Capacitive vs piezoresistive vs piezoelectric pressure sensors
The first pressure gauges were purely mechanical. They used mechanisms such as a diaphragm or a “Bourdon tube” that changed shape under pressure to move a pointer on a dial.
Various techniques have since been developed to convert mechanical displacements into electrical signals. Here we’ll consider the relative advantages of piezoresistive, capacitive and piezoelectric pressure sensors.
Principles of operation
In a piezoresistive strain gauge sensor, the change in electrical resistance of one or more resistors mounted on a diaphragm is measured. The change in resistance is directly proportional to the strain caused by pressure on the diaphragm. The resistors are connected in a Wheatstone bridge circuit, which is a very sensitive way of converting the small changes to an output voltage.
Capacitive pressure sensors measure changes in electrical capacitance caused by the movement of a diaphragm. A capacitor consists of two parallel conducting plates separated by a small gap. One of the plates acts as the diaphragm that is displaced by the pressure, changing the capacitance of the circuit. The resulting change of resonant frequency of a circuit can be measured. Or, in a digital system, the time taken to charge and discharge the capacitor can be converted to a series of pulses.
Piezoelectric sensors use materials, such as quartz crystals or specially formulated ceramics, which generate a charge across the faces when pressure is applied. A charge amplifier converts this to an output voltage proportional to the pressure. A given force results in a corresponding charge across the sensing element. However, this charge will leak away over time meaning that the sensor cannot be used to measure static pressure.
All three types of sensors can be miniaturised using silicon fabrication techniques and combined with electronics as microelectromechanical systems (MEMS). This allows very small sensing elements to be constructed and combined with the electronics for signal conditioning and readout.
Piezoresistive and capacitive pressure sensors can be used for absolute, gauge, relative or differential measurements.
Piezoelectric sensors are sensitive to changes in pressure so the output is usually treated as a relative pressure measurement, referenced to the initial state of the piezoelectric material.
Advantages and disadvantages of the three sensor types
Piezoresistive strain gauge sensors
These are the earliest and most widely used type of pressure sensor.
The simple construction means low cost and durability. The sensors are robust with good resistance to shock, vibration, and dynamic pressure changes.
The readout circuits are very simple and enable high-resolution measurement.
The output is linear with pressure and the response time is typically below one millisecond.
They can be used for a wide range of pressure measurements from 3 psi up to about 20,000 psi (21 kPa to 150 MPa). The output is also stable over time.
The resistive elements can be bonded to the diaphragm. This is a standard technique that has been in use for a long time but there can be problems with the adhesives at high temperatures and overpressure.
Alternatively, thin film resistors can be created directly on the membrane. These can operate at higher temperatures and are more suitable for use in harsh environments.
The main disadvantage is that the sensor has to be powered. This makes them unsuitable for low power or battery operated systems. Scaling down the size reduces the resistance and increases the power consumption.
There are also limitations on scaling because strain averaging reduces the sensitivity of the sensor. However, very small sensors can be fabricated as MEMS devices.
The sensor output is temperature dependent. This can be a big disadvantage for applications such as tyre pressure measurement where there are large temperature changes over the operating cycle.
Silicon strain gauges are much more sensitive and can measure pressures down to 2 kPa.
The accuracy of MEMS devices can be reduced by junction leakage current. This can be mitigated by using silicon on insulator (SoI) technology, but this adds to the cost.
Capacitive sensors
The capacitive element is mechanically simple and robust.
Capacitive sensors are able to operate over a wide temperature range and are very tolerant of short-term overpressure conditions.
They can be used to measure a wide range of pressure from vacuum (2.5 mbar or 250 Pa) to high pressures up to around 10,000 psi (70 MPa). They’re ideal for both lower-pressure applications and reasonably harsh environments.
Because no DC current flows through the capacitor, they are inherently low power.
Passive devices may not require a power source at all; the excitation signal can be provided by the external reader. This makes them suitable for wearable or implanted medical devices. These applications can be enhanced by new technologies that enable the construction of sensors that are flexible or moulded to shape.
Capacitive sensors exhibit low hysteresis and good repeatability of measurements. They also have low temperature sensitivity.
The response time is in the order of milliseconds, and even faster in the case of MEMS devices.
Because they’re inherently AC devices, capacitive sensors are suitable for wireless applications. They can be used in an oscillator circuit to generate a signal, with a frequency proportional to pressure, that can be received wirelessly.
Alternatively, the reader can use inductive coupling to measure the change in resonant frequency – this is particularly suitable for passive devices that require no power supply.
One of the main disadvantages of capacitive sensors is the non-linearity exhibited because the output is inversely proportional to the gap between the parallel electrodes. This can be improved by using the sensor in touch mode, where the diaphragm is in contact with the insulating layer on the lower electrode. However, this can reduce sensitivity and increase hysteresis. They are also sensitive to vibration.
The interface needs to minimise stray capacitance by having the electronics as close as possible to the sensor. This is another benefit of MEMS technology.
Piezoelectric sensors
The main advantages of piezoelectric sensors are robustness and low power.
The sensing elements are made of rigid materials, which can be natural crystals such as quartz or specially formulated ceramics. These require only a very small deformation to generate an output, so there are effectively no moving parts.
This means the sensors are extremely robust and suitable for use in a range of very harsh environments. They can also tolerate very high temperatures; some materials can be used at up 1,000ºC.
This makes piezoelectric sensors suitable for applications such as measuring pressures in jet engines.
The sensor elements are self-powered so they’re intrinsically low-power devices. It also means they’re insensitive to electromagnetic interference.
However, designing the electronic interface is more complex than the other sensor types. A charge amplifier is required to convert the very high impedance charge output to a voltage signal. This needs to be located close to the sensing element.
Some sensors include integrated electronics, which simplifies the use of the sensor but reduces the operating temperature range.
With ceramic materials, a usable output can be obtained with very small displacements. This means they can be used for measuring a very wide range of pressures, between 0.1 psi and 10,000 psi (0.7 kPa to 70 MPa), with very high accuracy.
The piezoelectric elements can be very small with an extremely fast response to changes in pressure. Some devices can measure rise times in the order of 1 millionth of a second. As a result, piezoelectric sensors are used for measuring pressure changes in explosions.
The sensors are simple to construct and can be made from inexpensive materials.
The main limitation of piezoelectric sensors is that they can only be used for dynamic pressure measurement.
The sensors are sensitive to vibration or acceleration, which may be common in the applications where they are used. This can be minimised by using an extra “compensation” sensor attached to a dummy mass. The output from this is used to correct for acceleration experienced by the sensor.
Overall, these three sensor types are robust and low cost. They function over a wide range of pressures and temperatures so there are suitable sensors available for almost every application. For more on each sensor technology refer to chapters 6.3, 6.4 and 6.5 respectively.
If you want to learn more about the core types mentioned here, as well as MEMS and optical technologies, 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.
