Capacitive pressure sensors
What are capacitive pressure sensors?
Capacitive pressure sensors measure pressure by detecting changes in electrical capacitance caused by the movement of a diaphragm.
Working principle
A capacitor consists of two parallel conducting plates separated by a small gap. The capacitance is defined by:
where:
- εr is the dielectric constant of the material between the plates (this is 1 for a vacuum)
- ε0 is the electric constant (equal to 8.854x1012 F/m),
- A is the area of the plates
- d is the distance between the plates
Changing any of the variables will cause a corresponding change in the capacitance. The easiest one to control is the spacing. This can be done by making one or both of the plates a diaphragm that is deflected by changes in pressure.
Typically, one electrode is a pressure sensitive diaphragm and the other is fixed. An example of a capacitive pressure sensor is shown to the right.
An easy way of measuring the change in capacitance is to make it part of a tuned circuit, typically consisting of the capacitive sensor plus an inductor. This can either change the frequency of an oscillator or the AC coupling of a resonant circuit.
Construction
The diaphragm can be constructed from a variety of materials, such as plastic, glass, silicon or ceramic, to suit different applications.
The capacitance of the sensor is typically around 50 to 100 pF, with the change being a few picofarads.
The stiffness and strength of the material can be chosen to provide a range of sensitivities and operating pressures. To get a large signal, the sensor may need to be fairly large, which can limit the frequency range of operation. However, smaller diaphragms are more sensitive and have a faster response time.
A large thin diaphragm may be sensitive to noise from vibration (after all, the same basic principle is used to make condenser microphones) particularly at low pressures.
Thicker diaphragms are used in high-pressure sensors and to ensure mechanical strength. Sensors with full-scale pressure up to 5,000 psi can readily be constructed by controlling the diaphragm thickness.
A cross section of capacitive sensor construction |
By choosing materials for the capacitor plates that have a low coefficient of thermal expansion, it’s possible to make sensors with very low sensitivity to temperature change. The structure also needs to have low hysteresis to ensure accuracy and repeatability of measurements.
Because the diaphragm itself is the sensing element, there are no issues with extra components being bonded to the diaphragm, so capacitive sensors are able to operate at higher temperatures than some other types of sensor.
Capacitive pressure sensors can also be constructed directly on a silicon chip with the same fabrication techniques that are used in manufacturing semiconductor electronic devices (see diagram to the left). This allows very small sensing elements to be constructed and combined with the electronics for signal conditioning and reporting. Pressure sensors using microelectronic mechanical systems (MEMS) are described in more detail in chapter 6.4.
Function
The change in capacitance can be measured by connecting the sensor in a frequency-dependent circuit such as an oscillator or an LC tank circuit. In both cases, the resonant frequency of the circuit will change as the capacitance changes with pressure.
An oscillator requires some extra electronic components and a power supply. A resonant LC circuit can be used as a passive sensor, without its own source of power.
The dielectric constant of the material between the plates may change with pressure or temperature and this can also be a source of errors. The relative permittivity of air, and most other gasses, increases with pressure so this will slightly increase the capacitance change with pressure. Absolute pressure sensors, which have a vacuum between the plates, behave ideally in this respect.
A more linear sensor can be constructed by using ‘touch mode’ where the diaphragm makes contact with the opposite plate (with a thin insulating layer in between) throughout the normal operating range (as shown to the right). The geometry of this structure results in a more linear output signal.
This type of sensor is also more robust and able to cope with a larger over-pressure. This makes it more suited to industrial environments. However, this structure is more prone to hysteresis because of friction between the two surfaces.
Design
An external antenna in some passive sensors to stimulate the tuned circuit |
The electronics for measuring and conditioning the signal need to be placed close to the sensing element to minimise the effect of stray capacitance.
Because they can be incorporated as components in high-frequency tuned circuits, capacitive sensors are well suited for wireless measurement.
In the case of passive sensors an external antenna can be used to provide a signal to stimulate the tuned circuit and so measure the change in resonance frequency (see diagram to the left). This makes them suitable for medical devices that need to be implanted.
Alternatively, for an active sensor, the frequency generated by the oscillator can be picked up by an antenna.
Applications
Capacitive pressure sensors are often used to measure gas or liquid pressures in jet engines, car tyres, the human body and many other places. But they can also be used as tactile sensors in wearable devices or to measure the pressure applied to a switch or keyboard.
They are particularly versatile, in part due to their mechanical simplicity, so can be used in demanding environments. Capacitive sensors can be used for absolute, gauge, relative or differential pressure measurements.
Advantages and disadvantages
Capacitive pressure sensors have a number of advantages over other types of pressure sensors.
They can have very low power consumption because there is no DC current through the sensor element. Current only flows when a signal is passed through the circuit to measure the capacitance. Passive sensors, where an external reader provides a signal to the circuit, do not require a power supply – these attributes make them ideal for low power applications such as remote or IoT sensors.
The sensors are mechanically simple, so they can be made rugged with stable output, making them suitable for use in harsh environments. Capacitive sensors are usually tolerant of temporary over-pressure conditions.
They have low hysteresis with good repeatability and are not very sensitive to temperature changes.
On the other hand, capacitive sensors have non-linear output, although this can be reduced in touch-mode devices. However, this may come at the cost of greater hysteresis.
Finally, careful circuit design is required for the interface electronics because of the high output impedance of the sensor and to minimise the effects of parasitic capacitance.
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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.
