it changes its size synchronously with the changes of the field – opposite Piezoelectric effect (used to generate audible and ultrasonic signals). When the crystal is deformed in an appropriate direction an electric charge is generated (straight Piezoelectric effect).

One of the commonly used structures for vibration measurement while controlling the state of the machine is shown in the Figure below. The Piezoelectric transducer is glued strongly at the one end of the bending under the forces of inertia plate and the free end is soldered seismic mass. Attenuation is achieved through the oil drops placed in the gap between the seismic mass and the attenuator. When the base is moving downside-up the inertia opposes and deforms the Piezoelectric transducer. This generates an electrical charge that is proportional to the acceleration. Typical values of the sensitivity of these sensors are 0,5-50 mVs2/m in the frequency range 0,1 Hz to 200 kHz.

 Between the vibrational displacement x, absolute displacement of the seismic mass y and its relative movement z exist dependence (Z = X - Y)

Below the Figure the internal components of the transducer

A piezoelectric vibration transducer generates an electrical charge that is proportional to the deflection of the piezoelectric transducer.

Advantages

The piezo wafers are configured to undergo shear deformation from acceleration (i.e. the piezo wafers are perpendicular to the base). This has become the most popular configuration because the piezo wafers are isolated from the base which helps reduce the temperature sensitivity and susceptibility to base strain. But this configuration typically has a relatively low sensitivity-to-mass ratio which means that you'll need a charge amplifier.

disadvantages

• With the piezo in compression, it is directly in line with the base and a seismic mass above. This provides a moderately high sensitivity-to-mass ratio but this results in effectively a spring-mass system between the piezoelectric element and the base. This can lead to easily produced erroneous results from base bending or thermal expansion. Therefore this configuration is rarely used except in high shock applications because of its robustness.
• Piezoelectric accelerometers are not capable of a true DC response. Piezos need dynamic forces to act upon it, a piezoelectric accelerometer can never directly measure the force exerted by gravity. The low-frequency rollover of the piezoelectric accelerometer is determined by the electronic circuit and can be tuned so that you can measure very-low-frequency (< 1 Hz) vibrations like those in seismic or large structure applications. But they will never be able to measure static acceleration levels - do not expect a 1g offset from a piezoelectric accelerometer due to gravity!