Pressure control
The pressure control has the task of maintaining a certain setpoint at the pressure. This is achieved, for example, by an automatic pump control with connected pressure sensor. Constant pressures are as important for the water supply in general as they are for saving energy.
- Pressure sensor
- Piezoresistive pressure sensor
- Piezoelectric pressure sensor
- Capacitive pressure sensor
Pressure sensor
For pressure control to a specific set pressure, the actual pressure must be measured with a pressure sensor.
There are various ways of measuring pressure. To make pressure detectable electronically, the force acting on a diaphragm is usually determined via its
deflection.
Piezoresistive pressure sensor
This type of sensor uses the piezoresistive effect. If a compressive or tensile force is applied to a piezoresistive (semiconductor) material, for example in the form of a strain gauge, the electrical resistance of the material changes.
For use as a pressure sensor, a membrane is coated on the back with the sensor material. When the membrane is deflected, the material is stretched and the resulting change in resistance is measured.
The temperature dependence of the resistances must be taken into account. This is technically compensated by a differential circuit, so that it has no influence on the measurement quality.
The advantages of the piezoresistive pressure sensor are its high sensitivity and measuring accuracy, which in combination with its very compact design makes it ideal for pressure control in finely tuned systems.
Piezoelectric pressure sensor
This type of sensor uses the piezoelectric effect. When a piezoceramic material is deformed by pressure, an electrical voltage is formed across it due to charge displacement.
For use as a pressure sensor, a piezoceramic crystal is placed on a solid base and covered with a diaphragm so that when the diaphragm is deflected, the crystal is centrally loaded.
Since unavoidable leakage currents, mainly due to voltage measurement, ensure that the voltage across the crystal is dissipated over time, this type of sensor is only suitable for detecting the magnitude of dynamic pressure changes, but not for measuring static pressures.
Capacitive pressure sensor
A capacitive sensor is based on the principle that the electrical capacitance of a plate capacitor can be affected in the following ways:
- Change of the effective plate area due to displacement of the plates (e.g. rotary capacitor)
- Change of the plate spacing, e.g. due to deformation of one of the plates
- Change in capacitance due to the presence of a dielectric or a conductive material
For use as a pressure sensor for pressure control, the side of a ceramic diaphragm facing away from the pressure is metallically coated so that, together with an underlying metallic plate, it forms a plate capacitor whose capacitance changes when the diaphragm is deflected.
Advantages are the simple and robust design as well as the temperature stability. Capacitive pressure sensors are also suitable for dynamic applications.
