Vacuum pressure switch capacitance sensing principle

The capacitive sensing principle of the vacuum pressure switch is based on the parallel plate capacitor principle. It measures pressure by changing the capacitance due to the alteration of the plate spacing or area under pressure. The following is a specific analysis:

Core structure

Capacitive pressure sensors are composed of deformable diaphragms, fixed electrodes, electrode clearances, dielectric materials and reference pressure chambers, etc. The deformable diaphragm is a pressure-sensing element, usually made of metal (such as stainless steel, nickel alloy) or insulating material (such as ceramic or silicon) plated with a metal layer, and has a certain degree of elasticity. The fixed electrode is located beneath the deformed diaphragm, parallel to it and opposite to it. It is a conductive surface and is usually machined on a strong insulating substrate. A parallel plate capacitor is formed between the deformed diaphragm and the fixed electrode. When the diaphragm is not compressed, the gap has an initial distance d

0

The medium filled in the gap is usually air, nitrogen or other inert gases, and there may also be transfer media such as oil. Its dielectric constant is ε. In terms of the reference pressure chamber, the back side of the diaphragm may be a sealed vacuum chamber (for measuring absolute pressure) or connected to the atmosphere (for measuring gauge pressure), or it can be another chamber that introduces the reference pressure (for measuring differential pressure).

Work process

When the external pressure P acts on the deformed diaphragm, the diaphragm will undergo elastic deformation and bend towards the fixed electrode (protrusion or depression depends on the structure), resulting in a decrease in the distance d between the diaphragm and the fixed electrode (the direction of deformation makes the gap smaller). According to the capacitance formula of the parallel plate capacitor, C=ε

cdotA/d (where ε is the dielectric constant of the medium between the electrodes and A is the effective area relative to the electrodes, which is usually considered constant), as the distance d decreases, the capacitance C increases. That is, the greater the pressure, the greater the deformation; the smaller the d, the larger the C. The applied pressure P is proportional to the change in capacitance ΔC.

Signal conversion

The change in capacitance value (ΔC) is very small and requires a dedicated electronic measurement circuit to convert it into a usable electrical signal. Common circuits include LC oscillation circuits, bridge circuits (such as Ventrobridge), capacitance-based oscillators (to change frequency), and charge and discharge circuits using operational amplifiers, etc. The measurement circuit eventually converts the capacitance change into a standard analog signal (such as 4-20mA current, 0-5V / 0-10V voltage) or a digital signal output, and this output signal corresponds to the measured pressure value.

Characteristics and precautions

Capacitive pressure sensors have the advantages of high sensitivity, low power consumption, good stability and repeatability, good linearity and resolution, etc. They can measure various pressure types, have a moderate response frequency, and have good anti-electromagnetic interference ability. However, there are also problems such as output nonlinearity, sensitivity to parasitic capacitance, the need for complex circuits, and being affected by temperature. At the same time, the installation requirements are high. It is necessary to ensure that the diaphragm has good contact with the medium to be measured, and there should be no solid particles damaging the diaphragm. Installation stress may also affect the accuracy (especially for small-range sensors).