Piezoelectric pressure sensors
What are piezoelectric pressure sensors?
Piezoelectricity is the charge created across certain materials when a mechanical stress is applied.
Piezoelectric pressure sensors exploit this effect by measuring the voltage across a piezoelectric element generated by the applied pressure. They are very robust and are used in a wide range of industrial applications.
When force is applied to a piezoelectric diaphragm, a
voltage proportional to the pressure is generated |
Working principle
When a force is applied to a piezoelectric material, an electric charge is generated across the faces of the crystal. This can be measured as a voltage proportional to the pressure (see diagram to the right).
There is also an inverse piezoelectric effect where applying a voltage to the material will cause it to change shape.
A given static force results in a corresponding charge across the sensor. However, this will leak away over time due to imperfect insulation, the internal sensor resistance, the attached electronics, etc.
As a result, piezoelectric sensors are not normally suitable for measuring static pressure. The output signal will gradually drop to zero, even in the presence of constant pressure. They are, however, sensitive to dynamic changes in pressure across a wide range of frequencies and pressures.
This dynamic sensitivity means they are good at measuring small changes in pressure, even in a very high-pressure environment.
Function
Unlike piezoresistive and capacitive transducers, piezoelectric sensor elements require no external voltage or current source. They generate an output signal directly from the applied strain.
The output from the piezoelectric element is a charge proportional to pressure. Detecting this requires a charge amplifier to convert the signal to a voltage.
Some piezoelectric pressure sensors include an internal charge amplifier to simplify the electrical interface by providing a voltage output. This requires power to be supplied to the sensor.
An internal amplifier makes the sensor simpler to use. For example, it makes it possible to use long signal cables to connect to the sensor. The amplifier can also include signal-conditioning circuitry to filter the output, adjust for temperature and compensate for the changing sensitivity of the sensing element.
The presence of the electronic components does, however, limit the operating temperature to not much more than 120ºC.
For higher temperature environments, a charge-mode sensor can be used. This provides the generated charge directly as the output signal. It therefore requires an external charge amplifier to convert this to a voltage.
Care is required in the design and implementation of the external electronics. The high impedance output of the sensor means the circuit is sensitive to noise caused by poor connections, cable movement, electromagnetic and RF interference.
The low frequency response of the sensor is determined by the discharge time of the amplifier.
Construction
The piezoelectric effect requires materials with a specific asymmetry in the crystal structure. This includes some natural crystals, such as quartz or tourmaline.
In addition, specially formulated ceramics can be created with a suitable polarisation to make them piezoelectric. These ceramics have higher sensitivities than natural crystals. A useful output can be generated with as little as 0.1% deformation.
Because the piezoelectric materials are rigid, only a very small deflection of the material is required to get a usable output signal. This makes the sensors very robust and tolerant of over-pressure conditions. It also means they respond rapidly to changes in pressure.
The pressure sensor can be affected by any external force on the piezoelectric element, for example, by forces caused by acceleration or noise.
Microsensors can be constructed using thin films. Zinc oxide was one of the first materials used. This has largely been replaced by ceramics made from materials such as lead zirconate titanate (PZT) because of their larger piezoelectric effect.
Micoelectromechanical systems (MEMS) can be created by combining piezoelectric thin films with micromachined silicon membranes.
Piezoelectric materials are also used in some other types of MEMS sensors. For example, the inverse piezoelectric effect is used to generate surface acoustic waves through a diaphragm. The distortion of the surface under pressure can then be detected by the changes it causes in the waves that are received by another piezoelectric element.
A cross-section of a piezoelectric presure sensor construction |
Design
Piezoelectric pressure sensors are often constructed in a threaded tube (as shown in the diagram below) to make it easy to mount them in equipment where pressure is to be monitored. Care is needed when installing these because over-tightening can affect the output sensitivity.
In some of the typical applications of piezoelectric sensors, they may be exposed to thermal shock (a sudden change in temperature) caused by either radiant heat or the flow of hot gases or liquids past the sensor.
This can cause changes to the output due to heating of the crystal, the diaphragm or the casing of the sensor. Note that this is not the same as the static temperature sensitivity of the sensor.
The effects of thermal shock can be minimised by the design of the enclosure and mounting the sensor to provide isolation.
Sensitivity
The output is linear over a wide range, typically 0.7 KPa to 70 MPa (0.1 to 10000 psi) with an accuracy of about 1%.
Ceramic sensors are subject to a loss of sensitivity over time. But this is usually quite small; typically less than 1% per year.
There may also be a small loss in sensitivity when first exposed to high pressure and temperature. The effects of this can be avoided by cycling the sensor through the maximum expected pressure and temperature before deploying them.
The frequency response of a piezoelectric sensor drops off at low frequencies because the generated charge cannot be retained.
At high frequencies there is a peak corresponding to the resonant frequency of the piezoelectric element. The sensor is normally used within the flat region of the response curve between these two extremes (see below).
The frequency response of a piezoelectric sensor
Applications
The robustness, high frequency and rapid response time of piezoelectric pressure sensors means they can be used in a wide range of industrial and aerospace applications where they’ll be exposed to high temperatures and pressures.
They are often used for measuring dynamic pressure, for example in turbulence, blast, and engine combustion. These all require fast response, ruggedness and a wide range of operation.
Their sensitivity and low power consumption also makes them useful for some medical applications. For example, a thin-film plastic sensor can be attached to the skin and used for real-time monitoring of the arterial pulse.
Advantages and disadvantages
One of the main advantages of piezoelectric pressure sensors is their ruggedness. This makes them suitable for use in a variety of harsh environments.
Apart from the associated electronics, piezoelectric sensors can be used at high temperatures. Some materials will work at up to 1,000ºC. The sensitivity may change with temperature but this can be minimised by appropriate choice of materials.
The output signal is generated by the piezoelectric element itself, so they are inherently low-power devices.
The sensing element itself is insensitive to electromagnetic interference and radiation. The charge amplifier and other electronics need to be carefully designed and positioned as close as possible to the sensor to reduce noise and other signal errors.
Piezoelectric sensors can be easily made using inexpensive materials (for example quartz or tourmaline), so they can provide a low cost solution for industrial pressure measurement.
Want to learn more about the other core technologies used in pressure sensors? Click the links below to jump to the section you're interested in.
Looking for more on pressure sensor technology? Check out the further chapters of this guide below, or if you're pressed for time you can download it in a PDF format here.
