Introduction of Capacitive Pressure Sensor

What is capacitive pressure sensor?

A pressure transducer is a device that measures the pressure of a fluid, indicating the force the fluid is exerting on surfaces in contact with it. The capacitance pressure sensor is a pressure measurement device, which converts an applied pressure into a current signal, Like 4-20mA. Capacitive pressure transducers are used in many control and monitoring applications, such as flow, airspeed, level, pump systems, or altitude. A capacitive pressure sensor consists of two main parts: an elastic material that will deform when exposed to a pressurized medium. And an electrical device that detects the deformation and converts it into a usable electrical signal.

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 below). This allows very small sensing elements to be constructed and combined with the electronics for signal conditioning and reporting.

A cross section of a capacitive MEMS sensor construction

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.

Design:

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 below). 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. The electronics for measuring and conditioning the signal need to be placed close to the sensing element to minimize the effect of stray capacitance.

Because they can be incorporated as components in high-frequency tuned circuits, capacitive pressure 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 below). This makes them suitable for medical devices that need to be implanted.

An external antenna in some passive sensors to stimulate the tuned circuit

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.

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