Capacitive sensor use various types of capacitors as sensing elements to convert the measured physical quantity or mechanical quantity into a conversion device for capacitance change, which is actually a capacitor with variable parameters. A capacitive pressure sensor contains a capacitor with one rigid plate and one flexible membrane as electrodes. The area of these electrodes being fixed, the capacitance is proportional to the distance between the electrodes. The pressure to be measured is applied to the flexible-membrane side, and the resulting deflection causes a change in capacitance that can be measured using an electrical circuit. Capacitive sensors are widely used in the measurement of displacement, angle, vibration, speed, pressure, composition analysis, medium properties, etc. The most commonly used are parallel plate capacitors or cylindrical capacitors.
Working principle of capacitive pressure sensor
Capacitive pressure sensor measures 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.
The diagram below illustrates the operating principle behind capacitive pressure sensing.
Advantages of capacitive pressure sensor
- 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.
- Low power
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.
- Low temperature sensitivity
Capacitive sensors exhibit low hysteresis and good repeatability of measurements. They also have low temperature sensitivity. 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.