Vibrating Sensor

8/12/2019 Vibrating Sensor

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Vibrating Sensor:

Vibration sensors are utilized in a number of applications to measure acceleration and/or

vibrational activity. Vibration sensors can be utilized to determine whether the is operating

properly. Vibration sensors can be useful for monitoring the condition of rotating machinery,

where overheating or excessive vibration could indicate excessive loading, inadequate

lubrication, or bearing wear. Such sensors are also utilized in geophysical and applications

requiring accelerometers. Vibration sensors are used as knock sensors in internal combustion

engines. In order to assure that an engine is operating under optimum conditions, it is necessary

to accurately monitor its actual operating state. One device known to be highly useful for this

purpose is the engine vibration sensor. Vibration or shock sensors are commonly used in to

activate an alarm whenever the devices to which they are attached are touched, moved, or otherwise

vibrated. For example, vibration sensors are commonly placed in windows of buildings to sense glass

breakage and in car alarm to detect vehicle tampering. Commercial vibration sensors use a piezoelectric

ceramic strain transducer attached to a metallic proof mass in order to respond to an externally imposed

acceleration. Piezoelectric vibration sensors used for detecting vibration from various vibration sources

are generally classified into two large types, resonant type and non resonant type. A capacitive vibration

sensor or an accelerometer is formed from a capacitor one plate of which is a proof mass, with the other

plate fixed to a substrate. Vibrations are typically measured using analog vibration sensing elements, such

as analog accelerometers, positioned on machinery at strategic locations.

A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure,

acceleration,strainorforceby converting them to anelectricalsignal.

A piezoelectric disk generates a voltage when deformed
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Appl icat ions

Piezoelectric disk used as aguitar pickup

The Piezoelectric sensors have proven to be versatile tools for the measurement ofvarious processes. They are used forquality assurance,process controland for research

and development in many different industries.

In theautomotive industrypiezoelectric elements are used as the standard devices forengine indicating in developinginternal combustion engines.The combustion processes

are measured with piezoelectric sensors. The sensors are either directly mounted into

additional holes into the cylinder head or the spark/glow plug is equipped with a built inminiature piezoelectric sensor.

The rise of piezoelectric technology is directly related to a set of inherent advantages.The high modulus of elasticityof many piezoelectric materials is comparable to that of

many metals and goes up to 105 N/m. Even though piezoelectric sensors areelectromechanical systems that react oncompression,the sensing elements show almost

zero deflection. This is the reason why piezoelectric sensors are so rugged, have anextremely high natural frequency and an excellent linearity over a wideamplituderange.Additionally, piezoelectric technology is insensitive to electromagnetic fields and

radiation, enabling measurements under harsh conditions. Some materials used

(especiallygallium phosphateortourmaline)have an extreme stability over temperatureenabling sensors to have a working range of up to 1000C. Tourmaline shows pyro

electricityin addition to the piezoelectric effect; this is the ability to generate an electrical

signal when the temperature of the crystal changes. This effect is also common topiezo

ceramicmaterials.

Principle Strain Sensitivity [V/*] Threshold [*] Span to threshold ratio

Piezoelectric 5.0 0.00001 100,000,000

Piezoresistive 0.0001 0.0001 2,500,000

Inductive 0.001 0.0005 2,000,000
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Capacitive 0.005 0.0001 750,000

One disadvantage of piezoelectric sensors is that they cannot be used for true static

measurements. A static force will result in a fixed amount of charges on the piezoelectric

material. While working with conventional readout electronics, imperfect insulating materials,

and reduction in internal sensorresistancewill result in a constant loss ofelectrons,and yield a

decreasing signal. Elevated temperatures cause an additional drop in internal resistance and

sensitivity. The main effect on the piezoelectric effect is that with increasing pressure loads and

temperature the sensitivity is reduced due to twin-formation. While quartz sensors need to be

cooled during measurements at temperatures above 300C special types of crystals like GaPo4

gallium phosphate do not show any twin formation up to the melting point of the material

itself.Anyhow, it would be a misconception that piezoelectric sensors can only be used for very

fast processes or at ambient conditions. In fact, there are numerous applications that show quasi-

static measurements while there are other applications that go to temperatures far beyond

500CPiezoelectric sensors are also seen in nature. Dryboneis piezoelectric, and is thought by

some to act as a biological force sensor.

Princip le of op erat ion

Depending on how a piezoelectric material is cut, three main modes of operation can be

distinguished: transverse, longitudinal, and shear.

Transverse effect

A force is applied along a neutral axis (y) and the charges are generated along the (x) direction,

perpendicular to the line of force. The amount of charge depends on the geometrical dimensions

of the respective piezoelectric element. When dimensions a,b,capply,

Cx= dxyFyb/ a,

where ais the dimension in line with the neutral axis, bis in line with the charge generating axis

and dis the corresponding piezoelectric coefficient.

Longitudinal effect
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The amount of charge produced is strictly proportional to the applied force and is independent of

size and shape of the piezoelectric element. Using several elements that are mechanically in

series and electrically inparallelis the only way to increase the charge output. The resulting

charge is

Cx= dxxFxn,

Where dxxis the piezoelectric coefficient for a charge in x-direction released by forces applied

along x-direction (inpC/N). Fxis the applied Force in x-direction [N] and ncorresponds to the

number of stacked elements.

Shear effect

Again, the charges produced are strictly proportional to the applied forces and are independent ofthe elements size and shape. For nelements mechanically in series and electrically in parallel

the charge is

Cx= 2dxxFxn.

In contrast to the longitudinal and shear effects, the transverse effect opens the possibility to fine-

tune sensitivity on the force applied and the element dimension.

Electr ical prop ert ies

Schematic symbol and electronic model of a piezoelectric sensor

A piezoelectric transducer has very high DC output impedance and can be modeled as a

proportional voltage source and filter network. The voltage V at the source is directly
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proportional to the applied force, pressure, or strain. The output signal is then related to this

mechanical force as if it had passed through the equivalent circuit.

Frequency response of a piezoelectric sensor; output voltage vs applied force

A detailed model includes the effects of the sensor's mechanical construction and other non-

idealities. The inductance Lm is due to the seismic mass and inertia of the sensor itself. Ce is

inversely proportional to the mechanical elasticity of the sensor. C0 represents the static

capacitance of the transducer, resulting from an inertial mass of infinite size. Riis the insulation

leakage resistanceof the transducer element. If the sensor is connected to a load resistance,this

also acts in parallel with the insulation resistance, both increasing the high-pass cutoff frequency.
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In the flat region, the sensor can be modeled as a voltage source in series with the sensor's

capacitance or a charge source in parallel with the capacitance

For use as a sensor, the flat region of the frequency response plot is typically used, between the

high-pass cutoff and the resonant peak. The load and leakage resistance need to be large enough

that low frequencies of interest are not lost. A simplified equivalent circuit model can be used in

this region, in which Csrepresents the capacitance of the sensor surface itself, determined by the

standardformula for capacitance of parallel plates.It can also be modeled as a charge source in

parallel with the source capacitance, with the charge directly proportional to the applied force, as

above.

Sensor design

Metal disks with piezo material, used in buzzers or ascontact microphones

Based on piezoelectric technology various physical quantities can be measured; the most

common are pressure and acceleration. For pressure sensors, a thin membrane and a massive

base is used, ensuring that an applied pressure specifically loads the elements in one direction.

Foraccelerometers,aseismic massis attached to the crystal elements. When the accelerometer

experiences a motion, the invariant seismic mass loads the elements according to Newtons

second law of motion F= ma.

The main difference in the working principle between these two cases is the way forces are

applied to the sensing elements. In a pressure sensor a thin membrane is used to transfer the force

to the elements, while in accelerometers the forces are applied by an attached seismic mass.
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Sensors often tend to be sensitive to more than one physical quantity. Pressure sensors show

false signal when they are exposed to vibrations. Sophisticated pressure sensors therefore use

acceleration compensation elements in addition to the pressure sensing elements. By carefully

matching those elements, the acceleration signal (released from the compensation element) is

subtracted from the combined signal of pressure and acceleration to derive the true pressure

information.

Vibration sensors can also be used to harvest otherwise wasted energy from mechanical

vibrations. This is accomplished by using piezoelectric materials to convert mechanical strain

into usable electrical energy.

Sensing mater ia ls

Two main groups of materials are used for piezoelectric sensors: piezoelectric ceramics and

single crystal materials. The ceramic materials (such as PZT ceramic) have a piezoelectric

constant / sensitivity that is roughly twoorders of magnitudehigher than those of single crystal

materials and can be produced by inexpensive sinteringprocesses. The piezo-effect in piezo

ceramics is "trained", so unfortunately their high sensitivity degrades over time. The degradation

is highly correlated with temperature. The less sensitive crystal materials (gallium phosphate,

quartz, tourmaline)have a much higher when carefully handled, almost infinite long term

stability.
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