The working principle of the piezoelectric effect of vacuum pressure switches

Vacuum pressure switches usually do not directly rely on the piezoelectric effect for pressure sensing. However, some piezoelectric pressure sensors used in vacuum pressure switches involve the piezoelectric effect, and their working principle is as follows:

Positive piezoelectric effect

When piezoelectric materials are subjected to external forces, polarization occurs inside them, and at the same time, opposite positive and negative charges appear on their two opposite surfaces. For instance, quartz crystals are composed of oxygen and silicon atoms. Oxygen and silicon atoms share electrons to form chemical bonds. Due to the smaller size of oxygen atoms, the shared electrons are closer to the oxygen nucleus. Oxygen has a greater electronegativity than silicon and carries a negative charge, while silicon atoms carry a positive charge. Together, oxygen and silicon atoms form dipoles in quartz crystals, which are connected end to end to form a specific arrangement. When no pressure is applied, the positive and negative charge centers of a crystal coincide, the total electric moment of the entire crystal is zero, and the crystal surface is not charged (it does not exhibit piezoelectricity). When pressure is applied along the x direction, the crystal deforms, and the centers of positive and negative charges separate (referred to as polarization), that is, the electric dipole changes, thus resulting in charge accumulation on the X plane. When pressure is applied along the Y-axis, the crystal deformation causes a change in the distribution of positive and negative charge centers. At this time, the total electric dipole distance changes and leads to the accumulation of charges with the opposite sign on the X-plane. At the same time, if the tensile force is used instead of the compressive force mentioned earlier, it indicates that the sign of the charge is reversed. In conclusion, when pressure is applied to a piezoelectric crystal, the piezoelectric effect may be caused.

Signal conversion and output

In vacuum pressure switches, piezoelectric materials serve as pressure-sensitive elements. When the pressure in the vacuum system changes, the pressure exerted on the piezoelectric materials also changes. According to the positive piezoelectric effect, charges will be generated on its surface, and the amount of these charges is directly proportional to the magnitude of the pressure. The generated charge will form an electrical signal. This electrical signal is processed by subsequent circuits, such as amplification and filtering, and is converted into standardized electrical signals, such as 4-20mA or 0-10V analog signals, or into digital signals. Then, the processed signal is compared with the preset pressure threshold. When the signal exceeds or falls below the preset threshold, the vacuum pressure switch will output the corresponding control signal, such as the on-off of the control circuit, to achieve the monitoring and control of the vacuum system.