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.
Working Principle
A capacitor consists of two parallel conducting plates separated by a small gap. The capacitance is defined by:
C=ereoA/d
- er is the dielectric constant of the material, between the plates (this is 1 for a vacuum)
- e0 is the electric constant, (equal to 8.854×1012 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 below:
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 pressure 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.
Circuits like the above diagram apply a constant current to the bridge. The output signal is then amplified and applied to the input of the measurement system. If your application is based upon a micro controller, the signal could be connected to an ADC input pin. Otherwise, there are many standalone ADCs with digital outputs that could be sourced as an alternative.
The analog front end (AFE) may also need to allow provision for offset voltage, temperature compensation, and span. If you’re looking for a simpler solution, many manufacturers provide fully integrated differential sensors. For example, some may include temperature compensation circuitry together with two stages of amplification, enabling it to be simply connected to the ADC input of a micro controller.